2.1. Thermal Behavior Study
The TG/DSC curves of pure SiO2
(S) and of the SiO2
/PCL hybrids (SP6, SP12, SP24 and SP50) have been reported in Figure 1
Initial and final temperatures of each process accompanied by a mass loss have been more clearly identified by the first-order derivative curves of TG (DTG) curves, displayed in Figure 2
The TG/DSC curves of all the materials in Figure 1
showed an initial mass loss (corresponding to the first DTG peak) accompanied by an endothermic DSC peak, ascribed to the simultaneous loss of water and alcohol up to 140 °C, except for SP50 (short dotted lines) for which the process ends at around 85 °C. It is clearly evident from Figure 2
(low-temperature region) that the SP hybrid materials, except for SP24 and SP50, show the same thermal behavior as pure S. At temperature higher than 180 °C, dehydration is completed and S undergoes dehydroxylation, elimination of water due to condensation of the hydroxyl surface groups, with a slow and quite constant mass loss rate (linear portion of the TG curve up to 600 °C not detectable by the DTG curve), as found in previous studies [19
]. SP materials (except for SP24 and SP50) show the same thermal behavior up to 300–400 °C, while at higher temperatures, a one- or two-step process took place up to 580–600 °C. This process is accompanied by an endothermic effect, and the intensity of the corresponding DSC peak was found to increase with the amount of PCL in the material, while the degradation temperature shifts towards lower values with an increase in the PCL content.
Similar to what has been observed in a previous study [40
], this process is attributable to the thermal degradation of PLC, which usually takes place in two steps of mass loss. On the other hand, the thermal behavior of the PCL-richer materials (SP24 and SP50) is remarkably different from those of the other SP materials. When dehydration is completed at about 85 °C, SP50 undergoes a two-step process up to 260 °C, which can probably be ascribed to dehydroxylation, followed by the two-step thermal degradation of PLC between 300 and 600 °C. The DSC curve recorded two endothermic effects, expressed by two partially convoluted broad peaks: the first one intense up to 455 °C, followed by a second that is a shoulder.
2.2. FTIR Evolution Gas Analysis to Provide a Mechanistic Interpretation of the Thermally Stimulated Processes
Vertical bars displayed in Figure 2
, close to the DTG peak temperatures where the reaction reaches the maximum rates, represent the temperatures at which the gas or gaseous mixture evolved from TG experiments was collected and sent to the FTIR device. The FTIR spectra of the mixtures collected from the TG/DSC experiments of all materials tested are shown in Figure 3
A confirmation of the mechanisms hypothesized was found by analyzing the FTIR spectrum of the gases evolved from the samples S during the TG experiment at low temperature (77 °C), showing the typical signals of water. Sharp peaks in the wavenumber regions 4000–3400 cm−1
and 2000–1200 cm−1
are visible due to the H–O–H stretching and bending vibrations. Moreover, the weak peak at about 1040 cm−1
suggests that ethanol [41
], used as solvent in the synthesis process and also formed by the hydrolysis reaction that involves the alkoxide precursor tetraethyl orthosilicate (TEOS), is also released in this temperature range from the material in which it was previously embedded in the gel form. A higher release of ethanol was detected in SP6, revealed by the presence of the C–H stretching at 2955 cm−1
, as well as by the peaks related to the C–C and C–O bonds at 1373, 1249, 1040 and 875 cm−1
. This can be explained by a decrease of hydrolysis degree and condensation rate caused by the interaction of the –OH groups of the forming inorganic network with the polymer chains in the sol. Therefore, the –OH groups involved in the H–bonds with the C=O of the PCL [26
] cannot react with other alkoxide precursors or other oligomers. As a consequence, a higher content of residual ethoxy group is retained in the gel. Moreover, the presence of water and CO2
(duplet at 2345–2300 cm−1
]) is also observed.
Similarly, the amount of ethanol and water released even at low temperature in PCL-rich OIHs (SP12, SP24 and SP50) is higher, due to the higher amount of PCL and, thus, to the higher amount of –OH bonded with it. Moreover, the higher amount of ethanol leads to the formation of a higher amount of CO2. The FTIR spectra of pure S and SP6 at 280 and 281 °C, respectively, show that a decrease of the bands attributed to water and ethanol, as well as the development of CO2, were observed. SP12 revealed a similar thermal behavior (with respect to those of S and SP6) at low temperatures (69.5 and 217 °C).
At higher temperatures (513 °C for SP6), ethanol is completely degraded, thus leading to the formation of ethylene (as proved by the bands in the following regions: 3300–2900 and 1430 cm−1, as well as the sharp band at 950 cm−1), CO2 and a low amount of CO. The higher amount of ethylene produced from the SP6 sample compared to that of S is due to the higher initial amount of ethanol developed from sample SP6.
FTIR spectra of SP12 at 411 °C showed new bands at 2940, 1770, 1150 and 1050 cm−1
, which can be ascribable to the formation of caproic acid and ε-caprolactone, both of which are produced from the thermal degradation of PCL, as affirmed by Persenaire and co-workers [40
]. Moreover, the bands of CO2
, CO and the sharp one of ethylene are also visible, even if with low intensity.
SP24 and SP50 showed the same thermal behavior as SP12, but the presence of 5-hexenoic acid in the FTIR spectrum at 410 °C is more evident in the former, while that at 495 °C showed the least decrease; and in the gas phase, ε-caprolactone is mainly present. By increasing the temperature, the band at 3570 cm−1
, present only in the spectrum of 5-hexenoic acid, decreases. This finding is in agreement with the mechanism of degradation of PCL reported in the literature [40
], which is reported to occur in two steps: in the first, the rupture of polyester chains via ester pyrolysis reactions is involved, leading to the formation of 5-hexenoic acid, H2
and a low amount of CO. The second step is attributed to the formation of ε-caprolactone by an unzipping depolymerisation process. Therefore, the intensity of CO2
and CO signals is higher in the spectra of those samples compared to those of S and SP6, because when the PCL degrades, CO2
and CO also are produced [40
Therefore, the obtained results suggest that in order to obtain OIHs free of internal toxic residual solvents, the materials should be heated at 400 °C.
2.3. XRD Analysis to Provide a Mechanistic Interpretation of the Thermally Stimulated Processes
shows the XRD spectra of both S and SP50 after their thermal treatment at 450 and 600 °C (plots (a
) and (b
), respectively). They are all practically amorphous, and only the broad characteristic peak of silica between 15 and 35° is observed [43
Furthermore, the S and SP materials are revealed to be amorphous, even after their treatment at 1000 °C, as is clearly evident from the XRD spectra in Figure 5
SP50 shows an initial crystalline structure (that of β-cristobalite, a high-temperature stable polymorph of silica). Crystallization of β-cristobalite seems to occur more evidently (especially in the case of SP50) in all the materials (even in pure S) after their treatment at 1200 °C, as the XRD spectra in Figure 5
b show clearly. This result partly confirmed what was obtained in a previous study [44
], where amorphous silica crystallized in cristobalite at 1000 °C due to a local rearrangement of the amorphous material (similar to β-cristobalite). The explanation for this is that the instantaneous local atomic arrangement of amorphous SiO2
is similar to that of β-cristobalite [45
]. Usually, a phase change transformation from quartz to β-cristobalite only takes place when the temperature is about 1470 °C [38
]. Then, at high temperature, it is easier to observe the crystallization of amorphous SiO2
into β-cristobalite than the phase change from quartz.