Rheological and film-forming properties of
carboxymethylcellulose and polyvinyl alcohol
(CMC/PVA) mixtures

Eya Chikhaoui, Emna Chérif, Mehdi Ammar, Julien Chaste, David Bouville, Etienne Herth

► To cite this version:

Eya Chikhaoui, Emna Chérif, Mehdi Ammar, Julien Chaste, David Bouville, et al.. Rheological and film-forming properties of carboxymethylcellulose and polyvinyl alcohol (CMC/PVA) mixtures. Journal of Molecular Liquids, 2025, 421, pp.126819. 10.1016/j.molliq.2024.126819. hal-05400841

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# Rheological and film-forming properties of carboxymethylcellulose and polyvinyl alcohol (CMC/PVA) mixtures

Eya Chikhaoui<sup>a</sup>, Emna Cherif<sup>a,*</sup>, Mehdi Ammar<sup>b</sup>, Julien Chaste<sup>b</sup>, David Bouville<sup>b</sup>, Etienne Herth<sup>b,*</sup>

<figure><img src="image_3.png" /></figure>

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## ARTICLE INFO

### Keywords:

Dynamic viscosity

Activation energy

Interaction

CMC

PVA

## ABSTRACT

Polysaccharides are common components in medical, technological, and industrial products. Mixtures of sodium carboxymethylcellulose (CMC) and polyvinyl alcohol (PVA) are widely used in various formulations. Understanding their interactions can help to control the physical properties and stability of these systems. In this study, CMC/PVA mixtures were prepared with different volume fractions of PVA, referred to as $X_{PVA}$ (0–1), to investigate these interactions. Mixed aqueous solutions of CMC/PVA (10 g/L) were analyzed at increasing temperatures from 25 °C to 40 °C using rheological and conductometric analyzes. The results showed that dynamic viscosity ($η$) ranged from 0.4 cP to 0.92 cP, while electrical conductivity ($σ$) increased from 1100 µS/cm to 1500 µS/cm with increasing temperature. The activation energies for dynamic viscosity ($ΔG_η$), electrical conductivity ($ΔG_σ$) and activation parameters ($ΔH_η$ and $ΔS_η$) were determined. Morphological and structural properties were analyzed by profilometry, contact angle measurement, Fourier transform infrared spectroscopy (FTIR) and Raman spectra of thin films of CMC/PVA deposited by spin coating. Our findings provide valuable insights into the thermal behavior and interaction mechanisms of CMC/PVA mixtures, contributing to the development of advanced materials with customizable properties for applications in biomedical and industrial fields.

## 1. Introduction

Materials play a crucial role in advancing human society, and polymers are at the forefront of modern technological developments due to their versatility and wide range of applications [1]. While synthetic polymers dominate the market, interest in biopolymers is growing due to their renewable origin, biodegradability, biocompatibility, and relatively low cost [2,3].

However, a major challenge in the use of biopolymers is their often-poor mechanical properties, including their low strength and limited durability. To address this limitation, researchers often resort to chemical modifications, often using crosslinking agents. Unfortunately, many crosslinking reagents are highly toxic, which limits their use, especially in applications that require biocompatibility, such as pharmaceuticals and food packaging. This problem has led to the search for alternative strategies to improve the mechanical properties of biopolymers without compromising safety.

Sodium carboxymethylcellulose (CMC), a cellulose-derived

biopolymer, and polyvinyl alcohol (PVA), a synthetic polymer known for its excellent film-forming properties, have gained attention as promising candidates for composite materials with enhanced performance [4–11]. CMC, with its abundant hydroxyl groups, is well-suited for the production of films and hydrogels, while PVA, a synthetic water-soluble polymer exhibits excellent film-forming properties and high compatibility with biopolymers [12–15]. Combining CMC and PVA, mixtures can be produced that offer enhanced mechanical strength, improved thermal stability, and controlled biodegradability [16–19].

Despite the promising potential of CMC/PVA composites, the relationship between mixing ratios, processing conditions, and the resulting physical properties is not yet fully understood. This knowledge is crucial for tailoring composites to specific applications in drug delivery, wound dressings, and eco-friendly packaging. One significant gap in the literature is the lack of comprehensive studies that systematically investigate the rheological, conductometric and film-forming properties of CMC/PVA blends, particularly under different temperature conditions and

<footnote><sup>a</sup> Faculté des Sciences de Tunis, LR99ES16, Laboratoire de la Physique de la Matière Molle et De la Modélisation Electromagnétique Université de Tunis el Manar, 2092 Tunis, Tunisie</footnote>

<footnote><sup>b</sup> Centre de Nanosciences et de Nanotechnologies, CNRS, Université Paris-Saclay, Palaiseau 91120, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: amna.cherif@yahoo.fr (E. Cherif), etienne.herth@cnrs.fr (E. Herth).</footnote>
PVA volume fractions.

The aim of our research is to address this gap by investigating the physical behavior of CMC after chemical modification and its combination with PVA to produce customizable composites. Through detailed miscibility studies and analysis of the effects of temperature and PVA volume fraction on the structural and functional properties of CMC/PVA blends, we aim to develop materials with specific performance profiles suitable for medical, engineering and industrial applications. Our analysis includes advanced techniques such as SEM, FTIR and Raman spectroscopy to provide insights into the morphological and structural properties of the resulting films. These findings will contribute valuable information for the development of advanced polymer-based materials with broad industrial potential.

