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

Abstract

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 X3V3=3.3 mol %, X5V3=5 mol %, and X7V3=7 mol %. LSCE networks are characterized by frozen-in orientation order, P. Xc is a constitutional critical point for LSCE networks. When X<Xc (P=0), there are no shape changes and a nematic-isotropic transition takes place at TNI. When X>Xc (P>0), supercritical behavior prevails over a range of temperatures, ΔTs. Taking ΔTs∝(X-Xc)1/2, we find a critical cross-linking density, Xc≈3%, for V3 LSCEs. 5CB diffuses ⊥n into V3 similar to isotropic gels with typical times, τ⊥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 τ⊥(2) (twist), and τ|| (13) (splay-bend) that both increase nearly parabolically with increasing (X-Xc). Front speeds are 50% faster in 5V3 than in 7V3 but no fronts were observed in 3V3. Compared to dry V3 LSCE volumes, Vd, the swollen volumes, Vs, increased as X→Xc: Vs/Vd∼(X-Xc)-0.27 reducing the frozen-in orientational order at X to \tildeX=X(Vd/Vs). The maximum electromechanical effect found in swollen V3 was δMAX∼(\tildeX-\tildeXc) with \tildeXc∼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.