1994 рік

Macroporous particles, prepared by the suspension polymerization of ethylene dimethacrylate (EDMA), possessing high specific surface areas (558 m 2 g-1) and pore volumes (6.9 ml g-1), contain unreacted double bonds which are accessible for free radical polymerization. The amount of unreacted double bonds was determined by 13C CP/MAS n.m.r. These bonds were used for the functionalization of poly(EDMA) particles by the solution grafting copolymerization of glycidyl methacrylate (GMA) into the pore structure of the particles. The poly(GMA) formed in this way is permanently attached to the particles. By this method composite particles, containing up to 77 wt% of oxirane groups (determined by i.r. spectroscopy), were obtained. However, the presence of grafted polymer in the pores leads to a decrease in both the pore volume and the specific surface area. The oxirane groups can be chemically modified, as evidenced by their reactions with hydrogen chloride and 2,2,6,6-tetramethyl-4-aminopiperidine

Детальніше...  

New high-capacity ion exchange materials have been prepared utilizing the structural feature of dendrimeric 'balloons' or'strings' attached to a polymer backbone. The dendrimeric regions have covalently incorporated ionic sites at the core and branch points (anion exchanging materials) or at the termini of the chains (cation exchanging materials).

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Polydiacetylene yellow solutions of the derivatives 2-, 3- and 4BCMU and 9PA in chloroform were prepared with a range of different molecular weights. The measured wavelength of the optical absorption spectrum peak maximum 2,,,x showed a continuous increase with increasing chain length up to lengths of nearly 5000 repeat units. A theory is proposed that this is due to the excluded volume effect. The persistence lengths of the polymer chains in the solutions were calculated from 2 .... using small angle neutron scattering data from the literature. The persistence length is shown to obey a direct power-law relationship with physical chain length, which is in good agreement with the theory. Numerical parameters in equations derived from these results, using the theory, are in good agreement with those calculated from independent light scattering and intrinsic viscosity measurements in the literature for other polydiacetylenes and polymers. Furthermore, it is shown that realistic values for the expansion coeflcient ~, can be calculated from the derived equations.

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The high pressure polymerization of aromatic dicyanates was carried out by using a piston cylinder type hot-pressing apparatus, which led to the formation of crosslinked polycyanurate resins. In a model compound study, aromatic cyanates were found to cyclotrimerize under high pressure to give aromatic cyanurates, with the application of high pressure accelerating the cydotrimerization process. Aromatic dicyanates polymerized at 150-280°C under pressures of 200-450MPa, affording polycyanurates through a cyclotrimerization reaction. Polymerization under higher pressure required both higher temperatures and longer reaction times. After further curing of these crosslinked polymers, very hard resins were produced, with Vickers hardness values of 330-530 MPa and with a high modulus (>2 GPa), when compared with the same polymers synthesized under normal atmospheric pressure

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The variation of tensile strength and elongation at break of styrene-butadiene-styrene triblock copolymer containing 34.1, 40.0 and 52.5 % styrene have been studied with respect to their morphology and the extent of crosslinking produced by irradiation with 6°Co rays. We assume that the existence of microphase separation between polybutadiene (PB) and polystyrene domains and entanglements between PB segments result in the unusual variation of the two characteristics with radiation dose, which is different from that in common elastomers and blends.

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A detailed study of the phase transitions in poly(octamethylene p,p'-dibenzoate) (P8MB) was carried out using differential scanning calorimetry. The thermodynamic parameters of the mesophase of P8MB have been determined by maintaining this phase at high temperatures to prevent its transformation into the crystal. If the crystal is formed, however, it exhibits a monotropic transition to the isotropic melt, and consequently the mesophase is not obtained on heating. This monotropic character is reflected in the free energy diagram of P8MB, as the crystal melting point is higher than the isotropization temperature (Tcr~ i > Tic.i). From the free energy diagram it is also deduced that the crystal phase is thermodynamically the most stable one at any temperature below its melting point, although at low undercoolings the mesophase is always formed first for kinetic reasons. Moreover, the liquid crystalline (LC) glass is obtained by fast quenching from the isotropic melt, as revealed by X-ray analysis. This LC glass is stable at room temperature owing to a glass transition at 42°C, although it undergoes a relatively rapid ageing process at that temperature. Finally, the viscoelastic relaxations have been analysed for two samples of P8MB with different thermal histories.

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The deformation mechanism of amorphous poly(ethylene terephthalate) (PET) has been studied as a function of molecular weight and entanglement density in predrawn films. PET chips (intrinsic viscosity = 0.6-4.9 dl g- 1) were dissolved in mixed solvent of 1,1,1,3,3,3-hexafluoro-2-propanol and dichloromethane. The polymer solutions (polymer conc. = 2-40 wt%) were frozen at -30°C and then most of the solvent was removed at -30°C under vacuum, resulting in amorphous films with various entanglement densities. The films were drawn by solid-state coextrusion at 70°C. It was found that the initial polymer concentration used for film preparation had a marked effect on the maximum achievable extrusion draw ratio (EDRma,) of the films, especially for higher molecular weights. The optimum concentration decreased and the highest EDR~a X increased with increasing molecular weight. The deformability was also affected by the stress-induced crystals, which might act as net points. The tensile modulus and strength of drawn films were related to draw ratio and molecular weight. The higher the draw ratio and molecular weight, the higher were the tensile properties of drawn samples. The improved efficiency of draw with higher molecular weight was explained by the suppression of disentanglement and relaxation of oriented amorphous molecules during deformation; both are important in the development of structural anisotropy and continuity along the draw direction

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The dependence of thermal degradation behaviour on vinyl acetate (VA) content of ethylene-vinyl acetate (EVA) copolymers was studied by thermogravimetric analysis (t.g.a.). Among the parameters investigated, the maximum rate of weight loss at the stage of acetic acid evolution was found to have a linear relationship with VA content and to be independent of molecular weight and heating rate. Thus, a new method based on t.g.a, for the prediction of VA content of EVA copolymers is suggested.

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The analysis of a polymer obeying the log-normal molecular weight distribution (MWD) by means of size exclusion chromatography with dual concentration/light scattering or viscosity detection was theoretically examined. The relations between the theoretical and experimental values of MWD parameters and Mark-Houwink-Kuhn-Sakurada constants calculated by comparing viscometric, light scattering and effective linear calibrations to the universal one were calculated as functions of the instrument parameters (interdetector volume and resolution factor). The linearity of calibrations calculated from the dual trace directly or indirectly, by combining with the universal calibration, is preserved.

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Extended periods of ionization of poly(p-phenylene terephthalamide) (PPTA), namely Kevlar 149, fibres by sodium methylsulfinyl carbanions leads to a significant deterioration in the fibre tensile properties. The non-chain-breaking nature of the ionization suggests that hydrogen bonds among the PPTA chains in the intercrystalline regions also contribute to the axial strength of the fibres. Rinsing with dimethylsulfoxide (DMSO) following the ionization initially improves the tensile strength. With DMSO immersions of 10 h or longer, the tensile strength loss resumes, and can be explained by hydrolytic chain scission of the PPTA molecules in the intercrystalline regions. Ionization which does not affect the fibre strength of Kevlar 49 is effective for structural differentiation between Kevlar 49 and 149 fibres. In a similar fashion to ionization, etching by argon glow discharge produces little change in Kevlar 49 fibre structure and strength. The fact that both ionization and glow discharge treatment affect PPTA fibre properties similarly demonstrates that both reactions depend on the accessibility and precise microstructure of these fibres.

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