Raman Spectra Used to Understand the Origins of Banding in Spherulites

The formation of spherulites in polymers is a well-known phenomenon; when the polymer is crystallized by cooling from the melt, crystal lamellae grow out from a nucleation site in a spherical pattern. If the material is annealed on a planar surface, and viewed between crossed polarizers in a microscope, a Maltese cross with a banding pattern is observed. Where the crystals grow in a direction not parallel to the polarizers, the sample lights up. Often banding of the lit regions is observed, and is believed to be due to rotations of the crystal lamellae around the growth direction. Because it is well known that polarized Raman spectra respond to crystal orientation, we thought it would be interesting to try to document the relationship between the banding behavior and Raman polarization/orientation behavior. In this column I will show results of such an investigation of spherulites of poly(hydroxybutyrate-co-hydroxyhexanoate) (PHBHx) with varying composition.
Those of you who follow my column know that PHBHx has been my 'favorite' polymer over the last few years, as PET (polyethylene terephthalate) was for many years previously. PHBHx [...]