nanomaterial reacted to temperature variations while display-
ing a simultaneous increase in its fluorescence intensity.¹⁵⁴
The latter was attributed to the thermoresponsiveness of
PNIPAM brushes, which altered the distance between the dye
groups.

**Dual pH- and Temperature-Responsive Hybrids.** Contributions of polymeric chains bearing different stimuli-responsive properties can result in copolymers with multiple response functions. In the domain of polymer-grafted CNCs, the first example of hybrids reporting such dual-responsiveness was CNC-g-P(NIPAM-ran-AA) containing different amounts of AA, which responded to pH and temperature variations.¹⁴² The CPT of this system was tuned by three factors: polymer tethering to CNCs, AA content, and pH (Figure 13). The CPT increased

<figure><img src="image_28.png" /><figcaption>**Figure 13.** LCST diagram of 0.2 wt % aqueous suspensions of P(NIPAM-ran-AA) as free copolymer (red) or grafted to CNCs (blue) bearing different AA content at low and high pH values. Reprinted with permission of ref 142. Copyright 2014, Royal Society of Chemistry.</figcaption></figure>

from 38 to 45 °C at high pH (ca. 7–8) while it decreased from
30 to 18 °C at lower pH values (ca. 3–4), when the AA content
of the grafts was increased from 5 to 30 mol %. The reduction
observed in the CPT was mainly attributed to the hydrophobic
behavior of AA in protic media. In addition, a comparison
between free and CNC-grafted copolymers demonstrated that
tethering copolymers to CNCs resulted in an increase up to 6 °C
in the CPT, with respect to the LCST of free copolymer.

Using the same monomers, albeit with block rather than
random distribution of comonomers and varying the inner
and outer block, Haqani et al. also reported thermoresponsive
systems. A dual-sensitive characteristic was only observed when
the PAA was at the outer block, i.e. CNC-g-PNIPAM-block-
PAA.¹⁷⁸ Furthermore, Tang et al. developed a hybrid, containing
POEGMA and PMAA grafted-CNCs, in order to control the
stability of particle-stabilized emulsions manipulated by pH and
temperature.¹³⁵ PMAA brushes formed hydrogen bonds with
POEGMA at low pH values (<4.5), leading to the formation of
aggregates, while the presence of electrostatic repulsions at higher
pH disrupted hydrogen bonding to yield stable dispersions.
Additionally, POEGMA chains exhibited temperature-dependent

conformational changes that influenced the stability of the
emulsion.

*Shape-Memory Hybrids.* Shape-memory polymers (SMPs) are those capable of being deformed and returned to their original state through a specific stimulus, for example, via thermal transitions or activation–deactivation of supramolecular interactions, often in elastomeric systems. Wang et al. proposed a shape-memory nanocomposite derived from biomass, i.e., soybean oil and CNC.¹⁵⁶ The system was composed of CNC-g-PSBAM (synthesized via SI-ATRP with a theoretical M~n~ = 22 kDa) and bis-TAD using PSBAM as a matrix (Figure 10B). The network of the hybrid was formed by using two types of cross-links within the material: one covalently linked by bis-TAD and another dynamic physically cross-linked via hydrogen bonding with the plant oil functional groups (and also the bis-TAD moiety). Here, the covalently bonded network was responsible for the permanent shape of the material, while the physical network provided multiresponsive and multi shape-memory properties. The material showed triple-shape-memory with two recovery steps performed at staged temperatures of 40 and 80 °C, respectively. Additionally, water and THF induced the shape-recovery of the material within 30 and 5 min of immersion, respectively.

The properties exhibited by the above-mentioned stimuli-
responsive nanohybrids are mainly attributed to the type of
polymer grafted on the CNC surface. Thus, the main function of
the CNCs is to provide either mechanical reinforcement¹⁵⁶ to
the material or enhance their solubility¹⁴² in polar solvents due
to the large amount of –OH groups at the surface of CNCs.
Most of these investigations have been focused on the synthesis
of stimuli-responsive materials and their behavior upon applica-
tion of different stimuli, but little is known about the minimum
amount of grafted polymer required to observe such stimuli-
responsive behavior. Furthermore, it is in many cases unclear
how the grafting density of the polymer influences the respon-
siveness of the material.

**4.3. Nanocomposites.** Nanocomposites are materials composed of two or more components, often a matrix and a filler, in the solid or gel-like state, the filler being on the nanoscale. There are different processing routes reported in the literature for the production of cellulose nanocomposites, such as solution casting, melt extrusion, compression molding, injection molding, ball milling, or even precipitation methods.<sup>4,5,7,203</sup> The outstanding mechanical properties and the high specific area suggest that CNCs would provide good reinforcement to other polymeric materials. The reinforcement potential of CNCs was first reported by Favier et al., who incorporated tunicate-derived CNCs into a copolymer of Sty and BA (P(Sty-co-BA)).<sup>204,205</sup> The reinforcement effect of CNCs is often explained by the formation of a rigid percolating network in which the filler nanoparticles interact via hydrogen bonding.<sup>5</sup> The experimental evidence and statistical models stipulate that the matrix, the filler characteristics, such as aspect ratio, dispersion and volume fraction, and their interactions (filler–matrix and filler–filler) all influence the mechanical properties of the nanocomposites.<sup>3</sup>

Even though unmodified CNCs have been reported to offer
reinforcement in some materials,²⁰⁴⁻²⁰⁶ the hydrophilicity of
the CNC surface makes their dispersion difficult in polymer
matrices, especially those that are nonpolar or hydrophobic.
In addition, the numerous –OH groups at their surface promotes
the agglomeration of CNCs via hydrogen bonding limiting the
extent of mechanical reinforcement. In that regard, surface modi-
fication of CNCs is a viable approach to create good interfacial
<figure><img src="image_25.png" /><figcaption>**Figure 14.** Reinforcement on the mechanical properties of nanocomposites by the incorporation of polymer-grafted CNCs. (A) Pictures of neat PCL (top left) and 1 wt % CNC-g-PCL/PCL (bottom left) and the storage modulus of CNC-g-PCL/PCL nanocomposites with different wt % of the hybrid (right). (B) Scanning electron microscopy (SEM) images of electrospun mats of neat PCL (top left) and 5 wt % CNC-g-PCL/PCL (bottom left), and the elastic modulus (E) and tensile at break (σ<sub>b</sub>) of samples bearing up to 5 wt % of CNC-g-PCL. Reprinted with permission from references 207 (A) and 80 (B) Copyright 2016, John Wiley and Sons.</figcaption></figure>

adhesion between CNCs and a (hydrophobic) polymer matrix, ideally without compromising the intrinsic properties of the CNCs and their filler-filler interactions. Many efforts have focused on the modification of the CNC surface in order to enhance their compatibility with hydrophobic media.⁴

The first nanocomposite containing polymers grafted from the surface of CNCs was reported by Habibi et al. in 2008, based on CNC-g-PCL dispersed in a PCL matrix.³⁰ Nanocomposites of PCL-grafted CNCs incorporated into PCL,³⁰,⁸⁰,⁸⁸,²⁰⁷,²⁰⁸ PLA,⁸¹ PCL/PLA 50/50 w/w %,¹⁰⁴ poly(butylene succinate-co-adipate) (PBSA),⁸² and segmented polyurethane (PCL/PU)²⁰⁹ matrices have also been investigated. The above-mentioned systems generally reported good filler-matrix compatibility due to the hydrophobicity of modified CNCs, with the exception of the PCL/PU matrix,²⁰⁹ where microphase separation was observed. For example, the incorporation of 2 wt % of CNC-g-PCL in PCL matrix showed an increase on the storage modulus (E': going from 1.95 to 2.6 GPa and 0.52 to 0.74 GPa at 30 and 80 °C, respectively) attributed to the gain in rigidity of the matrix through restriction of the mobility of the brushes (Figure 14A).²⁰⁷

In another study, the reinforcement effect of CNC-g-PCL was confirmed by 140% increase of Young's modulus (E) when 2 wt % or a higher amount of modified CNCs were added to PCL/PU matrix.²⁰⁹ The incorporation of homopolyester (PCL or PLA) or diblock copolymer (PCL-block-PLA) grafted-CNCs into PCL or PCL/PLA matrices enhanced the shear storage modulus (G'), recorded at low-frequency, by a factor 10–100 relative to the PCL/PLA blend loaded with unmodified CNCs.¹⁰⁴

Electrospun nanocomposites, based on CNC-g-PCL in PCL matrix, also demonstrated an improvement in stiffness (from 4 to 7 GPa) and strength (from 1.6 to 2.3 MPa) upon an increase in modified CNC content from 0 to 5 wt % (Figure 14B).⁸⁰ The improvements in mechanical behavior were attributed to the addition of modified CNCs in the nanofiber and the higher molecular weight of the grafted PCL chains, which perhaps cocrystallized with the PCL nanofiber matrix.

The effects of incorporating polymer-modified CNCs in polymer matrices were not only observed by mechanical testing, but also by DSC. For PLA reinforced by CNC-g-PLA, a reduction between 3 and 5 °C in thermal stability, with respect to neat PLA, was observed and attributed to either a plasticizing effect of

short PLA chains⁸⁸ or residual catalyst from the polymerization step.⁹⁰,⁹¹ However, this was not the case when D-LA was used as the monomer and loaded up to 5 wt % CNC-g-PLA in a PLA matrix.¹⁰⁹ The incorporation of CNC-g-PLA in poly(β-hydroxybutyrate) (PHB) resulted in a substantial reduction of the crystallization temperature of PHB varying from 2 to 8 °C with respect to the neat polymer, while the lamellar structure of the latter was unaffected.¹¹³ In another study, the grafting of P(MMA-co-BA) from the surface of CNCs resulted in self-reinforced thermoplastic elastomers, whose T<sub>g</sub> increased by a remarkable 19 °C in comparison to the free copolymer, as a consequence of the restricted mobility of the copolymer brushes on the surface of CNCs.¹⁶⁹

Polymer-grafted CNCs used in the preparation of nano-composites have been mainly synthesized via SI-ROP. Nevertheless, some examples have also been reported for SI-FRP<sup>94,133</sup> and SI-ATRP.<sup>157</sup> PGMA<sup>133</sup> and PSty<sup>94</sup> brushes showed a contribution in the increase of compatibility with PLA matrices. In addition, CNC-g-PGMA also showed a slight increase in the tensile strength (from 47.8 ± 0.6 to 52 ± 2 MPa) compared to equivalent nanocomposite made with unmodified CNC in PLA matrix.<sup>133</sup> In another study, CNC-g-PSty was incorporated in PMMA matrix exhibiting better dispersibility and thermal stability (50 °C higher than CNC/PMMA nanocomposite).<sup>157</sup> It is worth mentioning that the reinforcement effect in nano-composites based on polymer-grafted CNCs cannot always be explained by percolation theory, due to the disturbance of hydrogen bonding interactions between chemically modified CNCs, which may hinder their percolation.<sup>5</sup> Therefore, the reinforcement may involve other factors, such as better interfacial adhesion between filler and matrix or cocrystallization of grafted polymers with the polymer matrix.

