1 Introduction

Phenolic resins (PR) synthesized by phenolic (phenol and resorcinol, etc.) and aldehyde (formaldehyde and furfural, etc.), are important heat-resistant thermosetting materials. PR have been conventionally used as resin matrix for high performance thermal protection [1].

With development of sol–gel technology, PR aerogels with resorcinol and formaldehyde as precursor were produced by pioneer research of Pekala [2]. Abundant works have been focused on tailoring the structure by tuning sol–gel parameters [3,4,5,6,7]. PR aerogels and their carbonized derivatives, combining the advantages of heat insulation and thermal stability, attract intense interests in thermal protection [8,9,10]. However, PR aerogels have drawbacks of serious weight loss under high temperature, especially under thermal oxidation condition, and adsorption of water in moisture atmosphere. To enhance the thermal insulation property or mass retention under high temperature, composites of PR with heat-resistant fillers were prepared by copolymerization of PR with antidegradation components such as inorganic precursor, or blending particles or fibers in their matrix [11,12,13]. Silica was mostly studied for improving thermal stability of PR due to their similar sol–gel process. Binary PR/SiO2 composite aerogels were prepared by copolymerization of silica precursor with resorcinol–formaldehyde (RF) or doping silica aerogels in PR [14,15,16]. As-prepared aerogels have abundant porosity and thermal insulation property. However, as reported, traditionally prepared PR and SiO2 aerogels are hydrophilic [17, 18]. The studies showed that the thermal insulation property of materials was deteriorated due to adsorption of moisture [19]. In addition, hydrophilic property may result in crack of aerogel monolith during ambient drying. Therefore, silica and PR aerogels were usually modified by hydrophobic organic structure [20,21,22]. It is crucial to prepare PR aerogels with hydrophobic and thermal stability for obtaining intact monolith and applying in moisture condition and high temperature.

For enhancing thermal stability and hydrophobic property simultaneously, polysiloxane with nonpolar substitutes was used to modify PR. Polysiloxane as semi-inorganic polymer with Si–O backbone and organic side groups has advantages of thermal oxidation stability and moisture resistance over most of traditional polymer, investigated as heat-resistant and waterproof matrix to prepare coating, film, and composite materials widely [23, 24]. However, the study indicated that polysiloxane and PR tended to phase separation during silane hydrolysis and condensation process [25]. Coupling molecule was synthesized for resisting macroscale phase separation of phenolic formaldehyde from polysiloxane to obtain bi-continuous structure [26]. A method without solvent and coupling agent was proposed to synthesize thermal stable PR/silicone composites in which covalent bonds formed by reaction between phenolic hydroxyl groups and Si–OH [27].

However, texture structure of PR–polysiloxane composite aerogels was complicated influenced by reaction parameters. During sol–gel transformation, phase separation occurred not only between reactant and solvent, but PR and polysiloxane. Coupling molecule such as amino-functionalized silane was usually used in copolymerization with RF to form chemical cross linking [28, 29]. Methyl-substituted silane was mostly incorporated in RF aerogels for improving hydrophobic property. Similarly, amino-functionalized silane assisted sol–gel reaction of resorcinol–furfural and methyl-substituted silane [30]. When methyl-substituted silane was used as hydrophobic source of polysiloxane, phase structure of composite aerogels was changed by adjusting reaction conditions to produce uniform, core-shell or phase separation structure [31]. It was known that heat-resistant polysiloxane was usually comprised of phenyl groups within appropriate content [32]. The study on phase structure of phenyl-incorporated polysiloxane-modified RF is still imperative for avoiding macroscale phase separation by carefully tuning the component of polysiloxane and reaction parameters. In addition, carbonization treatment could remove heteroatoms and unstable component. The thermal stability of carbonized derivatives is improved than the PR [33, 34]. Therefore, it is necessary to investigate the structure and thermal stability of carbonized derivatives.

Herein, we prepared hydroxyl-terminated poly(methylphenylsiloxane) prepolymer which could be dissolved with RF in cosolvent. Poly(methylphenylsiloxane)-modified RF (RF-SP) composite wet gel monoliths were obtained by controllable reaction parameter. Ambient drying instead of supercritical drying was conducted to obtain xerogels for safety and low cost. The influence of phenyl and polysiloxane contents on structure, wettability and thermal stability of RF-SP composite xerogels were investigated. Furthermore, as-prepared RF-SP composite xerogels were carbonized under nitrogen atmosphere at 500 and 800 °C, respectively. The structure and thermal stability of derivatives were analyzed. The heat-resistant RF-SP composite xerogels and carbonized derivatives could be potentially applied in the area of thermal protection.

