Pure Polystyrene

There are three principal families of styrene containing polymers which are used to make commercial plastic products. The first family is pure polystyrene, the second family comprises random copolymers, and the final family consists of polystyrene chains grafted to blocks of rubbery polymers. There are also synthetic rubbers that contain significant concentrations of styrene, but these are outside the scope of this book. 

Small amounts of isotactic polystyrene have been synthesized in the laboratory using noncommercial polymerization techniques. These polymers are capable of partially crystallizing, albeit at a very slow rate. Syndiotactic polystyrene was available commercially for several years, but its continued production proved unprofitable. 

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The properties of styrenic block copolymers are dependent on many factors besides the polymerization process. The styrene end block is typically atactic. Atactic polystyrene has a molecular weight between entanglements (Me) of about 18,000 g/mol. The typical end block molecular weight of styrenic block copolymers is less than Mg. Thus the softening point of these polymers is less than that of pure polystyrene. In fact many of the raw materials in hot melts are in the oligomeric region, where properties still depend on molecular weight (see Fig. 1). 

In a block copolymer, a long segment made from one monomer is followed by a segment formed from the other monomer. One example is the block copolymer formed from styrene and butadiene. Pure polystyrene is a transparent, brittle material that is easily broken polybutadiene is a synthetic rubber that is very resilient, but soft and opaque. A block copolymer of the two monomers produces high-impact polystyrene, a material that is a durable, strong, yet transparent plastic. A different formulation of the two polymers produces styrene-butadiene rubber (SBR), which is used mainly for automobile tires and running shoes, but also in chewing gum. 

For the remaining three systems, styrene-vinyl acetate, vinyl acetate-vinyl chloride, and methyl acrylate-vinyl chloride, one reactivity ratio is greater than unity and the other is less than unity. They are therefore nonazeotropic. Furthermore, since both ri and 1/7 2 are either greater than or less than unity, both radicals prefer the same monomer. In other words, the same monomer—styrene, vinyl chloride, and methyl acrylate in the three systems, respectively—is more reactive than the other with respect to either radical. This preference is extreme in the styrene-vinyl acetate system where styrene is about fifty times as reactive as vinyl acetate toward the styrene radical the vinyl acetate radical prefers to add the styrene monomer by a factor of about one hundred as compared with addition of vinyl acetate. Hence polymerization of a mixture of similar amounts of styrene and vinyl acetate yields an initial product which is almost pure polystyrene. Only after most of the styrene has polymerized is a copolymer formed... 

By incorporating acrylonitrile into polystyrene we can depress the copolymer s glass transition temperature below that of pure polystyrene. When sufficient acrylonitrile is present, the copolymer s glass transition temperature falls below room temperature. The resulting copolymer is tough at room temperature and at higher temperatures. 

One of the principal weaknesses of pure polystyrene is its low impact resistance. To counteract this problem, we toughen it with various types of rubber. This is most effective when a portion of the rubber is chemically grafted to the polystyrene. The rubber forms small inclusions within a matrix of polystyrene. The presence of rubber also improves polystyrene s extensibility, ductility, and resistance to environmental stress cracking. 

The importance of the block and graft copolymers is that the resultant material tends to exhibit the properties of each homopolymer. For example, pure polystyrene is quite brittle, whereas polymerisation in the presence of about 5 per cent of rubber produces a material which is strong and tough. 

The polyphenylenes were brittle and did not form self-standing films when cast from solution. Therefore, they were considered poor materials. The use of these polymers was instead investigated as additives in polystyrene to improve processing and mechanical properties. A mixture of polystyrene and hyperbranched polyphenylene (5%) was studied and the results showed that the melt viscosity, especially at high temperatures and shear rates, was reduced by up to 80% as compared to pure polystyrene. Also, the thermal stability of polystyrene... 

The thermal stability of hyperbranched polymers is related to the chemical structure in the same manner as for linear polymers for example, aromatic esters are more stable than aliphatic ones. In one case, the addition of a small amount of a hyperbranched polyphenylene to polystyrene was found to improve the thermal stability of the blend as compared to the pure polystyrene [31]. 

Figure 3. Intrinsic viscosity of polystyrene samples, irradiated in benzene solution in the presence of initiator I-III. Polystyrene concentration 7.69x10 2 M, photoinitiator concentration 2.31 x 10-3 Ml p Pure polystyrene initiator I initiator II A initiator III. (Reproduced with permission from Polym. Deg. Stability Ref. 21).

