EPDM/PP Thermoplastic Vulcanizates As Studied by Proton NMR Relaxation: Phase Composition, Molecular Mobility, Network Structure in the Rubbery Phase, and Network Heterogeneity

Abstract

Proton solid-state NMR T2 relaxation analysis is used to study model oil-extended ethylene−propylene-diene rubber (EPDM) and thermoplastic vulcanizates (TPV) composed of polypropylene (PP), EPDM and oil. It is shown that the method allows selective characterization of crystalline and amorphous phases of PP, cross-linked EPDM, and oil in TPVs. The network density in the rubbery (EPDM) phase is determined in a wide range of the TPV compositions, as well as in the oil-extended EPDM vulcanizates containing different amount of oil. The entanglement density decreases with increasing oil content in oil-extended EPDM. This decrease can be described using a scaling approach. Molecular mobility of oil molecules decreases with increasing EPDM network density and is significantly lower than that of pure oil due to physical interactions with the host matrix. It is shown that the network density in the rubbery phase of TPV is composed of chemical cross-links, physical junctions at the EPDM/PP interface, and temporary and trapped chain entanglements. The density of chemical cross-links increases with the amount of cross-linker per weight unit of EPDM. The density of the physical junctions increases with increasing PP content. Crystallinity of PP in TPV is hardly affected by the TPV composition. A small fraction of oil molecules plasticizes the amorphous phase of PP and the plasticization effect is proportional to an oil:PP mass ratio in the TPV composition. Two NMR methods for improving the selectivity of the T2 relaxation analysis to different phases of TPV are evaluated. The methods are based on the inversion−recovery experiment and double-quantum (DQ) filtering of the decay of the transverse magnetization relaxation. The DQ filtered T2 relaxation experiment improves the selectivity of the method and could open new possibilities for characterization of the network heterogeneity in rubbery materials. Some relationships between the TPV composition, network density, and mechanical properties are shortly discussed.