Temperature-dependent transparency and structural changes in the hydrogels

The following figures include VT 1H MAS NMR spectra and full-width half maximum values of the 1H resonances. Images are obtained of the hydrogels after annealing at 100°C for 4 hours.

Figure 5.1 (a). Variable temperature 1H MAS NMR spectra of CS2 hydrogel acquired at an MAS rate of 10 kHz (b). Temperature dependence on FWHM values for the water protons and strongly hydrogen bonded silanol protons and (c). Image of the CS2 hydrogel at 100°C. The hydrogel was subjected to heating at 100°C for 4 hours.

Figure 5.1 (a). and (b). shows the change in 1H resonances and line widths respectively as a function of temperature for the CS2 hydrogel. The resonances for the water and silianol protons become narrow with increasing temperature, which is shown by a general decrease in FWHM values. The

1

H MAS NMR spectra show no additional resonances but a reduction in FWHM values as a function of temperature, indicating an increase in molecular motions. Figure 5.1 (c). Indicates that CS2 hydrogel remains transparent from room temperature up to 100°C.

Figure 5.2 Variable temperature 1H MAS NMR spectra of the CS2G hydrogel acquired at an MAS rate of 10 kHz (b). Temperature dependence on FWHM values for the water protons and glycerol protons and (c). Image of the CS2G hydrogel at 100°C. The hydrogel was subjected to 100°C heating for 4 hours.

In the glycerol hydrogel, the 1H MAS NMR spectra show that two components at ca. 6.7 and 4.0 ppm can be resolved at temperatures above 30°C. Figure 5.2 (b). shows a decrease in line-widths for water and glycerol protons with increasing temperature. This indicates a rise in molecular motion and, hence, a reduction in the strength of 1H-1H dipolar couplings. Indeed, at 100°C, the broad resonance at 4.0 ppm shows fine structure corresponding to the different proton environments in the glycerol molecule.

The hydrogel remains transparent from room temperature to 100°C which is shown in Figure 5.2 (c).

Figure 5.3 Variable temperature 1H MAS NMR spectra of the CS2EG hydrogel acquired at an MAS rate of 10 kHz; (b) Temperature dependence on FWHM values for the water protons, resonance at 4.8 ppm and –CH2-

protons in ethylene glycerol; (c) Image of the CS2EG hydrogel at 100°C. The hydrogel was subjected to 100°C heating for 4 hours.

Figure 5.3 (a). and (b). shows the 1H MAS NMR spectra and FWHM values for the CS2EG hydrogel as a function of temperature. There are significant changes in 1H spectra, FWHM values and optical properties for the CS2EG hydrogel: From 30 to 40°C onwards the –CH2- protons in

ethylene glycol manifest as a sharp resonance at 3.7 ppm whose FWHM values decrease from 630 to20 Hz indicating rapid molecular motion. This is also evidenced from the VT T1 relaxation measurement in the previous

chapter. We showed that the –CH2- protons are in the “fast regime” opposed

to the “slow regime” describing the mobility of water.

Figure 5.4 Variable temperature 1H MAS NMR spectra of the CS2EG hydrogel acquired at an MAS rate of 10 kHz

Figure 5.4 shows the 1H MAS NMR spectrum of the CS2EG hydrogel in more detail. There is an increase in signal intensity of the 4.8 ppm resonance as the 6.9 ppm resonance decreases as a function of temperature, on this basis, the resonance at 4.8 ppm is assigned to a second water domain, H2O(II). This is consistent with the results of Jeong et al.366

who investigated poly(vinyl butyral) (PVB) gels with different wt % water loadings using variable temperature 1H MAS NMR and gravimetric methods. They observe sharp resonances between 5.0 and 4.9 ppm assigned to different “free” water domains. These domains are consistent with the resonance at 4.8 ppm. The resonance is apparent above 30°C and is associated with the ethylene glycol molecules resulting in an ethylene glycol/H2O(II) component separated from the bulk water/silica phase.

Therefore, the change in optical properties of the hydrogel could be explained by phase separation and is discussed later in this chapter. The

process is reversible; when the hydrogel is cooled to room temperature transparency is restored. To investigate this hypothesis further, VT 1H-13C HETCOR experiments were used to clarify the polyol/silica proximity at room temperature and at 50°C.

Figure 5.5 1H-13C HETCOR NMR spectra of (top) CS2EG and (bottom) CS2G acquired at room temperature. The f1 projection shows the 1H indirect

The HETCOR spectra is a 2D contour plot that correlates 1H to 13C distances of ~ 0.3 nm367 through dipolar couplings. The spectra in Figure 5.5 consist of 1H (f1 dimension) and 13C (f2 dimension) resonances; those of which are in

close proximity to one another are represented by the blue cross peaks. In the f1 dimension, the proton resonances in both the CS2EG and CS2G

hydrogels are similar to those seen in Chapter 4, Figure 4.14, however, the

≡Si-OH···O environment at 15 ppm can also be resolved. In both hydrogels the polyol and water protons show strong correlations to ethylene glycol and glycerol carbons. The ≡Si-OH···O environment associated with silicon Q2, 3 sites also correlate to the polyols. This result confirms that a proportion of the polyols are located near the silica units at room temperature.

Figure 5.6 1H-13C HETCOR NMR spectra of (top) CS2EG and (bottom) CS2G acquired at 50°C.

Figure 5.6 (top) shows the HETCOR spectrum of the CS2EG hydrogel acquired at 50°C. The correlation between the polyol and water protons to the ethylene glycol carbons is present, but the ≡Si-OH···O to ethylene glycol interaction has disappeared. The HETCOR experiment relies on the presence of through space 1H–13C heteronuclear dipolar interactions and the strength of the correlation is, therefore, dependent upon distance and

mobility of the 1H-13C spin pair (see Chapter 2). Strong correlations are seen for short 1H-13C distances and slow mobility. Weak correlations are observed for longer distances and faster mobility. Indeed, as temperature is increased the ethylene glycol molecules become mobile and weaken the 1H-13C interaction but this alone does not account for the loss of correlation. There is also a physical separation between the polyol and the silica units. Conversely, a correlation between glycerol and the ≡Si-OH···O environment remains.