Analysis of several poly(organophosphazene) copolymers with variedmeta- andpara-substituted phenoxy repeat units indicated that the copolymers had random sequences, with compositions very close to the equimolar feed ratios used in their synthesis. DSC measurements indicated that they were semicrystalline in spite of this randomness. Networks prepared by cross-linking these copolymers were studied in elongation above their melting points, and stress-strain isotherms thus obtained showed the presence of strain-induced crystallization. Raising the temperature decreased their values of the modulus, presumably by melting some of these crystallites. Swelling the networks also decreased their values of the modulus (at least partly because of solvent-induced depression of the melting point) and decreased their values of the elongation at rupture (presumably because of the stretching-out of the chains by the imbibed solvent prior to the elongation measurements). 相似文献
A uniaxial deformation is performed on a styrene-butadiene elastomer and two nitril butadiene elastomers with two different crosslinking densities in their rubbery state. The slow α-relaxation process is investigated under constant deformation using low frequency dynamic mechanical analysis. No effect is observed in the stretched elastomers although microstructural changes are detected thanks to tensile tests and wide angle X-ray scattering experiments. A possible explanation of these results is that the inter-molecular interactions are not significantly modified in the stretched network. 相似文献
Linear segmented polyurethane elastomers have been prepared by reaction of polyethyleneoxide with 4,4′-diphenylmethanediisocyanate and the product was subsequently cured with diamines. Chemical analysis of the prepolymer formed using stoichiometric ratios of polyol and diisocyanate indicated that the product was deficient in ‘free’ isocyanate. A number of side reactions can lead to a reduction in the amount of isocyanate available for the chain extension reaction. Infrared, 1H- and 13C-n.m.r. spectroscopy were used to follow the initial end-capping and subsequent chain extension reactions. In order that an unambiguous assignment of the weak peaks observed in the prepolymer and final elastomer spectra could be made, a number of model compounds associated with the side reactions were synthesized and their spectra characterized. It was concluded that the loss of ‘free’ isocyanate in the formation of the prepolymer can be predominantly attributed to the effects of traces of water in the initial reaction mixture. 相似文献
The current state of the art is reviewed in respect of heat-resistant polymers, which are used as elastomers, and as thermoplastic or thermosetting compositions. 相似文献
The optimization of vulcanized EPM/EPDM mechanical properties must be a fundamental task for electric cables producers. Insulator mechanical properties depend on the vulcanization degree of the rubber, which is obtained by manufacturers by means of a number of different peroxides acting as reticulation inducers. The industrial production process consists in the extrusion coating of the conductor with elastomeric compounds, and in the successive vulcanization of the insulator rubber through the continuous vulcanization tube, a pressurized high temperature tube filled with nitrogen (the high temperature is a condition necessary to activate the cross-linking agent). Finally, water and/or air are used to cool the insulated cable at ambient temperature. Changes of process variables associated with the vulcanization tube can cause considerable changes in output mechanical properties of the elastomers.In this framework, in the present paper a novel genetic algorithm with zooming and elitist strategy is used for the determination of optimal production line parameters to use in order to maximize rubber output mechanical properties. Nitrogen temperature and rubber exposition time are assumed as production parameters to optimize, whereas EPM/EPDM final tensile strength is considered as objective function.The GA approach proposed exploits a specifically crafted zooming strategy, consisting in the subdivision of the population at each iteration into two sub-groups, depending on individual's grade of fitness (elitist strategy). Different genetic procedures are applied to the sub-groups, consisting of both two typologies of admissible mutations for the elite sub-population and mutation and reproduction for the remaining individuals. In order to improve algorithm convergence, a user-defined population percentage, depending on individual's fitness, is replaced with new phenotypes at the end of each iteration, enforcing in this way the chromosomes renewal.In order to test the reliability of the method proposed, a technical meaningful case study consisting of a high voltage cable used by Italian Railways and cross-linked with a number of different peroxides is discussed. Numerical results show that particular care has to be used by practitioners in the choice of nitrogen temperature and exposition time of production lines to obtain high quality products. 相似文献
Small-angle X-ray and neutron scattering are used in conjunction to study structures in a poly(dimethylsiloxane) network having a bimodal distribution of network chain lengths, a type of elastomer of considerable interest because of its unusually good mechanical properties. The scaling regimes for the scattering are compared and contrasted for varying degrees of network swelling. The excess scattering at small angles in the swollen state is associated with large-scale structures, likely high cross-link density clusters with reduced swelling, topologically trapped within the network mesh. These results indicate the presence of supramolecular structures with apparently smooth surfaces even in the non-swollen state for this type of elastomer. Optical microscopy of unimodal and bimodal elastomers having comparable crosslink staining does in fact suggest the presence of smooth surfaced, micron-scale islands of higher cross-link density. Finally, the effects of the molecular weight distribution of the network mesh demonstrate that bimodal networks can have supramolecular structures that are smaller but more random than their unimodal counterparts. 相似文献
CNT based elastomer‐hybrid‐nanocomposites prepared by melt mixing have been investigated showing promising results in technologically relevant electrical, mechanical, and fracture‐mechanical properties. It is demonstrated that the incorporation of CNT in silica‐filled natural rubber results in a good dispersion of the CNT. The materials show an enhanced mechanical stiffness and tensile strength, an increased modulus, and a high electrical conductivity with quite low amounts of CNT, though the tear resistance under dynamical loading is slightly reduced. Using DMA and dielectric spectra, a better understanding of the conduction mechanism, the polymer/tube interaction, and the filler networking in CNT nanocomposites is achieved.