The phase transition from tetragonal form II to hexagonal form I was studied for the butene-1/ethylene and butene-1/1,5-hexadiene random copolymers, which have comparable molecular weight but distinct linear ethylene and ringlike methylene-1,3-cyclopentane (MCP) structural co-units, respectively. It is known that this solid phase transition follows the nucleation-growth mechanism, so the stepwise annealing protocol was utilized to investigate the influences of co-units on the optimal nucleation and growth temperatures. Compared with optimal nucleation and growth temperatures of-10 and 35 °C, respectively, in polybutene-1 homopolymer,two butene-1/ethylene copolymers with 1.5 mol% and 4.3 mol% co-units have the slightly lower optimal nucleation temperature of-15 °C but much higher optimal growth temperature of 50 °C. Clearly, the effect of ethylene co-unit is more significant on varying optimal temperature for growth than for nucleation. Furthermore, when the incorporated co-unit is ringlike MCP, the optimal nucleation temperature is-15 °C for 2.15 mol% co-units, the same with above BE copolymers, but-13 °C for a very low concentration of0.65 mol%. Interestingly, the optimal growth temperature of butene-1/1,5-hexadiene copolymers with 0.65 mol%-2.15 mol% MCP counits increases to 55 °C, which is also independent on co-unit concentration. These obtained values of optimal temperatures provide crucial parameters for rapid II-I phase transition.
