Thermal Properties and Three-Phase Structure of cis-1,4-Polybutadiene

The thermal properties of cis-1,4-polybutadiene (cis-PBD) were investigated in dependence of content of cis units and of linearity of the main chain. cis-PBD grades containing a fraction of cis-segments ranging from 93 to 98 %, and with linear or branched chains, were analyzed. Upon isothermal crystallization from the melt at -26 °C for 1 h, followed by cooling to below the glass transition temperature, cis-PBD develops a three-phase structure, where the mobile amorphous phase is equal to 0.41, independently of chain structure, crystallinity ranges from 0.23 to 0.28, and the rigid amorphous fraction varies from 0.31 to 0.35. Up to three main endotherms can be evidenced by calorimetry at slow heating rate, depending on chain regularity. The multiple melting behavior was discussed on the basis of the three-phase structure, as well as of crystallization kinetics.