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Thermal stability, mechanical, and optical properties of novel RTV silicone rubbers using octa(dimethylethoxysiloxy)-POSS as a cross-linker

  • Xing Huang , Guomin Song , Jianjun Shi , Jiafei Ren , Ruilu Guo , Chunyuan Li , Guangxin Chen , Qifang Li EMAIL logo and Zheng Zhou EMAIL logo
Published/Copyright: April 6, 2022
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e-Polymers
From the journal e-Polymers Volume 22 Issue 1

Abstract

Octa(dimethylethoxysiloxy) POSS (ODES) was synthesized successfully and used as the novel curing agent to prepare RTV silicone rubber (SROD) with outstanding mechanical properties and thermal stability. Compared with the silicone rubber cross-linked by tetraethoxysilane (SRTE), the novel RTV silicone rubber using octa(dimethylethoxysiloxy) POSS as a cross-linker had better mechanical, thermal, and optical properties. The highest tensile strength of SROD reached 1.26 MPa, which is three times that of SRTE. Besides, the decomposition temperature of 10% weight loss reached 507.7°C, exceeding that of SRTE by nearly 150°C. In addition, it was remarkable that due to the good compatibility of ODES with the silicone rubber matrix, the series of SROD showed good transmittance, greater than 87%. The thermal decomposition process of SROD was investigated by TGA coupled with real-time FTIR, and the results revealed the rigid structure and large steric hindrance of ODES that efficiently blocked the “backbiting” of the polysiloxy chains and delayed the end-induced ring decomposition, and consequently, improved the thermal stability of SROD significantly.

Keywords: octa(dimethylethoxysiloxy) POSS; transmittance; RTV silicone rubber; mechanical properties; thermal stability

1 Introduction

Silicone rubber whose main chain consisted of alternating silicon and oxygen atoms had the characteristics of both inorganic and organic materials and had unique performance advantages. Because the energy of the Si–O bond is much larger than that of the C–C bond of typical rubbers, silicone rubber has many unique physical and chemical properties including high-and low-temperature resistance, low dielectric properties, and excellent chemical resistance (1,2,3,4,5); therefore, it has been widely used in construction, health care, aerospace, and electronics industries (6,7,8). Among the various characteristics of silicone rubber, more attention was paid to optical properties, with a view to applying it to electronic devices such as light-emitting diode (LED) covers, light guide films, and optical fibers (9,10). Research on the transparency of silicone rubber has been carried out for a long time. Xu et al. studied the effect of 1D carbon nanotubes and 2D graphene as the surface conductive coating on the transmittance of silicone rubber (11). Norihiro et al. studied the transparency of silicone rubbers loaded with various kinds of silica particles (9). Another reason to consider is that the electrical equipment field has higher requirements for the working temperature of silicone rubber. Therefore, it is necessary to develop a silicone rubber that has high thermal stability, excellent mechanical properties, and high transparency.

As an organic–inorganic nanomaterial, polyhedral oligomeric silsesquioxane (POSS) had attracted a lot of attention due to its unique structure and excellent performance (12,13,14). Owing to the special structure of POSS, the addition of POSS or POSS derivatives into the polymer system can significantly improve the properties of the polymer such as decomposition temperature, oxidation resistance, surface hardness, mechanical properties, flame retardancy, and other properties. These improvements have been achieved in a wide range of polymer systems, such as polyvinyl chloride (15), polycaprolactone, polyurethane (16), polyimide (17), and polymethyl methacrylate (18). Ding et al. reported that the addition of trisilanol polyhedral oligomeric silsesquioxane-containing phosphorus (DPCP-TPOSS) led to a considerable increase in the fire-retardant performance (14). Liu et al. prepared the room-temperature vulcanized hydroxyl-terminated polydimethylsiloxane (HPDMS) crosslinked with multi-ethoxy POSS (EOPS) coatings for developing advisable ablation resistance composites in aerospace applications (19).

