Abstract
1H spin lattice relaxation rate (R1) 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″(ωτs) master curves were constructed by shifting the total FC NMR susceptibility, χ″(ω) = ωR1(ω), curves along the frequency axis by the correlation times for glassy dynamics, τs. Longer τs 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, χpol″(ω), to χ″(ω) was singled out by subtracting the contribution of glassy dynamics, χglass″(ω), well represented using a Cole–Davidson spectral density. For all elastomers, χpol″(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.
Highlights
Linear polymer melts show dynamics over a wide frequency scale, from the fast scale of local segmental dynamics and internal motions to the very slow scale of diffusive motions of the center of mass of the polymer chains
Glassy and polymer dynamics were investigated on isoprene rubber (IR), butadiene rubber (BR), and styrene-co-butadiene rubber (SBR), three elastomers of technological interest for the tire industry, both uncured and cross-linked by sulfur curing under different conditions. 1H FC Nuclear magnetic resonance (NMR) relaxometry measurements over a wide range of temperatures and frequencies, combined on the basis of the FTS principle, allowed dynamics to be carefully investigated over a quite broad time scale, ranging from local segmental motions within the Kuhn segment to chain dynamics within the constraints imposed by entanglements and sulfidic cross-links
The effect of cross-linking on collective chain dynamics was investigated by separating the contribution of polymer dynamics to 1H longitudinal relaxation from that of glassy dynamics for uncured and, for the first time in this work, for vulcanized elastomers
Summary
Linear polymer melts show dynamics over a wide frequency scale, from the fast scale of local segmental dynamics and internal motions to the very slow scale of diffusive motions of the center of mass of the polymer chains. Segmental motions within the so-called Kuhn segment with characteristic time τs are responsible for “glassy dynamics” connected with the structural relaxation or α-process (τs = τα). If mobile side groups exist in the polymer, the main-chain local segmental motions may be supplemented and superimposed to reorientations of such groups. The connection of segments into chains results in slower polymer-specific collective dynamics, indicated in the following as “polymer dynamics”. For polymers with molar mass M below the critical value Me (average molar mass between entanglements), Rouse theory[1] well describes polymer dynamics.
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