Structure of polystyrene glasses

We present results from detailed atomistic simulations concerning the structural, conformational, dynamic, and barrier properties of the amorphous (glassy and melt) phases of two polyisomers, PET [poly(ethylene terephthalate)] and PEI [poly(ethylene isophthalate)]. First, well-relaxed atomistic configurations of the two polyesters are generated following a succession of equilibration stages consisting of energy minimizations, temperature annealings and coolings, and compressions−decompressions, as proposed by Hofmann et al. (Hofmann D.; et al. Macromol. Theory Simul. 2000, 9, 293). This equilibration cycle is significantly extended here by subjecting the resulting configurations to an additional molecular dynamics (MD) simulation at a high temperature for 2 ns, affording extra relaxation at all length scales. With the statistically uncorrelated configurations generated via these extended equilibration runs, isothermal−isobaric (NPT) at P = 1 atm as well as canonical (NVT) (with the densities set at the co...

Detailed Atomistic Simulation of the Segmental Dynamics and Barrier Properties of Amorphous Poly(ethylene terephthalate) and Poly(ethylene isophthalate)

In this chapter we review flow-induced mesophases in some crystalline polymers with rather differentcharacteristics: polyethylene terephthalate, isotactic polypropylene and polydiethylsiloxane. Becausethese three polymers are representatives of flexible and semi-rigid-chain polymers, the occurrenceof a mesophase suggests a similar behaviour for many other crystalline polymers. The questionmay be not so much whether crystalline polymers show mesophase behaviour, but rather under which conditionsit can be obtained. A flow field turns out to be an effective tool to unravel structural information,even if hidden in quiescent conditions. The emergence of a mesophase affects strongly any subsequentcrystallization. As a result of templating and nucleating effects, a mesophase may acceleratethe crystallization rate, lead to different crystal modifications, change the morphology and guidethe orientation of the crystals. On the basis of mesophase formation new insights into flow-inducedpolymer crystallization emerge, even though the nature of the mesophase ordering is still not fullyunderstood. A re-evaluation of models for the polymer melt (random coil and folded-chain fringed-micellargrains) is proposed to understand the combined experimental observations on mesophase ordering andcrystallization.

Flow-induced mesophases in crystallizable polymers

A conformational energy model has been set up for poly(ethylene terephthalate) (PET) for use in molecular dynamics simulations. The molecular dimensions, as expressed by the characteristic ratio, and the dipolar correlation factor of PET are key parameters in the simulations because they depend very differently on the conformational characteristics of the individual bond types in the chain. These parameters as well as the equation of state of the melt were emphasized in testing the model, and satisfactory representation was accomplished. It was found, however, difficult to achieve the experimental glass volume in MD cooling experiments below Tg. Packing features of bulk PET are discussed in terms of radial distribution functions. Dipolar correlation was found to be largely intramolecular in nature.

Molecular Dynamics Simulation of Amorphous Poly(ethylene terephthalate)

This is a review of atomic pair distribution function (PDF) analysis as applied to the study of molecular materials. The PDF method is a powerful approach to study short- and intermediate-range order in materials on the nanoscale. It may be obtained from total scattering measurements using X-rays, neutrons, or electrons, and it provides structural details when defects, disorder, or structural ambiguities obscure their elucidation directly in reciprocal space. While its uses in the study of inorganic crystals, glasses, and nanomaterials have been recently highlighted, significant progress has also been made in its application to molecular materials such as carbons, pharmaceuticals, polymers, liquids, coordination compounds, composites, and more. Here, an overview of applications toward a wide variety of molecular compounds (organic and inorganic) and systems with molecular components is presented. We then present pedagogical descriptions and tips for further implementation. Successful utilization of the method requires an interdisciplinary consolidation of material preparation, high quality scattering experimentation, data processing, model formulation, and attentive scrutiny of the results. It is hoped that this article will provide a useful reference to practitioners for PDF applications in a wide realm of molecular sciences, and help new practitioners to get started with this technique.

Structural Analysis of Molecular Materials Using the Pair Distribution Function.

Wide-angle x-ray scattering (WAXS) of glassy PET and two samples annealed at 190°C for 15 and 30 min were analyzed by differential radial distribution function (DRDF) methods. The corrected intensity profile of the glassy sample show three peaks at 4.19, 2.13, and 1.19 A. The sample annealed for 15 min shows splitting of the 4.19 A peak into three peaks at 3.50 (d100), 3.96 (d110), and 5.21 (mainly d010 and unresolved d011-and d-111) A. Longer annealing for 30 min produces two more peaks at 4.11 (d-111) and 5.40 (d011) A. Annealing does not affect the peaks at 1.19 and 2.13 A (which are caused by first and second nearest atoms in the chain). The DRDFs of all three samples are similar in some respects as all show intrachain peaks at ∼ 1.45, 2.45, and 4.05 A. From theoretical calculations of the intrachain distances and their contributions to the DRDF, the peak at the 4.90 A in the glassy sample was found to be mainly due to intermolecular distances. Since the peaks above 4.9 A are relatively weak,...

DRDF studies of glassy and partially crystalline poly(ethylene terephthalate)

The structure of amorphotized polyethylene (APE) obtained by irradiation of branched PE was studied by means of differential radial distribution function (DRDF) analysis of its WAXS. The experimental DRDF shows intramolecular peaks at 1.5 and 2.5 A and intermolecular peaks at 5.3, 9.8, 14.7, 18.6 and 23.4 A The experimental DRDF was compared with a synthetic DRDF based on the application of paracrystalline lattice theory to a distorted hexagonal lattice. The synthetic DRDF with the best fit has lattice constants, a = 4.94 and c = 2.54 A, and the fluctuation parameters of g11 ≃ g12 ≅ 18%, g13 ≃ g31 ≅ g32 ≃ 6%, and g33 ≃ 10%. Our analysis suggests that a hexagonal lattice, albeit distorted, is still present in the amorphotized state of PE. The extent of lateral order is estimated to be about 50 A, as compared to about 30 A for molten PE.

Mani R. Gupta

Papers

DRDF studies of glassy and partially crystalline poly(ethylene terephthalate)

Application of paracrystalline lattice theory to the analysis of differential radial distribution function of amorphotized polyethylene