Several research groups had introduced POSS into the PDMS system by physical blending or chemical bonding, including monovinyl-POSS (20), octaisobutyl-POSS (21), octasilane-POSS (22), vinyl POSS (23), vinyl POSS derivatives (24,25,26), tetrasilanol-phenyl-polyhedral oligomeric silsesquioxane (27), amino-POSS (28), etc. All the above research results showed that the introduction of POSS greatly improved the temperature resistance and mechanical properties of silicone rubber. But, there are a few studies on the transparency of POSS-modified silicone rubber. Of the above reports, only Chen et al. reported the transparency of POSS-modified silicone rubber, and they found that the introduction of POSS would greatly reduce the transparency of silicone rubber (25). Therefore, it is difficult to maintain high transparency while improving the performance of RTV silicone rubber by POSS.

In this article, using octa(dimethylsiloxy) POSS (ODHS) as the precursor, we synthesizedODES that had the same structure as the PDMS chain segment and had good compatibility for the first time. Then, the novel transparent RTV silicone rubbers were prepared using ODES as a cross-linking agent (Scheme 1). The optical transmittance and thermal stabilities of the RTV silicone rubbers were investigated by Haze Meter and thermal gravimetric analysis, respectively. In addition, the effects of ODES on the morphology, mechanical properties, and degradation mechanism of RTV silicone rubber were explored.

Scheme 1 Preparation of ODES and curing RTV silicone rubber.
Scheme 1

Preparation of ODES and curing RTV silicone rubber.

2 Experimental

2.1 Materials

ODHS was prepared in our laboratory. TEOS was purchased from Meryer Chemical Technology Co., Ltd., Shanghai, China. Activated carbon and tetramethylammonium hydroxide was purchased from Aladdin Reagent (Shanghai) Co., Ltd., Shanghai. Chlorodimethylsilane was supplied by Tai Cheng Silicone Co., Ltd., Kai Hua, Zhejiang. Catalyst and dibutyltin dilaurate (DBTDL, catalyst) were purchased from Sinopharm Chemical Reagent Co., Ltd., Beijing, China. Hydroxyl-terminated polydimethylsiloxane (HPDMS) (η 25°C = 20,000 mPa‧s) was provided by Jinan Ying Yu Chemical Co., Ltd., Ethanol, n-hexane, toluene, and ethyl acetate were 98% pure and purchased from Beijing Chemical Factory, Beijing, China. All chemicals were used directly.

2.2 Synthesis of ODES

About 41.0 mL of tetramethylammonium hydroxide, 51.0 mL of methanol, and 15.0 mL of deionized water were charged into a 500 mL flask. The mixture was stirred at 0–4°C for 30 min, and then 22.0 mL of TEOS was added to the flask and continued to stir at room temperature for 24 h. After the above reaction was completed, the product was added to a 1 L flask that had been charged with a solution of 600.0 mL of hexane and 42.9 mL of chlorodimethylsilane. Then, the mixture was allowed to react at room temperature for 3 h. Subsequently, the reaction mixture solution was subjected to liquid separation and solvent removal to obtain the white powder. The white powder was washed three times with methanol and dried in a vacuum drying oven at 60°C. Finally, 10.2 g of ODHS was obtained with a yield of 80.3%.

A mixture of 80 mL of toluene and 8 g of ODHS was charged into a 250 mL three-neck round-bottom flask and stirred at room temperature for half an hour under a nitrogen environment. After that, 0.16 g of the catalyst and 2.9 g of ethanol were charged into the flask and stirred at 40°C for 12 h. After the reaction was completed, the activated carbon was added to this flask and stirred for 2 h. Then, the mixture was filtered under reduced pressure to give a clear, colorless solution. Finally, 9.5 g of a pale yellow oily liquid (yield 88.2%) was obtained by moving solvent from this solution under reduced pressure.

2.3 Preparation of RTV silicone rubber

HPDMS and ethyl acetate, used to reduce system viscosity, were added to a three-necked flask and stirred for 15 min. Then, the cross-linking agent was added dropwise (2 drops per min) to the flask. After the cross-linking agent was added, stirring was continued for 1 h and then 100 μL of DBTDL was added and stirred under a dry nitrogen atmosphere for 15 min. Then, the mixture was quickly poured into a mold. After curing at room temperature for 2 days, the RTV silicone rubber film was obtained. Finally, the film was placed in an oven at 60°C for 12 h to be fully cured.

