Relationships between microvoid heterogeneity and physical properties in cross-linked elastomers: An NMR imaging study

H-1 NMR imaging experiments identify the presence of microvoids in cross-linked poly-(isobutylene-p-methylstyrene-p-bromomethlstyrene) (PIB-PMS/BrPMS) terpolymers, for both solid and solvent-swollen samples. Three-dimensional reconstruction of the sample images reveals that the voids are spherically shaped. A statistical analysis of the void density in six commercial (i.e., cured and carbon black filled) isobutylene-based elastomers indicates a strong linear correlation between the mechanical performance and the average number of voids per unit area (R = 0.972). In addition, the average number of neighboring voids within a radius of 1 mm per void was indicative of mechanical performance (R = 0.945). Our experimental results indicate that high microvoid density in cured elastomers leads to crack initiation and accelerated crack growth, thereby resulting in premature mechanical failure of the material. As this relationship is most likely general for many cured polymer compounds, we believe NMR imaging provides a particularly attractive approach to obtaining microvoid densities over bulk length scales in technically important materials.