A Novel And Self-Consistent Physical Framework To Characterize Molecular Structure And Dynamics Of Reinforced Rubbers Based On A Combination Of Experimental Techniques

Elasticity in rubber materials depends on molecular parameters that define the structure. Crosslink density is the most well-known, although it has been proven that the molecular weight of polymer chains (entanglements) or dangling chain ends (network defects) are also crucial to understand the structure-properties relationships on unfilled rubbers. Additional effects arise when reinforced rubbers are considered, such as hydrodinamic amplification, filler-filler interactions or filler-rubber interactions. The development of a self-consistent physical framework was required to quantify all the molecular parameters necessary to quantify the molecular structure of reinforced rubbers. In order to achieve this challenging goal, Multiple Quantum (MQ) NMR experiments and tensile tests were performed and their respective theoretical backgrounds were unified to accomplish the quantitative determination of all the molecular parameters that define the molecular structure. The methodology consisted in 4 separated phases: i) preparation of the samples, ii) tensile tests experiments, iii) MQ-NMR experiments and iv) data analysis. The preparation of the samples followed a standard method which can be found in the literature [1]. The tensile tests were performed at low-speed (10 mm/min) to minimize the viscous response of the material. MQ-NMR experiments provided structural information about the order parameter by the application of an advanced Baum-Pines pulse sequence. The data obtained were treated with a theoretical approach based on the order parameter and the tube model. The real crosslink density, the entanglement density, the finite extensibility parameter, both stress and strain amplification factors, the fraction of network defects and the density of filler-rubber interactions were determined for the first time. Some experimental approaches provide inconsistencies in the determination of the network structure of rubber compounds and a combination of experimental techniques is needed to characterize every parameter. MQ-NMR provides information on the segmental order at a molecular level, network imperfections and the spatial distribution of the restrictions of movement, while tube model discerns between crosslinks and entanglements and is easy to work with and generalize it to filled samples.