Crystallization in unstretched and in stretched rubber and its thermodynamic description

Thermodynamics of heat- and strain-induced crystallization of networks is presented which allows the description of both crystallization modes. The network is treated as a eutectoid copolymer. Chains of different length are components representing crystallizable “c-sequences”. For thermodynamic reasons, extended c-sequence crystals are formed in any case. It is the essential step to combine the van der Waals theory of networks and the theory of eutectoid copolymers. The solubility in the solid seems to be increased in presence of a mechanical field. The typical hysteresis in stress-strain cycles is then understandable only if “quasi-static constraints” are introduced. These constraints are responsible for the observation that strain-induced crystallization is enforced under large overdrawings. Melting during retraction seems to run under invariant constraints. With the aid of this approach, it is possible to describe the work and the heat exchanged during stress-strain cycles of natural rubber measured in a stretching micro-calorimeter. The origins of the constraints are discussed