Styrene-butadiene block copolymers. See

There are two mechanisms that may invalidate the above prediction (1) In spite of the best efforts of researchers and technologist, the added copolymer may prefer to form micelles inside one of the polymeric phases than to migrate to the interphase. This has been frequently observed in blends with block copolymers, e.g., for blends of PS with PE, compatibilized by addition of a hydrogenated styrene-butadiene block copolymer, SEES [Utracki and Sammut, 1988, 1990]. (2) Depending on the blend composition, the addition of compatibilizer may affect the total free volume of the system. These changes are difficult to predict. An increase of the free volume (evidenced by reduction of melt density) is expected to result in increased fluidity of the system. 

Transparent-, impact-, and blush -resistant blends were obtained by compounding linear low-density polyethylene, LLDPE, with hydrogenated styrene-butadiene block copolymer, SEES... 

Styrene—butadiene block copolymers are made with anionic chain carriers, and low molecular weight PS is made by a cationic mechanism (110). Analytical standards are available for PS prepared by all four mechanisms (see Initiators). 

Melt adhesives are based on thermoplastics, but usually contain a number of other components. The most commonly used melt adhesives are based on ethylene-vinyl acetate (EVA) copolymers, but polyethylene, polyesters, polyamides, and thermoplastic rubbers (e.g., styrene-butadiene block copolymers) are also used (see Adhesive Bonding of Plastics in Chapter 2). 

Styrene/butadiene triblock copolymer. See Styrene/butadiene/styrene block copolymer Styrene, p,a-dimethyl-. See p-a-Dimethylstyrene... 

In a sense, the styrene-butadiene block copolymers, SB or SBS, (first reported in 1956) constituted the next stage of PS modification. The triblock styrene-diene thermoplastic elastomers were patented in 1962, and soon incorporated in blends with PS, PP, LDPE, HDPE, PPE, PET, PBT, or PC, either as impact modifiers or compatibilizers [Bull and Holden, 1977]. In the 1977-78 patents (applications in 1976) it was disclosed that selective hydrogenation of these copolymers leads to new materials, with properties particularly attractive for polymer blends. For example, blending hydrogenated-SBS, or SEES, generated phase co-continuity in blends with PP, PA, PC, PBT, PES, etc. [Gergen et al., 1987]. More recent modification of these copolymers involved incorporation of acidic or acid-anhydride moieties. 

The SSBR material should not be confused with the styrene-butadiene block copolymer, a thermoplastic elastomer made from the same monomers also by an anionic polymerization mechanism (see the following). This material has very different mechanical properties and applications. 

A gel that is commercially available has the trade name ELAMUS. It is made of a hydrogenated styrene-butadiene block copolymer that is synthesized by solution polymerization. It is swollen by a liquid plasticizer. To maintain shape, a crosslinked structure resembling that of human tissue is formed (see Fig. 4) [5]. The characteristics of this gel are that even if the same polymer main chain is used, various properties can be tailored by changing the low molecular weight material that swells the polymer. By selecting appropriate compatibility and miscibility to allow the polymer molecules to be mobile, gel properties can be tailored. 

As discussed previously, thermoplastic elastomers are materials which have the functional properties of conventional vulcanized rubbers but which may be processed as normal thermoplastics (see section 2.9). Effects of this kind are shown by styrene-butadiene block copolymers. Two types of styrene-butadiene block copolymers are produced commercially, namely triblock and radial block copolymers. The triblock copolymers (denoted by SBS) consist of a centre block of butadiene units with two terminal blocks of styrene units. The radial block copolymers (denoted by (SB) X) consist of three or more styrene-butadiene diblock copolymers radiating from a central... 

If we look at Table 21.1 [64], we can see that there is a lower limit to how small a styrene endblock can be in a styrene-isoprene-styrene (SIS) styrenic block copolymer before the strength is severely reduced, or is reduced to that of chewing gum. There is a lower minimum molecular weight for the styrene block that will not yield a useful polymer or property set, let alone an elastomer. Thus, we have a lower boundary condition on how small the endblock is allowed to be. Similar results are found for styrene-butadiene-styrene (SBS) block copolymers [65]. 

Blends of polycarbonate and elastomers contain mostly graft copolymers based on butadiene and acrylate rubbers as the elastomeric component Polycarbonate blends with special thermoplastic elastomers, e.g., styrene/ethylene/butylene/styrene block copolymers (SEES) are another interesting product class. These blends exhibit improved resistance to gasoUne compared to polycarbonate, Figure 5.309. 

Early morphological studies to determine the nature of multiphase polymers and blends were reviewed by Folkes and Keller [363]. Many studies were of extruded block copolymers of materials such as SBS where the dispersed phase, an unsaturated rubber stained with OSO4 (see Section 4.4.2), was observed in the form of spheres, cylinders, or lamellae [364]. An excellent example is shown in a TEM micrograph of a thin section of a poly(styrene-butadiene) diblock copolymer, stained with OSO4 [365], which depicts the (100) projection of a body centered cubic lattice (Fig. 5.79). 

Butadiene copolymers are mainly prepared to yield mbbers (see Styrene-butadiene rubber). Many commercially significant latex paints are based on styrene—butadiene copolymers (see Coatings Paint). In latex paint the weight ratio S B is usually 60 40 with high conversion. Most of the block copolymers prepared by anionic catalysts, eg, butyUithium, are also elastomers. However, some of these block copolymers are thermoplastic mbbers, which behave like cross-linked mbbers at room temperature but show regular thermoplastic flow at elevated temperatures (45,46). Diblock (styrene—butadiene (SB)) and triblock (styrene—butadiene—styrene (SBS)) copolymers are commercially available. Typically, they are blended with PS to achieve a desirable property, eg, improved clarity/flexibiHty (see Polymerblends) (46). These block copolymers represent a class of new and interesting polymeric materials (47,48). Of particular interest are their morphologies (49—52), solution properties (53,54), and mechanical behavior (55,56). 

Closely related to these but thermoplastic rather than rubber-like in character are the K-resins developed hy Phillips. These resins comprise star-shaped butadiene-styrene block copolymers containing about 75% styrene and, like SBS thermoplastic elastomers, are produced by sequential anionic polymerisation (see Chapter 2). 

The heat of fusion AHf (obtained from the area under the DSC melting curve) and percentage crystallinity calculated from AHf is found to be linearly dependent on butadiene content, and independent of the polymer architecture. This is shown in Figure 3. Also, the density of the block copolymers was found to be linearly dependent on butadiene content (see Figure 4). The linear additivity of density (specific volume) has been observed by other workers for incompatible block copolymers of styrene and butadiene indicating that very little change in density from that of pure components has occurred on forming the block copolymers.(32) While the above statement is somewhat plausible, these workers have utilized the small positive deviation from the linear additivity law to estimate the thickness of the boundary in SB block copolymers.(32)... 

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