Light-stimulated mechanically switchable, photopatternable cellulose nanocomposites†
Abstract
We report light-responsive, mechanically switchable, photopatternable nanocomposites based on benzophenone-derivatized cellulose nanocrystals (Bp-CNCs). Bp-CNCs are highly photoreactive and undergo radical-mediated reactions upon UV exposure, which were exploited to create materials with switchable mechanical characteristics. The nanocomposites were fabricated by incorporating 10 or 20% w/w Bp-CNCs into a rubbery ethylene oxide/epichlorohydrin copolymer (EO-EPI) matrix. The introduction of Bp-CNCs caused a pronounced stiffness increase. The tensile storage modulus (E′) increased from 4 MPa (neat polymer) to 222 MPa and 407 MPa for nanocomposites with 10% w/w or 20% w/w Bp-CNCs. E′ further increased to 293 MPa and 508 MPa upon irradiation with 365 nm UV light, on account of formation of covalent bonds between the Bp-CNCs and between the Bp-CNCs and the matrix polymer. The photoreaction reduced the level of aqueous swelling of the nanocomposites, as well as the extent of water-induced softening. The properties can be changed in a spatially resolved manner and the new nanocomposites also exhibit shape memory properties.