RTV silicone rubbers cured with different amounts of ODES are listed in Table 1. HPDMS (10 g) and DBTDL (100 μL) were used in all samples. ODES as the cross-linking agent increased from 5 to 20 wt%, and the cured RTV silicone rubbers were named SROD-1, SROD-2, SROD-3, and SROD-4, accordingly. As a reference material, we also synthesized RTV silicone rubber using a conventional tetra-functional cross-linking agent (TEOS) according to the same method as above, and the formulation thereof is also shown in Table 1.

Table 1

Components of RTV silicone rubber

Sample HPDMS (g) Catalyst (μL) TEOS (g) ODES (g) ODES (%) Ethoxy (mmol) ethyl acetate (mL)
SRTE 10 100 1.11 0 0 21.35 10
SROD-1 10 100 0 0.53 5 3.10 10
SROD-2 10 100 0 1.11 10 6.49 10
SROD-3 10 100 0 1.76 15 14.62 10
SROD-4 10 100 0 2.50 20 21.37 10

2.4 Characterization and measurements

1HNMR spectra were recorded on a Bruker AV 11–400 spectrometer at 400 MHz in CDCl3 (0.05% TMS as an internal standard) at room temperature. Fourier transform infrared (FTIR) spectroscopy analysis of the RTV silicone rubber was conducted by a Bruker Tensor 27(Bruker Optics Inc.) in the transmission mode in the range 4,000–600 cm−1 with 32 scans by using the KBr crystal plate. The density of the RTV silicone rubber was carried out on a pycnometer. The hardness of the RTV silicone rubber was measured 5 times using a rubber hardness tester. Scanning electron microscopy (SEM) was performed on a Hitachi S-4700 scanning electron microscope at a voltage of 20 kV. Thermogravimetric analysis (TGA) was performed with a NETZSCH TG 209 instrument. About 10 mg of the sample was heated in an N2 atmosphere at a heating rate of 10°C·min−1 from 40°C to 800°C under nitrogen gas (flow rate: 70 mL·min−1). The mechanical properties of the specimens were tested on a universal testing machine (Shenzhen San Si Technology Co., Ltd. UTM4204) at room temperature and five specimens were tested per sample. The tensile strength, elongation at break, and modulus were measured at a tensile rate of 50 mm·min−1. The transmittance of the cured rubber was tested on a Haze Meter TH-110 (Hangzhou CHNSpec Technology Co., Ltd).

2.5 Measurement of the cross-linking density

The RTV silicone rubber was subjected to a swelling test to calculate the cross-linking density (29,30,31,32). The silicone rubber samples were weighed ( W 0 ′ ) and immersed in a 50 mL sample bottle containing 20 mL of toluene. After being immersed in the toluene for 48 h, the samples were taken out and weighed. Next, the excess solvent on the surface was removed by a filter paper and the samples were immersed in toluene and the above steps were repeated every 3 h until swelling equilibrium (W sw) was reached. Finally, the swollen samples were dried in a vacuum oven until the weight is constant (W 0). The degree of swelling (d sw) and dissolution fraction (s) of silicone rubber can be calculated using the following formulas:

(1) s = W 0 ′ − W 0 W 0 ′ × 100 %

(2) d sw = W sw − W 0 W 0 × 100 %

The cross-linking density can be expressed as the average molecular weight between two cross-linking points (Mc), so it can be calculated using the following equations:

(3) ν = ρ / Mc = − Ln ( 1 − φ ) + φ + χ φ 2 V 0 φ 1 3

where ρ is the density of the silicone rubber before swelling, V 0 (=106.54 cm3·mol−1) is the molar volume of the solvent, and χ represents the Flory–Huggins interaction parameter between the polymer and the solvent (this value is 0.465 in this study). In addition, φ is the volume percentage of silicone rubber in the swollen sample and can be calculated as follows:

(4) φ = W 0 ′ / ρ ( W sw − W 0 ′ ) / ρ 1 + W 0 ′ / ρ

where ρ 1 (= 0.87 g·cm−3) is the density of toluene.

