GENERATION AND EXTINCTION OF A CRYSTAL NUCLEUS BELOW THE GLASS-TRANSITION TEMPERATURE

The generation/extinction phenomenon of a crystal nucleus was studied for 4-dibenzylamino-2-methylbenzaldehyde-1, 1-diphenylhydrazone by usi ng a differential scanning calorimeter at 120-450 K. Class transition and fusing temperatures were found to be T-g = 306.6 K and T-fus = 422 .2 K, respectively, at the heating rate of 2 K min(-1). Bulk crystalli zation, observed in the range 390-400 K, was used to check whether the crystal nucleation did or did not proceed at low temperatures. The ef fective nucleus which had a radius larger than the critical value at 2 90 K was generated through annealing the sample for 1 h in the range 1 20-280 K or for 5 min at 170-270 K. The crystal nucleation process was thus found to proceed even at 120 K, lower by 186 K than the T-g. Thi s is interpreted as indicating that the nucleation rate is controlled by the characteristic time of the beta-molecular rearrangement process which could be much shorter than the annealing period. The crystal nu cleus generated during the annealing at 220 K for 1 h was found to be extinguished by rapid heating to temperatures above 300 K. This is qua litatively understood from a classical theory.