With the aim to fabricate viable alternatives to petroleum-
based composites, PLA-modified CNCs have gained interest
due to their biodegradability, renewability and nontoxicity.
Biodegradable CNC-based nanocomposites containing CNCs
grafted with homo-⁸⁸,⁹⁰,⁹¹,¹⁰⁴,¹⁰⁶⁻¹⁰⁹,¹³³ and random copolymers¹⁰⁵
dispersed in PLA matrices have been reported. As common
characteristics, these all showed good interfacial adhesion
between modified-CNCs and the matrix, stereocomplexation
from cocrystallization with the matrix, efficient nucleation induced
by grafted CNCs, as well as enhancement in the mechanical
properties.
<figure><img src="image_29.png" /><figcaption>**Figure 14.** Reinforcement on the mechanical properties of nanocomposites by the incorporation of polymer-grafted CNCs. (A) Pictures of neat PCL (top left) and 1 wt % CNC-g-PCL/PCL (bottom left) and the storage modulus of CNC-g-PCL/PCL nanocomposites with different wt % of the hybrid (right). (B) Scanning electron microscopy (SEM) images of electrospun mats of neat PCL (top left) and 5 wt % CNC-g-PCL/PCL (bottom left), and the elastic modulus (E) and tensile at break (σ<sub>b</sub>) of samples bearing up to 5 wt % of CNC-g-PCL. Reprinted with permission from references 207 (A) and 80 (B) Copyright 2016, John Wiley and Sons.</figcaption></figure>

adhesion between CNCs and a (hydrophobic) polymer matrix, ideally without compromising the intrinsic properties of the CNCs and their filler-filler interactions. Many efforts have focused on the modification of the CNC surface in order to enhance their compatibility with hydrophobic media.⁴

The first nanocomposite containing polymers grafted from the surface of CNCs was reported by Habibi et al. in 2008, based on CNC-g-PCL dispersed in a PCL matrix.³⁰ Nanocomposites of PCL-grafted CNCs incorporated into PCL,³⁰,⁸⁰,⁸⁸,²⁰⁷,²⁰⁸ PLA,⁸¹ PCL/PLA 50/50 w/w %,¹⁰⁴ poly(butylene succinate-co-adipate) (PBSA),⁸² and segmented polyurethane (PCL/PU)²⁰⁹ matrices have also been investigated. The above-mentioned systems generally reported good filler-matrix compatibility due to the hydrophobicity of modified CNCs, with the exception of the PCL/PU matrix,²⁰⁹ where microphase separation was observed. For example, the incorporation of 2 wt % of CNC-g-PCL in PCL matrix showed an increase on the storage modulus (E': going from 1.95 to 2.6 GPa and 0.52 to 0.74 GPa at 30 and 80 °C, respectively) attributed to the gain in rigidity of the matrix through restriction of the mobility of the brushes (Figure 14A).²⁰⁷

In another study, the reinforcement effect of CNC-g-PCL was confirmed by 140% increase of Young's modulus (E) when 2 wt % or a higher amount of modified CNCs were added to PCL/PU matrix.²⁰⁹ The incorporation of homopolyester (PCL or PLA) or diblock copolymer (PCL-block-PLA) grafted-CNCs into PCL or PCL/PLA matrices enhanced the shear storage modulus (G'), recorded at low-frequency, by a factor 10–100 relative to the PCL/PLA blend loaded with unmodified CNCs.¹⁰⁴

Electrospun nanocomposites, based on CNC-g-PCL in PCL matrix, also demonstrated an improvement in stiffness (from 4 to 7 GPa) and strength (from 1.6 to 2.3 MPa) upon an increase in modified CNC content from 0 to 5 wt % (Figure 14B).⁸⁰ The improvements in mechanical behavior were attributed to the addition of modified CNCs in the nanofiber and the higher molecular weight of the grafted PCL chains, which perhaps cocrystallized with the PCL nanofiber matrix.

The effects of incorporating polymer-modified CNCs in polymer matrices were not only observed by mechanical testing, but also by DSC. For PLA reinforced by CNC-g-PLA, a reduction between 3 and 5 °C in thermal stability, with respect to neat PLA, was observed and attributed to either a plasticizing effect of

short PLA chains⁸⁸ or residual catalyst from the polymerization step.⁹⁰,⁹¹ However, this was not the case when D-LA was used as the monomer and loaded up to 5 wt % CNC-g-PLA in a PLA matrix.¹⁰⁹ The incorporation of CNC-g-PLA in poly(β-hydroxybutyrate) (PHB) resulted in a substantial reduction of the crystallization temperature of PHB varying from 2 to 8 °C with respect to the neat polymer, while the lamellar structure of the latter was unaffected.¹¹³ In another study, the grafting of P(MMA-co-BA) from the surface of CNCs resulted in self-reinforced thermoplastic elastomers, whose $T_g$ increased by a remarkable 19 °C in comparison to the free copolymer, as a consequence of the restricted mobility of the copolymer brushes on the surface of CNCs.¹⁶⁹

Polymer-grafted CNCs used in the preparation of nanocomposites have been mainly synthesized via SI-ROP. Nevertheless, some examples have also been reported for SI-FRP⁹⁴,¹³³ and SI-ATRP.¹⁵⁷ PGMA¹³³ and PSty⁹⁴ brushes showed a contribution in the increase of compatibility with PLA matrices. In addition, CNC-g-PGMA also showed a slight increase in the tensile strength (from 47.8 ± 0.6 to 52 ± 2 MPa) compared to equivalent nanocomposite made with unmodified CNC in PLA matrix.¹³³ In another study, CNC-g-PSty was incorporated in PMMA matrix exhibiting better dispersibility and thermal stability (50 °C higher than CNC/PMMA nanocomposite).¹⁵⁷ It is worth mentioning that the reinforcement effect in nanocomposites based on polymer-grafted CNCs cannot always be explained by percolation theory, due to the disturbance of hydrogen bonding interactions between chemically modified CNCs, which may hinder their percolation.⁵ Therefore, the reinforcement may involve other factors, such as better interfacial adhesion between filler and matrix or cocrystallization of grafted polymers with the polymer matrix.

With the aim to fabricate viable alternatives to petroleum-based composites, PLA-modified CNCs have gained interest due to their biodegradability, renewability and nontoxicity. Biodegradable CNC-based nanocomposites containing CNCs grafted with homo-⁸⁸,⁹⁰,⁹¹,¹⁰⁴,¹⁰⁶⁻¹⁰⁹,¹³³ and random copolymers¹⁰⁵ dispersed in PLA matrices have been reported. As common characteristics, these all showed good interfacial adhesion between modified-CNCs and the matrix, stereocomplexation from cocrystallization with the matrix, efficient nucleation induced by grafted CNCs, as well as enhancement in the mechanical properties.
<figure><img src="image_26.png" /><figcaption>**Figure 15.** Three-component CNC-reinforced hydrogel based on (a) PVA containing methyl viologen functionalities as first-guest units, (b) CNC-g-P(DMAEMA-*ran*-NpMA) bearing NpMA units as second-guest and (c) the CB[8] host motif. The latter shows simultaneous binding of the guest entities resulting in the formation of (d) a dynamic hydrogel with supramolecular cross-linking. Reprinted with permission from ref 166. Copyright 2014, John Wiley and Sons.</figcaption></figure>

A new class of nanocomposites based on polymer-modified CNCs, referred to as one-component nanocomposites (OCNs) or cocontinuous composites, has recently started to attract attention. In contrast to conventional composites these materials feature (as the name implies) only one component, i.e., CNCs from which a significant amount of polymer chains have been grafted that serves as the “matrix”. As the polymer is covalently attached to CNCs, these systems do not typically suffer from phase separation or mixing problems, like conventional nanocomposites, while benefiting properties to which both components contribute.

The first example was reported by Chen et al. through the grafting of PCL chains from the surface of CNCs, via SI-ROP, as a thermoformable material.<sup>112</sup> The molecular weight of the grafts was unknown, however the content of the polymer brushes was calculated to be between 92 and 96 wt % for CNC-g-PCL, depending on the reaction time and the two different microwave irradiation powers (160 and 320 W) employed. In addition, a nonlinear dependence was observed in the mechanical properties of these OCNs, indicating that the reaction conditions may influence the number and length of the grafted chains.

Recently, another study showed that OCN materials based on CNC-g-PMMA displayed properties superior to equivalent conventional two-component nanocomposites for two different compositions, i.e., 10 and 20 wt % of CNC.<sup>93</sup> For example, the *E*′ at 25 °C of CNC<sub>10</sub>-g-PMMA was 3168 MPa versus 3018 for 10% w/w PMMA/CNC conventional nanocomposite. The authors suggested that the stress-transfer is mediated by the polymer chains attached to the CNCs. Indeed, OCNs are promising materials due to their easy-to-process characteristics, which also offer a unique combination of high stiffness, strength, and toughness.<sup>93</sup>

**4.4. Hydrogels.** Hydrogels are soft cross-linked three-dimensional networks mainly based on hydrophilic polymeric materials that have the capability of holding large amounts of water or biological solutions. Such soft materials have many potential applications, especially in the biomedical field.<sup>128</sup> The

hydrophilic character and the intrinsic properties of CNC make them an attractive candidate for the formation of reinforced or structured hydrogel nanocomposites.<sup>210</sup> Hydrogels based on polymer-grafted CNCs have generally reported enhanced mechanical properties due to the critical role of CNCs acting as multifunctional cross-linkers and bridges for the neighboring polymer chains.

