Thermomechanics of amorphous polymers and its applications to shape memory behaviors

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Johns Hopkins University
Amorphous polymers exhibit a wide range of complex temperature-dependent and time-dependent behaviors, from elastic and rubbery to viscoplastic and glassy. At high temperatures, the polymer structure has high mobility and is in the equilibrium rubbery state. The mobility decreases with temperature, and cooling drives the initially rubbery material out of equilibrium and induces the glass transition. The glass transition mechanism can be exploited to achieve the shape memory behaviors. The programmed shape of amorphous shape memory polymers can be stored by the tremendous decrease in chain mobility and recovered to an original shape in response to an environmental trigger, such as heat and solvent, which increases the chain mobility. Modeling the shape memory effect of amorphous polymers requires modeling the temperature-dependent and time-dependent behaviors of the glass transition. Simultaneously, the investigation on shape memory behaviors of amorphous polymers can also enrich the understanding the glass transition. In this work, we started with exploiting the glass transition to model the thermally-activated shape memory behaviors. The model adopted multiple discrete relaxation processes to describe the distribution of relaxation times for stress relaxation, structural relaxation, and stress-activated viscous flow. Experimental methods were also developed to obtain the stress and structural relaxation spectra, and viscoplastic parameters. The model was applied to study the deformation temperature and physical aging influence on the partially constrained recovery and fixed-strain recovery responses. The model was able to capture the main features of the shape memory recovery response observed in experiments. We further extended this model to describe the influence of solvent on the thermomechanical properties and shape memory behavior of amorphous polymers. The solvent increases the chain mobility, decreases the relaxation time and the glass transition temperature. The time-dependent diffusion process was also incorporated into the model. The model showed the ability to predict quantitatively the dramatic softening of the stress response of saturated specimen and the time-dependent solvent-driven shape recovery. In the last part of this work, we developed a thermomechanical theory that couples the structural evolution and inelastic deformation to describe the nonequilibrium behavior of amorphous polymers. We showed that this theory was able to reproduce the temperature-dependent and rate-dependent stress response spanning the glass transition and the effects of physical aging and mechanical rejuvenation on the stress response and enthalpy change observed in experiments.
shape-memory polymers, glass transition, physical aging, mechanical rejuvenation