NMR-based molecular rheology and structural characterization of model gels

Low-resolution proton NMR, performed on cost-efficient low-field instruments, holds great potential for the assessment of the relation between the NMR-detected monomer-level mobility and the relevant physical properties of polymer-based materials. In elastomers and gels, multiple-quantum (MQ) NMR allows for the precise measurement of residual dipolar couplings (RDCs) among the monomer protons, which reflects the conformational space of the highly mobile network chains. In this way, not only the average network chain length but also structural inhomogeneities become accessible. In model networks made by end-linking of star polymers, we can go as far as quantifying connectivity motifs in gels made by heterocomplementary [1,2] and homocomplementary coupling reactions [3], either in one-component [1,3] or amphiphilic co-networks [2].

While the measured RDC is consistently found to be proportional to the gels' elasticity modulus [3], the absolute-level interpretation of the measured average RDC to predict the modulus remains a challenge. We here report on recent experiments performed on a variety of PEO-based model gels to elucidate the effects of solvent on local conformational fluctuations, thermodynamic aspects (solvent quality) and the inhomogeneity of the gels.



References

[1] F. Lange et al., Macromolecules 44, 9666 (2011), DOI: 10.1021/ma201847v

[2] C. Bunk et al., Macromolecules 55, 6573 (2022), DOI: 10.1021/acs.macromol.2c00693

[3]Z. Meng et al., Macromolecules 57, 3058 (2024), DOI: 10.1021/acs.macromol.3c02514