The blending ability of poly(vinylidene fluoride) (PVF2) depends on the head-to-head (H-H) defect structure present in the chain. With increasing H-H defect content, the lower critical solution temperatures (LCST) of PVP2/PMA blends decrease. This has been explained from the polymer-polymer interaction parameter (chi(12)) values measured from the equilibrium melting point (T degrees(m)) depression of the a phase of PVF2. A critical analysis on the method of measuring the equilibrium melting point from the Hoffman-Weeks plot has been done. For Hoffman-Weeks plots the T-m (melting point) should be determined for the same and low level of crystallinity at each T-c (crystallization temperature) to obtain correct results. If T-m is counted for the same time of crystallization at each T-c, erroneous results are obtained. Extrapolation of the T-m-T-a (T-a = annealing temperature) plot to the T-m = T-a line also yields the correct T degrees(m). chi(12) values are determined with all these T degrees(m)s, and it has been observed that the annealing method and the same crystallinity method yield almost same value of chi(12), which can explain the LCST phase diagram of PVF2 blends. Annealing results yield that PVF2 up to 24 mol % H-H defect concentration would be miscible with PMA. chi(12) measured from T degrees(m)s of the same time of crystallization and from the apparent melting points of PVF2 blends yield incorrect results.