2 Experiments

2.1 Chemicals

Methyltriethoxysilane, dimethyldiethoxysilane, and phenyltriethoxysilane were supplied by Aladdin reagent Co., Ltd. Hexamethyldisilazane was purchased from Sinopharm Chemical Reagent Co., Ltd. Formaldehyde solution (37%) and absolute ethanol were provided by Tianli Chemical Reagent Co., Ltd. Ammonia solution (25%) was supplied by Tianjin Quartz Clock Factory Bazhou Chemical, hydrochloric acid (HCl) was purchased from Sinopharm Chemical Reagent Co., Ltd, and resorcinol was supplied by Tianjin Fuchen Chemical Reagent Factory. All the chemicals were used directly without further purification.

2.2 Synthesis of polysiloxane prepolymer

Polysiloxane prepolymer was synthesized according to a classical method by modification [35]. When the molar ratio of methyl and phenyl groups (R) to Si (R/Si) was fixed, prepolymer of polysiloxane with phenyltriethoxysilane molar ratio among monomers of 0%, 10%, and 20% were prepared and named as 0Ph-SP, 0.1Ph-SP, and 0.2Ph-SP, respectively. In a typical procedure, 0.1Ph-SP prepolymer was synthesized by mixing methyltriethoxysilane, dimethyldiethoxysilane, phenyltriethoxysilane, water, and concentrated HCl in a molar ratio of 7:2:1:33.6:0.336, respectively. Then the mixture was stirred at 60 °C for 3 h, and terminated by adding hexamethyldisilazane to tune pH value near to 7. After that, the solution was distilled to remove small molecules under reduced pressure at 35 °C. The obtained product was diluted by ethanol to 30 wt%.

2.3 Preparation of polysiloxane-modified resorcinol–formaldehyde (RF-SP) composite xerogel monoliths

RF solution was formed by dissolving resorcinol and formaldehyde with molar ratio of 1:2 in ethanol to form 30 wt% solution. 30 wt% RF and 0.1Ph-SP solution were mixed with different volume ratio of 1:0, 3:1, 1:1, 1:2, 1:3 and 0:1, denoted as RF-0, RF-SP-1, RF-SP-2, RF-SP-3, RF-SP-4 and SP, respectively. 30 wt% RF and ethanol were mixed with the volume ratio of 1:1, named as RF-2 for comparing with RF-SP-2 composite xerogel monoliths. The samples formed by 30 wt% RF solution and 0Ph-SP, and 0.2Ph-SP with volume ratio of 1:1 were abbreviated as RF-SP-5 and RF-SP-6, respectively. Ammonia aqueous solution (25%) was diluted by adding water to be 6.66 mol/L, and then adding two times volume ethanol in above solution. 1.0 mL diluted ammonia solution was dropped to 4 mL reactant solution under vibration for 60 s, and sonication for 60 s. The sol was sealed into plastic tube and heated at 60 °C for 24 h. The wet gel was dried by a procedure at 40 °C for 24 h, 60 °C for 1 h, 80 °C for 1 h, 110 °C for 2 h, 130 °C for 2 h, and 150 °C for 3 h. Finally, RF-SP composite xerogel monoliths were obtained.

2.4 Thermal conversion treatment of RF-SP composite xerogel monoliths

RF-SP-2 xerogel was treated under nitrogen atmosphere in tube furnace. The heating procedure was set from room temperature to 250 °C with heating rate 10 °C/min, keeping at 250 °C for 30 min, then raising the temperature from 250 to 500 °C or 800 °C, with a heating rate of 10 °C/min, and keeping for 30 min. The two samples were abbreviated as 500 °C-RF-SP-2 and 800 °C-RF-SP-2.

Muffle furnace was used for the thermal conversion treatment of xerogels at stationary air atmosphere. RF-2, RF-SP-1, RF-SP-2, and RF-SP-3 xerogels were heated from room temperature to 600 °C with a heating rate of 10 °C/min, and staying at 600 °C for 1 h.

