1999 рік

The emulsion polymerization of lauryl methacrylate and its copolymerization with trimethylolpropane trimethacrylate (TMPTMA) were investigated. The induction period (IP) for LMA polymerization was observed to be very long as compared to substantially no IP for methyl methacrylate polymerization. The addition of TMPTMA as a crosslinker clearly prolonged IP, owing to the reduced radical entry of oligomeric growing radical having rather hydrophilic and crosslinkable TMPTMA units into the polymer particle surface. Besides, no gelation was observed even above 90% conversion for LMA/TMPTMA(80/20) copolymerization. The initial abrupt increase in the conversion dependencies of weight-average molecular weight results from the compartmentalization of reaction loci and the gel effect.

Детальніше...  

The molecular interdiffusion across a poly(vinyl pyrrolidone)/vinyl ester monomer (PVP/VE) interface was investigated in situ by Fourier transform infrared attenuated total reflectance (FTi.r.-ATR) spectroscopy. In order to separate the effects of the vinyl ester monomer diffusion and the VE crosslinking reaction, ATR experiments were carried out at temperatures below the normal curing temperature of the VE. The infrared bands at 1717 and 1507 cm -¹, characteristic of the vinyl ester monomer, and the bands at 1669 and 1419 cm -¹, characteristic of the PVP, were used in a quantitative analysis. Diffusion coefficients were determined by following intensity variations in these selected characteristic bands as a function of time, and fitting this data to a Fickian model. A mutual diffusion coefficient on the order of 2 х 10 -8 cm 2/s was calculated at 100°C. Although the magnitude of the diffusion coefficients obtained was consistent with values found in the literature for diffusion of small molecules in polymers, the simple one-dimensional Fickian diffusion model employed as a first approximation had some limitations for the particular (PVP/VE) system. Because the glass transition temperature of PVP changed as diffusion proceeded, the mutual diffusion coefficient should not stay constant. In fact, it was shown that the T g can drop by as much as 140°C during the diffusion process.

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The tensile and compressive deformation behaviour of a new type of heterocyclic rigid-rod polymer fibre, polypyridobisimidazole 'PIPD’ or 'M5’, produced by the spinning of liquid-crystalline solutions, have been studied. The Young’s moduli and tensile strengths for the two different PIPD or M5 fibres have been determined to be greater than 300 GPa and 4 GPa, respectively. Molecular deformation in the fibres has been monitored using Raman spectroscopy and it has been shown that the peak positions of the Raman bands in the fibres are sensitive to the application of mechanical stress and strain. This has enabled a detailed study of molecular deformation to be undertaken for both tensile and compressive deformation. Full tensile-compression stress–strain curves have been derived and the compressive modulus and compressive strength of the PIPD fibres have been determined to be greater than 300 GPa and up to 1 GPa, respectively, which are both considerably higher than values for other high-performance polymer fibres. The potential use of PIPD fibres as reinforcing fibres in high-performance composites is discussed.

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We have performed EELS analysis of poly(methyl metacrylate) (PMMA) in the analytical TEM in order to evaluate the possibility to obtain chemical information on electron sensitive polymers. In the acquired spectra, we propose an identification to the different chemical bondings in agreement with molecular orbital calculations and with previous XANES experiments. Moreover, main results confirm that PMMA is very sensitive to electrons with a critical dose of about 10 2 C m 22. The evolution of the EELS spectra of PMMA during irradiation damage is found to be slightly dependant on the molecular weight of the polymer. Finally, a degradation process in agreement with EELS data is proposed.

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Polyamide 6 (PA6) hybrid composites reinforced with potassium titanate whiskers (K 2Ti 6O 13) and liquid crystalline polymer (LCP) were prepared in a twin-screw extruder followed by injection moulding. The whiskers were surface treated with tetrabutyl orthotitanate prior to blending. Static tensile measurements showed that the tensile strength and modulus of the composites tend to increase with increasing whisker content. However, Izod impact tests indicated that the hybrid composites show little variation in the impact strength with the addition of whisker content up to 10 wt%, thereafter the impact energy shows an obvious decrease with increasing whisker content. Scanning electron microscopic observations revealed that the oriented LCP fibrils were formed in the skin section of hybrid composites containing whisker content > 10 wt%, whilst spherical droplets were dispersed in the skin section of hybrids containing whisker content < 15 wt%. Torque measurements showed that the LCP is very effective in reducing the melt viscosities of PA6/K 2Ti 6O 13 composites. Finally, thermogravimetric analysis showed that the hybrid composites with higher whisker content exhibit good heat resistance properties. The hybrid effects of the two reinforcements on the mechanical properties and compatibility of these hybrid composites are discussed.

