Morphology of PEO/PDMS blends during shear: Coexistence of two droplet/matrix structures and additive effects

The morphologies of blends of polyethyleneoxide (PEO 37) and poly(dimethylsiloxane)s (PDSM), with viscosity ratios, λ, of approximately one (PDMS 230) or 2.8 (PDMS 314, being the component of higher viscosity) and interfacial tensions on the order of 10 mN/m, were investigated at 70 °C as a function of shear rate (up to 10 s−1) and of time. For the system PEO 37/PDMS 230 we have also studied the influence of the compatibilizer dimethyl-ethyleneoxide-copolymer (PDMS-co-PEO), which is only reasonably soluble in PEO. To investigate the morphologies we have used an optical shear cell in combination with a light microscope. The most important observation consists in the formation of two coexisting droplet/matrix structures for volume fractions of PDMS ranging from 0.4 to 0.6 for both λ values; the presence of the copolymer extends this region to 0.7. In the case of λ≈1 the average droplet radii are within experimental error independent of composition and morphology; for λ=2.8 they depend on the matrix phase in which they are contained and do again not vary with composition. The reduction in drop size caused by the copolymer is markedly larger if PEO forms the matrix. The present morphological observations suggest that the two coexisting droplet/matrix phases develop out of a single droplet/matrix structure via coalescence processes.