Local molecular dynamics and heterogeneity in PEO-NiCl<inf>2</inf> supramolecular networks

The melt dynamics of poly(ethylene oxide) [PEO]-nickel chloride [NiCl<inf>2</inf>] systems are analyzed by proton NMR relaxometry and rheology. A consistent picture of the corresponding microstructure is proposed based on the combined results. Rheology reveals the presence of a weak gel due to PEO-metallic salt complexation, evidenced by a low second storage modulus plateau (<10⁴ Pa) well below the time scale of PEO terminal relaxation. NMR relaxometry shows that transverse magnetization relaxes faster with increasing salt content. Eventually a biexponential behavior is observed, manifesting the coexistence of two distinct environments, with slow and fast dynamics. The chain dynamics in the slow (hindered) domains is temperature, salt content and molecular weight independent within experimental limits, whereas the fast (mobile) domains exhibit similar properties to that of pure PEO. The size of the hindered domains is calculated to be at least 4 times larger than that of the PEO chains, proving that hindered domains encompass many chains. The fraction of hindered domains shows an experimental upper limit of 63%. On the basis of these observations, we propose a microstructure picture for the PEO-NiCl<inf>2</inf> gels in which PEO chains are bound together via PEO-metallic salt complexes, and eventually form hindered mobility clusters. These clusters are immersed in the matrix of PEO (mobile domains). In this microstructure picture, in the linear viscoelastic regime the terminal flow of the PEO-NiCl<inf>2</inf> system is dominated by the dynamic constraint of PEO-NiCl<inf>2</inf> complexes, which arrest a fraction of PEO segments.