Study by adiabatic scanning calorimetry of phase transitions in liquid crystals and liquid crystalline mixtures with nonmesogenic solutes

The experimental work presented in this thesis can be divided into two main parts: one part concerning phase transitions in pure liquid crysalline systems and an other part regarding liquid crystalline mixtures with nonmesogenic sollutes. All experimental results were obtained by Adiabatic Scanning Calorimetry. First two ferroelectric liquid crystals were studied: S-(-)-4-(2’-methylbutyloxy)phenyl 4-n-octyloxybenzoate and S-(-)-2-methylbutyl 4-n-nonanoyloxybiphenyl-4’-carboxylate. The experiments showed that no latent heat was present for the smectic A to smectic C* (AC*) transition to within the experimental resolution. Therefore the AC* transition in both samples could be classified as a continuous one. By fitting the specific heat data we showed that this phase transition could be described by an extended mean field model augmented with Gaussian fluctuation terms in the specific heat capacity expression. The second pure system investigated in this thesis was the liquid crystal 4-butyloxyphenyl-4’-decyloxybenzoate with the emphasis on the nematic to smectic A (NA) and the nematic to isotropic (NI) phase transitions. Because of the high resolution ASC technique used in this investigation we were able to accurately determine the latent heat of the first order NI transition. The NA transition was found to be a continuous one within the experimental resolution and the value of the effective specific heat capacity critical exponent, a, was found to be 0.23 ± 0.01, thus between the 3D XY and tricritical value. In a last part the effect of nonmesogenic impurities on the order of the nematic to smectic A phase transition was studied in a liquid crystal. Three different nonmesogenic impurities being biphenyl, cyclohexane and water were added to a liquid crystal, octylcyanobiphenyl (8CB), resulting in three different conclusions. The NA transition in pure 8CB is continuous and for all concentrations of biphenyl studied this transition remained continuous. The effective specific heat critical exponent decreased from 0.31±0.03 for pure 8CB down to 0.08±0.03 for the highest mole fraction of biphenyl studied, x=0.1415. For mixtures of 8CB and cyclohexane, however, crossover from continuous to first order is observed at a tricritical point of the mole fraction, x, of cyclohexane around 0.0460. After investigating mixtures of 8CB and water, the effective critical exponent increased up to 0.36±0.03 for mixtures with a mole fraction of water x=0.1105 and it remained constant for all higher concentrations studied (up to x=0.3509). By means of visual inspection of the samples we were able to conclude that for mixtures with a mole fraction of water higher than x=0.10, phase separation occured. The difference between these three systems and the crossover from continuous to first order for the system 8CB and cyclohexane was explained in terms of a mean field free energy density expression including coupling terms of the mole fraction, x, with the nematic and smectic A order parameters.status: publishe