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

Abstract Gelation of tetramethoxysilane (TMOS) silica gels was monitored in situ using 2 H NMR relaxation measurements ( T 1 , T 1 ρ and T 2 ) of reaction mixtures prepared in D 2 O and CD 3 OD 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 T 1 and T 1 ρ 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 T 1 ρ and T 2 values (to about 20 ms for D 2 O), while T 1 decreases only slightly. Comparing D 2 O and CD 3 OD as gelation probes, the 2 H T 1 ρ values for the –CD 3 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.