J. Höpfner, G. Guthausen, W.Chassé, K. Saalwächter, M. Wilhelm.
Network Structure and Inhomogeneities of Model and Commercial
Polyelectrolyte Hydrogels as Investigated by Low-Field Proton NMR
Macromolecules 47, 4251-4265 (2014).
This work presents an experimental study of
the network structure of swollen, charged hydrogels. We
provide an in-depth comparison of two proton low-field NMR
methods, i.e., transverse relaxation and double-quantum
spectroscopy, that are both sensitive to residual orientation
correlations of the network chain segments. The results are in
both cases analyzed by help of integral inversion techniques,
and both methods are demonstrated to provide comparable
results despite the more qualitative nature of the transverse
relaxation. We investigate model samples that are similar to
commercial superabsorber materials based on partly neutralized and chemically cross-linked poly(acrylic acid) as prepared by free
radical polymerization. The degree of cross-linking and the monomer concentration are varied during synthesis. Both parameters
are found to affect the network structure and the amount of elastically inactive defects in a systematic way, and a comparison with
commercial samples with homogeneous or purposely inhomogeneous core?shell structures proves that the cross-linking
bimodality of the latter is readily revealed despite the rather large intrinsic inhomogeneity of the swollen gels.