ABSTRACT
Due to its stereoregularity Natural Rubber (NR) can partially crystallize at low temperature (−25C) or under strain. Strain Induced Crystallization (SIC) in NR presents a major interest in the rubber technology due to the unique physical properties it confers to NR (strength, crack resistance). Indeed, the formation of crystallites in a polymer network induces a strengthening of this material, giving NR a self-reinforcement character [(Chenal 2007)]. The SIC is a kinetic process which is activated at a critical elongation rate (for example 300% for NR in quasi-static experiments at room temperature) [(Candau 2012)], it is thus very sensitive to the parameters of the test (deformation velocity, temperature) but also to material parameters (crosslink density, physical entanglements network …) [(Mitchell 1968, Candau 2012)]. Although numerous attempts to develop a model able to describe the SIC have been made [(Flory 1947, Kilian 1980)], these
the system to its equilibrium. The microstructure evolution with time is thus a result of the model. The model we present here is based on a free energy functional that contains bulk and interface properties for the crystallites (enthalpy of melting, surface tension), but also the coupling to the local deformation field (as created by the applied external constraint, or generated by the growth itself). Topological constraints such as entanglements or crosslinks have an important impact on the microstructure, and the model accounts for the release of the topological constraints from the crystallites. The model is able to reproduce several interesting features such as the limited growth of the crystallites, with a size that depends on the applied constraint, or to predict the formation of spherulites in cold crystallization. Furthermore, it allows the investigation of transient effects and accounts for complex external forcing like in cyclic tensile tests.
