STUDY OF SOL EXTRACTION FROM MODEL POLY(DIMETHYLSILOXANE) NETWORKS USING PULSED GRADIENT NUCLEAR-MAGNETIC-RESONANCE DIFFUSION MEASUREMENTS

Model poly(dimethylsiloxane) (PDMS) networks have been prepared from a blend of hydroxyl- terminated molecules of two sizes, 85 mol % Mn = 550 and 15 mol % Mn = 18 000, using tetraethoxysilane as cross-linking agent at stoichiometric concentration in the presence of a catalyst. Sol was extracted for various lengths of time by swelling in toluene; up to 4.2% of sample weight was extractable in 5 days. The progress of extraction was followed by measuring NMR relaxation and diffusion of sol molecules in the un- swelled network, using the pulsed-gradient spin-echo method. The smaller, faster diffusing sol molecules were extracted in <1 h, while the largest molecules required days to extract. There was evidence that the largest sol molecules are entangled with the network. Results suggest that prolonged extraction is required for PDMS networks destined as host materials for the study of the kinetics of polymeric trace additives. © 1990, American Chemical Society. All rights reserved.