## 2. Experimental details

### 2.1. Synthesis of CMC/ PVA

We ensured that the molecular weights selected for CMC and PVA are carefully matched to the specific requirements of the intended applications, particularly in biomedical and industrial contexts. In biomedical applications, such as drug delivery systems or wound dressings, the molecular weight influences not only the viscosity and film-forming ability but also the biodegradability and biocompatibility of the polymer blends. Higher molecular weights tend to improve mechanical strength and elongation, both of which are crucial for the durability and effectiveness of biomedical materials under physiological conditions. In industrial applications, especially in packaging and film production, molecular weight influences processing behavior and final material properties. For instance, a balance between sufficient viscosity for processing and optimal mechanical properties for the end application is crucial importance. The selected molecular weights of 700,000 g/mol for CMC and 205,000 g/mol for PVA provide the necessary properties for the production of stable films with suitable barrier properties, which are essential for applications ranging from food packaging to environmental remediation. These specific molecular weights were selected based on their relevance to similar studies in the existing literature [20], and provide an optimal balance between film-forming ability, control over molecular viscosity and desired mechanical properties in CMC/PVA blends.

For this study, CMC was purchased from Sigma-Aldrich, and 0.5 g of CMC was dissolved in 50 ml of deionized water with magnetic stirring and adjusted to a pH of 6 to obtain a 10 g/L solution. The PVA used for this study was also purchased from Sigma-Aldrich. 0.5 g of PVA powder was dissolved in 50 ml of deionized water with magnetic stirring at a pH of 6, to prepare a 10 g/L concentration. Mixtures with different PVA fractions were then prepared to produce 11 daughter solutions.

The preparation of the CMC/PVA mixtures was strictly controlled to ensure precise volume fractions ($X_{PVA}$) in each sample. The volume fraction of PVA ($X_{PVA}$), ranging from 0 to 1 (0 % to 100 % PVA), was achieved by accurately measuring the volumes of stock solutions of PVA and CMC, both prepared at a concentration of 10 g/L. Each stock solution was obtained by dissolving the polymers in deionized water and stirring continuously for 24 h to ensure complete dissolution. The desired volume fraction of PVA in each mixture was calculated using the formula [20]:

$$X_{PVA} = \frac{V_{PVA}}{V_{PVA} + V_{CMC}} \quad (1)$$

where $V_{PVA}$ and $V_{CMC}$ represent the volumes of PVA and CMC solutions, respectively. By adjusting the volumes of these two solutions, we achieved mixtures with $X_{PVA}$ values between 0 (pure CMC) and 1 (pure PVA), with the intermediate compositions closely controlled.

To ensure homogeneity and prevent phase separation, the mixtures were stirred continuously stirred at room temperature for 12 h before

further analysis. The accuracy of the volumetric fractions was confirmed by measuring the weight and concentration of the components in the final mixtures, which matched the $X_{PVA}$ target values. This method allowed us to systematically investigate the effects of PVA content on the physical properties of the CMC/PVA blends, ensuring consistency and reproducibility of our experimental results.

Fig. 1 shows a photograph of the CMC/PVA mixtures with different $X_{PVA}$ fractions of PVA at 25 °C. All CMC/PVA mixtures were then deposited by spin-coating onto silicon samples at a speed of 200 rpm for 30 s. To ensure the evaporation of the remaining solvent, the samples were air-dried at room temperature for at least 72 h. The films were stored in a drying box for further use and characterization.

### 2.2. Characterization techniques

The dynamic viscosities ($η$) [21–25], electrical conductivities ($σ$) [26–28] and activation energies related to the dynamic viscosity ($ΔG_η$) and electrical conductivity ($ΔG_σ$) of the CMC/PVA mixtures were determined. The measurements showed that increasing the temperature significantly affects the dynamic viscosity and conductivity of the solutions. Thermodynamic activation parameters, such as the enthalpy of activation ($ΔH_η$) and entropy of activation ($ΔS_η$) were also calculated, revealing that two identified critical regions play a crucial role in the dynamics of the viscosity properties. These results indicate a structural reorganization in CMC/PVA blends as a function of temperature and PVA volume fraction.

In addition, the film-forming properties of CMC/PVA composite blends are investigated in this work. The morphological and structural properties of the resulting films were thoroughly analyzed using various advanced techniques. Scanning electron microscopy (SEM) model Magellan equipped with an energy-dispersive X-ray spectrometer (EDX) was used to investigate the surface morphology, while Fourier transform infrared spectroscopy (FTIR) analyzed the chemical interactions within the composites in the range of 4000 cm$^{-1}$ to 500 cm$^{-1}$ with a resolution of 4 cm$^{-1}$. Raman spectroscopy provided insights into the molecular structure. Contact angle measurements ($θ$) were performed at room temperature using a Data Physics OCA20 goniometer to evaluate the wettability of the prepared and modified CMC/PVA surfaces. A drop of 1 µL of isopropanol was applied to the needle tip at a rate of 0.5 µL/s. The thickness of the CMC/PVA films was measured using a Dektak XT profilometer.

The analyses provide valuable insights into polymer compatibility, the formation of polymer networks and film formation mechanisms. The rheological properties [29,30] were measured under the influence of a process to increase the volumetric fraction, referred to as $X_{PVA}$, and increasing temperature. Measurements were performed using a Brookfield DV-II (+Pro Co; Germany) cone-plate viscometer, with temperature precisely controlled to within ± 0.5 °C using a Haake D8 Circulator thermostatic water bath and a Haake L water bath (Germany). Electrical conductivity measurements were performed using a conductometer [31,32] equipped with a conductivity cell composed of circular stainless-steel electrodes with fixed spacing. Direct current measurements were carried out, and the cell was calibrated with liquids of known electrical conductivity, in particular a NaCl solution. For both the viscosity and conductivity experiments, each sample composition was tested with a minimum of three replicates at each temperature.

