Small-Angle Neutron Scattering Reveals the Structural Details of Thermosensitive Polymer-Grafted Cellulose Nanocrystal Suspensions.

Thanks to the use of small-angle neutron scattering (SANS), a detailed structural description of thermosensitive polymer-grafted cellulose nanocrystals (CNCs) was obtained and the behavior of aqueous suspensions of these derivatized biosourced particles upon temperature increase was revealed. Although literature data show that the surface grafting of thermosensitive polymers drastically enhances the colloidal properties of CNCs, direct space microscopic investigation techniques fail in providing sufficient structural information on these objects. In the case of CNCs decorated with temperature-sensitive polyetheramines following a peptide coupling reaction, a qualitative and quantitative analysis of SANS spectra shows that CNCs are homogeneously covered by a shell comprising polymer chains in a Gaussian conformation with a thickness equal to their radius of gyration in solution, thus revealing a mushroom regime. An increase of the temperature above the lower critical solution temperature (LCST) of the polyetheramine results in the formation of finite size bundles whose aggregation number depends on the particle concentration and suspension temperature deviation from the LCST. SANS analysis further reveals local changes at the CNC surface corresponding to a release of water molecules and a related denser polymer shell conformation. Noticeably, data show a full reversibility at all length scales when a sample was cooled down to below the LCST after being heated above it. Overall, the results obtained by SANS allow an in-depth structural investigation of derivatized CNCs, which is of high interest for the design of functional materials comprising these biosourced colloids.

Relationship between Young's Modulus and Film Architecture in Cellulose Nanofibril-Based Multilayered Thin Films.

Young's moduli of cellulose nanofibril (CNF)-poly(allylamine hydrochloride) (PAH) multilayered thin films were measured using strain-induced elastic buckling instability for mechanical measurements (SIEBIMM) and the quantitative nanomechanical mapping technique (PF-QNM). To establish the relationship between structure and mechanical properties, three types of films with various architectures were built using the layer-by-layer method by changing the ionic strength of the dipping solution. Both methods demonstrate that the architecture of a film has a strong impact on its mechanical properties even though the film has similar cellulose content, emphasizing the role of the architecture. Films with lower porosity (Φair = 0.34) and a more intricate network display the highest Young's moduli (9.3 GPa), whereas others with higher and similar porosity (Φair = 0.46-0.48) present lower Young's moduli (4.0-5.0 GPa). PF-QNM measurements indicate a reverse ranking that is probably indicative of the surface composition of the films.

Adjustment of the Chiral Nematic Phase Properties of Cellulose Nanocrystals by Polymer Grafting.

The self-organization properties of sulfated cellulose nanocrystals, TEMPO-oxidized cellulose nanocrystals and polymer-decorated cellulose nanocrystals suspensions in water were investigated and compared. Polarized light optical microscopy observations showed that these three systems phase separated to form a lower anisotropic chiral-nematic phase and an upper isotropic phase following a nucleation and growth mechanism, proving that surface-grafted polymer chains did not inhibit the self-organization properties of CNCs. The phase diagrams and pitch of the suspensions were shown to strongly depend on the surface chemistry of the nanoparticles and the nature of the interacting forces. Especially, the entropic repulsion contribution of the polymer chains to the overall interactions forces resulted in a decrease of the critical volume fractions due to an increase of the effective diameter of the rods. Additionally, above a cellulose volume fraction of 3.5% v/v, the pitch was significantly smaller for polymer-decorated CNC suspensions than for sulfated as-prepared CNC ones, revealing stronger chiral interactions with the surface-grafted chains. In all cases, the addition of small quantities of monovalent salt induced an increase of the critical concentrations, but values for polymer-decorated CNCs were always the smallest ones due to entropic repulsion forces. Overall, results show that polymer grafting provides more tunability to the chiral-nematic phase properties of CNCs, including an enhanced expression of the chirality.

Tunable Aggregation and Gelation of Thermoresponsive Suspensions of Polymer-Grafted Cellulose Nanocrystals.

The colloidal stability together with the tunable aggregation and viscoelastic properties of thermoresponsive polymer-grafted cellulose nanocrystals (CNCs) were investigated. TEMPO oxidation of CNCs followed by peptidic coupling in water were used to covalently graft thermosensitive Jeffamine polyetheramine M2005 chains onto the surface of CNCs. The resulting polymer-decorated particles (M2005-g-CNCs) exhibited new colloidal properties, by their ability to perfectly redisperse in water and organic solvents such as toluene, dichloromethane or DMF after freeze-drying. In addition, they presented an enhanced thermal stability when compared to that of sulfated or TEMPO-oxidized CNCs. Dynamic light scattering experiments were used to demonstrate that the thermally induced aggregation of M2005-g-CNCs was fully reversible and reproducible over many temperature cycles and that, most interestingly, the aggregation number could be tuned by varying the ionic strength and/or the pH of the medium, making the suspension multiresponsive. This property arises from the variations of the sign (attractive or repulsive) and the range of the different types (entropic, electrostatic, hydrophobic) of interaction forces between the thermosensitive polymer-decorated nanoparticles. The variation of the viscoelastic properties of M2005-g-CNCs suspensions as a function of temperature, probed by oscillatory rheology measurements of more concentrated suspensions, revealed a reversible temperature-triggered liquid-to-gel transition. Such enhanced functionalities pave the way to the design of advanced CNC-based materials benefiting both from the intrinsic characteristics of these biosourced particles and the new properties imparted by the stimuli-sensitive grafted chains.

