Photoresponsive Azobenzene Liquid Crystal Polymer Networks: In Situ Photogenerated Stress Measurement

Azobenzene liquid crystal polymers and polymer networks are adaptive materials capable of converting light into mechanical work. Often, the photomechanical output of the azobenzene liquid crystal network (azo-LCN) is observed as a bending cantilever. The response of these materials can be either static (e.g. a simple bending cantilever) or dynamic (e.g. oscillating cantilever of 20–270 Hz). The resulting photomechanical output is dependent upon the domain orientation of the polymer network and the wavelength and polarization of the actinic light. Polydomain azobenzene liquid crystal polymer networks, which have the capability of bending both backwards and forwards with the change of polarization angle, are of particular interest. In the current study, three azo-LCNs are compared — two of them are equivalent in all respects except for one contains pendant azobenzene mesogens (1azo, azo-monoacrylate) and the other contains crosslinked azobenzene mesogens (2azo, azo-diacrylate). The third specimen has a combination of both mesogens. The mechanical behavior at different temperatures and examination of structure-property relationships in the polymerization process, including curing temperatures and liquid crystal cell alignment rubbing methods, were explored. Using dynamic mechanical analysis (DMA) the mechanical properties and the photogenerated stress and strain in the polymer are examined. It is found the differences in chemistry do correlate to small variation in the speed of photodirected bending, elastic modulus, and glass transition temperature. Despite these differences, all three azo-LCNs display nearly equivalent photogenerated stresses.