In order to address the origin of the lamellar peak scattering and the fact that fibril aggregates are composed of polymers, we now use the FA model which explicitly considers such hierarchical composition based on scattering theory of particle assemblies . A scheme is shown in Figure demonstrating the model. It is assumed that polymers exhibit lamellar stacking within the fibril aggregate which also gives rise the structure factor peak at high 𝑞. We note that the scheme is simplified and that the polymer aggregate may not possess such high degree of positional or orientation order as is drawn. along with the mean fibril radius 𝑅 𝑓 ̅̅̅ derived from the parameters showing similar results for the fibril and polymer radii as compared to the 2SFC model. These quantitative results also validate that the high 𝑞 peak originates from internal lamellar stacking within the fibril aggregates. Since the fibril radius 𝑅 𝑓 ̅̅̅ = (𝜀𝑅 𝑝 2 𝑁 𝑝 /𝜙 𝑝 ) 1/2 is dependent on the product of 𝜀 and 𝑁 𝑝 /𝜙 𝑝 , one can be specified to determine the other under an additional constraint. Fits with the FA model can be made by fixing 𝜀 to 0.13 (the ratio of π-π to lamellar stacking distance) forcing a polymer minor diameter of ~4 Å, corresponding to the π-π stacking distance. This gives an estimate for the number of polymers within the fibril cross-section, 𝑁 𝑝 ~ 𝑁 𝑝 /𝜙 𝑝 . FA model fits with 𝜀 fixed to 0.13 are shown in Figure with results in Table . A scheme is shown in Figure depicting the key dimensions of the PII-2T solution state structure which consists of both fibrils composed of polymer chains and a separate polymer population. We again emphasize that the persistence length extracted from SAXS matches with our theoretical calculation and that the extracted dimensions agree with our imaging results as well.