Thermo-micromechanics of strain-induced crystallization

Strain-induced crystallization in natural rubber has long been modeled as an isothermal process. Recent calorimetric measurements (Samaca Martinez et al. 2013a,b) and a theoretical result (Khiem and Itskov 2018) revealed severe limitations of this approach. First, it was not able to describe the Gough-Joule effect as well as the rate of crystallization (melting) in natural rubber. Second, the stress-strain hysteresis of natural rubber was improperly attributed to the mechanical dissipation. In this contribution, we present a fully coupled thermo-micromechanical theory of strain-induced crystallization in natural rubber. Accordingly, deformation is accompanied by heat production/absorption. It potentially induces an evolution of the heat source which alters the temperature of the sample. In contrast to previous works on thermo-mechanics of rubber-like materials, the internal energy and the entropy are formulated in our model explicitly in terms of state variables. The crystallinity is not considered as an internal variable, and its evolution is elucidated by crystal nucleation in loading and crystal growth in unloading. The crystallization kinetics (Khi(boolean AND)em and Itskov 2018) is further extended beyond uniaxial deformation, which offers microscopic boundary conditions for the representative chain on the one hand, and provides a formulation for the crystallinity on the other hand. Model predictions are compared with comprehensive experimental results and shed new light on strain-induced crystallization of natural rubber.