A Study of the Degradation Mechanism of Ladder-like Polyhedral Oligomeric Silsesquioxane via Fourier Transform Infrared Spectroscopy

Abstract

As a result of global warming, fire outbreaks are becoming a common occurrence. There is, therefore, the need for an effective, low-cost and environmentally friendly fire-retardant material. Amine-terminated polyhedral oligomeric silsesquioxane, ATL-POSS, is a low-cost, water-soluble, fire-retardant material based on aminosilane coupling agents. Because of its solubility in water, it can serve as a general-purpose fire retardant. The ATL-POSS nanoparticles reported in this paper have high char retentions of about 75 and 54% in nitrogen and air atmospheres, respectively. Differential scanning calorimetry (DSC) was used to determine the phase transition temperatures. It was shown that ATL-POSS is an amorphous material. The thermal stability and rate of decomposition of POSS was determined by using thermogravimetric analysis (TGA). The TGA derivative curves (DTA) show that the degradation of ladder-like POSS occurred in multiple stages and that the rate of degradation is affected by the heating rate. The mechanism of decomposition of ATL-POSS was determined by using Fourier transform infrared spectroscopy, FTIR. The FTIR technique was chosen for this study because of its accessibility and ability to distinguish ladder-like POSS from the cage-type POSS structures. The FTIR spectra showed that the -Si-O-Si- cyclic structure was the predominant structure of POSS. By analyzing the FTIR spectra of the thermally treated POSS residues, obtained at the specified test temperatures, the detailed degradation mechanism of POSS was inferred. It was shown that the terminal silanol group was degraded at test temperatures below 400 °C. Silica was shown to be the final product of the pyrolysis of POSS. The presence of the FTIR transmission peaks at 1000 and 1100 cm⁻¹, due to asymmetric vertical and horizontal stretching vibrations of the Si-O-Si, respectively, was the key evidence used to infer the ladder-like structure of POSS.

1. Introduction

Polyhedral oligomeric silsesquioxanes (POSSs) are one of the smallest silica nanoparticles. The presence of ordered ladder-like or cage conformations in their structure makes POSSs more thermally stable than a linear polysiloxane. Reinforcement with nanoparticles is one of the strategies used to enhance the performance of polymeric materials. As a reinforcing filler, POSS can increase the thermal stability and corrosion resistance of the polymer matrix. Polymers are essential materials in the electronics, aerospace and construction industries. However, the existence of the carbon-based backbone in organic polymers is considered to be one of the weaknesses of polymer structures, because of its susceptibility to fire. One frequently suggested solution to this shortcoming is the addition of thermally stable fillers.

Silicone-based materials are well-known for their thermal stability. They are regarded as an environmentally friendly class of flame-retardant materials for carbon-based polymeric materials [1,2,3,4,5,6]. The favorable heat of degradation of silicone-based materials originates at the atomistic level from the superior bond strength of the silicon–oxygen (Si-O) bond when compared to that of the carbon–carbon (C-C) bond. The dissociation energy of Si-O bonds is 108 kcal/mol, while that for C-C and carbon–oxygen (C-O) bonds are 85.2 and 82.6 kcal/mol, respectively [7]. The formation of silica (SiO₂), which is the major product of the thermal degradation of silicone, involves a massive consumption of oxygen. This phenomenon is also referred to as the oxygen sink, which enables the protection of the polymer matrix by suppressing the cleavage of the carbon–carbon backbone [8]. A similar mechanism is also observed in other silicon bonds, including Si-Si and Si-N bonds. These types of silicon bonds can consume more oxygen during thermal degradation.

