Investigation of crystallization kinetics and its relationship with molecular dynamics for chiral fluorinated glassforming smectogen 3F5HPhH6

Abstract

The phase transitions, crystallization kinetics and molecular dynamics of (<i>S</i>)-4'-(1-methylheptylcarbonyl)biphenyl-4-yl 4-[5-(2,2,3,3,4,4,4-heptafluorobutoxy)pent-1-oxy] benzoate (3F5HPhH6) are investigated by differential scanning calorimetry, polarizing optical microscopy and broadband dielectric spectroscopy. The vitrification of the antiferroelectric hexatic phase is observed for cooling rates ≥5 K min^-1 and the fragility index determined from dielectric data is <i>m</i>_f ≈ 90. Two regimes of non-isothermal cold crystallization are distinguished using the Kissinger and Augis-Bennett methods in the heating rate ranges of 1-5 K min^-1 (larger activation energy) and 8-20 K min^-1 (lower activation energy). The correlation between the time of non-isothermal cold crystallization (using isothermal approximation) and relaxation time of the α-process is determined. The obtained coupling coefficient <i>ξ</i> ≈ 0.7 and temperature dependence of the crystallization rate <i>Z</i> from the Ozawa model imply a mainly diffusion-controlled cold crystallization below 275 K. The Avrami exponents <i>n</i> and Ozawa exponents <i>n</i>_O determined for isothermal melt crystallization and non-isothermal cold crystallization, respectively, weigh in favour of two- rather than three-dimensional crystal growth. The transition between crystal phases is observed on heating, with a lower activation energy for 1-3 K min^-1 than for 5-20 K min^-1 rates.