= Cross-link density inferred from rubber elasticity
{title2=$\nu_s=G/(k_BT)$}
The ideal <rubber elasticity> relation infers active strand <number density> from the <shear modulus>. If a quoted modulus is instead <Young's modulus>, nearly incompressible behavior gives $\nu_s\simeq E/(3k_BT)$. This is not automatically the junction density: for junction functionality $f$ and no dangling ends or loops, counting strand ends gives $\nu_s=f\nu_j/2$. Network fluctuations and inactive strands modify the modulus, so converting it into a chemical <cross-link> count requires a network model.
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