Polystyrene plastic phase, linking

For NR based IPNs, Mathew et al. had investigated the sorption and diffusion of three aromatic solvents (benzene, toluene, and xylene) through NR/polystyrene semi-, and full IPNs, and reported that as the polystyrene content increased, the solvent uptake value decreased. This was attributed to the fact that the introduction of plastic phase decreased the chain flexibility of the network. The study further showed that the nature and size of the penetrant molecule affected the transport behaviour. Temperature, it was observed, affected the transport properties. The solvent uptake was found to inerease with temperature up to 65 °C, and at 70 °C, a deerease in uptake was observed (Figure 22.4). The values of sorption and permeation eoeffieients obtained showed a direct dependency on sample characteristics, blend composition, and crosslink level. The study further showed that as the number of crosslinks increased in the blends, the resistance offered to solvent uptake increased since the solvent molecules have to overcome the dense barrier of polymer cross-linking and entanglements to diffuse into the blend. 

Styrene-butadiene block copolymer belongs to the A-B-A type thermoplastic elastomer. The principal structure of this type of polymer involves the thermoplastic rubber molecules terminated by the hard, glassy end blocks. The A and B copolymer block segments are incompatible and, consequently, separate spontaneously into two phases. Thus in the solid state, the styrene-butadiene (S-B-S) thermoplastic elastomer has two phases a continuous polybutadiene rubber phase and the dispersed glassy domains of polystyrene. The styrene plastic end blocks, called domains, act as cross-links locking the rubber phase in place. 

Other thermoplastic elastomer combinations, in which the elastomer phase may or may not be cross-linked, include blends of polypropylene with nitrile (26), butyl (28), and natural (29) rubbers, blends of PVC with nitrile rubber and plasticizers (30-32), and blends ofhalogenated polyolefins with ethylene interpolymers (30). Commercially important products (33,34) based on blends of polystyrene with S-B-S and oil and also on blends of polypropylene with S-EB-S and oil are described later in this article. They are also considered as thermoplastic elastomer combinations. The oils nsed in these prodncts are nsnally hydrocarbons but blends with silicone oils have also been described (35). Collectively, all thermoplastic elastomers of this type (both bends and dynamic vnlcanizates) are referred to herein as hard polymer/elastomer combinations. 

The macroscopic composites are those in which one or both phases tend to have structures large enough to be easily seen with the naked eye. These include paint hlms (paint itself is usually a particulate composite), adhesive joints, and foams (e.g., the familiar polystyrene foams used for coffee cups because of their low heat transfer coefficient). The fabric laminates alternate layers of inorganic cloth and plastic. An important subclass usually has multiple layers of glass cloth or chopped fiber interspersed with layers of unsaturated polyester-styrene AB-cross-linked materials see Section 2.7.3. These sheet molding compounds are widely used for the hulls of pleasure boats, because they are light weight and very puncture resistant, as well as for automotive bodies see Section 13.9.1. 

Styrenic block copolymer (SBS) TPEs are multiphase compositions in which the phases are chemically bonded by block copolymerization (see chapter Introduction to Plastics and Polymers). At least one of the phases is a hard styrenic polymer. This styrenic phase may become fluid when the TPE composition is heated. Another phase is a softer elastomeric material that is rubber-like at room temperature. The polystyrene blocks act as cross-links, tying the elastomeric chains together in a three-dimensional network. SBS TPEs have no commercial applications when the product is just a pure polymer. They must be compounded with other polymers, oils, fillers, and additives to have any commercial value. 

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