  Owing to the possibility of controlling its specific mechanical behaviors up
taken by irradiated by light at particular wavelengths, the photoisomerization
hydrogels have a broad range of potential applications. A theory connecting the
optical excitation to mechanical behavior is essential to precisely control the
photo-mechanical behaviors of the hydrogel. In this work, a photo-mechanical
coupling theory is developed to describe the photo-mechanical responses of
photoisomerization hydrogels within the framework of finite deformation
continuum thermodynamics. In the model, the deformation gradient is decomposed
into two parts to effectively model the light induced deformation and the
elastic one. To consider the effect of the optical excitation on mechanical
behaviors, we first investigate the transporting mechanism of light in
hydrogel, as well as the photochemical reaction process; and we then explore
the disturbance of light irradiation on the equilibrium of the thermodynamic
system of hydrogel, as well as the relationship of conformational entropy of
hydrogel network with the photochemical reaction; finally, based on the entropy
elasticity theory, we propose a new free energy function of the photosensitive
hydrogel to consider the effect of molecular chain distribution evolution on
the stiffness of the hydrogel network. With the implementation of the proposed
model, we study the photo-mechanical behaviors and mechanical properties of
photoisomerization hydrogels. The present research is helpful for understanding
the multi-field coupling behaviors of the photosensitive hydrogel, and then
providing guidelines for the application of photoisomerization hydrogel.