In this work, control experiments were performed with pure CMC and pure PVA (100 % concentrations) under the same experimental conditions as the mixtures. These controls were critical for evaluating the individual contributions of each polymer to the properties observed in the blends. The results provided valuable insight into how the combination of CMC and PVA altered the physicochemical properties compared to each polymer individually. In particular, the viscosity and conductivity measurements of the pure CMC and pure PVA solutions showed different behavior from those of the blends, confirming the presence of specific interactions between the polymers within the mixed
<figure><img src="image_4.png" /><figcaption>Fig. 1. Photograph of CMC/PVA mixtures at different fractions X~PVA~ of PVA at 25 °C.</figcaption></figure>

systems. The control experiments provide an important basis for the interpretation of the effects observed in CMC/PVA mixtures, particularly regarding their rheological properties and film-forming characteristics. Establishing this baseline enables a clearer understanding of polymer interactions and the resulting performance of CMC/PVA blends in different applications.

## 3. Results

### 3.1. The dynamic viscosities

In polymer mixtures such as CMC/PVA, dynamic viscosity is governed by molecular interactions, including hydrogen bonding and hydrophilic/hydrophobic effects between the polymer chains. In non-ideal polymer blends, viscosity often deviates from linearity. This is due to the molecular reconfigurations and specific interactions that can lead to phase separation or complex formation within the polymer network. These phenomena significantly influence the overall viscosity behavior of the mixture. The dynamic viscosities of the binary CMC/PVA mixture were measured at 25 °C, 30 °C, 35 °C, and 40 °C. Fig. 2 shows the experimental viscosity values as a function of the mole fraction of the PVA component in the binary CMC/PVA mixture. Where X~φ1~ and X~φ2~ represented respectively the CMC-rich region and the PVA-rich region. We found that the dynamic viscosity of the CMC/PVA mixture did not vary linearly with the volume fraction X~PVA~ of PVA, indicating complex interactions and non-ideal behavior within the mixture.

As shown in Fig. 2, three distinct regions emerge based on composition and electrostatic interactions within the solutions, each associated with different phase behaviors. In the CMC-rich region (X~φ1~), with 0 < X~PVA~ < 0.35, the dynamic viscosity η increases significantly, rising from 0.506 to 0.943 cP at 25 °C, from 0.496 to 0.925 cP at 30 °C, from 0.488

to 0.899 cP at 35 °C, and from 0.476 to 0.885 cP at 40 °C with the increase in X~PVA~ concentration in the range of 0 to 0.3. In the critical region, 0.35 < X~PVA~ < 0.45, the highest magnitude of dynamic viscosity η was obtained with a critical region centered around X~C~ = 0.4 vol fraction of PVA. This peak was expected due to the strong molecular correlations between PVA and CMC at this composition. In the PVA-rich region (X~φ2~), with 0.45 < X~PVA~ < 1, the dynamic viscosity η decreases significantly, dropping from 0.923 to 0.771 cP at 25 °C, from 0.925 to 0.653 cP at 30 °C, from 0.899 to 0.536 cP at 35 °C, and from 0.885 to 0.41 cP at 40 °C with the increase in X~PVA~ fraction in the range of 0.45 to 1.

### 3.2. The electrical conductivities

Electrical conductivity in polymer mixtures is influenced by ion mobility and the presence of charged functional groups. In CMC/PVA systems, conductivity can be influenced by interactions between the polymer chains and ions, hydrophilic/hydrophobic effects, and the overall molecular structure, all of which affect ion transport. Fig. 3 shows the experimental values of electrical conductivity (σ) as a function of the volumetric fraction of PVA in the binary CMC/PVA mixtures. We observed that σ in the CMC/PVA mixture does not follow a linear trend with the volume fraction X~PVA~ of PVA. Based on composition and electrostatic interactions, three distinct regions with different phase behaviors are distinguishable in the CMC/PVA mixture. In the CMC-rich region (X~φ1~), with 0 < X~PVA~ < 0.35, electrical conductivity (σ) decreases significantly. A possible interpretation is that the hydrophilic group of PVA primarily adsorbs on the walls of the conductivity cell. Consequently, the proportion of the hydrophobic group of PVA on the solid-liquid surface increases. Therefore, the interaction of the solid-liquid surface with the water solvent decreases significantly. On the other hand, the interaction of the solid-liquid surface with the hydrophobic group of PVA molecules in solution should increase accordingly.

<figure><img src="image_5.png" /><figcaption>Fig. 2. Dynamic viscosity of aqueous solutions of CMC/PVA mixtures as a function of the volume fraction X~PVA~ of PVA at different temperatures ranging from 25 °C to 40 °C.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. The electrical conductivities σ of aqueous solutions CMC/PVA mixtures as a function of the volume fraction X~PVA~ of PVA at different temperatures ranging from 25 °C to 40 °C.</figcaption></figure>
In the critical region, $0.35 < X_{PVA} < 0.45$, the highest electrical conductivity values ($\sigma$) occur around the critical composition, with a peak at $X_C = 0.4$ vol fraction of PVA. This result is anticipated due to the strong molecular correlations between PVA and CMC at this composition. In the PVA-rich region ($X_{\phi2}$) with $0.45 < X_{PVA} < 1$, electrical conductivity ($\sigma$) increases significantly. The increase in electrical conductivity of the CMC/PVA was attributed to the repulsive intermolecular interactions between PVA and CMC in solution. Tewari and Srivastava [33] pointed out that the most important polymer-polymer interaction was the thermodynamic interaction, which includes the intramolecular excluded volume effect leading to coil expansion in solution and the intermolecular excluded volume effect leading to coil contraction. In this context, the repulsive interactions between PVA and CMC in solution enhance the intermolecular excluded volume effect.

