Styrene content

Proportion of Hard Segments. As expected, the modulus of styrenic block copolymers increases with the proportion of the hard polystyrene segments. The tensile behavior of otherwise similar block copolymers with a wide range of polystyrene contents shows a family of stress—strain curves (4,7,8). As the styrene content is increased, the products change from very weak, soft, mbbedike materials to strong elastomers, then to leathery materials, and finally to hard glassy thermoplastics. The latter have been commercialized as clear, high impact polystyrenes under the trade name K-Resin (39) (Phillips Petroleum Co.). Other types of thermoplastic elastomers show similar behavior that is, as the ratio of the hard to soft phase is increased, the product in turn becomes harder. 

Butadiene and styrene may be polymerised in any proportion. The Tfs of the copolymers vary in an almost linear manner with the proportion of styrene present. Whereas SBR has a styrene content of about 23.5% and is rubbery, copolymers containing about 50% styrene are leatherlike whilst with 70% styrene the materials are more like rigid thermoplastics but with low softening points. Both of these copolymers are known in the rubber industry as high-styrene resins and are usually used blended with a hydrocarbon rubber such as NR or SBR. Such blends have found use in shoe soles, car wash brushes and other mouldings but in recent times have suffered increasing competition from conventional thermoplastics and to a less extent the thermoplastic rubbers. 

The outstanding morphological feature of these rubbers arises from the natural tendency of two polymer species to separate one from another, even when they have similar solubility parameters. In this case, however, this is restrained because the blocks are covalently linked to each other. In a typical commercial triblock the styrene content is about 30% of the total, giving relative block sizes of 14 72 14. At this level the styrene end blocks tend to congregate into spherical or rod-like glassy domains embedded in an amorphous rubbery matrix. These domains have diameters of about 30 nm. 

Figure 11.18 Influence of styrene content on properties of ethylene-styrene copolymers (based on...

More recently Fina Chemicals have introduced linear SBS materials (Finaclear) in which the butadiene is present both in block form and in a mixed butadiene-styrene block. Thus comparing typical materials with a total styrene content of about 75% by weight, the amount of rubbery segment in the total molecule is somewhat higher. As a result it is claimed that when blended with polystyrene the linear block copolymers give polymers with a higher impact strength but without loss of clarity. 

The last decade has also seen the advent of commercial SIS polymers with 15-18% styrene content, but with higher diblock content (typically 30-55%). These are available from many of the major suppliers. The higher diblock content makes them ideal for label applications where low modulus PSAs provide easy die cuttabilty in processing, and rapid wet out and bonding to the substrate in the label application. The shear holding power characteristic would be reduced, but this would not be a critical factor for many label applications. 

In the cationic copolymerization of DOL with styrene considerable cleavage of polymer chains occurs if the styrene content is high hut a molecular weight as... 

FIGURE 2.9 X-ray diffractogram (XRD) spectra of unmodified and modified nanoclays and styrene-butadiene rubber (SBR)-based nanocomposites with styrene content of (a) 15% and 40% and (b) 23%. (From Sadhu, S. and Bhowmick, A.K., J. Polym. Set, Part B Polym. Phys., 42, 1573, 2(304. Courtesy of Wiley InterScience.)... 

FIGURE 2.10 Variation in mechanical properties with styrene content in styrene-butadiene rubber (SBR)-based nanocomposites. 

FIGURE 11.12 Interaction energy density versus 4-methyl styrene content. (From Raboney, M., Gamer, R.T., Elspass, C.W., and Peiffer, D.G., Phase Behavior of Brominated Poly(Isobutylene-co-4-Methylstyrene)/ General Purpose Rubber Blends. Rubber Division, Proceedings of the American Chemical Society, Nashville, TN, Sept. 29-Oct. 2, 1998, Paper No. 36.)... 

FIGURE 27.1 ATR-IR spectra of sulfuric acid-treated thermoplastic rubbers (TRs) with different styrene content. Immersion time = 0.5 min. (From Cepeda-Jimenez, C.M., Pastor-Bias, M.M., Ferrandiz-Gomez, T.P., and Martm-Martmez, J.M., Int. J. Adhes. Adhes., 21, 161, 2001.)... 

However, in the anionic copolymerisation of styrene and MMA, jq r2/l and r < 1 and r2> 1. As it adds slower, the styrene content in the copolymer is always lower than in the monomer mixture. 

Variation of Styrene Content with Extent of Conversion. Figure 8 gives the relationship between copolymer composition and the extent of conversion for copolymers of butadiene and styrene (25 wt.7. styrene) prepared in toluene, at 30°C, with n-BuLi and barium salts of t-butanol and water. For comparison purposes, the copolymer composition curve is shown for the reaction initiated using n-BuLi alone. Copolymerization using n-BuLi results in very little incorporation of styrene into the copolymer chain until about 757. conversion, after which the styrene content increases very rapidly. In contrast, copolymerization using the barium salts and n-BuLi results in an increased incorporation of styrene at the same extents of conversion. 

The term star-block copolymer is used for a star architecture in which each arm is a diblock. The influence of chain topology on mechanical and morphological properties was investigated for copolymers composed of PS and PB with a constant styrene content of = 0.74 by Michler s group (Fig. 32) [101,102], While hexagonally packed cylinders of PB in a PS matrix were observed in a symmetric PS-fo-PB-fr-PS triblock copolymer, an L phase... 

Copolymers of styrene and butadiene with styrene content of 75-90%. They are organic nonblack reinforcing materials and find their greatest application in leather-type shoe soles. They facilitate the easy processing of relatively hard compounds due to a high degree of thermoplastic behaviour. 

A term applied to certain styrene-butadiene copolymers with a combined styrene content between 35% and 50%. 

Figure 5. Sediment volume of silica 186 in CCl solutions of polymers of various styrene contents. Squares, block polymers circles, random copolymers or homopolymers.

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