Styrene-butadiene copolymer cross-linking

FIG. 14-10. Storage compliance, reduced to 25°C, plotted logarithmically for styrene-butadiene copolymer cross-linked to three different extents as gauged by the values of log Je shown. (Mancke and Ferry. )... 

Synthetic polymers in general can be classified (1) by thermal behavior, i.e., thermoplastic and thermosetting (2) by chemical nature, i.e., amino, alkyd, acrylic, vinyl, phenolic, cellulosic, epoxy, urethane, siloxane, etc. and (3) by molecular structure, i.e., atactic, stereospecific, linear, cross-linked, block, graft, ladder, etc. Copolymers are products made by combining two or more polymers in one reaction (styrene-butadiene). See cross-linking block polymer epitaxy homopolymer plastics. 

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). 

In copolymers having both a readily cross-linked structure and an aromatic component, such as styrene-butadiene copolymers, it is found that radiation protection is greatest when the aromatic units are randomly dispersed in the copolymer rather than in segregated units, as with a block copolymer... 

Figure 8.29 plotted against Gg for three rubbers [polybutadiene (PB), styrene-butadiene copolymer (SBR), and natural rubber (NR)] at different stages of cross-linking by (O) dicumyl peroxide and ( ) sulfur. (From Ref. 44.)... 

An alternative method of initiation is through the use of the radical anion produced from the reaction of sodium (or lithium) with naphthalene. Such radical anions react with styrene by electron transfer to form styrene radical anions these dimerize to produce a dianion, which initiates polymerization as outlined in Scheme 14. One particular feature of this method is that polymerization proceeds outwards from the centre. Subsequent reaction of the living chains ends with another suitable monomer system produces a triblock copolymer. This is the principle by which styrene-butadiene-styrene triblock copolymers (formed when butadiene is polymerized in the same way. and styrene is added as second monomer) are produced commercially. This material behaves as a thermoplastic elastomer, since the rigid styrene blocks form cross-links at room temperature on heating these rigid styrene portions soften, allowins the material to be remoulded. ... 

Thermoplastic elastomers, for example certain styrene-butadiene copolymers (see Table 10), contain so-called hard and soft segments that react like crosslinks at low to medium temperatures, but fuse thermoplastically at higher temperatures and thus do not represent true chemical cross-links. 

Acrylonitrile-butadiene-styrene/acidic monomer, an elastomeric copolymer Cross-linked polyethylene Extra-strength molding compound... 

The binder, therefore, must have the following properties good adhesion to fibre and pigment in the dry state, and in water and solvent a soft, non-tacky handle elasticity lightfastness, clarity and no colour compatibility with, and stability towards, other ingredients in the mix. Some styrene butadiene copolymer rubbers and vinyl latices are used successfully but the most suited are the acrylics. The polymers are usually either self-cross-linking (with heat) or incorporate a cross-linkable resin such as melamine-formaldehyde. 

In this chapter the homopolymer, polystyrene, is considered together with styrene-acrylonitrile copolymers, acrylonitrile-butadiene-styrene copolymers and styrene-a-methylstyrene copolymers. The important styrene-butadiene copolymers are described with other diene polymers in Chapter 18. The use of styrene in the cross-linking of unsaturated polyesters is described in Chapter 10. 

FIG. 12-8. Diffusion coefficient of n-hexadccane at 2S°C through random styrene-butadiene copolymers plotted against reciprocal of fractional free volume calculated from equation 8. Open circles, uncross-linked samples black circles, cross-linked. Lines drawn with slope of — 1/2.3Q3 as specified by equation 7. (Rhee and Ferry. ) Reproduced, by permission, from the Journal of Applied Polymer Science. 

It has already been pointed out in Chapter 13, Section Al, that maxima in the loss compliance and the retardation spectrum are characteristic of network structures as predicted from the Rouse theory, suitably modified, in Fig. 10-7. Such maxima appear in moderately cross-linked polymers as well as in uncross-linked polymers of high molecular weight, and their shapes are remarkably similar. In Fig. 14-3, J" is compared for styrene-butadiene copolymer without cross-links and cross-linked to a value of Gg = 7.3 X 10 dynes/cm, characteristic of a well-vulcanized soft rubber e.g., curve VI of Figs. 2-1 to 2-8). The curves are both close in shape to that predicted by the Rouse-Mooney theory for a most probable distribution of network strands, i.e., curve D of Fig. 10-7, as already evident from Fig. 13-1. 

Styrene-Butadiene-Styrene Block Copolymers. Styrene blocks associate into domains that form hard regions. The midblock, which is normally butadiene, ethylene-butene, or isoprene blocks, forms the soft domains. Polystyrene domains serve as cross-links. 

Organic peroxides are used in the polymer industry as thermal sources of free radicals. They are used primarily to initiate the polymerisation and copolymerisation of vinyl and diene monomers, eg, ethylene, vinyl chloride, styrene, acryUc acid and esters, methacrylic acid and esters, vinyl acetate, acrylonitrile, and butadiene (see Initiators). They ate also used to cute or cross-link resins, eg, unsaturated polyester—styrene blends, thermoplastics such as polyethylene, elastomers such as ethylene—propylene copolymers and terpolymers and ethylene—vinyl acetate copolymer, and mbbets such as siUcone mbbet and styrene-butadiene mbbet. 

The term ABS was originally used as a general term to describe various blends and copolymers containing acrylonitrile, butadiene and styrene. Prominent among the earliest materials were physical blends of acrylonitrile-styrene copolymers (SAN) (which are glassy) and acrylonitrile-butadiene copolymers (which are rubbery). Such materials are now obsolete but are referred to briefly below, as Type 1 materials, since they do illustrate some basic principles. Today the term ABS usually refers to a product consisting of discrete cross-linked polybutadiene rubber particles that are grafted with SAN and embedded in a SAN matrix. 

S-B-S Triblocks are block copolymers consisting of a block of butadiene units flanked by blocks of styrene. Below the T, of polystyrene blocks from different chains congregate into domains which act both as cross-links and reinforcing fillers. The jDolymers will dissolve in hydrocarbon solvents. Hydrogenated S-B-S materials have better resistance to ageing. 

FIGURE 1.11 Tear energy Gc versus rate R of tear propagation for a cross-linked sheet of a high-styrene copolymer of butadiene and styrene (48% styrene Tg = —30°C). (From Gent, A.N. and Lai, S.-M., J. Polymer Sci., Part B Polymer Phys., 32, 1543, 1994. With permission.)... 

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