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 using a differential scanning calorimeter at 120 - 450 K. Glass transition and fusing temperatures were found to be and , respectively, at the heating rate of . Bulk crystallization, observed in the range 390 - 400 K, was used to check whether the crystal nucleation did or did not proceed at low temperatures. The effective nucleus which had a radius larger than the critical value at 290 K was generated through annealing the sample for 1 h in the range 120 - 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 . This is interpreted as indicating that the nucleation rate is controlled by the characteristic time of the -molecular rearrangement process which could be much shorter than the annealing period. The crystal nucleus 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 qualitatively understood from a classical theory.