Crystallization of hydroxybutyrate oligomers. Part 3. Unfolding transitions followed in real time using SAXS and WAXS

High resolution wide angle X-ray scattering has been used to follow changes in crystallinity and lattice parameter which occur on heating hydroxybutyrate oligomers. These effects can be correlated with changes in crystal thickness detected by small angle X-ray scattering. Melt-grown crystals from oligomers with 24 and 32 repeat units that initially contain once folded chains transform during slow heating to produce extended chain crystals. This chain unfolding occurs via a process of partial melting and recrystallization and is accompanied by an expansion in the crystal lattice of approximately 0.2% in the (110) fold direction. The presence of a benzyl protecting group on one end of the chain does not affect the transformation temperature but reduces the (020) and (110) lattice spacings very slightly, as well as increasing the rate of transformation. In all cases the stable crystal forms display specific crystal thicknesses simply related to the extended chain length, but the intermediate stages of thickening vary between samples.