1993 рік

The effects of poly(vinyl acetate) (PVAc) and CaCOa on the cure kinetics, mechanical properties and surface rugosities of unsaturated polyester (UP) resins were studied. When the ratio of styrene to UP unsaturation was kept constant, PVAc did not have any effect on cure kinetics, maximum conversion or glass transition temperature of the crosslinked UP resin. The addition of CaCO3 led to a decrease in the induction period of the copolymerization, without any significant action on the subsequent cure rate. The addition of PVAc was only effective in promoting an excellent copy of the mould surface at concentrations higher than the critical one leading to co-continuous structures (8 wt% for the formulation used in the present study). The use of PVAc at concentrations beyond the critical value did not produce further improvements in the surface quality. The simultaneous use of both PVAc and CaCO 3 in UP formulations led to materials with very low strength and impact resistance, and with only a moderate ability to reproduce the mould surface

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Morphology, dynamic mechanical analysis, differential scanning calorimetry (d.s.c.), physicomechanical properties and electrical resistivity were studied on different blends of ethylene vinyl acetate (EVA) (28% VA content) and low density polyethylene (LDPE). An interpenetrating polymer network (IPN) is formed with a minimum of 50 wt% EVA in the blend. The glass transition temperatures change with blend compositions according to the Fox equation or the Gordon-Taylor relation. This indicates intermiscibility (compatibility). On curing, tensile strength goes up dramatically, and a maximum tensile value is obtained for 50:50 blend composition. The electrical resistivity of the blends is explained by a simple model, and can be correlated to morphology

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Near infra-red spectroscopy techniques were used to study the cure reactions of various epoxy resin formulations based on diglycidyl ether of bisphenol A (DGEBA) resins cured with 4,4'-diaminodiphenyl sulfone (DDS) hardener. Stoichiometric and non-stoichiometric DGEBA/DDS resin formulations, using neat as well as thermoplastic toughened systems containing two phenolic hydroxyl terminated polysulfones with different molecular weights were involved in this study. The infra-red absorption spectra of the prepared formulations were obtained on an FTi.r. spectrometer operating in the region of 11000-4000 cm-1 and were analysed according to methods described in this paper. The chemical group peaks of interest in a DGEBA/DDS spectrum were identified by a comparative study with individual spectra of DGEBA and DDS monomers. Where necessary, special model compounds were used to identify unknown bands, such as the primary amine band at 4535 cm-1. The absorption bands of interest were integrated to quantify the areas and then converted to molar concentrations. This series of quantitative analysis of the major chemical groups in several resin systems, led us to understand not only the reaction mechanism in each system but also the cure kinetics, which showed strong dependence on the formulation of the system. In this paper, the details of the quantitative analysis of the infra-red spectrum for various systems and the reaction mechanisms observed in stoichiometric DGEBA/DDS resin formulations are described.

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Photoablation of spun polystyrene films was studied at 248 nm. The etching occurs on a submicrosecond time-scale, is strongly intensity dependent and is preceded by an incubation period. The chemical and photochemical properties of the polymer are not affected by its molecular weight. However the molecular weight changes the incubation behaviour of the samples. This last result is analysed as a physical effect in terms of the volume changes brought about by the ablation process.

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The influence of the position and length of alkoxy substitutents on the phase behaviour of poly(p-phenylene terephthalate)s was investigated by differential scanning calorimetry and rheological measurements. The two polymers with respectively dodecyloxy and hexyloxy side chains on the terephthalate moiety (PTA12HQ and PTA6HQ) both showed stable liquid-crystalline phases. For these two polymers the location of the transition temperatures as a function of the molecular weight (inherent viscosity) was shown to level off above an inherent viscosity of 2 dl g-1. From rheological measurements it is concluded that the layered mesophase, shown by PTA6HQ and PTA12HQ, behaves more like a solid than a liquid. For the nematic mesophase, only shown by PTA6HQ, a minimum in the viscosity-temperature curve was found. The polymer with dodecyloxy substituents on the hydroquinone moiety (PTAHQ12) only shows mesomorphic behaviour on cooling, while the polymer with double the amount of substituents (PTA12HQ12) does not show any liquid-crystalline behaviour

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The methodology for conversion of transient current data in the time domain into dielectric loss data in the frequency domain using the Brather approximation is described. A test of the accuracy of the approximation is made using the 'stretched exponential' relaxation function (Kohlrausch-Williams-Watts function) for which the Fourier transform has been previously tabulated for a range of exponent fl(0 < ]7 ~< 1 ). As an example of the application of the Brather approximation we report ultra-low frequency dielectric loss data (10-3.5 to 10-0.4) for a liquid-crystalline side-chain polymer as derived from transient current data.

