Atactic PS

Dissolution (toluene)/precipitation (methanol) of EPS beads (atactic PS foams with ca. 2wt.% HBCD and BaSCL) with TLC separation and IR identification allowed detection of hexabromocyclododecane for quantification XRF analysis (Br) was used [629]. Disso-lution/precipitation of PS/lubricants with evaporation to dryness and redissolution was followed by FT1R analysis [106],... 

Further evidence for a TiIII-centered pathway has been provided from model studies comparing the activity of analogous TiIV and Ti precatalysts for example, (C5Me5)Ti(OMe)2/MAO was reported to be more active than (C5Me5)Ti(0Me)3/MA0.371 More recently it has been shown that Tiiv catalysts only produce atactic-PS when the polymerization is performed in the absence of... 

In order to illustrate the fractionated crystallization behavior we will present here previous results on immiscible atactic PS and isotactic polypropylene blends (iPP) [68]. The cooling behavior of PS, iPP and an 80/20 PS/iPP blend is presented in Fig. 1, as well as that of an unmixed blend , labeled... 

Fig.l Differential scanning calorimetry (DSC) cooling scans from the melt, at 10°Cmin 1, of the following materials (from top to bottom) Isotactic polypropylene (iPP) iPP after self-nucleation treatment at TS = 162°C 80/20 polystyrene (PS)/iPP melt mixed blend 80/20 PS/iPP melt mixed blend after self nucleation treatment at Ts = 161 °C 80/20 PS/iPP unmixed blend (UB), see text and atactic PS homopolymer. (From [68] with permission)... 

Figure 4. C-13 spectra (25 MHz) of styrene-SOt copolymers of four different compositions. The aromatic carbons are on the left and the main chain carbons on tw right. R values are the mole ratios of styrene to SO in the copolymers. The spectrum at the bottom is that of atactic PS. (All observed as 25% solutions in CDCl at 55°C except PS, which was observed as 20% solution in cyclo-...

Tacticity and geometric isomerism affect the tendency toward crystallization the tendency increases as the tacticity (stereoregularity) is increased and when the geometric isomers are predominantly trans. Thus isotactic PS is crystalline, whereas atactic PS is largely amorphous and c/s-polyisoprene is amorphous, whereas the more easily packed trans isomer is crystalline. 

Although it is called crystal clear polystyrene, atactic PS is amorphous and clear, as are PMMA and polycarbonate (PC). In contrast, hdpe and nylon 66 are highly crystalline and opaque. The relationship of packing efficiency to specific gravity and crystallinity is readily illustrated by ldpe and HDPE, which are 60 and 95% crystalline, respectively, and have specific gravities of 0.91 and 0.97, respectively. 

Thus commercial atactic PS is transparent, but isotactic PS is translucent because of some amorphous areas present in the highly crystalline product. Also, high-impact PS is translucent because of the presence of two phases. 

Crystalline polymers such as high-density polyethylene (hope), PP, PTFE, and polyoxymethylene (POM) exhibit somewhat higher X values than amorphous polymers such as low-density polyethylene (ldpe), atactic PS,... 

Several reports have appeared in which the UV absorption spectrum of PS was found to be qualitatively similar to an alkyl benzene model compound. Vala and Rice15> demonstrated the similarity between atactic PS and ethyl benzene this was confirmed for atactic PS in cyclohexane by Hirayama 16) and in tetrahydrofuran by Abuin 17>. Bovey and co-workers20) found that absorbance spectra for isotactic PS, atactic PS, and styrene-methylmethacrylate copolymers in 1,2-dichloroethane were roughly similar to that of toluene. 

More quantitative studies have shown that, although some hypochromism effects exist, they are small. For example, Monnerie I9) examined the integrated molar extinction coefficient of atactic PS in chloroform and found only a 2 % decrease for the polymer relative to ethyl benzene. In addition, Vala and Rice 15 reported a 10% decrease in absorption for the 260 nm band of isotactic PS relative to atactic PS, which was qualitatively confirmed by Longworth18>. Similarly, Cantow 8> observed a 4% hypochromism of the 261.5 nm band of isotactic PS in dioxane relative to atactic PS. Finally, Cantow 8) observed strong hypochromism (relative to atactic PS) ranging from 19% at 262 nm to 32% at 269 nm for an alternating styrene-methyl methacrylate copolymer and for random copolymers having a low styrene content. 

In summary, the UV absorbance spectrum of atactic PS in both poor and good solvents was found to be substantially unchanged from that of ethyl benzene or isopropylbenzene. The hypochromism at 261 nm of isotactic PS relative to atactic... 

The calculated value of experimental values for these respective compounds 17 37 135) fall into two conflicting proportions, because of a wide range in the fluorescence ratio of atactic PS. The proportion 28 6 1 reflects (Pd/ = 17 for PS 17) yields the proportion 28 24 1. The data for PS are inconclusive, but it will be shown later that energy migration in PS is probably slower and less effective than in P2VN. 

The unperturbed chain dimensions of near-monodisperse atactic PS are evaluated from intrinsic viscosity measurements. Negative values for the temperature coefficient of chain dimensions are found. Under conditions where specific solvent effects are eliminated or minimized, measurements yield results in excellent agreement with the theoretical predictions for atactic PS. 

Whereas atactic PS is an amorphous polymer with a Tg of 100 CC, syndio-tactic PS is semicrystalline with a Tg similar to aPS and a Tm in the range 255-275 °C. The crystallization rate of sPS is comparable to that of polyethylene terephthalate). sPS exhibits a polymorphic crystalline behavior which is relevant for blend properties. In fact, it can crystallize in four main forms, a, (3, -y and 8. Several studies [8] based on FTIR, Raman and solid-state NMR spectroscopy and WAXD, led the a and (3 forms to be assigned to a trans-planar zig-zag molecular chain having a (TTTT) conformation, whereas the y and 8 forms contain a helical chain with (TTG G )2 or (G+G+TT)2 conformations. In turn, on the basis of WAXD results, the a form is said to comply with a unitary hexagonal cell [9] or with a rhombohedral cell [10]. Furthermore, two distinct modifications called a and a" were devised, and assigned to two limiting disordered and ordered forms, respectively [10]. 

The y peak intensity for isotactic PS is substantially lower than for atactic PS and also shifts to lower temperatures [10,17,24]. Moreover, some researchers have indicated that it is completely missing from isotactic PS [11,15]. 

The a relaxation in both isotactic and syndiotactic PS is broader than that in atactic PS and the actual location of the peak is slightly shifted to higher temperature. The broadening effect was attributed to restrictions imposed by crystallites on the amorphous phases [12,24,25], Nakatani et al. [26] showed how the broadness of a syndiotactic PS sample can be represented by the overlapping of two glass relaxation processes arising from one purely amorphous component and the other amorphous component, which is under restrain owing to the proximity of crystallites. 

Materials(PS/PVME). Atactic PS of weight-average molecu-... 

Materials(PS/PPE). Atactic PS of molecular weight Mw=50,000 and Mw/Mn=1.05 was prepared in the laboratory PPE was purchased from Aldrich-Chemie, Steinheim, FRG, with Mw=46,000 and Mw/Mn=2.4. Reagent grade ethylbenzene, toluene, benzene, cyclohexane, and n-octane from Fluka and Roth were used without further purification. 

A typical phosphorescence spectrum of atactic PS has been published by Vala, Haebig and Rice (19) together with the monomeric model ethylbenzene. The two spectra do not show substantial differences. PVCA is compared with N-isopropyl-carbazole and BCP in Figure... 

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