3 Results and discussion

3.1 Characterization of POSS: NMR analysis

The chemical structure of ODES was further confirmed by 1H-NMR and 29Si-NMR in CDCl3 (Figure 1c and d). Figure 1a shows that ODHS had two different hydrogen protons at chemical shifts of 0.26 and 4.73 ppm, corresponding to methyl protons (“a”) and hydrogen protons attached to silicon (“b”). Figure 1c shows that the methyl proton attached to the silicon moved from 0.26 to 0.15 ppm, indicating that ODHS had reacted and caused a change in the chemical shift of the methyl proton adjacent to the silicon. In addition, two new peaks appeared at 1.20 and 3.76 ppm due to the chemical shift of Si–O–CH2–CH3.

Figure 1 1HNMR spectra of (a) ODHS and (c) ODES, and 29SiNMR spectra of (b) ODHS and (d) ODES.
Figure 1

1HNMR spectra of (a) ODHS and (c) ODES, and 29SiNMR spectra of (b) ODHS and (d) ODES.

It can be seen from the 29Si NMR spectra of ODHS and ODES that the silicon on the silicon cage had a similar chemical shift (−108.66 and −110.03 ppm). The chemical shift of Si connected to the methyl group changed significantly from −1.40 to −9.91 ppm, which also indicated that ODHS reacted completely to form ODES.

3.2 Characterization of RTV silicone rubber

3.2.1 FTIR analysis

FTIR spectroscopy was used to demonstrate the presence of cross-linked structures. There were clear differences between the FTIR spectrum of the cured and the uncured HPDMS (Figure 2). Compared with the uncured HPDMS, the hydroxyl absorption peak at 3,425 cm−1 of the cured HPDMS disappeared obviously, which was mainly because the hydroxyl group undergoes a condensation reaction to form a Si–O–Si structure. The three absorption peaks appearing at 2,964, 2,904, and 1,260 cm−1 were due to the C–H stretching vibration in the methyl group. Since HPDMS was a linear polysiloxane structure with long segments, two Si–O–Si antisymmetric stretching vibration absorption peaks almost of equal strength appeared at 1,097 and 1,024 cm−1. ODES showed mainly a Si–O–Si cage structure with a strong absorption peak at 1,097 cm−1. So, with an increase of ODES in silicone rubber, the absorption peak near 1,097 cm−1 became stronger and that at 1,024 cm−1 weakened.

Figure 2 FTIR characterization of ODES, HPDMS (107#), and RTV silicone rubber (SRTE, SROD-1, SROD-2, SROD-3, SROD-4).
Figure 2

FTIR characterization of ODES, HPDMS (107#), and RTV silicone rubber (SRTE, SROD-1, SROD-2, SROD-3, SROD-4).

3.2.2 Solvent swelling and cross-linking density

The swelling test was used to demonstrate the formation of a three-dimensional cross-linked network, and the cross-linking density of the silicone rubber could be calculated with it. The results of the swelling test including swelling degree, dissolution fraction, and cross-linking density of the RTV silicone rubber are presented in Table 2. It can be clearly seen from Table 2 that with the increase of ODES, the density of the silicone rubber gradually increased. The changing trend of the degree of swelling and cross-linking density is displayed in Figure 3. The curves showed that SROD exhibited a lower swelling degree and higher cross-linking density than that of SRTE. In addition, SROD-3 exhibited maximum cross-linking density and was nearly twice that of SRTE. It can be seen from Table 1 that the number of functional groups of the cross-linking agent added to SROD was less than or equal to that of SRTE, while the cross-linking densities of all SROD were higher than that of SRTE. This showed that the cage structure of ODES can help increase the degree of cross-linking density of RTV silicone rubber (26). A downward trend shown for the cross-linking density of SROD-4 might be due to the fact that with the increase of the POSS content, POSS aggregated and underwent self-polycondensation, resulting in a decrease in the degree of reaction between the ODES and HPDMS system (33).