### 3.3. The activation energies Gibbs of the viscosity and electrical conductivity of CMC/PVA aqueous complexes

Blending two polymers can be an effective approach to tailoring materials by harnessing the synergistic properties of each component. Polymer blends are sometimes also called “polymers alloys”. However, this term is more accurately applied to compatible polymers, those that can form a relatively uniform mixture at the molecular level. Achieving a homogeneous and stable blend of two or more polymers requires that the dispersion of one polymer within the other is thermodynamically favorable. To investigate the temperature-dependent changes in viscosity and electrical conductivity of aqueous CMC/PVA solutions, the logarithmic form of the Arrhenius equation can be applied as follows [31]:

$$ \ln(y) = \ln(y_o) - DG_y/RT \quad (2) $$

where $\ln(y_o)$ is the intrinsic pre-exponential factor, $\Delta G_y$ is the intrinsic Gibbs free energy, and R is the ideal gas constant. Thus, to obtain $\Delta G_y$, we use the following equation (3) [26,28]:

$$ DG_y = -RT\ln(y)/\ln(y_o) \quad (3) $$

In Fig. 4, the calculated values of $\Delta G_{\eta}$ (where y refers to viscosity $\eta$) for different temperatures are plotted as a function of the volume fraction of PVA. As expected, we observed three regions: the CMC-rich region ($X_{\phi1}$), the critical region $X_C$, and the PVA-rich region ($X_{\phi2}$).

In the CMC-rich region ($X_{\phi1}$), particles in the liquid state exhibit mobility relative to each other, although they remain in close proximity, lacking any ordered structure. Consequently, the $\Delta G_{\eta}$ values decrease exponentially with increasing $X_{PVA}$. This was an effect on $\Delta G_{\eta}$ resulting from the strong correlation between PVA and CMC molecules near $X_C =$

<figure><img src="image_7.png" /><figcaption>Fig. 4. Energies of activation $\Delta G_{\eta}$ of aqueous solutions CMC/PVA mixtures as a function of the volume fraction $X_{PVA}$ of PVA at different temperatures ranging from 25 °C to 40 °C.</figcaption></figure>

0.4. At a fixed temperature and far away from $X_{\phi1}$, the volume fraction has no significant effect on the $\Delta G_{\eta}$ values. In the PVA-rich region ($X_{\phi2}$), it was no longer possible for the condensed phase matter (liquids) to consider individually the thermal kinetic energy of atoms and molecules. The intense interaction forces acting between these elements lead to a state of motion of one atom being perceived by a whole series of neighboring atoms, so the values of $\Delta G_{\eta}$ increase exponentially with $X_{PVA}$. This was an effect on $\Delta G_{\eta}$ resulting from the strong correlation between PVA and CMC molecules. The $\Delta G_{\eta}$ values of the CMC/PVA mixture decrease significantly with temperature. At 25 °C, the $\Delta G_{\eta}$ values of the CMC/PVA mixture are initially positive at the beginning of the CMC-rich region ($X_{\phi1}$), become negative in the critical region ($X_C$), and become positive again in the PVA-rich region ($X_{\phi2}$). The molecular interactions between demineralized water (W) and both CMC/W and PVA/W exceed the molecular interactions of the pure components [25]. The $\Delta G_{\eta}$ values are negative in the critical region $X_C$, and the characteristic behaviors of the CMC/PVA mixture show strong specific interactions. At 30 °C and 35 °C, the $\Delta G_{\eta}$ values are positive at the beginning of the CMC-rich region($X_{\phi1}$) but shift to negative. At 40 °C, the $\Delta G_{\eta}$ values in the CMC-rich region ($X_{\phi1}$) are negative, indicating a temperature-dependent shift in molecular interactions.

The calculated values of $\Delta G_{\sigma}$ (where $\sigma$ refers to conductivity) for different temperatures are plotted as a function of the volume fraction of PVA in Fig. 5. We observed three regions: the CMC-rich region ($X_{\phi1}$), the critical region ($X_C$), and the PVA-rich region ($X_{\phi2}$).

In the CMC-rich region ($X_{\phi1}$), the particles, which form a liquid, exhibit independent movement relative to each other, but remain in close proximity. There is no longer an ordered structure, leading to an exponential increase in the values of $\Delta G_{\sigma}$ with increasing $X_{PVA}$. This is an effect on $\Delta G_{\sigma}$ resulting from the strong correlation between PVA and CMC molecules near $X_C = 0.4$. At a fixed temperature and far from $X_{\phi1}$, the volume fraction does not have a significant effect on the $\Delta G_{\sigma}$ values.

In the PVA-rich region ($X_{\phi2}$), for the condensed phase matter (liquids), it was no longer possible to consider the thermal kinetic energy of atoms and molecules individually. The intense interaction forces acting between these elements cause the state of motion of one atom to be detected by a whole series of neighboring atoms, so the values of $\Delta G_{\sigma}$ decrease exponentially with $X_{PVA}$. This is an effect on $\Delta G_{\sigma}$ resulting from the strong correlation between PVA and CMC molecules. The $\Delta G_{\sigma}$ values of the CMC/PVA mixture decrease significantly with temperature. For all temperatures from 25 °C to 40 °C, the $\Delta G_{\sigma}$ values of the CMC/PVA mixture are negative in all three regions: CMC-rich $X_{\phi1}$, critical $X_C$, and PVA-rich $X_{\phi2}$. The molecular interactions in the CMC/PVA mixture are higher than the molecular interactions of the pure components in CMC/W and PVA/W water [34]. In the critical region $X_C$, the characteristic behaviors of the CMC/PVA mixture exhibit strong specific interactions.

