1999 рік

The d phase of syndiotactic polystyrene/ethylbenzene has previously been shown to involve a sheet-like arrangement of crystal stems, with superfolding at higher molecular weights. SANS measurements have been made using tilted samples in order to separate in-plane and out-ofplane radii of gyration. Use of intensities from the whole area of the detector allows the determination of these quantities with good precision. Comparison between the out-of-plane radius and the long spacing determined from small angle X-ray measurements shows close agreement for the lower molecular weight (M̄ w ¼ 42 700) in both d and g phases, while R z is a factor of 2 larger for the higher molecular weight (M̄ w ¼ 124 500). This indicates that each molecule occupies a single lamella at the lower molecular weight and, on average, two lamellae at the higher one, in both d and g phases. This provides clear evidence of tie molecules at the higher molecular weight, and also implies that the d to g phase transition only involves a reorganisation of chain segments within those lamellae which originally contained the molecule.

Детальніше...  

Wide angle X-ray scattering (WAXS) data recorded during the drawing of poly(ethylene terephthalate) (PET) under industrial processing conditions was analysed in terms of changes in the degree of polymer orientation and crystallinity over a wide range of draw temperatures and draw ratios. The actual draw rate and draw ratio at the region in the specimen from which X-ray data was recorded was determined independently with high consistency by direct video observation of strain and total X-ray scattering. In contrast to previous claims, the start of strain-induced crystallization coincided with the end of draw within the 40 ms time-resolution of the investigation. Crystallization has been shown to follow first order kinetics and the rate constant was determined over a range of temperatures from 858C–1258C. Above 1258C little orientation is observed with no evidence of strain-induced crystallization. A detailed determination was made of the critical value of draw ratio below which strain-induced crystallization does not occur and where a relaxation in molecular orientation is observed after the extension process. The implications of these experimental observations for existing theory of strain-induced polymer crystallization are discussed.

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The dielectric properties of oxyviscose samples prepared with different oxidizing agents were investigated. The results obtained are interpreted and compared from the viewpoint of the modifications that occur in the structure of the fibres during the oxidation processes. These modifications were determined by infra-red spectroscopy, X-ray diffraction, degree of polymerization, copper number, total carbonyl content and carboxyl content. The relative permittivity and loss index were measured over a frequency range from 0.1 to 100 kHz and temperature range from ¹ 50 to 100°C. The dipolar group involved in the secondary relaxation processes was concluded to be the methylol group in the non-crystalline regions.

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In this work, the immersion precipitation process was used to obtain porous poly(l-lactide) films which were tested as supports for culture of VERO cells. The mechanism of phase separation taking place during the film formation was investigated using light scattering measurements. A theoretical ternary phase diagram was proposed and correlated with the light scattering results. The film morphology was investigated using scanning electron microscopy. For films with a more porous surface, VERO cells grew with a round shape. For films with a lower surface porosity, the growth of flatter cells was observed. These preliminary tests show that the film morphology can affect the growth of cells, or can further change the cells’ functions.

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The helium and hydrogen gas permeabilities through proton exchange membranes, poly(vinylidene)-graft-polystyrene sulfonic acid membranes (PVDF-g-PSSA), were studied using a mass spectrometric leak detector. The membranes were prepared by electron beam irradiation followed by grafting with styrene and subsequent sulfonation. In order to separately evaluate the effect of grafting and sulfonation the unsulfonated membranes (PVDF-g-PS) were also studied. The permeability was found to increase with increasing graft content of the unsulfonated membranes. The PS grafts are likely to phase separate from the PVDF matrix during grafting because of the incompatibility of the polymers. Hydrogen and helium permeabilities of PS are higher than of PVDF. Thus the increase in gas permeabilities is most likely due to gas permeation through phase separated polystyrene domains. The gas permeabilities of the dried PVDF-g-PSSA membranes were considerably lower than the values for the unsulfonated membranes. The gas permeability of the dried PVDF-g-PSSA membranes decreased around d.o.g. 35% which is the graft penetration limit. Similarly the ion exchange capacity of the membranes increased at this point. In the water saturated state the permeabilities increased with increasing water content of the membrane. The gas permeability of the water saturated PVDF-g-PSSA membranes reached a plateau value at the graft penetration level. The gas permeation in the water saturated state is mainly governed by the diffusion of the gas in water. The highest permeabilities for PVDF-g-PSSA membranes were of the same order of magnitude as for the commercial Nafionw 117 membrane. These permeabilities were measured for membranes with d.o.g. , 70%. Thus the PVDF-gPSSA membranes are good candidates for the use as electrolytes in electrochemical cells.

