Haziness is defined as the fraction of the transmitted light that falls outside an angle of ±2.5o from the incident beam; the fraction of light that falls within that range is defined as clarity. Haziness is due to light scattering and it has two contributors: bulk scattering and surface scattering. Bulk scattering may depend on the wavelength (e.g., refer to Rayleigh scattering) while haze due to surface scattering is independent of the wavelength. In order to remove surface scattering from being a factor, in a recent study of the haziness of TMOS-co-APTES aerogels as a function of their density (i.e., total silane concentration in the sol) we used wet gels in specially constructed molds as a proxy of the aerogels. In this study here, however, haze from surface scattering is an issue that has to be reckoned with.
Haze of all samples (Figure 1) was measured according to ASTM D1003-13 using an integrating sphere as described recently (see Experimental section). To minimize reflection losses of transparency, all samples were prepared using molds constructed with Teflon tape-coated glass plates facing and held parallel to each other with circular spacers of constant width (see Experimental). The thickness of all aerogels was 7mm. Average haze in the 400-800 nm range was calculated by integrating the primary haze vs wavelength data. and reported Average haze data for all samples are given in Table S2 of the Supporting Information. The haze of the native samples increased from 17.4% (K aerogels)
to 23.2% (0.5K). The average haze of the crosslinked samples was significantly higher, reaching 45.6%. It was noted though that the haze of the K and 0.5K aerogels was significantly higher that the haze of the correspinding wet-gels (17.4% and 23.2%, respectively). The difference could not be reconsiled by the difference in the refractive index of the pore-filling fluid in the case of wet-gels and aerogels (acetonitrile vs air, respectively). Thereby, it was concluded that the haze of aerogels had a significant contribution from surface scattering. We set out to measure the latter by adopting the analytical technique referred to as the Method of Standrad Additions. For this we prepared several native aerogel samples with variable thickness based on the 0.857K formulation, measured their average haze and extrapolated to zero thickness (Figure 8). Surface haze was given by the intercept (14.41%).
Subsequently, with the understanding that surface haze comes from surface roughness carried over from the molding surfaces (Teflon tape), or random scratches introduced from the contact of the wet-gels with the containers during agitation and solvent exchanges, we considered that value of surface haze (14.41%) as representative for all samples (native and crosslinked) and it was subtracted from all data, thus yielding the bulk haze of each 7 mm thick sample (see Table S2).
Owing to the fragility of native wet-gels, our experimental technique did not allow fabrication of aerogels less than 7 mm thick. However, for application of these aerogels as a see-through thermally insulating layer in double-pane windows, the target thickness of the aerogel layer is 3 mm. The bulk haze of those samples was calculated using Eq 1, which is in effect Beer’s Law applied to transmission loss due to bulk haze caused by Rayleigh
scattering, as is the case here. The calculated bulk average haze values for 3 mm samples are plotted vs the explanatiry variables in Figure 6C (blue surface).
As our next level of refinement, we fabricated native TMOS-co-APTES aerogels at the K, 0.75K and 0.5K formulations, each one at different thicknesses, and we calculated (Method of standard additions) the surface haze. For those samples we did not use teflon tape on the molding plates and we minimized agitation during solvent exchange. The surface haze of K, 0.75K and 0.5K were found equal to 2.48%, 4.38% and 6.27%, respectively. The upward trend of surface haze from K to 0.5K is attributed to the deterioration of mechanical proporties as density decreases. Nevertheless, haze was measured according to the procedure described in ASTM D1003-13. The aerogel sample was placed at the transmittance port of the integrating sphere and the transmittance was recorded. When light passes through the sample, it scatters due to the inhomogeneity in the sample. Two transmittances were recorded for each sample. In first case, the total light that passes through the sample termed as total transmittance of sample (T2) was recorded by
placing the spectralon reference puck on the reflectance port of the integrating sphere and collecting the spectrum. In second case, only the scattered light was captured by removing the spectralon reference puck. That transmittance value was termed as sample scatter (T4).
In order to eliminate the scattering from the instrument, similar experiments were done without placing any sample in the transmittance port of the integrating sphere with the spectralon reference puck on the reflectance port termed as instrument radiant spectra (T1)
and without the spectralon reference puck on the reflectance port termed as instrument scatter (T3). Scheme 2 shows the configuration of the integrating sphere for measuring haze
in aerogels. Using the above four transmittance values, haze (H) was calculated according to equation 1.
(1) The total transmittance decreased with increase in sample scattering as silane concentration was decreased. Minimum haze was observed for K aerogels with average haze of 15 % and increased with decrease in silane concentration and increase in degree of crosslinking. When a silica sample is crosslinked, a thin conformal coating of polymer layer occurs on the surface of particles. Therefore, the particle size of crosslinked samples increased upon crosslinking. Those particles act as scatterers and hence due to increase in scatterer size, scattering increases there by making the crosslinked samples appear hazier than the native samples. Moreover, scattering further increased with change in refractive index from silica particle to conformal polymer layer to air. Average haze for all the samples is tabulated in Table 5.
Figure 6 shows the combined plot of Young’s modulus, thermal conductivity and %Average Haze. 0.875K-4.5% N3200 was the the near optimal sample with E = 70.29 ± 2.94 MPa, λ = 22.5 ± 0.8 mW m-1 K-1, and H = 16.2%.
3. EXPERIMENTAL