Stretchable composite hydrogels incorporating modified cellulose nanofiber with dispersibility and polymerizability: Mechanical property control and nanofiber orientation

We fabricated composite hydrogels using surface-modified cellulose nanofiber (CNF) as a multifunctional crosslinker and N-isopropylacrylamide (NIPAM) as a monomer. Surface modification of CNF was performed using maleic anhydride (MA), which endowed the CNF with not only aqueous dispersibility but also vinyl polymerizability. We characterized the MA-modified CNF (MACNF) using wet chemical analysis, spectroscopy, and morphological observations. Hydrogels were prepared by in situ radical polymerization of mixtures of MACNF aqueous suspensions and NIPAM monomer, using a radical initiator and a catalyst under various conditions. The obtained hydrogels were transparent and highly stretchable (to more than 20 times their original length), and their mechanical properties, in particular, their tensile strength, could be controlled by exploiting the high dispersibility of the MACNF. Optical anisotropy measurements of the stretched gel specimens revealed that the MACNF fibers were highly oriented due to the large deformation, which led to an increase in the tensile strength of the specimens. A clear temperature dependence of the optical transmittance of the hydrogel samples was also observed, which originates from the coil-to-globule transition of poly(NIPAM) in aqueous media at its lower critical solution temperature.