Photoactive Thermoplastic Elastomers of Azobenzene-Containing Triblock Copolymers Prepared through Atom Transfer Radical Polymerization

Atom transfer radical polymerization (ATRP) was used to prepare a new series of ABA triblock copolymers that are photoactive thermoplastic elastomers. The samples synthesized have the same rubbery midblock of poly(n-butyl acrylate) (PnBA) but differ in the degree of polymerization of the end blocks of a methacrylate-based azobenzene-containing side-chain liquid crystalline polymer (Azo-SCLCP). The coupling between elasticity, liquid crystallinity and photoactivity imparts interesting features to this type of thermoplastic elastomers. When the solution-cast film is stretched at T > Tg of the Azo-SCLCP whose microdomains act as physical cross-links, in contrast to conventional thermoplastic elastomers (such as styrene−butadiene−styrene triblock copolymer) that lose the elasticity, liquid crystalline microdomains can support part of the elastic extension of PnBA chains and, in the same time, deform to result in a long-range orientation of azobenzene mesogens. The liquid crystal orientation is retained in the relaxed film at T < Tg, which creates a thermoplastic elastomer whose glassy microdomains contain oriented azobenzene mesogens. Moreover, the reversible trans−cis photoisomerization of the azobenzene chromophore can be used to modulate the mechanically induced orientation.