1992 рік

The dielectric properties of homogeneous solutions of amine-terminated butadiene acrylonitrile (ATBN) elastomer, diglycidyl ether of bisphenol-A and Ancamide have been measured during their curing at 298 and 373 K. As curing progresses, the dielectric permittivity, e', decreases and dielectric loss, e", first decreases and then peaks. The e' and e" of the cured thermosets increase with increase in the elastomer content but decrease with increase in the temperature of cure for the same elastomer content. The rate of curing becomes slower with increased amount of ATBN in the thermoset. A prominent relaxation process in the cured thermoset is the a-relaxation of the elastomer, whose height increases with increase in the elastomer content. The broad shoulder to the low-temperature side of the peak is due to the y- and//-relaxation processes of the epoxy. The dielectric properties of the thermosets are relatively insensitive to the inversion of the thermoset matrix to an elastomer matrix as the elastomer concentration is increased.

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Polyetherethersulphone-polyethersulphone (PEES-PES) copolymers have been chloromethylated using chloromethyl butyl and octyl ethers as the source of the chloromethyl group and trifluoromethylsulphonic acid (triflic acid) as both cosolvent and acidic catalyst. This combination is successful, in contrast to other systems which failed to produce chloromethylated, uncrosslinked materials. Nuclear magnetic resonance studies indicate that the chloromethylation proceeds exclusively on the hydroquinone residue of the PEES unit

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Solubility coefficients for CO2, Ar and CH4 in polystyrene (PS) films with glass transition temperatures (Tg) of 52 to 107°C were measured in a pressure-decay sorption apparatus at 1 atm over a temperature range of 20 to 90°C. The gas solubility at 30°C increased as the T s of the PS increased; the heat of solution was largest for the PS with the highest Tg. This observed dependence of gas solubility on T s was analysed in terms of enthalpy-temperature relationships for glassy polymers. The solubilities for PS samples with different glass transition temperatures converged when comparisons were made on the basis of states of equal enthalpy instead of the temperature of measurement.

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The transport of small molecules in polyolefins is a subject of both technical and commercial importance. It is technically important because the factors, such as morphology and molecular interactions, that control diffusion are not well determined. Also, the existing theory of diffusion, based on macroscopic parameters, is difficult to interpret in terms of the interactions occurring between the polymer matrix and the diffusants. It is commercially important because the use of polyolefins in applications such as wire coatings, barrier layers and structures is extremely widespread. Failure of such applications can impact the lifetime of large amounts of capital investment. Examples such as wire and cable, landfill liners and automotive components all have expected lifetimes. Previous research indicates that loss of stabilizers may be the major factor in failures and that previous estimates of the loss mechanisms have been inaccurate due to a lack of basic knowledge about the transport properties of polyolefins. This review attempts to cover the previous work on transport in polyolefins and to correlate the results where possible.

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Investigated ternary composites assume two phase structures: (i) ethylene-propylene elastomer (EPR) and filler are dispersed separately in polypropylene (PP) matrix; (ii) EPR encapsulates filler particles, thus forming a low-modulus interlayer between the filler and PP. Formation of these phase structures is promoted by acid-base interactions when functionalized PP or functionalized EPR is added. PP functionalized with maleic anhydride or acrylic acid enhances the interfacial adhesion between matrix and filler, which prevents dewetting of filler particles and thus accounts for higher yield stress and alteration in failure mechanism. Alternatively, the addition of 5 to 20 vol% of an analogously functionalized EPR results in extensive filler encapsulation, i.e. formation of core-shell particles. The functionalized elastomers have lower molecular mobility manifested in smaller dissipative capacity and higher glass transition temperature. Thus, it seems advisable to combine functionalized species with a standard EPR to ensure good interfacial adhesion and high molecular mobility of the interlayer. Simple model calculations indicate that virtually complete encapsulation of filler can be achieved. Composites with core-shell particles have lower yield stress and higher impact strength than composites with functionalized matrix. These two mechanical properties were found to be inversely proportional in all series of tested composites. However, core-shell particles are apparently less effective impact modifiers than elastomer inclusions, presumably because of improper size (predestined by the size of filler cores) and/or shell modulus.

