Brownian dynamics simulations are used to study highly entangled solutions of semiflexible polymers. Bending fluctuations of semiflexible rods are significantly affected by entanglement only above a concentration $c^{**}$, where $c^{**}\sim {10}^3{L}^{-3}$ for chains of length $L$ similar to their persistence length. For $c>c^{**}$, the tube radius $R_e$ approaches a dependence $R_e\propto c^{-3∕5}$ and the linear viscoelastic response develops an elastic plateau that is absent for $c<c^{**}$. Experiments on isotropic solutions of F-actin are shown to span concentrations near $c^{**}$ for which the predicted asymptotic scaling of the plateau modulus, $G\propto c^{7∕5}$, is not yet valid.

• Received 22 January 2007

DOI:https://doi.org/10.1103/PhysRevE.76.010501