Influence of Sulfur-Curing Conditions on the Dynamics and Crosslinking of Rubber Networks: A Time-Domain NMR Study

Elastomers are polymeric materials extensively used for manufacturing a wide range of products for industrial applications, especially in the tire industry. These materials are obtained by vulcanization of one (or more) polydiene polymer(s) in the presence of sulfur and other additives (accelerators, activators, plasticizers, reinforcing fillers, etc.). During this process, chemical crosslinks are formed between the polymeric chains, which provide elasticity and durability to the final product. Furthermore, depending on the formulation and the vulcanization conditions, other mechanical properties required for industrial applications can be obtained. Importantly, such macroscopic properties are strongly related to the microscopic structure of the polymeric network [1]. Consequently, the investigation of microscopic and macroscopic properties giving access to information on the network structure in relation to the vulcanization conditions is fundamental for the optimization of processing and performance of elastomeric materials. In this context, 1H time-domain NMR (TD-NMR) represents a valuable tool to gain insights into the molecular dynamics of the polymeric chains. In fact, this technique allows to measure NMR observables (1H T1 and T2 relaxation times and 1H-1H residual dipolar couplings (Dres)), which depend on the modulation of 1H-1H dipolar couplings by segmental motions. These motions are quite fast in elastomers above glass transition temperature, but are anisotropic, resulting in residual 1H-1H dipolar interactions, which depend on the amount and distribution of topological constraints in the polymeric network [2]. In this work, natural and isoprene rubbers vulcanized at different curing temperatures and different sulfur contents have been investigated by exploiting 1H TD-NMR techniques, including 1H multiple-quantum experiments for the measurements of Dres, Carr-Purcell-Meiboom-Gill pulse sequence for the evaluation of 1H T2 relaxation times, and field cycling NMR relaxometry for the measurements of T1 relaxation times on a wide range of Larmor frequencies (10 kHz-35 MHz). The NMR observables were compared with the crosslink density or macroscopic properties of the material that depend on this quantity, obtained using routinely employed methods in industrial analyses, allowing to gain insight into the effects of the formulation and the vulcanization conditions on the structure and dynamics of the polymeric networks [3]. [1] S. P. O. Danielsen et al., Chem. Rev. 121 (2021) 5092 [2] K. Saalwächter, Rubber Chem. Technol. 85 (2012) 350 [3] F. Nardelli et al., Polymers 14 (2022) 767