<figure><img src="image_8.png" /><figcaption>Fig. 5. Energies of activation $\Delta G_{\sigma}$ for CMC/PVA mixtures versus the volume fraction $X_{PVA}$ of PVA at different temperatures range from 25 °C to 40 °C.</figcaption></figure>
### 3.4. State diagram of CMC/PVA solutions of the activation parameters ΔHη and ΔSη

According to Eyring's absolute rate theory [35], molecules of a pure liquid at rest are considered as rigid spheres that constantly move and rearrange themselves, shifting from one active site to another vacant site. However, due to intermolecular stacking, this movement was effectively limited to the vibration of each molecule within a space defined by surrounding molecules. This space per molecule is associated with potential energy, which represents the energy required for the molecule to move from one equilibrium position to another nearby unoccupied or vacant position. Like any other spontaneous process, self-assembly is driven by the minimization of the system's activation energy [31]. The variation in Gibbs activation energy of dynamic viscosity of a system at constant temperature and volume, ΔEη, is defined by equation (4) [30–32]:

$$DG_h = DH_h + TDS_h \quad (4)$$

where ΔHη is the activation enthalpy and ΔSη defines the change in dynamic viscosity entropy for the CMC/PVA mixture. We can use Eq. (4) to determine the activation parameters ΔHη and ΔSη. The obtained activation parameters ΔHη and ΔSη for the binary mixture are presented in Fig. 6. To study the stability of the CMC/PVA mixture in aqueous medium, Fig. 6 shows the morphological changes of the phases of the CMC/PVA mixture. The values of ΔHη for the CMC/PVA mixture are positive for all studied XpVA compositions. We observed that the ΔHη values of the pure components vary in the following order: ΔHη, CMC > ΔHη, PVA, indicating that the energy required for CMC/W to deform its bonds is higher than that of PVA/W, suggesting that the self-association of CMC/W is superior to that of PVA/W. The values of ΔSη for the CMC/PVA mixture are negative for all studied XpVA compositions. We found that the ΔSη values of the pure components vary in the following order: ΔSη, CMC < ΔSη, PVA, indicating that the formation of the activated complex increases when hydrogen bonds are broken. Based on the composition and hydrophobic interaction of the solutions, three distinct regions with different phase behavior can be distinguished for the CMC/PVA mixture in Fig. 6. In the CMC-rich regions (Xφ1) where 0 < XpVA < 0.35, ΔHη increases and ΔSη decreases. When PVA is added to CMC, an interaction occurs, causing association between the three types of molecules (PVA molecules, CMC molecules, and W molecules), resulting in entanglement effect. In the critical region where 0.35 < XpVA < 0.45, the greatest magnitude of positive values of ΔHη and the lowest magnitude of negative values of ΔSη are centered around Xc = 0.4 vol fraction of PVA. This result is not surprising as the molecular correlations between PVA and CMC are strong. In the PVA-rich region (Xφ2) where 0.45 <

<figure><img src="image_9.png" /><figcaption>Fig. 6. The enthalpy (ΔHη) and entropy (ΔSη) of activation of dynamic viscosity for CMC/PVA mixtures as function of the volume fraction XpVA of PVA.</figcaption></figure>

XpVA < 1, the values of ΔHη decrease and ΔSη increases as the volume fraction XpVA of PVA increases. As the volume fraction XpVA of PVA increases, restructuring of the polymer network occurs due to chain breaks, which is likely to induce the growth of water clusters and their contact with hydrophobic regions of the membranes, leading to a progressive change with stronger hydrogen bonding.

### 4. CMC/PVA films properties

#### 4.1. Spin coating of CMC/PVA thin-films on silicon samples

The present work investigates the interaction properties of CMC/PVA mixtures, and the preparation of CMC-PVA films for medical applications, such as the delivery of water-soluble basic drugs. A study of the thickness as a function of the coating parameters was carried out by characterizing the thicknesses films layers obtained. Fig. 7 shows the experimental thickness values as a function of the volume fraction XpVA of PVA for the CMC/PVA films.

As shown in Fig. 7 three different regions with different phase behavior can be distinguished depending on the composition and electrostatic interaction within the solutions [36]. In the CMC-rich region, 0 < XpVA < 0.35, the thickness ep decreases significantly, from 750 to 200 nm as XpVA increase. In the critical region, 0.35 < XpVA < 0.45, the minimum thickness ep was obtained centred around Xc = 0.4, a volume fraction of PVA. This result was expected, as the molecular correlations between PVA and CMC are strong. In the PVA-rich region, 0.45 < XpVA < 1, the thickness ep increases with increasing XpVA from 200 to 300 nm. The data in Fig. 2 reveal that the dynamic viscosity of aqueous CMC/PVA mixtures reaches a peak at PVA volume fractions between 0.3 and 0.5, attributed to strong interactions between the CMC and PVA chains, which create a more rigid internal structure. However, Fig. 7 shows that the film thickness of blends with these same volume fractions (0.3 to 0.5) is lower compared to other compositions. This apparent contradiction can be explained by the film formation process during spin coating. In this range of high viscosity, the spreading of the mixture is hindered, resulting in thinner films. Essentially, increased viscosity limits the flowability of the mixture, leading to less uniform deposition and therefore thinner film thickness. In addition, as shown in Fig. 1, samples containing 40 % to 50 % PVA suggesting partial phase separation or poor miscibility between CMC and PVA at these specific ratios. At these intermediate concentrations, the interactions between the two polymers are not sufficient to maintain a homogeneous mixture, likely resulting in micro-aggregates or suspended particles. This phenomenon could be due to hydrogen bonding between CMC and PVA molecules,

<figure><img src="image_10.png" /><figcaption>Fig. 7. Thickness ep for CMC/PVA films as function of volume fraction, XpVA of PVA at 25 °C.</figcaption></figure>
which promotes the formation of polymer complexes and reduces the solubility of one of the components. This behavior aligns with known properties of these polymers, where specific concentration ranges can lead to complexation or phase separation. Therefore, although the blends exhibit maximum viscosity within this volume fractions, this does not result in thicker films. Instead, the complex interactions between the polymers pose a challenge to achieving uniform film formation.

