= Solution
For two species of chains with the same polymerization degree $N$, the number of independently translating objects per occupied site is reduced by $N$. The <symmetric Flory-Huggins polymer blend> has
$$
\frac f{k_BT}=\frac{\phi\ln\phi+(1-\phi)\ln(1-\phi)}N+\chi\phi(1-\phi),\qquad \frac{f''}{k_BT}=\frac1{N\phi(1-\phi)}-2\chi.
$$
The symmetry still places the <thermodynamic critical point> at $\phi_c=1/2$, but now
$$
\boxed{\chi_c=\frac2N.}
$$
The <entropy of mixing> per monomer is much smaller than for simple liquids, so even a weak unfavorable interaction can cause <phase separation> for long chains. If the energetic interaction parameter is $\chi=A/T$ with $A>0$, this model gives $T_c=AN/2$; the conclusion relies on that specified temperature law.
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