The variability in the structures of silicone is partially responsible for their thermal behavior. For instance, the presence of silanol-terminated groups might result in a unique unzipping mechanism for polysiloxanes [9]. Generally, an inter- or intra-molecular structure redistribution usually occurs between siloxane backbones, resulting in the formation of cyclic oligomers and silica. This mechanism is also known as a random scission reaction. The high-temperature treatment of POSS produces thermally stable residues and silica. The fraction of residues formed is dependent on the heating conditions [7,10]. The possible pathways for the thermal oxidation of silicone-based materials are shown in Figure 1. Figure 1 suggests that intermediate materials such as cyclic and oligomeric siloxanes are amongst the products of the thermal decomposition of silicon-based materials. The final product of decomposition is the stable silicon dioxide (silica).

The synthesis and utilization of nanoparticles are of interest to the scientific community and are frequently reported in materials engineering journals. Nanomaterials such as organic–inorganic composites are important components for designing novel high-performance materials systems. The combination of a highly rigid inorganic filler and a flexible polymer matrix results in a synergistic enhancement in thermal stability [11,12,13,14,15]. Polyhedral oligomeric silsesquioxanes (POSSs), with the empirical formula of RSiO_1.5 (where R can be a hydrogen atom, alkyl, alkylene or acrylate), were first reported by Scott in 1946 [16]. They are regarded as some of the smallest silica nanoparticles, with diameters of 1–3 nm [17,18]. There are two structural forms of silsesquioxanes, including the ladder-like and cube-like structures (Figure 2). Compared to the traditional silicone-based materials, POSS has a higher thermal stability and good corrosion-inhibiting properties, and it is incorporated into an organic polymer matrix material to form nanocomposites [19,20,21,22,23,24,25,26,27,28,29,30,31,32,33]. POSS-based copolymers have been shown to enhance mechanical and thermal properties.

In this paper, amine-terminated ladder-like POSS (ATL-POSS) is synthesized from a commercially available aminosilane, 3-aminopropyltrimethoxysilane (APTMS). APTMS was initially hydrolyzed in an acidified solution. Then, the hydrolyzed silanol was heated in an organic base to form the ladder-like POSS structure via a condensation reaction. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to determine the thermal properties of POSS and its residue. The activation energy for the decomposition of POSS was calculated by using the Kissinger–Akahira–Sunose equation (KAS) (Equation (1)). For this purpose, the TGA was performed at different heating rates. Fourier transform infrared spectroscopy (FTIR) was used to determine the chemical structure of POSS and the residues resulting from the thermal degradation of POSS. The detailed degradation mechanism of amine-terminated ladder-like POSS was obtained by combining the results of the analysis of the TGA curves with the analysis of the FTIR spectra of the residues.

$$ln\frac{\beta }{T^2}=\ln \left(\frac{AR}{E_{a}g\left(\alpha \right)}\right)-\frac{E_a}{RT}$$

(1)

where α is the degree of conversion, β is the heating rate, g(α) is the integral conversion function, A is the pre-exponential factor, R is the gas constant and E_a is the apparent activation energy.

2. Materials and Methods

Materials: 3-aminopropyltrimethoxysilane (APTMS) was purchased from Gelest; toluene and triethyl amine (TEA) were purchased from Sigma-Aldrich. These chemicals were used as received. Hydrochloric acid was purchased from Fisher Scientific and diluted with deionized water. The concentration of the hydrochloric acid solution was maintained at 0.1 M.

Synthesis: APTMS (0.02 mol) was dissolved in 20 mL of toluene at room temperature. 25 mL of the HCl solution (0.1 M) was added in a drop-wise manner into the APTMS solution under stirring. The hydrolysis of APTMS was carried out for 24 h at 100 °C in an oil bath. Then, 0.1 g of TEA was added to the reactor after 24 h of hydrolysis. The temperature of the oil bath was raised to 120 °C and a condenser was used for refluxing. The reaction was carried out for another 72 h. The reaction scheme is shown in Scheme 1. The reaction mixture was separated into two layers. The organic layer was crystal clear, while the aqueous phase was cloudy. The aqueous layer was separated by means of a separating funnel and washed thrice in toluene to remove any remaining silane. The resulting liquid sample was dried at 120 °C for 14 h (overnight) under vacuum conditions. The final product was a clear transparent solid.

