1993 рік

S-Acryloyl O-ethyl xanthate was copolymerized with styrene and methyl methacrylate, giving rise to copolymers containing the photoactive acyl xanthate chromophore. Thin films of these copolymers, on irradiation, undergo phototransformations leading to crosslinked polymers. The photocrosslinking yield of the copolymers depends mainly on their molecular weight and not on the composition of the copolymer. The mechanism of photocrosslinking was investigated through nanosecond laser flash photolysis studies. The 308 nm laser pulse excitation of the copolymers in benzene produces radical intermediates, arising through a C(~--~O)-S bond fission. The assignment of the photogenerated (ethoxycarbonylthiocarbonyl)thiyl radical is based on its characteristic long wavelength absorption maximum (2=..~ 660 nm in benzene), ready quenching by nitroxy radicals and similarity in spectra and kinetics with those of the radical transient, photogenerated from the model substrate, S-isobutyryl O-ethyl xanthate.

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Polyesters derived from 2,5-furandicarboxylic acid and 1,4:3,6-dianhydrohexitols have been prepared. These polyesters are made solely from monomers derived from renewable resources such as starch and fructose. As shown by thermogravimetric analysis, all materials have excellent stability against thermal decomposition. For sufficient degrees of polymerization, the glass transition of these polyesters, as determined by differential scanning calorimetry, is about 185°C. The degree of crystallinity, as studied by wide-angle X-ray scattering, is low for all materials investigated.

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A variety of simultaneous and sequential interpenetrating polymer networks (IPNs) based on a polyurethane (PU) network and poly(methyl methacrylate) (PMMA) in linear and network forms were evaluated in terms of dynamic and static mechanical behaviour. IPNs of mid-range composition produced broad tan & peaks with significant intensities at room temperature to influence elongation to failure. Unusually high elongations at failure were recorded in the simultaneous IPNs due to improved homogeneity in the distribution of the component polymers. The patterns of behaviour of the elastic modulus, hardness and glass transition temperature suggest the occurrence of phase inversion in the simultaneous IPNs and dual-phase continuity in the sequential IPNs. Synergism was experienced in various IPN properties with maximum improvements occurring in the PU/PMMA composition range 30/70 to 40/60.

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Ellipsoidal polystyrene latex particles, a few micrometres in length and very suitable for use as models for colloid studies, have been prepared and characterized. Polystyrene spheres were embedded in poly(vinyl alcohol) (PVA) and the resulting film stretched at 200°C to two film draw ratios, 1.4 and 2.35. The deformed particles were examined by light scattering and some were extracted for transmission electron microscopy. The light scattering results, analysed using Rayleigh-Gans theory, were in very good agreement with the electron microscopy. The ellipsoids were biaxial (within the limits of experimental error). Those deformed to the highest draw ratio deviate slightly from true ellipsoids, having rather sharper ends. The PVA deforms by a process of necking, which is non-uniform on a large scale, so that over the whole film there can be a considerable variation in draw ratio. However, particles recovered from any small, local area of the PVA film show a size distribution almost as uniform as that of the starting material. The effective draw ratio of the particles is found to be considerably greater than that of the matrix

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High-resolution core-level and valence-band X-ray photoelectron spectroscopic measurements show that polystyrene surfaces become more heavily oxidized during ultraviolet photo-oxidation than under plasma oxidation. A greater proportion of-CO2- linkages are evident following the former treatment, along with the virtual absence of any phenyl features in the valence region and an attenuated n-z* shake-up satellite in the C(ls) spectra. This apparent paradox of the more reactive medium producing a less oxidized surface is explained in terms of the complex plasma-surface interactions associated with the exposure of a polymer surface to an oxygen glow discharge

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The high-temperature crystallization of poly(p-phenylene terephthalamide) (PPTA) from dilute organic solutions was achieved through the introduction of a non-solvent, or precipitating agent, at the desired crystallization temperature. The morphology and crystal structure were examined for crystals produced from PPTA polymer with two different molecular weights (M,=46000 and 3430gmo1-1), using transmission electron microscopy. For the high-molecular-weight polymer, ribbon-like crystals were produced, while the low-molecular-weight polymer yielded small needles or platelets. In both cases, electron diffraction showed that the Northolt allomorph was obtained. For the high-molecular-weight polymer, the molecular axes were parallel to the ribbon axes in a chain-extended type structure. A hypothesis for the orientation of the low-molecular-weight PPTA in the small platelets, is also given.

