Early stages of crystallization of polymers may be viewed as physical gelation. This is shown with four commercial isotactic polypropylenes, which have been studied by dynamic mechanical experiments at low degrees of undercooling, ∆T ) 10-26 K, below their nominal melting temperature. The physical gel point is manifested by slow power law dynamics, which expresses itself in a shear relaxation modulus G(t) ) St -n at long times, λ0 < t < λpg, where S is the gel stiffness, n is the relaxation exponent, λ0 is the crossover to short time dynamics (entanglements, glass modes), and λpg is the longest relaxation time, which can be considered to be infinite for our experiments due to the long lifetime of the physical bonds. The time to reach the gel point (gel time tc) decreases exponentially with ∆T, and the critical gel becomes stiffer (smaller n, larger S) with increasing ∆T. The absolute critical crystallinity at the gel point, Xc, was found to be only about 2% or less. This value was determined from published DSC data which, however, needed to be extrapolated to tc, as measured by mechanical spectroscopy. This very low crystallinity suggests that only a few junctions are necessary to form a sample spanning network. The network in this case is "loosely" connected, and the critical gel is soft.

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Studies of dilute polymer solutions in shear flow suggest that the mean fractional extension of molecules increases gradually with the Weissenberg number (Wi = shear rate × longest relaxation time) and approaches an asymptotic value of 0.4−0.5, while in elongational flow it approaches full contour length above a certain critical strain rate. In an entangled polymer melt, this behavior is more complex due to inter- and intramolecular interactions. In situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide-angle X-ray diffraction) experiments were performed to investigate the effects of shear rate, shear duration, and Wi on the extent of molecular orientation/extension and crystal orientation in an isotactic polypropylene (iPP) melt. Two series of experiments were designed:  (1) variation of shear rate (30, 45, and 60 s-1) at a constant shear duration (5 s) and (2) variation of shear duration (1.3, 3, and 5 s) at a constant rate (60 s-1). The degree of crystal orientation (Herman's orientation function, f) observed at 165 °C and fraction of oriented crystals (X o) observed in a fully crystallized sample at room temperature increased with both shear rate and shear duration. Interestingly, at a constant strain (rate × duration), short-duration shear at a high rate was found to be more effective (i.e., higher f and X o) than long-duration shear at a low rate. The longest relaxation time for the iPP sample and Wi were estimated from the dynamic moduli data. Both f and X o were found to gradually increase with Wi and approached plateau values at high values of Wi. Results indicated that, even under a very intense shear field (or high Wi values), molecules do not extend to full contour length, and there is a limiting value for mean orientation/extension and subsequent crystal orientation in a polymer matrix. Characteristic dimensions of the shish-kebab entity formed in a sheared iPP melt at 165 °C were determined from the rheo-SAXS data. It was found that the average shish length was 700−750 nm and the average spacing between adjacent kebabs was 60−70 nm.

Structure development during crystallization of isotactic polypropylene (iPP) at low undercooling is studied by small-angle light scattering (SALS) and light transmission. The structure development is related to network formation (gelation) as previously measured on the same sample in dynamic mechanical experiments. Close to the rheologically measured gel point, small-angle light scattering (SALS) patterns in both Hv and Vv modes show circular symmetry; density fluctuations (maximum in Vv invariant) as determined by SALS go through a maximum. Orientation fluctuations develop much more slowly and appear at much later stages of the crystallization process. Growing clusters are characterized by very low anisotropy and low internal crystallinity. The characteristic length scale associated with the cluster at early stages is about 1 µm. At later times, evolution of 2-fold and 4-fold symmetry patterns suggests the development of the anisotropic superstructures. The growth of crystalline clusters leads to a strong increase in turbidity as monitored by light transmission in the Vv mode.

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 (γa = 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. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3512–3519, 1999