Zhou et al. reported the synthesis of hydrogels from CNC-g-PAM in the presence of NMBA as cross-linker.<sup>128</sup> The grafted CNCs accelerated the formation of hydrogels and increased the effective cross-link density of the material. The interfacial interactions between PAM and CNCs were significantly improved and the reinforcement effect was confirmed by an increase in the compression stress of 2.5-fold for hydrogels containing 6.7 wt % of CNC-g-PMA. Later, a three-component supramolecular nanocomposite hydrogel showing high stiffness, rapid sol-gel transition and self-healing properties was reported.<sup>166</sup> The system was composed of poly(vinyl alcohol) (PVA) containing methyl viologen functionalities, CNC-g-P(DMAEMA-*ran*-NpMA) and cucurbit[8]uril (CB[8]) supramolecular cross-links, where CB[8] (host) selectively and simultaneously binded methyl viologen- and naphthyl-based guest units under stoichiometric molar ratio (Figure 15).

Even though the use of external cross-linkers is a known practice in the synthesis of CNC-based hydrogels, it can be avoided by designing a CNC-based system containing building blocks capable of physically or chemically cross-linking in three-dimensional networks. The first example of such systems was based on CNC-g-PMA.<sup>138</sup> This hydrogel presents elastomeric features, due to the properties of the PMA brushes, and high toughness, thanks to a unique core/shell network structure, where CNCs behave as multifunctional cross-links, resulting in unique mechanical properties.

Recently, an elastic macroporous composite hydrogel was prepared via aqueous phase polymerization of CNC-g-PUPyMA containing acrylamide and gelating methacrylate.<sup>123</sup> Their swelling ratio, pore size and mechanical strength
<figure><img src="image_30.png" /><figcaption>**Figure 15.** Three-component CNC-reinforced hydrogel based on (a) PVA containing methyl viologen functionalities as first-guest units, (b) CNC-g-P(DMAEMA-*ran*-NpMA) bearing NpMA units as second-guest and (c) the CB[8] host motif. The latter shows simultaneous binding of the guest entities resulting in the formation of (d) a dynamic hydrogel with supramolecular cross-linking. Reprinted with permission from ref 166. Copyright 2014, John Wiley and Sons.</figcaption></figure>

A new class of nanocomposites based on polymer-modified CNCs, referred to as one-component nanocomposites (OCNs) or cocontinuous composites, has recently started to attract attention. In contrast to conventional composites these materials feature (as the name implies) only one component, i.e., CNCs from which a significant amount of polymer chains have been grafted that serves as the “matrix”. As the polymer is covalently attached to CNCs, these systems do not typically suffer from phase separation or mixing problems, like conventional nanocomposites, while benefiting properties to which both components contribute.

The first example was reported by Chen et al. through the grafting of PCL chains from the surface of CNCs, via SI-ROP, as a thermoformable material.<sup>112</sup> The molecular weight of the grafts was unknown, however the content of the polymer brushes was calculated to be between 92 and 96 wt % for CNC-g-PCL, depending on the reaction time and the two different microwave irradiation powers (160 and 320 W) employed. In addition, a nonlinear dependence was observed in the mechanical properties of these OCNs, indicating that the reaction conditions may influence the number and length of the grafted chains.

Recently, another study showed that OCN materials based on CNC-g-PMMA displayed properties superior to equivalent conventional two-component nanocomposites for two different compositions, i.e., 10 and 20 wt % of CNC.<sup>93</sup> For example, the *E*′ at 25 °C of CNC<sub>10</sub>-g-PMMA was 3168 MPa versus 3018 for 10% w/w PMMA/CNC conventional nanocomposite. The authors suggested that the stress-transfer is mediated by the polymer chains attached to the CNCs. Indeed, OCNs are promising materials due to their easy-to-process characteristics, which also offer a unique combination of high stiffness, strength, and toughness.<sup>93</sup>

**4.4. Hydrogels.** Hydrogels are soft cross-linked three-dimensional networks mainly based on hydrophilic polymeric materials that have the capability of holding large amounts of water or biological solutions. Such soft materials have many potential applications, especially in the biomedical field.<sup>128</sup> The

hydrophilic character and the intrinsic properties of CNC make them an attractive candidate for the formation of reinforced or structured hydrogel nanocomposites.<sup>210</sup> Hydrogels based on polymer-grafted CNCs have generally reported enhanced mechanical properties due to the critical role of CNCs acting as multifunctional cross-linkers and bridges for the neighboring polymer chains.

Zhou et al. reported the synthesis of hydrogels from CNC-g-PAM in the presence of NMBA as cross-linker.<sup>128</sup> The grafted CNCs accelerated the formation of hydrogels and increased the effective cross-link density of the material. The interfacial interactions between PAM and CNCs were significantly improved and the reinforcement effect was confirmed by an increase in the compression stress of 2.5-fold for hydrogels containing 6.7 wt % of CNC-g-PMA. Later, a three-component supramolecular nanocomposite hydrogel showing high stiffness, rapid sol-gel transition and self-healing properties was reported.<sup>166</sup> The system was composed of poly(vinyl alcohol) (PVA) containing methyl viologen functionalities, CNC-g-P(DMAEMA-*ran*-NpMA) and cucurbit[8]uril (CB[8]) supramolecular cross-links, where CB[8] (host) selectively and simultaneously binded methyl viologen- and naphthyl-based guest units under stoichiometric molar ratio (Figure 15).

Even though the use of external cross-linkers is a known practice in the synthesis of CNC-based hydrogels, it can be avoided by designing a CNC-based system containing building blocks capable of physically or chemically cross-linking in three-dimensional networks. The first example of such systems was based on CNC-g-PMA.<sup>138</sup> This hydrogel presents elastomeric features, due to the properties of the PMA brushes, and high toughness, thanks to a unique core/shell network structure, where CNCs behave as multifunctional cross-links, resulting in unique mechanical properties.

Recently, an elastic macroporous composite hydrogel was prepared via aqueous phase polymerization of CNC-g-PUPyMA containing acrylamide and gelating methacrylate.<sup>123</sup> Their swelling ratio, pore size and mechanical strength
<figure><img src="image_27.png" /><figcaption>**Figure 16.** Hydrogel based on phosphorescent Eu²⁺/Dy³⁺-doped SrAl₂O₄ and CNC-g-PAA (top) showing fluorescent properties just after water absorption (bottom left), which get reduced with time (25 min later, bottom right). Adapted from ref 124. Copyright 2017, Springer Nature.</figcaption></figure>

were tunable by controlling the incorporated concentration of CNC-g-PUPyMA. The material achieved a swelling ratio of 70% within 10 min, while the pore size was reduced from 62 to 30 µm by increasing the concentration of modified CNCs from 0.1 to 0.5 wt %, respectively. The mechanical strength also significantly increased at higher CNC content, about 50 kPa when the concentration of CNC-g-PUPyMA was varied from 0.1 to 0.5 wt %. In addition, these materials exhibited good biocompatibility and promising cell adhesion and proliferation under *in vitro* cell culture.

Furthermore, a fluorescent nanohybrid hydrogel for water detection was developed by Wu et al. (Figure 16).¹²⁴ The structure was composed of a cross-linked CNC-g-PAA network in which Eu²⁺/Dy³⁺-doped SrAl₂O₄ NPs were fixed by the polymer chains through coordinate bonds preventing the aggregation and quenching in water. The fluorescence intensity of the material showed a direct linear relationship with water absorption and an excellent fluorescence stability, making them a suitable sensor material for water detection. In the domain of injectable hydrogels, CNC-g-PNIPAM could be potentially used for wound dressing due to the good drug-loading capacity at room temperature and a burst drug release at 37 °C, as shown by Zubik et al.¹²⁹

## 5. SUMMARY AND OUTLOOK

Since the seminal work of Habibi et al. in 2008 on SI-ROP of ε-CL from CNCs,³⁰ the scientific interest in utilizing surface-initiated polymerization from CNCs as a means to tailor their properties has manifested itself in over one hundred publications. Most investigations have dealt with the synthetic protocols to modify CNCs with high density polymer brushes, an objective appropriately reached using grafting from techniques. Upon a review of the literature, it appears that each of these

approaches has distinct advantages and disadvantages regarding number of reaction steps, conditions and polymer brush characteristics.

SI-ROP, for example, is advantageous in that it does not typically require the prior immobilization of initiators on the CNC surface, as SI-ROP can be conducted directly from the CNC -OH groups albeit in a dry organic solvent. Although there is only very limited information regarding the molecular weight and dispersity of such polymer grafts, relatively short polymer chains have been reported, e.g., DP = 12.¹⁰⁷ In contrast, high polymer contents (up to 96 wt %) were reported by others,¹¹² which could suggest high grafting density, high molecular weight, or both were obtained.

Analogously, SI-FRP can also be utilized without the prior immobilization of initiators on CNC surfaces at higher temperatures, however it is assumed that higher molecular weights are obtained, based on bulk FRP kinetics. In addition, the use of radical initiators attached to the surface of CNCs may be an alternative when high DP is desired despite the poor control over the molecular weight.⁹³ As many researchers have focused on using SI-FRP in order to produce nanocomposite hydrogels,¹²³,¹²⁴,¹²⁸,¹²⁹,¹³⁷,¹³⁸ little is known about the molecular weight and dispersity of the grafted polymers.

SI-CRP approaches can be used to grow polymer brushes with controlled molecular weight and dispersity, however usually require an appropriate initiator or CTA to be attached to the CNC surface beforehand. An exception to this is an S-RAFT approach in which the polymerization is conducted in the presence of a free macro-RAFT agent and initiated by CAN.⁹⁶,¹⁸⁰ Although initiator immobilization is a prerequisite to conduct Cu-mediated SI-CRP from CNCs, it has been shown that such initiators can be covalently attached *in situ* during the sulfuric acid hydrolysis process to produce functionalized CNCs.⁹⁵
<figure><img src="image_31.png" /><figcaption>**Figure 16.** Hydrogel based on phosphorescent Eu²⁺/Dy³⁺-doped SrAl₂O₄ and CNC-g-PAA (top) showing fluorescent properties just after water absorption (bottom left), which get reduced with time (25 min later, bottom right). Adapted from ref 124. Copyright 2017, Springer Nature.</figcaption></figure>

were tunable by controlling the incorporated concentration of CNC-g-PUPyMA. The material achieved a swelling ratio of 70% within 10 min, while the pore size was reduced from 62 to 30 µm by increasing the concentration of modified CNCs from 0.1 to 0.5 wt %, respectively. The mechanical strength also significantly increased at higher CNC content, about 50 kPa when the concentration of CNC-g-PUPyMA was varied from 0.1 to 0.5 wt %. In addition, these materials exhibited good biocompatibility and promising cell adhesion and proliferation under *in vitro* cell culture.

