Density studies on various smectic liquid crystals

Density measurements as a function of temperature for four homologues of the 5-n-alkyl-2-(4-n-alkyloxy-phenyl)-pyrimidines (PYP nOm) which exhibit nematic, smectic A and smectic C phases are reported. Furthermore 1-butyl-c-4- (4'-octyl-biphenyl-4-yl)-r-1-cyclo-hexan-carbonitrile (NCB 84) is studied; this has additionally a smectic G phase. From these data the thermal expansion coefficients are calculated. Comparing PYP 907 and PYP 709, differing in their exchanged alkyl chains, we observe in the smectic A and the smectic C phase a distinctly lower density for PYP 709 whereas their densities nearly agree in the isotopic phase. The pyrimidines PYP 709 and PYP 808 exhibit a continuous volume change on crossing the smectic A-smectic C transition which differs dramatically from PYP 909 which shows a small volume jump. Futhermore a binary mixture of PYP 708 and PYP 706 is analysed which shows only a nematic and a smectic C phase. The associated phase transition is probably first order reveal ing nearly no pretransitional behaviour. The smectic A-smectic C transition of NCB 84 seems to be second order exhibiting a continuous change of volume across the transition whereas the smectic C-smectic G transition shows a volume discontinuity and is first order. In order to induce ferroelectric smectic C asterisc phases all smectic C materials were doped with a chiral pyrimidine dopant. Astonishingly the thermal expansion coefficient across the smectic A-smectiv C asterisc transition is influenced by the dopant in a very different way.