Детальніше...  

Water is absorbed by most polymers, but a change of properties is induced only in specific types of polymers. Water is considered to be present in the free volume and active only when attached to polymer chains by hydrogen bonds. The materials considered are PEI, PEEK, PES, PC, PA 12, and PA 6. The changes in damping spectra, moduli, and thermal expansion due to the moisture absorption have been investigated in the broad temperature range of 4.2–320 K. It is surprising that properties are changed by water in a different or opposite ways at low and high temperatures. Even at very low temperatures, absorbed water influences the mechanical performance in an unexpected way. The results at low temperatures might be a tool for a better understanding of the features of the hydrogen bonded water, which itself could be a sensor for analysing molecular mobilities. The interpretation of results is generally not yet clear and some cryogenic results are in contradiction to well established correlations for dry polymers. Ideas for solving these problems are given in the conclusions of this study.

Детальніше...  

The gelation process in chainwise crosslinking polymerisation of methacrylate resins with average functionalities of 2.1–50 (1.05–25 methacrylate groups/molecule) was investigated using dynamic mechanical analysis. For all systems a crossover in tan d (tan d independent of frequency) was observed at gelation. The gel time was found to decrease with increasing functionality of the system. Within the measured frequency ranges, power law behaviour for the dynamic modulus was observed at the gel point in all systems. A value of the power law exponent n of 0.4 ^ 0.2 to 0.6 ^ 0.1 (increasing with increasing functionality of the system) was determined for gelation during reactions at T $ Tg∞. This trend suggests that the differences in screening between the systems dominate over the difference in fractal dimension. For reaction temperatures below Tg∞ a value of n ˆ 0.3 ^ 01 was obtained, which was attributed to the influence of micro-vitrification.

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Co-continuous morphologies are found over a wide composition range (30–80 vol%) in blends of a polystyrene thermoplastic and a poly(ether-ester) thermoplastic elastomer when they are blended below the block copolymer’s order–disorder transition (ODT). This range decreases with increasing processing temperature and becomes limited when both polymers show pseudo-plastic behaviour. Annealing of the co-continuous blends below the ODT hardly influences the phase sizes and composition range, but when annealing takes place at higher temperatures the blends show an increase of their phase domains and the composition range decreases. The condition for the formation of cocontinuous morphologies, especially at low volume fractions, is the existence of stable interconnected elongated structures, that do not show breakup or retraction. Breakup and retraction experiments on poly(ether-ester) fibres embedded in a polystyrene matrix make clear that these mechanisms can be severely limited or even stopped and therefore the condition for the existence of co-continuous morphologies is fulfilled.

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Two model linear polyethylenes of different molecular weights and very narrow molecular weight distributions were subject to g-ray irradiation under vacuum. Irradiations were conducted at two temperatures: 25 and 135°C. Number average and weight average molecular weight of irradiated samples was measured by LALLS and membrane osmometry, and the distribution of molecular weights was studied by GPC. The presence of a low but measurable amount of scission was verified. A mathematical model of the irradiation process was developed, which gave very good predictions of the measured values.

Детальніше...  

The effect of shear on the alignment of the lamellar phase in a concentrated aqueous salt solution of a poly(oxyethylene) and poly(oxybutylene) diblock copolymer is investigated in situ using small-angle X-ray scattering (SAXS). The alignment of the lamellae is deduced from the anisotropy of the scattering patterns. It is found that the orientation of the lamellae with respect to the shear direction alters with an increase in the shear rate: the lamellae are oriented transverse to the shear direction at low shear rates, and perpendicular to the vorticity direction at high shear rates. The transition between the two lamellar alignments occurs at a critical shear rate in the range: 10 s 21 , g c , 100 s 21.

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