Furthermore, a fluorescent nanohybrid hydrogel for water detection was developed by Wu et al. (Figure 16).¹²⁴ The structure was composed of a cross-linked CNC-g-PAA network in which Eu²⁺/Dy³⁺-doped SrAl₂O₄ NPs were fixed by the polymer chains through coordinate bonds preventing the aggregation and quenching in water. The fluorescence intensity of the material showed a direct linear relationship with water absorption and an excellent fluorescence stability, making them a suitable sensor material for water detection. In the domain of injectable hydrogels, CNC-g-PNIPAM could be potentially used for wound dressing due to the good drug-loading capacity at room temperature and a burst drug release at 37 °C, as shown by Zubik et al.¹²⁹

## 5. SUMMARY AND OUTLOOK

Since the seminal work of Habibi et al. in 2008 on SI-ROP of ε-CL from CNCs,³⁰ the scientific interest in utilizing surface-initiated polymerization from CNCs as a means to tailor their properties has manifested itself in over one hundred publications. Most investigations have dealt with the synthetic protocols to modify CNCs with high density polymer brushes, an objective appropriately reached using grafting from techniques. Upon a review of the literature, it appears that each of these

approaches has distinct advantages and disadvantages regarding number of reaction steps, conditions and polymer brush characteristics.

SI-ROP, for example, is advantageous in that it does not typically require the prior immobilization of initiators on the CNC surface, as SI-ROP can be conducted directly from the CNC -OH groups albeit in a dry organic solvent. Although there is only very limited information regarding the molecular weight and dispersity of such polymer grafts, relatively short polymer chains have been reported, e.g., DP = 12.¹⁰⁷ In contrast, high polymer contents (up to 96 wt %) were reported by others,¹¹² which could suggest high grafting density, high molecular weight, or both were obtained.

Analogously, SI-FRP can also be utilized without the prior immobilization of initiators on CNC surfaces at higher temperatures, however it is assumed that higher molecular weights are obtained, based on bulk FRP kinetics. In addition, the use of radical initiators attached to the surface of CNCs may be an alternative when high DP is desired despite the poor control over the molecular weight.⁹³ As many researchers have focused on using SI-FRP in order to produce nanocomposite hydrogels,¹²³,¹²⁴,¹²⁸,¹²⁹,¹³⁷,¹³⁸ little is known about the molecular weight and dispersity of the grafted polymers.

SI-CRP approaches can be used to grow polymer brushes with controlled molecular weight and dispersity, however usually require an appropriate initiator or CTA to be attached to the CNC surface beforehand. An exception to this is an S-RAFT approach in which the polymerization is conducted in the presence of a free macro-RAFT agent and initiated by CAN.⁹⁶,¹⁸⁰ Although initiator immobilization is a prerequisite to conduct Cu-mediated SI-CRP from CNCs, it has been shown that such initiators can be covalently attached *in situ* during the sulfuric acid hydrolysis process to produce functionalized CNCs.⁹⁵
In these Cu-mediated systems, the inherent surface chemistry of the CNCs, i.e. sulfate half-ester groups, also plays a role in the polymerization kinetics when performed in aqueous media, due to the formation of electrical double layers of Cu/ligand species.<sup>173</sup> Regarding SI-NMP, the synthetic strategy is simple in that, once the NMP initiator is grafted to the CNC surface, only monomer and heat are required to initiate polymerization.<sup>143,181</sup> Although information on the polymer graft characteristics is still lacking, free polymers synthesized *in situ* during SI-NMP had M<sub>n</sub> values in excess of 100 kDa.<sup>143</sup> In most cases of SI-CRP, molecular weight >100 kDa may be obtained with the appropriate reaction conditions. In general, there are few reports on the analysis of polymers cleaved from the surface of CNCs in order to accurately determine their molecular weight and grafting density,<sup>96,115,154,163,176</sup> suggesting that there is much to be understood regarding polymer brush growth kinetics from CNCs compared to analogous solution polymerizations.

As the kinetics of surface-initiated polymerizations from other types of planar and curved interfaces are very often reported to differ significantly from solution polymerizations,<sup>28</sup> this also likely applies to polymer brush growth from CNCs and will depend highly on the polymerization technique, monomer, solvent, reactions conditions, etc. Utilizing cleavable functional groups for covalent attachment of initiator species<sup>28,115</sup> and applying new techniques to determine polymer brush grafting density<sup>211</sup> are promising approaches to address this challenge.

In addition to the development of appropriate strategies for the chemical synthesis of polymer-modified CNCs and fundamental investigations of polymer brush growth kinetics, a majority of these research efforts have focused on exploiting the unique thermomechanical properties of CNCs as a reinforcing filler in hydrophobic polymer matrices through surface functionalization. Such nanocomposites have been primarily processed by solution casting and various forms of melt processing.

A few works have sought to fabricate nanocomposites that do not suffer some of the processing limitations associated with blending hydrophilic CNCs with hydrophobic polymers by removing the polymer matrix component altogether; so-called OCNs.<sup>93,112</sup> These systems are advantageous in that they are less affected by phase separation of the components and can offer superior mechanical properties provided the molecular weight of polymer grafts is sufficient to allow chain entanglements between neighboring hybrid nanoparticles. One of the challenges in this regard is obtaining such high molecular weight polymer grafts without compromising the structural integrity of the CNCs. Due to the limited molecular weight that is often obtained using SI-ROP,<sup>107</sup> it may not be ideal for this type of application. In comparison to SI-CRP, it is already clear that this goal is probably best realized through SI-FRP techniques, since good control over polymer graft dispersity may not be as critical. However, there are many open research questions; what is the relationship between internanoparticle chain entanglements, anisotropy, molecular weight and grafting density? How does the dispersity of grafted polymers affect the thermomechanical properties of OCNs of rod-shaped nano-objects? Furthermore, one can imagine OCNs composed of block or statistical copolymer grafts with dissimilar, e.g., hard and soft phases, which might show interesting phase separation behaviors.

From the standpoint of LC research, anisotropic colloidal hybrid materials are useful models for elucidating the relationship

between building block properties and self-assembly. Among this class, CNCs are unique in that they retain their chiral LC assemblies in the solid-state upon evaporation-induced self-assembly (EISA), which has opened new doors for the preparation of mesoporous inorganic solids with long-range chiral order.<sup>191</sup> Such chiral materials have great implications in enantioselective separations, asymmetric catalysis and chiral optics. Thus, far, such templating processes have been mostly limited to aqueous systems, and this puts a number of restraints on the types of chiral mesoporous solids one can fabricate. Moreover, the pitch of chiral nematic phases of CNCs is sensitive to the presence of additives, pH, ionic strength, external fields, and evaporation rate, due to the complex interplay between surface charge density, CNC chirality, aspect ratio, and dispersity.<sup>31,212-214</sup> Such factors have often been employed as a means to control the optical properties of CNC LC phases both in solution and in free-standing films. However, these means are not typically reversible processes, which are better realized by incorporating stimuli-responsive polymer systems.

In this regard, surface chemical modification with tethered polymers has not been widely explored. This is probably due to the disturbance that the macromolecular layers could cause on the self-assembly process. Nevertheless, simulations have shown that this effect is dependent on the packing density of the polymer tethers on the rods.<sup>215</sup> Therefore, selective modification at the reducing ends of CNCs could allow sufficient packing density and the formation of chiral nematic phases. Even though there is no experimental data for the formation of chiral nematic phases with such polymer end-group modified CNCs, Horsch et al. reported theoretical descriptions on chiral nematic phases of achiral end-tethered nanorods.<sup>215-217</sup> Furthermore, as described in section 3.4, Zoppe et al. proposed the starting point for symmetric (perhaps using CNCs from the cellulose II polymorph) and asymmetric CNCs (cellulose I polymorph) as new colloidal building blocks for self-assembled structures.<sup>172</sup> Moreover, much work remains in addressing some of the research challenges ranging from the synthesis of such polymer-modified CNCs to their self-assembly and applications.

In addition, CNC aspect ratio and polymer:tether length ratio will likely play a critical role in their self-assembly. Other questions include; what is the most promising approach to reduce the size distribution of CNCs? How does cellulose polymorph structure affect their native chiral morphology and assembly? Will end-tethered polymers allow polymer-modified CNCs to form chiral LC phases without sulfate half-ester groups? Will they phase separate into defined morphologies? These challenges will be investigated in our future research and we encourage others in the field to join us on this invigorating research path. Overall, our hope is that such end group-modified polymer-modified CNCs will open new avenues toward the fundamental understanding of the processes that govern the self-assembly of anisotropic colloids and a wide range of new nanocomposites and functional materials with programmed chirality.

<figure><img src="image_28.png" /></figure>

## ASSOCIATED CONTENT

<figure><img src="image_29.png" /></figure>

### Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00733.

Overview of all polymers grafted from CNCs cited in this Perspective summarized in a table by the polymerization technique employed, including substrate, monomer(s),
In these Cu-mediated systems, the inherent surface chemistry of the CNCs, i.e. sulfate half-ester groups, also plays a role in the polymerization kinetics when performed in aqueous media, due to the formation of electrical double layers of Cu/ligand species.<sup>173</sup> Regarding SI-NMP, the synthetic strategy is simple in that, once the NMP initiator is grafted to the CNC surface, only monomer and heat are required to initiate polymerization.<sup>143,181</sup> Although information on the polymer graft characteristics is still lacking, free polymers synthesized in situ during SI-NMP had M<sub>n</sub> values in excess of 100 kDa.<sup>143</sup> In most cases of SI-CRP, molecular weight >100 kDa may be obtained with the appropriate reaction conditions. In general, there are few reports on the analysis of polymers cleaved from the surface of CNCs in order to accurately determine their molecular weight and grafting density,<sup>96,115,154,163,176</sup> suggesting that there is much to be understood regarding polymer brush growth kinetics from CNCs compared to analogous solution polymerizations.

As the kinetics of surface-initiated polymerizations from other types of planar and curved interfaces are very often reported to differ significantly from solution polymerizations,<sup>28</sup> this also likely applies to polymer brush growth from CNCs and will depend highly on the polymerization technique, monomer, solvent, reactions conditions, etc. Utilizing cleavable functional groups for covalent attachment of initiator species<sup>28,115</sup> and applying new techniques to determine polymer brush grafting density<sup>211</sup> are promising approaches to address this challenge.