2.5 Characterization

The radial shrinkage was calculated by the difference in diameter between reaction tube and composite xerogels divided by the diameter of reaction tube. The bulk density was determined by mass per volume of monoliths. Gelation time was determined by the time interval from heating till not flowing by tilting the reaction tube to 90°.

Fourier transform infrared (FTIR) spectra were used to reveal the chemical structure of xerogels by using Fourier Transform Infrared Spectrometer (Nicolet iS50, Thermo Fisher Scientific, USA). Molecular weight of polysiloxane prepolymer was analyzed under room temperature by gel permeation chromatography (GPC) (Malvern Viscotek TDAmax, Malvern Panalytical, Britain) with tetrahydrofuran as solvent. Thermogravimetry analyses (TGA) of xerogels were tested in argon atmosphere and air atmosphere, respectively. TGA from room temperature to 1000 °C under argon atmosphere was carried out by using thermogravimetric analyzer (TA Q500, TA Instruments, Inc., USA) with a heating rate of 10 °C/min. TGA in air was carried out by using thermal analyzer (STA 449 C, NETZSCH Instrument Manufacturing Co., Ltd, Germany), and heating range was from 35 to 1000 °C with heating rate of 10 °C/min. Xerogels were observed with scanning electron microscope (Zeiss-supra 55, Carl Zeiss AG, Germany). Elemental dispersion was conducted by EDS analyses. In order to evaluate the hydrophobicity of xerogels, contact angle tester (SL200B, Shanghai Solon Information Technology Co., Ltd China) was used to test the water contact angle on xeorgels. Two microliters of water droplet was dropped on the surface of xerogels. X-ray photoelectron spectroscopy analysis (XPS) was used to characterize the chemical structure of xerogels. The test instrument is X-ray photoelectron spectrometer (ESCALAB 250, Thermo Fisher Scientific, USA). Thermal conductivity of RF composite monoliths was measured by laser thermal conductivity meter at room temperature (LFA457, NETZSCH Instrument Manufacturing Co., Ltd, Germany). The samples were cut into cylinder, with diameter 12.7 mm and thickness between 5 and 6 mm. The test atmosphere was argon atmosphere. Pore size distribution was texted by mercury injection apparatus (Autopore 9500, Micrometics Co., USA). XRD patterns were observed by X-ray diffractometer (X’PERT, Panalytical, Holland) at angle from 10° to 90°.

3 Results and discussion

3.1 Structure of RF-SP composite xerogels

Hydroxyl-terminated poly(methylphenylsiloxane) prepolymer was synthesized by acid-catalyzed reaction of silane monomer. The component of poly(methylphenylsiloxane) was controlled by tuning phenyl content in order to dissolve polysiloxane prepolymer with RF in cosolvent. The structure of typical prepolymer of 0.1Ph-SP was analyzed by FTIR and GPC. FTIR spectrum in Fig. 1a showed the strong absorption at 1000–1100 and 799 cm−1 was ascribed to Si–O–Si. The characteristic peak at 1271 cm−1 was C–H structure from Si–CH3. The peaks appeared at 699, 1431, and 1596 cm−1 were attributed to phenyl groups. C–H stretching vibration was observed by peaks at 2911 and 2969 cm−1. The broad band in the range from 3200 to 3700 cm−1 was ascribed to Si–OH structure [36,37,38,39]. The analyses indicated that silicone prepolymer was copolymer of methyl-substituted and phenyl-substituted siloxane with Si–OH as terminal groups. The number average molecular weight and dispersibility index of typical prepolymer of 0.1Ph-SP, measured by GPC, was about 990 and 3.62, respectively. FTIR and GPC results revealed hydrolysis and condensation of silane monomer to form hydroxyl-terminated poly(methylphenylsiloxane) prepolymer.