## 4.2. Surface properties of CMC/PVA thin-films

As shown in Fig. 8, the surface morphology of CMC/PVA films was analyzed as a function of the PVA volume fraction ($X_{PVA}$) [37–45]. It appears that the incorporation of PVA into CMC/PVA composite films significantly influences their surface morphology and structure. For pure CMC [46–50], the films exhibit irregular and rough surfaces, characterized by fibers and pores [51]. In contrast, with the addition of PVA, the SEM images reveal smooth, continuous and homogeneous surfaces, suggesting enhanced compatibility between the biopolymers. This observation suggests that PVA likely promotes the formation of hydrogen bonds between the O–H and C=O groups of the biopolymers, there by strengthening inter molecular interactions and contributing to a more uniform and continuous film structure [8]. In addition, the hydrophilic interactions dominate in the CMC/PVA mixtures with a higher CMC content, which leads to a lower contact angle. For example, the contact angle for pure CMC was 53.45°, indicating its strong hydrophilic nature. However, as the proportion of PVA increases, the hydrophobic interactions become more pronounced, leading to an increase in the contact angle, such as the contact angle of 61.7° observed for pure PVA. These variations in the contact angle reflect the different affinity of the mixture components for water and the solid surface, which results from their respective hydrophilic and hydrophobic properties.

Fig. 9 presents the FTIR spectra of CMC/PVA films as a function of the PVA volume fraction at 25 °C. Pure PVA and pure CMC thin films were characterized by the presence of hydroxyl groups (–OH), which form hydrogen bonds and impart rigidity to the polymer structure. As shown in Fig. 9.a, distinct characteristic peaks are observed in the FTIR spectrum of pure PVA: 611 $cm^{-1}$ (C–C bonds), 1110 $cm^{-1}$ (C–O–C bonds), 1733 $cm^{-1}$ (carbonyl ester groups –C=O), 2935 $cm^{-1}$ (C–H bonds of methyl and methylene groups), and 3378 $cm^{-1}$ (O–H bonds of hydroxyl groups) [52–54]. Similarly, the FTIR spectrum of pure CMC (Fig. 9.b) [55,56] shows prominent peaks: 3400 $cm^{-1}$ (O–H bonds), 2987 $cm^{-1}$ and 2912 $cm^{-1}$ (C–H bonds of methyl groups), 1608 $cm^{-1}$ (C=O bonds of carboxyl groups), and 1425 $cm^{-1}$ and 1371 $cm^{-1}$ (C–H bonds of methyl and methylene groups). In the FTIR spectrum of the CMC/PVA mixture (Fig. 9.c) [48], key peaks at 611 $cm^{-1}$ (C–C of PVA), 738 $cm^{-1}$ (C–H of PVA), 1112 $cm^{-1}$ (C–O–C of CMC), and 1606 $cm^{-1}$ (C=O of CMC) provide insights into the interactions between the two polymers. These spectral features indicate potential hydrogen bonding

or other intermolecular interactions between the functional groups of CMC and PVA. Analyzing these spectra offers valuable information about the composition and properties of the CMC/PVA mixture [57–59]. As shown in Fig. 9, FTIR spectra, the shifts of the hydroxyl (–OH) group peaks from 3378 $cm^{-1}$ for pure PVA and 3400 $cm^{-1}$ for pure CMC to an intermediate position in the mixture (about 3385 $cm^{-1}$) indicate the formation of new intermolecular interactions between the CMC and PVA chains. This shift indicates that hydrogen bonds are formed between the hydroxyl groups of PVA and the hydroxyl or carboxyl groups of CMC. These interactions lower the energy required for O–H bond vibrations, explaining the intermediate position of the peaks. The intensity variations of characteristic bands, particularly the carbonyl (–C=O) group peaks at 1733 $cm^{-1}$ in pure PVA and 1608 $cm^{-1}$ in pure CMC, provide further important insights. In the mixture, the relative intensity of these bands increases, indicating increased interactions between the carbonyl groups of PVA and the hydroxyl groups of CMC via hydrogen bonding. This behavior is consistent with a decrease in the mobility of the polymer chains due to the entanglement caused by these interactions, which reinforces the structure of the mixture. The formation of these new hydrogen bonds suggests that the polymer chains undergo significant restructuring to align their functional groups in close proximity to each other and maximize interaction. This restructuring improves miscibility and contributes to the formation of a uniform composite film. The overlapping peaks confirm that these polymers exhibit a high degree of compatibility through strong hydrophilic interactions, which are critical for stabilizing the mixture at the molecular level. These results indicate that CMC/PVA mixtures are promising candidates the production of protective and functional films in medical devices, thanks to their barrier properties and their capacity to incorporate therapeutic agents [60–63].

## 4.3. Structural properties of CMC/PVA thin-films

Raman spectroscopy, in contrast, provides insight into non-polar and symmetric vibrations that FTIR may not detect as effectively. Fig. 10 shows the Raman spectra of the CMC/PVA mixture for different molar fractions of PVA. These spectra provide valuable information about the molecular composition and the interactions within the mixture [59]. Distinct peaks at specific wavenumbers, such as 968 $cm^{-1}$ and 2922 $cm^{-1}$ for pure CMC, as well as at 965 $cm^{-1}$ and 2932 $cm^{-1}$ for pure PVA, provides insights into the structural properties of both polymers. Variations in the intensity or position of these peaks in the spectra indicate changes in the relative concentrations of the two polymers and point to possible interactions between them.