In addition to the development of appropriate strategies for the chemical synthesis of polymer-modified CNCs and fundamental investigations of polymer brush growth kinetics, a majority of these research efforts have focused on exploiting the unique thermomechanical properties of CNCs as a reinforcing filler in hydrophobic polymer matrices through surface functionalization. Such nanocomposites have been primarily processed by solution casting and various forms of melt processing.

A few works have sought to fabricate nanocomposites that do not suffer some of the processing limitations associated with blending hydrophilic CNCs with hydrophobic polymers by removing the polymer matrix component altogether; so-called OCNs.<sup>93,112</sup> These systems are advantageous in that they are less affected by phase separation of the components and can offer superior mechanical properties provided the molecular weight of polymer grafts is sufficient to allow chain entanglements between neighboring hybrid nanoparticles. One of the challenges in this regard is obtaining such high molecular weight polymer grafts without compromising the structural integrity of the CNCs. Due to the limited molecular weight that is often obtained using SI-ROP,<sup>107</sup> it may not be ideal for this type of application. In comparison to SI-CRP, it is already clear that this goal is probably best realized through SI-FRP techniques, since good control over polymer graft dispersity may not be as critical. However, there are many open research questions; what is the relationship between internanoparticle chain entanglements, anisotropy, molecular weight and grafting density? How does the dispersity of grafted polymers affect the thermomechanical properties of OCNs of rod-shaped nano-objects? Furthermore, one can imagine OCNs composed of block or statistical copolymer grafts with dissimilar, e.g., hard and soft phases, which might show interesting phase separation behaviors.

From the standpoint of LC research, anisotropic colloidal hybrid materials are useful models for elucidating the relationship

between building block properties and self-assembly. Among this class, CNCs are unique in that they retain their chiral LC assemblies in the solid-state upon evaporation-induced self-assembly (EISA), which has opened new doors for the preparation of mesoporous inorganic solids with long-range chiral order.<sup>191</sup> Such chiral materials have great implications in enantioselective separations, asymmetric catalysis and chiral optics. Thus, far, such templating processes have been mostly limited to aqueous systems, and this puts a number of restraints on the types of chiral mesoporous solids one can fabricate. Moreover, the pitch of chiral nematic phases of CNCs is sensitive to the presence of additives, pH, ionic strength, external fields, and evaporation rate, due to the complex interplay between surface charge density, CNC chirality, aspect ratio, and dispersity.<sup>31,212-214</sup> Such factors have often been employed as a means to control the optical properties of CNC LC phases both in solution and in free-standing films. However, these means are not typically reversible processes, which are better realized by incorporating stimuli-responsive polymer systems.

In this regard, surface chemical modification with tethered polymers has not been widely explored. This is probably due to the disturbance that the macromolecular layers could cause on the self-assembly process. Nevertheless, simulations have shown that this effect is dependent on the packing density of the polymer tethers on the rods.<sup>215</sup> Therefore, selective modification at the reducing ends of CNCs could allow sufficient packing density and the formation of chiral nematic phases. Even though there is no experimental data for the formation of chiral nematic phases with such polymer end-group modified CNCs, Horsch et al. reported theoretical descriptions on chiral nematic phases of achiral end-tethered nanorods.<sup>215-217</sup> Furthermore, as described in section 3.4, Zoppe et al. proposed the starting point for symmetric (perhaps using CNCs from the cellulose II polymorph) and asymmetric CNCs (cellulose I polymorph) as new colloidal building blocks for self-assembled structures.<sup>172</sup> Moreover, much work remains in addressing some of the research challenges ranging from the synthesis of such polymer-modified CNCs to their self-assembly and applications.

In addition, CNC aspect ratio and polymer:tether length ratio will likely play a critical role in their self-assembly. Other questions include; what is the most promising approach to reduce the size distribution of CNCs? How does cellulose polymorph structure affect their native chiral morphology and assembly? Will end-tethered polymers allow polymer-modified CNCs to form chiral LC phases without sulfate half-ester groups? Will they phase separate into defined morphologies? These challenges will be investigated in our future research and we encourage others in the field to join us on this invigorating research path. Overall, our hope is that such end group-modified polymer-modified CNCs will open new avenues toward the fundamental understanding of the processes that govern the self-assembly of anisotropic colloids and a wide range of new nanocomposites and functional materials with programmed chirality.

<figure><img src="image_32.png" /></figure>

## ASSOCIATED CONTENT

<figure><img src="image_33.png" /></figure>

### Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00733.

Overview of all polymers grafted from CNCs cited in this Perspective summarized in a table by the polymerization technique employed, including substrate, monomer(s),
reaction conditions, resulting properties, and applications
(PDF)

<figure><img src="image_30.png" /></figure>

AUTHOR INFORMATION

Corresponding Author

*(J.O.Z.) E-mail: justin.zoppe@unifr.ch. Telephone: +41 26 300 9563.

ORCID

Wim Thielemans: 0000-0003-4451-1964

Christoph Weder: 0000-0001-7183-1790

Justin O. Zoppe: 0000-0002-3599-9227

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

Biographies

<figure><img src="image_31.png" /></figure>

Originally from Venezuela, Sandra Wohlhauser received her Master's degree in chemical engineering from the University of Carabobo, Venezuela, in 2005. After a few years of professional experience, she returned to academia and completed a Master's degree in chemistry from the University of Fribourg, Switzerland, in 2015 while studying the potential mechano-responsive behavior of poly(sulfonate ester)s. She then joined the group of Prof. C. Weder in the Adolphe Merkle Institute at the University of Fribourg as a Ph.D. student, where she is currently working on the surface modification of CNCs with polymer chains to develop unique one-component nanocomposites in the solid state, which combine high strength, stiffness, and toughness.

<figure><img src="image_32.png" /></figure>

Gwendoline Delepierre obtained her Bachelor's degree at KU Leuven, where she worked in The Renewable Materials and Nanotechnology Research Group under the supervision of Prof. Wim Thielemans for

various research projects involving cellulose nanocrystals. During her Master's studies, she did an internship at Cambridge University in the research group of Dr. Silvia Vignolini. She worked on a project in collaboration with Dr. Wadood Hamad from FP Innovations on size sorting of cellulose nanocrystals. She went to Uppsala University for her Master's thesis to work on a collaboration between Dr. Lukazs Pilarski's group in Biomedicinskt centrum (BMC) and that of Dr. Anders Thapper at Uppsala's Ångström laboratory. She finished her Master's program in KU Leuven and started her Ph.D. work in the Adolphe Merkle Institute in October 2017 under the supervision of Dr. Justin Zoppe and Prof. Christoph Weder.

<figure><img src="image_33.png" /></figure>

Marianne Labet is a research associate working at KU Leuven (Belgium). She received her MEng in paper science, printing, and converting from the French Graduate School of Engineering in Paper, Print Media, and Biomaterials and her MRes in materials science and engineering from Grenoble INP in 2006. She obtained her Ph.D. in chemistry and chemical engineering from the University of Nottingham (U.K.) working on the SI-ROP of ε-caprolactone from polysaccharide nanocrystals under the supervision of Wim Thielemans. She then worked as a postdoctoral researcher with Chris Breen of the Materials and Engineering Research Institute at Sheffield Hallam University (U.K.) and with Xavier Coqueret of the Institut de Chimie Moléculaire de Reims at the University of Reims (France). Her research interests include cellulose based composites, the recycling of polymer-based materials and the development of materials for packaging applications.

<figure><img src="image_34.png" /></figure>

Gaëlle Morandi was born in Le Mans, France, and received her Ph.D. in polymer chemistry and physical chemistry in 2007 from the Université du Maine under the supervision of Prof. L. Fontaine. She then joined W. Thielemans' group in Nottingham, U.K., as a postdoctoral fellow, to develop cellulose nanocrystals grafted with polymeric chains through a SI-ATRP process. Since 2009, she is an assistant professor in INSA Rouen Normandie. Her research interests include the controlled
reaction conditions, resulting properties, and applications
(PDF)

## AUTHOR INFORMATION

### Corresponding Author

* (J.O.Z.) E-mail: justin.zoppe@unifr.ch. Telephone: +41 26 300 9563.

### ORCID

Wim Thielemans: 0000-0003-4451-1964

Christoph Weder: 0000-0001-7183-1790

Justin O. Zoppe: 0000-0002-3599-9227

### Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

### Notes

The authors declare no competing financial interest.

### Biographies

<figure><img src="image_34.png" /></figure>

Originally from Venezuela, Sandra Wohlhauser received her Master's degree in chemical engineering from the University of Carabobo, Venezuela, in 2005. After a few years of professional experience, she returned to academia and completed a Master's degree in chemistry from the University of Fribourg, Switzerland, in 2015 while studying the potential mechano-responsive behavior of poly(sulfonate ester)s. She then joined the group of Prof. C. Weder in the Adolphe Merkle Institute at the University of Fribourg as a Ph.D. student, where she is currently working on the surface modification of CNCs with polymer chains to develop unique one-component nanocomposites in the solid state, which combine high strength, stiffness, and toughness.

<figure><img src="image_35.png" /></figure>

Gwendoline Delepierre obtained her Bachelor's degree at KU Leuven, where she worked in The Renewable Materials and Nanotechnology Research Group under the supervision of Prof. Wim Thielemans for

various research projects involving cellulose nanocrystals. During her Master's studies, she did an internship at Cambridge University in the research group of Dr. Silvia Vignolini. She worked on a project in collaboration with Dr. Wadood Hamad from FP Innovations on size sorting of cellulose nanocrystals. She went to Uppsala University for her Master's thesis to work on a collaboration between Dr. Lukazs Pilarski's group in Biomedicinskt centrum (BMC) and that of Dr. Anders Thapper at Uppsala's Ångström laboratory. She finished her Master's program in KU Leuven and started her Ph.D. work in the Adolphe Merkle Institute in October 2017 under the supervision of Dr. Justin Zoppe and Prof. Christoph Weder.

<figure><img src="image_36.png" /></figure>

Marianne Labet is a research associate working at KU Leuven (Belgium). She received her MEng in paper science, printing, and converting from the French Graduate School of Engineering in Paper, Print Media, and Biomaterials and her MRes in materials science and engineering from Grenoble INP in 2006. She obtained her Ph.D. in chemistry and chemical engineering from the University of Nottingham (U.K.) working on the SI-ROP of ε-caprolactone from polysaccharide nanocrystals under the supervision of Wim Thielemans. She then worked as a postdoctoral researcher with Chris Breen of the Materials and Engineering Research Institute at Sheffield Hallam University (U.K.) and with Xavier Coqueret of the Institut de Chimie Moléculaire de Reims at the University of Reims (France). Her research interests include cellulose based composites, the recycling of polymer-based materials and the development of materials for packaging applications.

