Interrogating Differential Cross-link and Defect Densities in Elastomer Compounds by NMR Imaging

摘要

~1H spin-echo NMR micro-imaging has been used to identify and characterize chemical and physical network heterogeneities in synthetic polyisobutylene-based elastomer compounds. Spatial distributions of network density in two and three dimensions were obtained for both unfilled as well as carbon-black filled compounds. Chemical-shift selective imaging provided phase-specific information in cured, multicomponent blends. Different curative compounds lead to physically different morphologies in terms of the network heterogeneity. In addition to chemical cross-link variations, the imaging experiments clearly revealed the presence of microvoids in many materials. Unequivocal identification of these defects as polymer-free voids was obtained based on comparison of images obtained for solvent-swollen vs. neat, dry compounds. Systematic comparison of solvent images and dry images revealed the optimum experimental conditions for enhancing cure state vs. void distribution information. Detailed statistical analysis of microvoid densities in several materials clearly showed that void density strongly correlated (R = 0.972) with mechanical performance. Through these experiments, NMR imaging has been shown to provide key network information over statistically relevant sampling volumes for both neat and compounded polymers. As such, we expect that these methods will continue to influence the development of technically important elastomers.