1994 рік

In the present work the thermally stimulated currents technique was used to study the upper glass transition relaxations in two side-chain liquid crystalline polysiloxanes. The nature of those relaxations at the molecular level is a controversial subject, but our results lead us to suggest thay they correspond to the reorientational motions in the liquid crystalline phase of the longitudinal and transverse components of the dipole moment of the mesogenic side group. This assignment is compatible with the conclusions from extensive studies of the dipolar relaxation behaviour of side-chain liquid crystalline polymers by dielectric relaxation spectroscopy.

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A variety of diblock copolymers containing a high density polyethylene block were synthesized directly by a transformation reaction from an anionic to a Ziegler-Natta type polymerization. A wide range of reaction parameters were investigated and optimized. Efficiencies of conversion of the anionic first block to diblock were as high as 33% (100% normalized), consistent with the known 3/1 Li-R/Ti relationship in the active catalyst species. The activity of the polymeric Li-R/Ti catalyst with regards to ethylene polymerization was as high as 40 000 g of polymer per mole Ti. Molecular characterization of the resultant polymers is consistent with diblock copolymer structures, especially the characteristic microphase-separated structure exhibited in transmission electron micrographs. Detailed microscopic and X-ray analyses show distinct differences, however, between the morphological behaviour of these materials and the now well established morphological behaviour of amorphous-amorphous diblock copolymers. These differences are readily explained by consideration of the interactions between the competing thermodynamic processes of microphase separation and crystallization in these copolymers.

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In this work, nylon-6,6/Kevlar short fibre composites have been processed by injection moulding. Their microstructure, and fibre and matrix orientation, measured by wide-angle X-ray diffraction (WAXD), have been studied as a function of two major processing variables, i.e. the injection speed and the mould temperature. At low mould temperatures, skin, underskin and core structures have been observed, while at high mould temperatures only skin and core structures were found. Fibre orientation patterns exhibit both a processing and a geometrical dependence. Matrix orientation patterns are different from those of the pure polymer moulded material and are mainly influenced by the fibre orientations, which are themselves affected by the processing conditions

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A comparison of band dichroism in the transmission and attenuated total reflection infra-red spectra of poly(phenyl-p-phenylene terephthalate) fibres, prepared by jet drawing, showed low molecular orientation in a surface layer of about 1 pm thickness

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Values of intrinsic viscosity I-q] and Huggins' constant K n were measured for cellulose, polyacrylonitrile and their mixtures in dimethylacetamide/7% LiC1. The trend of [q] is linear with the weight fraction of the two polymers. The Huggins' constant values correspond to the case of ideal mixtures. Observations are made concerning deviations which arise in results obtained at higher concentrations.

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The community of scientists and engineers associated with composite materials has reached a level of understanding of composite material systems such that we can now consider how composite material systems should be made, not just how they can be made. In particular, we can now address this question not only for quasi-static stiffness characteristics, but also in order to achieve long-term performance characteristics such as desired remaining strength and life, resistance to environment, and reliability and safety. However, this opportunity presents challenges that are substantial and complex. The present paper addresses an approach to the problem of combining our understanding, and our fundamental science and technology, in such a way that we can make such global predictions with reasonable accuracy for engineering utility. The approach proposed is the use ofmicromechanical models that represent the strength of composite material systems in terms of the parameters that control the manner in which the systems are synthesized and manufactured.

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We exploit the utility of infra-red dichroism for studying the orientation in thin films of side-chain liquid crystalline polymers (LCPs). Mechanical stretching of these polymers is difficult to perform because of difficulties in preparing the actual film samples necessary for the experiments. The method we used in this study consists of casting a thin LCP film onto the surface of a supporting conventional polymer film which can easily be prepared and stretched over a wide temperature range. Three nematic polyacrylates were investigated in this way, using poly(vinyl alcohol) as the supporting polymer. It is shown that stretching of the LCP film is sustained by extension of the supporting film and, similar to cross-linked nematic elastomers, an order parameter of approximately 0.4 for the mesogenic groups, determined by infra-red dichroism measurements, is obtained at a draw ratio as small as 1.5, i.e. only 50% deformation, indicating a rapid macroscopic alignment of the nematic domains along the mechanical field direction. Comparisons show that the mechanical stretching is particularly efficient for obtaining high orientation in side-chain LCPs having a short flexible spacer, the orientation of which is difficult to develop in a magnetic field, revealing an opposite influence of the length of the flexible spacer on the alignment of the nematic domains in both magnetic and mechanical fields. The mechanism of the orientation process in this stretching method is also examined. It is proposed that the stress field is essentially transferred to the nematic polymer through the interface between the nematic and supporting films, and that the alignment of the nematic domains, which starts in the interfacial region, could propagate throughout the sample by virtue of a strong cooperative movement among the nematic domains. The efficiency of the alignment propagation is higher for LCPs containing a short flexible spacer because of the stronger couplings between the chain backbone and the mesogenic groups.

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Bisphenol A (BPA) carbonate cyclic oligomers have been found to be miscible with a wide spectrum of glassy polymers from polystyrene to aromatic polysulfones. The surprising breadth of the cohesive energy density (CED) range corresponding to this spectrum is roughly comparable to the breadth of the CED range of small molecule solvents that swell a given glassy polymer heavily, and much smaller than the breadth of the CED range of miscibility seen in a polymer/polymer blend series. Possible sources of this behaviour are indicated

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The structure and temperature-induced structural changes of three polyamides, PA 6, PA 6.6 and PA 6.10, were investigated by X-ray diffraction. In all three polyamides a reduction of the distance in the hydrogen-bonded sheets was measured. A discontinuity in the temperature dependence of the d-spacings dl o0 and dl ~ 0 - 0 ~ 0 was found only in PA 6.10 and 6.6. This can be interpreted by a change of the arrangement of the hydrogen bonds. Because of the existence of two equatorial peaks up to the melting point in PA 6 and PA 6.10, a non-statistic distribution of the hydrogen bonds can be assumed. In PA 6 the planar arrangement of the hydrogen bonds is maintained.

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Double torsion tests were used to evaluate the fracture toughness of a brittle organic network, poly[diethylene glycol bis(allyl carbonate)] commonly known as CR-39 resin, and to evaluate the enhancement in fracture toughness due to the incorporation of various additives. These additives were either polymerized to form a second network, an interpenetrating polymer network, or added as a second monomer which was copolymerized with the diethylene glycol bis(allyl carbonate) to form a random copolymer. The additives, which comprised two urethane acrylates, an allyl urethane and an epoxy acrylate, were found to have little effect on toughness at levels of around 2 wt% and l0 wt%. However, the stiffness of the polymer network was reduced by all four additives. Dynamic mechanical analysis revealed that all the additive agents had two competing influences on toughness. Toughening due to the lowering of an upper glass transition was counteracted by an antiplasticization effect, whereby the presence of the second network reduced the available free volume for segmental motion which caused an inherent decrease in toughness. Scanning electron microscopy revealed little evidence to support crazing as being an important mechanism in the deformation of CR-39 resins.

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