<figure><img src="image_37.png" /></figure>

Gaëlle Morandi was born in Le Mans, France, and received her Ph.D. in polymer chemistry and physical chemistry in 2007 from the Université du Maine under the supervision of Prof. L. Fontaine. She then joined W. Thielemans' group in Nottingham, U.K., as a postdoctoral fellow, to develop cellulose nanocrystals grafted with polymeric chains through a SI-ATRP process. Since 2009, she is an assistant professor in INSA Rouen Normandie. Her research interests include the controlled
synthesis of stimuli-sensitive amphiphilic copolymers and their association into mixed micelles of tunable stability.

<figure><img src="image_35.png" /></figure>

Wim Thielemans is Professor, Odysseus Fellow, and Province Chair of Advanced Materials at KU Leuven, Belgium where he leads the Renewable Materials and Nanotechnology research group. Wim received his Master in Chemical Engineering from KU Leuven (BE) in 1999 and his Ph.D. in Chemical Engineering from the University of Delaware (USA) in 2004. After postdoctoral work as a Marie Curie research fellow at the School of Engineering in Paper, Print Media, and Biomaterials of the National Polytechnic Institute of Grenoble (France), he became a lecturer, then associate professor at the University of Nottingham (U.K.) in 2006. In 2013, Wim completed the circle by moving back to KU Leuven with an Odysseus fellowship from Research Foundation Flanders as associate professor and was promoted to professor in 2017. Wim is currently also an honorary professor with the University of Nottingham and an affiliate professor with the University of Auburn. His research group focusses on the development of renewable (nano)materials.

<figure><img src="image_36.png" /></figure>

Christoph Weder is Professor for Polymer Chemistry and Materials and Director of the Adolphe Merkle Institute (AMI) at the University of Fribourg (Switzerland). He also directs the Swiss National Center of Competence in Research Bio-Inspired Materials. Chris studied chemistry at ETH Zurich where he also earned his doctoral degree. After a postdoctoral stay at MIT, he held positions at ETH Zurich and Case Western Reserve University, before joining the AMI in 2009. Chris' current research efforts are focused on stimuli-responsive polymers and nanomaterials. He coauthored some 250 research papers and book chapters and is coinventor of 20 patents. Chris has as mentored about 70 Ph.D. students and postdoctoral researchers. He serves as an Associate Editor of ACS Macro Letters and as codirector of the Swiss Chemical Society's Polymer and Colloid Division and is a member of the Swiss Academy of Technical Sciences and a Fellow of the American Chemical Society's Division of Polymer Chemistry.

<figure><img src="image_37.png" /></figure>

Justin O. Zoppe was born in Flint, Michigan (USA) in 1983 and graduated from the University of North Carolina at Wilmington with a Bachelor of Science degree in chemistry and a minor in mathematics. He was awarded a Ph.D. in 2011 from the Department of Forest Biomaterials at North Carolina State University under the supervision of Prof. Orlando Rojas. He then joined the Polymer Technology group of Prof. Jukka Seppälä at Aalto University as a postdoc before working as a Research Scientist at Clariant Specialty Chemicals in Frankfurt, Germany, in 2013. From 2014-2017, Dr. Zoppe was an EPFL Fellow cofunded by Marie Curie in the Polymers Laboratory of Prof. Harm-Anton Klok at the Swiss Federal Institute of Technology in Lausanne. Dr. Zoppe moved to the Polymer Chemistry & Materials Laboratory of Prof. Christoph Weder at the Adolphe Merkle Institute, University of Fribourg, in 2017 as a Group Leader and Ambizione Fellow funded by the Swiss National Science Foundation. In addition to surface modification of cellulose nanomaterials, he is interested in the fundamentals of transition metal-mediated SI-CRP carried out in polar media, surface forces, and colloidal liquid crystal assembly.

## ACKNOWLEDGMENTS

Financial support from the Swiss National Science Foundation (SNSF) (Ambizione Grant No. PZ00P2_167900; J.O.Z. and G.D.) is gratefully acknowledged. S.W. and C.W. acknowledge financial support through the Army Research Office under Grant No. W911NF-15-1-0190. M.L. and W.T. acknowledge funding from Flanders Innovation & Entrepreneurship through the Plast-i-Com project (IWT 135100). W.T. also thanks the Provincie West-Vlaanderen for his Chair in Advanced Materials and Research Foundation Flanders for his Odysseus Fellowship (Grant G.0C60.13N).

## ABBREVIATIONS

$^{13}$C,carbon isotope 13; $^{1}$H NMR,proton nuclear magnetic resonance; 4-SS,sodium 4-vinylbenzenesulfonate; A(R)GET,activators (re)generated by electron transfer; AA,acrylic acid; Ac,sodium triacetetoxyborohydride; ACVA,4,4'-azobis(4-cyanovaleric acid); AEM,2-aminoethyl methacrylate; AEMA,2-aminoethyl methacrylamide; AgNPs,silver nanoparticles; AGU,anhydroglucopyranose unit; AIBN,2,2'-azobis(2-methylpropionitrile); AM,a-crylamide; (AMIM)Cl,1-allyl-3-methylimidazolium chloride; APS,ammonium persulfate; ATRP,atom transfer radical polymerization; AuNPs,gold nanoparticles; BA,butyl acrylate; BIBA,α-bromoisobutyric acid; bis-TAD,4,4'-(1,4-phenylene)-bis(1,2,4-triazoline-3,5-dione); BlocBuilder,4-(diethoxyphosphinyl)-2,2,5,5-tetramethyl-3-azahexane-N-oxyl; Br,bromine; BiBB,2-bromoisobutyryl bromide; CAN,ceric(IV) ammonium nitrate; CB[8],cucurbit[8]uril; CDCl₃,deuterated chloroform; Ce⁴⁺,ceric ion; Cl,chlorine; Cn,sodium cyanoborohydride;
synthesis of stimuli-sensitive amphiphilic copolymers and their association into mixed micelles of tunable stability.

<figure><img src="image_38.png" /></figure>

Wim Thielemans is Professor, Odysseus Fellow, and Province Chair of Advanced Materials at KU Leuven, Belgium where he leads the Renewable Materials and Nanotechnology research group. Wim received his Master in Chemical Engineering from KU Leuven (BE) in 1999 and his Ph.D. in Chemical Engineering from the University of Delaware (USA) in 2004. After postdoctoral work as a Marie Curie research fellow at the School of Engineering in Paper, Print Media, and Biomaterials of the National Polytechnic Institute of Grenoble (France), he became a lecturer, then associate professor at the University of Nottingham (U.K.) in 2006. In 2013, Wim completed the circle by moving back to KU Leuven with an Odysseus fellowship from Research Foundation Flanders as associate professor and was promoted to professor in 2017. Wim is currently also an honorary professor with the University of Nottingham and an affiliate professor with the University of Auburn. His research group focusses on the development of renewable (nano)materials.

<figure><img src="image_39.png" /></figure>

Christoph Weder is Professor for Polymer Chemistry and Materials and Director of the Adolphe Merkle Institute (AMI) at the University of Fribourg (Switzerland). He also directs the Swiss National Center of Competence in Research Bio-Inspired Materials. Chris studied chemistry at ETH Zurich where he also earned his doctoral degree. After a postdoctoral stay at MIT, he held positions at ETH Zurich and Case Western Reserve University, before joining the AMI in 2009. Chris' current research efforts are focused on stimuli-responsive polymers and nanomaterials. He coauthored some 250 research papers and book chapters and is coinventor of 20 patents. Chris has as mentored about 70 Ph.D. students and postdoctoral researchers. He serves as an Associate Editor of ACS Macro Letters and as codirector of the Swiss Chemical Society's Polymer and Colloid Division and is a member of the Swiss Academy of Technical Sciences and a Fellow of the American Chemical Society's Division of Polymer Chemistry.

<figure><img src="image_40.png" /></figure>

Justin O. Zoppe was born in Flint, Michigan (USA) in 1983 and graduated from the University of North Carolina at Wilmington with a Bachelor of Science degree in chemistry and a minor in mathematics. He was awarded a Ph.D. in 2011 from the Department of Forest Biomaterials at North Carolina State University under the supervision of Prof. Orlando Rojas. He then joined the Polymer Technology group of Prof. Jukka Seppälä at Aalto University as a postdoc before working as a Research Scientist at Clariant Specialty Chemicals in Frankfurt, Germany, in 2013. From 2014-2017, Dr. Zoppe was an EPFL Fellow cofunded by Marie Curie in the Polymers Laboratory of Prof. Harm-Anton Klok at the Swiss Federal Institute of Technology in Lausanne. Dr. Zoppe moved to the Polymer Chemistry & Materials Laboratory of Prof. Christoph Weder at the Adolphe Merkle Institute, University of Fribourg, in 2017 as a Group Leader and Ambizione Fellow funded by the Swiss National Science Foundation. In addition to surface modification of cellulose nanomaterials, he is interested in the fundamentals of transition metal-mediated SI-CRP carried out in polar media, surface forces, and colloidal liquid crystal assembly.

## ACKNOWLEDGMENTS

Financial support from the Swiss National Science Foundation (SNSF) (Ambizione Grant No. PZ00P2_167900; J.O.Z. and G.D.) is gratefully acknowledged. S.W. and C.W. acknowledge financial support through the Army Research Office under Grant No. W911NF-15-1-0190. M.L. and W.T. acknowledge funding from Flanders Innovation & Entrepreneurship through the Plast-i-Com project (IWT 135100). W.T. also thanks the Provincie West-Vlaanderen for his Chair in Advanced Materials and Research Foundation Flanders for his Odysseus Fellowship (Grant G.0C60.13N).