PEG-grafted polystyrene is also well suited for reactions with highly reactive orga-nometallic reagents, provided that the support has been dried. PEG-containing polymers are generally more difficult to dry than pure polystyrene. Cross-linked PEG is stable towards Lewis acids, and can be used for SnCl4-mediated allylations of aldehydes with allyl silanes [21],... 

This value has been checked very recently (169), by polymerizing methyl methacrylate in the presence of varying amounts of pure polystyrene oligomers of different molecular weights (ranging from 1000 to 5000). After separating carefully the resulting polymers, it was shown... 

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... 

Koberstein and coworkers121 have examined the effects of a polydimethylsiloxane-polystyrene (PDMS-PS) block copolymer on the interfacial tensions of blends of PDMS and polystyrene. As little as 0.002 wt% of the copolymer, added to the siloxane phase, was sufficient to lower the interfacial tension by 82% in the case of a blend of polystyrene (Afn = 4,000) and PDMS (Mn = 4,500). No further reduction in interfacial tension was observed at higher copolymer levels due to micelle formation. Riess122 has polymerized styrene in the presence of a silicon oil and a polydimethylsiloxane-polystyrene block copolymer to obtain a polystyrene in which 0.1-1 pm droplets of silicone oil are dispersed. This material displayed a lowered coefficient of kinetic friction on steel compared to pure polystyrene. 

Figure I 1.5. Viscosity of polystyrene with dissolved R-152a at 150 °C measured as a function of shear rate. The Schummer correction and a pressure correction have been applied, as described in the text, to obtain isobaric viscosity curves corresponding to the back-pressures applied during the measurements. The R-152a composition and back-pressure values are O 5.6 wt% R-152a, 12.06 MPa 7.0 wt%, 12.48 MPa A 8.3 wt %, 17.18 MPa V 10.4 wt%, 16.41 MPa. The solid curve is the viscosity curve for pure polystyrene at 150 °C and 1 atm pressure. Data from Kwag (1998).

Figure I 1.6. Master viscosity curve produced by superposing all data for all systems. Viscosity data taken at 175 °C have been shifted to 150°C by employing the temperature scaling factor aT for pure polystyrene. The master viscosity curve is identical to the viscosity curve for pure polystyrene at 1 atm and 150 °C, which is displayed as the solid line. Data from Kwag (1998).

FIGURE 11.20 TGA curves and the CCA curves for pure polystyrene and three PS/clay nanocomposites obtained with in situ bulk polymerization, VB16 and OH16 are two ammonium-modified MMT and P16 is phosphonium-modified MMT. (From Zhu, J. et al., Chem. Mater., 13, 3774, 2001. With permission.)... 

However, to illustrate the effect of solvent on viscosity, we measured the intrinsic viscosities of three copolymers in a polar solvent—benzyl alcohol at 80°C. The two solvents are compared in Table B. Although the intrinsic viscosities of the hydroxyl-containing copolymers are similar in the two solvents, the value for pure polystyrene is much reduced in the polar solvent. This provides indirect evidence for the existence of a smaller coil volume for the copolymer molecules in the nonpolar solvent. 

The specific volume and expansion coefficient of the solution-blended material are shown in Figure 6, along with data for pure polybutadiene and pure polystyrene. None of the three polymers has any distinguishing features below the polystyrene Tg> illustrating that the observed transition and minimum are the results of the unique structural morphology of the block copolymers. It should be noted that the substantial difference in the thermal expansion coefficients of polybutadiene and polystyrene can be expected to be an important factor affecting the structure and properties of block copolymer samples prepared under various conditions. 

Interesting thermal response of the polystyrene nanocomposites was reported when untreated and polystyrene grafted nanotubes were used for the reinforcement of polymer (49). The glass transition temperature of the pure polystyrene matrix was observed to be 99 °C. Similar transition temperature of 98 °C was observed for the composites containing 2.5 vol% of the untreated nanotubes. The nanocomposites containing polymer functionalized nanotubes... 

The determination of the microstructure of polybutadiene, i.e., the distribution of cis and frans-1,4-polybutadiene, as well as that of trans-XA- and 1,2-polybutadiene in polystyrene presents an analytical challenge. Fig. 5.1-12 shows the spectrum of a mixture of polystyrene and polybutadiene, obtained in CS2 solution. Difference spectroscopy with pure polystyrene as a standard affords the spectrum of the polybutadiene fraction, from which the microstructure can easily be determined (Peitscher, 1979). 

Pure polystyrene is too brittle for many uses, so most polystyrene-based polymers are actually copolymers of styrene and butadiene,... 

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