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.