A NOTE ON TWIST-BANDING IN SPHERULITES OF POLY(3-HYDROXYBUTYRATE)

A mechanism is suggested for twist-banding in spherulites of poly(3-hy droxybutyrate) (PHB), which involves elastic bending of lamellar ribbo n crystals caused by chainfolding in different directions on opposite faces of lamellae. This folding behaviour is considered to be a conseq uence of the stereoregularity of bacterially produced PHB, whose molec ules adopt a helical chain form. The model is used to predict values f or the band spacing P in terms of crystal elastic moduli, lamellar sur face energy density and lamellar thickness. Measured values of P are s omewhat larger than predicted, and too small a temperature dependence is predicted at low supercoolings. Reasons for the discrepancies are s uggested and discussed. This proposed mechanism for helicoidal twistin g could be appropriate for many polymers, in particular for chiral pol ymers, and in cases where fold staggering does not play a role. (C) 19 97 Elsevier Science Ltd.