Rheology of Miscible Blends:  SAN and PMMA

The linear viscoelasticity of miscible blends of a random copolymer of 80% styrene and 20% acrylonitrile and poly(methyl methacrylate) has been investigated using oscillatory shear. The Flory−Huggins interaction parameter of this blend is weakly negative. The glass transitions of the pure components are very close (ΔT<INF>g</INF> = 20 K). The blends are thermorheologically simple, in that the oscillatory shear response at different temperatures can be superimposed with the empirical time−temperature superposition principle with a precision similar to that for the pure component polymers. These results are anticipated by a theory of concentration-fluctuation-induced dynamic heterogeneities in miscible polymer blends. While sizable concentration fluctuations are present in this blend system, they do not complicate the dynamics, because all compositions have similar local dynamics. We suggest a simple phase diagram based on this model, that should be useful for deciding whether time−temperature superposition will be valid for a given blend with weak energetic interactions. Regions of thermorheological complexity are separated from regions of thermorheological simplicity on a plot of the range of blend free volume studied against the glass transition contrast of the components (ΔT<INF>g</INF>).