Characterization: A differential scanning calorimeter (DSC) (TA Q20), was used to observe the phase transition of POSS. Using the TA Q20 DSC model, three ramping sequences were carried out. The sample temperature was raised up to 100 °C, 300 °C and 700 °C, respectively, in three cycles with a ramping rate of 10 °C/min, and then cooled down to 80 °C at the same rate. A thermogravimetric analyzer (TGA) (TA Q50) was the major characterization tool for performing the degradation study of ladder-like POSS. During the TGA run, the samples were heated from room temperature (~25 °C) to 800 °C at 5, 10, 15, 20, 25, 30, 40 and 50 °C/min, respectively. Because TGA is performed up to the degradation temperature of a material of ≥500 °C, a heating rate of 30 °C/min is considered adequate. It is unusual to perform TGA at a heating rate lower than 5 °C/min. For the determination of the activation energy of decomposition, it is the practice to perform TGA at four or more heating rates. A higher heating rate is often chosen, to mimic the fire burning rate. The TGA study was performed in an inert nitrogen atmosphere as well as in an oxidizing air atmosphere. A Fourier transform infrared spectrometer (FTIR) (Nicolet 6700), equipped with an attenuated surface reflectance (ATR) module, was used to determine the functional group and chemical structure of the freshly prepared POSS as well as the residues obtained after thermal testing. The samples were heated up to the target temperatures of 100, 150, 200, 250, 300, 350, 400, 450, 500 and 600 °C, respectively, at a heating rate of 30 °C/min, and the final testing temperature was maintained for an additional 30 min.

3. Results and Discussion

General thermal behavior study

The comparison of the TGA traces of ATL-POSS obtained in nitrogen and air atmospheres is shown in Figure 3. Both curves indicate excellent thermal stability for the samples at a heating rate of 10 °C/min. The char yield of the ATL-POSS is about 74% in nitrogen and 54% in an air atmosphere. In comparison, a linear amine-terminated PDMS had a char retention of about 4.3 and 10.5% in nitrogen and air atmospheres, respectively [34]. The significant increase in char yield of ATL-POSS over the linear amine-terminated polysiloxane is indicative of the superior thermal stability of the ladder-like conformation. The weight loss in the early stage (peak I) of heating (T < 150 °C) is believed to be mainly due to the condensation of unreacted silanol and the evaporation of solvent residue. This assertion is supported by the presence of Si-OH in the IR spectrum of the low-temperature-treated samples. In Figure 4, the derivative TGA curves (DTA) of ATL-POSS are shown. There are three major derivative peaks labelled as peaks II, III and IV located in the temperature range of 300 °C to 600 °C. For the tests carried out in air atmosphere, DTA peaks II and III are slightly shifted towards higher temperatures, while peak IV is very intense and shifted slightly towards lower temperatures. As shown in Figure 4, there is a noticeable increase in the peak height for the samples tested in an oxygen atmosphere. Under inert nitrogen conditions, cyclic siloxanes are formed through an intramolecular reaction, resulting in structural redistribution. The presence of oxygen can catalyze this process of self-assemblage.

For a given testing atmosphere, the degradation process varies with the heating rate. Accordingly, the DTA peaks are shifted to higher temperatures with increasing heating rates in both nitrogen and air atmospheres. Because of the changing degradation rate, the final char yields were affected by the heating rate, as shown in Figure 5. Based on the integration of the DTA peaks, the degradation rate at the different degradation stages were calculated and plotted as a function of the heating rate (Figure 6 and Figure 7). The plots of the degradation rate against the heating rate show that increasing the heating rate resulted in an increased degradation rate. This is especially true for decomposition stage II. The activation energies for decomposition, obtained from using the KAS model, are listed in

Table 1. Activation energies for ATL-POSS based on the KAS model.

