Melting transitions in diene polymers

The melting transitions of natural rubber, cis‐polyisoprene, gutta‐percha, balata and synthetic trans‐polyisoprene, and cis‐polybutadiene have been examined by means of differential thermal analysis. Natural rubber melts over a wide range of temperature giving several transitions, the magniture and temperature of which depend on the experimental conditions. At heating rates feasible for differential temperature measurements the values observed are markedly below the equilibrium melting points. Natural and synthetic trans‐polyisoprenes possess similar melting points and extents of crystallinity. The ratio of the peaks associated with the α and the β forms is not only dependent on the rate of cooling and temperature of crystallization, but also on the temperature to which the sample was heated before crystallization. This is thought to be due to the persistance of crystalline nuclei in the polymer melt. Under certain conditions a small melting transition occurs at a lower temperature. Synthetic cis‐polyisoprenes show melting characteristics similar to those of natural rubber, but the upper transition is lower than that of the natural product and correlates with the cis content of the polymer. cis‐Polybutadienes melt slowly over a wide range of temperature below the characteristic melting transition which is dependent on the cis content. The highest melting point observed for a polymer of 98% cis content was +1°C.