Cross-linked elastomers, combined polymers

It is possible to classify polymers by their structure as linear, branched, cross-linked, and network polymers. In some polymers, called homopolymers, merely one monomer (a) is used for the formation of the chains, while in others two or more diverse monomers (a,p,y,...) can be combined to get different structures forming copolymers of linear, branched, cross-linked, and network polymeric molecular structures. Besides, on the basis of their properties, polymers are categorized as thermoplastics, elastomers, and thermosets. Thermoplastics are the majority of the polymers in use. They are linear or branched polymers characterized by the fact that they soften or melt, reversibly, when heated. Elastomers are cross-linked polymers that are highly elastic, that is, they can be lengthened or compressed to a considerable extent reversibly. Finally, thermosets are network polymers that are normally rigid and when heated do not soften or melt reversibly. 

While IPNs can be and have been made extremely tough and impact resistant, many of the proposed applications involve such diverse fields and sound and vibration damping, biomedical materials, and nonlinear optics. This is because the presence of cross-links in both polymers reduces creep and flow, allowing relatively stable materials with a wide range of moduli to be prepared. Thus, those materials with leathery mechanical behavior, combinations of elastomers and plastics, are especially interesting to scientists, inventors, and engineers. 

Other thermoplastic elastomer combiaations, ia which the elastomer phase may or may not be cross-linked, include blends of polypropylene with nitrile (30,31), butyl (33), and natural (34) mbbers, blends of PVC with nitrile mbber (35,36), and blends of halogenated polyolefins with ethylene interpolymers (29). Collectively, thermoplastic elastomers of this type ate referred to herein as hard polymer/elastomer combinations. Some of the more important examples of the various types are shown in Table 3. 

In the hard polymer/elastomer combinations, the elastomer is often chosen to be a polar mbber or it is cross-linked in some cases it is both. Either of these features improves the resistance to oils and solvents (44). 

The production of the hard polymer/elastomer combinations is more simple. The two components are mixed together under conditions of intensive shear. In some cases, grafting may occur. In a variation of this technique, the elastomer can be cross-linked while the mixing is taking place, a process described as dynamic vulcanization (32). 

Radiation-induced modification or processing of a polymer is a relatively sophisticated method than conventional thermal and chemical processes. The radiation-induced changes in polymer materials such as plastics or elastomers provide some desirable combinations of physical and chemical properties in the end product. Radiation can be applied to various industrial processes involving polymerization, cross-linking, graft copolymerization, curing of paints and coatings, etc. 

In contrast, organophilic PV membranes are used for removal of (volatile) organic compounds from aqueous solutions. They are typically made of rubbery polymers (elastomers). Cross-linked silicone rubber (PDMS) is the state-of-the-art for the selective barrier [1, 43, 44]. Nevertheless, glassy polymers (e.g., substituted polyacetylene or poly(l-(trimethylsilyl)-l-propyne, PTMSP) were also observed to be preferentially permeable for organics from water. Polyether-polyamide block-copolymers, combining permeable hydrophilic and stabilizing hydrophobic domains within one material, are also successfully used as a selective barrier. 

Thermoset polyurethanes are cross-linked polymers, which are produced by casting or reaction injection molding (RIM). For cast elastomers, TDI in combination with 3,3,-dichloro-4,4,-diphen5lmethanediamine (MOCA) are often used. In the RIM technology, aromatic diamine chain extenders, such as diethyltoluenediamine (DETDA), are used to produce poly(urethane ureas) (47), and replacement of the polyether polyols with amine-terminated polyols produces polyureas (48). The aromatic diamines are soluble in the polyol and provide fast reaction rates. In 1985, internal mold release agents based on zinc stearate compatibilized with primary amines were introduced to the RIM process to minimize mold preparation and scrap from parts tom at demold. Some physical properties of RIM systems are listed in Table 7. 

Cross-linked LC elastomers from combined LC polymers were mostly obtained by means of the hydrosililation reaction shown in Figure 7 [3-7, 10]. Later, a thermal or photochemical polymerization of acrylates was used [9, 11]. As can be seen from Figure 7, this cross-linking process, which... 

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