In situ observation of heterogeneous melting of poly[( R)-3-hydroxybutyrate] single crystals by temperature-controlled atomic force microscopy

Abstract

Morphological changes of lath-shaped single crystals of poly[( R)-3-hydroxybutyrate] (P(3HB)) during annealing were followed in situ by a temperature-controlled atomic force microscopy (TC-AFM) in order to elucidate thermal stability or chain-mobility within the individual single crystal. Two kinds of the solution-grown single crystals prepared under different crystallization conditions were used to also investigate the influence of crystallization temperature on the annealing behavior. The significant morphological changes during annealing were observed at temperatures above the original crystallization temperature. The TC-AFM observations have revealed that ridges on the lamellar surface running across the crystal width (parallel to the b-axis), and cracks along the long axis of the crystal (parallel to the a-axis) are occasionally formed by thermal treatment. At a higher annealing temperature, numerous notches along the b-axis were frequently formed at the lateral sides of crystal. The notch formation may be due to preferential melting of the defective or less perfect regions within the lamellar crystal. The whole crystal became narrow while the notches progressed along b-axis, and some cavities developed at the central portion of the crystal as annealing temperature was elevated. The isothermal annealing experiment has demonstrated that the melting of P(3HB) crystals gradually progresses from the lateral outer faces. On the basis of the results, it has been concluded that the less perfect regions along the a-axis and b-axis exist within the P(3HB) single crystal, and that melting of the regions at the lateral sides of crystal is more easily initiated.