For some cases this is sufficient, however there are two shortcomings with this model. The first is that it only considers the external shape of the fibrils and ignores that they are composed of polymer chains and the second is that it does not explain the origin of the lamellar structure factor peak that has been seen in D-A conjugated polymer scattering . To address these issues we developed the FA model based on small angle scattering theory for particle assemblies .