Structural Investigation of Poly(3-hydroxybutyrate) Spherulites by Microfocus X-Ray Diffraction

A banded spherulite of bacterial poly(3-hydroxybutyrate) (band spacing 120 μm) was linearly scanned along the radial direction in 3 μm steps by means of microfocus X-ray diffraction using a synchrotron source with a beam diameter of 3 μm. A large number of X-ray patterns was collected inside each band. Those taken at the center of the bands represented the two limiting diffraction patterns from which the exact orientation of the unit cell was inferred. The intensity of the reflections (020) and (002), taken as indicators of parallelism to the X-ray beam of the unit cell's c- or b-axis, respectively, changed periodically along the spherulite radius, alternating maxima with zero-intensity zones. When the reflection (020) showed a maximum, the reflection (002) was practically nil and vice versa. This behavior clearly shows that the unit cell's mean position smoothly rotates around the a-axis with increasing distance from the spherulite center. The identity of the cell rotation period with the band spacing derived from optical microscopy observations provides structural evidence that the extinction bands originate from a regular twisting of the lamellar crystals during their growth.