Table 2

Cross-linking density when swollen at equilibrium

Sample Density (g·cm−3) Dissolution fraction (%) Degree of swelling (%) Volume percentage Cross-linking density (×10−4 mol·cm−3)
SRTE 0.9668 8.08 378 0.1921 0.66
SROD-1 0.9741 6.16 304 0.2272 1.01
SROD-2 0.9854 5.60 301 0.2281 1.02
SROD-3 1.0022 5.35 267 0.2454 1.22
SROD-4 1.0145 5.54 284 0.2314 1.05
Figure 3 Degree of swelling and cross-linking density of RTV silicone rubber.
Figure 3

Degree of swelling and cross-linking density of RTV silicone rubber.

3.2.3 Morphologies

In this study, ODES was chemically incorporated into the HPDMS polymeric system. However, the agglomeration of POSS and phase separation might still occur, which had a great impact on the performance of the materials. Therefore, it was important to study the dispersion of ODES in the matrix. In this work, the subsequent morphologies of the cured silicone rubbers were investigated by SEM to evaluate the dispersion of ODES. Figure 4 shows the cold-fracture surface images of silicone rubbers. TEOS is a small molecule cross-linker and had good dispersibility and compatibility in HPDMS, as shown in Figure 4b. In the previous study, we had analyzed that TEOS might be aggregated into a larger cross-linking center, but no aggregation and phase separation were observed as shown in Figure 4b, indicating that the cross-linking center was small. Like SRTE, there was no aggregation and phase separation in the fractured surfaces of the SROD-1 and SROD-2 illustrating that with the amounts of ODES less than 10%, it could achieve good dispersion in the matrix and could be attributed to the good compatibility of ODES and HPDMS. Unlike the above samples, significant micron-sized spherical domains appeared in SROD-3 and this phenomenon was more prominent in SROD-4. The micron-sized spheres were likely to be POSS-rich clusters, which were ascribed to the self-crosslinking of ODES.

Figure 4 Visual images and morphologies of RTV silicone rubber: (a) visual images of SRTE and SROD; (b) SEM images of SRTE; (c) SEM images of SROD-1; (d) SEM images of SROD-2; (e) SEM images of SROD-3; and (f) SEM images of SROD-4.
Figure 4

Visual images and morphologies of RTV silicone rubber: (a) visual images of SRTE and SROD; (b) SEM images of SRTE; (c) SEM images of SROD-1; (d) SEM images of SROD-2; (e) SEM images of SROD-3; and (f) SEM images of SROD-4.

3.2.4 Optical transmittance

From Figure 4a, it can be seen that all the samples that had a thickness of 0.5 mm showed a transparent state. To further obtain the change in the optical transmittance with the ODES content, the silicone rubber was tested using Haze Meter. Figure 5 shows the transmittance of all samples. The measured transmittance of the sample is the weighted average in the wavelength range of 400–700 nm. The figure shows that the SRTE had a transparency of 93.79%, and the transmittance of the SROD series was above 87%, indicating that the compatibility of ODES and HPDMS was ideal, but there were still some tiny aggregation phenomena seen on the SEM image that might cause a slight decline of light transmittance. With the addition of ODES, the transmittance showed a slight downward trend. When ODES in the RTV silicone rubber reached a relative saturation state, it caused agglomeration of ODES, resulting in the phase separation in rubber due to a decrease in transmittance, which was consistent with the SEM results.

Figure 5 Optical transmittance image of RTV silicone rubbers (SRTE, SROD-1, SROD-2, SROD-3, SROD-4).
Figure 5

Optical transmittance image of RTV silicone rubbers (SRTE, SROD-1, SROD-2, SROD-3, SROD-4).

3.2.5 Mechanical properties

The effects of ODES on the mechanical properties of RTV silicone rubber were evaluated as illustrated in Table 3 and Figure 6. The stress–strain curves (Figure 6) showed that the RTV silicone rubber using ODES as the cross-linking agent possessed better mechanical properties than SRTE. Among them, SROD-3 had the best mechanical properties, with a tensile strength of 1.26 MPa, which was three times higher than the SRTE. Meantime, its elongation at break, modulus, and hardness reached 381.18%, 0.22 MPa, and 21.3, which were much larger than that of SRTE. It was difficult to achieve both reinforcement and toughening using traditional modification methods, but the addition of POSS introduced this interesting and valuable effect in this study.

