Temperature, effect polystyrene

With these catalysts, the cation complexes with the monomer so weakly that a solid surface and low polymerization temperatures are required to achieve sufficient orientation for stereospecificity. Braun, Herner and Kern (217) have shown that lower polymerization temperatures are required (in n-hexane diluent) to obtain isotactic polystyrene as the alkyl metal becomes more electropositive (RNa, —20° C. RK, —60° to —70° C. and RRb, —80° C.). They correlate isotacticity with the polymerization rate as a function of catalyst, temperature or solvent. However, with Alfin catalysts, stereospecific polymerization of styrene is unrelated to rate (226). A helical polymerization mechanism as proposed by Ham (229) and Szwarc (230) is also inadequate for explaining the temperature effects since the probability for adventitious formation of several successive isotactic placements should have been the same at constant temperature in the same solvent for all catalysts. 

Figure 23.7 The temperature effects on product phase yields of PS. (Reproduced from Journal of Analytical and Applied Pyrolysis, 60 (2), A. Karaduman, Flash pyrolysis of polystyrene wastes in a free-fall reactor under vacuum, 179-186(2001), with permission from Elsevier)...

The fluorescence intensity of quinoline derivatives has been found to increase dramatically with an increase in the molecular weight of the host polystyrene. " This is attributed to a decrease in the free volume in the polymer matrix restricting molecular rotation/motion of the fluorophore. Similar effects have been observed for juliodinemalononitrUe in different stereo-regular poly(methyl methacrylates), and temperature effects on the luminescence properties of indole and coumaric acid derivatives in different polymer matrices showed abrupt changes in emission intensity at temperatures which correspond to the onset of local relaxation processes in the polymer. ... 

These temperature effects on solvent quality of cyclopentane for polystyrene should have interesting consequences on the cyclization behaviour of the polymer. We carried out experiments on DMT-PS-Py over a wide temperature range. These are the same samples shown in Figure 2. Since the polymer concentrations in these samples were so low, they could be studied at temperatures where higher concentrations would precipitate from solution. At each temperature, plots of log vs log N were linear. 

Takashika, K., Oshima, A., Kuramoto, M., Seguchi, T., Tabata, Y, 1999. Temperature effects on radiation induced phenomena in polystyrene having atactic and syndiotactic structures. Radiat. Phys. Chem. 55 399 3. 

Fig. 38. Solvent composition and temperature effects for the fractionation of six polystyrene samples of different molecular weights (1) 2.5 x 10 , (2) 12 x 10 , (3) 29 x...

In Figure 2.4 the pressure-volume isotherm is shown at 25°C for polystyrene using the FOV equation of state. The characteristic parameters were obtained from Table 2.2. The EOS does not explicitly account for molecular weight and glass transition temperature effects. 

As in the case of gaseous media, the effect of liquids depends on their activity towards the macroradicals formed by mechanical action and on the nature of the polymer chain. Generally, the ESR spectra obtained on radicals from polymers do not seem to depend on solvent [32, 33], but exceptions are noted below. There are also related temperature effects when mastication is performed at temperatures below the solvent freezing point, the macromolecules are distributed in the solvent crystal lattice. During vibromilling of polystyrene and poly(methyl methacrylate), in frozen polymer solutions [32], the radical concentration is 10-100 times higher since the recombination reaction is reduced. Degradation in a frozen polymer solution permits... 

Since the ends of the polyisoprene chains are covalently bonded into the polystyrene glass, they cannot move away, and the glassy polystyrene phase effectively cross-links the rubber polyisoprene chains. However, at temperatures above the glass transition of polystyrene, the chains can be pulled out of the polystyrene domains, and so the article can be reshaped, becoming cross-linked once again on cooling. 

Chiu, F., Shen, K., Tsai, S. H. Y, Chen, C. Pre-melting temperature effect on the isothermal melt crystallization and multiple melting behavior of syndiotactic polystyrene. Polym. Eng. Sci., 41(5), 881-889 (2001). 

