The SAXS data (Figure 5 set) were fitted using a model of interacting rigid cylinders with elliptical cross sections and uniform scattering length density. 
The fitting parameters for the form factor of the fibrils are the major and minor radii of the cross-section of the cylinders, Rmax and Rmin respectively, and the length of the fibrils, L. Polydispersity of the cross section was not modelled. The effect of interactions on the scattering was calculated using the PRISM model, with the parameter νRPA , proportional to the concentration of cylinders, describing the strength of the interactions between the cylinders (the local excluded volume radius around each site being neglected for weak interactions).