Activation Energy/(kJ/mol)	Peak II	Peak III	Peak IV
Nitrogen	100	82	223
Air	94	142	92

and based on the following heating rates: 5, 10, 15, 20, 25 and 30 °C/min. The plots of ln(β/T²) vs. 1/T that were used to determine the activation energy are shown in Figure 8 and Figure 9. The activation energy (E_a) for decomposition is obtained from the slope, −Ea/R, of the ln(β/T²) vs. 1/T curve (where R is the gas constant).

The DSC thermogram is compared with the TGA derivative curve (DTA), as shown in Figure 10. The absence of a phase transition is indicative of the amorphous nature of ATL-POSS. For the DSC tests carried out up to 300 °C, a peak located at around 200 °C is associated with the early stage of decomposition. For the samples heated up to 700 °C, a large exothermic peak is observed between 400 and 700 °C, in same region where the DTA curves showed intense and strong peaks. The exothermic peak shown around 400 °C is suggestive of the redistribution of the Si-O cyclic structure, and the endothermic peaks located between 400 and 600 °C may be ascribed to the heat of decomposition of ATL-POSS.

FTIR analysis: FTIR is a powerful tool for chemical structure determination, especially for organic insoluble solids. By analyzing the spectrum of ATL-POSS and the residue remaining after thermal treatment, it is possible to deduce the detailed chemical structural changes that occurred. By analyzing the combined DTA curves and the respective FTIR spectra, the thermal degradation mechanism of ATL-POSS was inferred.

The FTIR spectra of ATL-POSS is shown in Figure 11. The FTIR transmittance peaks shown around 1000 cm⁻¹ and 1100 cm⁻¹ are due to asymmetric vertical and horizontal stretching vibration peaks of the Si-O-Si functional group [35]. The presence of these two FTIR transmittance peaks is indicative of the ladder-like structure. The other characteristic FTIR transmittance peaks, shown in Figure 11, are the 3200 cm⁻¹ and 1600 cm⁻¹ peaks due to the terminal amine functional group. The transmittance peak around 900 cm⁻¹ is due to the terminal Si-OH functional group. The FTIR transmittance peaks in the wavenumber range of 2800–3000 cm⁻¹ are due to the Si-C-H asymmetric stretching vibration.

As was discussed in the previous section, the thermal behavior of ATL-POSS is affected by the atmosphere, whether oxidizing or inert, under which the test is carried out. Figure 12 and Figure 13 show the FTIR spectra of ATL-POSS that were thermally treated at various temperatures ranging from 100 to 600 °C under a nitrogen atmosphere (Figure 12) and an air atmosphere (Figure 13). As shown in Figure 12 and Figure 13, the peak intensity for the -Si-C, -NH₂ and -Si-O-Si functional groups decreased with the increasing testing temperature due to their decomposition and the volatilization of the gaseous residues.

There is one common trend that is observed in the change in the intensity of the FTIR peaks: that is, the disappearance of the transmittance peaks due to Si-OH (~900 cm⁻¹), occurring at temperatures >150 °C and ≤300 °C (Figure 14). The condensation of silanol (Si-OH) seems to be nearly completed at about 250 °C in both nitrogen and air atmospheres. The condensation of Si-OH is correlative to the slight weight loss preceding the major weight loss. Another early decomposition process is the cleavage of the alkyl side group bonded to ATL-POSS and marked by the sharp decrease in the intensity of the FTIR transmittance peaks around 2900 cm⁻¹ (Figure 12 and Figure 13). The intensity of the Si-C-H functional group transmittance at 2900 cm⁻¹ decreased with the increasing testing temperature and was reduced to zero at 300 °C and 400 °C in air and nitrogen atmospheres, respectively. The presence of oxygen accelerates this process and results in the formation of a minor DTA peak at 250 °C. This peak is also observed in the DTA peak for the samples tested at a low heating rate of <20 °C/min. A weak transmittance peak is also observed between 940 and 950 cm⁻¹ due to C-C bond vibration. This peak is attributed to the formation of an intermediate cyclic and oligomeric structure, and is observed in the temperature range between 400 and 500 °C in the air atmosphere (Figure 15). The FTIR transmittance peak for this structural group is observed at relatively higher testing temperatures in the nitrogen atmosphere. The occurrence of this peak can be attributed to the decomposition of the alkyl pendant group in POSS. Part of the alkyl pendant group is volatilized, while the others are trapped in the silicone cyclic structure. The amine terminal group is also decomposed in the same temperature range, as shown through the disappearance of the amine transmittance peak at 3200 cm⁻¹. The presence of a peak at around 1600 cm⁻¹ (Figure 16) also confirms the presence of the primary amine structure in the low-temperature-treated samples. This peak shifted to higher wavenumber at high testing temperatures, indicating the transformation of primary amines to C=N structures, which is one of the products of the oligomeric volatiles.

