We present polymeric MEMS materials which reversibly respond to either thermal or UV stimuli by moving between nearly flat (r ~ infinity) and tightly curled states (r ~ 5mm) with variations in the radiation environment or temperature. The molecular orientation gradient of a liquid crystal network controls the primary bending axes, while controlled order parameter variations are responsible for the degree of deformation. In the case of thermal activation, these order changes are dominated by thermal motion, while UV-switchable defects bring about reduced network order in the case of UV actuation. We report fabrication and operation of the actuators and supplementary data regarding alignment configurations for controllable deformations, the phase behaviour of the liquid crystal constituents, thermal expansions, and absorption of the UV dyes are included. We find that splayed molecular configurations are preferred over twisted modes due to their single deformation axis, and that the optimum concentration of active molecules for UV-driven actuation is on the order of 7-8wt.%.
Polymer helices with submicron dimensions have been fabricated from a variety of isotropic and liquid crystalline polymers with storage moduli ranging from 38MPa to 1.9GPa (measured at 1Hz, room temperature). These helices are made using a double templating process, in which a thin film comprised of independent helical structures deposited using glancing angle deposition (GLAD) acts as the master. In our process the 'positive' structure of the master is copied into a polymer 'negative', which then itself acts as a template for the final film of polymer helices. Liquid crystalline polymers are of particular interest for use in MEMS because highly ordered liquid crystalline polymers can be actuated by exposing them to a stimulus (such as heat) that causes a decrease in order, leading to a reversible, macroscopic change in shape. The phase behavior, optical properties, and mechanical properties of planar aligned monoacrylate liquid crystalline polymers with varying crosslinker content are investigated, in order to determine the composition that will yield the largest deformations upon heating. We find that films with the lowest crosslinker content investigated (2.5%) undergo the largest reduction in birefringence as they are heated, corresponding to a loss in order. However, we also observe that the films with the highest crosslinker content investigated (10%) undergo the largest physical deformation upon heating. SEM images illustrating the deformation of liquid crystalline polymer helices as they are heated are also presented.
Liquid crystal networks change their dimensions when the degree of order is altered. Upon decreasing order, e.g. as a result of temperature increase, the linear dimension decreases in the direction along the director and increases orthogonal to that. When the director changes as a function of position, the local dimensional changes cause stresses that effect in deformation of the sample. In the case of thin films with a twisted molecular orientation over their cross-section a change in the order parameter results in a double, saddle-like, bending of the film as the linear expansion is different for both in-plane axes. For geometric reasons this bending is uncontrolled and irregular. When the linear expansion is chosen to be different along one in-plane axis, but is kept the same for the other axis, the deformation becomes orderly and controlled. Therefore, films of liquid crystal networks with a splayed molecular alignment over their cross-section provide a well-controlled bending deformation as a function of a changing order parameter. In a liquid crystal network the order parameter can be modulated by temperature. The direction- and order parameter dependent linear expansion than comes on top of the volume expansion as caused by induced thermal molecular motions and decreased secondary molecular forces. Besides by temperature the order parameter can also be modulated by light in the presence of photo-sensitive moieties in the liquid crystal network. The deformation behavior is anticipated to be of relevance for polymer based MEMS technology.