= Flory–Huggins solution theory
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{title2=$f/(k_BT)=(\phi/N)\ln\phi+(1-\phi)\ln(1-\phi)+\chi\phi(1-\phi)$}
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<Flory–Huggins solution theory> is an incompressible lattice description of mixing a <polymer> with a <solvent>. A chain occupies $N$ connected sites, while a solvent molecule occupies one; the displayed mixing <free energy> is per site. The first two terms are translational <entropy of mixing> contributions and the last is a mean-field excess contact interaction. A positive interaction parameter $\chi$ penalizes polymer–solvent contacts. If the site volume is $v_0$, the physical free-energy density is $f/v_0$ and the polymer <volume fraction> gives monomer <number density> $\phi/v_0$. Composition-dependent interaction parameters require extra derivative terms in stability calculations.
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