# How the shape of fillers affects the barrier properties of polymer/ non-porous particles nanocomposites: A review

Caroline Wolf, Helene Angellier-Coussy, Nathalie Gontard, F. Doghieri, Valérie Guillard

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Caroline Wolf, Helene Angellier-Coussy, Nathalie Gontard, F. Doghieri, Valérie Guillard. How the shape of fillers affects the barrier properties of polymer/ non-porous particles nanocomposites: A review. Journal of Membrane Science, 2018, 556, pp.393-418. 10.1016/j.memsci.2018.03.085. hal-02626375

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How the shape of fillers affects the barrier
properties of polymer/ non-porous particles
nanocomposites: A review

C. Wolf, H. Angellier-Coussy, N. Gontard, F. Doghieri, V. Guillard

<figure><img src="image_2.png" /></figure>

www.elsevier.com/locate/memsci

PII: S0376-7388(18)30097-8

DOI: https://doi.org/10.1016/j.memsci.2018.03.085

Reference: MEMSCI16079

To appear in: *Journal of Membrane Science*

Received date: 11 January 2018

Revised date: 29 March 2018

Accepted date: 30 March 2018

Cite this article as: C. Wolf, H. Angellier-Coussy, N. Gontard, F. Doghieri and V. Guillard, *How the shape of fillers affects the barrier properties of polymer/non-porous particles nanocomposites: A review*, *Journal of Membrane Science*, https://doi.org/10.1016/j.memsci.2018.03.085

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# How the shape of fillers affects the barrier properties of polymer/ non-porous particles nanocomposites: A review

C. Wolf¹, H. Angellier-Coussy¹, N. Gontard¹, F. Doghieri², V. Guillard¹

¹UMR IATE, University of Montpellier, INRA, 2 place Pierre Viala, 34060 Montpellier Cedex 1, France

² DICAM University of Bologna, Via Terracini 28, I-40131 Bologna, Italy.

**Abstract:**

More than 1000 published experimental data of gas ($O_2$ and $CO_2$) and vapor ($H_2O$) permeability in nanocomposites containing either spherical, elongated or platelet particles were collected, assorted and compared in order to decipher the role of particle shape on the reduction of the relative permeability of the nanocomposite. It is well known that inclusion of homogeneously dispersed and oriented impermeable fillers with high aspect ratio, such as platelets or elongated particles, should significantly increase the diffusion path of gas and vapors and yield to improve barrier properties. Results revealed that this expected impact was not systematically achieved, even for impermeable lamellar fillers that usually displayed the highest aspect ratio. More specifically, an unexpected increase of the permeability in the nanocomposite was often observed. To explain this deviation of the 'ideal behavior', this paper discusses extensively the impact of the nanoparticle shape on the nanocomposite permeability along with structural aspects, related to both the particle nature and size, and the nanocomposite processing routes. Deviations from expected results of enhanced barrier effect are also discussed in correlation with unexpected variations in gas selectivity for $O_2$/ $CO_2$ pair. Lastly, this review aims at drawing meaningful conclusions on the structure/mass transfer relationships and giving directions for the development of the next generation of packaging materials with tailored mass transfer properties.

**Keywords:**

Particle shape, Nanocomposites, Structure & mass transfer relationships, Permeability
In the objective of developing efficient and optimal packaging, one of the main challenges is to design and provide food packaging materials able to protect the food from the external environment and to maintain food quality and safety throughout its shelf life [1,2]. The mass transfers are thus at the heart of the feature of the food packaging, especially transfers of water vapor, oxygen and/or carbon dioxide, which condition the rates of numerous reactions of food degradation (oxidation, microbial development, physiological reactions, etc.). The development of bulk nanocomposite structures by introducing nanoparticles, i.e. fillers having at least one dimension lower than 100 nm, in polymeric matrices appeared as one of the most promising directions in the development of packaging materials with advanced mass transfer properties.

The expected role of these inclusions is to achieve a significant decrease of the mass transfer properties as compared to the neat matrix by acting as physical obstacles to the diffusion and permeation of diffusing molecules, which have to follow a more tortuous pathway. This phenomenon is called the tortuosity effect.

Many types of nanoparticles have already been tested to modulate mass transfer properties, with hundreds of permeability data published in literature. Nanoparticles can be classified into three major categories according to their particle shape (Figure 1): (i) *isodimensional particles*, that have the same size in all directions and an aspect ratio close to unity (such as spherical silica, TiO₂ nanoparticles, carbon black and fullerenes); (ii) *elongated particles* that consist in fibrils with a diameter ranging between 1 and 100 nm and length up to several hundred nanometers (such as carbon nanotubes or cellulose nanofibers); and (iii) *layered particles* that are characterized by one dimension ranging from several angstroms to several nanometers (such as layered silicates or starch nanocrystals).

<figure><img src="image_3.png" /><figcaption>(ii) Elongated particle

$\ge 100 \text{ nm}$</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>(iii) Layered particle

$\approx 1-10 \text{ nm}$</figcaption></figure>

<figure><img src="image_5.png" /></figure>
<caption>**Figure 1.** Schematic representation of the three categories of nanoparticles.</caption>

Among the three aforementioned types of particles, it is widely recognized that inclusion of impermeable platelets such as clays in the polymer should significantly enhance its barrier properties. However, an in-depth study of the available data in the literature revealed that it was not systematically the case and, even, on the contrary could lead to a deterioration of the barrier properties. Many reviews deal with the functional properties of nanocomposites, by mainly focusing on mechanical properties [3–14]. Among these studies, some of them focused slightly on the mass transfer properties [5,6,8–12] but without deepening the topic. These studies generally focused on one type of nanoparticle, i.e. spherical nanoparticles or nano-platelets for instance, particularly for membrane applications. Few of them focused on food packaging applications [13,15–18]. The last exhaustive review in the field is that of Cui et al. in 2015 [19] that focused on gas barrier properties of clay-based polymer nanocomposites. However, the effect of the platelet-shape has never been compared and discussed along with results obtained with spherical and elongated particle shape. In spite of all these preceding reviews, there is still a huge lack of analysis of all available data in the perspective of deciphering the role of particle shape on the mass transfer properties of the resulting nanocomposite materials, even if it has been highlighted as a key input parameter in mathematical models. As far as we knew, there is no review that proposes an exhaustive analysis of the experimental permeability values from the available literature.

In order to better correlate the mass transfer properties in nanocomposite materials with their multi-scale structure, the objective of the present review is to decipher and comprehensively discuss the role of the nanoparticle shape (either isodimensional, elongated or layered) on the modulation of the mass transfer properties in nanocomposites, as a function of filler volume fraction and in the light of the nanocomposite structure achieved. For that purpose, more than 1000 values (i.e. about 170 articles) of the 1995-2015 period containing measured values of O₂, CO₂ and H₂O permeability in polymer-based nanocomposites were collected from the available literature and capitalized in a dedicated on-line database [20]. All data could be uploaded from the permalink provided in figure caption (https://doi.org/10.5281/zenodo.1136269). The proposed on-line database consists in the first and unprecedented compilation of permeability values
for nanocomposite based materials. The current work is focused on direct analysis and comparison of experiments, without any mediation which could be provided by quantitative models. It must be specified that, in order to build up the database for analysis, which is of interest here, recent data for gas barrier in polymer based materials with dispersed graphene and graphene derivatives were not taken into consideration. In fact, while the latter represent nanocomposite materials with dispersed layered domains, the extremely high aspect ratio and flexibility of graphene layers reduces the threshold value for percolation in these materials to much lower values than in traditional layered particles nanocomposites [21]. Under those conditions, in the volume concentration range for layered particles which is of reference in this work (in the order of 10% by volume) layers arrangement and layer defects are significantly more relevant to the barrier effect than the aspect ratio (particle shape) itself. For a comparison between gas barrier properties in polymer/clay and polymer/graphene nanocomposite see the review by Tan [22]. For the above reason, although graphene has attracted much attention in recent years, here also for the production of polymer based composites for packaging [] analysis of pertinent results for gas barrier properties is left to a different discussion and the interested reader is addressed to recent reviews in the field [23].

Besides the meaningful conclusions on the role of nanoparticle shape, the main output of this paper is to give some recommendations for the design of nanocomposite packaging materials with tunable mass transfer properties.

## 2. Brief overview of nanocomposites processing and morphologies

All the processes used to prepare polymer-based nanocomposites have already been extensively detailed in dedicated reviews and are not further detailed here. Examples of reference publications for layered silicates-based nanocomposites are those of Alexandre & Dubois [4], Ray et al. [11], Pavlidou et al. [6], Mittal et al. [9] and Cui et al. [19], for natural fiber-based nanocomposites, those of Saheb et al. [24] and Siqueira et al. [25] and for inorganic spherical particle-based nanocomposites, those of Cong et al. [8] and Chung et al. [5]. Due to the large range of polymer matrices and nanoparticles, different processing routes have been proposed to produce nanocomposites based on their convenience according to the nature of
the raw constituents and their field of application (Figure 2). At laboratory scale, four main processes are most commonly used to prepare polymer-based nanocomposites from all the types of nanoparticles. Melt processing and solution blending are two processes based on the use of polymers, which lead to the formation of Van der Waals interactions between the constituents, while in-situ polymerization and sol/gel methods are based on the use of monomers or oligomers, leading to the formation of covalent bonds between the polymer and the nanoparticles (Figure 2). At industrial scale, melt processing is generally preferred due to the difficulty to implement processes requiring the use of huge volumes of solvent.

Depending on the processing conditions, the particle shape and the affinity between constituents, *i.e.* the polymer matrix and the nanoparticle, different morphologies can be obtained. For isodimensional and elongated nanoparticles, three arrangements can be observed, *i.e.* (i) well-dispersed, (ii) agglomerated, or (iii) percolating systems. For platelets, specific names are given to the various possible morphologies: (i) micro-composite, (ii) intercalated nanocomposite or (iii) fully exfoliated nanocomposites. The micro composite structure corresponds to the formation of tactoïds and appears when the polymer chains do not manage to penetrate into the layered sheets due to, mainly, a poor particle/polymer matrix affinity, an insufficient interlayer distance and/or an inefficient shearing during the process. Intercalated nanocomposite structures are obtained when the polymer chains have diffused between silicate layers, leading to an increase of the interlayer distance. Exfoliated nanocomposite structures are characterized by a complete delamination and homogeneous dispersion of the layered sheets. Depending on the process, an isotropic (random) or anisotropic orientation could be achieved (for example platelets orthogonal to the diffusion flux).

A good knowledge of the morphologies obtained is an indispensable prerequisite to well understand variations of mass transfer properties in the nanocomposite materials.
<figure><img src="image_6.png" /><figcaption>**Figure 2:** Outline of the preparation techniques of nanocomposite based materials</figcaption></figure>

### 3. Effect of the nanoparticle shape on the modulation of mass transfer properties

#### 3.1. Theoretical background

A great amount of effort has been made in the literature to describe the effect on transport properties of the inclusion of non-conducting particles into a conductive matrix and the effect of particle shape has been a focus since the beginning of these studies. Similar analyses of the problem can be found in the field of electrical properties (electrical conductivity/permittivity) as well as in that of heat or mass transfer (thermal conductivity/mass diffusivity). Exact solutions in terms of effective conductivity of the composite medium have been obtained already by Maxwell in 1873 [26] for the case of dilute mixture of spherical domains ("non-interacting particles") perfectly adhering to the matrix. These exact solutions assume that the properties of dispersed and continuous phase in the composite are the same than the one measured in the corresponding pure materials ("ideal interface condition").

Wiener [27] in 1912 extended the results to the case of dilute dispersion of long cylinders in the matrix, all aligned in the direction normal to the overall flux. While exact solutions are also available for the case of spheroids (dilute mixture, perfect alignment), empirical relations as well as numerical result have been discussed for the case of different shapes of the inclusions. Among the best, known results of this kind are those obtained by Nielsen [28] in 1967 following empirical ideas about the increase in contour length of
path of diffusing molecules in the composite medium due to the presence of impermeable inclusions in the form of bricks. The specific expression for effective conductivity as function of particle concentration in the composite as derived from the above models is not here presented and discussed, as the direct comparison of available experimental results with model predictions is not in relevant to this work. On the other hand, it is useful to remember here that all the above approaches consistently predict a decrease in the conductivity when the concentration of impermeable particles in the composite increases. In general terms, it can be said that, assuming ideal interface and non-interacting particle condition, the effective conductivity of the composite medium is expected to be half the value of the pure matrix when the concentration of dispersed phase approaches 40% vol for isometric particles, 30% vol for elongated particles aligned normally to the flux, and a much lower value, only few percent, depending on the exact shape, for layered particles lying in planes orthogonal to the flux.

It should be finally mentioned that more complex analyses are definitely needed to consider the interaction between particles in the medium (concentrated mixtures), the distribution in their orientation, as well the non-ideal interaction with the matrix which can ultimately result in regions close to the interface with properties different from that of pure materials [29].

## 3.2. Explanation of the analysis approach

The publications considered in the following part were collected from peer-reviewed scientific journals published between 1995 and 2015 in different fields of research such as membrane, materials and packaging sciences. All these papers, dealing with nanocomposite materials, presented at least one value of permeability in relation to their composition. The nanoparticles considered in works examined in this review are those assumed as impermeable and non-porous with respect to the application that is focused on: barrier membranes mainly for packaging application. Moreover, it must be noted that mass transfer does not follow the same mechanisms in dense and porous material and that mixing porous and non-porous materials may disturb the analysis. Only gases ($O_2$ and $CO_2$) and water vapor permeability data were considered because of their high interest for food packaging applications. These data were analyzed as regard to the particle shape (isodimensional, elongated, or platelet), the filler volume fraction, the eventual
chemical and functionalization treatments applied to the nanoparticle and the elaboration technique used to prepare materials. The main characteristics of the nanocomposites studied in the present review are gathered in Tables 1, 2 & 3.

