A model for inhomogeneous large deformation of photo-thermal sensitive hydrogels

The current study develops a 3D constitutive model for photo-thermal sensitive hydrogels based on free energy decomposition. The hydrogel under study is PNIPAM network with copper chlorophyllin nanoparticle agents attached to the network. The effect of light intensity is considered as a rise in temperature since chlorophyllin nanoparticle agents absorb light irradiation and convert it to heat. Moreover, it is necessary to consider the effect of dissociation of these agents on the hydrogel's free energy function; therefore, a term is added to the free energy function. After introducing the model, some problems, including the free swelling and uniaxial loading problems, are studied, and the obtained results are compared with experimental data to validate the model. The results of the model are in good agreement with experiments, which confirms the validity of the model. Next, to develop a numerical tool to study problems with complicated boundary conditions, the model is implemented in ABAQUS by developing a user-defined UHYPER subroutine, and several practical problems are studied. For example, the deformation of a bilayer made of a sensitive hydrogel attached to a neutral elastomer and the behavior of a self-folding structure is investigated with respect to temperature and light intensity changes. Thereafter, the problem of coexistent phases in a rod due to the light irradiation is investigated. The obtained results confirm the performance of the presented model for use in complicated boundary value problems.