Morphology of Photopolymerized End-Linked Poly(ethylene glycol) Hydrogels by Small-Angle X-ray Scattering

Because of the biocompatibility of poly(ethylene glycol) (PEG), PEG-based hydrogels have attracted considerable interest for use as biomaterials in tissue engineering applications. In this work, we show that PEG-based hydrogels prepared by photopolymerization of PEG macromonomers functionalized with either acrylate or acrylamide end-groups generate networks with cross-link junctions of high functionality. Although the cross-link functionality is not well controlled, the resultant networks are sufficiently well ordered to generate a distinct correlation peak in the small-angle X-ray scattering (SAXS) related to the distance between cross-link junctions within the PEG network. The cross-link spacing is a useful probe of the PEG chain conformation within the hydrogel and ranges from approximately 6 to 16 nm, dependent upon both the volume fraction of polymer and the molecular weight of the PEG macromonomers. The presence of a peak in the scattering of photopolymerized PEG networks is also correlated with an enhanced compressive modulus in comparison to PEG networks reported in the literature with much lower cross-link functionality that exhibit no scattering peak. This comparison demonstrates that the method used to link together PEG macromonomers has a critical impact on both the nanoscale structure and the macroscopic properties of the resultant hydrogel network.