1993 рік

Complex mechanical properties of dodecane/polystyrene systems were studied. The temperature and frequency dependence of the shear modulus was studied for various samples containing different amounts of dodecane. Addition of dodecane lowered the temperature at which mechanical transitions occurred and broadened the transition region. The modulus decreased with increasing dodecane content. The temperature and concentration dependences of the shift factor were determined by time-temperature and time-concentration superposition and analysed by a free-volume model and an entropy model. The concentration effect on the temperature-dependent shift factor was validated by an entropy model. The frequency-dependent complex moduli master curves experimentally determined were converted to a discrete relaxation time spectrum; subsequently, the time-dependent material function could be determined from the relaxation spectrum. The relaxation spectrum shifted significantly to shorter times with increasing concentration of dodecane

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The influence of enthaipic interactions on interracial adhesion between immiscible polymer matrices and reinforcing block copolymer segments has been studied using the transmission electron microscopic iTEM) methodology of Creton et al. We examined the behaviour of four statistical styrene-acrylonitrile (SAN) copolymers, each having different acrylonitrile CAN) content, blended with polystyrene (PS) as the minor component, and reinforced by three poly(methyl methacrylate-b-styrene) (PMMA-b-PS) block copolymers of differing molar masses, viz. 20000, 65 000 and 680 000 g moi-1. These observations were compared with similar experiments on poly(methyl methacrylate) (PMMA) blended with PS and reinforced by PMMA-b-PS. Emulsification was observed with all three PMMA-b-PS copolymers. Crazes were formed in the SAN matrices and a statistical evaluation of interfacial failures was performed on the discrete PS domains that lay within the crazes. For the two block copolymers of higher molar mass, optimal reinforcement of the interfaces was observed independent of the SAN composition. With the 20000 block copolymer, however, the pattern of the interfacial failure depended strongly on the SAN composition. Specifically, it was observed that the fraction of the discrete particles that suffered interfacial failure, and led to the creation of large voids in the crazes in these blends, increased with increased AN content of the SAN matrix. Thus, we found that the fraction of discrete PS particles that produce large voids in crazes of blends containing SAN33 is always higher than in blends containing SAN15, when reinforced with the 20000 PMMA-b-PS. We infer that the critical molar mass required of a mechanically reinforcing copolymer depends on the short-range (attractive and repulsive) interactions between the blend components in the interfacial region. The TEM method could not, however, distinguish between reinforced and neat PMMA/PS blends, all of which showed strong adhesion. This is attributed to the comparatively diffuse interface in the PMMA/PS system, a consequence of the relatively weak repulsion between these two polymers.

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A systematic study of the healing process of polymers with different chemical composition, structure and properties is carded out. It is shown that welding can be due to a number of physical and/or chemical factors. Diffusion alone leads to relatively weak welding under the selected conditions. A jump is observed in the temperature dependence of the shear stress of Kepton-H and Upilex-R. The increase of the activation energy is explained by chemical reactions taking place at the contacting surfaces. For the first time the healing process is carded out at temperatures below the glass transition temperature. In this way, the contribution of the physical processes to the healing is distinguished from that of the chemical processes.

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Polymeric mesophases with positional order in two dimensions are considered in relation to the established liquid crystal phases. It is proposed that the mesophase with hexagonal symmetry observed in polymers as flexible as polyethylene and poly(diethylsiloxane) is columnar liquid crystal. The relationship is discussed with columnar phases in non-discotic low molar compounds and, particularly, with polymers like poly(dialkylsilane)s or alkylated polyglutamates. It is argued that the presently considered hexagonal phases are properly classified as liquid crystalline in spite of the absence of both rigid moieties and amphiphilic character from some of the polymers concerned. Since the phases described here have in the past prompted the introduction of 'condis crystals' as the third class of mesophases, alongside plastic and liquid crystals, some comments are made about the general classification of mesomorphic states.