Tables 1, 2 and 3 (these tables are provided at the end of the manuscript for the sake of clarity)

In the following, the evolution of the relative permeability, i.e. the ratio of the composite permeability to the permeability of the neat matrix (P/P₀), was represented as a function of the nanoparticle volume fraction (Φₗ), which is considered as input parameter in all mathematical models. This implied to systematically calculate Φₗ values from weight fractions (Φₘ) values given in the original papers, based on the knowledge of the true density of each constituent (equation 1):

$$ \Phi_{\text{vol}} = \frac{\frac{\Phi_{\text{wt}}}{\rho_{\text{nanoparticle}}} \frac{1-\Phi_{\text{wt}}}{\rho_{\text{matrix}}}}{\frac{\Phi_{\text{wt}}}{\rho_{\text{nanoparticle}}} + \frac{1-\Phi_{\text{wt}}}{\rho_{\text{matrix}}}} \quad [1] $$

where ρₘ and ρₙ are the true density of the matrix and the nanoparticles respectively. Such a conversion led to unavoidable uncertainty on Φₗ.

3.3. Modulation of mass transfer properties in nanocomposites: global evolution of P/P₀

Most of the studies of O₂, CO₂ and H₂O permeability in nanocomposites were carried out in platelets-based nanocomposites with about 100 publications on the topic resulting in a total of 710 permeability values. All of them have been represented in Figure 3c leading to a scatter graph in which each individual study could not, of course, be distinguished. For comparison, 45 publications were found for isodimensional nanoparticles-based nanocomposites against 22 for elongated nanoparticles-based nanocomposites, resulting in about 380 and 120 permeability values respectively (Figures 3a and 3b). Papers dealing with elongated particle displaying micrometric sizes (13 papers more) were voluntary excluded from this review. Due to the high number of studies found for each type of particle shape (for O₂, CO₂ and H₂O permeability), the figures are overloaded and thus not easily readable. Three complementary figures have been drawn for each type of particle shape and for each molecule studied, i.e. O₂, H₂O and CO₂ respectively. Analysis of these general figures permits to highlight the intensive work on the topic and to draw the general
tendencies of P/P₀ variation as a function of nanoparticle shape and Φvol and nature of the penetrant. The
following observations can be made:

* The investigated Φvol range is higher for isodimensional and elongated nanoparticles than for platelets with maximal Φvol values of 40 vol% and 45 vol% respectively (Figures 3a and 3b), against 15 vol% for nanoplatelets (Figure 3c). Only 3 publications (i.e. 11 permeability values) were in the range of 15-40 vol% for nanoplatelets-based nanocomposites (not shown in Figure 3c). This feature could be ascribed to the fact that usually, only a small weight fraction of nanoplatelets (generally 1 to 5 wt%) is necessary to obtain a significant effect on the material functional properties.

* The evolution and variation amplitude of P/P₀ as a function of the nanoparticle volume fraction is really complex and does not follow the same trend according to the shape of nanoparticles (Figure 3). Figures 3a and 3b permit to point out that the addition of isodimensional or elongated nanoparticles in a polymer generally provokes either a monotonic decrease or increase of P/P₀ with an amplitude of variation ranging from 0.15 to 16 and from 0.1 to 18, respectively. This leads to a scatter graph of P/P₀ with a general aspect in herringbones centered on P/P₀ =1. On the contrary, Figure 3c shows that for nanoplatelets, except for one or two cases, P/P₀ values remain always below 1.

* The maximum drop in P/P₀ is much higher for nanoplatelets-based nanocomposites, with P/P₀ reaching a minimum value of 0.01, i.e. up to 10 folds lower than the minimum value reached with elongated nanoparticles, and in spite of the lower filler volume fractions used.

* For all the kinds of nanoparticles, some non-monotonic variations of P/P₀ are noted with simultaneous increase and decrease of the relative permeability for the nanocomposites. It represents approximately 12% of the studies for isodimensional nanoparticles, 21% of the study for elongated nanoparticles and only 7% of the studies for nanoplatelets.

* Analysis of the trends per type of penetrant, O₂, H₂O and CO₂ respectively, did not lead to find some difference on P/P₀ regarding the molecules studied related to geometry. We also noted that the nature of the material depending of particle geometry did not influence the trends observed on the general figure.
Among the aforementioned conclusions of this first general analysis of P/P₀ evolution, some trends were expected, including the fact that nanoplatelets generate, more often and with a higher amplitude than elongated and/or isodimensional nanoparticles, a decrease of the permeability. This could be easily ascribed to the particle geometry and the more pronounced tortuosity effect induced by platelets as compared to spheres or cylinders. To go further, an in-depth, quantitative analysis of the data collected and capitalized was carried out.

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>
<figure><img src="image_9.png" /></figure>

a. Isodimensional particles

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>
<figure><img src="image_13.png" /></figure>

b. Elongated particles

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>
<figure><img src="image_17.png" /><figcaption>c. Layered particles</figcaption></figure>

**Figure 3:** Evolution of the relative permeability $P/P_0$ as a function of particle volume fraction in nanocomposite material containing either (a) iso-dimensional, (b) elongated or (c) layered particles. From references listed in Tables 1, 2 and 3 respectively. Raw data can be uploaded from https://doi.org/10.5281/zenodo.1136269.
3.4. In-depth analysis of the modulation of mass transfer properties per type of nanoparticle

In order to perform a quantitative analysis of the efficiency of each type of nanoparticle shape on P/P₀, all the permeability data collected in the literature were gathered on a histogram displaying the percentage of values per class of modulation, i.e. per range of P/P₀ values. The maximum number of permeability values collected for isodimensional (43%), elongated (29%) and layered (30%) nanoparticles is obtained for the classes 1 < P/P₀ < 2, 1 < P/P₀ < 2, and 0.6 < P/P₀ < 0.8 respectively, confirming that layered nanoparticles were more prone to improve the barrier properties of a given polymer (Figure 4).

Besides this main impact, secondary peaks are observed on **Figure 4**. In the case of isodimensional nanoparticles, 17% of values are obtained for the class 2 < P/P₀ < 3 and 14% for 0.8 < P/P₀ < 1, indicating that the permeability in these nanocomposites could be equally multiplied by a factor of 3 or slightly decreased by a factor of 0.8. This feature confirms well the herringbone scatter graph observed when all P/P₀ data are plotted as a function of Φ<sub>vol</sub> (Figure 3a).

In the case of elongated nanoparticles, 17% of collected data belong to the class 0.6 < P/P₀ < 0.8 and another 17% to the class 0.8 < P/P₀ < 1. This indicated that even if the main observed effect of the addition of elongated nanoparticles is a weak increase of permeability up to a factor 2, it could also statistically lead to a decrease of the permeability.

In the case of layered nanoparticles, the overall trend of decreasing permeability is confirmed by
the occurrence of secondary peaks, which reveals that 23% of permeability values were obtained
for 0.4 < P/P₀ < 0.6 and 21% for 0.8 < P/P₀ < 1. In all the cases, P/P₀ values higher than 3 are very
rare (less than 6% of the studies for every particle shape).
<figure><img src="image_18.png" /><figcaption>**Figure 4.** Percentage of permeability values collected in literature for nanocomposites filled with either iso-dimensional/spherical (light-grey), elongated (dark-grey) or layered (black) nanoparticles as a function of the classes of the relative permeability $P/P_0$</figcaption></figure>

To verify if the conclusions drawn from this global analysis were confirmed for all the tested filler contents, permeability values were also apportioned into classes of filler volume fraction in addition to the classes of $P/P_0$ values (Figure 5). The classes chosen for filler fractions were 0-5 vol%, 5-10 vol%, 10-20 vol% and 20-30 vol%. For this analysis, the range for $P/P_0$ was limited to a maximal value of 3, considering that a higher increase of the permeability ratio was an exceptional behavior. In the case of isodimensional nanoparticles, exactly the same evolution of $P/P_0$ is obtained for each filler fraction, with an increase up to a factor 2 of the permeability in almost 45 % of the cases (Figure 5a). For elongated nanoparticles, the same increase of $P/P_0$ is obtained but only for filler contents lower than 20 vol%. For higher filler contents, no clear tendency is evidenced, with a disordered shape of the curve. This could be due to agglomerated or percolated structures that become frequent for such high concentration in fillers whose effect
on permeability is more unpredictable (Figure 5b). Finally, for nanoplatelets-based nanocomposites, for filler contents lower than 5 vol%, the evolution of P/P₀ is the same as the general one previously observed on Figure 4 and is characterized by a main peak (30% of data) centered on the class 0.6 to 0.8. This main peak shifts to lower P/P₀ values, i.e. better efficiency, for filler contents ranging from 5 to 10 vol%, with a main peak (38%) for P/P₀ values in the range of 0.4 to 0.6. For filler content higher than 10 vol%, the decrease in permeability is not confirmed anymore. We noted that studies conducted with such high amounts of platelets (> 10 vol%) were scarce, which makes it difficult to generalize the effect of high contents of platelets on the permeability of resulting nanocomposites (Figure 5c).

To sum up, beyond the fact that modulation of the permeability is clearly affected by the shape of the nanoparticles, three conclusions remain:

(i) whatever the filler content, the addition of isodimensional particles does not statistically impact the permeability of resulting nanocomposites very much,

(ii) a significant decrease of permeability could be achieved by nanostructuring polymer matrices with the introduction of either elongated or nanoplatelets-shaped nanoparticles, with a much higher efficiency of layered nanoparticles. Indeed, the use of filler contents higher than 20 vol% are necessary to achieve a significant decrease of P/P₀ in the case of elongated nanoparticles while less than 10 vol% are sufficient in the case of nanoplatelets,

(iii) the barrier effect of nanoplatelets is even more pronounced for filler contents ranging from 5 to 10 vol%.

This analysis has also revealed that the behavior of P/P₀ is more complex than expected, with the occurrence of non-monotonic behaviors, especially in the case of elongated nanoparticles. In the following, lines of explanation will be tentatively brought in order to relate this “shape effect” to the morphology of the particle and the (nano)-structure of the composite.
<figure><img src="image_19.png" /></figure>

?

?

<figure><img src="image_20.png" /></figure>

?

<figure><img src="image_21.png" /></figure>

?
**Figure 5:** Percentage of permeability values collected for nanocomposites filled with spheres (a), cylinders (b) and platelets (c) -based composites according to the volume filler content and as a function of the relative permeability P/P₀

**4. In-depth investigation and understanding of the mass transfer properties/structure relationships in nanocomposites**

In the following section, a case by case qualitative explanation of the modulation of the permeability according to particle morphology will be proposed by focusing on the nanostructure obtained and its relationship with mass transfer properties. Three cases will be considered: (1) “ideal” case of monotonic decrease of P/P₀, (2) monotonic increase of P/P₀ and (3) non-monotonic variation of P/P₀. A critical discussion relying on the mechanisms proposed by the different authors will be offered.

**4.1. Monotonic decrease of the permeability**

Theoretically, the incorporation of impermeable particles displaying a good compatibility with the polymer matrix should favor the decrease of the permeability of gases and water vapor through an increase of the tortuous path for the diffusing molecules. Such “ideal” cases were largely observed in data collected from literature, as illustrated on **Figure 3** for a large range of nanoparticles displaying various shapes, either isodimensional, elongated or nanoplatelets. For the sake of clarity, the exhaustive list of these references is in footnote<fnref n="1" />. The amplitude of this

**For spherical particles:** TiO₂ [Hu et al. [84]], SiO₂ nanoparticles [Zoppi et al. [50]], Patel et al. [89,90], Vladimorov et al. [44], Vassiliou et al. [45], Zhu et al. [40], Sadeghi et al. [43], Sadeghi et al. [81]

**For elongated nanoparticles:** cellulose nanocrystals [George et al. [99]], cellulose nanowhiskers [Saxena et al. [65], Sanchez-Garcia et al. [105,107], Bilbao-Sainz et al. [51]], cellulose micro-fibres [Fendler et al. [174]],

**For nanoplatelets:** montmorillonites [Gatos et al. [30], Strawhecker et al. [160], Chang et al. [63,144], Xu et al. [169], Choi et al. [118], Chien et al. [116,175], Yeh et al. [170], Herrera-Alonso et al. [128,176], Katiyar et al. [134], Alboofetileh et al. [110], Abdollahi et al. [109]], rectorite silicate [Wang et al. [177]], mica silicate [Sanchez-Garcia et al. [178],

<footnote n="1">¹ Among studies that observed a decrease of P/P₀, we can cite the following papers:</footnote>
decrease largely depends on the particle shape and/or particle volume fraction used. The authors mainly attributed this result to (1) a tortuosity effect induced by the nanostructure created by the incorporation of an impermeable particle, and to a lesser extent to (2) the modification of the polymer matrix properties by the presence of nanoparticles, or (3) the modification of the availability of specific sorption sites for the sorption of the diffusing molecule. These three assumptions will be discussed in the following paragraphs.

### 4.1.1. Tortuosity effect.

When impermeable nanoparticles are incorporated within a polymer matrix, they act as an obstacle to the diffusion and thus the permeation of diffusing molecules, which have to follow a more tortuous pathway. Mathematically, tortuosity (τ) represents the ratio of the distance that a molecular specie must follow through the nanocomposite thickness to the distance through the neat polymer.

This tortuosity effect is influenced by the volume fraction and the dispersion and distribution states of nanoparticles within the polymer matrix. Concretely, it could be related to two quantitative parameters, i.e. the in-situ size aspect ratio of the particle and the orientation of nanoparticles.

cellulose nanocrystals [Fortunati et al. [125]], waxy maize starch nanocrystals [Angellier et al. [113]], boron nitride [Swain et al. [161], Kisku et al. [136]] or silicon carbide [Kisku et al. [137], Dash et al. [120]].
In-situ particle aspect ratio.

The size aspect ratio of a nanoparticle is defined as the ratio of its longest dimension to its lowest dimension. Considering the different possible shapes of nanoparticles, it is obvious that the size aspect ratio of a sphere, equal to 1, is lower than that of elongated nanoparticles and nanoplatelets for which very high size aspect ratio could be achieved.

