XRay Scattering Study of Nematic and Smectic  a Critical Behavior in Liquid Crystals.
Abstract
Xray measurements of susceptibility and correlation length exponents have been carried out on mixtures of hexylphenyl cyanobenzoyloxy benzoate (DB6) and terephtalbisbutyl aniline (TBBA). Pure DB6 has a nematic to smecticAd to smecticA2 phase sequence. Mixtures with > 12 m% TBBA concentration have a smecticA1 phase between nematic and smecticA2 phases. For the NAd transition in pure DB6, and along the NA1 line in mixtures, measured exponents are for smectic fluctuations in the nematic phase. Along the line of A1A2 transitions in mixtures, the exponents are for A2 fluctuations, in the A1 phase. The NAd transition in pure DB6 has exponents (gamma) = 1.29 (+OR) .05, (nu)(,(PARLL)) = .67 (+OR) .03, and (nu)(,(PERP)) = .52 (+OR) .03. Close to the multicritical point (12 m% TBBA) where a second order NA1 line meets the first order part of the A1A2 line, the NA1 exponents are (gamma) = 1.09 (+OR) .05, (nu)(,(PARLL)) = .57 (+OR) .03, and (nu)(,(PERP)) = .43 (+OR) .03. Anisotropy persists along the entire NA1 line, with exponents decreasing as the multicritical point is approached. The A1A2 line has both first and second order parts. Exponents are invariant along the second order part of the A1A2 line. Correlation length exponents are isotropic. The result is (gamma) = 1.46 (+OR) .05, (nu)(,(PARLL)) = (nu)(,(PERP)) = .74 (+OR) .03. These numbers also hold, close to the tricritical point where the transition turns first order. Measurements have been made of the integrated scattering intensity from Bragglike peaks in the smectic A phase, and pretransitional smectic fluctuations in the nematic phase. This has been done on oxtyloxycyanobiphenyl (80CB), butyloxybenzylidene heptylaniline (40.7), and 4 npentylphenylthiol4'nnonyloxybenzoate ((')9S5). For a given material, the intensity exponent, x, is the same for both sides of the transition. The results are: x = .67 (+OR) .05 for 80CB, .70 (+OR) .05 for 40.7, and .63 (+OR) .05 for (')9S5.
 Publication:

Ph.D. Thesis
 Pub Date:
 1984
 Bibcode:
 1984PhDT........98C
 Keywords:

 Physics: Condensed Matter