New photoactive polymers azobenzene elastomers
Autre titre : Nouveaux polymères photoactifs : élastomère d'azobenzène
Date de publication2003
Azobenzene-containing elastomers have been synthesized for the first time. By free radical polymerization of methacrylate or acrylate-based monomers bearing an azobenzene moiety in solution with dissolved styrene-butadiene-styrene (SBS) triblock copolymer, azobenzene side-chain liquid crystalline polymer (SCLCP) was grafted on the rubbery polybutadiene (PB) central block of SBS. Similar to SBS, the resultant azobenzene SCLCP-grafted SBS was a thermoplastic elastomer, for which polystyrene (PS) cylindrical microdomains behave as physical cross-links. Three different azobenzene SCLCCs, differing in chain backbone or terminal group on the azobenzene moiety, were used to prepare azobenzene elastomers. Their thin films obtained from solution casting were transparent and could be repeatedly stretched at room temperature. Polarized UV-vis and infrared spectroscopy were used to investigate the coupling effects from film deformation and photoisomerization of azobenzene chromophore in the elastomer. Stretching the film effectively induces a long-range orientation of azobenzene mesogenic groups (in the trans form) along the strain direction due to the liquid crystalline nature of the azobenzene polymer. When the stretched film is exposed to polarized or unpolarized UV light (360 nm), the orientation of trans-azobenzene is erased as a result of the trans-to-cis photoisomerization that corresponds to a photochemical liquid crystal-to-isotropic phase transition. On polarized or unpolarized visible irradiation (440 nm), the cis-to-trans back-isomerization recovers the orientation of trams-azobenzene mesogens. This coupled mechanical and optical effect allows a reversible optical switching between orientation-on and orientation-off states to be realized on stretched films. The study shows that the azobenzene SCLCP grafted on SBS is important since its glass and mesophase transition temperatures determine greatly the degree of stretching induced orientation while its cis isomer concentration at the photostationary state affects the erasure of orientation on UV light irradiation. Moreover, experiments found no evidence of reorientation of azobenzene mesogens in stretched films under linearly polarized UV irradiation, but during the orientation recovery with linearly polarized visible light, a contribution from photoinduced orientation of azobenzene becomes significant.
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