Grietje N. Mol

Synthesis and photoinitiated polymerization of liquid crystalline diepoxides

Abstract Four different liquid crystalline diepoxides were synthesized. Replacement of a methylene group by an oxygen atom in the spacer segment of the molecule decreased the thermal stability of the mesophase. In order to produce highly anisotropic networks, these compounds were cationically photopolymerized in the nematic phase. Differential scanning calorimetry and Fourier transform infra-red were used to monitor the polymerization. The degrees of conversion calculated from both techniques were in good agreement with each other. The replacement of a methylene group by an oxygen atom in the spacer part of the molecule also increased the rate of the polymerization reaction. Post-curing was sometimes necessary to produce highly stable networks. Furthermore, some results suggest that liquid crystalline nematic ordering increases the polymerization rate.

Temperature effects on the kinetics of photoinitiated polymerization of dimethacrylates

Abstract The photoinitiated polymerization of three bisphenol-A-based dimethacrylates is studied. The polymerization rate of the dimethacrylate increases with temperature up to 160°C. Above this temperature depropagation starts to dominate the polymerization and the rate approaches zero above 210°C. In the case of the diacrylates three temperature regions are distinguished. Below 90°C the polymerization rate increases with temperature. Between 90 and 145°C chain transfer suppresses the autoacceleration which results in a decreasing polymerization rate with increasing temperature. Above 145°C depropagation causes an additional rate decrease.

Molecular orientation control for thermal and UV-driven polymer MEMS actuators

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.%.