## ABBREVIATIONS

$^{13}$C,carbon isotope 13; $^{1}$H NMR,proton nuclear magnetic resonance; 4-SS,sodium 4-vinylbenzenesulfonate; A(R)GET,activators (re)generated by electron transfer; AA,acrylic acid; Ac,sodium triacetetoxyborohydride; ACVA,4,4'-azobis(4-cyanovaleric acid); AEM,2-aminoethyl methacrylate; AEMA,2-aminoethyl methacrylamide; AgNPs,silver nanoparticles; AGU,anhydroglucopyranose unit; AIBN,2,2'-azobis(2-methylpropionitrile); AM,a-crylamide; (AMIM)Cl,1-allyl-3-methylimidazolium chloride; APS,ammonium persulfate; ATRP,atom transfer radical polymerization; AuNPs,gold nanoparticles; BA,butyl acrylate; BIBA,α-bromoisobutyric acid; bis-TAD,4,4'-(1,4-phenylene)-bis(1,2,4-triazoline-3,5-dione); BlocBuilder,4-(diethoxyphosphinyl)-2,2,5,5-tetramethyl-3-azahexane-N-oxyl; Br,bromine; BiBB,2-bromoisobutyryl bromide; CAN,ceric(IV) ammonium nitrate; CB[8],cucurbit[8]uril; CDCl$_3$,deuterated chloroform; Ce$^{4+}$,ceric ion; Cl,chlorine; Cn,sodium cyanoborohydride;
CNC,cellulose nanocrystal; CNC-1,cellulose nanocrystals with
high surface density of sulfate half-ester groups; CNC-
2,cellulose nanocrystals with low surface density of sulfate
half-ester groups; CNC-COOH,carboxylate functionalized
cellulose nanocrystals; CNC-SH,thiolated cellulose nanocryst-
als; CNF,cellulose nanofibrils; CO₂,carbon dioxide; CPADB,4-
cyano-4-(phenylcarbonothioylthio)pentanoic acid; CP-MAS,-
cross-polarization magic angle spinning; CPT,cloud-point
temperature; CSC,cellulose synthase complex; CTA,chain
transfer agent; Cu,copper; DCC,N,N'-dicyclohexylcarbodiim-
ide; DDMAT,2-(dodecylthiocarbonothioylthio)-2-methyl-
propionic acid; DEAEMA,2-(diethylamino)ethyl methacrylate;
DEPN,N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethyl-
propyl)] nitroxide; D-LA,D-lactide; DLS,dynamic light scatter-
ing; DMAEMA,2-(dimethylamino)ethyl methacrylate;
DMAM,N,N-dimethylacrylamide; DMAP,4-dimethylaminopyr-
idine; DMAPMAM,N-[3-(dimethylamino)propyl]-
methacrylamide; DMF,N,N-dimethylformamide; DMSO,di-
methyl sulfoxide; DMSO-d₆,deuterated dimethyl sulfoxide;
DP,degree of polymerization; DSC,differential scanning calo-
rimetry; E,elastic modulus; E',storage modulus; EANI,4-ethoxy-
9-allyl-1,8-naphthalimide; EBIB,ethyl α-bromoisobutyrate;
EDA,ethylenediamine; EDC,N-(3-(dimethylamino)propyl)-N-
ethylcarbodiimide hydrochloride; EGDMA,2-(2-methyl-
acryloyloxy)ethyl 2-methyl-acrylate; EISA,evaporation-induced
self-assembly; Fe₃O₄,magnetite; FRP,free radical polymeriza-
tion; FTIR,Fourier transformed infrared; G',shear storage
modulus; gg,gauche—gauche; gt,gauche—trans; HBr,hydrobro-
mic acid; HCl,hydrochloric acid; KPS,potassium persulfate;
LC,liquid crystal; LCEs,liquid crystal elastomers; LCST,lower
critical solution temperature; L-LA,L-lactide; MA,methyl
acrylate; MADIX,macromolecular design via the inter-
change of xanthates; METAC,[2-(methacryloyloxy)ethyl]-
trimethylammonium chloride; MgH₂,magnesium hydride; MMA,-
methyl methacrylate; MMAZO,6-[4-(4-methoxyphenylazo)-
phenoxy] hexyl methacrylate; Mₚ,number-average molecular
weight; D,dispersity; Mo(CO)₆,molybdenumhexacarbonyl;
MPC,2-methacryloyloxyethyl phosphorylcholine; Mw,weight-
average molecular weight; N₂,nitrogen; NaCl,sodium chloride;
NaClO₂,sodium chlorite; NH₂(CH₂)₆SH·HCl,6-amino-1-hex-
anethiol hydrochloride; NHS,N-hydroxysuccinimide; NHS-
BiB,2-bromoisobutanoic acid N-hydroxysuccinimide ester;
NIPAM,N-isopropylacrylamide; NMBA,N,N'-methylenebis-
(acrylamide); NMP,nitroxide-mediated polymerization;
NMR,nuclear magnetic resonance; NPs,nanoparticles; NpMA,-
naphtyl methacrylate; NVC,N-vinylcaprolactam; OCNs,one-
component nanocomposites; OEGMA,oligoethylene glycol
methacrylate; OEGMEMA,oligoethylene glycol methyl ether
acrylate; -OH,hydroxyl; P(MAA-co-VSA),poly(methacrylic
acid-co-vinylsulfonic acid); P4VP,poly(4-vinylpyridine);
PAA,poly(acrylic acid); PAM,polyacrylamide; PBSA,poly-
(butylene succinate-co-adipate); Pc,2-picoline—borane com-
plex; PCL,poly(ε-caprolactone); PDMAEMA,poly-
[(dimethylamino)ethyl methacrylate]; PGMA,poly(glycidyl
methacrylate); pH,potential of hydrogen; PHB,poly(β-hydrox-
ybutyrate); PHEMA,poly(2-hydroxyethyl methacrylate); PHMA,poly(hexyl methacrylate); pKₐ,acid dissociation con-
stant; PLA,poly(lactic acid); PMMA,poly(methyl methacry-
late); PMAA,poly(methacrylic acid); PNVC,N-vinylcaprolac-
tam; POEGMA,poly(oligoethylene glycol) methacrylate;
POEGMEMA,poly[(oligoethylene glycol) methyl ether acryl-
ate]; POM,polarized optical microscopy; PPEGEEMA,poly-
[poly(ethylene glycol)ethyl ether methacrylate]; PSBAM,poly-

(soybean amide methacrylate); PSty,polystyrene; PtBA,poly(
(tert-butyl acrylate); PU,polyurethane; PVA,poly(vinyl
alcohol); PVAc,poly(vinyl acetate); RAFT, reversible addi-
tion-fragmentation chain transfer; ROP,ring-opening polymer-
ization; S, sulfur; SARA ATRP, supplemental activator and
reducing agent atom transfer radical polymerization; SBAM,-
soybean amide methacrylate; SBS,sodium bisulfite; SEC, size-
exclusion chromatography; SEM, scanning electron microscopy;
SET LRP, single-electron transfer living radical polymerization;
SI-AGET-ATRP, surface-initiated activators generated by elec-
tron transfer atom transfer radical polymerization; SI-ATRP, sur-
face-initiated atom transfer radical polymerization; SI-CRP, sur-
face-initiated controlled radical polymerization; SI-FRP, surface-
initiated free radical polymerization; SI-NMP, surface-initiated
nitroxide-mediated polymerization; SI-ROP, surface-initiated
ring-opening polymerization; SMA,stearyl methacrylate;
SMPs,shape-memory polymers; Sn(Oct)₂,tin(II) 2-ethylhexa-
noate; S-RAFT, surface reversible addition-fragmentation chain
transfer; Sty, styrene; tBA,tert-butyl acrylate; T<sub>d</sub>, degradation
temperature; TEM, transmission electron microscopy;
TEMPO, (2,2,6,6-tetramethylpiperidin-1-yl)oxy; T<sub>g</sub>,glass tran-
sition temperature; tg,trans-gauche; TGA,thermogravimetric
analysis; THF, tetrahydrofuran; TMEDA, N,N,N',N'-tetrame-
thylethane-1,2-diamine; UCST, upper critical solution temper-
ature; UPyMA,2-(3-(6-methyl-4-oxo-1,4-dihydropyrimidin-2-
yl)ureido)ethyl methacrylate; UV,ultraviolet; VAc,vinyl acetate;
wt %, weight percent; XPS, X-ray photoelectron spectroscopy;
ΔCₚ, heat capacity variation; ε-CL,ε-caprolactone; σₚ, tensile at
break

■ REFERENCES

(1) Sanchez, C.; Belleville, P.; Popall, M.; Nicole, L. Applications of advanced hybrid organic-inorganic nanomaterials: from laboratory to market. *Chem. Soc. Rev.* **2011**, *40* (2), 696–753.

(2) Dufresne, A. *Nanocellulose: from nature to high performance tailored materials*; Walter de Gruyter GmbH: Berlin, 2012.

(3) Eichhorn, S. J. Cellulose nanowhiskers: promising materials for advanced applications. *Soft Matter* **2011**, *7* (2), 303–315.

(4) Eichhorn, S. J.; Dufresne, A.; Aranguren, M.; Marcovich, N. E.; Capadona, J. R.; Rowan, S. J.; Weder, C.; Thielemans, W.; Roman, M.; Renneckar, S.; Gindl, W.; Veigel, S.; Keckes, J.; Yano, H.; Abe, K.; Nogi, M.; Nakagaito, A. N.; Mangalam, A.; Simonsen, J.; Benight, A. S.; Bismarck, A.; Berglund, L. A.; Peijs, T. Review: current international research into cellulose nanofibres and nanocomposites. *J. Mater. Sci.* **2010**, *45* (1), 1–33.

(5) Habibi, Y.; Lucia, L. A.; Rojas, O. J. Cellulose Nanocrystals: Chemistry, Self-Assembly, and Applications. *Chem. Rev.* **2010**, *110* (6), 3479–3500.

(6) Klemm, D.; Kramer, F.; Moritz, S.; Lindstroem, T.; Ankerfors, M.; Gray, D.; Dorris, A. Nanocelluloses: A New Family of Nature-Based Materials. *Angew. Chem., Int. Ed.* **2011**, *50* (24), 5438–5466.

(7) Moon, R. J.; Martini, A.; Nairn, J.; Simonsen, J.; Youngblood, J. Cellulose nanomaterials review: structure, properties and nano-composites. *Chem. Soc. Rev.* **2011**, *40* (7), 3941–3994.

(8) Kontturi, E.; Laaksonen, P.; Linder, M. B.; Nonappa; Gröschel, A. H.; Rojas, O. J.; Ikkala, O. Advanced Materials through Assembly of Nanocelluloses. *Adv. Mater.* **2018**, *30*, 1703779.

(9) Klemm, D.; Heublein, B.; Fink, H.-P.; Bohn, A. Cellulose: Fascinating biopolymer and sustainable raw material. *Angew. Chem., Int. Ed.* **2005**, *44* (22), 3358–3393.

