Abstract
Abstract We present results of a small-angle neutron scattering (SANS) study of the structure and thermodynamic properties of symmetric blends of deuterated poly(dimethylsiloxane) (d-PDMS) and poly(ethylmethylsiloxane) (PEMS) as a function of temperature ( T ) (40≤ T ≤300 °C) and the molecular weight ( M w ) (4700≤ M w ≤23,200). The radius of gyration ( R g ) of d-PDMS was measured using the high-concentration labeling method and revealed unperturbed chain dimensions at all temperatures regardless of the polymer M w . The random phase approximation (RPA) fits the data for low M w blends, however it fails to describe the SANS data for M w >10,000 g/mol. This observation is explained by the fact that for high M w blends the correlation length of the concentration fluctuations ξ is always large ( ξ > R g ), implying that these blends remain microscopically inhomogeneous at all temperatures studied in this work. At the same time, the low M w blends are randomly mixed ( ξ R g ) at all T and can reach the ‘ideal mixing’ or Θ condition ( χ =0).
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