A new contrast parameter for visualization of the cross-link density in rubber based on the dipolar-correlation effect.
A new parameter for NMR mapping is suggested on the basis of the mean squared dipolar fluctuation (MSDF). The MSDF characterizes the relaxation mechanism due to ultra-slow dipolar fluctuations in liquids subject to local anisotropy of molecular motions. These fluctuations can be monitored on the time scale exceeding a few microseconds. In rubber materials, the MSDF is a function of the density of chemical cross-links strongly affecting (anisotropic) mesh chain fluctuations. Experimentally, the MSDF is determined from the attenuation curves of the quotient of the amplitudes of the stimulated and the primary echoes produced by the three 90 degrees radio-frequency pulse sequence. In order to evaluate the MSDF maps, the latter sequence was combined with the standard scheme of the magnetic field gradients providing a spatial resolution. The pixel values of the MSDF are "visualized" using grey shades related to the equidistant intervals covering the whole range of the measured values. The MSDF maps are demonstrated for the two composite samples. The first sample consists of a water filled tube in the middle part surrounded by high molecular mass polyisoprene (PI) in the outer part. The relaxation weighted spin density image of this sample is dominated by a water signal with PI producing a much weaker intensity. The MSDF map, on the contrary, enhances the relative intensity of the outer, PI, part while scaling the middle, water, part down to the level of noise. The second sample consists of the four rubber pieces with different cross-link density. This sample thus models an inhomogeneous rubber object. The MSDF map produces clear contrast for the relevant regions. The advantages of employing this kind of NMR mapping for a characterization of materials are discussed.