| We use Brownian dynamic simulations to study entangled solutions of semiflexible rods; with contour length L comparable to their persistence length Lp. We explore concentrations comparable to those studied in experiments on solutions of Fd-virus, F-actin and poly (benzyl-L-glutamate) (PBLG). We find a clear crossover with increasing number concentration c from a regime of loosely entangled rods, in which rotational diffusion is hindered by topological constraints but transverse bending fluctuations are not, to a tightly entangled regime in which bending fluctuations are also restricted, and can relax only by reptation. This crossover occurs at a dimensionless concentration cL3 ∼ 1000 for chains with Lp = L. In the tightly entangled regime, the tube radius Re is found to depend on c and Lp with the predicted scaling relation Re ∝ c-3/5Lp-1/5 for c > c**. Although rotational diffusivity Dr for semiflexible rods decreases with increasing concentration, it is found to be higher than that for rigid rods at all c. At high concentrations, Dr for semiflexible rods approaches a concentration independent value. Simulations of excluded volume chains indicate that excluded volume does not drastically affect dynamics in entangled solutions. The dynamic modulus G(t) has been obtained by studying stress relaxation after subjecting the polymer solution to a small amplitude step deformation. For c ≥ c**, G(t) exhibits a plateau which is attributed to deviations of chain curvature from equilibrium. The curvature plateau increases steeply with concentration, and at higher concentrations approaches the c1.4 scaling predicted by theory. Comparisons of simulation results for dynamics and viscoelasticity with experimental measurements on entangled solutions of rodlike polymers such as poly (benzyl-glutamate), Fd-virus and F-actin has met with reasonable success. |
