LIQUID-CRYSTALLINE SOLVENTS AS MECHANISTIC PROBES .50. KINETICS OF ISOTHERMAL CRYSTALLIZATION OF A SMECTIC PARTIALLY FLUORINATED ALCOHOL BYH-2 NMR

The rates of primary and secondary crystallization of F(CF2)7CD(OH)(CH 2)8H and F(CF2)7CH(OH)CD2(CH2)7H from their more stable monotropic sme ctic (Sm2) phase have been measured in situ at a variety of temperatur es using a deuterium saturation recovery quadrupole echo NMR technique . Application of the Avrami equation to primary crystallization and th e temperature dependence of the associated crystallization time consta nts provide insights into the mode of nucleation. The deuteron T1 spin -lattice relaxation time of F(CF2)7CH(OH)CD2(CH2)7H at 300 K is found to be 141 ms (Sm2) and 77 s (crystalline). A large difference between the T1 times for the initial and final phases and a relatively long ph ase-to-phase transformation time (greater-than-or-equal-to 0.5 h) are necessary to apply this NMR method to other materials. A much more lab orious and less precise differential scanning calorimetric method has been used to estimate the time constant (tau1/2 approximately 4 min at 300 K) for conversion of the less table monotropic smectic (Sm1) phas e to Sm2.