Molecular dynamics study of polymeric photoactuators: coupling between photochemical reaction and polymer relaxation

Photoactuators are materials that can change their own macroscopic shape to perform mechanical work, when subjected to a light stimulus. In this study, we investigate the properties of such a model photoresponsive material made of an azobenzene photochrome (4,4'-dihydroxyazobenzene) embedded in a cis-1,4-polybutadiene matrix, by considering the coupling between molecular photoisomerization and polymer matrix relaxation. We propose a strategy to bridge molecular and material scales: a classical molecular dynamics approach is developed to simulate the trans-cis photoisomerization by dynamically switching between ground and excited state potential energy surfaces, while allowing the polymer chains to respond simultaneously. Our results reveal that the photoisomerization has negligible influence on the mechanical (total and local pressures) and structural (atomic and angular displacements) properties of the surrounding polymer matrix. In contrast, the reverse impact of the polymer on the photoisomerization process is significant: increasing the strength of the photochrome-matrix interactions, from van der Waals to hydrogen bonding and then to covalent bonding, strongly modifies or slows down the photoisomerization dynamics. These findings highlight the importance of molecular environment in controlling the photoactuating properties.