H NMR Characterization of Swelling in Cross-Linked Polymer Systems

A 1 H NMR method capable of determining the level of swelling of microscopic volume elements (about 20 μm in diameter) within cross-linked materials is described. The fact that it is a microscopic swell measurement makes it extremely useful for the characterization of the swelling heterogeneities which may exist within common network systems, such as core/shell or other morphologies. The method utilizes the differences in chemical shift between solvent absorbed into the cross-linked polymer and that of solvent outside the polymer. This chemical shift difference is then correlated to macroscopic swelling (rather than cross-linking) through a simple model which encompasses both the effective chemical cross-links and the entanglement cross-links in the manner of classical swelling experiments. The analysis is demonstrated for styrene, divinylbenzene copolymer beads, cross-linked polycarbonates, ion-exchange cation resins and cross-linked poly(acrylic acid). A calibration is, in each case, developed with a series of standard materials whose bulk swelling characteristics were determined. An example of the analysis of the cross-linking morphology within a single cation-exchange bead is also presented. The analysis of swelling by this 1 H NMR method appears to be applicable to any network system with aromatic or acid functionality. Its application is expected to enable identification of new structure/property relationships critical for developing advanced materials.