Critical analysis of network defects in cross-linked isobutylene-based elastomers by NMR imaging

摘要

The analysis of chemical and physical network heterogeneities has been carried out for synthetic isobutylene-based elastomers using 1H spin echo NMR imaging. Spatial homogeneity was compared for isobutylene-p-methylstyrene-p-bromomethylstyrene (PIB-PMS/BrPMS) terpolymers cured with inorganic versus organic curatives and unfilled materials versus those filled with carbon black. Surprising differences in the network structure were found for materials cured with an inorganic curative (ZnO) relative to those cured with an organic curative (1,6-hexamethylenediamine). Voids and void distributions were found to be an important performance variable on the basis of comparisons between the NMR imaging data and mechanical testing. Systematic comparison of solvent images and polymer images revealed the optimum experimental conditions for enhancing network density contrast or detecting microvoids. For the first time, direct 1H images of polymer spins in a fully compounded, commercial polyisobutylene-based elastomer were acquired in the absence of any swelling solvent. Multislice imaging experiments were investigated as a method to construct three-dimensional void densities in elastomer compounds. Chemical-shift selective polymer imaging was used to selectively obtain network density information for either component of a two-phase blend containing the PIB-PMS/BrPMS terpolymer and polybutadiene. The ability of NMR imaging to provide data over statistically relevant sampling areas (hundreds of mm3) for both neat polymers and fully compounded commercial materials (containing carbon black) represents a unique advantage over microscopic imaging methods for material property analyses.