Rheology and structure of isotactic polypropylene near the gel point: quiescent and shear-induced crystallization

Abstract

Crystallizing isotactic polypropylene (iPP) develops large-scale spherulites and thick threads, large enough for observation by optical microscopy, and undergoes a liquid-to-solid transition as an expression of increased connectivity of the structure. In order to relate the time scales of structural and rheological changes, we measured time-resolved small-angle light scattering (SALS) and transmittance properties in a single experimental run, which then was repeated in an optical microscope for direct observation of growth of large-scale structures, and in a rheometer for mechanical spectroscopy. The results for quiescent and shear-enhanced melt crystallization of a high molar mass iPP are presented. In quiescent crystallization, iPP nuclei are only formed in the beginning (and not in later stages) and grow simultaneously at the same rate, which leads to spherulites of equal size. The critical gel point is reached close to the instant of the maximum of density fluctuations, but before spherulites impinge. Crystallinity estimates from Hv SALS (estimation method of Stein) are much higher than values from DSC. The discrepancy may be caused (in addition to the simplifying assumptions in the estimate) by the enhanced crystallization in the rheo-optical cell due to surface and sample loading effects. Shear flow induces anisotropic molecular conformations, preferably in the high molecular weight component of the iPP sample. The resulting orientation fluctuations (of highly oriented long chains and less oriented short chains) cause (1) an increase in nucleation rate, (2) possibly an increase in crystallization rate and (3) formation of highly elongated structures (threads) which are visible in the optical microscope and in anisotropic SALS patterns. The threads thicken until, at later stages, additional spherulites start to grow, presumably from the shorter chains and nucleated on by the threads.