Structure of Isotropic Solutions of Rigid Macromolecules via Small-Angle Neutron Scattering: Poly(y-benzyl L-glutamate)/ Deuterated Dimethylformamide

We have measured the structure factor for rigid-rod macromolecules in solution, poly(γ-benzyl L-glutamate) (PBLG) in deuterated dimethylformamide (d-DMF), in the isotropic phase. Small-angle neutron scattering measurements of a concentration series, from the dilute through the concentrated isotropic phase, have been analyzed using the recent modification of the random-phase approximation by Doi, Shimada, and Okano (DSO-RPA) for rodlike macromolecules, which includes nematic interactions. The results and comparison to theory are in good agreement with reported values from previous light scattering measurements performed by De Long and Russo at lower wave vectors. The concentrated isotropic phase exhibits an intensity increase that grows with increasing concentration, a pretransitional effect consistent with increased fluctuations in orientational order. Additional orientational fluctuation correlations are introduced into the theory through a phenomenological dependence of the excluded-volume parameter on the scattering wave vector, and good agreement with the measured scattering is found. Attempts to verify this behavior with lower molecular weight samples showed further anomalies.