NMR Imaging: Application to Elastomeric Materials

Abstract

We have examined a number of elastomeric systems of similar glass transition temperature T_g but differing chain segmental mobilities. Usable hydrogen images were obtained at ambient temperatures in reasonable times (one half hour to overnight) for bulk cis-polybutadiene, natural rubber and a cured, carbon-black-filled cis-polybutadiene. Image quality is, as expected, highly dependent on the relative values of T₂, the spin-spin relaxation time, and TE, the time to echo. Thus, under comparable conditions pure cis-polybutadiene gave a better image than natural rubber, which in turn gave a better image than the cured, filled cis-polybutadiene. Polyisobutylene, a rubber with the same T_g as natural rubber, but a T₂ < 1 ms, gave no image under the same conditions. The polymers were imaged using a spin-echo sequence, a gradient-echo sequence, and 3-D thick-slice sequence. The gradient-echo sequence may be better than the spin-echo sequence for detection of macroscopic defects such as voids, gaps, or small foreign particles in elastomers. A crude resolution test was performed on sheets of cured, filled cis-polybutadiene separated by spacers of variable width. Features as small as 0.07 mm could be detected even though the pixel size was considerably larger.