SANS and SLS Studies on Tetra-Arm PEG Gels in As-Prepared and Swollen States

A series of model networks consisting of polyethylene glycol (PEG), tetra-PEG gels, have been prepared and their structure and dynamics have been investigated by small-angle neutron scattering (SANS) and static light scattering (SLS). The Tetra-PEG gels were prepared by cross-end coupling of two types of tetra-arm PEG macromers with molecular weights, M w, of (5 to 40) × 103 g/mol. In the SANS regime, the structure factors of both as-prepared and swollen gels can be represented by Ornstein−Zernike-type scattering functions and superimposed to single master curves with the reduced variables, ξq and I(q)/ϕ0ξ2, irrespective of the molecular weight of tetra-PEG, where q, ξ, I(q), and ϕ0 are the magnitude of the scattering vector, the correlation length, the scattering intensity, and the polymer volume fraction at preparation, respectively. In the SLS regime, however, a power-law-type upturn was observed, indicating the presence of PEG chain clusters. Interestingly, these inhomogeneities disappear by swelling. It is concluded that Tetra-PEG gels can be an “ideal polymer network” with a self-similar structure with respect to M w without significant entanglements and/or defects. This explains why Tetra-PEG gels have high mechanical strength as reported elsewhere (Macromolecules 2008, 41, 5379).