In the last decades, proton time-domain NMR experiments performed on low-field spectrometers have shown their potential applicability on polymer science and technology. This feasible and versatile experimental approach allows us to obtain molecular-level information from the dynamics and structure of polymer samples with the aim of obtaining a better and more complete understanding of structure-property relationships for a rational design and development of polymer materials. It is important to point out the versatility of this experimental methodology that have been applied to characterize a wide variety of polymer materials, from thermoplastics to polymer networks (gels, rubbers or thermosets), to provide novel insights in different fundamental fields of polymer science and technology, e.g., polymer dynamics, network formation (gelation/vulcanization/curing), rubber reinforcement, ageing and recycling.
In this contribution we would like to revise some fundamental aspects on the relationship of the NMR observable, i.e., dipolar couplings, with the molecular order parameter of macromolecules and its link with polymer dynamics and the structure of polymer networks. Combination of different time-domain NMR experiments provide the opportunity to obtain a quantitative and complete characterization of the most important factors that determine polymer network structures: dangling chain ends, number of cross-links and their spatial distribution and their effect on the polymer dynamics, observing the suppression of entanglement effects on the segmental dynamics when the molecular weight between cross-links is much shorter than the molecular weight between entanglements.
Finally, we would like to show some new insights in the field of rubber reinforcement. It is a complex phenomenon that if fundamental to understand macroscopic properties of rubber materials and their applications, but it is based on molecular arguments, such as the number of cross-links and entanglements in the rubber network, the filler-rubber interactions, the filler-filler interactions and the hydrodynamic effect. More recently, this theory has been extended to include the fraction of network defects (dangling chain ends) and the stress and strain amplification factors induced by the addition of fillers. Nevertheless, the characterization of all the structural parameters that define the reinforcement mechanism in rubber is still a complex challenge due to the number of parameters, the different length and time scales where they are active and the interrelation between them in the currently measurable physical properties. In this sense, a pioneering experimental methodology has been developed by our Group to characterize the rubber-filler interface at the nanoscale by combining 1H Multiple-Quantum (MQ) NMR, equilibrium swelling and mechanical experiments, hence establishing a common physical framework between the three approaches and developing a novel combined analysis protocol. Additionally, the fabrication of a device that deforms the rubber samples inside the NMR spectrometer provides access to the stress and strain amplification factors at the rubber-filler interface. With these tools we are able to obtain, for the first time, the complete quantification of all the molecular parameters that determine the reinforcement mechanism of rubber materials.