Small-angle neutron scattering from symmetric blends of poly(dimethylsiloxane) and poly(ethylmethylsiloxane)

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 (Mw) (4700≤Mw≤23,200). The radius of gyration (Rg) of d-PDMS was measured using the high-concentration labeling method and revealed unperturbed chain dimensions at all temperatures regardless of the polymer Mw. The random phase approximation (RPA) fits the data for low Mw blends, however it fails to describe the SANS data for Mw>10,000 g/mol. This observation is explained by the fact that for high Mw blends the correlation length of the concentration fluctuations ξ is always large (ξ>Rg), implying that these blends remain microscopically inhomogeneous at all temperatures studied in this work. At the same time, the low Mw blends are randomly mixed (ξ<Rg) at all T and can reach the ‘ideal mixing’ or Θ condition (χ=0).