Characterization of Elastomers Based on Monitoring Ultraslow Dipolar Correlations by NMR

Abstract

Owing to enormous industrial use, elastomers remain an important field of intensive NMR studies [1–15] since more than three decades. The major goal is to establish a correlation between the molecular structure, molecular dynamics and the macroscopic mechanical properties of final materials. On this basis, material properties can be tailored using an optimization of the technological procedures and an application of cross-linking agents and special additives [16,17]. Undisputable advantage of NMR in comparison to other conventional methods of material characterization (like swelling or stress–strain measurements [16,17]) is that it is fully non-destructive and does not require any special sample treatment. Various NMR methods proposed as the characterization tools are based mainly on probing stochastic molecular motions via the measurements of the line shapes, nuclear magnetic relaxation rates and translational displacements. At room temperatures, the NMR response of elastomers is predominantly determined by the so-called residual dipolar couplings (RDCs). Finite RDCs are characteristic of many “soft” materials like liquid crystals or polymers in which molecular motions are restricted by anisotropic molecular alignment or topological constraints [18,19]. In isotropic non-viscous liquids, in contrast, dipolar interaction are averaged out completely [20,21]. In polymers, restrictions to chain re-orientations arise owing to the excluded volume effects or “entanglements”. The constraints of this art can also be produced artificially by confining polymeric materials in the nanoscopic pores [22,23]. In rubber, especially severe constraints on isotropic chain re-orientations are imposed by chemical cross-links. The consequence is that any fast chain fluctuations between the cross-links remain strongly anisotropic in the time scale of typical NMR experiments and cannot completely average out the secular part of the dipolar Hamiltonian. This feature is exploited in various experimental NMR approaches deployed in study of elastomers. The influence of topological constraints on “residual” dipolar fluctuations was elaborated many years ago in works by Cohen-Addad [1–4]. In these works, a given chain is assumed to have a mean position in space which is determined by the cross-links and/or the constraints formed by the so-called chain entanglements. The chain is described in terms of N freely jointed Kuhn (or statistical) segments of length b. If chain meshes between the cross-links are not too long, an end-to-end vector R is determined mainly by the two adjacent cross-links. In the time scale of typical NMR pulse sequences, the mean orientation of the end-to-end vector can be considered as constant, whereas fast intrachain fluctuations tend to be anisotropically restricted in space. Averaging over fast fluctuations with respect to a local symmetry axis determined by the fixed vectorRgives for the “residual” dipolar coupling constant in elastomers [4]: