1994 рік

Time dependent-measurements of the dynamic shear modulus in the splitting region a,--,~ + fl of poly(n-butyl methacrylate) (PnBMA) and poly(ethyl methacrylate) (PEMA) are reported. Non-equilibrium effects in the frequency-temperature region around the onset of the ct transition are discussed. Physical ageing effects on the intensity, shape and position of the ~ and fl peaks of the shear loss modulus are reported. For the relaxation a shift of the mean relaxation frequency to lower values, a peak sharpening, and an increase in intensity during ageing are observed, whereas the fl intensity decreases significantly. Long times are necessary to reach the equilibrium state a few kelvins below the onset temperature (~ glass transition temperature for PnBMA). A small amplitude fine structure was found in the frequency sweeps at temperatures below the onset temperature, which is discussed in terms of McCrum's sequential ageing.

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Styrene-acrylonitrile-glycidyl methacrylate (SAG) copolymer with ethyltriphenyl phosphonium bromide (ETPB) catalyst has been used successfully as in situ compatibilizer to compatibilize the immiscible and incompatible blends of poly(butylene terephthalate) (PBT) and acrylonitrile-butadiene-styrene (ABS). This SAG copolymer contains reactive glycidyl groups that are able to react with PBT end groups (-COOH or -OH) under melt conditions to form SAG-g-PBT copolymer. However, the rate of the grafting reaction is slow without the presence of catalyst. The ETPB catalyst is highly efficient: even at a concentration as low as 50 ppm, it is able to promote the grafting reaction to obtain the maximum achievable properties. The compatibilized PBT/ABS blend has smaller phase domain, higher viscosity, and significantly improved mechanical properties compared to the corresponding non-compatibilized blend.

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The isothermal crystallization process from the melt in nylon-6,6 was followed in situ by X-ray diffraction. At all the crystallization temperatures studied, the samples crystallized into the pseudohexagonal (hexagonally packed) Brill structure, which transformed into the triclinic structure on subsequent cooling to room temperature from the crystallization temperature. This transition occurs about 40°C lower than the respective crystallization temperature. Also the transformation temperature (TB) displays a hysteresis effect on heat cycling, with Ts being higher on heating than on cooling. These findings affirm the first-order transition character of TB, contrary to some previous suggestions. They also imply gradation in perfection and/or crystal size while in the Brili structure. It follows further that any difference in structure and property, as determined at room temperature, must reside in the Brill structure at the stage of formation and can only be reflected indirectly by the transformed structures observed at room temperature. With reference to spherulites, the latter has important implications for the arrangement of hydrogen bonding within the Brill structure. On the other hand, crystallization directly into the triclinic Bunn structure remains a possibility for crystallization from solution, awaiting confirmation by X-ray diffraction

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A substituted poly(cyclohexadiene) precursor polymer has been pyrolysed (aromatized) in N-methylpyrrolidinone solution to poly(p-phenylene). For aromatizations up to 40% the polymer remains in solution, but size exclusion chromatography shows that the initially narrow molecular weight distribution fractions become bimodal with a high molecular weight peak appearing even at low percentage aromatizations. At the same time, the main component of the polymer rapidly decreases in molecular weight. Size exclusion chromatography using detection specific for phenylene residues indicates that their concentration is the same across the whole distribution. These results were confirmed by fractionating a partially aromatized polymer. The data indicate that aromatization occurs both randomly and in sequences and is accompanied by scission. Molecules with long sequences of phenylene groups aggregate, forming the high molecular weight fraction observed by size exclusion chromatography. The low molecular weight fraction has the same degree of aromatization but the phenylene units are randomly distributed and do not aggregate intermolecularly.

