The thermomechanical behavior of glassy polymers: applications to modeling shape memory behavior and 3D-printed polymers
Glassy polymers are amorphous polymers that have been driven out of equilibrium by cooling to below the glass transition temperature and thus experience a wide range of time-dependent and temperature-dependent mechanical behavior. In the nonequilibrium state, the polymer chains continue to slowly rearrange towards a lower entropy state. This process causes physical properties, such as the viscosity, yield strength, and enthalpy, to change with time in a process commonly referred to as physical aging or structural relaxation. Physical aging can be reversed by plastic deformation, which moves the material further away from equilibrium. This mechanical rejuvenation process is also responsible for post-yield dynamic softening observed in the stress-strain behavior glassy polymers, the extent of which determines the toughness and failure response of the material. Though structural relaxation and viscoplasticity are interdependent phenomenon, they have been treated as separate processes and described by different theoretical approaches. I will present a new theory that strongly couples both viscoplasticity and structural relaxation through an effective temperature thermodynamic framework. Using this framework, we developed a new unifying thermomechanical theory for amorphous polymers that describes a wide range of nonequilibrium phenomena, including the glass transition, viscoplasticity, physical aging, mechanical rejuvenation, and orientation hardening, using a common set of physically measurable parameters. I will also present applications of the theory to two problems that require this unified thermomechanical description: predicting the shape memory behavior of amorphous polymers for morphing and deployable structures and modeling the mechanical behavior of polymers printed by fused deposition modeling.