Thermodynamically-consistent constitutive modeling of moisture- and thermo-responsive shape memory polymers

Taking into account that shape memory polymer (SMP)-based devices are often subject to multiple environmental conditions during application, it is difficult to accurately predict their shape memory effect (SME). Thus, constitutive modeling for SMPs in multi-field environments is of great importance. However, most of the models available are limited to describing the temperature-driven SME and do not refer to multi-field conditions. In this paper, a constitutive model for SMPs in hygrothermal environments is developed under a consistent thermodynamic framework. The derivation is based on an additive decomposition of the Helmholtz free energy density and satisfying the first law and second law of thermodynamics. In this paper, the absorbed moisture is categorized into free and bound phases and it is considered that they have different effects on the material properties. Accordingly, it is the first time to study the variation of configurational entropy with different phases in the polymer-moisture system during the moisture diffusion process. For the first time, the validity of the constitutive model proposed in this paper can be confirmed by systematically comparing the modeling results and experimental data of various types of hygrothermal-induced shape memory cycles.