The melting process and the rigid amorphous fraction of cis-1,4-polybutadiene

A thorough analysis of the melting behavior of cis-1,4-polybutadiene (cis-PBD) is detailed in this contribution. Isothermal crystallization at 26 C, followed by cooling, provides a three-phase structure composed of a mobile amorphous fraction equal to 0.413, a crystallinity of 0.277, and a rigid amorphous fraction of 0.310. Similar to many other polymers, cis-PBD displays multiple melting after isothermal crystallization, and up to three main endotherms can be evidenced by calorimetry, in dependence of the scanning rate. The results of conventional and temperature-modulated calorimetry analyses presented in this contribution suggest a link between multiple melting and devitrification of the rigid amorphous fraction in cis-PBD. The small endotherm located a few degrees above the crystallization temperature appears to be caused by concurrent partial mobilization of both the crystal and the rigid amorphous portions. Additional partial mobilization of rigid amorphous segments seems to take place at around 11 C, and it is only above this temperature that large reorganization of the crystal phase becomes possible, allowing partial melting and recrystallization/annealing/crystal perfection.