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The compatibility of melt-mixed blends of a partially neutralized poly(ethylene-co-methacrylic acid-co-isobutyl acrylate) Zn 2þ ionomer with a polyester-type polyurethane (PU), reported in Part 1 of this work, allowed application of the ionomer as a compatibilizer for incompatible PU/high-density polyethylene (HDPE) blends. The techniques applied were dynamic mechanical analysis (d.m.a.), tensile testing, differential scanning calorimetry and scanning electron microscopy (SEM) of cryofractured surfaces. Once optimization of mixing conditions was established, various composition parameters were examined such as PU/HDPE ratio and compatibilizer content. Blend performance was assessed by means of tensile testing and d.m.a., while morphology was studied mainly by SEM and indirectly by large- and small-deformation behaviour. Although all ternaries studied at ionomer contents of ca. 15 wt% had a satisfactory level of ultimate tensile properties, the PU-rich polymeric alloys performed best.

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This paper shows that a quartz crystal resonator is applicable to the investigation of rheological property changes of polymer blend and polymer films by the simultaneous measurement of resonant frequency and resonant resistance of the quartz crystal resonator. Phase transition phenomena of PMMA/PVAc blend films were studied in the thermal cycles. Viscoelastic property changes of PMMA homopolymers including a certain amount of plasticizer and the dependence of molecular size in blend film on phase transition temperature were also investigated. Both the resonant frequency and resonant resistance of film coated quartz crystals changed critically at the glass transition temperature. The responses of the resonant resistance showed more clear change than those of the resonant frequency because the resonant resistance directly reflects the information of energy dissipation in the coated films. The responses in the thermal cycle were represented as diagrams of the resonant frequency versus the resonant resistance and dR/dF versus temperature for detailed explanations. These diagrams showed clear glass transition temperatures of the films in the thermal cycles.

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High molecular weight segmented polyurethaneurea (PUU) copolymers based on an aliphatic diisocyanate, bis(4-isocyanatocyclohexyl)methane and mixed hydrophilic and hydrophobic soft segments were prepared. Hydrophilic blocks consisted of poly(ethyleneoxide) (PEO) of molecular weight 1450 g/mol, whereas the hydrophobic blocks were poly(tetramethylene oxide) of molecular weight 2000 g/mol. Ethylene diamine was used as the chain extender. Hard segment contents of the copolymers were kept constant at 18%, whereas PEO contents were varied between 0% and 50% by weight. Water vapor permeation rates (WVPR) of thin films (23–178 mm) cast from dimethylformamide solutions were determined. In studies performed at 238C and 50% relative humidity, the relationship between PEO content and WVPR followed an S-shaped curve. For copolymers containing up to about 15% by weight of PEO, WVPR were fairly low. This was followed by a region where WVPR increased continuously for membranes containing between 15% and 30% PEO. Further increase in PEO content above 30% did not influence the WVPR substantially. There was also a dramatic increase in WVPR with an increase in temperature from 238C to 378C. Activation energy of permeation was determined to be 91.5 kJ for PUU containing 22.0% by weight of PEO. Equilibrium water absorption levels of PUU containing different levels of PEO in their backbone structures followed a similar trend to that of WVPR. Hydrophilic PUUs showed good tensile properties and mechanical integrity even at very high levels of water absorption.

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Hydrogels with varying crosslink densities and ionic moieties were prepared from the ternary systems N-vinyl 2-pyrrolidone/itaconic acid/ water by irradiation with g rays at ambient temperature. The influence of external stimuli such as pH, temperature and ionic strength of the swelling media and the type of buffer on the equilibrium swelling properties were investigated. Hydrogels showed typical pH response and temperature responses, such as high-pH and low-temperature swelling and low-pH and high-temperature deswelling. A change in the ionic strength of the swelling solution from 0.01 to 0.20 caused a decrease in the equilibrium degree of swelling of hydrogels. Oscillatory swelling behaviour was also observed and investigated in response to changes in the pH of the solution.

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The aggregation behavior of gel microspheres of it-poly(methyl methacrylate) (it-PMMA) from xylene solution was studied using optical microscopy, electron microscopy, DSC and wide-angle X-ray diffractometry. When the dilute solution (0.05–2.0 wt%) of it-PMMA (M n ¼ 64 000, triad tacticity, 92% isotactic) was cooled to 25°C, the it-PMMA precipitated as swollen, crystalline gel-microspheres (G-MSs) with a diameter of about 1 mm, which stick together to form complex dendritic clusters. It has been found that the resulting clusters are well described as fractals. The fractal dimension of the aggregates is estimated to be about 1.5. On the other hand, when the rather concentrated solution (⬃2.0 wt%) was cooled to 10–15°C, G-MSs aggregate to form the porous, highly swollen gel. The process of aggregation of the G-MSs and the structures of gel of it-PMMA are important technologically and scientifically.

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