H NMR Characterization of Swelling in Cross-Linked Polymer Systems

Abstract

A 1H 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 1H 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.

Dynamics of Soft and Hairy Polymer Nanoparticles in a Suspension by NMR Relaxation

“…The variation of T 1 with the correlation time of the local relaxation process describes a parabola with a frequency-dependent minimum, and for relatively slow local subsegmental motions, like those observed in the systems under study, one would expect T 1 to decrease as the subsegmental dynamics becomes faster. 35 The increase in T 1 observed with the increasing cross-linking density was in agreement with the expected trend for macromolecules and gels, 36 where, as the network becomes more cross-linked and rigid, the local subsegmental dynamics slows down leading to an increase in T 1 .…”

Nitroxide-Controlled Free-Radical Copolymerization of Vinyl and Divinyl Monomers. 2. Gelation

“…These experimental results indicate that the spatial distribution of pendant vinyls (around the polymer radical), and hence that of cross-links in the gel, is highly heterogeneous. Inhomogeneous distributions of cross-links were experimentally observed by several methods. − For example, in a uniaxially deformed gel, Bastide et al observed “butterfly patterns”, unusual scattering spectra arising from a spatially inhomogeneous deformation, which originates from an inhomogeneous cross-link distribution. For poly( N -isopropylacrylamide) gels, Shibayama et al observed that the scattering intensity from such a gel varies from position to position of the sample due to a nonuniform density distribution.…”

Impact of Hydrophilic Surfaces on Interfacial Water Dynamics Probed with NMR Spectroscopy

“…The only reports found in literature that had recorded such multiple resonances of water are associated with biological tissues or polymer systems that are quite similar to the ion exchange resins or Nafion that were used in this study. − These studies, however, did not address the dynamics of water lying outside the polymers or tissues.…”