“…The two simulations with cyclization agree reasonably well up to about 50% conversion, and then the difference becomes large. Figure also illustrates that the simulation results taking account of cyclization compare favorably with experimental data (points) of Kopylov et al and Grubisic-Gallot et al discussed below. 6 Simulated weight-averaged degree of polymerization as a function of siloxane bond conversion.…”
Section: Resultssupporting
confidence: 79%
“…Results of cyclization and cage models differ only little in this range of conversions (R < 0.35), so that Figure 9 can be used to estimate the silica DP w values up to 4. When the monomer conversions from 29 Si NMR are used as the x-axis, the agreement is good between our simulations and DP w from Kopylov et al 47 (Figure 9). pH Effects.…”
Section: Resultssupporting
confidence: 77%
“…See Table 1 for more details on conditions. 15 1.3 10 3 a Kopylov et al 47 2.3 0.2-1.5 2.3 Sanchez et al 29 2.1 2.0 2.6…”
Section: Introductionmentioning
confidence: 99%
“…1.9 5.1 1.0 Brinker and Assink 17 2.3 1.5 2.8 Brinker and Assink 17 2.3 1.7 2.8 Devreux et al 1.3 10 3 a Kopylov et al 2.3 0.2−1.5 2.3 Sanchez et al …”
Section: Introductionmentioning
confidence: 99%
“…Figure 8. Comparison of "cyclization" DMC results (solid curves) to data (open circles with dashed lines) of Kopylov et al47 for tetraethoxysilane polycondensation. The filled circles in the monomer figure are the collected data from29 Si NMR (Figure1).…”
Sol−gel processing provides a useful route to novel metastable materials such as molecular hybrids of silica
and either transition metal oxides or organic components. Because the properties of these materials depend
critically on how the components are combined, we present calculations of the size and structure of silica
building blocks that may be prepared as precursors to nanocomposites. By fitting existing silicon-29 NMR
data, we find kinetic parameters applicable for acid catalyzed hydrolytic polycondensation of tetraethoxysilane
and tetramethoxysilane precursors. In a wide range of conditions, the local connectivity of silicon sites evolves
in approximately the same way with respect to the siloxane bond conversion. Using dynamic Monte Carlo
simulations, including nearest-neighbor effects and cyclization, we calculate the molecular weight distribution
of silica as a function of conversion, in reasonable agreement with available experiments. At low siloxane
bond conversions (α ≤ 0.6) the mass weighted degree of polymerization DPw is less than 10. When enough
water for alkoxide hydrolysis is available, gel precursors containing 8−20 silicon sites form quickly up to
conversion α = 0.65−0.75 and then slowly react together until gelling at α = 0.82. The calculated distributions
provide a quantitative road map for forming composite materials with well-defined silica blocks, where the
siloxane bond conversion can be used as an indicator of the progress of structure development.
“…The two simulations with cyclization agree reasonably well up to about 50% conversion, and then the difference becomes large. Figure also illustrates that the simulation results taking account of cyclization compare favorably with experimental data (points) of Kopylov et al and Grubisic-Gallot et al discussed below. 6 Simulated weight-averaged degree of polymerization as a function of siloxane bond conversion.…”
Section: Resultssupporting
confidence: 79%
“…Results of cyclization and cage models differ only little in this range of conversions (R < 0.35), so that Figure 9 can be used to estimate the silica DP w values up to 4. When the monomer conversions from 29 Si NMR are used as the x-axis, the agreement is good between our simulations and DP w from Kopylov et al 47 (Figure 9). pH Effects.…”
Section: Resultssupporting
confidence: 77%
“…See Table 1 for more details on conditions. 15 1.3 10 3 a Kopylov et al 47 2.3 0.2-1.5 2.3 Sanchez et al 29 2.1 2.0 2.6…”
Section: Introductionmentioning
confidence: 99%
“…1.9 5.1 1.0 Brinker and Assink 17 2.3 1.5 2.8 Brinker and Assink 17 2.3 1.7 2.8 Devreux et al 1.3 10 3 a Kopylov et al 2.3 0.2−1.5 2.3 Sanchez et al …”
Section: Introductionmentioning
confidence: 99%
“…Figure 8. Comparison of "cyclization" DMC results (solid curves) to data (open circles with dashed lines) of Kopylov et al47 for tetraethoxysilane polycondensation. The filled circles in the monomer figure are the collected data from29 Si NMR (Figure1).…”
Sol−gel processing provides a useful route to novel metastable materials such as molecular hybrids of silica
and either transition metal oxides or organic components. Because the properties of these materials depend
critically on how the components are combined, we present calculations of the size and structure of silica
building blocks that may be prepared as precursors to nanocomposites. By fitting existing silicon-29 NMR
data, we find kinetic parameters applicable for acid catalyzed hydrolytic polycondensation of tetraethoxysilane
and tetramethoxysilane precursors. In a wide range of conditions, the local connectivity of silicon sites evolves
in approximately the same way with respect to the siloxane bond conversion. Using dynamic Monte Carlo
simulations, including nearest-neighbor effects and cyclization, we calculate the molecular weight distribution
of silica as a function of conversion, in reasonable agreement with available experiments. At low siloxane
bond conversions (α ≤ 0.6) the mass weighted degree of polymerization DPw is less than 10. When enough
water for alkoxide hydrolysis is available, gel precursors containing 8−20 silicon sites form quickly up to
conversion α = 0.65−0.75 and then slowly react together until gelling at α = 0.82. The calculated distributions
provide a quantitative road map for forming composite materials with well-defined silica blocks, where the
siloxane bond conversion can be used as an indicator of the progress of structure development.
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