Fig. 1
figure 1

a FTIR spectrum of 0.1Ph-SP and inset photograph of 0.1Ph-SP dissolved in ethanol. b Photograph of RF-0, RF-SP-1, RF-SP-2, RF-SP-3, and SP. c Water contact angle of RF-0, RF-SP-1, RF-SP-2, and RF-SP-3. d Photograph of RF-0, RF-SP-1, RF-SP-2, and RF-SP-3 put on the surface of water

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The solubility of polysiloxane prepolymer with RF depended on solvent and structure of polysiloxane. It was found in inset of Fig. 1a that the prepared prepolymer of poly(methylphenylsiloxane) (0.1Ph-SP) well dispersed in ethanol due to the low molecular weight and hydroxyl groups. Thus, ethanol was used as cosolvent to mix 0.1Ph-SP prepolymer and RF monomer to form homogeneous solution. After being catalyzed by ammonia and heating at 60 °C, gels of RF-SP-1, RF-SP-2, and RF-SP-3 quickly formed in 5 min, but RF-SP-4 could not form intact wet gel monolith and just produced precipitate. It took as long as 12 h for polysiloxane to transform into gel. The xerogel monoliths of RF-0, RF-SP-1, RF-SP-2, RF-SP-3, and SP were obtained by ambient pressure drying. Figure 1b showed photographs of as-prepared xerogels. SP was transparent with apparent shrinkage. RF-0 and RF-SP composite monoliths were in dark red by aging and postheated treatment [40]. The observation revealed that RF-SP composite monoliths were formed without macroscopic separation of RF from SP.

The characteristic parameters of xerogels were listed in Table 1. Radial shrinkage of RF-0 xerogel was about 7.3%, and its density was 0.212 g/cm3. After diluting RF-0 solution with equal volume of ethanol, the density of obtained RF-2 xerogel was down to 0.121 g/cm3. The radial shrinkage of RF-SP composite xerogel was in the range from 10 to 20%. Density of composite xerogel monoliths was increased, but lower than 0.300 g/cm3, indicating the samples were lightweight. Blank polysiloxane shrank about 50% in diameter after drying. It was deduced that silicone formed dense structure according to the density. The wettability of xerogels was evaluated by testing water contact angle. Water droplet quickly spread on the surface of RF-0 xerogel and the contact angle was near to zero due to the hydrophilic property. In the case of freshly prepared RF-2, it was shown in Fig. S1 that the surface layer was hydrophobic when the drying temperature above 130 °C in air condition, but the internal structure was still hydrophilic and the contact angle was similar to that of RF-0. It spent about 3 s for water droplet permeating in interior of RF-2, more slowly than RF-0. It indicated that wettability of RF xerogels changed with different reaction parameters. After incorporating polysiloxane, RF-SP composite monoliths were hydrophobic with water contact angle about 130° as shown in Fig. 1c and the internal structure was hydrophobic. The lightweight and hydrophobic properties of RF-SP composite xerogels were directly certified by photograph in Fig. 1d. Hydrophilic RF-0 monolith almost immersed in water, but RF-SP composite monoliths with similar density to RF-0 could float on the surface of water.

Table 1 Characteristic parameters of RF, SP, and RF-SP composite xerogels
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SEM images in Fig. 2 were used to observe the microscale structure of xerogels. In order to analyze the influence of polysiloxane on structure of RF-SP-2 xerogels, RF solution with same content in RF-SP-2 was used to prepare RF-2 xerogel as controlled sample. Figure 2a revealed that RF-2 xerogel was mainly composed by microspheres with diameter in the range from 2 to 5 µm. The dense morphology of SP was observed in Fig. 2b. SEM image in Fig. 2c indicated that RF-SP-2 had similar structure with RF-2, and the colloids of RF-SP-2 tended to be adhesive together. EDS and silicone elemental mapping results in Fig. 2d certified that polysiloxane coated along RF microspheres without individual SP particles in RF-SP-2. However, larger spheres about 20 μm in diameter were observed in RF-SP-3 by Fig. 2e. According to the concentrated dispersion of silicone element of larger spheres in Fig. 2f, those spheres were presumed to be SP particles.

