Entangled Chain Structure Trapped in a Styrene-Butadiene Random Copolymer by Cross-Linking in Simple Extension

Rubber networks were prepared by cross-linking linear styrene-butadiene random copolymer (67% styrene content) with γ-radiation. Before cross-linking, the polymer was stretched in simple extension, allowed to relax at constant strain and temperature for a controlled time, and then quenched 20°C below T_g and maintained at this temperature during cross-linking. Upon warming above T_g, the equilibrium length was intermediate between the stretched and unstretched lengths and was describable as the result of additive contributions from the cross-link and trapped entanglement networks. The fractions of trapped unrelaxed entanglements T_e and trapped relaxed entanglements T_e,r were estimated and compared with the Langley theory. For minimal terminal zone relaxation before quench, T_e agreed rather well with the Langley theory and T_e,r was zero. With increasingly long terminal zone relaxation before quench, T_e increased and T_e,r became finite and increased. This behavior is consistent with the terminal mechanism of relaxation in the tube theory of Doi and Edwards. These experiments confirm the presence of permanently trapped entangled chain structure (or equivalent trapped topological restraints), in agreement with earlier extensive work on 1,2-polybutadiene.