The achievement of high size aspect ratios theoretically favors the establishment of a more tortuous pathway. This effect is all the greater for nanoplatelets that allow the creation of 2D obstacles. This has been experimentally validated by Gatos et al. [30] who prepared by melt processing fully-exfoliated nanocomposites from a hydrogenated acrylonitrile butadiene rubber polymer and two layered silicates displaying different sizes, i.e. octadecylamine modified montmorillonites with a size aspect ratio of 100 and synthetic fluorohectorite with a size aspect ratio of 200. They observed that the highest decrease of oxygen permeability was achieved using the fluorohectorite as filler, which was ascribed to its higher aspect ratio [30]. It must be noted that the achievement of such high in-situ size aspect ratios is exceptional. Indeed, the formation of stacks or tactoids within the polymer matrix is usually obtained due to the agglomeration and/or incomplete exfoliation of nanoplatelets, resulting in a decrease of the theoretical size aspect ratio. An intercalated nanocomposite structure is thus less tortuous than a full-exfoliated structure. This implies that the in-situ size aspect ratio is usually different from the one of the native nanoparticle. As an example, Angellier-Coussy et al. [31] found average values of in-situ size aspect ratio of 17 and 23 for 10.8 and 2.2 vol% of filler, respectively in wheat gluten/montmorillonite based nanocomposites while the size aspect ratio of one native nanoparticle was about 80. This issue is the same for elongated nanoparticles where agglomeration often occurs.

In practice, the knowledge of the *in-situ* size aspect ratio is thus necessary to fully understand and predict the impact of tortuosity on mass transfer properties. It is usually evaluated using image
analysis of TEM pictures when nanoparticles can be easily distinguished from the matrix as is the case for montmorillonite [31]. This analysis is tricky and time consuming. Indeed, a large number of images must be analyzed to obtain a significant representativeness of the whole material. In addition, it is well-known that TEM analysis has some inherent limitations due to the fact that it projects three-dimensional (3D) objects onto a two-dimensional (2D) plane, resulting in TEM cross-sections that do not necessarily represent the actual size of all the platelets [32]. Furthermore, an ideal image would exhibit sharp transitions from black to white, what is not the case in reality, necessitating the conversion of the original gray-scale TEM micrograph into a black and white image using a semi-automated approach, this step being inevitably accompanied by errors. And finally, this evaluation must be done for each filler content, since the dispersion state strongly depends on the filler content. All these limitations probably explain why the evaluation of the *in-situ* size aspect ratio is rarely experimentally determined. It is usually an imprecise, unrealistic (if the size aspect ratio of the native nanoparticle is considered) or missing data.

**In-situ size aspect ratio and dispersion state.**

As explained above, the *in-situ* aspect ratio strongly depends on the particle dispersion state. Contrary to the *in-situ* size aspect ratio, which is rarely determined, the impact of the particle dispersion state on nanocomposite mass transfer properties has been largely demonstrated. For example, Koh et al. [33] demonstrated with PLA/organo-modified montmorillonites nanocomposites that the highest decrease of the oxygen and carbon dioxide permeability was obtained for the most exfoliated structure. The same positive impact of exfoliation on permeability decrease was also obtained by Sanchez-Garcia et al. [34] in PHBV-PCL nanocomposites and by Sun et al. [35] for α-zirconium phosphate/epoxy resin nanocomposites.

It is worth noting that the achievement of a good dispersion state is all the more difficult as the filler content increases due to agglomeration phenomena, as evidenced in many systems such as agar- (Rhim et al. [36]), polyester- (Bharadwaj et al. [37]), polyamide- (Picard et al. [38]) or
polyimide- (Chang et al. [39]) based nanocomposites with montmorillonite as nanoparticles.

Chemical modification of nanoparticles does not always prevent agglomeration, as evidenced by Zhu et al [40] with modified SiO₂ particles in poly (vinyl chloride) matrix.

**Orientation.**

To obtain an optimal decreasing effect on permeability, the largest surface of each nanoparticle must be oriented perpendicular to the direction of the gas diffusion or permeation. Well-oriented and aligned particles perpendicularly to the permeation flux have more impact on the decrease of the permeability than disoriented particles as regards the tortuosity effect. This effect of orientation is only valid for elongated particles and nanoplatelets and, of course, all the more significant for particles with high size aspect ratios. This effect of orientation is also theoretically much more efficient for nanoparticles displaying high size aspect ratio in two dimensions (such as platelets) than only in one dimension (such as elongated particles). In practice, it is an extremely challenging task to achieve a regular arrangement of nanoparticles within the polymer matrix. It is strongly dependent on processing methods. Solution intercalation and in-situ-polymerization are more prone to produce nanostructures with well-aligned and oriented particles than melt intercalation as observed by several authors. It is all the more difficult that particles display high aspect ratio in 2-dimension such as platelets. For example, Messersmith et al. [41] showed that montmorillonites adopted a planar orientation in the poly(ε-caprolactone) polymer matrix after in-situ-polymerization and solution casting processing while Sanchez-Garcia et al. [42] noted that it was difficult to obtain, using melt processing and compression molding, well aligned and oriented montmorillonites in PHBV-PCL polymer and that a random orientation is at best the most often achieved.

**4.1.2. Modification of the polymer matrix.**
The incorporation of nanoparticles can modify the structure and thus the mass transfer properties of the polymer matrix itself. The two main structural modifications reported to contribute to the decrease of P/P₀ are (i) a reduction of polymer free volume associated to an increase in the glass transition temperature and/or (ii) an increase in crystallinity due to a nucleating effect of nanoparticles. The effect of polymer free volume was for example evidenced in the study of Sadeghi et al. [43] by a reduction of the polymer chain mobility in the amorphous phase in the case of polyurethane-silica nanocomposites. The impact of nanoparticles on polymer crystallinity was largely reported for either spherical particles such as silica nanoparticles (Vladimirov et al. [44]; Vassiliou et al. [45]) or layered particles such as montmorillonite (Sanchez-Garcia et al. [42]; Ghasemi et al. [46]). Crystallites act as obstacles to migrant diffusion.

#### 4.1.3. Decrease of polymer sorption sites.

The chemical modification of the nanoparticle surface is generally performed to reach a better filler/matrix affinity and thus a better dispersion of nanofillers within the polymer matrix. A side effect of this higher affinity is a decrease of the availability of the sorption sites in the polymer. It is particularly true for highly interactive molecules such as water vapor that interacts though hydrogen bonding with hydrophilic sites. These sites bonded with the nanoparticles are not any more available for water sorption. Such changes in specific interactions between the migrant and the polymer were proposed to explain the decrease of water vapor permeability observed by Tunc et al. [47] in wheat gluten/montmorillonite or by Lee et al. [48] in soy protein/montmorillonite nanocomposites.

### 4.2. Monotonic increase of the permeability

Several assumptions are proposed in literature to explain either the absence of significant modification or even an increase of the permeability in nanocomposites: (1) an insufficient tortuosity effect, (2) the occurrence of macroscopic structural defects in the nanocomposite
material induced for instance by particles agglomeration, (3) a significant modification of the structure of the polymer matrix such as a decrease of crystallinity or molecular weight, and (4) an increase of migrant sorption induced by nanoparticles (case of water vapor for example, for water sensitive materials). It is worth noting that some mechanisms assumed to explain an increase of P/P₀ were previously discussed for explaining a decrease of P/P₀ (§ 3.1).

### 4.2.1 Lack of tortuosity effect.

In some cases, the dispersion state of nanoparticles did not allow to achieve a sufficient tortuosity effect to slow down the diffusion of migrating molecules, especially for small molecules such as gases. Such an explanation is widely proposed to explain why values of permeability did not change or only slightly decreased in the nanocomposite as compared to the neat matrix. This has been, for example, proposed by Tunc et al. [47] to explain the constancy of O₂ permeability in wheat gluten/montmorillonite nanocomposites. In the same article, authors however demonstrated that the achieved tortuosity was nevertheless sufficient for decreasing permeability of bigger molecules such as aroma compounds. This highlighted that the nature, molecular weight and/or steric hindrance of the migrant are also key parameters governing the mass transfer properties in nanocomposites.

### 4.2.2. Macroscopic structural defects

#### Particle agglomeration.

The presence of both nano- or micro-sized agglomerates and aggregates of particles could induce macroscopic structural defects, which play a major role in the increase of P/P₀, as already shown for either spherical-based composites with SiO₂ [Cornelius et al. [49]], TiO₂ nanoparticles [Zoppi et al. [50]] or cylinder-based composites with cellulose fibrils [Bilbao-Sainz et al. [51]]. These agglomeration phenomena are related to a bad dispersion of the nanoparticles which is mainly ascribed to (1) mismatching of the particle polarity, as compared to that of the polymer, resulting
in a low affinity between constituents and thus a poor particle/polymer adhesion [50] and/or (2)
inefficiency of the elaboration technique to separate aggregates.

For example, insufficient shearing forces during the melt processing could cause agglomeration of
particles. During casting, sol-gel method and in-situ polymerization, coalescence of the particles
during drying of the solution could appear through insufficient particle/particle interactions ("bad
compatibility"). This last phenomenon is normally limited by chemical modification of the surface
of the particles. But when the effect of this chemical modification is wrongly anticipated it could
even amplify/trigger coalescence of particles as evidenced by Zhu et al. [40] in their study of H₂O
and O₂ permeability of silica/PVC nanocomposites with SiO₂ surface grafted with polymethyl
methacrylate (PMMA). They noticed that high content of filling (> 2.5 vol/vol%) increased the
permeability of both O₂ and H₂O due to an agglomeration phenomenon owing to the tendency of
nanoscale silica modified nanoparticles to be fused together at this concentration. This
phenomenon was not anticipated.

Agglomeration could lead to the formation of “interfacial voids” at the particle/polymer matrix interface and to the formation of a preferential pathway for the migration of the molecular penetrant [Ismail et al. [52], Rafiq et al. [53]]. This is further detailed below in the section dedicated to the “interphase”.

Interphase.

An interphase, i.e. a “third compartment” with its own properties, is generated at the
particles/polymer interface due to the establishment of either weak or strong interfacial
interactions between the particles and the polymer matrix. Depending on its nature and thickness
(or volume fraction), the interphase could significantly participate to the overall mass transfer
properties and strongly influence the overall permeability of the composite as evidenced by Liang
et al. [54] in their poly(ethersulfone)/montmorillonite nanocomposites. These authors
hypothesized that the interphase in their nanocomposite structure was made of interface voids
due to a poor adhesion between hydrophilic nanoparticles and hydrophobic polymers
contributing to the high gas permeability observed.

4.2.3. Modification of the polymer matrix.

The incorporation of nanoparticles could result in the disruption of polymer chain packing,
causing an increase of polymer matrix free volume. This increase in free volume leads to an
increase in permeability or to a lesser decrease than expected. This impact on free volume is
often just a hypothesis that is not always confirmed experimentally, due to the difficulty to
experimentally access such information.

Such hypothesis of polymer matrix modification with increase of free volume was speculated by
several authors for isodimensional particles-based composites with titanium dioxide [Matteucci et
al. [55], Moghadam et al. [56]] or silica nanoparticles [Dougnac et al. [57], Romero et al. [58]] and
in elongated particles-based nanocomposites with carbon nanotube [Murali et al. [59]]. An
increase of free volume was also hypothesized to occur when using compatibilizing agents due to
bad compatibility between the compatibilizer and the surface treated particles (Mittal et al. [60]).
The addition of nanoparticles in a polymer matrix could also hinder the crystallization process if
chain mobility is hindered, leading thus to a decrease of the overall crystallinity of the polymer
that could contribute to enhance the overall mass transfer within the composite [Dougnac et al.
[57]].

We noted that this increase of free volume is often considered as a global explanation resulting
from different phenomena: in addition to the modification of polymer matrix, increase in free
volume could also result in void space formation by nanoparticles aggregation and weak
interaction between polymer-nanoparticles at interface [Moghadam et al. [56]]. All these
mechanisms are not easily distinguished. Therefore, authors stressed an overall increase of free
volume that they experimentally assessed by density measurements, without distinguishing all
underlying mechanisms. Indeed, if the density of the nanoparticles, crystalline polymer and
amorphous polymer phases has their pure component values in the nanocomposites, then the
density of a nanocomposite sample, would be equal to the theoretical additive density. The
difference between theoretical and experimental densities can be rationalized by the overall
increase of free volume as evidenced by [Matteucci et al. [55]] in spherical-based composites with
titanium dioxide.

**4.2.4. Increasing migrant sorption.**

The increase of P/P₀ could be related to the establishment of specific interactions between the
molecular penetrant and the material constituents, as largely observed for water vapor and
hygroscopic nanoparticles such as silica nanoparticles [Bracho et al. [61], Dougnac et al. [57]] or
microcrystalline cellulose [Dogan et al. [62]], and also for O₂ and organo-modified montmorillonite
[Chang et al. [63]], for O₂ or modified silica nanoparticles [Iwata et al. [64]] or CO₂ and titanium
dioxide nanoparticles [Matteucci et al. [55]].

Different mechanisms could occur concomitantly, explaining why significant effects on P/P₀ are
not systematically observed. For instance, Dogan et al. [62] did not notice any significant change
of the water vapor permeability in hydroxyl propyl methyl cellulose/microcrystalline cellulose
nanocomposites in spite of a tortuosity effect. This was ascribed to the occurrence of a
competitive effect, i.e. an increased water affinity with the materials. The same happened in the
study of Matteucci et al. [55]: they observed an enhancement of CO₂ permeability in spherical-
based composites with titanium dioxide due to an increase in gas solubility in the nanocomposite
films upon incorporation of highly sorbing nanoparticles into the polymer. This effect is
overlapping with the creation of voids and global increase in free volume with increasing particle
loading. It should be noted that all these explanations are always hypotheses that are usually not
experimentally validated.
Figure 6 is providing summary of the two previous 4.1 and 4.2 paragraphs with key take home messages about the main mechanisms explaining monotonic increase of decrease of the ratio P/P₀.

