Morphology influences most technological properties of polymer materials. The study of morphological aspects of macromolecular materials is therefore relevant to material applications. Polymers tend to crystallise from the melt with spherulitic morphology. Spherulites are polycrystalline aggregates with spherical symmetry constituted of ribbon-like crystals (lamellae) that grow outwards from a central nucleus. Some polymer spherulites, when observed between crossed polarisers in an optical microscope, show concentrical extinction bands. Investigation of structural details in banded spherulites at the micron level by means of X-ray diffraction has been severely limited in the past by the unavailability of small enough X-ray beams.

Poly(3-hydroxybutyrate), PHB, is a natural polyester that easily develops a high degree of crystallinity and forms large banded spherulites upon isothermal crystallisation from the melt at high temperature. An insight into structural variations within banded PHB spherulites was recently obtained at the ESRF by investigating a 300 µm x 300 µm area of a practically two-dimensional spherulite portion, using microfocus X-ray diffraction [1]. The results showed a close correlation between the observed structural changes and the morphological features responsible for banding.

Fig. 49: Polarised optical micrograph of the investigated PHB spherulite (left). To the right, the 100 patterns registered along the yellow line. The coloured spots indicate the positions where the evidenced patterns were taken.

In the present experiment, PHB was isothermally crystallised under the constraint of two parallel surfaces at 140°C, obtaining a spherulite section with very wide extinction bands (band spacing 120 µm). The microfocus ID13 beamline was used to analyse a 300 µm segment along the radius of the spherulite. The sample was tracked across the incident beam in 3 µm steps in order to collect a large number of diffraction patterns inside each band. Figure 49 shows the polarised optical micrograph of the PHB spherulite investigated by microfocus X-ray diffraction. The radial segment analysed is highlighted in the micrograph. The 100 X-ray diffraction patterns collected during the radial scan are shown in the background of Figure 49. It is quite clear that the same type of pattern appears two/three times at different positions along the scan, indicating a radial periodicity in the spherulite microstructure. The frames highlighted in pink and green (see magnification) represent the two limit patterns. The reflections in each of the 100 frames collected were indexed and the unit cell orientation at each point was determined. Figure 50 shows the change of intensity of reflections (020) and (002) while moving from frame to frame during the radial X-ray scan. The intensity of both reflections shows a periodicity that perfectly reproduces the band spacing derived from optical microscope observations (120 µm). The great number of diffraction patterns collected inside each band of the PHB spherulite by the microfocus X-ray technique yields conclusive evidence that during lamellar growth the unit cell smoothly rotates around the radially-oriented a-axis [2].

Fig. 50: Normalised intensities of reflections (0 2 0) and (0 0 2) as a function of the distance from the origin of the scanned segment (see Figure 49).

[1] M. Gazzano, M.L. Focarete, C. Riekel and M. Scandola, Biomacromolecules, 1, 604-608 (2000).
[2] M. Gazzano, M.L. Focarete, C. Riekel, A. Ripamonti and M. Scandola, Macromol. Chem. Phys., 202, 1405-1409 (2001).

M. Gazzano (a), M.L. Focarete (b), C. Riekel (c), A. Ripamonti (b) and M. Scandola (a,b).
(a) CSFM, CNR, Bologna (Italy)
(b) Universita' di Bologna (Italy)
(c) ESRF