= Binary Flory-Huggins polymer blend
{title2=$f/(k_BT)=(\phi/N_A)\ln\phi+[(1-\phi)/N_B]\ln(1-\phi)+\chi\phi(1-\phi)$}
An incompressible, equal-site-volume <mean-field approximation> for two <polymers> of degrees of polymerization $N_A,N_B$. Here $f$ is mixing <free energy> per site and $\phi$ is the <volume fraction> of species $A$. For a composition-independent <Flory interaction parameter>, local stability requires $1/(N_A\phi)+1/[N_B(1-\phi)]-2\chi>0$. The <thermodynamic critical point> is $\phi_c=\sqrt{N_B}/(\sqrt{N_A}+\sqrt{N_B})$ and $\chi_c=(N_A^{-1/2}+N_B^{-1/2})^2/2$.
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