D. H. Reneker

Small defects in crystalline polyethylene

Abstract A family of five crystallographic defects of three classes (two dislocations, two dispirations and one disclination) which primarily involve only one polymer chain is described for polyethylene. The extra energy associated with each defect in a perfect polyethylene crystal was computed. The crystal model used consisted of a central chain containing the defect with 18 zig-zag chains in two shells around the central chain. The zig-zag chains each had 60 carbon atoms. The conformation of each defect, placed near the centre of the central chain, was adjusted to minimize the sum of the interatomic interactions. A closely related procedure was used to calculate the energy per chain at boundaries where each chain contains an identical ‘partial’ dislocation or disclination. The characterization of these well defined defects and partial defects greatly simplifies the establishment of connections between atomic and macroscopic scale phenomena in crystalline polyethylene.

Erratum: Energetics of defect motion which transports polyethylene molecules along their axis

Defect energy was calculated as a function of dihedral angles of the bonds in a point dislocation for sequences of conformations that resulted in motion of the dislocation along the polyethylene chain. Paths that presented low barriers to diffusive motion of the defect were found by incrementing, in a particular sequence, selected dihedral angles around two separated bonds near the opposite ends of the defect as the computer searched for the lowest energy conformation of all the other parts of the defect. Thus, the diffusion of a point dislocation provides a plausible mechanism for diffusion of the chain along its axis.

Dispirations, disclinations, dislocations, and chain twist in polyethylene crystals

Abstract It is proposed that the twist in polyethylene chains that can result from crystallization and subsequent deformation aggregates at boundaries and becomes a template for further reorganization that results in the long period observed in polyethylene fibres. The observed lower density at the boundaries requires the transport of free volume to the twist boundaries. Dispirations, disclinations and dislocations are crystallographic defects that are involved in the necessary transport mechanism. Twist and bend, derived from the Eulerian angles which are computed from the sets of chain internal coordinates, relate the orientation of different segments of a chain. Twist and bend are useful for the characterization of both crystallographic defects and arbitrary conformations of polymer chains. Defects, along with folds, chain ends, and ordinary edge and screw dislocations provide a basis for interpretation of structure-property relationships in solid polyethylene.

Effects of defects on the longitudinal acoustic mode of n‐alkane chains

Normal mode calculations show that a single conformational defect in an otherwise all‐trans n‐alkane molecule disrupts the longitudinal acoustic mode (LAM) associated with the all‐trans molecule. Similar calculations for a chain in the conformation produced by smoothly twisting a planar zigzag through 180 ° about the chain axis show that the LAM amplitudes and frequencies are unaffected by this gentle twist. These calculations indicate that the decrease in the LAM intensity observed in some polyethylene samples cannot be accounted for by smoothly twisted chains, but can be accounted for by defects which involve large localized departures from the all‐trans conformation.

Modelling of chain twist boundaries in poly(vinylidene fluoride) as a mechanism for ferroelectric polarization

Abstract It is assumed that the process of ferroelectric polarization of the beta phase of poly(vinylidene fluoride) (PVF2) in response to the action of the external electric field in direction perpendicular to the molecular axis and to the film, involves movement of the chain twist boundaries. These boundaries, at which every chain is twisted by 180 degrees, separate domains of opposite polarization. The energy barriers that are surmounted as the boundary was advanced one repeat unit were calculated and compared with the energy gained by reversing the polarization of an unfavourably oriented repeat unit in an electric field that produces polarization in PVF2. It is suggested that the movement of chain twist boundaries, in contradistinction to previously postulated models in which only one chain is twisted at a time, provides a model for the poling of PVF2 that is feasible energetically and kinetically. Theoretical modelling, analogous to that for Bloch wall that separates domains in magnetic materials, suggest that the process of polarization might be described either as a diffusion process or as the propagation of a soliton along the chains.

Stochastic defect diffusion model for relaxation effects in crystalline polyethylene

Abstract It is suggested that some relaxation processes observed in crystalline polyethylene are consequences of the diffusive motion of a particular defect called a point dislocation or twist dispiration loop along the polyethylene stems in lamellar crystals. The motion of the defect, characterized by a diffusion coefficient and a mobility, is described by solutions of the Smoluchowski diffusion equation with boundary conditions that constrain the defect to move along routes that produce experimentally observable results. The fact that passage of the defect causes both a 180° rotation of the chain and moves an extra CH 2 group in the direction of the chain axis is important to the interpretation of the data according to this model. The diffusion coefficient for a defect is estimated to be around 2 × 10 −9 cm 2 s −1 at 70°C. This value is shown to be reasonable both from the viewpoint of detailed computer modelling of defect motion and contemporary ideas about scaling.

Polyoxymethlene Crystals Grown Within Irradiated Trioxane Crystals

The morphological development of polyoxymethylene crystals polymerized from and grown within trioxane crystals was observed following irradiation of the trioxane with alpha particles and a subsequent heat treatment. A novel method of sample preparation enabled the morphology to be observed in detail and resulted in the discovery of new forms of polyoxymethylene crystals as well as an improved description of previously reported forms. A solid‐gas‐solid crystal growth mechanism was postulated to account for the predominantly acicular habit of the polyoxymethylene crystals.

Distribution of straight-chain lengths in unannealed and annealed solution-crystallized polyethylene by Raman spectroscopy

Abstract The effect of annealing on the morphology of solution-crystallized polyethylene has been studied by analysing the shape of the low-frequency Raman LAM-1 band. The distributions of lengths of straight-chain segments have been determined for samples annealed at different temperatures. Unannealed samples, which have distribution peaks Lmax near 100 A, have halfwidths ΔL 1 2 less than 20 A. However, this narrow distribution is drastically broadened when the sample is annealed. The broadening is less if the breadth of the distribution of the unannealed sample is initially less. For equilibrium crystallized samples, the observed halfwidth and peak position of LAM-1 are found to be related. This relation can be understood quantitatively if it is assumed that ΔL 1 2 and L−1max are linearly related as is indeed found to be the case for solution-crystallized samples. As L−1max becomes very large, ΔL 1 2 approaches a limiting value near 300 A.

Effect of Atomic Oxygen on the Surface Morphology of Polyethylene

Abstract The chemical species created in a low-pressure electrical discharge in oxygen attack the polymer at the surface, converting it to gaseous products. This process is interesting because: 1) the chemical changes on the resulting surface facilitate the formation of strong adhesive bonds and provide sites for the chemical attachment of other molecules, 2) significant morphological features lying below the surface may be revealed, 3) polymer can be cleanly removed from surfaces which are resistant to oxidation, and 4) dielectric breakdown frequently is preceded by the attack on the polymer of chemical species created in a corona discharge. Atomic oxygen is an important chemical species created in such a discharge. It reacts with organic substances rapidly at room temperature, but lives long enough in the low-pressure gas that it can be separated from many other reactive species created in the discharge. “Titration” with NO2 provides a straightforward chemiluminescent means for determining the concentra...

ANNEALING AND MELTING OF POLYOXYMETHYLENE CRYSTALS POLYMERIZED WITHIN IRRADIATED TRIOXANE CRYSTALS.

Morphological changes produced by the annealing and melting of the polyoxymethylene crystals that polymerize inside irradiated trioxane crystals were observed. The crystals in which the polyoxymethylene chains were oriented parallel to the threefold axis of the trioxane underwent major reorganization when held at 184°C for 30 sec. The polyoxymethylene crystals in which the chain axis was inclined at a large angle with respect to the threefold axis of the trioxane transformed and melted at temperatures approximately 5°C lower.