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Structural development in injection moulded poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) was studied as a function of processing parameters including mould temperature, injection speed and holding time. This polymer exhibits a relatively low thermal crystallization rate and as a result quenches into wholly amorphous form when moulded into thick cavities at low mould temperatures. It, however, exhibits a three-layer structural gradient (an amorphous skin, a shear crystallized intermediate layer and an amorphous core) when moulded into thin cavities at mould temperatures up to about its cold crystallization temperature. Above this temperature, thermally activated crystallization starts playing a role particularly in the structural formation at the interior of the samples. At these temperatures the holding time in the mould becomes an important factor and as it gets longer the overall crystallinity in the sample increases. Thermal analysis results suggested that at mould temperatures up to its glass transition temperature the crystal structure of PEN consists of a mixture of ordinary ct form and fl form that exhibits high melting temperature. Mechanical properties of injection moulded PEN were found to mostly depend on mould temperature and at high moulding temperatures on holding time

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Physicochemical phenomena at the polymer-metal interface have been observed by the use of surfaceenhanced Raman scattering (SERS). SERS spectra are compared with the infra-red reflection-absorption spectra for coatings of polymer on metals. Results show that SERS is an effective in situ analytical technique which is highly informative in that the authentic adsorption state and the microstructure of the first monolayer near the metal surface can be revealed with much less interference from the polymer bulk.

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High pressure crystallization has been used to compare three linear low-density polyethylenes because of its potential for forming lamellae whose thicknesses are equal to the inter-branch separation. Major differences are revealed between one polymer prepared using a metallocene catalyst and the other two which have been generated by conventional Ziegler-Natta synthesis. The broad distribution of inter-branch lengths in the latter two polymers and the narrow spectrum of the former are revealed both by temperature rising elution fractionation (TREF) thermograms and the melting endotherms of the respective polymers after high pressure crystallization. The metallocene-catalysed polymer shows unprecedented behaviour in that its lamellar thickness is invariant. In consequence, it crystallizes at high pressure entirely as orthorhombic lamellae; the hexagonal phase is not formed

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The dual melting behaviour of a rigid aromatic polyimide synthesized from 1,4-bis(4-aminophenoxy-4'- benzoyl)benzene and oxydiphthalic dianhydride was analysed. Film samples were synthesized through a stepwise imidization procedure wherein molecular weight control was effected through stoichiometric offset. The films display a glass transition temperature of 217°C and dual endothermic transitions at ca. 334°C and 364°C. Transmission electron microscopy, wide angle X-ray diffraction and small angle X-ray scattering (SAXS) studies, in conjunction with dynamic scanning calorimetry (d.s.c.) analysis, show that the higher melting transition results from the melting of lamellae which were melt recrystallized during heating in the d.s.c.D.s.c, heating rate studies reveal that the rate of transformation from thin to thick lamellae is dependent upon molecular weight, as expected. Synchrotron SAXS analysis confirms that the melt recrystallization process is very rapid for this high glass transition, low molecular weight polymer

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The well resolved Raman spectra of electrochemically synthesized conducting and non-conducting poly(N-vinylcarbazole) were obtained. The Raman and i.r. spectra revealed that the conducting polymer had a 3,3'-dicarbazyl structure formed by dimerization at the 3,6 position and an appropriate mechanism was proposed. The conducting polymer was also obtained by electrolysis of the non-conducting polymer dissolved in dichloromethane. The conducting polymer obtained by this method was found to have a similar structure to that obtained by direct electrolysis of the monomer. The polymerization was monitored by the use of a specially designed cell, with in situ Raman spectra being taken from the electrode surface at different intervals. These spectra showed that there were no significant structural changes occurring in the conducting polymer during the polymerization process. In addition, the Raman image of the conducting polymer was taken by using the intense band which was located at 1600 cm-1.

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The process of isothermal crystallization and phase separation in poly(tetramethylene isophthalate) (PTMI) and multiblock copolymers of PTMI with poly(tetramethylene oxide) (PTMO) is examined by differential scanning calorimetry. Morphology development is studied from the supercooled liquid to the completion of crystallization for compositions ranging from 30 to 100wt% PTMI over a wide range of isothermal crystallization temperatures. A model is suggested whereby phase separation in the copolymers proceeds in a manner similar to crystallization of the homopolymer, implying phase-mixed non-crystalline regions with inhomogeneous mobilities. The experimentally observed multiple endotherm behaviour in the PTMI/PTMO copolymers is a characteristic of the hard segment, and is also observed in the PTMI homopolymer.

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