Influence of cellulose nanocrystals concentration and ionic strength on the elaboration of cellulose nanocrystals-xyloglucan multilayered thin films.

The effect of the variation of CNC concentration on the growth pattern of CNC-XG films is investigated. We found that a transition in the growth slope occurs at a CNC concentration of roughly 3-4gL(-1). A close effect can be obtained by the increase of the ionic strength of the CNC suspensions, suggesting that electrostatic interactions are involved. Static light scattering investigation of CNC dispersions at increasing concentrations demonstrated that the particle-particle interactions change as the CNC concentration increases. Neutron Reflectivity (NR) was used to probe the internal structure of the films. The increase of the CNC concentration as well as the increase of the ionic strength in the CNC suspension were found to induce a densification of the adsorbed CNC layers, even though the mechanisms are not strictly identical in both cases. Small changes in these parameters provide a straightforward way of controlling the architecture of CNC-based multilayered thin films and, as a result, their functional properties.

Reversible modification of structure and properties of cellulose nanofibril-based multilayered thin films induced by postassembly acid treatment.

A postassembly acid-treatment consisting of an immersion in 5 mM HCl solution was applied to carboxylated cellulose nanofibrils (CNF)-poly(allylamine) hydrochloride (PAH) multilayered thin films. Our results show that the treatment did not affect the overall thickness of the films without any loss of the components. However, a modification of the surface morphology was observed, as well as the swelling behavior. The process was perfectly reversible since the original structure was recovered when the thin films were rinsed by ultrapure water. Moreover, a more pronounced antireflective character was detected for the treated films. The origin of these reversible modifications was discussed. Notably, the scattering length density (SLD) profiles of the films before and after treatment support the idea of a structural reorganization of the components within the film driven by the change of their charge densities induced by the acid treatment.

Cellulose nanofibril-based multilayered thin films: effect of ionic strength on porosity, swelling, and optical properties.

TEMPO-oxidized cellulose nanofibrils (CNF) and synthetic poly(allylamine) hydrochloride (PAH) were used to build multilayered thin films via the dipping-assisted layer-by-layer technique. We used the ionic strength, in both CNF suspension and PAH solution, as a key parameter to control the structure of the films. Three systems with different ionic strength parameters were investigated. We studied the growth of the films and their surface morphology by ellipsometry and AFM and investigated their porosity and swelling behavior using neutron reflectivity. Our results showed that the PAH conformation is a determining factor not only for film growth but also for structural properties: with salt-free PAH solution where chains have extended conformation, the resulting films have lower porosity and higher swelling ratios, compared to the ones made using high ionic strength (1 M) PAH solution, where chains have a coiled conformation. The slight aggregation of CNF, induced by adding a small amount of salt (12 mM), has less influence on film growth and porosity, whereas it has a greater impact on swelling. The origin of these differences is discussed. The structure of the films obtained was linked to their optical properties and, in particular, to their antireflective character.

Preparation by grafting onto, characterization, and properties of thermally responsive polymer-decorated cellulose nanocrystals.

The grafting of thermosensitive amine-terminated statistical polymers onto the surface of cellulose nanocrystals (CNCs) was achieved by a peptidic coupling reaction, leading to unusual properties like colloidal stability at high ionic strength, surface activity, and thermoreversible aggregation. We have used a large variety of experimental techniques to investigate the properties of the polymer-decorated CNCs at different length-scales and as a function of the different reaction parameters. A high grafting density could be obtained when the reaction was performed in DMF rather than water. Infrared and solid-state NMR spectroscopy data unambiguously demonstrated the covalent character of the bonding between the CNCs and the macromolecules, whereas TEM images showed a preserved individualized character of the modified objects. Dynamic light scattering and zeta potential measurements were also consistent with individual nanocrystals decorated by a shell of polymer chains. Surface tension measurements revealed that CNCs became surface-active after the grafting of thermosensitive amines. Decorated CNCs were also stable against high electrolyte concentrations. A thermoreversible aggregation was also observed, which paves the way for the design of stimuli-responsive biobased nanocomposite materials.