Jianning Liu

Morphology and crystal structure in single crystals of poly(p-phenylene terephthalamide) prepared by melt polymerization

Using the confined thin film melt polymerization technique lamellar single crystals, of two morphologies, have been grown for poly(p-phenylene terephthalamide). Electron diffraction (e.d.) patterns from these crystals and from fibre-like samples polymerized from sheared monomer, are in best agreement with a modified phase I cell. The lattice parameters (a = 7.88, b = 5.22, c = 12.9 A; α, β, γ = 90°) are similar to those in previously proposed cells, but the Pla1 space group rather than Pn or P21/n space groups more closely fits the e.d. data. In particular, 210, 120, 320 and 410 reflections are present with moderate intensity, which are forbidden for the latter structures and do not result from double diffraction.

Fingerprinting Molecular Relaxation in Deformed Polymers

The flow and deformation of macromolecules is ubiquitous in nature and industry, and an understanding of this phenomenon at both macroscopic and microscopic length scales is of fundamental and practical importance. Here we present the formulation of a general mathematical framework, which could be used to extract, from scattering experiments, the molecular relaxation of deformed polymers. By combining and modestly extending several key conceptual ingredients in the literature, we show how the anisotropic single-chain structure factor can be decomposed by spherical harmonics and experimentally reconstructed from its cross sections on the scattering planes. The resulting wavenumber-dependent expansion coefficients constitute a characteristic fingerprint of the macromolecular deformation, permitting detailed examinations of polymer dynamics at the microscopic level. We apply this approach to survey a long-standing problem in polymer physics regarding the molecular relaxation in entangled polymers after a large step deformation. The classical tube theory of Doi and Edwards predicts a fast chain retraction process immediately after the deformation, followed by a slow orientation relaxation through the reptation mechanism. This chain retraction hypothesis, which is the keystone of the tube theory for macromolecular flow and deformation, was critically examined by analyzing the fine features of the two-dimensional anisotropic spectra from small-angle neutron scattering by entangled polystyrenes. It is shown that the unique scattering patterns associated with the chain retraction mechanism were not experimentally observed. This result calls for a fundamental revision of the current theoretical picture for nonlinear rheological behavior of entangled polymeric liquids.

Failure behavior after stepwise uniaxial extension of entangled polymer melts

This work studies how stepwise extension of various well-entangled polymer melts produce mechanical/structural breakdowns during stress relaxation. Depending on how stepwise extension is imposed on five different styrene-butadiene random copolymers, two different forms of specimen failure are observed. When a step extension is produced with a low Hencky rate or to a low strain below some thresholds, the sample breaks up rather sharply after an appreciable period of induction during which the stress relaxes quiescently. After step extension, the sample draws and undergoes unsustainable necking due to shear yielding, if the step extension is produced with a Hencky rate higher than the Rouse relaxation rate and the magnitude is beyond a Hencky strain of 1.5. Moreover, introduction of long-chain branching suppresses the elastic breakup, postponing it to Hencky strains beyond 2.5. The clearly identifiable characteristics of the elastic yielding may be understood in terms of some speculative interpretations. More convincing explanations have yet to come from future computer experiments that hopefully the present work is able to motivate.