<table><tr><td>► P/P₀</td><td><strong>Efficient tortuosity effect</strong><br>high in-situ particle aspect ratio<br>Good dispersion of particles<br>Orientation [1] flux [2]<br><br><strong>Modification of the polymer matrix</strong><br>↓ polymer free volume & Tg<br>↑ crystallinity (nucleating effect) of particles<br><br><strong>Decrease of polymer sorption sites</strong><br>better filler/matrix affinity [3]</td></tr></table>

<table><caption>**Figure 6:** Main mechanisms explaining monotonic increase of decrease of the ratio P/P₀ in nanocomposites</caption><tr><td>P/P₀</td><td><strong>Lack of tortuosity effect</strong><br><br><strong>Macroscopic structural defects</strong><br>Particle agglomeration<br>Presence of an interphase<br><br><strong>Modification of the polymer matrix</strong><br>☐ free volume<br>void space formation<br><br><strong>Increasing migrant sorption</strong><br>specific interactions, penetrant material</td></tr></table>

### 4.3. Non-monotonic variation of P/P₀

The permeability towards gases and water vapor did not always monotonically vary. In a general manner, theories already provided above for explaining monotonic decrease or increase of P/P₀ remain valid in that case and apply to a specific part of the curve.

The most observed non-monotonic variation of P/P₀ is first a decrease of P/P₀ until reaching a threshold value of filler content, followed by a sudden increase of P/P₀. This evolution is generally ascribed to a change in the nanostructure as a function of filler content. For example, in the work of Sanchez-Garcia et al. [42] dealing with montmorillonite-based nanocomposites, the permeability first decreased due to a tortuosity effect and then increased for filler contents higher than 5 vol% due to the formation of agglomerates. In the case of elongated particles-based nanocomposites [Dogan et al. [62], Saxena et al. [65], Paralikar et al. [66]], a similar evolution was observed with the formation of particle agglomerates at higher particle volume fractions (generally > 10 vol%). In addition to agglomeration that could reverse the trend of variation of P/P₀, some extensive cracking of the composite film could occur and lead to a complete loss of
barrier properties [Chaiko et al. [67]], which was attributed by the authors to excessive particle agglomeration at high particle fraction.

An opposite evolution has been also noted in the case of water vapor as migrant: first, an increase of the water vapor permeability probably caused by enhanced affinity between water vapor and composite constituents displaying a hydrophilic nature and, second, a decrease of the permeability induced by the tortuosity that finally is sufficient enough at high loading rates to decrease the permeability value. This has been observed by Follain et al. [68] for poly(ε-caprolactone)/cellulose nanocrystals.

### 4.4. Explanation of the evolution of the permeability in line with the evolution of solubility and diffusivity coefficients

In this section, it is considered that the properties of the polymer matrix are not affected by the incorporation of nanoparticles and that nanocomposites are two-phases systems with a perfect adhesion at the particle/matrix interface. This makes the relation P=D×S, where P, D and S are respectively the permeability, diffusion and solubility coefficients, valid [49,69]. In order to explain the complex evolution of P/P₀ as a function of filler content, some authors tried to decompose the permeability into diffusivity and solubility and to relate the apparent evolution of P/P₀ to change in D or S or both. For that purpose, they generally determined the diffusivity and/or solubility coefficients independently of the permeability.

In nanocomposites, a decrease in permeability should be ideally related to a decrease in both diffusivity and solubility coefficients due to the formation of a more tortuous pathway (influence on D) and to the diminution of the volume for the sorption of gases or vapors. However, this ideal case was not always observed. In most cases, solubility did not systematically decrease and tended either to remain constant or even to increase.
For example, O₂/CO₂ solubility was found to remain constant in poly(amide-6-b-ethylene oxide) [70] or brominated poly(2,6-diphenyl-1,4-phenylene oxide) [71] filled with silica nanoparticles while CO₂ solubility was found to increase with the addition of silica particles in polybenzimidazole [72]. Despite the fact that the same nanoparticle was used in these three studies, the effect on gas solubility obtained was not. This demonstrated that if the particles could be considered as impermeable (D=0), it is not true for their solubility. As mentioned above in § 3.2, increase of S could also be related to the progressive uncovering of active sorption sites in the polymer matrix as a consequence of particle addition and to the creation of specific sites at the particle/polymer interface that favor the sorption of molecular penetrants as hypothesized by Sadeghi et al. [72] and Suzuki et al. [73].

As regards the diffusivity coefficient, the expected behavior is a decrease due to the restriction of the motion of molecular penetrant and the creation of a more tortuous path caused by the presence of the particles [72,73]. But the opposite phenomenon could also occur. Sadeghi et al. [74], seeking to understand the increase of the permeability in polyvinyl acetate with the addition of silica nanoparticles, made the assumption that the diffusivity coefficient could increase either due to a reduction of the packing density of the polymer which provides further open structures for the diffusion of CO₂ or to a reduction of the crystallinity which lead to more amorphous phases where diffusion happens. The latter effect was also hypothesized by Kono et al. [75] and Ahn et al. [76] on silica nanoparticle based nanocomposite. The diffusivity could also remain constant or increase with the addition of impermeable particles due to bad adhesion or compatibility between constituents or the creation of interconnected cavities/channels as evidenced by Suzuki et al. [73], Kim et al [70] (2001) and Cong et al. [71].

To sum up, following analysis of all aforementioned experimental evidences, we can conclude that there is generally a competition between an increase of the solubility and a decrease of the diffusivity. Therefore, non-monotonic changes of P/P₀, in peculiar, decrease of P/P₀ followed by
an increase for higher particle volume fraction, could be generally ascribed to an increase of S that could, from a threshold value of particle volume fraction, counteract the decrease of D.

**4.5. Discussion of the selective transport characteristics of nanocomposite membranes**

In the context of developing barrier films, it is also interesting to discuss the selective transport characteristics of nanocomposites membranes. It is indeed of high interest in many applications to develop nanocomposites that are designed to provide a very strong barrier to one component (for instance oxygen) while remaining permeable to another (such as carbon dioxide or water vapor). In this purpose, CO₂/O₂ selectivity and H₂O/O₂ selectivity were calculated, when possible, from the data gathered in the framework of this study and plotted as a function of the nanoparticle volume fraction (Figures 7 and 8). In most of cases, CO₂/O₂ and H₂O/O₂ selectivity was not initially in the objective of the original works used to derive it. Therefore, it was not systematically possible to calculate it from the permeability data available per reference. It must be noted that for the sake of clarity, for H₂O/O₂ selectivity, the ratio of nanocomposite selectivity to neat polymer selectivity is used in Figure 8 while absolute values of PCO₂/PO₂ are directly plotted in Figure 7, because they display less range of variation.

As it can be seen in **Figure 7**, and in the logic of result of the Figure 3 analysis, there are much more data of CO₂/O₂ permselectivity available for layered particles than for iso-dimensional particles and elongated ones where only four references have permitted to calculate this value. It is thus not possible to draw meaningful conclusions about the evolution of CO₂/O₂ permselectivity for elongated nanoparticles-based nanocomposites (**Figure 7b**).

Logically, CO₂/O₂ permselectivity of layered particles-based nanocomposites was centered around a mean value of 4, which is usually the permselectivity of oil-based polymers used as neat matrix in most of the studies (**Figure 7c**). In most cases, this permselectivity did not change so much as a function of the nanoparticle volume fraction (up to 8 vol/vol% for the study of Jacquelot et al
[77]) suggesting that the addition of layered nanoparticles would not affect the CO₂/O₂ permselectivity of the nanocomposites and that this property would remain governed by the matrix properties even in nanocomposites. Two studies nevertheless lead to an increase of the CO₂/O₂ permselectivity as a function of nanoparticle volume fraction [78,79]. In the work of Meera et al [79], the decrease of permeability as a function of filler content is much higher for O₂ than for CO₂ leading to an increase of CO₂/O₂ permeability ratio while it seems to be the contrary for Merinska et al [78]. In the same time and for another combination of filler / polymer (Nanofill 5 as MMT/PP), the data of Merinska et al [78] permitted to obtain a decrease of CO₂/O₂ permselectivity. Alena et al [80] also observed either an increase or decrease of CO₂/O₂ permselectivity depending on the type of nanoparticle (MMT) used in LDPE matrix. These last features confirm the random nature of the evolution of CO₂/O₂ permselectivity as a function of nanoparticle volume fraction. We noted that this notion of permselectivity and its evolution as a function of filler content was never explained nor even highlighted by the aforementioned group of authors.

For isodimensional particles-based nanocomposites, the CO₂/O₂ permselectivity is also centered around a value of 5 (Figure 7a) and kept almost constant as a function of filler volume fraction. Nevertheless, a group of 5 studies displays very high CO₂/O₂ permselectivity (> 15). Among this group of studies, we noted a general trend toward an increase of permselectivity with nanoparticle volume fraction (4 studies [43, 70, 81, 82] observing an increase against only one [50] observing a decrease). Variation of CO₂/O₂ permselectivity in the aforementioned studies was principally due to the fact that O₂ and CO₂ permeability did not decrease as much as a function of filler content resulting in a non-constant PCO₂/PO₂ ratio. Results of Zoppi et al [50] and Kim et al [70] with high CO₂/O₂ permselectivity were obtained on PEBAX® (copolymer with polyamide-6 and poly(ethyleneoxide)) which is known to display high CO₂/O₂ selectivity in packaging application. These authors did not try to explain why the CO₂/O₂ permselectivity varied in their sample: Kim et al [70] focused only on CO₂/He, CO₂/N₂ and O₂/N₂ selectivity and did not notice
any variation of these selectivities as a function of nanoparticle volume fraction (about 6 for CO₂/He, 70 for CO₂/N₂ and 3 for O₂/N₂ respectively) ; Zoppi et al [50] focusing on O₂/N₂, CO₂/CH₄, CO₂/N₂ and CO₂/H₂ selectivity did not notice so much variation of this value as a function of nanoparticle content except for very high volume ratio (15 vol/vol%). Only Semsarzadeh et al [82] calculated the CO₂/O₂ permselectivity in their study on polyurethane-silica/polyvinyl alcohol mixed matrix membranes. They noticed that by increasing the silica content in membranes, all gas permselectivities including CO₂, namely CO₂/N₂, CO₂/CH₄ and CO₂/O₂ increased much more than selectivity including other gases such as O₂/N₂, up to 136, 69 and 47 %, respectively. They explained that, while O₂, N₂ and CH₄ displayed decreasing permeability, CO₂ permeability demonstrated an increase upon increasing silica content due to an increase in the number of the active sites (polar OH groups) for gas dissolution in polymer matrix. The same reduced decrease in CO₂ permeability compared to other gases such as O₂ or N₂ was observed by Sadeghi et al [43,81] in polyether-based polyurethane-silica and polyurethane-silica nanocomposite leading to moderate increase of CO₂/O₂ selectivity as a function of nanoparticle content (Figure 7a).

The comparative examination of permeability and selectivity variation of data collected in the database considered in this work is now in order. Indeed, for the case of inclusion of impermeable particles in a permeable polymer matrix, the anticipated mechanism of induced tortuosity in path of gas molecules is expected to reduce the permeability of different gases by the same extent. A further analysis was thus performed to explore possible correlation between relative permeability and permselectivity in the available set of data. The examination is confined to the case of CO₂/O₂ selectivity in nanocomposite obtained from the inclusion of isodimensional or platelet shaped nanoparticles, as only for those cases is a sufficient number of data available to insure the significance of the statistical analysis. In Figure 9 the relative permeability for CO₂ in the nanocomposites is related to the corresponding square variation of permselectivity δ defined as follows:
$$ \delta = \left( \frac{\alpha_M}{\alpha_{NC}} - 1 \right)^2 \qquad [2] $$

where $\alpha_{NC}$ and $\alpha_M$ are CO₂/O₂ permselectivity in nanocomposite and neat matrix, respectively. Despite the scattering, data in **Figure 9** put in evidence that, both for the case of isodimensional particles and platelets, relative permeability higher than one is typically accompanied by a significant deviation of nanocomposite permselectivity from the value of neat matrix, while an interval exists for relative permeability of nanocomposite moderately lower than 1, for which the deviation in permselectivity from the value of neat matrix is negligible. Significantly, the above interval for relative permeability extends to lower values for the case of nanocomposites including platelets with respect to the case of isodimensional particles. It can be thus concluded that a good number of studies considered in this work shows results which are at least qualitatively consistent with the picture resulting from the tortuosity effect induced by the dispersed phase to the motion of gas molecules. On the other hand, a large number of nanocomposites exhibiting either relative permeability values higher than one or very low seems to be often characterized by significant variation of the permselectivity, indicating the relevance of additional phenomena hiding the effect of a pure increase in tortuosity path for the motion of gas molecules in the system.

As it can be seen in **Figure 8**, data available to calculate H₂O/O₂ selectivity are much scarcer than for CO₂/O₂ and, for this reason, the correlation between relative permeability and deviation of permselectivity is not discussed here for this kind of data. No values were even found in the collected references for calculated H₂O/O₂ selectivity in iso-dimensional nanoparticle based nanocomposites. Only three references were found for elongated ones. O₂ permeability is rarely investigated concomitantly to H₂O permeability although H₂O/O₂ selectivity is a very important value in many applications such as packaging where sometimes a high barrier property to O₂ is
required at the same time as low barrier to H₂O. From **Figure 8**, it can be concluded that the H₂O/O₂ selectivity did not vary so much as a function of nanoparticle volume fraction except for the study of Kasirga et al [74], Chang et al [39] and Lee et al [48] where H₂O/O₂ selectivity tended to dramatically increase then decrease for at least Chang et al [39] and Lee et al [48]. Even if none of these authors intentionally measured nor explained the evolution of H₂O/O₂ selectivity, this behavior could be generally ascribed to a better tortuosity effect obtained for H₂O than for O₂ and, for non-monotonic behavior, to nonlinear decrease in permeability with clay loading.

To conclude about the selective transport characteristics of nanocomposites membranes, the previous analysis has highlighted that a correlation exists between permeability and the corresponding selectivity value which could be used to emphasize that deviations from expected behavior for selectivity (either positive or negative variation of selectivity) are often associated with deviations from expected behavior for permeability (very high reduction or increase).

