Prediction of elastic properties of a poly(styrene-butadiene-styrene) copolymer using a mixed finite element approach

Despite several decades of research, the nature of linear elasticity in microphase-separated copolymers with chemically connected glass-rubber phases is still not fully understood. In this paper we investigate the linear elastic properties of a poly(styrene-butadiene-styrene) triblock copolymer using a mixed finite element approach. The technique permits phases of full incompressibility as well as phases of near incompressibility as they occur in this two-component system to be dealt with. Strikingly and contrary to the common belief, we find that the continuum description is accurate and that no additional detailed molecular information is needed to reproduce the available linear elastic experimental data. Our investigation indicates that the anomalous Poisson ratio of the polybutadiene phase of 0.37, determined by previous authors and attributed to molecular characteristics of the polybutadiene phase, might be related to sample end effects arising in their tensile and torsional experiments. We also test the suitability of several semiphenomenological models in reproducing the experimental measurements. We find that some of the methods provide reliable results of accuracy comparable to results from our mixed finite element approach.