Spiralling optical morphologies in spherulites of poly(hydroxybutyrate)

Some rather unusual optical morphologies in thin films of poly(hydroxybutyr ate) in which a temperature gradient is imposed across the thickness of the film are reported. Spherulites in which the Maltese cross degenerates into a continuous spiral grow under these conditions, and the direction of the rotation of the spirals depends on the direction of heating. These morpholo gies are explained with the existing understanding of spherulite optics. Th e helicoidally twisting crystallites are modeled as twisting around an axis at a fixed angle to the radius of the spherulite (and the plane of the fil m). The possible implications for future, inclusive models of banding in sp herulites are discussed. Further observations on the temperature dependence of the optical banding pattern in poly(hydroxybutyrate) are also reported, and an unexpected minimum in band spacing and fine optically visible fibri llar texture is discussed. (C) 2000 John Wiley & Sons, Inc.