1994 рік

The structural changes that occur during the incubation stage of tensile fatigue failure of glassy polycarbonate are investigated by means of small-angle X-ray scattering. The results are interpreted as the formation of voids, or 'proto-crazes', with initial dimensions of ~ 60nm and 150-200nm in directions parallel and perpendicular to the load axis, respectively. These values are in the range expected for a void in a static equilibrium, and practically coincide with the initial void dimensions measured in glassy polymers stretched under monotonic loading.

Детальніше...  

The crystallization of poly(vinylidene fluoride) (PVF2) in homogeneous blends with poly(methyl methacrylate) (PMMA), a copolymer of styrene and MMA (PSMMA) or poly(methyl acrylate) (PMA) was investigated to establish the conditions leading to dendritic crystals. These unusual crystals feature branches, from which grow twigs, so that a tree-like, i.e. dendritic, structure results. In blends with PMMA or PSMMA, but not in blends with PMA, the ct modification of PVF2 forms such dendrites, while the modification forms more or less perfect spherulites. The dendritic crystals appear at moderate undercoolings, up to surprisingly high PVF2 concentrations. The various spherulitic and dendritic structures observed in the blends can be explained in terms of a competition of crystallization and diffusion that produces concentration gradients in the melt.

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A soluble chloromethylated polystyrene has been used as a support for the stepwise synthesis of oligoethers. The building block was the monotrityl ether of tetraethylene glycol, and coupling, deprotection, activation and elongation steps have been successfully developed. The progress of reactions has been demonstrated by elemental analysis of the polymer and FTi.r. spectral analysis. Most powerful, however, has been IH n.m.r, spectroscopy, made possible by the soluble nature of the polymer. While successive reactions on the polymer appear to be highly efficient, work-up and isolation inevitably result in loss of polymer mass, and preclude this from being a viable methodology for scale-up

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This present work, dealing with filler/rubber interactions, provides a new approach for investigating the behaviour of elastomeric chains (styrene-butadiene rubber, SBR) in the close vicinity of carbon black surfaces. Pulsed nuclear magnetic resonance measurements have been carried out on pure SBR, and on carbon gels obtained by solvent extraction of the carbon black filled elastomers. The initial concentration of carbon black in the filler/rubber mixtures was varied between 20 and 100 parts per hundred parts by weight (phr). The high resolution proton spin-spin relaxation time, T2, has been measured for each of the individual resonance species belonging to the SBR chain. High resolution was achieved by rapidly spinning the sample at the magic angle. It has been observed that a very high magic-angle spinning (MAS) rate (> 18 kHz) is necessary to achieve a fully resolved spectrum of SBR. However, at a spinning rate of ~ 15 kHz it is possible to avoid all of the spinning side bands and to deconvolute, unambiguously, the different resonance peaks that are present in the spectrum. In the unfilled elastomer, at temperatures much higher than the T~, the chain segmental motions are anisotropic and deviate from true liquid-like behaviour. The adsorbed rubber chains are found to consist of loosely and tightly immobilized segments. The relative immobilization of the different protons has evidenced the methine ~H to be much more immobilized than the aromatic or methylene species. Therefore, the olefinic part of the butadiene segment of the elastomeric chain appears to be the most affected by the carbon black surface. Moreover, T2 is found to be independent of the filler concentration in the 30-80 phr range, and the relative concentration of the tightly bound rubber in the composite shows a maximum at a filler content of ~ 50phr, where maximum reinforcement is normally observed. As far as the molecular dynamics is concerned, highly filled systems (> 80 phr carbon black) behave differently from low and medium filled systems (< 80 phr).

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The microstructure and conformation of highly piezoelectric poly(vinylidene cyanide-co-vinyl formate), prepared by radical copolymerization, were studied by 13C and two-dimensional IH J-resolved n.m.r. spectroscopy, respectively. This copolymer was found to have a highly alternating and stereochemically atactic sequence and a predominantly trans-rich conformation about the main-chain methylene-methine bond in deuterated dimethyl sulfoxide solution. These structural characteristics do not differ greatly from those of other previously reported piezoelectrically active vinylidene cyanide copolymers with vinyl acetate, vinyl benzoate, and vinyl pivalate or from those of piezoelectrically less active vinylidene cyanide copolymers with styrene and methyl methacrylate.

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This investigation relates to the synthesis and characterization of four linear polyelectrolytes containing crystal violet and bipyridinium moieties in the main chain, with molecular weights ranging from 1154 to 3815. Of these, two polyelectrolytes had N-alkylammonium ions and the other two involved typical 2,5-diacylthiophenylene linkages with N-acylammonium and N-acyliminium ions in their respective structures. The solvation characteristics of these systems are explored through density measurements in dimethylformamide and dimethylsulfoxide solvents over the small temperature range 25___ 5°C, by analysing the concentration dependence of apparent molar volume data at various temperatures in terms of the competing solvophobic and electrostrictional interactions with the media.

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Metal-containing polyimides having ionic linkages in the main chain, and their parent polyimides, were synthesized. The structures of these materials were investigated by using solid-state n.m.r, spectroscopy and some characteristic features of the 1 s C n.m.r, spectra of the polyimides are described.

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Surface enhanced Raman scattering (SERS) studies of chemisorption of acrylonitrile and poly(acrylonitrile) on Ag are presented. The SERS spectra obtained at room temperature with low laser power irradiation are markedly different from normal Raman spectra of neat acrylonitrile and poly(acrylonitrile). This is ascribed to surface reactions after adsorption. Under high laser power irradiation, the SERS spectra show graphitic formation. When the temperature is raised, graphitization of poly(acrylonitrile) on the Ag surface is nearly complete. The reaction mechanisms consistent with the spectral results are proposed

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Polytetrafluoroethylene (PTFE) is considered to be a typical radiation degradative polymer. In this work the polymer was crosslinked by means of radiation at 335 + 5°C under vacuum. Crosslinked PTFE showed a great improvement in high temperature properties and radiation stability.

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Several novel thermotropic liquid-crystalline polymers (TLCPs) were blended with poly(ethylene terephthalate) (PET) and spun into fibres in an effort to improve fibre performance and processability. The TLCPs were screened to determine the optimal molecular architecture for fibre enhancement and compatibilty. At concentrations up to 20 wt%, blends of the block copolymers PME-b-PET and PME-b-PBT (PME=poly(ethoxy 1,4-phenylene terephthalate), PBT=poly(butylene terephthalate)) with PET showed minor improvements in modulus. Another TLCP, PBEM (poly(butoxyethoxy 1,4-phenylene terephthalate)), proved to be an excellent processing aid for PET, since it reduced the melt viscosity of PET by approximately 75 % at a concentration of only 2.5 wt%. The third class of TLCP investigated was a 'Random Copolymer' system. At a concentration of only 5 wt%, this blend exhibited a 50% modulus improvement (25 GPa) and an equivalent ultimate strength (ll00MPa) compared to neat PET fibres. SEM experiments exposed TLCP fibrils uniformly dispersed within the PET matrix with aspect ratios ranging between 20 and 200. The thermal stability of these fibres was similar to the PET control.

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