  We derive the linear force-extension relation for a wormlike chain of
arbitrary stiffness including entropy elasticity, bending and thermodynamic
buckling. From this we infer the plateau modulus $G^0$ of an isotropic
entangled solution of wormlike chains. The entanglement length $L_e$ is
expressed in terms of the characteristic network parameters for three different
scaling regimes in the entangled phase. The entanglement transition and the
concentration dependence of $G^0$ are analyzed. Finally we compare our findings
with experimental data.