Chiu, F-C., Peng, C-G., Fu, Q. Non-isothermal crystallization and multiple melting behavior of syndiotactic polystyrene— Pre-melting temperature effects. Polym. Eng. ScL, 40(11), 2397-2406 (2000). 

Polymer concentration and temperature effects on solvent self-diffusion were examined by Pickup and Blum(25), who made pulsed-field-gradient NMR measurements on toluene 270 kDa polystyrene. Figure 5.2 shows representative measurements. At each temperature, D (c) of the solvent is a simple exponential for c < 0.4 (weight fraction) and a stretched exponential at larger c. The slopes of the exponentials as seen in the figixre are very nearly the same at all T, but v of the larger-c form increases with increasing T. 

Figure 10.8 shows two sets of data plotted according to these conventions, after correction for the effect of interference. In Fig. 10.8a, HC2/T is plotted against C2 for three different fractions of polystyrene in methyl ethyl ketone. Figure 10.8b shows Kc2/Rg versus C2 for solutions of polystyrene in cyclohexane at five different temperatures. These results are discussed further in the following example.

Physical or chemical vapor-phase mechanisms may be reasonably hypothesized in cases where a phosphoms flame retardant is found to be effective in a noncharring polymer, and especially where the flame retardant or phosphoms-containing breakdown products are capable of being vaporized at the temperature of the pyrolyzing surface. In the engineering of thermoplastic Noryl (General Electric), which consists of a blend of a charrable poly(phenylene oxide) and a poorly charrable polystyrene, experimental evidence indicates that effective flame retardants such as triphenyl phosphate act in the vapor phase to suppress the flammabiUty of the polystyrene pyrolysis products (36). 

Commercial polystyrenes are normally rather pure polymers. The amount of styrene, ethylbenzene, styrene dimers and trimers, and other hydrocarbons is minimized by effective devolatilization or by the use of chemical initiators (33). Polystyrenes with low overall volatiles content have relatively high heat-deformation temperatures. The very low content of monomer and other solvents, eg, ethylbenzene, in PS is desirable in the packaging of food. The negligible level of extraction of organic materials from PS is of cmcial importance in this appHcation. 

When used alone at low temperatures, diaLkyl thiodipropionates are rather weak antioxidants. However, synergistic mixtures with hindered phenols are highly effective at elevated temperatures and are used extensively to stabilize polyolefins, ABS, impact polystyrene (IPS), and other plastics. 

Some interesting differences are noted between amorphous and crystalline polymers when glass fibre reinforcement is incorporated into the polymer. In Figure 9.2 (ref. 10) it will be seen that incorporation of glass fibre has a minimal effect on the heat deflection temperature of amorphous polymers (polystyrene,... 

Because of the chain-stiffening effect of the benzene ring the TgS of commercial materials are in the range 90-100°C and isotactic polymers have similar values (approx. 100°C). A consequence of this Tg value plus the amorphous nature of the polymer is that we have a material that is hard and transparent at room temperature. Isotactic polystyrenes have been known since 1955 but have not been of commercial importance. Syndiotactic polystyrene using metallocene catalysis has recently become of commercial interest. Both stereoregular polymers are crystalline with values of 230°C and 270°C for the isotactic and syndiotactic materials respectively. They are also somewhat brittle (see Section 16.3). 

In Chapters 3 and 11 reference was made to thermoplastic elastomers of the triblock type. The most well known consist of a block of butadiene units joined at each end to a block of styrene units. At room temperature the styrene blocks congregate into glassy domains which act effectively to link the butadiene segments into a rubbery network. Above the Tg of the polystyrene these domains disappear and the polymer begins to flow like a thermoplastic. Because of the relatively low Tg of the short polystyrene blocks such rubbers have very limited heat resistance. Whilst in principle it may be possible to use end-blocks with a higher Tg an alternative approach is to use a block copolymer in which one of the blocks is capable of crystallisation and with a well above room temperature. Using what may be considered to be an extension of the chemical technology of poly(ethylene terephthalate) this approach has led to the availability of thermoplastic polyester elastomers (Hytrel—Du Pont Amitel—Akzo). 

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