An AC Electrooptic Technique for Measuring the Flexoelectric Coefficient and Anchoring Energies of Nematics

(Received 30 April 1997, revised 29 July 1997, accepted 11 September 1997)

Abstract
We report an AC technique to measure both the sign and magnitude of (e₁ + e₃) using electrooptic measurements on a hybrid aligned nematic cell. A detailed numerical analysis of the optical signal allows us to obtain the temperature dependences of (e₁ + e₃), the rotational viscosity coefficient as well as the anchoring strengths at the two surfaces. Our measurements on CCH-7 show the following interesting results. a) The extrapolation length L₁ at the homeotropic surface shows a small decrease as the temperature is increased to 66 C beyond which it shows a sharp increase which we interpret as arising from a softening of the chains of the aligning ODSE layer. b) The slope of the temperature dependence of the extrapolation length . at the homogeneous surface changes sign around 72 C. c) (e₁ + e₃)/S is a constant upto 72 C and decreases rapidly beyond this temperature. The last two results are attributed to an increase in the conformational freedom between the rings of CCH-7 molecules beyond that temperature.