Trifunctionally Cross-Linked Liquid Single Crystal Elastomers: Swelling Dynamics and Electromechanical Effects

Here, we study the influence of cross-linking density, X, on the swelling, thermal and electric field response of trifunctionally cross-linked liquid single crystal elastomers (V3 LSCEs) swollen with low molecular weight liquid crystals, 4-n-pentyl-4-cyanobiphenyl (5CB). The cross-linker concentrations are X(3V3) = 3.3 mol %, X(5V3) = 5 mol %, and X(7V3) = 7 mol %. LSCE networks are characterized by frozen-in orientation order, P. X, is a constitutional critical point for LSCE networks. When X < X(c) (P = 0), there are no shape changes and a nematic-isotropic transition takes place at T(NI). When X > X(c) (P > 0), supercritical behavior prevails over a range of temperatures, Delta Ts. Taking Delta T(s) alpha(X - X(c))(1/2), we find a critical cross-linking density, X(c) approximate to 3%, for V3 LSCEs. 5CB diffuses perpendicular to n into V3 similar to isotropic gels with typical times, tau(perpendicular to n) in minutes, that decreases with increasing (X - Xc). Swelling with reorientation effects is initiated by 5CB propagating into V3 followed by relaxation of the front profile with tau((2))(perpendicular to) (twist) and tau((13))(parallel to) (splay-bend) that both increase nearly parabolically with increasing (X - X(c)). Front speeds are 50% faster in 5V3 than in 7V3 but no fronts were observed in 3V3. Compared to dry V3 LSCE volumes, V(d), the swollen volumes, Vs, increased as X -> X(c) : V(s)/V(d) similar to (X - X(c))(-0.27) reducing the frozen-in orientational order at X to = (X) over tilde =X(V(d)/V(s)). The maximum electromechanical effect found in swollen V3 was delta(MAX) similar to((X) over tilde - (X) over tilde (c)) with (X) over tilde (c) similar to 0.25%. The size of the effect is much smaller than observed in 8A2 which has a larger P. For 7V3 the effect is about 4 times smaller and for 5V3, it is an order of magnitude smaller.