Strain effects on physical gelation of crystallizing isotactic polypropylene

Abstract

Early stages of crystallization of polymers may be viewed as thermoreversible physical gelation in which molecular connectivity is introduced by crystallization. Effects of shear strain on the early stages of crystallization of a commercial isotactic polypropylene are studied by dynamic mechanical experiments. Shear creep with large strains (up to γ = 300) on the undercooled melt for short times (the time did not exceed 100 s) was followed by small amplitude oscillatory shear (SAOS) at a strain amplitude (γ a = 0.01) for gel-point detection. The imposed shear strongly accelerates gelation; gel times decrease in a power law with increasing strain. Strain applied during the crystal growth stage enhances gelation much stronger than strain applied in the earlier nucleation stage. For rapid gelation, frequency sweeps are not possible and new methods for gel-point detection need to be explored; here, we propose to estimate the gel point from the storage modulus growth at a single frequency. A value of 10% of the total growth of G' was found to be a good estimate for the gel point. High strain experiments show the complexity of underlying mechanisms of strain-enhanced crystallization and reveal at least two sequential stages in the structure development under shear: at the first stage, crystalline regions connect molecules into a loose network; at the second, stage-intense crystallinity growth within the network proceeds. Results have industrial importance in predicting/tuning structure development and connectivity growth during nonisothermal processing. Morphological study of the early stages of crystallization under strain is underway to explore molecular mechanisms, which govern the gelation process.