Poly(hydroxybutyrate) (PHB) is a biodegradable thermoplastic. Melt-cast PHB sheets normally become brittle on storage. The embrittlement has been associated with cracks formed during cooling due to differences in radial and circumferential thermal expansion coefficients. We show that the cracks are in fact due to differences in thermal expansion between the PHB him and the constraining glass slides. The circumferential features, previously identified as cracks, are shown to be surface features formed due to volume reduction during crystallization constrained by the substrate.