Polymer Composites Reinforced by Locking-In a Liquid-Crystalline Assembly of Cellulose Nanocrystallites

Abstract

An attempt was made to synthesize novel composites comprising poly(2-hydroxyethyl methacrylate) (PHEMA) and cellulose nanocrystallites (CNC) (acid-treated cotton microfibrils) from suspensions of CNC in an aqueous 2-hydroxyethyl methacrylate (HEMA) monomer solution. The starting suspensions (∼5 wt % CNC) separated into an isotropic upper phase and an anisotropic bottom one in the course of quiescent standing. By way of polymerization of HEMA in different phase situations of the suspensions, we obtained films of three polymer composites, PHEMA-CNC(iso), PHEMA-CNC(aniso), and PHEMA-CNC(mix), coming from the isotropic phase, anisotropic phase, and embryonic nonseparating mixture, respectively. All the composites were transparent and, more or less, birefringent under a polarized optical microscope. A fingerprint texture typical of cholesteric liquid crystals of longer pitch spread widely in PHEMA-CNC(aniso) but rather locally appeared in PHEMA-CNC(iso). Any of the CNC incorporations into the PHEMA matrix improved the original thermal and mechanical properties of this amorphous polymer material. In dynamic mechanical measurements, the locking-in of the respective CNC assemblies gave rise to an increase in the glass-state modulus E' of PHEMA as well as a marked suppression of the E'-falling at temperatures higher than T(g) (≈ 110 °C) of the vinyl polymer. It was also observed for the composites that their modulus E' rerose in a range of about 150-190 °C, which was attributable to a secondary cross-linking formation between PHEMA chains mediated by the acidic CNC filler. The mechanical reinforcement effect of the CNC dispersions was ensured in a tensile test, whereby PHEMA-CNC(aniso) was found to surpass the other two composites in stiffness and strength.

Different orientation patterns of cellulose nanocrystal films prepared from aqueous suspensions by shearing under evaporation

“…The procedure for preparing the CNC used in this work was essentially the same as that described previously (Tatsumi et al 2012). In brief, CNC particles were isolated from cotton cellulose powder (Whatman, CF11) by acid hydrolysis with 65 wt% sulfuric acid, with mechanical stirring at 70°C for 15 min.…”

“…3b. This morphology is apparently like the banded structure characteristic of shear-deformed polymer liquid crystals (Navard 1987;Nishio et al 1985), but, rather, it may be regarded as the trace of a helical arrangement of CNCs (Majoinen et al 2012;Tatsumi et al 2012); viz., the chiral nematic structure could develop in the process of condensation of the sheared suspension and partly remain in the dry sample, as discussed below. Another attention should be paid to the fact that no trace of such a mesomorphic organization was observed in the CNC-6.7 film.…”

Liquid Crystals of Cellulosics: Fascinating Ordered Structures for the Design of Functional Material Systems

“…In many cases, CNCs were compounded with thermoplastic polymers by simple mechanical mixing. As another route to microcomposition of CNCs, Tatsumi et al demonstrated a successful synthesis of polymer composites containing a fixed liquid crystalline arrangement of CNCs via bulk polymerization of monomer solvent [120]. From phase-separated suspensions of cotton CNCs in an aqueous 2-hydroxyethyl methacrylate (HEMA) monomer solution (water to HEMA ratio was 0.5-1.2:1 by volume), polymer composites PHEMA-CNC iso and PHEMA-CNC aniso coming from the upper isotropic phase and the lower anisotropic phase, respectively, were prepared in film form by photopolymerization of HEMA followed by oven-curing and drying.…”

“…An extensive effort was made to synthesize further novel composites from similar CNC suspensions in water/monomer using a magnetic alignment [120] with an adequate modification) technique [123]. As shown in Fig.…”

Tungsten oxide–cellulose nanocrystal composite films for electrochromic applications

“…34,35 For instance, the Yamazaki group 36 used tungsten methylcellulose composite films as photochromic material.…”