STRUCTURE OF ISOTROPIC SOLUTIONS OF RIGID MACROMOLECULES VIA SMALL-ANGLE NEUTRON-SCATTERING - POLY(GAMMA-BENZYL L-GLUTAMATE) DEUTERATED DIMETHYLFORMAMIDE/

We have measured the structure factor for rigid-rod macromolecules in solution, poly(gamma-benzyl L-glutamate) (PBLG) in deuterated dimethyl formamide (d-DMF), in the isotropic phase. Small-angle neutron scatter ing measurements of a concentration series, from the dilute through th e concentrated isotropic phase, have been analyzed using the recent mo dification of the random-phase approximation by Doi, Shimada, and Okan o (DSO-RPA) for rodlike macromolecules, which includes nematic interac tions. 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 isotropi c phase exhibits an intensity increase that grows with increasing conc entration, a pretransitional effect consistent with increased fluctuat ions in orientational order. Additional orientational fluctuation corr elations are introduced into the theory through a phenomenological dep endence of the excluded-volume parameter on the scattering wave vector , and good agreement with the measured scattering is found. Attempts t o verify this behavior with lower molecular weight samples showed furt her anomalies.