Shape-Stabilized Monolithic Composite for Switchable Stiffness Based on Cellulose and Poly(stearyl methacrylate)

Stiffness-changing materials (SCMs) have recently attracted attention because their mechanical strength can be changed in response to external stimuli. Although the stiffness of SCMs can be adjusted considerably, these materials do not necessarily possess sufficient mechanical robustness in their soft state, thereby limiting their practical applicability. In this study, a stimuli-responsive polymer was incorporated into a porous supporting material to fabricate SCMs with high structural stability. Poly(stearyl methacrylate) (PSMA) was utilized as the responsive domain owing to the phase-changing behavior of its long alkyl side chain. A cellulose monolith (CM) was selected as the support owing to its outstanding chemical and physical stability despite its high porosity. The PSMA-modified CM (PSMA-CM) exhibited clearly detectable stiffness changes in response to temperature as a result of melting and crystallization of the PSMA domain. The Young’s modulus of PSMA-CM at 60 °C was half as large as that at 20 °C. Furthermore, during cyclic compressive loading–unloading tests, PSMA-CM displayed good shape recoverability, and the changes in its thermoresponsive stiffness were highly repeatable owing to both the stability of CM and the reversible responsive properties of PSMA. In addition, as opposed to CM, the stiffness of PSMA-CM decreased as the polarity of the surrounding solvent decreased. This result indicates that PSMA was composited not only onto the surface but also into the interior of the CM skeletal backbone. PSMA clearly reflects the variable stiffness of the monolith derived from the PSMA domain without impeding the dimensional stability of the material.