Small-Angle X-ray Scattering Study of Gelation and Aging of Eu3+-Doped Sol−Gel-Derived Siloxane−Poly(oxyethylene) Nanocomposites

The aggregation, gelation, and aging of urea-cross-linked siloxane−poly(oxyethylene) nanohybrids [(U600) n ] containing two different amounts of europium triflate initially dissolved in an ethanol−water mixture were investigated by in situ small-angle X-ray scattering (SAXS). For both low ( n = [O]/[Eu] = 80) and high ( n = 25) europium contents, the SAXS intensity was attributed to the formation of siloxane clusters of about 8−11 Å in size. Siloxane cluster formation and growth is a rapid process in hybrids with low Eu contents and slow in Eu-rich hybrids. An additional contribution to the scattering intensity at very low angles was attributed to the formation of a coarse structure level. At this secondary level, the structure can be described as a set of dense domains containing siloxane clusters embedded in a depleted matrix composed of unfolded polymer chains and solvent. By fitting a theoretical function for this model to the experimental SAXS curves, relevant structural parameters were determined as functions of time during the sol−gel transition and gel aging. For hybrids with low europium contents ( n = 80), the size of the siloxane clusters remains essentially invariant, whereas the dense segregation domains progressively grow. In hybrids with high doping contents ( n = 25), the preponderant structure variation during the first stages of the sol−gel transformation is the slow growth of siloxane clusters. For these hybrids, the segregation of siloxane clusters forming dense domains occurs only during advanced stages of the process.