The structural details of thick polymer films made from poly(Z-L-lysine)-polystyrene rod-coil block copolymers, the polydispersity index being in the range similar to1.01-1.64, were investigated by small-angle X-ray scattering techniques. All films were found to have a hexagonal-in-zigzag lamellar morphology. The zigzag superstructure, a plane lamellar structure disrupted by kinks, results from the hexagonal packing of the polydisperse set of polypeptide a-helices, fractionated according to the length of helices. The data reveal a correlation between the molecular weight distribution of the polypeptide and the interface-curvature properties of the morphology.