Polydimethylsiloxane networks at equilibrium swelling: Extracted and nonextracted networks

The influence of the extractable components in the microscopic structure of swollen polymer networks was assessed by measuring the small-angle neutron scattering function of extracted and nonextracted networks. The study was done as a function of cross-link density. The scattering data are analyzed using a model cross section that introduces two characteristic length scales: ξS, distance below which single chain behavior is dominant, and ξL, distance above which large-scale fluctuations of polymer concentration are no longer observed. The correlation length ξL, and the intensity scattered in the limit of zero scattering vector obtained from the scattering data, for both the extracted and nonextracted systems, scale with polymer concentration with a power law of (-3/2). Furthermore, in a double logarithmic plot versus concentration, the experimental points obtained from the extracted and nonextracted systems for ξL and I(q → 0) fall on the same line within experimental accuracy, suggesting that, in the nonextracted swollen system, the extractable components conform essentially as network strands. The results are compared to predictions of theories based on the percolation model, and with another model that describes the fluctuations in polymer concentration in the swollen system as resulting, for distances below ξL, from excluded volume interactions established between large, connected, "hairy-like" units, that result from the attachment of pendant chains and clusters.