Glassy and Polymer Dynamics of Elastomers by 1H Field-Cycling NMR Relaxometry: Effects of Cross-Linking

High Resolution Image Download MS PowerPoint Slide 1 H spin lattice relaxation rate ( R 1 ) dispersions were acquired by field-cycling (FC) NMR relaxometry between 0.01 and 35 MHz over a wide temperature range on polyisoprene (IR), polybutadiene (BR), and poly(styrene- co -butadiene) (SBR) rubbers, obtained by vulcanization under different conditions, and on the corresponding uncured elastomers. By exploiting the frequency–temperature superposition principle, χ″(ωτ s ) master curves were constructed by shifting the total FC NMR susceptibility, χ″(ω) = ω R 1 (ω), curves along the frequency axis by the correlation times for glassy dynamics, τ s . Longer τ s values and, correspondingly, higher glass transition temperatures were determined for the sulfur-cured elastomers with respect to the uncured ones, which increased by increasing the cross-link density, whereas no significant changes were found for fragility. The contribution of polymer dynamics, χ pol ″ (ω), to χ″(ω) was singled out by subtracting the contribution of glassy dynamics, χ glass ″ (ω), well represented using a Cole–Davidson spectral density. For all elastomers, χ pol ″ (ω) was found to represent a small fraction, on the order of 0.05–0.14, of the total χ″(ω), which did not show a significant dependence on cross-link density. In the investigated temperature and frequency ranges, polymer dynamics was found to encompass regimes I (Rouse dynamics) and II (constrained Rouse dynamics) of the tube reptation model for the uncured elastomers and only regime I for the vulcanized ones. This is clear evidence that chemical cross-links impose constraints on chain dynamics on a larger space and time scale than free Rouse modes.