Effect of high pressure on relaxation dynamics and crystallization kinetics of chiral liquid crystal in its smectic...

The impact of high pressure on molecular dynamics and the crystallization process in the smectic phase with antiferroelectric properties of partially fluorinated liquid crystal (S)-4'-(1-methyloctyloxycarbonyl)biphenyl-4-yl-4-[7-(2,2,3,3,4,4,4-heptafluorobutoxy)heptyl-1-oxy]-benzoate (3F7HPhH7) was studied by broadband dielectric spectroscopy (BDS). By analyzing dielectric spectra measured under isobaric and isothermal conditions, the changes of the activation volume vs. temperature and the activation enthalpy vs. pressure have been determined to better understand the molecular system's behaviour in terms of its thermodynamic properties. The isothermal and isobar crystallization was studied by a BDS method along the trajectory of constant relaxation time τ on the (T, p) plane. The kinetics of this process was compared to that at ambient pressure, derived from the results of differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The melt crystallization depends primarily on the formation of nuclei with the activation energy of approx. 50 kJ mol^-1. This energy corresponds with the intramolecular movements of the carbonyl group in the rigid core. The behaviour of the apparent activation energies suggests that this process becomes easier with the progressive crystallized volume fractions. The obtained values of the Avrami exponent n_A suggest that the crystal growth is three-dimensional. Additionally, we successfully used the scaling of dielectric response for experimental data. The scaling of the dielectric relaxation processes indicates that the dynamics and the behaviour of dielectric permittivity have the same origin for all phases regardless of the change in temperature and/or pressure.