Stressed polystyrene, mechanically

Mechanically stressed polystyrene without the application of any chemical compound can also produce crazing. Figure 23 shows crazing and its diffraction pattern attributed to bending the polystyrene sheet surface crazing appears on only one side. This contrasts the development of crazing in depth by simple tensile stress (see Figure 24) where crazes developed on both sides of, and sometimes in between, the surfaces. 

Macromolecular materials react quite differently to mechanical stresses. Beakers of conventional polystyrene are very brittle, and a short, quick blow will break them. In contrast, beakers of nylon 6 are very tough. Weakly cross-linked natural rubber expands on stretching by several hundred percent after being released, it returns to what is practically its original form. When plasticine is deformed, on the other hand, it completely retains its new shape. 

Stress-cracks are formed in parts molded from polystyrene when they are exposed to mechanical stresses and certain media. Here, influence of media that would normally not or only lightly attack stress-free molded parts can lead to crack formation. Fibrillated crazes are typical of polystyrene Table 5.71 provides typical dimensions [193]. 

The isothermal curves of mechanical properties in Chap. 3 are actually master curves constructed on the basis of the principles described here. Note that the manipulations are formally similar to the superpositioning of isotherms for crystallization in Fig. 4.8b, except that the objective here is to connect rather than superimpose the segments. Figure 4.17 shows a set of stress relaxation moduli measured on polystyrene of molecular weight 1.83 X 10 . These moduli were measured over a relatively narrow range of readily accessible times and over the range of temperatures shown in Fig. 4.17. We shall leave as an assignment the construction of a master curve from these data (Problem 10). 

The mechanical properties of two-phase polymeric systems, such as block and graft polymers and polyblends, are discussed in detail in Chapter 7. However, the creep and stress-relaxation behavior of these materials will be examined at this point. Most of the systems of practical interest consist of a combination of a rubbery phase and a rigid phase. In many cases the rigid phase is polystyrene since such materials are tough, yet low in price. 

CNT can markedly reinforce polystyrene rod and epoxy thin film by forming CNT/polystyrene (PS) and CNT/epoxy composites (Wong et al., 2003). Molecular mechanics simulations and elasticity calculations clearly showed that, in the absence of chemical bonding between CNT and the matrix, the non-covalent bond interactions including electrostatic and van der Waals forces result in CNT-polymer interfacial shear stress (at OK) of about 138 and 186MPa, respectively, for CNT/ epoxy and CNT/PS, which are about an order of magnitude higher than microfiber-reinforced composites, the reason should attribute to intimate contact between the two solid phases at the molecular scale. Local non-uniformity of CNTs and mismatch of the coefficients of thermal expansions between CNT and polymer matrix may also promote the stress transfer between CNTs and polymer matrix. 

Mechanical data like stress/strain behavior, impact resistance in comparison to polystyrene... 

Although many biochemical reactions take place in the bulk aqueous phase, there are several, catalyzed by hydroxynitrile lyases, where only the enzyme molecules close to the interface are involved in the reaction, unlike those enzyme molecules that remain idly suspended in the bulk aqueous phase [6, 50, 51]. This mechanism has no relation to the interfacial activation mechanism typical of lipases and phospholipases. Promoting biocatalysis in the interface may prove fruitful, particularly if substrates are dissolved in both aqueous phases, provided that interfacial stress is minimized. This approach was put into practice recently for the enzymatic epoxidation of styrene [52]. By binding the enzyme to the interface through conjugation of chloroperoxidase with polystyrene, a platform that protected the enzyme from interfacial stress and minimized product hydrolysis was obtained. It also allowed a significant increase in productivity, as compared to the use of free enzyme, and simultaneously allowed continuous feeding, which further enhanced productivity. 

Schremp,F. W., Ferry, J.D., Evans, W. W. Mechanical properties of substances of high molecular weight. IX. Non-Newtonian flow and stress relaxation in concentrated polyisobutylene and polystyrene solutions. J. Appl. Phys. 22,711-717 (1951). 

The mechanical properties of Shell Kraton 102 were determined in tensile creep and stress relaxation. Below 15°C the temperature dependence is described by a WLF equation. Here the polystyrene domains act as inert filler. Above 15°C the temperature dependence reflects added contributions from the polystyrene domains. The shift factors, after the WLF contribution, obeyed Arrhenius equations (AHa = 35 and 39 kcal/mole). From plots of the creep data shifted according to the WLF equation, the added compliance could be obtained and its temperature dependence determined independently. It obeyed an Arrhenius equation ( AHa = 37 kcal/mole). Plots of the compliances derived from the relaxation measurements after conversion to creep data gave the same activation energy. Thus, the compliances are additive in determining the mechanical behavior. 

Shen and Kaelble (29) found the same linear dependence in the region —60° and 60°C but state that below —50°C and above 80°C the temperature dependence of Kraton 101 could be described by the WLF equation with cx = 16.14, C2 = 56, and Tr — — 97°C below —50°C, and Tr — 60°C above 80°C. They ascribe the temperature dependence below —50 °C to the pure polybutadiene phase and that above 80 °C to the pure polystyrene phase. They then assume that at temperatures between —50° and 80°C the molecular mechanisms for stress relaxation are being contributed by an interfacial phase visualized as a series of spherical shells enclosing each of the pure polystyrene domains and characterized... 

In the case of CSCs, under conditions where there is a low mobility of polymer chains within fibrils, it is likely [19] that under the applied surface stress ctcsc. the polymer chain, elongated between entanglements, breaks down, thus decreasing the number of load-bearing chains in the fibril. Estimates performed on a blend of high MW (Mw = 3 x 105 g moD1) and low MW (Mw = 1.9 x lO gmol ) polystyrenes [19] support such a mechanism. 

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