Critical fluctuations and molecular dynamics at liquid-crystalline phase transitions. II. Electron spin resonance experiments

Abstract

Electron spin resonance (ESR) relaxation studies at nematic–isotropic (N–I), and nematic–smectic‐A (N–S_A ) phase transitions in two liquid crystals, 4O,6 and 6OCB–8OCB, using the three spin probes, PD‐tempone, MOTA, and P are described. In general, one finds that (i) at the N–I transition, as T_NI is approached, the linewidths diverge with a critical exponent of 1/2; (ii) at the N–S_A transition, the linewidths diverge with a 1/3 power law as the transition is approached from the nematic side. The nature of the critical divergences in the relaxation parameters is interpreted and analyzed in terms of fluctuations in the nematic and smectic order parameters at the respective transitions and the coupling of the orientational dynamics of the probe to these modes. Good quantitative agreement with theory for the N–I transition required the inclusion of the effects of asymmetric probe ordering. The theory developed in detail in paper I is applied to interpret the results at the N–S_A transition. This theory is extended to include the effects of the measured anisotropies in (a) translational diffusion of the probe, (b) smectic correlation lengths, and (c) dynamic scaling exponents. In general, the magnitudes of the observed effects as well as their critical exponents are of the order expected, provided the averaging of the effects of density fluctuations within a smectic layer by probe diffusion is incomplete as a result of hindered diffusion.