1 H NMR Studies of the Sol-Gel Transition
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lH NMR STUDIES OF THE SOL-GEL TRANSITION* ROGER A. ASSINK AND BRUCE D. KAY Sandia National Laboratories Albuquerque, NM 87185, USA ABSTRACT 1 High resolution H NMR spectroscopy has been employed to study the dynamics of the sol-gel transition in simple silicates. High magnetic fields (360 MHz) were used to detect and identify the various chemical species present in the Si(OCH 2CH3 ) 4 :C 2H5OH:H 2 0 sol-gel system. Using these techniques, the time evolution of the reactant and product species were monitored. The results of these studies have shown that acid and base catalyzed systems react along very different chemical pathways and that the elementary hydrolysis and condensation reactions occur on widely different time scales.
INTRODUCTION In previous publications [1,2] Brinker and colleagues reported on polymer growth during acid and base catalyzed hydrolysis of tetraethylorthosilicate (TEOS) in alcoholic solutions and proposed that the form of the resulting polymers was governed by the relative rates of hydrolysis and condensation. We have extended our investigation by employing proton nuclear magnetic resonance (lH NMR) spectroscopy to probe both the temporal and chemical nature of the sol-gel reactions. As will be shown, considerable new kinetic and chemical information has been gained. EXPERIMENTAL 0 A two-step process carried out at 25 C was used to prepare all the soluIn the first step TEOS, CH3 CH2OH(EtOH), H2 0, and HCZ were mixed tions [1,2]. in the mole percents displayed in Table I. This amount of water is one-half that required to form anhydrous silica via the stoichometric reaction
Si(OEt) 4 + 2H20
-
SiO2 + 4EtOH.
(1)
After 90 minutes, additional water plus HCi or NH4 0H were added comprising the second step. The concentrations studied are shown in Table I and were chosen to match those previously examined [1,2]. 1H NMR spectra were recorded at 360 MHz at the Colorado State University 0 NSF Regional NMR Facility. All spectra were recorded at 25 C using 4 pulses All resonances were referenced to tetramethyl at a .5 Hz repetition rate. silane (TMS) via the ethanol methylene group whose chemical shift is reported as 3.70 ppm relative to TMS [3].
*This work supported by the U. S. Department DE-AC04-76DP00789. Mat. Rs. Soc.
Symp. Proc. Vol. 32 (1984)
of Energy under Contract No.
Pobjished by Elsevier Science Publishing Co., Inc.
302
TABLE I.
Compositions (mole %).
STEP
EtOH
H20
TEOS
HC£
1st Acid
65.4
17.9
16.7
0.012
---
2nd Acid
22.7
71.6
5.8
0.004
---
2nd Base
45.5
42.9
11.6
0.005
0.014
NH4 0H
RESULTS AND DISCUSSION General Findings Figure la displays the entire proton spectrum for the initial hydrolysis after the reaction has proceeded 12 min. Figs. lb and c show an expansion of the 3.5 to 4.1 ppm spectral region for short (12 min) and long (90 min) times during the initial stage. The quartet centered at 3.90 ppm is the resonance of methylene (-CH2 -) protons associated with a silicon while that centered at 3.70 ppm is associated with ethanol. Two changes are clearl
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