Physical properties of some organophosphazene copolymers. I. Structural aspects, transition temperatures, and stress-strain isotherms of the cross-linked networks

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).     

Deformation,yield and fracture of elastomer‐modified polypropylene

In recent decades, great attention has been devoted to the toughening of isotactic poly(propylene) (PP) with elastomers such as ethylene–propylene rubber (EPR). The most important reasons for this interest are the moderate cost and favorable properties of PP. This article is focused on the role of EPR in the deformation and fracture mechanism of PP/EPR blends with different volume fractions of elastomer phase. Differential scanning calorimetry (DSC), tensile tests, and microscopy techniques were used in this study. The fracture mechanism of isotactic PP toughened by EPR (PP/EPR) has also been studied by three point bending (3‐PB) and four point bending (4‐PB) tests. Rubber particle cavitation appears to be the main mechanism of microvoid formation, although some matrix/particle debonding was observed. The investigation of the toughening mechanism shows that a wide damage zone spreads in front of the pre‐crack. Optical microscopy (OM) illustrates that, in pure PP, crazing is the only fracture mechanism, and no evidence of shear yielding is found, while in PP blends craze‐like features associated with shear yielding are observed, which have been identified as high shear localized dilatational bands. This type of deformation pattern supports a model previously proposed by Lazzeri 1 to explain the interparticle distance effect on the basis of the stabilization effect on dilatational band propagation exerted by stretched rubber particles. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3767–3779, 2003     

Molecular mobility of crosslinked elastomers stretched above Tg

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.     

1H and 13C nuclear magnetic resonance studies of the synthesis of linear segmented polyurethane elastomers

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.     

Heat-resistant polymers - current status

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.     

Genetic algorithm for the optimization of rubber insulated high voltage power cables production lines

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.     

Large-Scale Structures in Bimodal Poly(dimethylsiloxane) Elastomers

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.     

CNT Based Elastomer‐Hybrid‐Nanocomposites with Promising Mechanical and Electrical Properties

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.

     

Tensile and elastic properties of triblock copolymer based on aramide end‐segments and polyether mid‐segments

The tensile and elastic behavior of triblock copolymers containing uniform aramide (TΦB) hard end‐segments (HS) and poly(tetramethylene oxide) (PTMO, M_n = 2900 g/mol) soft segments (SSs) was studied. The molecular weight of the copolymer was varied by changing the length of the soft mid‐segment; by extending the PTMO₂₉₀₀ with terephthalate units, the SS length was increased from 2900 g/mol to 21,000 g/mol and concurrently the aramide concentration decreased from 18 to 3 wt %. The mechanical properties were investigated by means of tensile testing, stress relaxation (SR) experiments, and cyclic tensile set (TS) tests. The E‐modulus was found to increase with increasing aramide content. The low molecular weight copolymers were brittle whereas the high molecular weight copolymers displayed large fracture strain values. The transition from brittle to ductile seemed to occur at a triblock copolymer molecular weight of 6600 g/mol. A strain‐induced crystallization was observed at strains above 250%, and both the fracture strain and stress were found to be highly dependant on the molecular weight of the copolymer. Cyclic tensile experiments showed that the materials had low TS values up to the strain hardening point. On the other hand, the SR data at 10% strain seemed to be little dependant on the molecular weight. The higher molecular weight copolymers did not display lower SR values than their low molecular weight counterparts. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

端羟基液体橡胶聚氨酯弹性体与金属粘接的研究

端羟基液体橡胶聚氨酯弹性体为柔性材料,丁羟聚氨酯弹性体硬度比丁腈羟聚氨酯弹性体小,前者的拉伸强度比后者稍大．伸长率也大;其作为粘合剂与金属粘接时,金属表面在分别经过酸化、磷化,喷砂、清洗,喷砂、清洗、涂偶联剂（KH-550）3种方法处理,其中金属表面进行喷砂、清洗、涂覆偶联剂时粘接性能最好。