Elastomeric property

If a modest number of cross-links between the polymer backbone are introduced, the polymer Hquid crystal takes on elastomeric properties. The useflilness of these materials probably Hes in the coupling of mechanical and optical effects. 

Polyuretha.ne, A type of spunbonded stmcture has been commercialized in Japan based on thermoplastic polyurethanes (15). This represents the first commercial production of such fabrics, although spunbonded urethane fabrics have been previously discussed (16). The elastomeric properties claimed are unique for spunbonded products and appear to be weU suited for use in apparel and other appHcations requiring stretch and recovery. Polyurethanes are also candidates for processing by the meltblown process. 

Copolymers of S-caprolactone and L-lactide are elastomeric when prepared from 25% S-caprolactone and 75% L-lactide, and rigid when prepared from 10% S-caprolactone and 90% L-lactide (47). Blends of poly-DL-lactide and polycaprolactone polymers are another way to achieve unique elastomeric properties. Copolymers of S-caprolactone and glycoHde have been evaluated in fiber form as potential absorbable sutures. Strong, flexible monofilaments have been produced which maintain 11—37% of initial tensile strength after two weeks in vivo (48). 

Pipe fittings require quite high tensile and creep resistance they are thus molded from compounds that have less melt flow than for thin-waHed housings. Plasticized compounds are injection-molded into a variety of parts requiring elastomeric properties. 

The properties of elastomeric materials are also greatly iafluenced by the presence of strong interchain, ie, iatermolecular, forces which can result ia the formation of crystalline domains. Thus the elastomeric properties are those of an amorphous material having weak interchain iateractions and hence no crystallisation. At the other extreme of polymer properties are fiber-forming polymers, such as nylon, which when properly oriented lead to the formation of permanent, crystalline fibers. In between these two extremes is a whole range of polymers, from purely amorphous elastomers to partially crystalline plastics, such as polyethylene, polypropylene, polycarbonates, etc. 

Fluoiocaibon elastomeis aie synthetic, noncrystaUine polymers that exhibit elastomeric properties when cross-linked. They are designed for demanding service appHcations in hostile environments characterized by broad temperature ranges and/or contact with chemicals, oils, or fuels. 

Over the past 40 years there have been a number of developments that have resulted in the availability of rubbery materials that are thermoplastic in nature and which do not need chemical cross-linking (vulcanisation or setting) to generate elastomeric properties (see also Section 11.8 and 31.2). This approach has been extended to the fluoroelastomers. 

Applications of NBR adhesives are based on the excellent elastomeric properties of the polymer coupled with its polarity, which provides good solvent resistance... 

In silicone adhesives used to bond structural glazing assemblies, the silicone network is made of very long PDMS chains and is filled with silica that improves the elastomeric properties of the adhesive. The strength of such an adhesive is strongly enhanced through various mechanisms of energy absorption. 

Although the primary function of sealants is to seal, adhesion promoters are often added, which allows them to adhere to the adjoining base materials. It is therefore sometimes difficult to distinguish between an adhesive and a sealant. For example, structural silicone adhesives are used in the building construction industry owing to their sealing, adhesive, elastomeric properties, and their resistance to harsh environmental conditions [67,70,77]. 

Polyary lsiloxanes Carboranes Good thermal stability 400-500°C (752-932°F) coatings, adhesives Stable in air and nitrogen at 400-450°C (752-842°F) elastomeric properties for silane derivatives up to 538°C (1000°F) adhesives. 

Amine-terminated siloxane oligomers have also been utilized in the synthesis of various siloxane-amide and siloxane-imide copolymers, High molecular weight siloxane-amide copolymers have been synthesized by the solution or interfacial co-polymerization of siloxane oligomers with sebacoyl chloride or terephthaloyl chloride respectively 1S5,165). In some reactions diamine chain extenders have also been utilized. Thermal and dynamic mechanical characterization of these copolymers have shown the formation of multiphase systems160). Compression molded films displayed very good elastomeric properties. 

Similar types of lamellar morphologies were observed for triblock copolymers of diphenylsiloxane and dimethylsiloxane having 40 wt% polydiphenylsiloxane, using electron microscopy, 47-148>. The lamellae thickness was approximately equal to the chain length of the rigid polydiphenylsiloxane blocks. These copolymers showed elastomeric properties comparable to those of conventional silica-reinforced, chemically crosslinked silicone rubbers. Tensile tests yielded an initial modulus of 0.5-1 MPa, tensile strength of 6-7 MPa and ultimate elongation between 400 and 800 %. 

Incorporation of flexible siloxane spacers into side chain or main chain liquid crystalline polymers have been shown to drastically reduce the transition temperatures 255,267,271,272,277) anc[ aiso increase the response time of the resultant systems to the applied thermal, optical or electrical fields 350-353>. In addition, siloxanes also provided elastomeric properties and improved the processibility (solution or melt) of the resulting liquid crystalline copolymers. 

Polymers containing each of these configurations are known, the most common being the cis- A and the 1,4-isomers. The first of these, poly(c/ -l,4-isoprene), is the macromolecular constituent of natural rubber the second is the material known as gutta percha. The latter, unlike natural rubber, has no elastomeric properties, but has a leathery texture. It has been used for diverse applications such as golf-ball covers and as an insulating material for the trans-Atlantic cables of the late nineteenth century. 

The presence in these copolymers of hetero-substituted monomeric units randomly dispersed along the phosphazene skeleton brings about the extreme difficulty of the polymeric chains to be packed in regular structures. They lose, therefore, the original stereo-regularity of the parent phosphazene homopolymers (microcrystalline materials), and show only amorphous structures, with sharp decrease in the values of the Tg (collapsed up to about -90 °C) and with the onset of remarkable elastomeric properties [399,409,457]. 

Fig. 20 Modular resilin-like polypeptide containing domains conferring elastomeric properties, heparin molecule interaction, cell adhesion, and matrix metalloproteinase (MMP) proteolysis. Lysine residues are encoded periodically to permit crosslinking...

Elastin is a structural protein with outstanding properties, and therefore it has inspired many investigations, with special interest in its elastomeric properties. This behavior can be mimicked with ELPs and these have found wide application in... 

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