Kinetics of phase transition between two crystalline modifications of p-n-hexyloxybenzylideneamino-p′-benzonitrile (HBAB) as studied by nuclear magnetic relaxation

Abstract

Pulsed proton n.m.r. was used to study the kinetics of the first-order phase transition of a nematogen compound, HBAB. The change in behaviour of magnetization recovery was measured as the transition proceeded from the undercooled crystal I to the stable crystal II. The transition kinetics is analysed by the nucleation/growth theory with a constant nucleation rate. A distinct deviation from the well-known master curve of Johnson and Mehl was observed when ca. 70 % transition was achieved. This slowing-down of the transition rate is interpreted by introducing an improved treatment of impingement among growing domains of the stable crystal. A possible mechanism, related to motional disorder in the parent crystal, is suggested for the transition kinetics.