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

Francesca Nardelli,Luca Giannini,Marco Geppi,Francesca Martini,Elena Rossi,Marco Arimondi,Silvia Borsacchi,Elisa Carignani,Lucia Calucci,Antonio Susanna,Mattia Cettolin

doi:10.1021/acs.jpcb.1c00885

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 rubber (IR), either unfilled or filled with different amounts of carbon black, silica, or a combination of both, and sulfur cured. By exploiting the frequency–temperature superposition principle and constructing master curves for the total FC NMR susceptibility, χ″(ω) = ωR1(ω), the correlation times for glassy dynamics, τs, were determined. Moreover, the contribution of polymer dynamics, χpol″(ω), to χ″(ω) was singled out by subtracting the contribution of glassy dynamics, χglass″(ω), well represented by the Cole–Davidson spectral density. Glassy dynamics resulted moderately modified by the presence of fillers, τs values determined for the filled rubbers being slightly different from those of the unfilled one. Polymer dynamics was affected by the presence of fillers in the Rouse regime. A change in the frequency dependence of χpol″(ω) at low frequencies was observed for all filled rubbers, more pronounced for those reinforced with silica, which suggests that the presence of the filler particles can affect chain conformations, resulting in a different Rouse mode distribution, and/or interchain interactions modulated by translational motions.

Highlights

Particles of solids, such as carbon black and silica, are usually dispersed in elastomers as reinforcing fillers to improve mechanical, thermal, and rheological properties of the polymeric matrix
1H field-cycling nuclear magnetic resonance (FC NMR) relaxometry was successfully applied to investigate glassy and polymer dynamics in polymer melts[44−48,62−67] and, less extensively, in cross-linked elastomers.[69−73] In particular, in previous work, we investigated the effect of introducing cross-links by sulfur curing on glassy and polymer dynamics of elastomers.[73]
1H FC NMR relaxometry experiments were performed at 303 and 373 K on IR rubbers filled with different amounts of carbon black (IR_S2_CB20, IR_S2_CB40, IR_S2_CB60, and IR_S2_CB80), silica (IR_S2_Si50), or a mixture of silica and carbon black (IR_S2_Si15_CB20); nuclear magnetic relaxation dispersion (NMRD) curves are reported in Figure 1 together with those of unfilled IR rubber vulcanized in the same conditions (IR_S2)

Summary

■ INTRODUCTION

Particles of solids, such as carbon black and silica, are usually dispersed in elastomers as reinforcing fillers to improve mechanical, thermal, and rheological properties of the polymeric matrix. Filler− rubber interactions impose constraints to chain dynamics in elastomers,[6,8] in addition to those arising from entanglements and, in cured compounds, chemical cross-links, which constitute the microscopic origin of macroscopic properties required for specific applications. It is, thereby, of both scientific and technological interest to investigate the effects of fillers on elastomer dynamics. At high frequency and low temperature, where ωτs ≅ 1, NMRD curves are dominated by fast intrasegment conformational fluctuations connected to glassy dynamics, well described by the Cole−Davidson spectral density.[60] At lower frequencies and higher temperatures (ωτs ≪ 1), dipolar translational (Ctrans(t)) and rotational (Crot(t)) autocorrelation functions associated to polymer dynamics are expressed as proportional to power laws of ⟨r2(t)⟩. The temperature was controlled within ±0.1 °C with a Stelar VTC90 variable temperature controller

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES