Crystallization kinetics of polypropylenes. Effect of nucleating agents ?

Thermal conditions and formulation affect the crystallization mechanisms of polymers and the associated kinetics in a coupled manner. In that field, the objective of this investigation is to compare overall crystallization kinetics and structural organization of one clarified polypropylene (specifically designed for stretch-blow molding) and a homopolypropylene. Liquid/solid transitions are investigated in- and ex-situ under isothermal and non-isothermal conditions combining crossed-polarized optical microscopy, differential scanning calorimetry and X-ray diffraction. Clarified polypropylene has a very 'singular behavior' compared to homopolymer since no spherulites can be observed. However, it exhibits a semi-crystalline structure. The major alpha-phase coexists with some gamma-phase even under quiescent conditions. Overall crystallization kinetics is rapid suggesting the existence of very efficient nucleating agent(s) and resulting in an increase of crystallization temperature. In parallel, the melting temperature of copolymer decreases by 20 degrees C compared to homopolymer, suggesting a drastic change in lamellae thickness. It is concluded that this unusual structure results from nucleation, which enforces high temperature crystallization, and copolymerization, which constraints the crystalline organization.