M. A. Malmierca, A. González-Jiménez, I. Mora-Barrantes, P. Posadas, A. Rodríguez, L. Ibarra, A. Nogales, K. Saalwächter, J.L. Valentín.
Characterization of Network Structure and Chain Dynamics of
Elastomeric Ionomers by Means of 1H Low-Field NMR.
Macromolecules 47, 5655-5667 (2014).
The network structure and chain dynamics of ionic
elastomers based on carboxylated nitrile rubber (XNBR) cross-linked with
different content of magnesium oxide (MgO) have been studied by different
low-field time-domain NMR experiments. Ionic contacts created during the
vulcanization tend to aggregate trapping some polymer segments that show
restricted mobility as it was quantified by analyses of refocused free induction
decays. Increasing the MgO content above the stoichiometric fraction has no
effect on the amount of trapped polymer segments, but it increases the
network cross-link density as measured by multiple-quantum (MQ) NMR
experiments. The central finding of this work is that MgO addition above the
stoichiometric content enhances the mechanical properties by creating a larger number of smaller ionic clusters, which act as
dynamic cross-links, but are not readily seen by other techniques. Changes in the network structure and morphology of
segregated thermolabile ionic domains have an impact on the ionic rearrangement dynamics and, in consequence, on the
thermoplastic behavior of these materials at elevated temperatures.