<figure><img src="image_22.png" /><figcaption>a- Isodimensional particles</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>**Figure 7:** Evolution of the CO₂/O₂ permselectivity as a function of particle volume fraction in nanocomposite materials containing either (a) iso-dimensional, either (b) elongate or (c) layered particle</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>**Figure 8:** Evolution of the H₂O/O₂ selectivity ratio* as a function of particle volume fraction in nanocomposite materials containing either elongated or layered particles</figcaption></figure>

*note that for the sake of clarity, the ratio between selectivity of the nanocomposite to that of the neat polymer was preferred in the representation, instead of true selectivity values
<figure><img src="image_25.png" /><figcaption>**Figure 9:** Correlation plot for relative permeability $P/P_0$ for CO$_2$ and square deviation of CO$_2$/O$_2$ permselectivity in isodimensional particles (open circles) and layered particles (filled squares) nanocomposites.</figcaption></figure>

**5. Conclusion**

The recently-measured values of gas (O$_2$, CO$_2$) and water vapor permeability in agro-, bio- and synthetic-based nanocomposites have been comprehensively reviewed with emphasis on the link between their micro- and nano-structure and their barrier performance.

This review has revealed that despite numerous and intensive studies on the optimization of nanocomposite processing and structure in the perspective of modulating their barrier properties, the expected decreasing effect on the permeability is often not so high as expected and sometimes completely the opposite. This review confirmed that layered nanoparticles are more efficient than isodimensional and elongated nanoparticles to decrease the relative permeability
(P/P₀). But, this effect is difficult to anticipate due to numerous interferring mechanisms that provoke modifications of tortuosity, sorption, polymer matrix (crystallinity, free volume, molar mass), creation of an interphase, voids and cracks, and more that lead to unexpected behaviours of the permeability in nanocomposites. Investigation of the selective transport of CO₂ to O₂ and H₂O to O₂ did not reveal a general trend either and it is also difficult to anticipate.

It was concluded that tortuosity, the main mechanism proposed to explain permeability decrease in nanocomposites, often fails in explaining the effect of nanoparticles on nanocomposite barrier properties. This is particlarly important knowing that most of the mathematical models used to predict permeability in nanocomposites are based on this tortuosity effect.

<table><caption>**Table 1:** Characteristics of the iso-dimensional particles-based nanocomposites of the publications quoted in this review: Nature, size and chemical modification of the particles, processing routes and applications of nanocomposites.</caption><thead><tr><th>Reference</th><th>Particle</th><th>Matrix</th><th>Size before/after process</th><th>Chemical modification</th><th>Process</th><th>Film thickness</th><th>Molecular penetrant</th><th>Application</th></tr></thead><tbody><tr><td>Ahn et al. [76,83]</td><td>Silica nanoparticles</td><td>Polysulfone (PSF)</td><td>11.1-13.3 /</td><td>Trimethylsilyl<br/>Tetraethoxysilane</td><td>Solution / Casting</td><td>60 - 80 µm</td><td>Oxygen/ Carbon dioxide</td><td>Membrane</td></tr><tr><td>Bracho et al. [61]</td><td>Silica nanoparticles</td><td>Poly(propylene)</td><td>20 and 100nm /</td><td>Trimethylchlorosilane<br/>Dimethyloctylchlorosilane<br/>Unmodified</td><td>Melt mixing / Thermopressing</td><td>1000 µm</td><td>Water vapor</td><td>Generative<br/>engineering</td></tr><tr><td>Cong et al. [71]</td><td>Silica nanoparticles</td><td>Brominated Poly(2,6-diphenyl-1,4-phenylene oxide)</td><td>10nm /</td><td>Trimethylsilyl<br/>Triphenylsilyl</td><td>Sol-gel / Solution casting</td><td>50 - 90 µm</td><td>Carbon dioxide</td><td>Membrane</td></tr><tr><td>Cornelius et al. [49]</td><td>Silica nanoparticles</td><td>Poly(imide)</td><td>-</td><td>Tetraethoxysilane<br/>Phenyltrimethoxylane<br/>Methyltrimethoxysilane<br/>Tetramethoxysilane</td><td>Sol gel / Solution casting</td><td>80µm</td><td>Oxygen / Carbon dioxide</td><td>Membrane</td></tr><tr><td>Cornelius et al. [49]</td><td>Silica nanoparticles</td><td>Poly(imide)</td><td>-</td><td>Phenyltrimethoxylane<br/>Methyltrimethoxysilane</td><td>Sol gel / Solution casting</td><td>80µm</td><td>Oxygen / Carbon dioxide</td><td>Membrane</td></tr><tr><td>Dougnac et al. [57]</td><td>Silica nanoparticles</td><td>Poly(propylene)</td><td>10-150nm /</td><td>Tetraethoxysilane</td><td>Sol gel / Melt mixing / Pressing</td><td>5000 µm</td><td>Oxygen</td><td>Generative<br/>engineering</td></tr><tr><td>Hu et al. [84]</td><td>Titanium dioxide nanoparticles</td><td>Poly(amid e imide)</td><td>-</td><td>Unmodified</td><td>Sol-gel / Solution casting</td><td>15 - 17 µm</td><td>Oxygen / Carbon dioxide</td><td>Membrane</td></tr><tr><td>Iwata et al. [64]</td><td>Silica nanoparticles</td><td>Poly(acrilonitrile)</td><td>56.7nm /</td><td>Tetraethoxysilane</td><td>Sol gel / Solution casting</td><td>60 - 110 µm</td><td>Oxygen</td><td>Membrane</td></tr></tbody></table>
<table><caption>Table 1 (continue) ...</caption><tr><td>Joly et al. [85]</td><td>Silica nanoparticles</td><td>Polyimide Poly(4,4-oxydiphenylene pyromellitic imide)</td><td>/ 50-150nm</td><td>Tetramethoxysilane</td><td>Solution / Casting</td><td>25 - 30 µm</td><td>Oxygen / Carbon dioxide</td><td>Membrane</td></tr><tr><td>Kim et al. [70]</td><td rowspan="2">Silica nanoparticles</td><td>Poly(amid e-6-butylene oxide)</td><td>/ 33 and 517nm</td><td rowspan="2">Tetraethoxysilane</td><td rowspan="2">Sol-gel / Solution casting</td><td rowspan="2"></td><td rowspan="2">Oxygen / Carbondioxide</td><td rowspan="2">Membrane</td></tr><tr><td>Kim and Marand [86]</td><td>Polysulfone</td><td></td></tr><tr><td>Kong et al. [87]</td><td>Titanium dioxide nanoparticles</td><td>Poly(imide)</td><td>/ 10nm</td><td>Unmodified</td><td>Sol-gel / Solution casting</td><td>-</td><td>Oxygen</td><td>Membrane</td></tr></table>

<table><thead><tr><th>Publication</th><th>Particle</th><th>Matrix</th><th>Size before/after process</th><th>Chemical modification</th><th>Process</th><th>Film thickness</th><th>Molecular penetrant</th><th>Application</th></tr></thead><tbody><tr><td>Kono et al. [75]</td><td>Silica nanoparticles</td><td>Poly(1-chloro-2-phenylacetylene) Poly[1-phenyl-2-(4-trimethylsilyl)phenylacetylene] Poly[1-(trimethylsilyl)-1-propyne]</td><td>13 nm /</td><td>Trimethylsilyl</td><td>Solution / Casting</td><td>80 - 120µm</td><td>Oxygen / Carbon dioxide</td><td>Membrane</td></tr><tr><td>Matteucci et al. [88]</td><td>Titanium dioxide nanoparticles</td><td>1,2 Poly(butadiene)</td><td>&lt; 50nm / 9(+/-)4nm</td><td>Unmodified</td><td>Solution / Casting</td><td>180 - 220 µm</td><td>Carbon dioxide</td><td>Membrane</td></tr><tr><td>Moghadam et al. [56]</td><td>Titanium dioxide nanoparticles</td><td>Poly(imide)</td><td>3 nm /</td><td>Unmodified</td><td>Solution / Casting</td><td>80 - 120µm</td><td>Oxygen / Carbon dioxide</td><td>Membrane</td></tr><tr><td>Patel et al. [89]</td><td>Silica nanoparticles</td><td>Poly(ethylene glycol)</td><td>12nm /</td><td>Methacrylate</td><td>Solution / Casting</td><td>40 - 200 µm</td><td>Oxygen / Carbon dioxide</td><td>Membrane</td></tr><tr><td>Patel et al. [90]</td><td>Silica nanoparticles</td><td>Crosslinked Poly(ethylene glycol)</td><td>12nm /</td><td>Methacrylate</td><td>Solution / Casting</td><td>40 - 200 µm</td><td>Carbon dioxide</td><td>Membrane</td></tr><tr><td>Rafiq et al. [53]</td><td>Silica nanoparticles</td><td>Poly(sulfone) Poly(imide)</td><td>/ &lt;100nm</td><td>Tetraethoxysilane 3-amino-propyltrimethoxysilane</td><td>Solution / Casting</td><td>150 µm</td><td>Carbon dioxide</td><td>Membrane</td></tr><tr><td>Romero et al. [58]</td><td>Silica nanoparticles</td><td>Poly(etherimide)</td><td>-</td><td>Tetraethoxysilane 3-amino-propyltrimethoxysilane</td><td>Solution / Casting</td><td>25 - 70 µm</td><td>Oxygen / Carbon dioxide</td><td>Membrane</td></tr><tr><td>Sadeghi et al. [74]</td><td>Silica nanoparticles</td><td>Poly(ethylene vinyl acetate)</td><td>-</td><td>Tetraethoxysilane</td><td>Sol-gel / Solution casting</td><td></td><td>Oxygen / Carbon dioxide</td><td>Membrane</td></tr><tr><td>Sadeghi et al. [72]</td><td>Silica nanoparticles</td><td>Poly(benzimidazole)</td><td>-</td><td>Tetraethoxysilane 3-glycidyloxypropyltrimethoxysilane</td><td>Sol-gel / Solution casting</td><td>40 µm</td><td>Carbon dioxide</td><td>Membrane</td></tr><tr><td>Sadeghi et al. [81]</td><td>Silica nanoparticles</td><td>Poly(ether-based polyurethane)</td><td>-</td><td>Tetraethoxysilane 3-glycidyloxypropyltrimethoxysilane</td><td>Sol-gel / Solution casting</td><td>100 µm</td><td>Oxygen / Carbon dioxide</td><td>Membrane</td></tr><tr><td>Sadeghi et al. [43]</td><td>Silica nanoparticles</td><td>Polycaprolactone-based polyurethane</td><td>-</td><td>Tetraethoxysilane</td><td>Sol-gel / Solution casting</td><td>100 µm</td><td>Oxygen / Carbon dioxide</td><td>Membrane</td></tr></tbody></table>
es

3-glycidyloxy-
propylterimeth
oxysilane

<table><caption>**Table 1 (continue) ...**</caption><thead><tr><th>Publication</th><th>Particle</th><th>Matrix</th><th>Size before/after process</th><th>Chemical modification</th><th>Process</th><th>Film thickness</th><th>Molecular penetrant</th><th>Application</th></tr></thead><tbody><tr><td>Semsarzad<br>eh et al.<br>[82]</td><td>Silica<br>nanoparticles</td><td>Polyurethane</td><td>-</td><td>Tetraethoxysilane /Polyvinyl alcohol</td><td>Sol-gel / Solution<br>casting</td><td>120 µm</td><td>Oxygen / Carbon<br>dioxide</td><td>Membrane</td></tr><tr><td>Suzuki et<br>al. [73]</td><td>Silica<br>nanoparticles</td><td>Poly(imide)</td><td>-<br>18-26nm /<br>11-13nm /<br>25-31nm /</td><td>Tetramethoxysilane<br>Methyl<br>Trimethylsilyl<br>Polydiméthylsiloxane</td><td>Sol-gel / Solution<br>casting<br>Solution / Casting<br>and<br>Melt processing /<br>Compression<br>moulding</td><td>-<br>50-70<br>µm /<br>200-300<br>µm</td><td>Oxygen / Carbon<br>dioxide</td><td>Membrane</td></tr><tr><td>Takahashi<br>et al. [69]</td><td>Silica<br>nanoparticles</td><td>Poly(etherimide)</td><td>-</td><td>3-amino-propyltrimethoxysilane</td><td>Solution Casting<br>and<br>Melt processing /<br>Compression<br>moulding</td><td>30-50<br>µm /<br>200 µm</td><td>Oxygen / Carbon<br>dioxide</td><td>Membrane</td></tr><tr><td>Takahashi<br>et al. [69]</td><td>Silica<br>nanoparticles</td><td>Poly(etherimide)</td><td>-</td><td>Unmodified</td><td>Melt-mixing /<br>Thermopressing</td><td>300-500<br>µm</td><td>Water vapor</td><td>Membrane</td></tr><tr><td>Ulutan et<br>al. [91]</td><td>Silica<br>nanoparticles</td><td>Poly(vinyl<br>chlorure)</td><td>-</td><td>Unmodified</td><td>Melt mixing /<br>Thermopressing</td><td>45-55<br>µm</td><td>Oxygen</td><td>Membrane</td></tr><tr><td>Vassiliou<br>et al. [45]</td><td>Silica<br>nanoparticles</td><td>Isostatic<br>poly(propyl<br>ene)</td><td>12nm /<br>Isostatic<br>poly(propyl<br>ene)</td><td>Unmodified</td><td>Melt mixing /<br>Thermopressing</td><td>45-55<br>µm</td><td>Oxygen</td><td>Membrane</td></tr><tr><td>Vladimirov<br>et al. [44]</td><td>Silica<br>nanoparticles</td><td>Poly(vinyl<br>chlorure)</td><td>12nm /<br>Isostatic<br>poly(propyl<br>ene)</td><td>Unmodified</td><td>Melt mixing /<br>Thermopressing</td><td>45-55<br>µm</td><td>Oxygen</td><td>Membrane</td></tr><tr><td>Yu et al.<br>[92]</td><td>Silica<br>nanoparticles</td><td>Poly(sulfone)</td><td>16nm /<br>12nm</td><td>Hydroxyl<br>Trimethylsilyl</td><td>Solution / Casting</td><td>50 nm</td><td>Carbon dioxide</td><td>Membrane</td></tr><tr><td>Zhou et al.<br>[93]</td><td>Titanium<br>dioxide<br>nanoparticles</td><td>Whey<br>protein</td><td>&lt; 20nm /<br>Isostatic<br>poly(propyl<br>ene)</td><td>Unmodified</td><td>Solution / Casting</td><td>50 µm</td><td>Water vapor</td><td>Packaging</td></tr><tr><td>Zhu et al.<br>[40]</td><td>Silica<br>nanoparticles</td><td>Poly(vinyl<br>chlorure)</td><td>25nm /<br>Isostatic<br>poly(propyl<br>ene)</td><td>3-<br>(Trimethoxysilyl)<br>-propyl<br>methacrylate</td><td>Melt mixing /<br>Compression<br>moulding</td><td>450-550<br>µm</td><td>Oxygen / Water<br>vapor</td><td>General<br>engineer<br>ring</td></tr><tr><td>Zoppi et al.<br>[50]</td><td>Silica<br>nanoparticles</td><td>Poly(ethylene<br>oxide-b-amine-6)</td><td>-</td><td>Tetraethoxysilane</td><td>Sol-gel / Solution<br>casting</td><td>-</td><td>Oxygen / Carbon<br>dioxide</td><td>Membrane</td></tr><tr><td>Zoppi et al.<br>[50]</td><td>Titanium<br>dioxide<br>nanoparticles</td><td>Poly(ethylene<br>oxide-b-amine-6)</td><td>-</td><td>Tetraethoxysilane</td><td>Sol-gel / Solution<br>casting</td><td>-</td><td>Oxygen / Carbon<br>dioxide</td><td>Membrane</td></tr></tbody></table>

