1999 рік

A model for hysteresis loss of rubber vulcanizates at medium strain (less than 100%) under dynamic condition has been proposed by using Boltzmann superposition principle, statistical theory of rubber elasticity and phenomenological theory. The theory incorporates both experimental and analytical parameters to quantify hysteresis loss. The model with no adjustable parameter has been successfully tested using the experimental results for natural rubber (NR) and styrene-butadiene rubber (SBR) vulcanizates having variations of loading of carbon black, silica, clay, resin and curatives, at various strain rates, strain levels and temperatures. The model is virtually indistinguishable from other well-known models at low strain.

Детальніше...  

This paper will discuss the nature of the gas induced damage in a poly(vinylidene fluoride) (PVDF) exposed to 'explosive’ or rapid pneumatic decompression in a carbon dioxide environment. Generally, under high pressure, the polymer absorbs significant amounts of gas. The large quantity of gas absorbed may induce a dilatation of the specimen and also influence the mechanical properties of the polymer. When the ambient pressure is reduced, the polymer will often suffer irreversible mechanical damage; this process is usually termed explosure decompression failure or 'XDF’. This paper is presented in four parts. The first part will describe the variation of the mass of the PVDF polymeric sample in a carbon dioxide medium during the compression and the decompression process at 80°C in the pressure range of 0.1– 30 MPa. It is shown that the mass of the specimen increases as the pressure increases but, upon decompression under the conditions described, it is quite constant until 15 MPa and then it falls rapidly. The second part describes the corresponding volumetric change of the polymer, in situ, during the compression and the decompression at 80°C. The decompression curve exhibits a dilatation peak centred at around 14 MPa. The decrease of the mass and the size of the sample is related to the generation of cracks in the polymer matrix during decompression. The third part will describe the results of a mechanical deformation experiment on samples exposed to compression– decompression cycles which is used to establish a relationship between the loss of mechanical properties of the sample and the loss of internal coherence due to the presence of internal cracks. A final part summarises the major conclusions which indicate conclusively that the polymer is irreversibly damaged in the decompression process adopted in the current study.

Детальніше...  

The morphology and the coarsening behavior with and without compatibilizer were investigated by using scanning electron microscope. As a new compatibilizer, styrene–acrylonitrile–maleic anhydride terpolymer (S–AN–MAH) was used. For Polycarbonate (PC) /Nylon6-co12(50/50) blend, modulated structure was observed. And coarsening behavior was similar to that of spinodal decomposition. However, for PC/Nylon6-co-12(50/50) blend with S–AN–MAH, sea–island structure was developed and the size of domain did not increase although the sample was annealed for 90 min, at above Tm. It was confirmed that S–AN–MAH retards coarsening of morphology.

Детальніше...  

The structural and dynamic properties of polymers, with excluded volume interactions, in simple shear flow, are investigated by a MonteCarlo algorithm. Chain lengths of 5, 100 and 150 beads have been studied. For middle-sized polymers (100 or 150 beads), in strong shear, after the polymers have reached maximum elongation, beyond a shear rate threshold, the rear back of the polymer chains begins to fold and to spiral, in the wake of the running elongated polymer head. The potential part of the polymer’s shear viscosity is evaluated at different shear rates, using non-equilibrium Monte-Carlo simulation. In spite of large statistical fluctuations of the shear viscosity computations, simulations for middle-sized polymers (100 or 150 beads) show a shear thinning behaviour. The simulation results should be compared with experiments.

Детальніше...  

Aramid fibre, poly-m-phenylene isophthalamide (Teijin-Conex), was used to reinforce thermoplastic elastomer, styrene (ethylene butylene) styrene (SEBS). It was found that the moduli at 100 and 300% elongation of the composite increased linearly with increasing fibre loading. On the other hand, tensile strength of the composites decreased as the fibre content was increased. Improvement of interfacial adhesion was carried out by, first, slightly hydrolysing the fibre with sodium hydroxide solution to increase the number of reactive amino end groups and then mixing with the matrix and compatibiliser, maleic anhydride grafted SEBS (MA-g-SEBS), at various concentrations. Tensile strength of the compatibilised composite was found to increase and then level-off at 5 wt% compatibiliser. Fractured surface of composite containing compatibiliser showed more fibre breakage than the uncompatibilised one. Examination of the extracted fibre revealed that some fraction of rubber was chemically bonded to the fibre surface. These results suggest good compatibilising performance of MA-g-SEBS for the system studied.

