Wormlike micellar solutions form two different network topologies, entangled and branched, which are often difficult to distinguish. Neutron spin echo (NSE) can be used to probe the dynamics of wormlike micellar systems[1, 2]. The semi-flexible chain model by Zilman and Granek[2] has been successfully used to describe the high q scaling of the dynamic structure factor of both nonionic and cationic wormlike micelles[1, 3]. However, this scaling is only valid when the inverse of the mesh size (ξm) is much less than the scattering vector, q. When ξm-1~q, a cross-over to a Zimm scaling is expected. Also, when ξm becomes comparable to the persistence length (lp) of the micelles, the chains can no longer be described as Gaussian, and a cross-over to the Zimm scaling is expected. Branched and entangled networks have different stress relaxation mechanisms. Entangled networks relax by reptation and micelle breakage, while branched networks have the additional relaxation mechanism of branch points sliding along the contour of the micelle. Because branch point sliding is nearly frictionless, it is faster than reptation[4] and therefore should lead to changes in the scaling cross-over condition compared to purely entangled networks. We have studied the dynamics of the cationic wormlike micellar system of cetyltrimethyl ammonium bromide (CTAB) and sodium salicylate (NaSal) in D2O using NSE. Using rheological data, samples expected to be in both the entangled and the branched network regimes were investigated using NSE. A comparison of the dynamic behavior of these two network topologies will be presented.

1.Seto, H., et al., Collective motions of a network of wormlike micelles. Journal of Physics and Chemistry of Solids, 1999. 60(8-9): p. 1371-1373.

2.Zilman, A.G. and R. Granek, Undulations and dynamic structure factor of membranes. Physical Review Letters, 1996. 77(23): p. 4788-4791.

3.Nettesheim, F. and N.J. Wagner, Fast dynamics of wormlike micellar solutions. Langmuir, 2007. 23(10): p. 5267-5269.

4.Constantin, D., et al., Structural Transition in the Isotropic Phase of the C12EO6H2O Lyotropic Mixture: A Rheological Investigation. 2003. p. 2554-2559.