Structure of solution-grown trans-1,4-polyisoprene crystals: 2. D.s.c. studies of crystal form transformation

Trans-1,4-polyisoprene (TPI) crystals were grown from hexane and amyl acetate solutions at various temperatures (−30 to 35°C) for periods of 0.5 to 263 h. The melting temperature (Tm) of the crystals was independent of the crystallization temperature (Tc) up to 5°C for hexane solution and 15°C for amyl acetate solution. Above these temperatures (Tr), Tm increased with Tc. On the other hand, below Tr a mixture of the α- and β-forms was obtained and above Tr only the α-form crystals were obtained. The transformation from the β- to the α-form was found to occur during crystallization. In the samples crystallized at 0°C, cyclic transformation between the two forms was observed. The inverse transformation from the α- to the β-form was observed during storage at room temperature for TPI crystals grown from amyl acetate solution at temperatures below Tr. This transformation was also observed for naturally occurring TPI (gutta percha and balata) crystals.