Srivatsan Srinivas

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.

Phase transformation in quenched mesomorphic isotactic polypropylene

Abstract The transformation of mesomorphic phase to α-monoclinic crystal phase in quenched isotactic polypropylene has been investigated by TEM, DSC and time-resolved SAXS and WAXD methods. It is found that even though the initial appearance of the cluster structure in mesomorphic i-PP seems to support the model of a multi-step process for polymer crystallization, results indicate that the transformation is not spontaneous. In the cluster domains, a significant fraction of chain segments must undergo a reorganization process in order to establish the correct registration of helical hands for crystallization. In addition, a fraction of the ordered chains with correct registration of helical hands should also serve as primary nuclei to initiate crystallization. However, the entire cluster domains should not be considered as ‘precursors’. The growth process via secondary nucleation eventually transforms the cluster structure to the lamellar structure.

A simultaneous small- and wide-angle X-ray scattering study of the early stages of melt crystallization in polyethylene

The early stages of isothermal melt crystallization in linear polyethylene (PE) were investigated via simultaneous synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. During these stages, noticeable short-range density fluctuations with periodic spacing from 40 to 80 nm were detected (by SAXS) before the identification of three-dimensional crystal ordering (by WAXD). These results are consistent with the recent findings in several other polymers (polypropylene, poly(aryletherketone), poly(ethylene terephthalate) and poly(butylene terephthalate) by our and other laboratories. Some groups have proposed that the spinodal decomposition due to chain conformation in the molten state may act as a precursor to crystallization based on these findings. Detailed examination of the SAXS and WAXD data, however, indicated that the early stages of crystallization also follow the classical nucleation and growth behavior with a simple Avrami expression. The earlier detection of density fluctuations can be attributed to the lower detection limit of crystallinity in SAXS (0.1%) than in WAXD (1%). In addition, we found that the long spacing associated with the SAXS peak decreased slightly with time, which opposed the behavior of the spinodal decomposition.