(10) Sacui, I. A.; Nieuwendaal, R. C.; Burnett, D. J.; Stranick, S. J.; Jorfi, M.; Weder, C.; Foster, E. J.; Olsson, R. T.; Gilman, J. W. Comparison of the Properties of Cellulose Nanocrystals and Cellulose Nanofibrils Isolated from Bacteria, Tunicate, and Wood Processed Using Acid, Enzymatic, Mechanical, and Oxidative Methods. *ACS Appl. Mater. Interfaces* **2014**, *6* (9), 6127–6138.
CNC,cellulose nanocrystal; CNC-1,cellulose nanocrystals with
high surface density of sulfate half-ester groups; CNC-
2,cellulose nanocrystals with low surface density of sulfate
half-ester groups; CNC-COOH,carboxylate functionalized
cellulose nanocrystals; CNC-SH,thiolated cellulose nanocryst-
als; CNF,cellulose nanofibrils; CO₂,carbon dioxide; CPADB,4-
cyano-4-(phenylcarbonothioylthio)pentanoic acid; CP-MAS,-
cross-polarization magic angle spinning; CPT,cloud-point
temperature; CSC,cellulose synthase complex; CTA,chain
transfer agent; Cu,copper; DCC,N,N'-dicyclohexylcarbodiim-
ide; DDMAT,2-(dodecylthiocarbonothioylthio)-2-methyl-
propionic acid; DEAEMA,2-(diethylamino)ethyl methacrylate;
DEPN,N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethyl-
propyl)] nitroxide; D-LA,D-lactide; DLS,dynamic light scatter-
ing; DMAEMA,2-(dimethylamino)ethyl methacrylate;
DMAM,N,N-dimethylacrylamide; DMAP,4-dimethylaminopyr-
idine; DMAPMAM,N-[3-(dimethylamino)propyl]-
methacrylamide; DMF,N,N-dimethylformamide; DMSO,di-
methyl sulfoxide; DMSO-d₆,deuterated dimethyl sulfoxide;
DP,degree of polymerization; DSC,differential scanning calo-
rimetry; E,elastic modulus; E',storage modulus; EANI,4-ethoxy-
9-allyl-1,8-naphthalimide; EBIB,ethyl α-bromoisobutyrate;
EDA,ethylenediamine; EDC,N-(3-(dimethylamino)propyl)-N-
ethylcarbodiimide hydrochloride; EGDMA,2-(2-methyl-
acryloyloxy)ethyl 2-methyl-acrylate; EISA,evaporation-induced
self-assembly; Fe₃O₄,magnetite; FRP,free radical polymeriza-
tion; FTIR,Fourier transformed infrared; G',shear storage
modulus; gg,gauche-gauche; gt,gauche-trans; HBr,hydrobro-
mic acid; HCl,hydrochloric acid; KPS,potassium persulfate;
LC,liquid crystal; LCEs,liquid crystal elastomers; LCST,lower
critical solution temperature; L-LA,L-lactide; MA,methyl
acrylate; MADIX,macromolecular design via the inter-
change of xanthates; METAC,[2-(methacryloyloxy)ethyl]-
trimethylammonium chloride; MgH₂,magnesium hydride; MMA,-
methyl methacrylate; MMAZO,6-[4-(4-methoxyphenylazo)-
phenoxy] hexyl methacrylate; Mₚ,number-average molecular
weight; D,dispersity; Mo(CO)₆,molybdenumhexacarbonyl;
MPC,2-methacryloyloxyethyl phosphorylcholine; Mw,weight-
average molecular weight; N₂,nitrogen; NaCl,sodium chloride;
NaClO₂,sodium chlorite; NH₂(CH₂)₆SH·HCl,6-amino-1-hex-
anethiol hydrochloride; NHS,N-hydroxysuccinimide; NHS-
BiB,2-bromoisobutanoic acid N-hydroxysuccinimide ester;
NIPAM,N-isopropylacrylamide; NMBA,N,N'-methylenebis-
(acrylamide); NMP,nitroxide-mediated polymerization;
NMR,nuclear magnetic resonance; NPs,nanoparticles; NpMA,-
naphtyl methacrylate; NVC,N-vinylcaprolactam; OCNs,one-
component nanocomposites; OEGMA,oligoethylene glycol
methacrylate; OEGMEMA,oligoethylene glycol methyl ether
acrylate; -OH,hydroxyl; P(MAA-co-VSA),poly(methacrylic
acid-co-vinylsulfonic acid); P4VP,poly(4-vinylpyridine);
PAA,poly(acrylic acid); PAM,polyacrylamide; PBSA,poly-
(butylene succinate-co-adipate); Pc,2-picoline-borane com-
plex; PCL,poly(ε-caprolactone); PDMAEMA,poly-
[(dimethylamino)ethyl methacrylate]; PGMA,poly(glycidyl
methacrylate); pH,potential of hydrogen; PHB,poly(β-hydrox-
ybutyrate); PHEMA,poly(2-hydroxyethyl methacrylate); PHMA,poly(hexyl methacrylate); pKₐ,acid dissociation con-
stant; PLA,poly(lactic acid); PMMA,poly(methyl methacry-
late); PMAA,poly(methacrylic acid); PNVC,N-vinylcaprolac-
tam; POEGMA,poly(oligoethylene glycol) methacrylate;
POEGMEMA,poly[(oligoethylene glycol) methyl ether acryl-
ate]; POM,polarized optical microscopy; PPEGEEMA,poly-
[poly(ethylene glycol)ethyl ether methacrylate]; PSBAM,poly-

(soybean amide methacrylate); PSty,polystyrene; PtBA,poly(
(tert-butyl acrylate); PU,polyurethane; PVA,poly(vinyl
alcohol); PVAc,poly(vinyl acetate); RAFT, reversible addi-
tion-fragmentation chain transfer; ROP,ring-opening polymer-
ization; S, sulfur; SARA ATRP, supplemental activator and
reducing agent atom transfer radical polymerization; SBAM,-
soybean amide methacrylate; SBS,sodium bisulfite; SEC, size-
exclusion chromatography; SEM, scanning electron microscopy;
SET LRP, single-electron transfer living radical polymerization;
SI-AGET-ATRP, surface-initiated activators generated by elec-
tron transfer atom transfer radical polymerization; SI-ATRP, sur-
face-initiated atom transfer radical polymerization; SI-CRP, sur-
face-initiated controlled radical polymerization; SI-FRP, surface-
initiated free radical polymerization; SI-NMP, surface-initiated
nitroxide-mediated polymerization; SI-ROP, surface-initiated
ring-opening polymerization; SMA,stearyl methacrylate;
SMPs,shape-memory polymers; Sn(Oct)₂,tin(II) 2-ethylhexa-
noate; S-RAFT, surface reversible addition-fragmentation chain
transfer; Sty, styrene; tBA,tert-butyl acrylate; T<sub>d</sub>, degradation
temperature; TEM, transmission electron microscopy;
TEMPO, (2,2,6,6-tetramethylpiperidin-1-yl)oxy; T<sub>g</sub>,glass tran-
sition temperature; tg,trans-gauche; TGA,thermogravimetric
analysis; THF, tetrahydrofuran; TMEDA, N,N,N',N'-tetrame-
thylethane-1,2-diamine; UCST, upper critical solution temper-
ature; UPyMA,2-(3-(6-methyl-4-oxo-1,4-dihydropyrimidin-2-
yl)ureido)ethyl methacrylate; UV,ultraviolet; VAc,vinyl acetate;
wt %, weight percent; XPS, X-ray photoelectron spectroscopy;
ΔCₚ, heat capacity variation; ε-CL,ε-caprolactone; σₚ, tensile at
break

■ REFERENCES

(1) Sanchez, C.; Belleville, P.; Popall, M.; Nicole, L. Applications of advanced hybrid organic-inorganic nanomaterials: from laboratory to market. *Chem. Soc. Rev.* **2011**, *40* (2), 696–753.

(2) Dufresne, A. *Nanocellulose: from nature to high performance tailored materials*; Walter de Gruyter GmbH: Berlin, 2012.

(3) Eichhorn, S. J. Cellulose nanowhiskers: promising materials for advanced applications. *Soft Matter* **2011**, *7* (2), 303–315.

(4) Eichhorn, S. J.; Dufresne, A.; Aranguren, M.; Marcovich, N. E.; Capadona, J. R.; Rowan, S. J.; Weder, C.; Thielemans, W.; Roman, M.; Renneckar, S.; Gindl, W.; Veigel, S.; Keckes, J.; Yano, H.; Abe, K.; Nogi, M.; Nakagaito, A. N.; Mangalam, A.; Simonsen, J.; Benight, A. S.; Bismarck, A.; Berglund, L. A.; Peijs, T. Review: current international research into cellulose nanofibres and nanocomposites. *J. Mater. Sci.* **2010**, *45* (1), 1–33.

(5) Habibi, Y.; Lucia, L. A.; Rojas, O. J. Cellulose Nanocrystals: Chemistry, Self-Assembly, and Applications. *Chem. Rev.* **2010**, *110* (6), 3479–3500.

(6) Klemm, D.; Kramer, F.; Moritz, S.; Lindstroem, T.; Ankerfors, M.; Gray, D.; Dorris, A. Nanocelluloses: A New Family of Nature-Based Materials. *Angew. Chem., Int. Ed.* **2011**, *50* (24), 5438–5466.

(7) Moon, R. J.; Martini, A.; Nairn, J.; Simonsen, J.; Youngblood, J. Cellulose nanomaterials review: structure, properties and nano-composites. *Chem. Soc. Rev.* **2011**, *40* (7), 3941–3994.

(8) Kontturi, E.; Laaksonen, P.; Linder, M. B.; Nonappa; Gröschel, A. H.; Rojas, O. J.; Ikkala, O. Advanced Materials through Assembly of Nanocelluloses. *Adv. Mater.* **2018**, *30*, 1703779.

(9) Klemm, D.; Heublein, B.; Fink, H.-P.; Bohn, A. Cellulose: Fascinating biopolymer and sustainable raw material. *Angew. Chem., Int. Ed.* **2005**, *44* (22), 3358–3393.

(10) Sacui, I. A.; Nieuwendaal, R. C.; Burnett, D. J.; Stranick, S. J.; Jorfi, M.; Weder, C.; Foster, E. J.; Olsson, R. T.; Gilman, J. W. Comparison of the Properties of Cellulose Nanocrystals and Cellulose Nanofibrils Isolated from Bacteria, Tunicate, and Wood Processed Using Acid, Enzymatic, Mechanical, and Oxidative Methods. *ACS Appl. Mater. Interfaces* **2014**, *6* (9), 6127–6138.
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