Pulsed NMR studies on long-term crystallization behavior and melting process of natural rubber under elongation

Long-term crystallization and melting process of natural rubber under elongation was investigated by measuring spin-spin relaxation time T₂ by pulsed nuclear magnetic resonance (NMR). We examined the dependence of the both processes on extension ratio, crystallization temperature and crosslink density. It was found that the crystallization of natural rubber continued over three months even at 10 °C. According to the two-step melting process found for the first time under elongation, we proposed a model that crystalline part of natural rubber under large elongation is composed of two kinds of crystallites from highly extended and loose chains. The situation is caused by the microscopic distribution of extension ratio of the chains between crosslink points.