<table><caption>**Table 2:** Characteristics of the elongated particles-based nanocomposites of the publication quoted in this article; information about the nature, the size and the modification of the particles, and overview of the composites processing and applications</caption><thead><tr><th>Reference</th><th>Particle</th><th>Matrix</th><th>Size before/after process</th><th>Chemical modification</th><th>Process</th><th>Film thickness</th><th>Molecular migrant</th><th>Application</th></tr></thead><tbody><tr><td>Azeredo et al.<br>[94]</td><td>Cellulose fibre</td><td>Mango puree</td><td>Length: 82.6(+/-)4.3<br>nm<br>Diameter 7.2(+/-)0.3<br>nm</td><td>Unmodified</td><td>Solution / Casting</td><td>Water vapor</td><td>Packaging</td><td></td></tr></tbody></table>
<table><caption>Table 2 (continued ...)</caption><tr><td>Azeredo et al. [95]</td><td>Cellulose fibre</td><td>Chitosan</td><td>Length: 98.1(+/-)4.7 nm<br/>Diameter: 8.0(+/-)3.4 nm</td><td>Unmodified</td><td>Solution / Casting</td><td>29 µm</td><td>Water vapor</td><td>Packaging</td></tr><tr><td></td><td>Cellulose fibrils<br/>Eucalyptus sulphite wood pulp<br/>Oxidized cellulose fibrils</td><td>Hydroxy propyl methyl cellulose</td><td>Diameter: 35(+/-)9 nm</td><td>Unmodified</td><td>TEMPO sodium bromide</td><td>Solution / Casting</td><td>1150 µm</td><td>Water vapor</td><td>Packaging</td></tr><tr><td>Bilbao-Sainz et al. [51]</td><td>Softwood cellulose fibre<br/>Cellulose whisker<br/>Microcrystalline cellulose</td><td></td><td>Diameter: 67(+/-)34 nm<br/>Length: 301(+/-)67 nm<br/>Diameter: 28(+/-)9 nm</td><td>Unmodified</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Chang et al. [96]</td><td>Cellulose particle</td><td>Wheat starch</td><td>50-100nm / Length: 0.1-1 µm</td><td>Unmodified</td><td>Solution / Casting</td><td></td><td>Water vapor</td><td>Packaging</td></tr><tr><td>Cong et al. [97]</td><td>Carbon nanotube</td><td>Brominated poly(2,6-diphenyl-1,4-phenylene oxide)</td><td>Average diameter: 0.8-1.2 nm<br/>Length: 0.5-50 µm<br/>Average diameter: 40-60 nm</td><td>Unmodified</td><td>Solution / Casting</td><td>50-80 µm</td><td>Carbon dioxide</td><td>Membrane</td></tr><tr><td>Dogan et al. [62]</td><td>Microcrystalline cellulose</td><td>Hydroxy propyl methyl cellulose</td><td>Average size 0.5-3µm / Average length: 243.9 (+/-) 48.5 nm</td><td>Unmodified</td><td>Solution / Casting</td><td></td><td>Water vapor</td><td>Packaging</td></tr><tr><td>Espino Perez et al. [98]</td><td>Cellulose whisker</td><td>Poly(lactic acid)</td><td>Average diameter: 9.4 (+/-) 2.5 nm</td><td>n-octadecyl isocyanate</td><td>Solution / Casting</td><td>40-120 µm</td><td>Oxygen / Water vapor</td><td>General engineering</td></tr><tr><td>Follain et al. [68]</td><td>Cellulose nanocrystal</td><td>Poly(ε-caprolactone)</td><td>Average length: 290(+/-)130 nm</td><td>Long-chain isocyanate</td><td>Solution / Casting</td><td>300-400 µm</td><td>Water vapor</td><td>General engineering</td></tr><tr><td>George et al. [99]</td><td>Bacterial cellulose nanocrystal</td><td>Food gelatin</td><td>Average diameter: 20(+/-)5 nm</td><td>Unmodified</td><td>Solution / Casting</td><td></td><td>Water vapor</td><td>Packaging</td></tr></table>

<table><thead><tr><th>Reference</th><th>Particle</th><th>Matrix</th><th>Size before/after process</th><th>Chemical modification</th><th>Process</th><th>Film thickness</th><th>Molecular migrant</th><th>Application</th></tr></thead><tbody><tr><td>Ismail et al. [52]</td><td>Carbon nanotube</td><td>Poly(ethersulfone)</td><td>Average inner and outer diameter: 3.5-15 nm</td><td>3-aminopropyl-triethoxysilane (APTES)</td><td>Solution / Casting</td><td></td><td>Oxygen / Carbon dioxide</td><td>Membrane</td></tr><tr><td>Kim et al. [100]</td><td>Carbon nanotube</td><td>Poly(imide siloxane)</td><td>Average diameter: 1.4 (+/-) 0.2 nm</td><td>Acid treatment</td><td>Solution / Casting</td><td></td><td>Oxygen / Carbon dioxide</td><td>Membrane</td></tr></tbody></table>
<table><caption>Table 3: Characteristics of the platelet particles-based composites of the publication quoted in this article; information about the nature, the size and the modification of the particles, and overview of the composites processing and applications</caption><tr><td>Kim et al. [101]</td><td>Carbon nanotube</td><td>Poly(sulfone)</td><td>Average diameter: 1.2 nm</td><td>Wet-oxidation Acid treatment</td><td>Solution / Casting</td><td>Oxygen / Carbon dioxide</td><td>Membrane</td></tr><tr><td>Mondal et al. [102]</td><td>Carbon nanotube</td><td>Poly(urethane)</td><td>Diameter: 2-15 nm<br>Length: 1-10 µm<br>Length: 5-15 µm</td><td>Aniline solution diluted in DMF</td><td>Solution / Casting</td><td>90 µm</td><td>Water vapor</td><td>General engineering</td></tr><tr><td>Murali et al. [59]</td><td>Carbon nanotube</td><td>Poly(ether-block-amide)</td><td>Diameter: 10-20 nm<br>Length: up to around 100 nm</td><td>Unmodified</td><td>Solution / Casting</td><td>15-20 µm</td><td>Oxygen / Carbon dioxide</td><td>Membrane</td></tr><tr><td>Pantani et al. [103]</td><td>ZnO particle</td><td>Poly(lactic acid)</td><td>Diameter: 15-30 nm<br>Length: 50-200 nm</td><td>Unmodified</td><td>Melt compounding / Compress ion moulding</td><td>150 µm</td><td>Water vapor</td><td>Packaging</td></tr><tr><td>Paralikar et al. [66]</td><td>Cellulose nanocrystal</td><td>Poly(vinyl alcohol)<br>Crosslinked Poly(acrylic acid)</td><td>Diameter: 5-10 nm<br>Length: 0.1-10 µm</td><td>Unmodified<br>Free acid</td><td>Solution / Casting</td><td>10 µm</td><td>Water vapor</td><td>Membrane</td></tr><tr><td>Pradhan et al. [104]</td><td>Carbon nanotube</td><td>Poly(acrilonitrile)</td><td>Diameter: 10-15 nm<br>Length: 1-10 µm</td><td>Sulphuric and nitric acid</td><td>Free emulsion polymerization / Thermopressing</td><td>500 µm</td><td>Oxygen</td><td>General engineering</td></tr><tr><td>Sanchez-Garcia et al. [105]</td><td>Carbon nanofibre</td><td>Polyhydroxybutyrate-co-valerate</td><td>Diameter: 5-20 nm<br>Length: 50-100 µm</td><td>Unmodified</td><td>Solution / Casting</td><td>100 µm</td><td>Oxygen</td><td>Packaging</td></tr><tr><td>Sanchez-Garcia et al. [106]</td><td>Cellulose whisker</td><td>Poly(Lactic acid)</td><td>Diameter: 70-200 nm<br>Length: 60-160 nm</td><td>Unmodified</td><td>Solution casting</td><td>Oxygen / Water vapor</td><td>Packaging</td><td>Packaging</td></tr><tr><td>Sanchez-Garcia et al. [107]</td><td>Cellulose whisker</td><td>Hybrid Carrageenan</td><td>Thickness: 10-20 nm<br>Length: 25-50 nm</td><td>Unmodified</td><td>Solution / Casting</td><td>50 µm</td><td>Water vapor</td><td>Packaging</td></tr><tr><td>Saxena et al. [65]</td><td>Cellulosic whisker</td><td>Oat spelt xylan</td><td>Diameter: 5 nm<br>Length: 150-200 nm</td><td>Sulphuric acid</td><td>Solution / Casting</td><td>89-93 µm</td><td>Water vapor</td><td>General engineering</td></tr><tr><td>Yu et al. [108]</td><td>ZnO-carboxymethylcellulose sodium</td><td>Glycerol plasticized-pea starch</td><td>Width < 20 nm<br>Length: 30-40 nm</td><td>Unmodified</td><td>Solution / Casting</td><td>Water vapor</td><td>Water vapor</td><td>General engineering</td></tr></table>

