Nucleation activity at high supercooling: Sorbitol-type nucleating agents in polypropylene

Crystallization kinetics of polypropylene (PP) nucleated with sorbitol-type nucleating agents was studied withfast differential scanning calorimety (FDSC) at high supercooling and correlated to the optical characteristics ofmold injected parts. FDSC was used for determination of the critical cooling rate for occurrence of non-isothermalcrystallization and the time to maximum crystallization rate during isothermal crystallization after rapidcooling. The nucleating agents studied were 1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol (DMDBS) and 1,2,3-tridesoxy-4,6:5,7-bis-O-[(4-propylphenyl) methylene]-nonitol (TBPMN). In contrast to particulate nucleatingagents, where the number of particles determines the number of heterogeneous nuclei, sorbitol-type nucleatingagents self-assemble a nano-fibrillar super-structure by phase separation of sorbitol-type molecules that werepreviously dissolved in the polymer melt. An approach is proposed to describe the efficiency of nucleating agentsself-assembly. In combination with haze data two different crystallization mechanisms are identified, which areinterpreted as three-dimensional spherulitic crystallization at high haze and one-dimensional (rod or shishkebab-like) crystallization at low haze.