Small-Angle Neutron Scattering from Peptide Nematic Fluids and Hydrogels under Shear

Abstract

Small-angle neutron scattering has been used to study the mesoscopic structure responsible for the nonlinear rheology of aqueous dispersions of self-assembling peptide fibrils (nematic-fluid states) and fibrillar networks (nematic hydrogels). The orientation of the nematic director has been studied as a function of the shear rate (0−500 s-1) in both gel and fluid phases. The powder-averaged scattering data from the fluid nematic phase has been modeled using a Kratky−Porod wormlike chain and the cross-sectional area (a stack of 8−10 tapes) of the self-assembled fibrils measured under flow in the direction of the velocity gradient. A separate observation of the aggregate structure in both the fluid and the gel states was possible in the tangential-velocity direction. An apparent increase in the diameter of the fibrils upon gelation is, in view of the electron micrographs, attributed to fiber formation by the entwining of pairs of fibrils: it is estimated that approximately 10% of the total contour length of the fibrils participates in these fiberlike junctions. The stress-controlled nonlinear rheology has also been studied, and the nematic fluids were found to display Type I shear thinning behavior, typical of liquid-crystalline polymers. Stress relaxation in the form of a loss of orientation only occurred at high concentrations (6 mM) in nematic gels. Furthermore, the gels were observed to display Lozenge shaped scattering at intermediate shear rates, indicative of mobile oriented fibrils in an unoriented network.