Thermoporosimetry and 1H solid-state NMR for the monitoring of heterogeneous thermo-oxidation of polyisoprene: a critical study

In this paper, thermoporosimetry and 1 H DQ NMR were implemented to investigate elastomeric network changes (chain scission and cross-linking reaction) of crosslinked polyisoprene upon thermo-oxidation. We aimed to evaluate not only how these techniques could probe network structural changes but also their limits when used in aging monitoring. Insoluble fraction measurements unambiguously point out that chain scissions are predominant until 50 h of aging. In agreement, the thermoporosimetry shows that the average mesh radius <R P > value increases while the derived mesh size distribution broadens more and more. 1 H DQ NMR measurements, performed on dried extracted samples, greatly mitigates this last result. This feature is ascribed to a heterogeneous aging process generating chain scissions mainly at the surface of the sample. The extraction procedure thus removes most of the surface. As the result, 1 H NMR performed on the extracted samples tends to underestimate chain scission degradation, depicting a less modified sample core. After 50 h of aging, cross-linking reactions become predominant as highlighted by an increase of the insoluble fraction and a shift of <R P > to lower values. NMR measurements surprisingly exhibit a similar trend but a significantly less modified network through the monitoring of ν (NMR). Furthermore, a significant increase of the dangling chain fraction, not observed through the other approaches, is evidenced. Such results underline that aging is heterogeneous in the considered sample due to an oxidation gradient (the core of the sample is slowly oxidized while its surface becomes “glassy” due to cross-linking). This assumption is confirmed by FTIR mapping and 1 H solid-echo experiments, showing a strong mobility restriction at the surface of the sample. The combination of the two approaches, thermoporosimetry and NMR measurements, seems relevant to carefully monitor heterogeneous aging processes in elastomeric networks. • Thermoporosimetry and 1 H DQ NMR combination was relevant to follow heterogenous aging. • Thermoporosimetry is unsuitable unlike 1 H DQ NMR for following architectural changes. • Increase of average mesh radius and dangling chain fraction due to chain scissions.