In the Raman spectra, bands associated with C–H bond vibrations, observed at 2932 $cm^{-1}$ for both polymers, also exhibit intensity and position changes in the mixture. These changes reflect adjustments in the chemical environment of methyl and methylene groups, related to

<figure><img src="image_11.png" /><figcaption>Fig. 8. Scanning electron microscopy micrographs of CMC/PVA films as function of the volume fraction $X_{PVA}$ of PVA.</figcaption></figure>
<figure><img src="image_12.png" /></figure>

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

<figure><img src="image_14.png" /><figcaption>**Fig. 9.** FTIR spectra of: (a) pure PVA, (b) pure CMC and (c) CMC/PVA mixture with different PVA volume fractions.</figcaption></figure>

increased intermolecular interactions. Specifically, the slight shifts in the bands at 965 cm⁻¹ (PVA) and 968 cm⁻¹ (CMC), representing vibrations of C–C and C–O bonds, confirm that polymer–polymer interactions subtly alter the vibrational energies of these bonds [60]. These spectral shifts and intensities enhance our understanding of the composition and dynamic behavior of the CMC/PVA mixture across different ratios, which is critical for optimizing its properties and

<figure><img src="image_15.png" /><figcaption>**Fig. 10.** Raman spectra for CMC/PVA mixtures versus the volume fraction X~PVA~ of PVA at 25 °C.</figcaption></figure>

potential applications [61]. The shifts in these regions indicate that the hydrophobic interactions, especially with methyl and methylene groups, are influenced by the presence of the other polymer. This indicates a partial reorganization of the non-polar regions, which contributes to improved mechanical properties. The overlapping regions in the spectra indicate that the functional groups of PVA and CMC not only coexist but are actively involved in cooperative interactions. The molecular interactions observed in the FTIR and Raman spectra (see Fig. 9 and Fig. 10) directly influence the physical properties and functional performance of the CMC/PVA films. The strong hydrogen bonds observed between the hydroxyl (–OH) and carbonyl (–C=O) groups of CMC and PVA contribute to the cohesive integrity of the polymer matrix. The intermolecular hydrogen bonds act as reversible cross-links that provide elasticity to the polymer network. These interactions enable the films to sustain mechanical deformation without breaking. This is an essential property for applications such as wound dressings, that require conformability to irregular surfaces. The interactions between hydrophilic groups (such as –OH) reduce the free volume within the polymer network and restrict the diffusion pathways for water molecules. This improves the material’s ability to act as a barrier against moisture, a desirable property for packaging or pharmaceutical applications. Similarly, the restructuring of polar and non-polar regions, indicated by the shift in C–C bonds in the Raman spectra, can restrict gas diffusion, making the films suitable for controlled drug delivery systems or protective coatings. This spectral shift, indicative of robust CMC-PVA interactions, suggests an optimized balance between flexibility and tensile strength. This is crucial for the production of films that are both durable and able to withstand dynamic environmental conditions.

The use of FTIR and Raman spectroscopy to analyze the structural properties of CMC/PVA films provides complementary insights into the chemical interactions in the mixtures. Although both techniques identify molecular vibrations and functional groups, they differ in their sensitivity to specific interactions and structural details. Together, these techniques confirm the presence of hydrogen bonding and other molecular interactions between CMC and PVA. These results highlight the utility of Raman spectroscopy as a powerful tool for the analysis of polymers and complex materials, enabling precise characterization and facilitating their use in various scientific and industrial applications.

Analyzing both spectra together enable a more comprehensive understanding of the molecular structure of CMC/PVA mixtures. Discrepancies or confirmations between the two techniques highlight the complexity of the polymer interactions and validate the results obtained from one technique against the other. Furthermore, FTIR and Raman spectroscopic analyses confirm the chemical stability of the films, showing minimal changes in chemical bonding under varying
temperatures and PVA volume fractions. This robustness demonstrates the resistance of the films to different environmental conditions.

This work demonstrates that CMC/PVA mixtures possess rheological, electrical, and thermodynamic properties ideal for various biomedical and pharmaceutical applications [64,65]. The films produced were stable, uniform, and exhibited adjustable properties, making them well-suited for meeting the specific demands of modern medical applications. Furthermore, these findings open avenues for exploring new applications in the field of biomaterials and medical devices.

## 5. Discussion

Polymers play a crucial role in various industries and have a significant impact on the development of products that meet the changing needs of society. Among these, biopolymers from renewable sources such as sodium carboxymethyl cellulose (CMC) and polyvinyl alcohol (PVA) have gained increasing attention due to their biodegradable, biocompatible and cost-effective properties. These properties make CMC and PVA suitable for a wide range of applications, particularly in the biomedical and industrial sectors. In biomedicine, CMC/PVA blends are of particular interest due to their potential for use in drug delivery systems, wound dressings and scaffolds for tissue engineering. The ability to customize the physical properties of these blends allows for the optimization of drug release profiles, enhanced healing properties and improved biocompatibility, which are essential for effective therapeutic outcomes. For example, the hydrophilic nature of CMC combined with the film-forming ability of PVA can result in materials that not only support cell adhesion and growth, but also enable controlled drug release. In an industrial context, CMC/PVA blends offer a versatile platform for the development of advanced materials that require specific performance characteristics, such as increased mechanical strength, thermal stability and controlled biodegradability. These properties make them suitable for environmentally friendly packaging materials, adhesives and coatings that can replace conventional synthetic polymers, reducing the environmental impact associated with plastic waste.

