Novel Class of Nanostructured Hydrogels Reinforced with Modified Cellulose Nanofibers

In this work we report a novel example of optically transparent nano-structured composite which consists of surface-modified bacterial cellulose (B.C.) nanofibers reinforced in poly(hydroxyethyl methacrylate) (PHEMA) hydrogel matrix. To promote the interfacial strength between the nanofibers and the PHEMA phases, surface of the cellulose nanofibers was modified by fibrous acetylation to preserve the B.C. nanofibrillar morphology. The modified nanofibers were then graft copolymerized with acrylic based PHEMA hydrogel via free-radical mechanisms using a benzoyl-peroxide initiator. Several samples of grafted nanofibers were prepared with different degrees of acetylation and graft yields, which were characterized using 13C solid-state NMR, FTIR, and gravimetric method. The maximum degree of acetylation was quantified to be 2.3, while the gravimetrical graft yield was 82.35 %. The modified nanofibers were then reinforced into a polymeric matrix of PHEMA to form the final designed biocomposite.

The nanofiber-network-reinforced PHEMA polymer composite maintained its transparency up to more than 1% nanofibrillar content. The biocomposite material transmitted more than 80 % of the light when the nanofiber network content was 1 wt%. The light transmittance was higher with samples with lower nanofibrillar content. The loss of transparency in B.C.-based nanocomposite was small, despite the differences in B.C. and PHEMA refractive indices. This clearly indicates the size effect of the nano-scaled fibres. These results clearly indicate the advanced properties of the nanocomposite which allow it to be combined with various optically functional materials having significantly different refractive indices and maintain the transparency of the final material. This novel nanocomposite is light in weight, highly flexible, and easy to be mould. Considering the unique properties of the cellulose nanofibers, in addition to the optically transparency of the final material, the nanocomposite is expected to have significant improvements in the thermal and mechanical properties, which will allow a wide range of applications.