Table 3

Mechanical properties of RTV silicone rubber

Sample Tensile strength (MPa) Elongation at break (%) Modulus (MPa) Shore hardness
SRTE 0.40 ± 0.06 320.36 ± 5.59 0.09 ± 0.02 14.0 ± 0.6
SROD-1 0.73 ± 0.02 275.53 ± 4.65 0.20 ± 0.01 18.1 ± 0.2
SROD-2 0.90 ± 0.08 301.01 ± 2.51 0.22 ± 0.02 19.2 ± 0.7
SROD-3 1.26 ± 0.13 381.18 ± 6.10 0.22 ± 0.01 21.3 ± 0.3
SROD-4 1.02 ± 0.08 357.77 ± 16.25 0.25 ± 0.04 22.2 ± 0.3
Figure 6 Stress–strain curves of RTV silicone rubbers.
Figure 6

Stress–strain curves of RTV silicone rubbers.

The enhancement of strength, modulus, and hardness of RTV silicone rubber was closely related to the rigid cage structure of ODES (21). Moreover, cross-linking density was an important factor that influenced the mechanical properties of silicone rubber, and the effect of the rigid cage of ODES on the cross-linking density must be reflected in the mechanical properties of silicone rubber. SROD-3 had the highest cross-linking density in all sample tests, which was consistent with the mechanical properties test. It needs to be further explained that the elongation at break of ODES-modified silicone rubber had a tendency to fluctuate, as the elongation at break of SROD-3 and SROD-4 was higher than that of SRTE. The micro-aggregates of ODES in RTV silicone rubber acted as nanoparticles, which might increase the interaction forces between molecules in the system, thereby achieving the effect of toughening and other mechanical properties (16,20,34). However, this enhancement was limited. It could be seen that with the emergence of excessive aggregation, the toughness began to decrease, Such as SROD-4.

3.2.6 Thermomechanical properties

The thermomechanical properties of RTV silicone rubber determined by dynamic mechanical analysis (DMA) are shown in Figure 7a–c, The storage modulus (E′) of SROD-4 was the highest at −130°C, in which all the chain segments were completely frozen. The E′ (at −130°C) of SROD-3 was the lowest, and E′ decreased by degrees from SROD-1 to SROD-3. For RTV silicone rubbers prepared with ODES as the cross-linking agent, two apparent peaks appeared in the tanδ–temperature curve, whereas the peak around −110°C should be the secondary transition, which was attributed to the intrinsic Si–O motion. Meanwhile, the second peak appeared at about −42°C, which was due to the glass transition temperature (T g), meaning that the Si–O segments started to move.

Figure 7 (a) Storage modulus, (b) loss modulus, and (c) tan δ curves of RTV silicone rubber measured by DMA.
Figure 7

(a) Storage modulus, (b) loss modulus, and (c) tan δ curves of RTV silicone rubber measured by DMA.

The glass transition temperature of SRTE cannot be seen from the graph of tan δ–temperature, owing to its flexibility and low strength. The peak of the secondary transition was higher than the glass transition because the intensity was not high enough. The relatively high secondary transition peak was because below the glass transition temperature, the movement of segments was restricted and there was certain internal friction, whereas, above the glass transition temperature, the segments could move freely, and the friction between molecular chains was particularly small.

The cross-linking density (υe) is measurable by applying a rubber elasticity model. The υe can be calculated from Eq. 5:

(5) E ′ = 3 ν e RT

where E′, R, and T, respectively, represent the E′ at T = (T g + 30)°C, gas constant, and absolute temperature. The calculated υe of the silicone rubbers are listed in Table 4. The cross-linking density calculated by DMA was basically consistent with the swelling test. SROD-3 had the highest cross-linking density in all samples, which was also consistent with the mechanical performance properties.

