Shear-Induced Conformational Ordering, Relaxation, and Crystallization of Isotactic Polypropylene

Abstract

The shear-induced coil-helix transition of isotactic polypropylene (iPP) has been studied with time-resolved Fourier transform infrared spectroscopy at various temperatures. The effects of temperature, shear rate, and strain on the coil-helix transition were studied systematically. The induced conformational order increases with the shear rate and strain. A threshold of shear strain is required to induce conformational ordering. High temperature reduces the effect of shear on the conformational order, though a simple correlation was not found. Following the shear-induced conformational ordering, relaxation of helices occurs, which follows the first-order exponential decay at temperatures well above the normal melting point of iPP. The relaxation time versus temperature is fitted with an Arrhenius law, which generates an activation energy of 135 kJ/mol for the helix-coil transition of iPP. At temperatures around the normal melting point, two exponential decays are needed to fit well on the relaxation kinetic of helices. This suggests that two different states of helices are induced by shear: (i) isolated single helices far away from each other without interactions, which have a fast relaxation kinetic; (ii) aggregations of helices or helical bundles with strong interactions among each other, which have a much slower relaxation process. The helical bundles are assumed to be the precursors of nuclei for crystallization. The different helix concentrations and distributions are the origin of the three different processes of crystallization after shear. The correlation between the shear-induced conformational order and crystallization is discussed.

Ordered long‐helical conformation of isotactic polypropylene obtained in constrained environment of nanoclay

“…As shown by the literature, Fourier transform infrared (FTIR) spectroscopy is an effective approach for in situ investigation of crystallization processes in semicrystalline polymers. For example, the crystallization and/or melting process of isotactic polypropylene (iPP) has been well described by in situ FTIR experiments;13, 14 the exact correlation between special regularity band and helical sequence length (monomeric units in helix sequence) has been established;15, 16 and FTIR was even used to inspect shear‐induced conformational ordering of iPP 17. In addition, FTIR has been employed to evaluate nanoclay intercalation/exfoliation degree,18 orientation structure,19, 20 and displacement of tensile deformation21 in PLSN.…”

A few rediscovered and challenging topics in polymer crystals and crystallization

“…Based on the above experimental observations, a schematic multi‐scale ordering process for flow‐induced crystallization was proposed by Su et al (see Figure ), which showed a sequential ordering process as the conformational order‐density fluctuation‐preordering‐crystallization. Imposing flow on entangled polymer melts leads to intra‐chain conformational ordering or coil‐helical transition, which has been demonstrated by both theoretical predictions and experimental observations mainly from in‐situ spectroscopic methods . Flow‐induced coil‐helical transition has also been confirmed by molecular dynamic computer simulations .…”

Correlations between microstructure of α‐row nuclei and polymorphism of shear‐induced iPP/carbon fiber cylindrite

“…With the growth of α‐row nuclei, the β‐iPP nuclei formed at the front of a single fibril lamella through the α‐ to ‐β transition. It has been reported that the occurrence of the α‐ to ‐β transition is due to the reversal of the helical conformation . The initial β‐iPP lamellae proliferate through branching and result in the formation of a fan‐shaped β‐iPP region.…”