Properties of many crystalline and semicrystalline materials strongly depend on crystal morphology. Crystal nucleation, as the primary step, is of particular importance. 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 crystal nuclei growth at the transfer from 60 °C to 125 °C and 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. In a second experiment, from the remaining fraction of crystal nuclei after a short (1 ms) exposure to a temperature above the development temperature, the cluster size distribution after isothermal nucleation of PLLA at 60 °C for 1000 s was estimated. In the temperature interval from 125 °C to 145 °C, the density of critical nuclei decreases from the initial value ~4*10²⁴ m^-3 to zero, as nuclei with R > 2 nm, the critical radius for 145 °C, were not formed at the nucleation temperature.

Normalized enthalpy of crystallization of the crystals developed at <i>T</i><sub>d</sub> = 125 °C and <i>t</i><sub>d</sub> = 70 s as a function of the transfer-heating rate ß<sub>t</sub>.

Normalized enthalpy of crystallization of the crystals developed at T_d = 125 °C and t_d = 70 s as a function of the transfer-heating rate ß_t.

Rene Androsch

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