SORPTION KINETICS OF POLY(DIMETHYLSILOXANE) NETWORKS - DIFFUSION OF POLYDISPERSE PDMS BLENDS

Using the pulsed-gradient spin-echo NMR method, we have studied the di ffusivity D of two highly polydisperse OH terminated dimethylsiloxane blends (M-n = 15 900 and 26 400) in the melts and sorbed into networks made by linking the same material with tetraethoxysilane; the sol was carefully extracted before diffusant was added. Sorption took either 3 days for a 5% weight gain or 60 days for material weighing 5% of the network to be completely absorbed; samples were then equilibrated. Fi tting an echo attenuation model based on M distribution to the data sh ows that for both systems the diffusion in the 3-day sample is several times faster and more broadly distributed than in either its melt or the 60-day sample, reflecting the tendency for smaller molecules to be sorbed more rapidly. This finding complements our earlier similar fin ding for sol extraction. The D distributions in the 60-day samples are identical in shape, but slightly lower in mean rate, to those in the neat melts, and both in excellent correspondence with the original M d istributions, showing that the observed permselectivity is satisfactor ily explained on the basis of sorption kinetics alone.