Table 4

Thermomechanical properties of RTV silicone rubber

Sample SROD-1 SROD-2 SROD-3 SROD-4
T g (°C) −41.40 −45.85 −41.19 −42.21
υ e (×10−4 mol·cm−3) 0.88 1.10 1.33 1.31

3.2.7 Thermal stability

To investigate the enhanced effect of ODES as the cross-linker on the silicone rubber system, the thermal degradation of the HPDMS polymer system was evaluated by TG. The TG and DTG curves of RTV silicone rubber are shown in Figure 8a and b, and only a single degradation step was observed in all DTG curves. The results showed that the introduction of ODES significantly enhanced the thermal properties of silicone rubber.

Figure 8 Thermal stabilities of RTV silicone rubber: (a) TG curves of RTV silicone rubber; (b) DTG curves of RTV silicone rubber; (c) T 10% of RTV silicone rubber; and (d) T max of RTV silicone rubber.
Figure 8

Thermal stabilities of RTV silicone rubber: (a) TG curves of RTV silicone rubber; (b) DTG curves of RTV silicone rubber; (c) T 10% of RTV silicone rubber; and (d) T max of RTV silicone rubber.

Generally, the temperature of the 10% weight loss (T 10%) corresponded to the temperature at which the RTV silicone rubber began to thermally decompose. The T 10% of silicone rubber containing ODES was significantly higher than that of SRTE and reached the maximum at SROD-3, as shown in Figure 8c. The T 10% of SROD-3 was 507°C, which was nearly 150°C higher than that of SRTE, which could be ascribed to the introduction of ODES rigid cage structures.

It is known that polysiloxanes exhibit dissociation routes with low energy in the presence of hydroxyl impurities (35). The hydroxyl groups could participate in the “backbiting” reaction to form volatile cyclic products at elevated temperatures. The stable and large ODES cage structure was difficult to be bitten into a ring like a normal molecular chain in the cross-linked structure, which blocked the normal ring-forming decomposition process. In the SEM section, aggregated particles appeared in SROD-3 and SROD-4. The continued improvement in thermal stability might be attributed to the large size of nanoparticles that could better block “backbite.” The excess ODES not only caused severe agglomeration but also had more unreacted alkoxy functional groups that would preferentially decompose. Therefore, the T 10% of SROD-4 was lower than that of SROD-3.

The greatest rate of mass loss temperature (T max) also increased from 448°C (SRTE) to 560°C (SROD-3), which was an enhancement of 112°C (Figure 8d), and all the T max values of silicone rubber containing ODES were significantly higher than that of SRTE. This illustrated that the incorporation of ODES may improve thermal stability (14). Due to the weaker intermolecular forces, traces of residual hydroxyl groups in SRTE can accelerate the decomposition of silicone rubber, resulting in rapid degradation at 358°C. ODES could hinder the molecular chain from breaking down at a lower temperature. Only at relatively high temperatures, it might facilitate random degradation of the cross-linked structure because the chain mobility and molecular motion were relatively enhanced at this time.

3.2.8 Degradation mechanism of RTV silicone rubber

In general, PDMS is degraded by three reaction mechanisms in the inert environment, namely “unzipping” (36,37), “random fracture” (38), and “external catalysis” (39). The above mechanisms do not occur independently, sometimes they occurred simultaneously.

The thermal decomposition processes of SRTE and SROD-3 samples were monitored by real-time FTIR being coupled with TG. The main thermal weight losses of SRTE and SROD-3 in different temperature ranges were demonstrated by TG analysis. Therefore, the results of FTIR analysis including the main thermogravimetric stages of SRTE and SROD-3 are shown in Figure 9. It can be clearly seen that cyclic dimethylsiloxane (2,966, 1,261, 1,083, 1,022, and 814 cm−1) was the main product in the thermal degradation process of SRTE and SROD-3 (37). The infrared absorption peak intensity (Figure 9) could represent the degradation process of the two silicone rubbers at their respective main thermal degradation stages.

Figure 9 Real-time FTIR spectra of the (a) SRTE and the (b) SROD-3 sample thermal degradation (conditions: 10°C·min−1 and N2 atmosphere).
Figure 9

Real-time FTIR spectra of the (a) SRTE and the (b) SROD-3 sample thermal degradation (conditions: 10°C·min−1 and N2 atmosphere).

