Evolution Processes in Polymeric Networks followed through low-field TD-NMR

Polymeric networks, especially rubbers and hydrogels, are ubiquitous materials. The determination of their structure-property relationships is fundamental for their rational design. Low Field (LF) Time Domain (TD) Nuclear Magnetic Resonance (NMR) is a powerful technique to probe molecular-level dynamics, analysing 1H relaxation times T1 e T2. That becomes very useful when it is necessary to monitor time-dependent processes. Thermo-gelation has been investigated on methyl-cellulose (MC) gels (1) with the study of T2 variation with temperature. T2 increases with temperature until a sudden drop, associated to the gel point, which is concentration dependent. The reversibility of the process, the formation of a metastable phase and the associated hysteresis phenomena can be observed, obtaining analogous results to rheological tests (Fig.1a,1b). TD-NMR can also measure residual dipolar couplings (Dres), which are linearly proportional to the crosslinks density (CLD), according to the equation CLD=rac{D_{res}}{4pi A} (2). This has been applied to evaluate the increasing number of crosslinks during the vulcanization process in a commercial rubber compound, with the concurrent reduction in the number of uncoupled chains with progression of the curing (Fig.1c). That reflected also on the more homogeneous distribution of crosslinks at end of the curing process. Even the effect of overcuring at high temperatures in reversion conditions on network morphology can be monitored (Fig.1d). The overcuring induces a drastic change in the morphology with a relevant, but homogeneous, reduction in the CLD respect to the highest CLD accessible at that temperature. Moreover, a peak at even higher Dres appears for the overcured sample, indicating the formation of a much more inhomogeneous network, whose causes could be attributed either to the formation of clusters of monosulfidic bonds or to the association of freed chains to the surface of filler particles.