Детальніше...  

This paper describes the synthesis of interpenetrating polymer networks (IPN) materials based on polyethylene (PE) and polymethacrylates and the effect of ionising irradiation on degradation and grafting reactions in these materials. Two IPN systems, polyethylene/poly(butyl methacrylate-co-methyl methacrylate) and polyethylene/poly(dodecyl methacrylate-co-ethyl methacrylate), were synthesized by the in situ method using a peroxidic initiator and a divinyl crosslinker. During the synthesis the PE crosslinks and grafting reactions between PE and polymethacrylates occur. By electron beam irradiation the methacrylate phase partially degrades. After extraction with xylene a porous structure in the material is observed. The effect of the IPN composition, the synthesis conditions, and the electron beam irradiation on the amount of extractable material and the morphology of the IPN has been studied.

Детальніше...  

Small amounts of a reactive terpolymer added to a binary blend of polyamides and thermotropic liquid crystalline polyesters prior to injection moulding yielded in situ composites with significantly higher modulus and strength along the machine/reinforcement direction and substantially higher strength and toughness along the transverse direction of injection moulded parts. These were attributed to enhanced fibrillation of the liquid crystalline polymer (with fibril formation observed along the core regions of injection moulded parts also) when injection moulded in the presence of the terpolymer and improved interfacial adhesion characteristics between the components of the in situ composite. Finally, in situ composites injection moulded in the presence of the terpolymer displayed smoother surfaces.

Детальніше...  

Bilayer films between thermoset epoxy and thermoplastic poly(vinylpyrrolidone) (PVP) were prepared by casting a stoichiometric mixture of the uncured diglycidyl ether of bisphenol A epoxy (DGEBA) and 4,4 0 -diaminodiphenylsulfone (DDS) on a PVP film and then curing the system in a two-step process under an inert gas atmosphere. The resultant interface was analyzed by electron microprobe analysis (EMP). The EMP results demonstrated that the interface formation was predominantly controlled in the initial cure stage, and that the diffusion at the interface was suppressed significantly after the epoxy formed the network structure. New EMP techniques were developed to analyze the polymer–polymer interface and to obtain not only the concentration profile but also the image of the interface. The limit of the resolution was 1.6 mm under the experimental conditions employed in this study.

Детальніше...  

The effects of crosslinking on solution cast polyaniline films were investigated by means of X-ray photoelectron spectroscopy, atomic force microscopy, bulk elemental analysis, resistivity and tensile strength measurements. By comparing the anion/N molar ratio at the surface and bulk of the film, it was found that crosslinking of the films retards the dopant infusion process during reprotonation. The extent of doping has a significant effect not only on the film’s resistivity, but also its tensile strength. Under simulated weathering, the degradation mechanisms of the non-crosslinked and crosslinked films are similar. As a result of the reactions occurring in the polyaniline chains under simulated weathering, profound changes in the chemical state of the C and N species are evident. In addition to the changes in the film’s physical and electrical characteristics, the surface morphology of the films also undergoes substantial changes, especially for the reprotonated crosslinked films.

Детальніше...  

The kinetics of transesterification in blends of thermotropic copoly(oxybenzoate-ethylene terephthalate) and polycarbonate at melt state were analysed quantitatively with 13C nuclear magnetic resonance, and were then modelled by a phenomenological equation. It was found that the activation energy for the transesterification reaction between the poly(oxybenzoate) segment in copoly(oxybenzoate-ethylene terephthalate) and polycarbonate was more than twice that for the transesterification reaction between the poly(ethylene terephthalate) segment and polycarbonate (i.e. 94.6 versus 41.8 kcal/mol). This is probably due to the stronger chemical bond in oxybenzoate-oxybenzoate that in ethylene-terephthalate. Additionally, owing to the rigid-rod nature of the poly(oxybenzoate) segment, the frequency factor in the transesterification reaction in poly(oxybenzoate) segment and polycarbonate is much higher than that in poly(ethylene terephthalate) segment and polycarbonate.

Детальніше...