UC Riverside

Photomechanical materials interact with light to elicit a mechanical response. They are the basis of a new class of smart materials which can serve as the active element in actuators, thus creating light-powered devices. The origin of the mechanical response is a volume change induced by a solidsolid or solidliquid phase transition in the material. This light-induced volume change can be performed two different ways: through a photochemical reaction or through the photothermal effect. In this thesis, both pathways are discussed, and photomechanical actuation is demonstrated in piston-type actuators and polymer films. In Chapter 3, the photoisomerization kinetics of a third-generation donor acceptor Stenhouse adduct (DASA) are examined over a range of concentrations. DASAs are a novel photoswitch which absorb in the visible and near-infrared spectral regions. It switches efficiently at micromolar concentrations in both liquid solution and in polymers, but as the photochrome concentration is increased there is a dramatic inhibition of the photoisomerization. The physical origin of the inhibition of photoswitching at high photochrome concentrations must be understood if the DASA class of molecules is to be used for applications like photomechanical actuation. In Chapter 4, a photothermal solidliquid transition is studied because of its large volume expansion. Photoactive composites made of small molecules doped in a phase change material (PCM) are used in a commercial wax actuator to generate useful mechanical work. It is demonstrated that small molecules can act as absorbers to enable a photothermal solidliquid melting transition in eicosane, a low molecular weight PCM. The stability, work density, and efficiency of the actuator containing these PCMs are investigated, and it is found that the actuators are competitive with electrically powered devices. In Chapter 5, further investigation of the small molecule-PCM composite is conducted using a custom-built actuator to gain insight on how the actuator should be optimized to maximize its work output. This new actuator consists of a simpler geometry which allows for the comparison with theoretical work density models. In Chapter 6, an azobenzene derivative (AZO) that can undergo photochemical melting at room temperature, is investigated as a potential candidate for photomechanical actuation. AZO is incorporated in a variety of polymer hosts to create thin films capable of bending when irradiated with light. We find that doping polymers with AZO can provide a general strategy to make photomechanical polymers using a reversible photochemical reaction that induces a solidliquid phase transition.