Formation of Crystallites with Low Surface Energy-A Key to Understand the Crystallization of Oriented Polymer

In order to demonstrate that lower surface energies of bundle-like crystals have a dominant effect for the enhancement of strain-induced crystallization of natural rubber (NR), an estimation of the end-surface energy (sigma(e)) was attempted. NR samples were quickly stretched at predetermined crystallization temperature (T-c) to a prescribed draw ratio (alpha(s)), and the wide angle X-ray diffraction patterns during the subsequent stepwise heating process were investigated. Melting temperature (T-m) was found to depend solely on alpha(s) and to be independent of T-c. Simultaneously, average crystallite size in the chain direction (< L-c >) was primarily determined by network-chain density and was also independent of T-c. As a result, the plot of T-m < L-c >(-1) versus suggested that sigma(e) is negligibly low and estimated T-m is an approximation of the equilibrium melting temperature.