Spin diffusion effects in partially spin–lattice relaxed NMR spectra of solid block copolymers

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Room-temperature proton NMR measurements of 65 wt % poly(dimethylsiloxane) (DMS) and 35 wt % bisphenol-A polycarbonate (BPAC) block copolymers with number average DMS block lengths ranging from 20 to 100 monomer units have shown effects which can be explained in terms of a gradient of net spin population along the DMS blocks during recovery from a nonselective 180° spin inverting pulse. This gradient occurs because the spin–lattice relaxation of the BPAC component is faster than that of the DMS component and spin diffusion takes place along the DMS blocks to the BPAC, causing spins at the centers of the DMS blocks to relax slower than those at the ends. High-resolution solid NMR spectra were obtained with a multiple-pulse technique, which permitted independent measurements of the DMS and BPAC relaxations. Unusual line shapes in partially relaxed broadline spectra were interpreted as being caused by enhanced motional narrowing of the protons at the centers of the DMS blocks in addition to the gradient of net spin population along the DMS blocks. The results of a spin diffusion model, which was developed to aid interpretation of the experimental results, were in quantiative agreement with the data. This model fits the data only if the diffusion process is limited by diffusion along the length of the DMS blocks and not by a single diffusion barrier between the DMS and BPAC blocks.