Subsequently, the infrared spectra of the solid degradation residues of the SRTE and SROD-3 samples were characterized by infrared spectroscopy, as shown in Figure 9. Both test results show similar trends, and the significant characteristic peaks in the spectrum were assigned to the asymmetric Si–O–Si stretching vibration in the vicinity of 1,072 cm−1. The characteristic stretching vibration peak of C–H, the Si–CH3 peak at 2,964 cm−1, and the characteristic vibration peak of Si–O–CH2 at 2,850 cm−1 can be seen in Figure 10. The characteristic stretching vibration peak of Si–C at 796 cm−1 also is also clearly seen. These might indicate that the degradation residues of SRTE and SROD-3 in N2 contained the same silicon-oxycarbide complex. The black degradation residue in N2 was mainly silicon-oxycarbide (4). The reason why the residual rate of SROD was higher than that of SRTE could be attributed to the addition of cage stable structures.

Figure 10 FTIR spectra of degradation residues of SRTE and SROD-3 samples.
Figure 10

FTIR spectra of degradation residues of SRTE and SROD-3 samples.

It is well known that the thermal degradation of PDMS (107#) in the inert gas was mainly due to the end-induced ring decomposition degradation mechanism, which was also known as the “backbiting” process (27,35). SROD-3 and SRTE had the same decomposition products and the same single thermal degradation step, which was sufficient to demonstrate that they had the same thermal degradation process. Therefore, it was certain that the thermal degradation of the SROD-3 was also ascribed to the end-induced ring decomposition degradation mechanism (39). But, the difference was that the degradation of SROD-3 occurred at higher temperatures. Therefore, it was speculated that at a lower temperature, the stable large cage structure of POSS blocked the “backbite” process of the molecular chain as shown in Figure 11 (40). At high temperatures, the cross-linked structure underwent random breaking and the molecular chain motion was enhanced at this time, which made the degradation work normally.

Figure 11 RTV silicone rubber degradation mechanism diagram: (a) without ODES and (b) with ODES.
Figure 11

RTV silicone rubber degradation mechanism diagram: (a) without ODES and (b) with ODES.

4 Conclusion

A liquid POSS cross-linker was prepared and a series of transparent RTV silicone rubbers with high thermostability and mechanical strength were prepared. The mechanical and thermal properties of RTV silicone rubbers were also significantly improved compared with SRTE. The tensile strength, elongation at break, and hardness had a clear improvement, and the tensile strength of SROD-3 was nearly three times that of SRTE. Moreover, compared with the performance of SRTE, the T 10% and T max of the SROD-3 increased nearly 150°C and 100°C, respectively. The thermal degradation mechanism was analyzed by real-time FTIR coupled with TG, which proved that the stable cage structure of the ODES could block the degradation of silicone rubber. At relatively high temperatures, as the molecular chain breaks randomly, the silicone rubber will still degrade according to the end-induced ring decomposition degradation mechanism.

  1. Funding information: This study was supported by financial assistance from the National Natural Science Foundation of China (Grant No. 51403013 and Grant No. 51702076).

  2. Author contributions: Xing Huang: writing – original draft, methodology, writing – review and editing, data curation, formal analysis; Guomin Song: writing – review and editing, conceptualization, methodology; Jianjun Shi: resources; Jiafei Ren: software, methodology; Ruilu Guo: conceptualization; Chunyuan Li: investigation; Guangxin Chen: supervision; Qifang Li: project administration, supervision; Zheng Zhou: funding acquisition, project administration, supervision.

  3. Conflict of interest: The authors state no conflict of interest.

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Received: 2021-09-18
Revised: 2021-11-09
Accepted: 2021-12-15
Published Online: 2022-04-06

© 2022 Xing Huang et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/epoly-2022-0022
Keywords for this article
octa(dimethylethoxysiloxy) POSS; transmittance; RTV silicone rubber; mechanical properties; thermal stability
Creative Commons
BY 4.0

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