Despite the promising applications of CMC/PVA blends, understanding their interactions remains crucial for optimizing their performance in various applications. This study aims to investigate the physical behavior of CMC after chemical modification and its combination with PVA to produce composites with tailored properties. By conducting comprehensive miscibility studies and evaluating the effects of temperature and PVA volume fraction on the rheological, conductometric and film-forming properties of CMC/PVA blends, this work will contribute to the development of innovative materials for industrial and biomedical applications.

The selection of the temperature range of 25 °C to 40 °C in this study is based on its relevance to the potential applications of CMC/PVA mixtures in both the biomedical and industrial fields. This range includes ambient conditions (around 25 °C) and physiological temperatures (around 37 °C), which are critical for biomedical applications such as wound dressings or drug delivery systems that need to remain stable and functional at these temperatures. In addition, many industrial processes, such as the manufacturing of films and packaging, require materials to maintain stable properties within moderate temperature ranges. Studying this specific range allows us to assess the stability of materials under typical operating conditions. Finally, studying this temperature range helps observe changes in rheological behavior, particularly dynamic viscosity and electrical conductivity, which reflect structural reorganizations relevant to practical use. This approach ensures that the CMC/PVA mixtures perform consistently in real-world applications.

The identification of “critical regions” in the dynamic viscosity and electrical conductivity of CMC/PVA mixtures is based on both empirical data and theoretical considerations of polymer interactions. These regions are characterized by significant changes in physical properties, indicating structural transitions in the mixtures. The definition of these

“critical regions” is therefore based on both theoretical predictions from polymer science and the empirical data from our experiments. For dynamic viscosity, the critical region is around $X_c = 0.4$, where we observe a peak value at all tested temperatures (see Fig. 2). This indicates that the molecular interactions, such as hydrogen bonding and electrostatic forces between CMC and PVA, are strongest at this composition. The sharp increase in viscosity at this point was probably due to the formation of a semi-interpenetrating network structure, which leads to increased flow resistance.

Similarly, the critical region for conductivity (Fig. 3) occurs around $X_c = 0.4$. In this composition, strong intermolecular interactions reduce ion mobility, resulting in lower conductivity. These sharp changes in viscosity and conductivity indicate a significant structural reorganization of the polymer mixture in this range. These critical regions are defined by transitions in the experimental data that are consistent with theoretical models of polymer miscibility and phase behavior. The combination of electrostatic interactions, hydrogen bonding, and polymer chain entanglement plays a crucial role in determining the properties of the mixture at different PVA fractions. Similar behavior was observed for other polyelectrolyte systems, supporting our interpretation [53]. Rheological analysis revealed that an increase in CMC concentration leads to a higher dynamic viscosity, indicating enhanced intermolecular interactions [54]. Indeed, these rheological properties are crucial for tissue engineering and controlled drug delivery, as optimal viscosity is essential for the production and handling of biomaterials [55,56]. The results for electrical conductivity and activation energy $\Delta G_\sigma$ showed that the CMC/PVA mixture has tunable thermodynamic and electrical properties, which is beneficial for applications such as biomedical sensors and health monitoring devices [57–59]. Hydroxyl and carbonyl groups, which are primarily responsible for hydrophilic interactions through hydrogen bonding, improve the compatibility between the two polymers. This is also supported by the reduced intensity of the bands corresponding to free (unbound) groups, suggesting that these groups are actively involved in intermolecular bonding within the mixture. The peak shifts in conjunction with the intensity variations thus support the idea that the interactions in the mixture are dominated by intermolecular hydrogen bonding between the functional groups of the two polymers. These interactions are strong enough to influence the macroscopic structure, as shown by the homogeneous film properties observed in the SEM images (see Fig. 8). Furthermore, they explain the viscoelastic behavior and viscosity variations observed in CMC/PVA mixtures, where the formation of dense polymer networks by hydrogen bonding restricts chain mobility.

## 6. Conclusions

Our study on CMC/PVA mixture solutions provides valuable insights into the relationships between dynamic viscosity ($\eta$), electrical conductivity ($\sigma$) and activation energies ($\Delta G_\eta$, $\Delta G_\sigma$) at different PVA mole fractions and temperatures. The results show that both temperature and PVA content have a significant effect on the physical properties of the blends, with three distinct phases emerging: a CMC-rich phase, a critical transition zone (centered around $X_{PVA} = 0.4$) and PVA-rich phase. These phases are primarily governed by molecular reorganization and long-range polymer interactions. The observed changes in activation energies, coupled with thermodynamic parameters such as activation enthalpy ($\Delta H\eta$) and entropy ($\Delta S\eta$) highlight the significant structural transformations occur within the blends. These findings demonstrate the potential of CMC/PVA blends as highly customizable, biodegradable and biocompatible materials with promising applications in biomedical, industrial and technical fields. The tunable properties of these blends particularly their viscosity, conductivity and film-forming ability, make them exceptional candidates for next-generation materials engineered for advanced functionalities.
CRediT authorship contribution statement

**Eya Chikhaoui:** Writing – original draft, Investigation, Formal analysis, Data curation. **Emna Cherif:** Writing – original draft, Validation, Supervision, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Mehdi Ammar:** Supervision, Resources, Investigation, Conceptualization. **Julien Chaste:** Resources, Investigation, Funding acquisition, Conceptualization. **David Bouville:** Validation, Resources, Investigation. **Etienne Herth:** Writing – original draft, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors thank the staff of the clean room and characterization laboratory. This work was partially supported by the French RENATECH network of technological facilities.

Data availability

Data will be made available on request.

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