Growth and dissolution of crystal nuclei in poly( L-lactic acid) (PLLA) in Tammann’s development method*

Tammann’s nuclei development method allows for a detailed study of crystal nucleation by letting the nuclei formed at some nucleation temperature grow to measurable sizes at a higher development temperature. In this method, however, the problem occurs to what extent the crystal nuclei formed in nucleation retain their stability in the course of transfer to the development temperature. By fast scanning calorimetry (FSC), the influence of the transfer of nuclei from the nucleation stage at low temperature to the growth stage at higher temperature was systematically studied. Poly(L-lactic acid) (PLLA), a slowly crystallizing polymer, was chosen for the experiments performed in a wide transfer-heating-rate range. At heating rates between 1 K s^-1 and 20,000 K s^-1, there occurs stabilization/growth of crystal nuclei at low heating rates and their dissolution/melting at high heating rates. Heating rates above 1000 K s^-1 are sufficient to prevent growth of crystal nuclei at the transfer from 60 °C to 125 °C and to higher temperatures. The critical heating rate for preventing nuclei growth is about 1000 times higher than the critical heating rate to prevent crystal growth in a nucleated sample on heating.