Structure of Trifunctional End-Link Polymer Gels Studied by SANS

Abstract

We investigate, using small-angle neutron scattering (SANS), the polymer concentration fluctuations in a particular type of end-linked polystyrene gels, in the swollen state. These gels are obtained from the cross-linking of well-defined end-capped low-polydispersity chains end-linked through a trifunctional cross-linking agent. Polystyrene precursor chains of three different molecular weights, namely 5.5K, 10K, and 15.2K, were used. The scattering at low scattering vectors is expected to come from static concentration fluctuations. Here the intensity displays a dominant scattering characterized either at low q by a unique correlation length or on a wider q range by a set of two correlation lengths, one of which has been locked to the correlation length in the semidilute solution of same polymer concentration. We note ξhet as a characterization of swelling heterogeneity, the unique low q correlation length, or the larger one in the double length fit. For both fits, the variation of ξhet with the polymer volume fraction φ is almost the same for all precursor molecular masses; it scales close to ξhet ∼ φ-5/3. Such a variation is surprisingly identical to that observed in “statistical gels” randomly cross-linked from semidilute solutions of very large polymer chains. Results of this paper are thus different from the observations on other end-linking gels, the scattering of which seems to belong to other “classes”, displaying two distinct correlation lengths or a maximum corresponding to soft order between nodule-like junctions.