The divergence of the helical pitch near the cholesteric-smectic-A transition has been measured in cholesteryl nonanoate doped with controlled amounts of cholesteryl chloride. The apparent critical exponent v is found to increase sensitively but smoothly with chloride concentration. While the physical origin is unclear, the results indicate the value of v in pure cholesteryl nonanoate is nonclassical.
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Recently, we have shown \[1\]t hat the temperature dependence of the cholesteric pitch p of the liquid crystal cholesteryl nonanoate (CN) follows a power-law behavior near the smectic-A transition with a critical exponent v of 0.67, in keeping with current concepts of mesomorphic transitions \[2-4\]. It has also been reported that the divergence of the bend elastic constant near a nematic-smectic-A transition is strongly dependent on impurity content \[5\ ]. We report here a measurement of the effect of adding cholesteryl chloride (CC) to CN on the pitch divergence near the smectic phase. The monotropic cholesteric-smectic-A transition temperature of mixtures of CN and CC decreases with the weight concentration x of CC, from 74°C for x = 0 to 30°C for x ---0.17 \[6\].D ifferential scanning calorimetric measurements gave apparently broader transition peaks and smaller transition entropies with increasing x, but quantitative analysis is difficult because of the small latent heats involved. The selective reflection method \[1\]w as used to measure the optical pitch P = np, where n is an average refractive index. The pitch increases near the smectic phase in the mixtures as in pure CN, but over a wider temperature range as x increases. The widths of the reflection peaks of the mixtures are actually narrower than those of pure CN, indicating that they are probably limited by a slight temperature gradient of no more than 0.01°C. We have analysed the data with the power law * Work supported by the National Science Foundation under Grant GH-33633 and a DuPont Young Faculty Grant.