Pendant acrylate groups, elastomers

Scheme 4. Cross-linking of LC elastomers by radical polymerization of pendant acrylate groups.

X-ray measurements on LC elastomers have shown [6-8] that the reversible transition between a chiral smectic C phase with and without a helical superstructure can be induced mechanically. The helix untwisted state corresponds in this case to a polar ferroelectric monodomain. The piezoelectricity arising from this deformation of the helical superstructure (which does not require a complete untwisting) has been demonstrated [9] for polymers cross-linked by polymerization of pendant acrylate groups (Figure 15). 

A copolymer of ethyl acrylate with 5% chloroethyl vinyl ether has been marketed as an oil-resistant, high-temperature-stable, specialty elastomer. The pendant chlorine group gives a preferred cross-linking site with polyfunctional amines. The relatively high Tg of the rubber (about 28°C) can be lowered by plasticization, but this can be temporary if oils or solvents are in contact with the product and leach out the plasticizer. Interpolymers of ethyl acrylate with methyl methacrylate and with higher acrylates and methacrylates are widely used as latex paints and as additives for paper and textiles. The Tg of an interpolymer can be tailored for a particular application by varying the ratio of acrylate to methacrylate monomers. 

PMA is a tough leathery resin with a low Tg and a solubility parameter of 10.5 H. In polymers of alkyl acrylates the solubility parameter decreases as the size of the alkyl group increases. The flexibility also increases with the size of the pendant groups but because of side chain crystallization this tendency is reversed when the alkyl group has more than ten carbon atoms. Polyalkyi acrylates are readily hydrolyzed by alkalis to produce salts of polyacrylic acid. The copolymer of ethyl acrylate (95%) and chloroethyl vinyl ether (5%) is a commercial oil-resistant elastomer. 

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