For blends of a poly(methyl methacrylate) (PMMA) and a poly(vinylidene fluoride-co-hexafluoro acetone) [P(VDF-HFA)], we examined phase behavior and crystalline melting (T-m) and glass transition (T-g) temperatures. In the range 130-160 degrees C, which is a miscible one-phase region between their lower critical solution temperature (LCST: T-c = 220 degrees C; phi(c) congruent to 0.5) and T-m (congruent to 120 degrees C) of P(VDF-HFA), simultaneous measurements of transient tensile stress sigma(t) and birefringence Delta n(t) were conducted via elongational flow opto-rheometry (EFOR) on the blends under uniaxial elongation at constant Hencky strain rates. The stress optical coefficient C(t)(drop Delta n(t)/sigma(t)) increased monotonically with increasing volume fraction phi(P(VDF-HFA)) of P(VDF-HFA) in the blend. Molten PMMA/P(VDPHFA) blends in the one-phase region appear to follow the stress optical mle with C(I) obeying the simple additivity: C(t) = C(P(VDF-HFA)phi P(VDF-HFA)) + C(PMMA)phi(PMMA) with the suffices being relevant to each component. The value of C(t) extrapolated to phi(P(VDF-HFA)) = 1 yielded CP(VDF-HFA) = 6.5 x 10(-9) Pa-1. The C(t) vs phi(P(VDF-NFA)) behavior suggested that C(t) can be zero for the (97/3) blend or the addition of only 3% P(VDF-HFA) to PMMA makes the blend non-birefringent. Thus, P(VDF-HFA) can be an optimal modifier when PMMA is used as a high-technology optical material, e.g., optical discs and lenses.