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 γ1 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 Lu. 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.
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Warrier et al. (1997) studied this question.