<table><thead><tr><td>Reference</td><td>Particle</td><td>Matrix</td><td>Size before/aft er process</td><td>Chemical modification</td><td>Process</td><td>Film thickne ss</td><td>Molecular penetrant</td><td>Application</td></tr></thead><tbody><tr><td>Abdollahi et al. [109]</td><td>Montmorillo nite</td><td>Alginate</td><td>Average length: 300nm</td><td>Unmodified</td><td>Solution / Casting</td><td>–</td><td>Water vapor</td><td>Packaging</td></tr></tbody></table>
<table><thead><tr><th>Reference</th><th>Particle</th><th>Matrix</th><th>Size before/after process</th><th>Chemical modification</th><th>Process</th><th>Film thickness</th><th>Molecular penetrant</th><th>Application</th></tr></thead><tbody><tr><td>Alboofetileh et al. [110]</td><td>Montmorillonite</td><td>Sodium alginate</td><td>Average diameter: 40nm / -</td><td>Unmodified</td><td>Solution / Casting</td><td>40 - 60 µm</td><td>Water vapor</td><td>Packaging</td></tr><tr><td>Alena et al. [80]</td><td>Montmorillonite</td><td>Linear low density polyethylene Poly(chlorure vinyl)</td><td>-</td><td>Unmodified<br>Methyl bis-2-hydroxyethyl tallow ammonium Dimethyl, hydrogenated tallow, 2-ethylhexyl quaternary ammonium Methyl, dehydrogenated tallow ammonium</td><td>Extrusion / roll-pressing</td><td>100 µm</td><td>Oxygen / Carbon dioxide / Water vapor</td><td>Packaging</td></tr><tr><td>Alexandre et al. [111]</td><td>Montmorillonite</td><td>Poly(amide 12)</td><td>/ Thickness: 2-6nm</td><td>Methyl bis-2-hydroxyethyl tallow ammonium</td><td>Melt processing / Compression moulding</td><td>200 - 250µm</td><td>Water vapor</td><td>Membrane</td></tr><tr><td>Ali et al. [112]</td><td>Montmorillonite</td><td>Starch / Poly(vinyl alcohol)</td><td>-</td><td>Unmodified</td><td>Solution / Casting</td><td>-</td><td>Water vapor</td><td>Packaging</td></tr><tr><td>Angellier et al. [113]</td><td>Waxy maize starch nanocrystals</td><td>Natural rubber</td><td>Thickness: 6-8nm<br>Length: 40-60nm<br>Width: 15-30nm/<br>Aspect ratio 16</td><td>Unmodified</td><td>Solution / Casting</td><td>200 - 1000 µm</td><td>Oxygen / Water vapor</td><td>General engineering</td></tr><tr><td>Bae et al. [114]</td><td>Montmorillonite</td><td>Fish gelatin</td><td>-</td><td>Dimethyl dehydrogenated tallow ammonium</td><td>Solution / Casting<br>Melt processing / Compression moulding</td><td>-</td><td>Oxygen / Water vapor</td><td>Packaging</td></tr><tr><td>Balachandran et al. [115]</td><td>Montmorillonite</td><td>Nitrile rubber</td><td>-</td><td>Methyl bis-2-hydroxyethyl tallow ammonium chloride 1-1-hydroxydodecane-1,1-diphosphate ammonium salt</td><td>Melt processing / Compression moulding</td><td>2000 µm</td><td>Oxygen</td><td>General engineering</td></tr><tr><td>Bharadwaj et al. [37]</td><td>Montmorillonite</td><td>Polyester resin</td><td>-</td><td>Dimethyl dehydrogenated tallow ammonium</td><td>Melt processing / Compression moulding</td><td>-</td><td>Oxygen</td><td>General engineering</td></tr><tr><td>Chaiko et al. [67]</td><td>Montmorillonite</td><td>Paraffin wax</td><td>10-20nm /</td><td>Dimethyl dehydrogenated tallow ammonium</td><td>Melt processing</td><td>50 µm</td><td>Oxygen</td><td>General engineering</td></tr><tr><td>Chang et al. [39]</td><td>Montmorillonite</td><td>Poly(imide)</td><td>-</td><td>Dodecylamine ammonium salt Hexadecylamine ammonium salt Methyl bis-2-hydroxyethyl tallow ammonium</td><td>Solution / Casting</td><td>10 - 15 µm</td><td>Oxygen / Water vapor</td><td>General engineering</td></tr><tr><td>Chang et al. [63]</td><td>Montmorillonite</td><td>Poly(lactic acid)</td><td>/ Thickness: 1-5nm</td><td>Hexadecylamine Dodecyltrimethyl ammonium bromide-montmorillonite</td><td>Solution / Casting</td><td>10 - 15 µm</td><td>Oxygen</td><td>Packaging</td></tr><tr><td>Chien et al. [116]</td><td>Montmorillonite</td><td>Poly(vinyl acetate)</td><td>/ Aspect ratio 320</td><td>-</td><td>Soap free emulsion polymerization / Casting</td><td>150 µm</td><td>Water vapor</td><td>General engineering</td></tr><tr><td>Chivrac et al. [117]</td><td>Montmorillonite</td><td>Wheat starch</td><td>-</td><td>Organomodification</td><td>Melt processing /</td><td></td><td>Water vapor</td><td>Packaging</td></tr></tbody></table>
<table><thead><tr><th>Reference</th><th>Particle</th><th>Matrix</th><th>Size before/aft er process</th><th>Chemical modification</th><th>Process</th><th>Film thickness</th><th>Molecular penetrant</th><th>Application</th></tr></thead><tbody><tr><td>Choi et al. [118]</td><td>Montmorillonite</td><td>Poly(urethane)</td><td>-</td><td>Organofier UE400</td><td>Compression moulding</td><td>30 -50 µm</td><td>Oxygen</td><td>General engineering</td></tr><tr><td>Choi et al. [119]</td><td>Montmorillonite</td><td>Poly(propylene)</td><td>-</td><td>Dimethyl di(hydrogenated tallowalkyl) quaternary ammonium</td><td>Melt-compound/g / Injection moulding Solution / Thermopressing</td><td>600 - 800 µm</td><td>Oxygen</td><td>Packaging</td></tr><tr><td>Dash et al. [120]</td><td>Silicon carbide</td><td>Starch</td><td>50nm /</td><td></td><td></td><td>500 µm</td><td>Oxygen</td><td>Packaging</td></tr><tr><td>Duan et al. [121]</td><td>Montmorillonite</td><td>Poly(lactic acid)</td><td>Average thickness: 1.6nm Average length:80nm</td><td>Methyl bis-2-hydroxyethyl tallow ammonium chlorid</td><td>Melt processing / Compression moulding</td><td>600 µm</td><td>Water vapor</td><td>Packaging</td></tr><tr><td>Dunkerley et al. [122]</td><td>Montmorillonite</td><td>Poly(styrene)</td><td>-</td><td>Dimethyl di(hydrogenated tallowalkyl) quaternary ammonium</td><td>Solution spraying</td><td>60 - 270 µm</td><td>Oxygen</td><td>Packaging</td></tr><tr><td>Echeverria et al. [123]</td><td>Montmorillonite</td><td>Soy protein isolate</td><td>-</td><td>Unmodified</td><td>Solution / Casting</td><td>75 -90 µm</td><td>Water vapor</td><td>Packaging</td></tr><tr><td>Fasihi et al. [124]</td><td>Montmorillonite</td><td>Poly(amide 6)</td><td>Average particle size 8µm /Aspect ratio 210</td><td>Organomodification</td><td>Melt processing / Compression moulding</td><td>150 µm</td><td>Oxygen</td><td>Packaging</td></tr><tr><td>Fortunati et al. [125]</td><td>Cellulose nanocrystals</td><td>Poly(lactic acid)</td><td>Length:10 0-200nm Width: 5-10nm /</td><td>Unmodified Acid phosphate ester of ethoxylated nonyphenol</td><td>Solution - Casting</td><td>150 - 250 µm</td><td>Oxygen / Water vapor</td><td>Packaging</td></tr><tr><td>Gain et al. [126]</td><td>Montmorillonite</td><td>Poly(e-caprolactone)</td><td>-</td><td>Unmodified Methyl bis-2-hydroxyethyl tallow ammonium</td><td>Melt blending - In situ polymerization / Compression moulding</td><td>100 µm</td><td>Carbon dioxide</td><td>General engineering</td></tr><tr><td>Gatos et al. [30]</td><td>Fluorohexaite</td><td>Hydrogenated acrylonitirle butadiene rubber</td><td>/ Aspect ratio 200</td><td>Synthetic fluorohectorite Synthetic fluorohectorite / Octadecylamine Quaternary ammonium salt Quaternary ammonium salt / Octadecylamine</td><td>Melt mixing / Thermopressing</td><td>1000 µm</td><td>Oxygen</td><td>General engineering</td></tr><tr><td>Ghasemi et al. [46]</td><td>Montmorillonite</td><td>Poly(ethylene terephthalate)</td><td>-</td><td>Methyl bis-2-hydroxyethyl tallow ammonium</td><td>Melt processing / Casting</td><td>25 - 90 µm</td><td>Oxygen</td><td>Packaging</td></tr><tr><td>Guo et al. [127]</td><td>Montmorillonite</td><td>Fish gelatin</td><td>-</td><td>Unmodified</td><td>Solution / Casting</td><td>300 µm</td><td>Water vapor</td><td>Packaging</td></tr><tr><td>Herrera-Alonso et al. [128]</td><td>Montmorillonite</td><td>Poly(n-butylacrylate)</td><td>/Average particle diameter 140nm</td><td>[2-(Acryloyloxy)ethyl]-trimethylammonium</td><td>Emulsion polymerization / Casting</td><td>180 - 240µm</td><td>Oxygen / Carbon dioxide</td><td>Membrane</td></tr><tr><td>Herrera-Alonso et al.</td><td>Montmorillonite</td><td>Poly(n-butylacrylate)</td><td>-</td><td>[2-(Acryloyloxy)ethyl]-</td><td>In-situ polymerization</td><td>180 - 240µm</td><td>Oxygen / Carbon</td><td>Membrane</td></tr></tbody></table>
<table><thead><tr><th>Reference</th><th>Particle</th><th>Matrix</th><th>Size before/aft er process</th><th>Chemical modification</th><th>Process</th><th>Film thickness</th><th>Molecular penetrant</th><th>Application</th></tr></thead><tbody><tr><td>[128]</td><td></td><td></td><td></td><td>trimethylammonium</td><td>n / Casting</td><td></td><td>dioxide</td><td></td></tr><tr><td>Horst et al. [129]</td><td>Montmorillonite</td><td>Poly(propylene)</td><td>-</td><td>Quaternary ammonium salt</td><td>Melt-mixing / Injection moulding</td><td>140 - 200 µm</td><td>Oxygen</td><td>General engineering</td></tr><tr><td>Hotta et al. [130]</td><td>Montmorillonite</td><td>Linear low density polyethylene</td><td>-</td><td>Dimethylbis(hydrogenated-tallow ammonium</td><td>Melt processing / Compression moulding</td><td>150 µm</td><td>Oxygen / Carbon dioxide</td><td>General engineering</td></tr><tr><td>Huang et al. [131]</td><td>Montmorillonite</td><td>Poly(imide)</td><td>-</td><td>Hexadecyltrimethyl ammonium bromide</td><td>Solution / Casting / Thermal imidization</td><td>60 µm</td><td>Oxygen / Water vapor</td><td>Membrane</td></tr><tr><td>Ito et al. [132]</td><td>Montmorillonite</td><td>Poly(amide 6)</td><td>-</td><td>4,4' - oxydianiline</td><td>Extrusion / Compression moulding</td><td>200 µm</td><td>Oxygen</td><td>General engineering</td></tr><tr><td>Jacquelot et al. [77]</td><td>Montmorillonite</td><td>High and low density polyethylene</td><td>-</td><td>Hexadecyltrimethyl ammonium bromide / 4,4' - oxydianiline</td><td>Melt processing / Compression moulding</td><td>150 µm</td><td>Oxygen / Carbon dioxide</td><td>General engineering</td></tr><tr><td>Kasirga et al. [133]</td><td>Montmorillonite</td><td>Chitosan</td><td>Mix nano- / micro- &lt;100nm to 2µm</td><td>12-aminolauric acid ammonium salt</td><td>Solution / Casting</td><td></td><td>Oxygen / Water vapor</td><td>Packaging</td></tr><tr><td>Katiyar et al. [134]</td><td>Montmorillonite</td><td>Poly(L-lactic acid)</td><td>-</td><td>Dimethyl, hydrogenated tallow, 2-ethylhexyl quaternary ammonium</td><td>Extrusion / Rolling compression</td><td>400 µm</td><td>Oxygen / Water vapor</td><td>Packaging</td></tr><tr><td>Ke et al. [135]</td><td>Montmorillonite</td><td>Poly(ethylene terephthalate)</td><td>-</td><td>Unmodified</td><td>Melt mixing - In situ polymerization / Thermopressing</td><td>25 µm</td><td>Oxygen</td><td>Packaging</td></tr><tr><td>Kisku et al. [136]</td><td>Boron nitride</td><td>Chitosan</td><td>-</td><td>Organomodification</td><td>Solution / Thermopressing</td><td>500 µm</td><td>Oxygen</td><td>Packaging</td></tr><tr><td>Kisku et al. [137]</td><td>Silicon carbide</td><td>Cellulose</td><td>70nm /</td><td></td><td>Solution / Thermopressing</td><td>500 µm</td><td>Oxygen</td><td>Packaging</td></tr><tr><td>Kim et al. [138]</td><td>Montmorillonite</td><td>Epoxy resin</td><td>-</td><td>Quaternary alkylamine</td><td>Solution / Casting</td><td>1000 - 3000 µm</td><td>Water vapor</td><td>General engineering application</td></tr><tr><td>Koh et al. [33]</td><td>Montmorillonite</td><td>Poly(lactic acid)</td><td>-</td><td>Octadecylamine</td><td>Solution / Casting</td><td>25-40 µm</td><td>Oxygen / Carbon dioxide</td><td>Membrane application</td></tr><tr><td>Kristo et al. [139]</td><td>Starch crystals</td><td>Pullulan</td><td>Thickness: 6-8nm</td><td>Dimethyl di(hydrogenated tallowalkyl) quaternary ammonium Methyl bis-2-hydroxyethyl tallow ammonium chloride</td><td>Solution / Casting</td><td>69-75 µm</td><td>Water vapor</td><td>General engineering</td></tr></tbody></table>
