Bidirectional photoresponse of surface pretreated azobenzene liquid crystal polymer networks

We report on the photodriven, polarization-controlled response of UV-pretreated azobenzene-based liquid crystal polymer networks (azo- LCN) of polydomain orientation to higher wavelength CW argon-ion laser light (Ar(+)) of 457-514 nm. The significant absorbance of the azo-LCN cantilever in the UV is used to form an approximately 1 microm thick cis-isomer rich skin. Subsequent exposure to the Ar(+) laser drives a bidirectional bending process that is the result of two distinguishable photochemical processes. First, 457-514 nm laser light (regardless of polarization state) drives cis-trans photoisomerization of the UV-pretreated surface, restoring the order of the azobenzene liquid crystalline moieties. Mechanically, the cis-trans process results in an expansion on the exposed surface that forces the cantilever to undergo a rapid bend away from the laser source. Once a sufficient number of trans-azobenzene moieties are regenerated, continued Ar(+) illumination promotes both the trans-cis and cis-trans processes enabling trans-cis-trans reorientation. In this particular system and conditions, trans-cis-trans reorientation enables polarization controlled mechanical bending of different angles towards the Ar(+) source. Photomechanical responses of UV-pretreated azo-LCN demonstrate the viability of photogenerated effects in UV-rich environments such as space.