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 (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

Thermal stability of shear‐induced precursor structures in isotactic polypropylene by rheo‐X‐ray techniques with couette flow geometry

“…) corroborated the earlier observations, not only in PE but also in a variety of other polymers, such as iPP, 43,44,50,[52][53][54][55][56][57][70][71][72][73][75][76][77][78] isotactic poly-1-butene (iPB), 90 polyethylene terephthalate (PET), 91 polydiethylsiloxane (PEDS), 91 etc. In some report, the quasicrystallographic flowinduced structures are referred to as ''mesophase.''…”

“…It was shown that a fraction of PE molecules adopted all-trans sequence configurations upon shearing, which remained stable for several hours even after cessation of shear. The recent results of the techniques that couple flow (parallel plate, [42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57]66,68,[74][75][76][77][78] fiber pullout, 14,79,80 die extrusion 15,[70][71][72][73][81][82][83] ) with a physical probe of the structure development (transmitted light, [70][71][72][73][81][82][83] optical microscopy, 14,[75][76][77][78]79,80 X-ray diffraction, 42-57 atomic force microscopy, …”

Using rheometry to determine nucleation density in a colored system containing a nucleating agent

“…One could think of using other approaches by applying more simple models (Boutahar et al 1996(Boutahar et al , 1998Tanner 2002Tanner , 2003van Ruth et al 2006) or empirically based scaling laws between the space filling and the storage modulus (see Coppola et al 2006;Khanna 1993;Pogodina et al 1999). However, it was demonstrated in Steenbakkers and Peters (2008) that such approaches do perform less than the suspension model as used here, and therefore, we will not apply them to our results.…”

Modeling Flow-Induced Crystallization

“…The wellknown Kolmogorov-Avrami equation [56,57] accurately captures the crystallization kinetics of spherulites, even when their number density is strongly increased by flow [35,58]. However, for shish-kebab structures, in-situ XRD has revealed that the Avrami model often fails [49,[59][60][61].…”

“…The most straightforward way of implementing the proposed effect, requiring the least number of adjustable parameters, is to adjust the viscosity of the material via the modulus. Therefore, following Pogodina et al [58], we express the moduli of the material as…”