P. E. Cladis

Fluid Biaxial Banana Smectics: Symmetry at Work

Abstract Perhaps one of the most surprising results in the past 10 years was the prediction of Brand et al. [1] that fluid biaxial smectics made from comopunds without asymmetric carbons could, by symmetry, have a spontaneous polarization, P, in the layer plane. They called this construct, which can be either ferroelectric or antiferroelectric, smectic C9. Advances gained from their prediction include the development of highly efficient electrets for broad-band telecommunications [2, 3] as well as a basic non-linear model [4] for TLAFs, thresholdless antiferroelectrics [5], now seen in beatiful, CMOS compatible, active matrix liquid crystal displays [6] (figure 1).

Fluid Biaxial Banana Phases: Symmetry at Work

Abstract Fluid biaxial smectics made from compounds without asymmetric carbons but nevertheless with a spontaneous polarization are now known as banana smectics because of their molecular shape. Here we show that symmetry changes under parity (r ← - r) are an efficient way to summarize and differentiate their electro-optic properties. Typical of fluid biaxial smectics, there are also a large number of stacking options endowing them with opto-electric properties spanning a broad range of economically viable applications. “Value-added features” of some banana smectics include a faster electro-optic response than found in liquid crystals with a helix structure, their steric property allowing rotations about an axis in a layer plane without compromising smectic layer stability and ambidextrous chirality.

Hedgehog-antihedgehog pair annihilation to a static soliton

We experimentally demonstrate that, for sufficiently small perturbations, a hedgehog (H) and antihedgehog (H) point defect in nematic liquid crystals can be asymptotically free. For larger external perturbations, we find that the annihilation dynamics of an HH pair to a static soliton is asymmetric and shows two time regimes. A model for the experimental observations in the absence of smectic A fluctuations is proposed.

Multifunctional liquid crystal elastomers: Large electromechanical and electro-optical effects

A multifunctional main chain liquid crystal elastomer (MCLCE) with large mechanical and optical effects in applied electric fields is investigated, when MCLCE is swollen in a low molecular weight liquid crystal, 4-n-pentyl-4-cyanobiphenyl, a nematic solvent. The size change by the field effects is linearly proportional to the transmittancy change. This suggests the possibility of broad application as a field-induced-optical actuator device.

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 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), 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.

Deformable tetrahedratic phases: The effects of external fields and flows

Abstract.We discuss changes in the symmetry and physical properties of an isotropic phase which has initially tetrahedral symmetry characterized by four unit vectors. In its undeformed state, these four vectors are at the tetrahedral angle (

Shape Anisotropy and Optical Birefringence Measurements of Dry and Swollen Liquid Single Crystal Elastomers

109.47^circ

Selected Macroscopic Consequences of Tetrahedratic Order

) to each other. We find that this optically isotropic phase becomes uniaxial under the influence of an external electric field,

Electrooptical Effects of Swollen Polydomain Liquid Crystal Elastomers

{bf E}

Main Chain Liquid-Crystalline Elastomers: Swelling Dynamics and Electromechanical Effects

, resulting in a phase with C3v symmetry. For an applied simple shear flow, the system becomes biaxial and a time-dependent state with C1 symmetry arises. We discuss to what extent deformations induced by external forces and flows on this optically isotropic phase, which we call a deformable tetrahedratic phase, are consistent with observations at the isotropic-B7 transition found recently in compounds composed of banana-shaped molecules and suggest a number of experiments to test the conclusions of this model.