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

Gelation of tetramethoxysilane (TMOS) silica gels was monitored in situ using 2H NMR relaxation measurements (T1, T1ρ and T2) of reaction mixtures prepared in D2O and CD3OD solvents. Self-diffusion coefficients for both methanol 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 condensation occurs slowly and gelation did not take place during the NMR monitoring 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 T1 and T1ρ is observed after 24 h reaction at 320 K. For TMOS reactions at pH 6, gelation is much faster. The sol–gel transition is associated with a sharp decrease in both T1ρ and T2 values (to about 20 ms for D2O), while T1 decreases only slightly. Comparing D2O and CD3OD as gelation probes, the 2H T1ρ 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 more distinct making the gelation point easier to detect. For TMOS 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.