Homogeneous Crystal Nucleation of Poly(3-hydroxybutyrate): Kinetics, Stability, and Cluster-Size Distribution

The kinetics of homogeneous crystal nucleation in poly-(3-hydroxybutyrate) (P3HB) near its glass transition temperature (<i>T</i> _g) was investigated using fast scanning calorimetry (FSC) combined with Tammann's two-stage crystal nuclei development method. Crystallization on cooling is fully suppressed at rates of a few K/s, while the formation of crystal nuclei is inhibited only at higher rates, above around 100 K/s. The formation of homogeneous nuclei is fastest at around 40 °C (approximately 30 K above <i>T</i> _g), with an onset time of their formation of about 1 s, which is two orders of magnitude shorter than the crystallization onset time. To evaluate their thermal stability, a modified Tammann nucleation experiment involving a temperature spike before the growth stage was employed. The number of homogeneous nuclei of supercritical size at the growth temperature formed under the given conditions remained nearly constant upon heating up to about 100 K above their formation temperature. Beyond this limit, the number of surviving nuclei drastically decreased, and complete dissolution was observed at about 120 K above the temperature of their formation. Besides the finding of an exceptionally high stability of homogeneous crystal nuclei of P3HB, the sharp disappearance of the number of nuclei within a narrow temperature range of 10-15 K suggests a sharp truncation of the nuclei-size distribution for larger nuclei.