It is shown that, for the tests performed under a nitrogen atmosphere, the ladder-like structures change into a complex Si-O_X-Si structure similar to (-Si(OH)-O-Si-), resulting in the first DTA peak. The existence of such a structure is corroborated by the presence of a broad FTIR transmittance peak in the wavenumber range between 3680 and 2900 cm⁻¹. This structure is formed at high temperatures up to about 500 °C in a nitrogen atmosphere, and eventually decomposes into silica. The shift of the Si-O-Si peak from 990 to 1017 cm⁻¹ and from ~746 to 790 cm⁻¹ (Figure 17) suggests that the structural redistribution or re-assemblage occurred in this temperature range. The FTIR transmittance peak shifts to higher wavenumbers, mainly occurring in the temperature range of 250–450 °C. As the testing temperatures reach about 400 °C, an intermolecular reconfiguration seems to occur, resulting in the formation of an -OSi-C_n-SiO- structure, resulting in the second DTA peak at about 500 °C.

A similar phenomenon occurred in samples tested under an oxidizing air atmosphere. However, the presence of oxygen catalyzed the reactions. For instance, the FTIR transmittance peak shift for Si-O-Si from 746 cm⁻¹ to higher wavenumbers occurred earlier, in the temperature range from 200 to 250 °C, compared to samples tested in a nitrogen atmosphere. An important factor that affects the location and intensity of the second DTA peak is whether oxidizing oxygen is used or not during pyrolysis. The results obtained from the activation energy calculation using the KAS model indicated that the activation energy for forming -OSi-C_n-SiO- structures is the highest in all the tests carried out under an air atmosphere. However, the decomposition of the -OSi-C_n-SiO- structure requires the highest energy in a nitrogen atmosphere. This observed behavior is in accordance with the oxygen sink fire-retardant model that requires a reaction with oxygen to form a compound that is thermally stable.

4. Conclusions

Amine-terminated ladder-like POSS was synthesized via the condensation of hydrolyzed aminopropylsilane. The thermal behavior of ATL-POSS was studied through TGA and DSC. The DSC analysis confirmed the absence of any phase transition. The TGA results indicated that the char yield of ATL-POSS is much higher than that for linear amine-terminated PDMS. The thermal degradation rate of ATL-POSS is significantly affected by the heating rate. With the aid of FTIR analysis of the resulting ATL-POSS residue, details on the nature of the thermal decomposition of POSS were determined. The condensation of terminal silanol units mainly occurs at about 100 °C, resulting in the formation of the first DTA peak. The scission of side chains is completed at about 350 °C in nitrogen and 300 °C in an air atmosphere, respectively. The scission of the ladder-like backbone structure starts at around 250 °C in a nitrogen atmosphere and 200 °C in an air atmosphere. Once the testing temperature reaches 400 °C, the reorganization and re-assembly of the ATL-POSS structure occurred. The formation of a temporary intermediate structure was inferred from the FTIR spectral analysis. Finally, the ATL-POSS structure was fully decomposed into a silica-like structure at a testing temperature T ≥ 600 °C. The possibility of forming SiC-SiO₂ and SiC-SiO₂-N= structures exist at temperatures above 700 °C.