A.P. Unwin

Experimental and theoretical longitudinal chain moduli of copolymers of hydroxybenzoic acid with 2-hydroxy-6-naphthoic acid

Abstract The changes with stress of the meridional X-ray scattering patterns from liquid crystalline copolymers of hydroxybenzoic acid with hydroxynaphthoic acid have been used to measure the average chain moduli. The values obtained have been compared with theoretical estimates of the chain moduli and in general found to be greater than the latter at low temperatures. It is concluded that, although an isolated chain has a comparatively low modulus, in the polymer environment the need to overcome intermolecular shear forces results in an enhancement of the modulus. The importance of the shear modulus in determining sample properties is discussed and the conformational implications for the copolymer molecules are examined.

On the influence of initial morphology on the internal structure of highly drawn polyethylene

Morphological investigations on a range of polyethylenes, which differ with respect to their molecular weight characteristics and which possess quite distinctly different superstructures, non-spherulitic and banded spherulitic, have been performed to follow structural developments during drawing. Of particular significance is the detailed examination of transverse sections, in addition to the more commonly undertaken longitudinal studies. The key point that emerges is the retention of a memory of the original superstructure by the highly drawn products. This is strikingly evident in the lateral morphology, which clearly transforms in a continuous fashion, in contrast to the axial morphology, which is transformed out of all recognition by drawing.

An aggregate model for random liquid crystalline copolyesters

Abstract Several aromatic liquid crystalline copolyesters with different compositions have been examined in an attempt to reach a molecular understanding of their mechanical properties. This has been aided by use of a simplified aggregate model and by theoretical calculations. Replacement of hydroxynaphthoic acid residues by straighter intrinsically stiffer biphenyl residues enhances the chain modulus. At the same time the macroscopic shear modulus drops, presumably because the straighter chains can move relative to one another more easily. The tensile compliance of the sample depends on both these properties with the contribution from shear decreasing with increasing orientation. Improved molecular orientation caused by straightening of chains or fibrils probably explains the stress dependence of the extensional moduli of these materials.

Remnant morphologies in highly-drawn polyethylene after annealing

Abstract Specimens of the polyethylene Sclair 2907, full of large banded spherulites, have been drawn to high extensions, and parts of these annealed close to their melting points under constraint. Etching with permanganic reagents and examination under the scanning electron microscope revealed that in transverse sections of the unannealed specimens, the legacy of the banded spherulitic morphology is seen at draw ratio 10× but appears to have been overwritten by subsequent developments at 20× and 33×. However, after annealing, the specimens show recrystallized regions which follow the pattern of the original banded spherulites drawn affinely. This is so even for 33× drawn specimens where the unannealed longitudinal morphology has been overwritten by a new structure of density-deficient regions arranged in parallel bands. It is inferred that different deformation mechanisms operate on bundles of lamellae oriented differently in the original material, and that legacies of the original morphology may be retained at much higher deformations than previously assumed, but they may require appropriate treatments to make them visible.

The structure and mechanical properties of syndiotactic polypropylene

The drawing behaviour of several syndiotactic polypropylenes is examined as a function of molecular weight and initial morphology. It is found that these materials can be drawn to a much lesser extent than comparable isotactic polypropylene. This limited drawability is attributed to the absence of an effective crystallinec-slip process, a view supported by dynamic mechanical measurements. Highest draw ratios are achieved in gel spun high molecular weight material where the degree of molecular entanglement is low enough to suppress the undesirable increase in drawing stress with extension.

