THE WIDTH OF MISCIBILITY GAPS, MEASURED BY SCATTERING

Incompatible polymers yield two-phase binary blends at almost all comp ositions. In equilibrium, the blends consist of two coexisting phases which have frequently very one-sided compositions: Much of one compone nt contains little of the other. The small content of the minor compon ent, the so-called ''partial miscibility'', is important for some prop erties. But it is difficult to measure. Conventional techniques to det ermine miscibility gaps which rely on transparency and turbidity fail when the gaps are too wide. More reliably, the composition of the coex isting phases can be extracted from X-ray or neutron scattering data. A series of blends of poly(methyl methacrylate) (PMMA) and random copo lymers S(x)MMA(1-x) of styrene and MMA, of which the degree of incompa tibility was varied via the copolymer composition x, was studied by sm all angle neutron scattering. The interactions of the polymer componen ts were measured in homogeneous and demixed blends. The demixed blends yield at high wave vectors a scattering equal to the scattering of th e two coexisting phases, superposed. The composition of the coexisting phases was extracted from the slope of the Zimm curve which responds very sensitively even when the compositions are extreme. Miscibility g aps as wide as 99.7% could be determined.