1994 рік

In this paper theoretical and experimental studies of the phase behaviour of ternary polymer blends are reviewed. Particular emphasis is placed upon the effect of specific interactions (hydrogen bonds). The association model developed to predict the phase behaviour of binary hydrogen-bonded polymer blends has been extended to ternary polymer blends. Simulations have been performed to illustrate the major effects of 'physical' (primarily dispersion) and 'chemical' (hydrogen bonding) forces on phase behaviour. Theoretically predicted phase diagrams have also been compared to experimental results obtained from five hydrogen-bonded ternary polymer blend systems. Overall, the agreement is satisfactory. One general conclusion is that it will be very difficult to find ternary ,polymer blends that exist in a single phase over a wide composition range. Furthermore, in most cases an immiscible binary blend cannot be made homogeneous by introducing a small amount of a third polymer ('compatibilizer').

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When a linear polyethylene is blended with two homogeneous copolymers, of different comonomer content, the resulting ternary phase diagram shows three distinct regions of liquid-liquid phase separation. It is argued that we may usefully employ a ternary phase diagram to understand the degree of melt demixing occurring in some commercial polymers, such as linear low-density polyethylenes (LLDPEs), which are blends in themselves. Results obtained on blending LLDPEs with linear or randomly branched polyethylenes are presented, and it is shown that the observed phase behaviour may be interpreted as cloud point curves arising from sections through phase diagrams of ternary blend systems. The ternary phase diagram used in this work was originally determined experimentally for octene copolymers, but it is now shown to be equally applicable to butene copolymers

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Spinodal decomposition has been simulated in two spatial dimensions for ternary polymer mixtures over a wide range of volume fractions and Flory-Huggins interaction parameters. A great wealth of morphologies was observed: some closely approximate morphologies seen in cast film experiments, while others invite experimental confirmation. A taxonomy of phase structures is proposed

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The effect of temperature and pressure on necking behaviour of polycarbonate was studied in tension. Following the concepts elucidated by Coates and Ward, neck geometry and neck propagation were related to the materials parameters: strain rate sensitivity and strain hardening parameter. The relationship between neck profile and the true stress-true strain curve was established by combining an analytical expression for the macroscale mechanics of the flow process with a constitutive equation describing the true stress, true strain and true strain rate dependence of the material. Having assumed a constitutive equation, the materials parameters were obtained from the stable neck geometry. The strain hardening parameter was found to be insensitive to changes in pressure and temperature in the range studied and had a value of approximately 3.3. Since the strain hardening parameter affected primarily the draw ratio, the latter was essentially independent of temperature and pressure. The increase in strain rate sensitivity parameter with pressure and decreasing temperature from 0.028 to 0.035 was manifest as a gradual broadening of the neck. The molecular significance of the materials parameters is discussed in terms of the contributions of two separate processes to plastic deformation: thermally activated yielding and the temperature and pressure independent stretching of a molecular network.

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A new approach to the study of the local organization in amorphous polymer materials is presented. The method couples neutron diffraction experiments that explore the structure on the spatial scale 1-20 A with the reverse Monte Carlo fitting procedure to predict structures that accurately represent the experimental scattering results over the whole momentum transfer range explored. Molecular mechanics and molecular dynamics techniques are also used to produce atomistic models independently from any experimental input, thereby providing a test of the viability of the reverse Monte Carlo method in generating realistic models for amorphous polymeric systems. An analysis of the obtained models in terms of single chain properties and of orientational correlations between chain segments is presented. We show the viability of the method with data from molten polyethylene. The analysis derives a model with average C-C and C-H bond lengths of 1.55,~ and 1.1 A respectively, average backbone valence angle of 112:, a torsional angle distribution characterized by a fraction oftrans conformers of 0.67 and, finally, a weak interchain orientational correlation at around 4 A.

