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Changing liquid crystal elastomer ordering with light – a route to opto-mechanically responsive materials

Long, flexible chains with rods and spacers give nematic polymer melts. Crosslinking yields highly extensible nematic elastomers. Molecular fluidity and orientational order are retained. Uniquely, these elastomers change shape drastically when orientational order is lost with temperature since chains lose their elongation. Reversible changes of 400% are easy to achieve. de Gennes envisaged this coupling between macroscopic strain and nematic (and other) liquid crystalline order. Dye molecular rods are excited by photons into bent conformations. Bent rods depress orientational order and large thermal shape changes will be duplicated by light. Equally, rods may rotate away from the optical electric vector to avoid bending, thus also leading to mechanical strain. Mechanical recovery follows decay back to the molecular ground state. We sketch how these effects arise, explain how even polydomain networks deform under polarised and unpolarised light, and present new results on how nuclear magnetic resonance can reveal details of polydomain optical response. We explore non-linear absorption (photo-bleaching) leading to mechanical response where, by Beer's law, little light should theoretically penetrate. In actuation, light penetrates quickly, is easy to deliver remotely, both excites and stimulates decay, and offers polarisation control over mechanics. Non-uniform director fields also control response. We illustrate novel photo-mechanical effects.