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Incompatible polymers yield two-phase binary blends at almost all compositions. In equilibrium, the blends consist of two coexisting phases which have frequently very onesided compositions: Much of one component contains little of the other. The small content of the minor component, the so-called "partial miscibility", is important for some properties. But it is difficult to measure. Conventional techniques to determine miscibility gaps which rely on transparency and turbidity fail when the gaps are too wide. More reliably, the composition of the coexisting phases can be extracted from X-ray or neutron scattering data. A series of blends of poly(methy1 methacrylate) (PMMA) and random copolymers S,MMA,, of styrene and MMA, of which the degree of incompatibility was varied via the copolymer composition x, was studied by small angle neutron scattering. The interactions of the polymer components were measured in homogeneous and demixed blends. The demixed blends yield at high wave vectors a scattering equal to the scattering of the 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 gaps as wide as 99.7% could be determined.

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A new technique for measurements of extremely slow chain diffusion in polymer blends is discussed. It involves the analysis of "chain fragment diffusion by small-angle neutron scattering (CFD-SANS)". The principle is that polymer chains are split in the glassy state in a condition where the chain conformation does not immediately respond to the fragmentation. The subsequent separation of chain fragments can then be followed with SANS. In order to obtain contrast for SANS measurements, chain fragmentation of a deuterated polymer was made in a hydrogenous matrix. Experiments are described on a copolycarbonate containing a thermosensitive comonomer in a matrix of tetramethyl polycarbonate. Diffusion coefficients in the range 10"18-10"14 cm2/s were measured in a temperature range from 24 K below to 14 K above the glass transition. Evaluation of the data was based on a statistical analysis of the CFD process.

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