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The effects on poly(ether ether ketone) (PEEK) of thermal treatment in a particular temperature range in air have been studied. When PEEK is annealed in a partially melted state at a very high temperature, above its original melting point, both crosslinking and crystallization occur. The crosslinking takes place in the melted part in this case, while the crystallization occurs on the surface of the unmelted crystal residues. They lead to the characteristic double-melting peaks of PEEK, representing two distinct crystals. The low-temperature peak arises from the crystals formed during cooling after the treatment, and the higher peak is associated with crystals thickened in the process of annealing. With increasing the annealing time, the low-temperature peak decreases due to the variation in crosslinking of the melted part of PEEK, while the high-temperature peak, which is usually above 350°C, increases and may exceed the equilibrium melting point of PEEK if the annealing time is long enough.

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Novel aromatic polyimides prepared via the polymerization of monomeric reactants (PMR) approach and incorporating anthraquinone diamines in the main chain have been characterized. Imidization and crosslinking reaction profiles have been delineated by d.m.t.a, and the high-temperature mechanical and thermal properties of the thermoset resins examined. Elevated glass transition temperatures (> 300°C) are apparent in some resins while impairment' of mechanical integrity is not evident below 450°C. All resins show good thermal and thermo-oxidative stability.

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This work deals with the reaction of succinoylated dextran with glycidyl acetate (a model compound of glycidyl bioactive derivative) using benzyltriethylammonium chloride as catalyst. The reaction shows second order kinetics and depends on the glycidyl acetate and catalyst concentrations, showing no dependence on the acid groups' concentration. On the basis of these findings a probable mechanism of the reaction is suggested.

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A series of methacrylate-based side-chain liquid crystal polymers has been prepared with a range of molecular weights. For the high molecular weight polymers a smectic phase is observed with a very narrow nematic range; however, for low molecular weight polymers only the nematic phase is observed. A marked reduction in the glass transition temperature, TsN and TNI is observed with a reduction in the molecular weight. The orientational order parameters for these polymers in the liquid crystal phase have been determined using infra-red dichroism. It is found that the higher the molecular weight of the polymer, the greater is the threshold voltage of the electro-optic response and the lower the order parameter. The increase in the threshold voltage with increasing molecular weight may be related to the intrinsic curvature elasticity and hence to the coupling between the mesogenic units and the polymer backbone

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The properties of u.v. cured poly(ethylene glycol) diacrylates (PEGDA) having MWs in the range 200-2000 and acrylic functionality near to two were evaluated. First, the properties of pure PEGDA oligomers, subjected to u.v. curing, were considered. Then, mixtures of different PEGDA oligomers with a typical epoxy acrylate resin were prepared and investigated. The double-bond conversion during the curing process was determined by FTi.r. analysis. D.s.c., t.m.a, and d.m.t.a, were used to evaluate the thermal and dynamic-mechanical behaviour of the networks obtained. As far as the pure PEGDA oligomers are concerned, the final double-bond conversion was always found to be complete, with the exception of the lowest MW oligomers. By using oligomers having MWs up to 1000, after the u.v. curing process, films having a completely amorphous structure were obtained. Oligomers having MWs higher than 1000 show, after curing, a partial PEG crystallinity. The Tg values of amorphous films were found to decrease by increasing the MW of the oligomer, i.e. by decreasing the crosslinking density of the network. Good agreement with the Nielsen equation was observed. The equilibrium swelling values in water were found to increase by increasing the oligomer MW. The investigation of mixtures of PEGDA oligomers with a typical epoxy acrylate resin confirms the above reported results as far as the double-bond conversion, the thermal and the dynamic-mechanical behaviour are concerned. By changing the oligomer MW it is possible to change the properties of the u.v. cured films over a broad range

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The fracture toughness of poly(ethylene terephthalate) (PET) is correlated with various morphologies produced upon annealing. Annealing PET at a Hoffman regime III crystallization temperature (120°C) or inducing solid-state polymerization result in materials of high fracture toughness (Kc=8.7 MPa m 1/2 and 9.5 MPam 1/2, respectively); these high toughness values are related to multiple-crazing mechanisms produced by high tie-molecule densities. Annealing fully dried PET at 200°C transforms the material to a Hoffman regime I/II structure, and the fracture toughness decreases to 6.5 MPam 1/2. Hydrolysis also reduces the fracture toughness (Kc < 3 MPa m~/2). In these low toughness materials, multiple crazing is not observed. In addition, the fatigue crack propagation response of PET is found to be superior in samples annealed to produce high fracture toughness as compared with samples annealed to produce low fracture toughness values.

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