MEAM Seminar: “Some Investigations of Phase Transitions in Rod-like Macro-molecules and Fibrous Gels”

Stress-induced solid-to-solid phase transitions form a large class of phase transitions in nature. They occur in materials that can exhibit different crystal structures under different conditions (stress and temperature), and when the stress and temperature condition changes, the material may undergo a transformation from one phase to another. Despite its considerable potential in application, some fundamental aspects about the basic physical mechanisms of stress-induced solid–solid phase transitions remain poorly understood. In this work we are particularly interested in phase transitions in macromolecules and fibrous gels.

First, we conduct Langevin dynamics calculations on a chain of masses and bistable springs in a viscous fluid and extract a temperature dependent kinetic relation by observing that the dissipation at a phase boundary can be estimated by performing an energy balance. Using this kinetic relation, we solve boundary value problems for a bistable bar immersed in a constant temperature bath and show that the resultant force-extension relation matches very well with the Langevin dynamics results. We estimate the force fluctuations at the pulled end of the bar due to thermal kicks from the bath by using a partition function. We also show rate dependence of hysteresis in cyclic loading of the bar arising from the stick-slip kinetics.

Second, we use a double-well stored energy function in a chemo-elastic model of gels to capture the existence of two phases of the network. We model cyclic compression/decompression experiments on fibrous gels and show that they exhibit propagating interfaces and hysteretic stress-strain curves that have been observed in experiments. We can capture features in the rate-dependent response of these fibrous gels without recourse to finite element calculations.