Small quartz silica spheres induced disorder in octylcyanobiphenyl (8CB) liquid crystals: A thermal study - art. no. 061713

A photopyroelectric technique has been applied to the study of specific hea t and thermal conductivity of homeotropically aligned mixtures of small qua rtz spheres (aerosil) and octylcyanobiphenyl (8CB) with concentration 0 les s than or equal to rho (s) less than or equal to 0.04 g/cm(3). Thermal cond uctivity data show that, even at these very low concentrations, an annealin g of the disorder introduced by the aerosil takes place on cooling at the s mectic-A-nematic (Sm-A-N) phase transition and not only at the nematic-isot ropic (N-I) one. This means that there is some elastic strain in the nemati c phase of the sample which is not quenched. Accordingly the suppression of the N-I transition temperature as a function of rho (s) does not fit a ran dom field with a random dilution model that accounts for random quenched di sorder only. High resolution specific heat measurements at the A-N and N-I transition show the effect of the aerosil is not the same. While in the fir st case its peak is suppressed with increasing concentration, in the second case there are some indications that outside the two-phase coexistence reg ion it is enhanced. The effect of surface-induced alignment is also discuss ed to explain some discrepancies between our data and the ones reported in literature. It is concluded that the amount of disorder in the sample does not depend on rho (s) only, but also on other variables such as external fi elds. Finally, a relaxation phenomenon in the aerosil network that partiall y compensate the disordering effect of the particles is suggested to explai n the concentration dependence of the transition temperatures.