Density studies on various smectic liquid crystals

Up to now the nature of the phase transitions SA - SC, SC - SG and N - SC has been characterized mainly by calorimetric studies. Investigations of the associated density changes however, are very scarce (1-3). If the density measurement is carried out with high precision one can obtain very useful complementary information about the phase transitions mentioned above. We have measured the density as a funtion of temperature for vorious homologues of the 5-n-alkyl-2-(4-n-alkyloxyphenyl)-pyrimidines (PYP nOm) possessing nematic, SA and SC-phases. Furthermore 1-butyl-c-4-(4'-octylbiphenyl)-4-yl) -r-1-cyclohexan-carbonitrile (NCB84) was studied showing additionally a SG phase. Comparing the homoloques of PYP nOm we observed striking differences in the calculated isobaric expansion coefficients near the SA - SC transition. NCB 84 exhibits a second order SA - SC transition and a first order SC - SG transition. Furthermore we have analyzed a binary mixture of PYP 708 and PYP 706 comprising onl y a nematic and a SC phase. The associated N - SC phase transition is of the first order showing weak pretransitional behaviour. In order to unduce ferroelectric smectic C asteriks phases all SC-materials were doped by a chiral pyrimidine dopant. Remarkably, near the SA - SC asteriks transition the influence of the dopant on the expansion coefficients is very different for the various homologues of PYP nOm and will be discussed.