Polystyrene yield behaviour

Shear yield behaviour of polymer melts containing plate-like filler particles is also prevalent and is clearly shown in Fig. 8 for talc-filled polystyrene. In this system an estimate was made of shear yield values, which were found to increase with increasing particle loading and decreasing particle size. These results are compared with reported yield values for other particulate-filled polymers in Table 2. It is evident that shear yield values also depend on the particle type and thermoplastic matrix used. 

One of the most important methods for controlling the yield behaviour of polymers is rubber modification, which is widely used to increase fracture resistance. The technique was first used in 1948 to modify the properties of polystyrene, and has since been extended to most of the major plastics, including polypropylene, polycarbonate, and rigid PVC, and to a number of the less highly crosslinked thermosets, notably epoxy resins. Between S and 20 % of a suitable rubber is added in the form of small particles, which are typically between 0.1 and S /im in diameter. Chemically reactive rubbers are preferred, because they form bonds with molecules of the surrounding matrix which can withstand tensile stress. The rubber particles have low moduli, and therefore act as stress concentrators. Accelerated deformation in the matrix adjacent to the rubber particles results in a lowering of the yield stress. 

In order to determine which is the most appropriate yield criterion for a particular polymer it is necessary to follow the yield behaviour under a variety of states of stress. This is most conveniently done by working in plane stress = 0) and making measurements in pure shear (o- = -0-2) and biaxial tension (o-i, 02 > 0) as well as in the simple uniaxial cases. The results of such experiments on glassy polystyrene are shown in Fig. 5.28. The modified von Mises and Tresca envelopes are also plotted. In both cases they have been fitted to the measured uniaxial tensile and compressive yield stresses, oy, and oy. It can be seen that the von Mises... 

Fig. 5.28 Modified von Mises and Tresca criteria fitted to experimental data upon the yield behaviour of polystyrene. (Data taken from Whitney and Andrews J. Polym. ScL, C-16 (1967) 2981.)...

The flow behaviour of polymeric electrophotographic toner systems containing carbon black varying in surface area and concentration were determined using a cone and plate rheometer [51]. As the concentration of carbon black was increased, the viscosity at low shear rates become unbounded below a critical shear stress. The magnitude of this yield stress depended primarily on the concentration and surface area of the carbon black flller and was independent of the polymer (polystyrene and polybutyl methacrylate) and temperature. It was postulated that at low shear rates the carbon black formed an independent network within the polymer which prevented flow. 

The difficulties encountered in LLC can be overcome by the use of chemically bonded stationary phases or bonded-phases. Most bonded phases consist of organochlorosilanes or organoalkoxysilanes reacted with micro-particulate silica gel to form a stable siloxane bond. The conditions can be controlled to yield monomeric phases or polymeric phases. The former provides better efficiency because of rapid mass transfer of solute, whereas the polymeric phases provides higher sample capacity. BPC can be used in solvent gradient mode since the stationary phase is bonded and will not strip. Both normal-phase BPC (polar stationary, non-polar mobile) and reversed-phase BPC (non-polar stationary, polar mobile) can be performed. The latter is ideal for substances which are insoluble or sparingly soluble in water, but soluble in alcohols. Since many compounds exhibit this behaviour, reversed phase BPC accounts for about 60% of published applications. The main disadvantage of silica bonded phases is that the pH must be kept between 2 to 7.5. However, bonded phases with polymer bases (polystyrene-divinylbenzene) can be used in the pH range of 0 to 14. 

The thermal volatilization analysis of a mixture of polyvinylchloride and polystyrene is given in Fig. 81. The first peak corresponds to the elimination of HC1 and the second to that of styrene. Dehydrochlorination is retarded in the mixture. The production of styrene is also retarded styrene evolution, in fact, does not occur below 350°C. This contrasts with the behaviour of polyvinylchloride-polymethylmethacrylate mixtures for which methacrylate formation accompanies dehydrochlorination. The observed behaviour implies that, if chlorine radical attack on polystyrene occurs, the polystyrene radicals produced are unable to undergo depolymerization at 300° C. According to McNeill et al. [323], structural changes leading to increased stability in the polystyrene must take place. This could also occur by addition of Cl to the aromatic ring, yielding a cyclohexadienyl-type radical which is unable to induce depolymerization of the styrene chain. 

When polystyrene foams are compressed, the 1-5 p,m thick, biaxially oriented, cell faces form permanent plastic hinges at intervals (Fig. 8.20), in directions perpendicular to the compression axis. The deformation mechanism is similar to that when thin sheet steel crumples in a car crash. This behaviour contrasts to the crazing and fracture that occurs when 2 mm thick polystyrene sheet is bent. Thirty-two micrometers thick, biaxially oriented polystyrene film, used in window envelopes, yields in tension rather than crazing and fracturing. In closed-cell polyethylene foams... 

The diffusion coefficient of n-tetradecane on polystyrene in a fluid-viscous state calculated from free volume shows a sudden increase near Tg this behaviour was marked by its temperature dependence [219]. The introduction of diffusion coefficients into kinetic theory (which governs the shape of peaks [218]) yields very broad and asymmetric peaks above T,. This simplified model ignores adsorption and therefore fails to explain the behaviour of the retention volume at and below Tg. 

The choice of an appropriate electrolyte for the doping of polypyrrole during electrochemical polymerization can improve the mechanical properties of the films obtained. Thus, polypyrrole films electrogenerated in the presence of large electrolytes, as /7-toluensulpho-nate, tosylate, benzene sulfonate, etc., yield flexible films with tensile strengths similar to polystyrene [113]. We can extrapolate this behaviour by combining the... 

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