The role of polydispersity in the lamellar mesophase of model diblock copolymers

High-resolution small angle X-ray scattering (SAXS) measurements were performed on two series of poly(ethylene-alt-propylene)-b-poly(D,L-lactide) (PEP-PLA) diblock copolymer materials exhibiting differences in the widths of the poly(D,L-lactide) block molecular mass distributions as measured by their polydispersity indices (PDI _PLA), At symmetric compositions of PEP-PLA (f _PLA ≈ 0.5), all SAXS data were successfully fit to an established model describing the small angle scattering from lamellar mesostructures. According to this model, the increase in the PDI _PLA negligibly affected the amount of lattice disorder. The apparent asymmetry of the poly(ethylene-alt-propylene)-block lamellae (φ), also determined by the fitting procedure, were more substantially affected; increasing the PDI _PLA resulted in a decrease in φ. At asymmetric compositions of PEP-PLA (f _PLA ≈; 0.67), only the data at the highest values of the PDI _PLA could be reasonably fit to this model.