AAB sols

Because such a heat treatment is desla-uctiveto organic dyes, we added a second acid-catalyzed hydrolysis step (the AAB sol) to promote more complete reaction and to reduce the proportion of small, incompletely-hydrolyzed species. NMR studies of the AAB sols [16] showed that the second acid-catalyzed step promotes... 

Film Formation For all sols, films were deposited on polished.single stal <100> silicon substrates by dip-coating in a dry nitrogen atmosphere, using substrate withdrawal rates of 20 (A2 and AAB sols) or 25 cm/min (B2 sols). A2 films were... 

B2 and AAB Sols. The NMR spectra of the unaged and aged (39h at 50 Q B2 sols both show broad features due to q3 and silicon species. However the spectra appear dominated by much sharper resonances associated mainly with monomers and weakly hydrolyzed and silicon species in particular the unhydrolyzed monomer (-82.1 ppm), the unhydrolyzed dimer (-89.1 ppm), the unhydrolyzed cyclic tetramer Q 4c (-95.4 ppm), and an unhydrolyzed species possibly associated with a trimer (-96.5 ppm). Figure 4 shows the evolution of the species distribution with aging time. We see that although the percent of species increases at the expense of and species, consistent with further condensation, the percentage of species remains virtually constant (30%), and the concentration of monomer increases from about 2 to 7%. 

The 29Si NMR spectrum of an AAB sol after the second hydrolysis step (r = 2.5, dilution = 2 1) exhibits two broad peaks attributable to and Q3 species. There is no evidence of monomer or end groups/dimers consistent with a more extensive state of hydrolysis and condensation promoted by the additional acid-catalyzed hydrolysis step. The spectrum of an AAB sol (dilut 5 1) three hours after the third (base-catalyzed) hydrolysis step shows and (J species present in the proportions... 

Table n. Refractive indices and vol% porosities for films dip-coated from B2 sols and AAB sols (diluted S l) as a function of aging time (or aging time normalized by the gelation time). Mass fractal dimension values are for sols aged for comparable norooaliz aging times. 

AAB sols were prepared by a three-step process involving acid-catalyzed hydrolysis of aged stock solution (r = 2.5) followed by a base-catdyzed hydrolysis step (r s= 3.7). The final hydrolysis ratio, pH, and silicon concentration were identical to those of B2 sols. Comparing the NMR spectra of the stock solution and the AAB sol, we observed that the effect of the second (acid-catalyzed) hydrolysis step was to promote extensive hydrolysis and condensation. Monomer and species are consumed to produce primarily a variety of and species. Various cyclic tetramers are prominent sol species after the second hyckolysis step as was evident from a strong q2 resonance observed near -95 ppm. 

Comparing the results of the A2 and AAB series of films, we observe what appears to be conflicting behavior, viz. A2 sols exhibiting D = 1.7 yield rather dense films consistent with mutual transparency, whereas AAB sols exhibiting D = 1.7 yield porous films consistent with mutual opacity. This apparent inconsistency is... 

The values of the mass fractal dimension (obtained from SAXS [16]) show how the structure of the silicate polymers in the sol varies with the different routes these sols take to the same final dilution, H20/Si ratio and pH. The mass fractal dilnensions are considerably different. We would expect AAB, which is right at the borderline of D = 1.5 for Equation 2, to exhibit different behavior than B2 with D = 2.3. However, Table I shows that both sols exhibit an increase in porosity with sol age (once the pore-plugging species are burned out of B2). The reason for this is the tradeoff between cluster porosity and cluster interpenetration. B2 clusters are less porous but pack less efficiently than AAB clusters. This tradeoff is illustrated schematically in Figure 1. 

Heating the films may be a simple way to remove the salt. Additional XRD studies showed that washing an AAB gel with ethanol eliminates the salt, so washing the films might also work. It would be better, however, if we found the irfini-mum catalyst level that eliminates the small pore-plugging species. A new sol with less catalyst might also have D values that fall between those of B2 and AAB, and an intermediate value of D should produce more porous as-deposited films. 

We prepared a series of sols from the stock solution with various fractions of the total moles of HCI in AAB. The base-catalyzed step was adjusted to produce a final pH of 5.5 for each sol. For sols with [HCI] = 1/4 or more of the [HCI] in AAB, XRD detected the presence of NH4CI in the dried gels. For sols with [HCI] = 1/10 of the [HCI] in AAB, the XRD was amorphous, the NMR showed mainly Q24 cyclics, and sols at pH 5.5 did not gel after 24 days at 50°C. For sols with [HCI] = 1/5 of the original AAB, the XRD was amorphous, and the sols gelled in less than a week. After the second acid-catalyzed step, which was refluxed for 1 h, the NMR showed 17% Ql, 62% Q2 and 21% Q3, and the Q1 species were more fully hydrolyzed than in B2 sols. After the base-catalyzed step, AAB(l/5) had 24% Q2, 49% Q3 and 27% Q4 species. 

Figure 3. NMR spectra of (a) an AAB( 1/5) sol, (b) a sol with identical composition made by reversing the order of the second and third steps, and (c) a sol with identical composition made all in one step. The sols with differences in the timing and order of steps were refluxed for 15 h before the spectra were taken.

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