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The results of a comprehensive small-angle X-ray scattering study of linear polyethylene crosslinked using a peroxide have been combined with a study of the crystallization kinetics to obtain a generalized picture of the effects of limited mobility and comonomer content on crystallization. It is shown that there are three regions of behaviour: (i) an initial region in which the erosslinks are soluble in the crystal to a certain limiting composition; (ii) a region in which the lamellar thickness decreases in a continuous manner with increasing crosslink density; and (iii) a region where the lamellar thickness is virtually independent of crosslink density. The results are interpreted using the Sanehez-Eby theory, the Andrews theory and regime theory to generate a comprehensive description of the behaviour

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Microstructurai deformation behaviour in polystyrene-based compatible polymer blend systems was studied using transmission electron microscopy (TEM) and microdensitometry. Four different binary compatible blend systems were employed and characterized in this investigation: polystyrene (PS) and poly(2,6-dimethyl- 1,4-phenylene oxide) (PPO), PS and poly(vinyl methyl ether) (PVME), PS and poly(a-methylstyrene) (P~MS), and PPO and P~MS. Individual craze and shear deformation zone (DZ) microstructures were examined by TEM. For TEM observations, specimens deformed in situ on a TEM grid (loaded condition) were utilized. TEM micrographs showed that, for PS/PPO blends, deformation mode transition from crazing to shear DZ occurred around 25% PPO inclusion. For PS/PVME blends, this transition occurred around 20% PVME inclusion. For PS/P~MS blends, the deformation mode was totally controlled by crazing regardless of composition. For PPO/P~MS blends, deformation mode transition from shear DZ to crazing occurred around 25% P~MS inclusion. Quantitative analyses of these crazes and shear DZs were conducted utilizing microdensitometry of the TEM negatives in the manner developed by Lauterwasser and Kramer. From the microdensitometry, molecular parameters such as fibril extension ratios (2s) were determined. Microdensitometry results showed that 2 decreased as the PPO content increased in the PS/PPO blends, and, for 100% PPO, only shear DZs were observed. For PS/PVME blends, 2 also decreased as the PVME content increased. For PS/P~MS and PPO/P~MS blends, 2 increased as the P~MS content increased. These results were analysed in terms of existing entanglement and intermolecular interaction models in compatible blends. From this analysis, it is concluded that the overall microstructurai deformation behaviour of binary compatible blends cannot be fully explained by either entanglement density or intermolecular interaction model alone. Rather, the combined entanglement density and intermolecular interaction model can explain the microstructural deformation behaviour in binary compatible blends well.

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The impact behaviour of three linear low density polyethylenes (LLDPE) has been studied using the energy partitioning methodology recently developed in this laboratory. The aim was to elucidate the origin of the high toughness of this class of polymers and to establish whether a rubber toughening mechanism is involved. It has been found that LLDPEs have an unexpectedly high intrinsic toughness, and that there is a significant ductile contribution to the total fracture energy from shear lip energy absorption. No evidence of a conventional rubber toughening mechanism has been found. However, the shear lip area (and hence this specific component of the total fracture energy) increases with the concentration of the segregated 'rubbery' phase.

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Small-angle X-ray scattering was carried out on as-spun and heat-treated poly(p-phenylene benzobisoxazole) (PBO) fibre. While as-spun PBO fibre exhibits only an equatorial streak, generally attributed to elongated voids or fibrils, the heat-treated fibre shows in addition a radial four-point pattern. The long period and radial spacing of this pattern are 28 and 15 nm, respectively. The spacings do not vary with heat-treatment temperature. As the angle defined by the plane normal to the fibre axis and the X-ray beam is increased from 0 ° to 30 ~ to 60 °, the four-point pattern gradually changes to a two-point pattern. The measured long period in the tilted fibre is a projection of that in the untilted fibre. Various possible morphological models are discussed.

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We first show that the classical gas-like theory of rubber elasticity is incorrect because the force-extension relation for an individual polymer chain is dependent on the interaction of its chain segments with segments of other chains in its neighbourhood. Two previous attempts by the author to formulate a liquid-like theory are discussed and improved. It is then argued that chain entanglements result in the value of the number of segments N in the expression f= 3kTx/N increasing with chain extension x or with increasing force f The entropic contribution to the free energy of a rubber from this effect is then shown to be given by B(2x2y+ 2y2~+ 2=2=,) , where B is proportional to kTand to the number of chains per unit volume. In order to facilitate comparison with experiment, the stress-strain equations for simple extension, biaxial extension and for pure shear are displayed for three different choices of strain invariants: those of Rivlin, those of this paper, and those of the localization model of Gaylord, Douglas and McKenna. Comparisons with experiment are discussed.

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