Nucleation behaviour of poly-3-hydroxy-butyrate

Abstract

Poly-3-hydroxy-butyrate (PHB) is a thermoplastic polyester produced by bacterial fermentation. Because of this bacterial origin PHB is a very pure polymer. This high purity in turn leads to very few (if any) heterogeneous nuclei, which gives a much wider scope for a systematic study of nucleation behaviour and the effect of nucleating agents than was possible before. It is shown that in pure PHB nucleation is sporadic. The nucleation rate may be measured over a temperature range of some 100° C. The nucleation rate data are recorded in the range where homogeneous nucleation is possible and are at least consistent with it. When foreign particles are added the nucleation rate is modified. Two distinct types of behaviour are observed. One may be interpreted quantitatively as the local raising of the crystal melting point due to the constraints of the actual presence of a surface; and the other as being due to epitaxial growth on the foreign surface. Detailed kinetic data are presented to support these conclusions.