1993 рік

The crystallization kinetics of poly(phenylene sulfide) (PPS) in blends with high density polyethylene (HDPE) and poly(ethylene terephthalate) (PET) is reported. Although the crystallization of PPS took place in the presence of the molten phase of the second component in both cases, the changes observed in the crystallization behaviour of PPS were different. Isothermal crystallization studies indicated that the crystallization of PPS was accelerated in PPS/PET blends whereas it was retarded in PPS/HDPE blends. The presence of the molten phase of the second component was found to affect the crystallization of PPS through changes in both nucleation and growth processes. The differences observed in the crystallization behaviour of PPS are explained on the basis of the physical state of the melt and the possibility of chemical interactions.

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Storage hardening of natural rubber is a complex phenomenon involving crosslinking reactions due to certain abnormal groups on the rubber chains. The various mechanisms put forward can each account for certain of the experimental observations. Recently, an ionic crosslinking mechanism involving carboxylate and metal ions was proposed. It is being investigated by treating latex with a series of selected cations. While monovalent cations are found to have no effect on the crosslinking reaction, some divalent cations are able to reduce the storage hardening. Certain transition metal ions catalysed the degradation of natural rubber. Infra-red spectra show that the main chain structure remains the same after the degradation, and the abnormal groups appear unaffected

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The wettability of microfiltration membranes has been examined. The analytical method used is an application of the Wilhelmy plate procedure. The use of a new sample holder especially designed for this purpose enables the surface analysis of polymer films even if they present two physically different faces. The evolution of wettability properties during the membrane production process has been determined for nuclear microfiltration membranes obtained from poly(ethylene terephthalate) (PET) and polycarbonate (PC). A grafting procedure in solution, which can be applied to PET- and PC-based materials, enhances the wetting properties of membranes and make them similar to those reached by poly(vinylpyrrolidinone) adsorption.

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Transport equations are derived in the framework of the rational thermodynamics approach for membrane processes involving a dense (active) polymer layer. The influence of the deformation of the polymer matrix, formation of interacting species, changes in polymer configuration, etc., due to mixing of the penetrant and the polymer can be accounted for in the equations. Studies of permeation fluxes in the transient regime in which the membrane is suddenly put into contact with a solvent can give useful information on the behaviour of polymer-penetrant systems. Experimental results obtained for the transient permeation of alcohols and heptane through different cellulose ester membranes are reported and discussed in the framework of this approach.

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Monomeric and dimeric Schiff bases with cyanate terminals were prepared and characterized. Polarized optical microscopy revealed the narrow mesomorphic ranges for the corresponding monomer and dimers. Cyclotrimerization of cyanate groups was identified by differential scanning calorimetry (cure exotherms between 160 and 270°C) and infra-red spectroscopy (triazine absorptions at 1567 and 1370 cm-~). The cured dimers are hard and transparent.

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A relationship between the macroscopic toughness, the intrinsic network (entanglement and/or crosslink) density and the relative thickness of polymeric systems, is presented. Toughness of amorphous, glassy polymers is mainly determined by the strain to break, since the yield stress generally only varies between 50 and 80 MPa. It was found that the strain to break strongly depends on the absolute thickness of the specimen or, equivalently, the local thickness within the (micro) structure of the material. Only below a certain critical thickness can the intrinsic strain at break of a polymer be reached. The absolute value of this critical thickness and the intrinsic strain at break of a polymer are both determined by the network density. In this paper polystyrene (PS), a polymer that is generally considered to be very brittle, was investigated with respect to the influence of absolute thickness on its strain to break. For thin isotropic tapes of PS it was demonstrated that this critical thickness is below 1/~m. Based on experiments with macroscopically 'thick' PS samples (3 mm), which are made locally thin by the introduction of small, non-adhering rubbery particles ('holes'), we could identify that the critical thickness is 0.05 pm for PS.

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The neoprene and polyacrylamide system was studied at various blend ratios. Infra-red studies indicate the reactive nature of neoprene, polyacrylamide and their blends. Inter-crosslinking between neoprene and polyacrylamide segments occurs by chemical reaction through the imide groups formed from polyacrylamide and the halogen atoms of neoprene. Studies with a differential scanning calorimeter and a dynamic mechanical analyser clearly indicate the apparent incompatible nature of the blend and show it to be a three-phase system. The third phase is a compatibilized interfacial zone of inter-crosslinked material. Preferential etching of the phases with solvents is possible. A scanning electron microscopy study also confirmed the multiple-phase structure of the cured blends

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The molecular structure and conformational energies of methyl acrylate, methyl methacrylate, vinyl acetate, methyl propionate, methyl 2-methylpropionate and styrene have been investigated using PCILO (perturbative configuration interaction with localized orbitals) and AM1 (Austin model 1) semiempirical methods with total optimization of geometry. The minimum-energy conformations obtained with both methods have been compared. The heats of formation calculated with the AM 1 method are in excellent agreement with experimental values. The rotational barriers were also calculated and there are similarities in shape between the various compounds that do not involve an ester oxygen

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Transmission electron microscopy investigations of deformation in films of amorphous and semicrystalline isotactic polystyrene have shown crazing to dominate up to the glass transition temperature, T v the craze fibrils becoming more crystalline as the strain rate is lowered and the temperature raised. Above Tg semicrystalline films show fibrillar shear.

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Densities, glass transition temperatures and notched impact strengths (NIS) (before and after storing the test samples at 150°C) have been measured for several series of poly(arylene ether sulfone)s. Consideration of these data together with those presented in Part 4 and by other workers suggests that for polymers with molecular weights (as indicated by solution viscosity) above the entanglement molecular weight, the symmetry of the polymer chains is the most important structural factor deciding NIS. Polymers with the more symmetrical chains provided test samples showing relatively high values for NIS, while the less symmetrical ones, especially those containing phenylene rings linked by ortho- or meta-oriented inter-ring bonds, gave samples for which lower NIS values were obtained. Following recent advances defining the importance of crazing in brittle failure, it is suggested that this effect of symmetry on NIS arises because increasing asymmetry leads to higher values for the chain contour length between entanglements.

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