H-2 NMR relaxation monitoring of gelation in tetramethoxysilane sol-gels

Gelation of tetramethoxysilane (TMOS) silica gels was monitored in situ usi ng H-2 NMR relaxation measurements (T-1, T-1p and T-2) of reaction mixtures prepared in D2O and CD3OD solvents. Self-diffusion coefficients for both m ethanol and water in these systems are relatively insensitive to gelation. Reactions were carried out at pH 2 and 6, under both variable and constant temperature (320 K) conditions. At pH 2, TMOS hydrolysis is rapid but conde nsation occurs slowly and gelation did not take place during the NMR monito ring period. Evidence for formation of silica aggregates at pH 2 is seen in the reaction of TMOS containing 5 mol% poly(ethyleneglycol) (PEG) where a distinct and irreversible differentiation between T-1 and T-1p is observed after 24 h reaction at 320 K. For TRIOS reactions at pH 6, gelation is much faster. The sol-gel transition is associated with a sharp decrease in both T-1p and T-2 values (to about 20 ms for D2O), while T-1 decreases only sli ghtly. Comparing D2O and CD3OD as gelation probes, the H-2 T-1p values for the -CD3 and -OD groups show very similar rates of change leading up to the gel transition point. However, the break in the -OD relaxation curves is m ore distinct making the gelation point easier to detect. For TRIOS at 320 K , the gelation time was 21 min. The addition of low levels (5 mol%) of PEG did not alter the gelation time significantly. (C) 2000 Elsevier Science B. V. All rights reserved.