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Fibres and films were formed from mesophase solutions of cellulose in ammonia/ammonium thiocyanate solvent. The physical properties of the fibres and films were discussed on the basis of the properties of the mesophase solutions used. Cholesteric and nematic phases were formed in the solutions. Fibres were extruded from both kinds of solution. The fibres from the nematic solutions were more highly oriented, had a more fibrillar texture and were apparently stiffer than those from the cholesteric solutions. The fibres spun from the nematic solutions using the technique of dry jet-wet spinning had moduli comparable to those of Fortisan ~.

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The gas sorption and transport properties of a series of polysulphones with tetramethyl ring substitution are reported. The results for tetramethylhexafluorobisphenol A polysulphone (TMHFPSF) and tetramethylbisphenol F polysulphone (TMPSF-F) are compared to tetramethylbisphenol A polysulphone (TMPSF) and bisphenol A polysulphone (PSF). The effect of the substituents on chain mobility and chain packing has been related to the gas transport properties. Dynamic mechanical thermal analysis and differential scanning calorimetry were used to judge chain mobility, while X-ray diffraction and free-volume calculations give information about chain packing. Permeability measurements were made for He, H2, 02, N2, CH4 and CO2 at 35°C over a range of pressures up to 20 arm. Sorption experiments were also done for N2, CH4 and CO2 under the same conditions. The permeability coefficients of the methyl-substituted polymers rank in the order TMHFPSF >> TMPSF > TMPSF-F for all of the gases. This is the same order as the unsubstituted materials, hexafluorobisphenol A polysulphone (HFPSF), PSF and bisphenol F polysulphone (PSF-F), which suggests that the effects of tetramethyl substitution are somewhat additive with other structural modifications. Relative gas permeability values are in agreement with relative fractional free-volume values as well as with ~, transition temperatures.

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The rate of diffusion of ethyl acetate (EtAc) in poly (vinylidene fluoride) (PVF 2 ) has been monitored using a newly developed rheo-photoacoustic Fourier transform infra-red technique. The rate depends on the degree of applied strain, the structural changes occurring during the polymer plastic deformation, the concentration of EtAc in PVF 2 as well as the morphology of the polymer. These relationships are discussed in terms of the fractional free volume (FFV) theory of transport properties in polymers. The data indicate that for larger FFV, the rate of diffusion increases. Furthermore, the elongations at which plastic strain reaches a maximum, the changes that occur during plastic deformation and the difference between relaxed and non-relaxed plastically deformed films can be determined using specially designed rheo-photoacoustic Fourier transform infra-red apparatus.

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The uniaxial compression stress-strain curves of the loading and unloading cycle and the corresponding maximum points of yield for poly(methyl methacrylate) in the temperature range, T= 233-343 K and engineering strain range, ~t= 10-4-10 -1 s -1 are measured. A micromechanics model for the thermally activated plastic deformation of glassy polymer is suggested and the exact theoretical expression for plastic strain rate, ~p, is obtained. The thermodynamic parameters characterizing the plastic deformation process, including activation volume, V~, activation enthalpy, AH, and activation entropy, AS, and their curved profiles changing with condition variables T and ~t are provided. The number of structural repeat units in a V~ for various T and ~t values are calculated. The changing trends of parameters AH and AS with V~ and the relationships between AH and AS for various T and ~t are presented and compared with that between AHm and ASm for viscous flow in the molten state of the same polymer. The results show that the plastic deformation of glassy polymer can be regarded thermodynamically as the non-Newtonian viscous flow of overcooled melt.

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The ability of nine amphiphilic quaternary ammonium derivatives of cholesterol to form small unilamellar vesicles in water has been investigated using the entrapment of [14C]sucrose coupled with gel filtration. In all cases vesicles were detected and the proportion of [14C]sucrose entrapped suggested that these were indeed of the small unilamellar type. In the case of one amphiphile with a pyridinium ion head-group, studies over a period of four weeks showed the vesicles to be very stable, although there was some evidence for fusion and growth of the lamellar structures. Five of the amphiphiles also contained a polymerizable methacrylate group. Photoinitiation of polymerization of these in aqueous dispersions using 2,2-dimethoxy-2-phenylacetophenone as the initiator yielded only oligomeric products with DP,~5-13. In contrast polymerization in isotropic benzene solutions using thermal fragmentation of azobisisobutyronitrile yielded polymers with DP, up to ~ 180. In aqueous solution the 'rigid' bilayer structures may impose some additional steric restriction to polymer growth, a restriction that is absent in benzene solutions.

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