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The relationship between processing parameters, morphology and properties has been investigated for injection mouldings and extrudates of filled and unfilled grades of a thermotropic copolyester. Low injection speeds were found to result in a significant increase in tensile strength and modulus in the flow direction. This was correlated with an increase in the thickness of the highly oriented skin layer and also with improved molecular orientation of the core regions. Tensile tests on single layers confirmed the skin to have a higher tensile strength and modulus than the core, and the tensile properties to correlate well with the local orientation. With the addition of mineral filler, the influence of injection speed on tensile properties became less marked, and the properties within the mouldings were more homogeneous. The adhesion between the layers also improved in the presence of filler. Additional tests were carried out on extruded tubes. The extensional flows associated with the extrusion process lead to relatively high molecular orientation in the extrusion direction and therefore to good mechanical properties

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The molecular orientation in thick polyethylene samples has been studied by wide-angle X-ray diffraction, i.r. dichroism and Raman spectroscopy. The original specimens, with dimensions of the order of a centimetre, were cut to obtain 1 mm thick platelets on which the measurements were made. The mean coefficient of the second-order Legendre polynomial, (P2), was calculated from X-ray diffraction and from the 1894 cm- 1 i.r. band for the crystalline phase, from the 909 cm-1 i.r. band for the vinyl end groups and from the 1130 and 1060 cm-1 Raman bands for the all-trans C-C conformers. The fourth-order coefficient, (P4), was also determined from X-ray diffraction and Raman spectroscopy for a series of cylindrical rods of draw ratios (2) ranging from 6 to 20. An excellent correlation is observed between the (P2) coefficients measured from different X-ray reflections and from the 1894cm-1 i.r. band. The Raman spectroscopy results show that the all-trans bonds located in the amorphous phase are aligned perpendicular to the extrusion direction for the 2=6 rod, and gradually reorient towards the fibre axis for 2 values up to 20, while the (P2) and (P4) coefficients calculated for the crystalline phase remain constant at 2/>12. The variation of the orientation through the thickness of the samples was investigated for the cylindrical rods and for an H-shaped moulding produced by extrusion and rolling. Minor differences in the degree of molecular orientation were detected between the centre and the surface of the rods, whereas important variations were measured for the H-shaped sample.

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Sorption and diffusion of dichloromethane vapour were measured in poly(aryl ether ether ketone) films after different ageing times. In the fresh samples the diffusional behaviour is characterized by two stages: in the first stage the diffusion coefficient D is weakly dependent on concentration, and transport mainly occurs through a frozen system; whereas in the second stage a steep dependence of D on concentration was found. The aged samples also show an intermediate stage, characterized by an anomalous or non-Fickian sorption behaviour. Furthermore the diffusion coefficients of the aged samples are lower than those of the fresh samples, in agreement with a reduced fractional free volume. Ageing at a temperature slightly lower than the glass transition temperature decreases the sorption at low activity, leading to the suggestion that ordered domains, impermeable to the vapour at low activity, are formed at this temperature. The process of solvent-induced crystallization on the amorphous samples was investigated, and it was found that crystallization is induced after activity 0.7. Both the fresh and the aged samples show the same behaviour, although the level of crystallinity attained was found to be higher for the latter samples.

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The novel radical polymerization of an unsaturated monomer in the hydrophobic part of vesicles is attempted using the experience gathered from kinetic and thermodynamic studies of the emulsion polymerization process. The influence of the monomer content of the system and also the initiation of the polymerization are discussed. A free radical polymerization of styrene in dioctadecyldimethylammonium bromide vesicles is performed. Evidence of the formation of polymer-containing vesicles is obtained by means of electron microscopy and the comparison of the diffusion rate of a paramagnetic probe over the bilayer of vesicles and polymer-containing vesicle

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