Light-induced chain cross-linking

Changes also occur as the polymer ages under the influence of time, air, light and heat. Cross-linking or oxidation of the chains may cause a polymer to become insoluble in solvents that can be safely used on an object (Sease, 1981). The polymer may also cause the object to degrade more rapidly than it otherwise would have done (Berger and Zeliger, 1975 Baer et al 1976). The cause of these deleterious effects is probably a combination of chemical reactions induced by the polymer and the physical restraint imposed by the polymer on a very different type of material. 

The second example used visible light absorption that increased the temperature locally within the thermosensitive gel [39]. The gel consisted of a covalently cross-linked copolymer network of N-isopropylacrylamide and chloro-phyllin, a combination of a thermo-sensitive gel and a chromophore. In the absence of light, the gel volume changed sharply but continuously as the temperature was varied. Upon illumination the transition temperature was lowered, and beyond a certain irradiation threshold the volume transition became discontinuous. The phase transition was presumably induced by local heating of polymer chains due to the absorption and subsequent thermal dissipation of light energy by the chromophore. The details will be discussed in a later section. 

As mentioned earlier, ferroelectric LC elastomers were first synthesized by Zentel et al. [94-96] the synthesis and properties of LC elastomers are reviewed by Gleim and Finkelmann [97]. One of the synthetic routes to ferroelectric LC elastomers is preparation of side-chain polymers (Fig. 10) or combined polymers (Fig. 12b) containing active groups in side chains, with further cross-linking. The ultraviolet (UV) light-induced radical polymerization of acrylamide or acrylate active groups has been used in the former case [83,95,98] and hydrosilation addition in the latter case [94]. 

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