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Core-shell impact modifiers have the advantage of a predetermined particle size; however, they must be appropriately dispersed in and coupled to the matrix polymer in order to be effective for toughening engineering plastics which are notch-sensitive. These functions can result from physical interaction of the shell material with the matrix or by chemical reaction. For the latter the most obvious route is to combine reactive moieties into the shell chains during fabrication by emulsion polymerization that subsequently react with the matrix during melt processing. An alternative is considered here for dispersing and coupling modifiers with shells based on poly(methyl methacrylate), PMMA, in a polyamide matrix. Certain styrene-maleic anhydride copolymers, SMA, are miscible with PMMA, yet through the maleic anhydride moiety these copolymers can react with polyamides during melt processing (via amine end groups). This concept leads to very tough blends based on nylon 6 but not to those based on nylon 6,6. Evidence for the proposed role of the SMA copolymer was demonstrated by transmission electron microscopy. Properties and morphology were examined for two different SMA copolymers, two core-shell impact modifiers (butadiene-based and acrylic-based rubber cores), various mixing protocols and component proportions. An attempt is made to explain the responses observed.

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Semicrystalline poly(ethylene terephthalate) was hydrolysed in water at 180°C under elevated pressure and subsequently treated with ethanol, following the etching process first developed by Miyagi and Wunderlich. The weight loss, the wide-angle X-ray scattering and the molecular weight were measured as a function of etching time. It was found that even at the end of the etching process not all the amorphous material could be removed by the hydrolysis treatment. By comparing the obtained results with those derived from an elaborate small-angle X-ray scattering study and with wide-angle X-ray scattering measurements, it was concluded that only those amorphous regions lying outside of the lamellar stacks were removed. Subsequently, the lamellar stacks themselves were attacked. It was also found that at the very beginning of the hydrolysis process additional crystals were formed in the material

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Photosensitive materials that consist of photopolymerizable monomeric components (sensitizer, initiator, monomer, chain-transfer agents, etc.) dissolved in a solid plasticized polymeric matrix are called 'photopolymers'. We investigated structural changes occurring upon exposure to light in DuPont holographic, proofing and electrostatic photopolymers. In most cases, exposure of photopolymer films and coatings is unidirectional through some type of mask or target. Patterned exposure to light leads to concentration gradients between illuminated regions of photopolymer film and unexposed regions. Similarly, due to the high optical density of these materials, photopolymerization occurs faster near the illuminated surface creating a depthwise gradient in reactant and product concentration. High molecular weight products have lower diffusivity than low molecular weight reactants. Slow product outflow from the reaction zone and a difference in mobility of reactants migrating towards reaction regions in plasticized polymer upon unidirectional exposure lead to a variety of unusual structural changes. We report observation of swelling of the film regions exposed to light, and alignment of the polymeric molecules perpendicular to the illuminated surface. This observation is in agreement with the predictions of our previous model computations. Liquid crystalline mixtures or dichroic dyes added to the formulation align during film exposure to light perpendicular to the surface, parallel to the direction of light incidence. The direction of alignment is parallel to that of the forming polymeric molecules. This directly confirms the migratory mechanism of photopolymerization in films

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The excimer fluorescence of a triblock copolymer, styrene-butadiene-styrene (SBS) containing 48 wt% polystyrene was used to investigate its miscibility with poly(vinyl methyl ether) (PVME). The excimer-tomonomer emission intensity ratio I~I E can be used as a sensitive probe to determine the miscibility level in SBS/PVME blends: lr.t/lr~ is a function of PVME concentration, and reaches a maximum when the blend contains 60% PVME. The cloud point curve determined by light scattering shows a pseudo upper critical solution temperature diagram, which can be attributed to the effect of PB segments in SBS. The thermally induced phase separation of SBS/PVME blends can be observed by measuring lrJIE, and the phase dissolution process was followed by measuring I~I,~ at different times.

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