Extending the q-region in the scattering experiments: The pure silica network

The formation of a gel, derived from an ethylene glycol modified silane, was followed for a composition of EGMS(Si)/P123/1 M HCl = 48(4.7)/30/70 in the low q-region (0.018<q<0.6 nm^-1) at ID02/ESRF. Macroscopically, the system gelled after approximately 65 min. Phase separation, indicated by a colour change to white, was not observed for another 150 min. After aging for one week at 40 °C the gel exhibits three distinct Bragg reflections, which can be indexed according to (10), (20) and (30) and a (11) reflection that is very weak (Figure 3.4.17 (a)). The repeating unit distance, d, of the aged gel equals 11.74 nm.

3.4 IN-SITU SYNCHROTRON SAXS/XRD STUDY

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(30) (20) (10)

dried gel; surfactant removed by tmcs

log Intensity / a.u.

log q / nm^-1 wet gel;

after 1 week of aging

2 μm

SEM

2 μm 2 μm 2 μm

SEM

100 nm

TEM

100 nm 100 nm 100 nm

TEM

(a) (b)

Figure 3.4.17. (a) Small angle X-ray scattering profiles for the dilute EGMS/P123/1 M HCl system after one week of aging and after surface silylation (tms) and drying. For the latter, only one broad reflection could be observed. (b) SEM images show the macroporous network of the dried gel. TEM images reveal regions with highly ordered periodic mesopores but also regions with unordered structure.

After treatment with trimethylchlorosilane, washing with petroleum ether, ethanol and slow heating, the long range order seems nearly destroyed, as can be seen in the SAXS curve of the dried gel (Figure 3.4.17 (a) upper curve). Only one weak peak corresponding to a repeating unit distance of 10.47 nm is visible. However, TEM investigations have shown that the material is inhomogeneous with small domains and regions with less pronounced ordering.

Time-resolved small angle X-ray profiles of the synthesis are depicted in Figure 3.4.18.

The evolution of the porous network was followed during a time interval of 7 min up to 140 min after mixing the components, thus stopping prior to phase separation as well as long range order-formation. The broad peak(s) dominating the scattering pattern at the beginning of the measurement were fitted with two Gaussian functions, one for short range order and one for the slowly evolving long range order. The peak resulting in the first reflection of the periodic mesostructure is plotted in Figure 3.4.19 III. The fit data of the latter suggests an increase in intensity 65 to 75 min after homogenisation. This is in accordance with the gelation time of approximately 65 min. Parallel to the increase in intensity, a decrease in d-spacing and FWHM was observed as shown in Figure 3.4.19.

3.4 IN-SITU SYNCHROTRON SAXS/XRD STUDY

0,1

1 10 100

Time / m in

log q / nm^-1

log Intensity / a.u.

(10) Peak at low q

Particles

Mesostructure (10)

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1 10 100

Time / m in

log q / nm^-1

log Intensity / a.u.

(10) Peak at low q

Particles

Mesostructure (10)

Figure 3.4.18. Time-resolved small angle X-ray scattering measurement of an EGMS/P123/1 M HCl system.

Besides the evolution of a periodic mesostructure, again the formation of an intermediate phase is observed.

In addition, a peak, respectively, a shoulder starts to evolve at low q, which shifts to even lower values of q throughout the synthesis (and leaves the investigated q-range).

Besides the formation of a periodic mesostructure, again an evolution of disordered objects was found in the SAXS-profiles, equivalent to the previously described intermediate phase in case of the phenylene-bridged precursor. That is, the first twenty minutes of synthesis can be described by a Porod regime and two independent overlapping broad peaks. The fit results depicted in Figure 3.4.19 I show the trend of the evolution of the Porod amplitude over time. The point of gelation is indicated with a dotted line. Parallel to gelation the contribution of large scattering objects starts to increase. After 20-25 min the evolving of a Guinier region with corresponding Porod region was observed in the scattering curves. The Guinier radius, Rg, estimated from the SAXS pattern at the starting point of the intermediate phase has a value of approximately 6 nm. The resulting fit parameters are plotted in Figure 3.4.19 II (void circles) together with exponent n of the characteristic slope (I ∝Bq⁻ⁿ; full quads). In contrast to the study of the phenylene-bridged system, the fit data does not support a maximum and subsequent dissolution of the phase. This is most likely due to the fact, that no spontaneous transition into periodically ordered cylindrical micelles had occurred at this stage of the synthesis. However, the observed shoulder is growing in intensity as well as dimension. Determination of the equivalent Porod radius, Rp, after 63 min gives a radius of 8.7 nm. After 58 minutes, an additional bump evolves at approximately q = 0.048 nm^-1. This corresponds to a periodic unit distance of ~130 nm.

3.4 IN-SITU SYNCHROTRON SAXS/XRD STUDY

Therefore, the fit of the curves measured 60 to 110 minutes after homogenization, were extended by an additional gaussian function. The fit results describing the shoulder at low values of q are displayed in Figure 3.4.19 III (void triangles). The position shifts to even lower q in the course of the experiment, leaving the observed q-range (d>300 nm) after approximately 90 min. The evolution of this maximum is a possible indication for the formation of macrostructure.

Figure 3.4.19. Fit results for the time resolved measurements of Figure 3.4.18. I: Porod amplitude (P) from the low q-region of the curves. II (o): Fit amplitude (G) and radius (Rg) of the intermediate phase derived from Eq. 2.3). II (É): Fit parameters for the characteristic slope of the intermediate phase fitted according to I∝Bqⁿ. III: Fit amplitude (A), coherence length (L) and repeating unit distance (d) of the peak resulting in the first reflection of the periodic mesostructure (É), and the peak observed at low q (ó).

The shoulder evolving at low values of q after 60 min can also be approximated as second maximum of scattering obtained by spherical particles with a radius of about 132 nm. This attempt is shown in Figure 3.4.20. The experimental curve was taken 63 min after mixing of the components. Both shoulders are clearly visible. Fitting was performed by adding

3.4 IN-SITU SYNCHROTRON SAXS/XRD STUDY

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Experimental curve (after 63 min) Contribution of spherical particles (R₁) Contribution of spherical particles (R₂)

and Porod scattering Resulting fit curve

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log q /nm^-1

0,2

two contributions of polydisperse spherical particles (Eq. 2.5 combined with Eq. 2.8) and a small Porod contribution. The assigned radii were R1=12≤1 nm and R2=132≤20 nm. The compliance between fit and measured curve is shown in Figure 3.4.20.

Figure 3.4.20. Experimental curve measured 63 minutes after mixing together with the estimated fit curve (full line). The measured curve was approximated by scattering contributions of two kinds of polydisperse spherical particles (R1=12≤1 nm, R₂=132≤20 nm) which are depicted as dotted and dashed lines.

Further in-situ SAXS investigations on the low q-region of the EGMS system are necessary to draw definitive conclusions on the differences and similarities of the formation mechanism of the purely inorganic and the hybrid network. However, as was shown in these experiments, extending the studies to the USAXS-region provides crucial information on the interplay of mesostructure and macromorphology formation.