A new semi-phenomenological approach to predict the stress relaxation behavior of thermoplastic elastomers

In this paper we report on a new semi-phenomenological approach to predict the stress relaxation behavior of thermoplastic elastomers at long times. This approach relies on the method of Gurtovenko and Gotlib [J Chem Phys 115 (2001) 6785], which has originally been conceived to describe the relaxation dynamics of inhomogeneously crosslinked polymers forming agglomerations of crosslinks. In this work we demonstrate that the method can be extended to predict the stretched exponential stress decay of homogeneously crosslinked thermoplastic elastomers, which are subjected to an extensional strain pertaining to the nonlinear regime of mechanical properties. In our approach thermal fluctuations induce fluctuations in size of domains of crosslinks via a chain-pullout mechanism. We compare our theoretical predictions to the experimental measurements of Hotta et al. [Macromolecules 35 (2002) 271] performed on poly(styrene-isoprene-styrene) triblock copolymers, which are composed of hard domains of polystyrene embedded in a rubbery polyisoprene matrix. Our study confirm the importance of the chain-pullout mechanism in the stress relaxation process and demonstrates the involvement of multiple time- and structural-length-scales.