Concentration polydispersity in anisotropically swollen polymer networks

Unlike polymer solutions, the component chains in swollen networks have limited freedom of translation. In such gels radiation is scattered by two types of concentration fluctuation: dynamic thermal agitation of the polymer segments, which confers osmotic properties on the gel; and static fluctuations, due to elastic constraints frozen in by crosslinks when the gel is formed. In small angle scattering experiments, these two components of the scattering spectra are separable. In this Communication, the mean square amplitude of the static concentration fluctuations 〈δφ2〉s in polydimethyl siloxane networks swollen anisotropically in octane is determined from small angle x-ray scattering. Although the characteristic length of these fluctuations is strongly anisotropic, 〈δφ2〉s remains approximately constant. This finding provides a mechanism for the ‘‘butterfly’’ effect observed in small angle neutron scattering spectra.