Indentation of shape memory polymers: Characterization of thermomechanical and shape recovery properties

Abstract

A framework for understanding the structure (shape memory and thermomechanical)–property relationships in shape memory polymers (which could be reasonably modeled as comprising of a soft phase and a hard phase) using indentation is developed. A finite element model is developed to predict the complete stress–strain–temperature characteristics of shape memory polymers under uniaxial and indentation loading conditions. By invoking the indentation load–depth response characteristics for a range of temperatures, it is demonstrated that the indentation method predicts the variation of the mechanical properties of shape memory polymers (i.e., elastic modulus) as a function of temperature and microstructural state (i.e., 'glassy’ vs. 'rubbery’) for materials that exhibit a wide range of transitions (i.e., gradual and sharp). By invoking the indentation deformation characteristics of shape memory polymers for a range of indenter geometries and temperatures, it is demonstrated that the extent of shape recovery is strongly dependent on temperature, microstructural state and constraint loads. By invoking the spatial evolution of stresses during the indentation deformation process and the subsequent recovery process, it is demonstrated that the increase in the internal stresses during the deformation process and the rate of dissipation of the internal stresses during the recovery process is dependent on the indenter geometry and temperature. The predictions of the finite element model for the variations of the mechanical properties and shape recovery characteristics of shape memory polymers with temperature are in reasonable agreement with the trends observed in experiments.