<table><thead><tr><th>Reference</th><th>Particle</th><th>Matrix</th><th>Size before/aft er process</th><th>Chemical modification</th><th>Process</th><th>Film thickne ss</th><th>Molecular penetrant</th><th>Application</th></tr></thead><tbody><tr><td>Kumar et al. [140]</td><td>Montmorillo nite</td><td>Soy protein isolate</td><td>-</td><td>Unmodified</td><td>Melt processing / Casting</td><td>-</td><td>Water vapor</td><td>Packaging application</td></tr><tr><td>Kumar et al. [140,141]</td><td>Montmorillo nite</td><td>Soy protein isolate</td><td>-</td><td>Dimethyl di(hydrogenated tallowalkyl) quaternary ammonium Methyl bis-2-hydroxyethyl tallow ammonium</td><td>Melt processing / Casting</td><td>-</td><td>Water vapor</td><td>Packaging application</td></tr><tr><td>Lan et al. [142]</td><td>Montmorillo nite</td><td>Poly(imide)</td><td>-</td><td>Polyamic acid</td><td>Solution - Casting</td><td>25 µm</td><td>Carbon dioxide</td><td>General engineerin g application</td></tr><tr><td>Lavorgna et al. [143]</td><td>Montmorillo nite</td><td>Chitosan</td><td>-</td><td>Unmodified</td><td>Solution / Casting Melt processing / Compression moulding</td><td>115 - 125 µm</td><td>Water vapor</td><td>Packaging application</td></tr><tr><td>Lee et al. [144]</td><td>Montmorillo nite</td><td>High density polyethylene</td><td>-</td><td>Octadecylamine</td><td>Soap free emulsion polymerization / Casting</td><td>50 µm</td><td>Carbon dioxide</td><td>General engineerin g application</td></tr><tr><td>Lee et al. [145]</td><td>Montmorillo nite</td><td>Poly(methyl acrylate-co- methyl methacrylate )</td><td>-</td><td>Unmodified</td><td>Solution / Casting</td><td>200 µm</td><td>Water vapor</td><td>Packaging application</td></tr><tr><td>Lee et al. [48]</td><td>Montmorillo nite</td><td>Soy protein</td><td>-</td><td>Unmodified</td><td>Solution / Casting</td><td>90 µm</td><td>Water vapor</td><td>Packaging application</td></tr><tr><td>Liang et al. [54]</td><td>Montmorillo nite</td><td>Poly(ether sulfone)</td><td>/ Aspect ratio 7.4 - 20</td><td>Unmodified</td><td>Solution / Casting</td><td>21 - 27 µm</td><td>Carbon dioxide</td><td>Membrane application</td></tr><tr><td>Luecha et al. [146]</td><td>Montmorillo nite</td><td>Zein</td><td>-</td><td>Methyl dihydroxy ethyl hydrogenated tallow ammonium</td><td>Solution / Casting Blown - Extrusion</td><td>40 - 80 µm</td><td>Water vapor</td><td>Packaging application</td></tr><tr><td>Masclaux et al. [147]</td><td>Montmorillo nite</td><td>Native potato starch</td><td>-</td><td>Unmodified</td><td>Solution / Casting</td><td>50 µm</td><td>Oxygen</td><td>General engineerin g application</td></tr><tr><td>Meera et al. [79]</td><td>Montmorillo nite</td><td>Natural rubber</td><td>-</td><td>Octadecyl ammonium chloride</td><td>Melt processing / Compression moulding</td><td>1000 µm</td><td>Oxygen / Carbon dioxide</td><td>Membrane application</td></tr><tr><td>Merinska et al. [78]</td><td>Montmorillo nite</td><td>Poly(ethylene)</td><td>-</td><td>Methyl bis-2-hydroxyethyl tallow ammonium Dimethyl, hydrogenated tallow, 2- ethylhexyl quaternary ammonium Methyl, dehydrogenated tallow ammonium Organo-modification</td><td>Extrusion / Thermopressing</td><td>50 µm</td><td>Oxygen / Carbon dioxide / Water vapor</td><td>Packaging application</td></tr><tr><td>Messersmith et al. [41]</td><td>Montmorillo nite</td><td>Poly(ε-caprolactone )</td><td>-</td><td>Protonated amino acid</td><td>Solution / Casting</td><td>200 µm</td><td>Water vapor</td><td>General engineerin g application</td></tr><tr><td>Mittal et al. [148]</td><td>Montmorillo nite</td><td>Poly(propylene)</td><td>-</td><td>1-decyl-2-methyl-3-octadecylimidazolium bromide</td><td>Melt processing / Compression moulding</td><td>100 µm</td><td>Oxygen</td><td>General engineerin g application</td></tr></tbody></table>
<table><thead><tr><th>Reference</th><th>Particle</th><th>Matrix</th><th>Size before/aft er process</th><th>Chemical modification</th><th>Process</th><th>Film thickne ss</th><th>Molecular penetrant</th><th>Application</th></tr></thead><tbody><tr><td>Mittal et al. [60]</td><td>Montmorillo nite</td><td>Poly(propyle ne)</td><td>-</td><td>Dimethyldioctadec ylammonium bromide</td><td>Melt processing / Compression moulding</td><td>100 µm</td><td>Oxygen</td><td>General engineerin g application</td></tr><tr><td>Monsivais- Barron et al. [149]</td><td>Montmorillo nite</td><td>High density polyethylene</td><td>-</td><td>Dimethyl di(hydrogenated tallowalkyl) quaternary ammonium Quaternary ammonium salt Unmodified</td><td>Melt-compoundin g / Thermopressi ng</td><td>100 µm</td><td>Oxygen</td><td>Packaging application</td></tr><tr><td>Müller et al. [150]</td><td>Montmorillo nite</td><td>Cassava starch</td><td>-</td><td>Methyl bis-2- hydroxyethyl tallow ammonium</td><td>Melt processing / Compression moulding</td><td>350 - 550 µm</td><td>Water vapor</td><td>General engineerin g</td></tr><tr><td>Nazarenko et al. [151]</td><td>Montmorillo nite</td><td>Poly(styrene)</td><td>Thickness: 1nm Length: 80-300nm</td><td>Unmodified<br/>Octadecyldimethyl betaine<br/>Vinylbenzyldimeth yldodecyl- ammonium chloride<br/>Octadecyltrimethyl ammonium chloride<br/>Dioctadecyldimeth yl- ammonium bromide<br/>Methyltriocadecyl ammonium bromide<br/>Tetraoctadecylam monium bromide<br/>Octadecyltrimethyl ammonium</td><td>Melt processing / Compression moulding</td><td>-</td><td>Oxygen</td><td>General engineerin g</td></tr><tr><td>Osman et al. [152]</td><td>Montmorillo nite</td><td>Linear high density polyethylene</td><td>-</td><td>Dimethyldioctaecl ammonium<br/>Methyltriocadecyl ammonium<br/>Benzylhexadecyldi methylammonium<br/>Docosyltriethylam monium</td><td>Melt processing / Compression moulding</td><td>60 µm</td><td>Oxygen</td><td>General engineerin g</td></tr><tr><td>Osman et al. [153]</td><td>Montmorillo nite</td><td>Poly(propyle ne)</td><td>-</td><td>Dihydroxy mthyl tallow quaterny ammonium</td><td>Melt processing / Compression moulding</td><td>75 - 85 µm</td><td>Oxygen / Water vapor</td><td>General engineerin g</td></tr><tr><td>Picard et al. [38]</td><td>Montmorillo nite</td><td>Poly(amide 6 )</td><td>-</td><td>Dihydroxy mthyl tallow quaterny ammonium</td><td>Melt processing / Compression moulding</td><td>100 µm</td><td>Oxygen</td><td>General engineerin g</td></tr><tr><td>Picard et al. [154]</td><td>Montmorillo nite</td><td>Poly(lactic acid)</td><td>/ Aspect ratio 24</td><td>Organically modified synthetic fluorine mica</td><td>Melt processing / Compression moulding</td><td>300 µm</td><td>Oxygen</td><td>Packaging</td></tr><tr><td>Ray et al. [146]</td><td>Montmorillo nite</td><td>Poly(lactic acid)</td><td>-</td><td>Unmodified<br/>Dimethyl, hydrogenated tallow, 2- ethylhexyl quaterny ammonium</td><td>Solution / Casting</td><td>60 - 70 µm</td><td>Water vapor</td><td>Packaging</td></tr><tr><td>Rhim et al. [155]</td><td>Montmorillo nite</td><td>Chitosan</td><td>-</td><td>Unmodified</td><td>Solution / Casting</td><td>80 µm</td><td>Water vapor</td><td>Packaging</td></tr><tr><td>Rhim et al. [155]</td><td>Montmorillo nite</td><td>Poly(L-</td><td>-</td><td>Unmodified</td><td>Solution / Casting</td><td>80 µm</td><td>Water vapor</td><td>Packaging</td></tr></tbody></table>
<table><thead><tr><th>Reference</th><th>Particle</th><th>Matrix</th><th>Size before/aft er process</th><th>Chemical modification</th><th>Process</th><th>Film thickne ss</th><th>Molecular penetrant</th><th>Application</th></tr></thead><tbody><tr><td>[36]</td><td>nite</td><td>lactide)</td><td></td><td>Dimethyl di(hydrogenated tallowalkyl) quaternary ammonium Bis-(2-hydrox- yethyl)methyl (hydrogenated tallowalkyl) quaternary ammonium</td><td>Casting</td><td></td><td></td><td></td></tr><tr><td>Rhim et al. [36]</td><td>Montmorillo nite</td><td>Food grade Agar Blend</td><td>-</td><td></td><td>Solution / Casting</td><td>-</td><td>Water vapor</td><td>Packaging</td></tr><tr><td>Sanchez- Garcia et al. [42]</td><td>Montmorillo nite</td><td>(80%wt)PHB / (20%wt)PCL</td><td>-</td><td>Organophilic surface modified kaolinite</td><td>Melt processing / Compression moulding</td><td>100 - 700 µm</td><td>Oxygen</td><td>Packaging</td></tr><tr><td>Sanchez- Garcia et al. [34]</td><td>Montmorillo nite</td><td>Poly(hydroxy butyrate- covalerate) Poly(e- caprolactone ) Poly(lactic acid) Low density polyethylene</td><td>-</td><td>Organo- modification</td><td>Solution / Casting</td><td>100 µm</td><td>Oxygen / Water vapor</td><td>Packaging</td></tr><tr><td>Shah et al. [156]</td><td>Montmorillo nite</td><td>Sodium ionomer of poly(ethylen e-co- methacrylic acid)</td><td>-</td><td>Dimethyl di(hydrogenated tallow) ammonium</td><td>Melt processing / Blown processing</td><td>25.4 - 76.2µm</td><td>Oxygen / Carbon dioxide / Water vapor</td><td>Packaging</td></tr><tr><td>Slavutsky et al. [157]</td><td>Montmorillo nite</td><td>Brea gum</td><td>-</td><td>Unmodified</td><td>Solution / Casting</td><td>-</td><td>Oxygen / Carbon dioxide / Water vapor</td><td>Packaging</td></tr><tr><td>Sothornvit et al. [158]</td><td>Montmorillo nite</td><td>Whey protein</td><td>-</td><td>Unmodified Dimethyl di(hydrogenated tallowalkyl) quaternary ammonium Bis-(2-hydrox- yethyl)methyl (hydrogenated tallowalkyl) quaternary ammonium Bis-(2-hydrox- yethyl)methyl (hydrogenated tallowalkyl) quaternary ammonium</td><td>Solution / Casting</td><td>150- 190 µm</td><td>Water vapor</td><td>Packaging</td></tr><tr><td>Sothornvit et al. [159]</td><td>Montmorillo nite</td><td>Whey protein</td><td>-</td><td></td><td>Solution / Casting</td><td>170 - 210 µm</td><td>Water vapor</td><td>Packaging</td></tr><tr><td>Strawhecker et al. [160]</td><td>Montmorillo nite</td><td>Poly(vinyl alcohol)</td><td>-</td><td></td><td>Solution / Casting</td><td>89 µm</td><td>Water vapor</td><td>General engineerin g</td></tr><tr><td>Sun et al. [35]</td><td>α-zirconium phosphate</td><td>Epoxy resin</td><td>Thickness: 1nm Length: 100nm - 1µm</td><td>Polyoxyalkyleneamine Tetra-n-butyl ammonium hydroxide</td><td>Solution / Casting</td><td>1000 µm</td><td>Oxygen</td><td>General engineerin g</td></tr><tr><td>Swain et al. [161]</td><td>Boron nitride</td><td>Cellulose</td><td>70nm /</td><td></td><td>Solution / Thermopressi ng</td><td>5000 µm</td><td>Oxygen</td><td>Packaging</td></tr><tr><td>Rodriguez-</td><td>Montmorillo</td><td>Rice flour</td><td>-</td><td>Unmodified</td><td>Solution /</td><td>180 µm</td><td>Water vapor</td><td>Packaging</td></tr></tbody></table>
<table><thead><tr><th>Reference</th><th>Particle</th><th>Matrix</th><th>Size before/aft er process</th><th>Chemical modification</th><th>Process</th><th>Film thickne ss</th><th>Molecular penetrant</th><th>Application</th></tr></thead><tbody><tr><td>Marin et al. [162]</td><td>nite</td><td>Banana flour</td><td></td><td>Citric acid</td><td>Casting</td><td></td><td></td><td></td></tr><tr><td>Tang et al. [163]</td><td>Montmorillo nite</td><td>Corn starch Wheat starch Potato starch</td><td>-</td><td>Citric acid / Sulphuric acid Unmodified Onium ion modified MMT Dimethyl, hydrogenated tallow, 2- ethylhexyl quaterny ammonium</td><td>Melt processing / Casting</td><td>-</td><td>Water vapor</td><td>Packaging</td></tr><tr><td>Thellen et al. [164]</td><td>Montmorillo nite</td><td>Poly(L- lactide)</td><td>-</td><td></td><td>Melt processing / Blown processing</td><td>75 µm</td><td>Oxygen / Water vapor</td><td>Packaging</td></tr><tr><td>Tsai et al. [165]</td><td>Montmorillo nite</td><td>Poly(methyl methacrylate )</td><td>/ Aspect ratio 100- 150</td><td>Organo- modification</td><td>In situ free radical polymerization / casting</td><td>100- 115 µm</td><td>Oxygen</td><td>Membrane</td></tr><tr><td>Tsai et al. [166]</td><td>Montmorillo nite</td><td>Poly(methyl methacrylate )</td><td>-</td><td>Organo- modification</td><td>In situ polymerization / casting</td><td></td><td>Oxygen</td><td>Membrane</td></tr><tr><td>Tunc et al. [47]</td><td>Montmorillo nite</td><td>Wheat gluten</td><td>Particle size distributio n: 5-10µm</td><td>-</td><td>Solution - Casting</td><td>200 µm</td><td>Oxygen / Carbon dioxide / Water vapor</td><td>Packaging</td></tr><tr><td>Villaluenga et al. [167]</td><td>Montmorillo nite</td><td>Isotactic polypropylen e</td><td>-</td><td>Unmodified Octadecyl ammonium chloride</td><td>Melt processing / Compression moulding</td><td>100- 170 µm</td><td>Oxygen</td><td>Membrane</td></tr><tr><td>Wang et al. [168]</td><td>Rectorite</td><td>Styrene butadiene rubber</td><td>/ Aspect ratio 20</td><td>Unmodified</td><td>Melt-compoundin g / Thermopressi ng</td><td>1000 µm</td><td>Oxygen</td><td>General engineerin g</td></tr><tr><td>Xu et al. [169]</td><td>Montmorillo nite</td><td>Poly(urethan e urea) segmented block copolymers</td><td>-</td><td>Octadecyl ammonium chloride</td><td>Solution / Casting</td><td>250 µm</td><td>Water vapor</td><td>General engineerin g</td></tr><tr><td>Yeh et al. [170]</td><td>Montmorillo nite</td><td>Epoxy resin</td><td>-</td><td>Tetradecyltrimeth lammonium chloride</td><td>Solution / Casting</td><td>250 µm</td><td>Oxygen / Water vapor</td><td>Membrane</td></tr><tr><td>Zehetmeyer et al. [171]</td><td>Montmorillo nite</td><td>Poly(propyle ne)</td><td>-</td><td>Octadecyl ammonium chloride</td><td>Melt-compoundin g / Thermopressi ng</td><td>25 µm</td><td>Oxygen / Water vapor</td><td>Packaging</td></tr><tr><td>Zhang et al. [172]</td><td>Magnetic iron oxide Fe<sub>3</sub>O<sub>4</sub></td><td>Zein</td><td>Long diagonal size: 200nm Height: 20nm /</td><td>Amphilic oleic acid</td><td>Solution / Casting</td><td>600- 1300 µm</td><td>Oxygen / Water vapor</td><td>Packaging</td></tr><tr><td>Zhon et al. [173]</td><td>Montmorillo nite</td><td>Ethylene vinyl acetate Low density polyethylene High density polyethylene</td><td>-</td><td>Dimethyl di(hydrogenated tallowalkyl) quaternary ammonium</td><td>Melt processing / Blown processing</td><td>-</td><td>Oxygen</td><td>General engineerin g</td></tr></tbody></table>

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Highlights:
- More than 1000 experimental data of O₂, CO₂ and H₂O permeability are presented

- Impact of nanoparticle shape on permeability is analyzed

- Expected impact of tortuosity on permeability is not systematically achieved

- Selective transport characteristics of nanocomposites is discussed

- Meaningful conclusions on the structure/mass transfer relationship are offered

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