Structural Transition of Poly[(<i>R</i>)-3-hydroxybutyrate-<i>co</i>-(<i>R</i>)-3- hydroxyvalerate] Single Crystals on Heating As Revealed by Synchrotron Radiation SAXS and WAXD

Abstract

The annealing behavior of poly[( R )-3-hydroxybutyrate- co -( R )-3-hydroxyvalerate] (P(3HB- co -3HV)) crystal was studied by synchrotron radiation small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). Solution-grown single crystals of P(3HB- co -3HV) with various composition of 3HV (5.3−16.0 mol %) were used in order to examine the effect of 3HV unit on the structural changes on heating. The two-dimensional (2D) SAXS patterns demonstrated that the lamellar thickness of P(3HB- co -3HV) single crystal starts to increase discontinuously at an annealing temperature and that two populations with two different thicknesses of original and reorganized crystals coexist in a temperature range, independent of 3HV composition. This fact means that the 3HV unit with longer side chain does not hinder the jump of lamellar thickness. The 2D WAXD and differential scanning calorimetric experiments indicated that the discontinuous thickening is caused by partial melting and recrystallization. The depression of transition temperature from the original crystal to the thicker one and the incompletion of reorganization with increase in 3HV unit can be attributed to the lowering of the interaction between the copolymer chain stems and to the difficulty in incorporation of the chains to crystal lattice, respectively.