Temperature-Dependent Photoswitching Kinetics of Semicrystalline Main-Chain Poly(azobenzene)s

Photoresponsive semicrystalline polymers, although promising as photomechanical materials thanks to their high degree of molecular ordering and large phase transition enthalpy, have, in some cases, shown limited photoresponse. One major reason is that photochemical reactions involving large changes in molecular shape, such as trans-cis isomerization, can be greatly hindered in densely packed crystalline environments. Here, we focus on a recently developed set of semicrystalline poly(azobenzene)s prepared by thiol-Michael step growth polymerization of azodiacrylate monomers with alkyl dithiol chain extenders, which show strongly temperature-dependent photoswitching kinetics, even well below the melting temperature of the crystalline phase. A two-phase model is developed, incorporating a stretched exponential function in the slow phase to capture the dispersive kinetics of azobenzene in these regions. The model can be used to simulate the photoswitching behavior of optically thick macroscopic actuators, enabling prediction of photomechanical performance under various UV and thermal conditions. Although we focus in detail on the polymer prepared using a six-carbon chain extender, we show that similar behavior exists across a series of polymers with different length chain extenders, although with substantial odd–even effects.