Irreversible frustrated spinodal decomposition in simultaneous interpenetrating polymer networks: Small-angle x-ray scattering

A structure factor is derived in the hydrodynamic limit which reflects the development of a microphase structure during the formation of simultaneously cross-linked interpenetrating polymer networks (IPN’s). This structure formation is a competitive process of spinodal decomposition and frustration of the fluctuations by increasing topological restraints due to network growth. The structure factor is compared with the scattering intensities from small-angle x-ray scattering performed on simultaneously cross-linked IPN’s of poly(carbonate-urethane) and poly(methyl methacrylate). Experimental data and theory are in good agreement for all compositions. The results give an averaged characteristic length of the frozen fluctuations which is about 2 nm for samples having one thermodynamic glass transition, as determined by differential scanning calorimetry, and between 15 and 35 nm for samples having two glass transitions. In contrast to the structure factor for the microphase separation transition in homogeneous (ideal) IPN’s or block copolymers which shows a ${\mathit{q}}^2$ and ${\mathit{q}}^{\mathrm{-}2}$ dependence at small and large scattering vectors q, respectively, the structure factor reveals a finite contribution in the limit q→0 and a ${\mathit{q}}^{\mathrm{-}4}$ behavior at large q values. A comparison of the data with the Debye-Bueche law yields also a good approximation at intermediate q values and indicates agreement with a more general treatment of scattering by an inhomogeneous solid. © 1996 The American Physical Society.