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Axial tensile moduli, compressive moduli and compressive strengths of rigid-rod poly(p-phenylene benzobisoxazole) and stiff-chain poly(p-phenylene terephthalamide), Kevlar TM, fibres were measured with a Tecam Micro-tensile Testing Machine. This machine was configured to allow for testing of single fibres in direct tension and compression at extremely small gauge lengths. The appropriate gauge lengths were estimated based on the discussion of Euler buckling and non-uniform stress distribution in anisotropic materials. The measured tensile and compressive moduli were analysed for corrections to machine compliance and possible gauge length error. The corrected compressive moduli were slightly lower than the corrected tensile moduli, probably due to fibre misalignment under compression and the fibrillar nature of the fibres. The fibre axial compressive strengths measured by direct compression were comparable to those measured from recoil and composite tests but lower than those from cantilever beam and elastica loop tests.

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The creep and stress and strain relaxation behaviours of two different types of polymer have been investigated; polyethylene in strip form and polystyrene in fibre form. All measurements have been carried out before and after exposure to an oxygen ion beam of current densities 100 and 200/~A cm -2 at an energy of 0.5 keV for 1.8 x 10 3 and 0.9 x l0 3 S, respectively. Young's modulus has been measured as a function of temperature. The activation energies have been calculated from stress and strain relaxation measurements. The results obtained made it possible to determine a complete set of mechanical parameters, e.g. relaxation time z, critical strength tr c and structural sensitive parameter y, for both before and after ion exposure. The mechanical properties of the polymers investigated were found to be improved after exposure to oxygen ions due to structural changes.

Детальніше...  

A range of side chain liquid crystal copolymers have been prepared using mesogenic and non-mesogenic units. It is found that high levels of the non-mesogenic moieties may be introduced without completely disrupting the organization of the liquid crystal phase. Incorporation of this comonomer causes a marked reduction in the glass transition temperature (Tg), presumably as a result of enhanced backbone mobility and a corresponding lowering of the nematic transition temperature, thereby restricting the temperature range for stability of the liquid crystal phase. The effect of the interactions between the various components of these side-chain polymers on their electro-optic responses is described. Infrared (i.r.) dichroism measurements have been made to determine the order parameters of the liquid crystalline side-chain polymers. By identifying a certain band (C~N stretching) in the i.r. absorption spectrum, the order parameter of the mesogenic groups can be obtained. The temperature and composition dependence of the observed order parameter are related to the liquid crystal phase transitions and to the electro-optic response. It is found that the introduction of the non-mesogenic units into the polymer chain lowers the threshold voltage of the electro-optic response over and above that due to the reduction in the order parameter. The dynamic electro-optic responses are dominated by the temperature-dependent viscosity and evidence is presented for relaxation processes involving

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Simple considerations of the molecular-weight dependences of the intrinsic viscosity and melt viscosity of chain polymers are given.

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This paper concerns mainly the effect of the shear rate on the reduced viscosity of sodium poly(styrene sulphonate) (Na PSS) in salt-free solution. A maximum in reduced viscosity is obtained for a critical concentration attributed to a preferential intermolecular distance. This maximum seems to have the same origin as that observed by light or neutron scattering. Then, in excess of external salt, the viscosity of Na PSS is compared with that of polystyrene in organic solvent. The main difference between these two polymers is the large increase in the stiffness of the chain in the presence of ionic charge, causing large non-Newtonian behaviour of the polyelectrolyte when the polymer concentration increases even in 0.1 M NaC1.

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Crystalline polyhydroxyalkanoates (PHAs) such as poly-D(-)3-hydroxybutyrate (PHB) and its copolymers with poly-3-hydroxyvalerate (P(HB-HV)) are produced by a wide variety of bacteria and have uses in controlled drug delivery systems. The crystallization kinetics and morphology of P(HB-HV) polyesters both with and without the incorporation of a model drug, Methyl Red, have been investigated, as they are thought to influence drug release characteristics. The influence of copolymer composition and incorporation of Methyl Red on radial growth rates, G, of PHB and P (HB-HV) copolymer spherulites were investigated using polarized light video-microscopy. Growth curves could be obtained over a wide range of undercoolings for all the copolymers studied. At a given crystallization temperature, G decreased with increasing HV content and with increasing drug concentration in polymer spherulites. Spherulite morphology appeared to be a complex function of polymer molecular weight, copolymer composition, drug loading and crystallization temperature (T c). Release of Methyl Red from melt-crystallized matrices of these PHAs was a function of T? and copolymer composition. Drug release from isothermally crystallized copolymer films occurred progressively more rapidly with increasing HV content. This could be explained by the progressively poorer drug entrapment within copolymer matrices with increasing HV content. In PHB, Methyl Red was thought to be largely entrapped within spherulites (interlamellar regions) but for copolymers with increasing HV content, progressively greater amounts of drug were excluded at the spherulite surface and at interspherulitic boundaries.

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The miscibility of poly(tetrahydropyranyl-2-methyl methacrylate) (PTHPMA) with poly(styrene-coacrylonitrile) (SAN) and poly(p-methylstyrene-co-acrylonitrile) (pMSAN) has been studied by differential scanning calorimetry. PTHPMA is miscible with SAN having an acrylonitrile (AN) content between 0 wt% and 31wt%, and with pMSAN having an AN content between 5wt% and 29wt%. Three segmental interaction parameters, ~(THPMA/S, •THPMA/pMS and ~THPMA/AN, were found to be -0.011, 0.019 and 0.64, respectively.

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The miscibility behaviour of poly (cyclohexyl methacrylate ) (PCHMA) with poly (styrene-co-acrylonitrile) (SAN) and poly(p-methylstyrene-co-acrylonitrile) (pMSAN) was investigated by differential scanning calorimetry. PCHMA was found to be miscible with SAN having acryionitrile (AN) content up to 20 wt%, and with pMSAN having AN content up to 22 wt%. All miscible blends were examined for the lower critical solution temperature (LCST) behaviour and the results were used to construct the phase diagrams.

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Fourier transform infrared spectroscopy (FTi.r.) and electron spin resonance spectroscopy (e.s.r.) have been used to follow the kinetics of the polymerization of methyl methacrylate to high conversion at 60°C. The FTi.r. absorbance at 6152 cm-~ was used to monitor the time dependence of the concentration of double bonds, and the concentration of the polymer-chain propagation radicals was monitored using the e.s.r. absorption spectrum. These data were analysed to obtain instantaneous estimates of the kinetic rate parameters for propagation and termination across the range of conversion, and of the initiator efficiency at high conversion. The kinetic parameters were found to be consistent with values obtained by other methods and with the predictions of recent theories.

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The effects of various casting parameters were measured for films of novolacs (cresol-formaldehyde resins) containing pyrene groups either covalently bound to the polymer chain or physically dispersed in the novolac matrix. Aggregation of the dispersed pyrene was observed, as measured by excimer fluorescence and crystallite formation of the pyrene. The phase separation was enhanced in static-cast films over spin-cast films. Pyrene aggregation in films was also affected by the choice of casting solvent even though no significant solvent effects were observed for the tagged polymer or free pyrene in solutions of the casting solvents. A minimum in the plot of the excimer-to-monomer ratio for the films versus the dipole moment of the casting solvent occurred at about the dipole moment expected for the novolac. Pyrene aggregation appears to be more dependent on the interactions of the casting solvent and the host polymer rather than on the interaction of either with the pyrene itself

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