The determination of molecular orientation in oriented polypropylene by wide angle X-ray diffraction, polarized fluorescence and refractive index measurements

Abstract Orientation studies have been undertaken in uniaxially oriented drawn tapes of isotactic polypropylene, using the techniques of wide angle X-ray diffraction, polarized fluorescence and optical birefringence. The results have been interpreted on the basis of a simple two-phase model for orientation of the crystalline and noncrystalline regions. The rise in the orientation of the crystalline regions with increasing draw ratio is extremely rapid, even compared with the pseudo-affine deformation scheme. This result, taken in conjunction with the discontinuous change in the long period, supports the view that the plastic deformation involves a complete breakdown of the original lamellar structure and subsequent recrystallization of the highly oriented drawn material. Further support for this conclusion is obtained from the observation that the orientation of the long fluorescent probes, which might be expected to be constrained to the more highly oriented material, is similar to that measured for the crystalline material by X-ray diffraction.

The role of pressure annealing in improving the stiffness of polyethylene/hydroxyapatite composites

The effect of the combination of pressure annealing and subsequent hydrostatic extrusion on some mechanical properties of composites of hydroxyapatite in a polyethylene matrix is examined. Both linear and branched polyethylenes have been used as the matrix and it is found that composites with the former can be processed to give the higher modulus. An important practical finding is that products with a Youngs modulus well into the range shown by cortical bone can be produced. The critical step in the enhancement of the modulus is pressure annealing which alters the morphology of the linear polyethylene, encouraging the development of both crystallinity and crystallite size. The presence of butyl branches along the molecular backbone limits the extent to which these can be developed by pressure annealing and accounts for the failure of the process to improve the modulus of composites using the branched material. Comparison with similarly prepared samples of the pure polyethylenes shows that the development of orientation in the polyethylene is considerably restricted by the presence of hydroxyapatite particles, irrespective of whether pressure annealing is performed prior to extrusion. Consequently, the properties of these composites are less than might be expected from studies on the isolated polyethylenes.

Morphological changes in pressure annealed polyethylene

Polyethylene samples have been annealed under high pressures in order to transform the morphology and thereby make specimens, which on subsequent orientation by hydrostatic extrusion, produce high-modulus materials for comparatively low deformation ratios. The pre-extrusion morphology, as revealed by permanganic etching followed by transmission electron microscopy, is shown to be highly sensitive to the annealing conditions, especially in the vicinity of the orthorhombic/hexagonal phase boundary. The morphological changes have also been studied by gel permeation chromatography of nitrated samples and by differential scanning calorimetry. The moduli of samples, extruded to a standard extrusion ratio of 7, reflect differences in the morphology of the starting material. It is shown that the highest modulus is not obtained by annealing totally within the hexagonal phase but rather in the changeover region between it and the orthorhombic phase. Although the lamellar size can be dramatically increased by annealing within the hexagonal phase, this development occurs at the expense of reducing the integrity of the molecular network so that for high lamellar size the modulus falls and the samples become increasingly incoherent.

The effect of high pressure annealing on the molecular network in polyethylene

The changes in the molecular network of several grades of polyethylene subjected to high pressure annealing are followed using a simple model which combines thermal expansion and mechanical measurements. It is found that annealing reduces the degree of molecular entanglement, particularly when performed well within the intermediate hexagonal phase. It is shown that this process can be used to improve the drawability and hence the ultimate physical properties of high molecular weight material. In low molecular weight material it is necessary to retain a sufficiently intact network which can orient the crystallites effectively and which prevents the material from becoming friable.

The mechanical behaviour of oriented high-pressure annealed polyethylene

Four different grades of polyethylene have been annealed under high pressures, to transform the morphology, and then oriented by hydrostatic extrusion. The pre-extrusion morphology is shown to have a significant influence on the mechanical properties of the extrudates. At high crystallinities the mechanical behaviour of the extrudates can be interpreted using a simple aggregate model. This model suggests that high mechanical moduli can be obtained by orienting the crystallites more effectively and by increasing the crystallinity, which results in a higher shear modulus. However, the developments of crystallinity and lamellar size occur at the expense of reducing the integrity of the molecular network. Eventually this limits the efficiency of the orientation process and has a detrimental effect on the shear modulus and hence the extensional modulus of these oriented materials.