Fig. 2
figure 2

SEM images, EDS spectra, and Si element mapping of xerogels. a RF-2, b SP, c SEM image, and d EDS spectrum and Si element mapping of RF-SP-2. e SEM image and f EDS spectrum and Si element mapping of RF-SP-3

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According to the difference in gelation rate of RF and SP, combined with SEM images and EDS analyses in Fig. 2, formation mechanism of RF-SP composite monolith was illustrated in Fig. 3. It was deduced that RF quickly formed gel skeleton by aggregation of colloids, and SP prepolymer adsorbed on RF skeleton by interaction between Si–OH and phenolic hydroxyl groups. SP condensed on RF skeleton without forming new nuclei when content of SP was low for RF-SP-1 and RF-SP-2. When SP content was increased up to that in RF-SP-3, isolated nucleation of SP resulted in formation of larger particles among RF microspheres. The structure of RF-SP-2 was constructed by outer SP layer connecting RF colloids, but extra SP microspheres existed when higher ratio of SP to RF. Thus, the volume ratio of RF solution to SP should be controlled larger than 1:2 for avoiding the individual SP spheres.

Fig. 3
figure 3

Schematic illustration of formation mechanism of RF-SP composite xerogels

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3.2 Thermal stability of RF-SP composite xerogels

Thermal property of xerogels was investigated by TGA under argon and air atmosphere in Fig. 4, respectively. The detailed degradation parameters were concluded in Table 2. Initial degradation temperature is defined as temperature with 5% weight loss of xerogels (T5%) [25, 41, 42]. T5% tended to enhance with the controlled addition of SP under argon and air atmosphere. TGA curves in Fig. 4a showed that weight loss had the tendency to decrease with increase of 0.1Ph-SP content. The content of 0.1Ph-SP should be controlled higher than that in RF-SP-1 for improving the thermal stability of composite xerogels in argon atmosphere. DTG in Fig. S2a showed that SP had three main degradation steps peaked at about 300, 600, and 750 °C. As discussed, SP degraded by release of cyclic molecules, benzene and methane with increase of temperature [37]. RF-2 presented the broad weight loss band, corresponding to the release of volatiles including adsorbed molecules and pyrolyzed molecules to remove heteroatoms [43]. After incorporation of SP in RF, the degradation process was overlapped and weight loss inclined to be finished after 800 °C.

Fig. 4
figure 4

a TGA curves of RF-SP composite xerogels with different ratios of RF to SP(0.1Ph-SP) tested under argon atmosphere and b air atmosphere. c TGA curves of RF-SP-5 and RF-SP-6 tested under argon atmosphere and d air atmosphere

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Table 2 Thermal degradation parameters of RF-2, SP, and RF-SP composite xerogels under argon and air atmospheres
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Incorporation of SP is advantageous for antioxidation property of RF xerogels in practical application at air condition. Thermal stability of xerogels under air condition was revealed by TGA and DTG in Figs. 4b and S2b. The results indicated that thermal oxidation degradation mainly occurred in the range from 200 to 600 °C. TGA curves indicated that weight loss was decreased by increasing content of SP. Mass retention of RF-SP composites increased by 20.5–50.5 wt% with varying SP contents. It was shown that degradation process of RF-2 left off when heating temperature was about 600 °C from TGA curve. TGA indicated that RF-SP composite xerogels transformed into thermal-resistant residuals under air condition, but RF was decomposed into small molecules and completely removed without residue at 1000 °C.

The effects of phenyl groups on thermal property were analyzed by TGA and DTG under argon and air atmosphere in Figs. 4c, d and S2c, d. Thermal degradation parameters were listed in Table 2. Comparing RF-SP composite xerogels with different phenyl contents, the degradation behavior of RF-poly(methylsiloxane) and RF-poly(methylphenylsiloxane) was obviously different in argon atmosphere. The sharp weight loss was observed at about 700 °C in RF-poly(methylsiloxane) composite (RF-SP-5). The thermal degradation mechanism of polysiloxane and influences of phenyl groups on thermal stability were widely discussed by previous reports [44,45,46]. Chain mobility of polysiloxane was resisted due to the rigidity of phenyl groups, and further cross linking reaction by removing phenyl groups under heating also restrained the degradation of SP. Observed from TGA of RF-SP-2, RF-SP-5, and RF-SP-6 in Fig. 4b, d, RF-SP had similar degradation process with different phenyl contents under air atmosphere. The oxidation process removed organic groups and transformed polysiloxane into heat-resistant component [45]. The results certified that phenyl group favored in reducing the thermal degradation during 600 to 800 °C under argon atmosphere. In addition, when content of phenyl-substituted monomer was beyond 20%, macroscale phase separation of RF from SP tended to happen. Thus, polysiloxane of 0.1Ph-SP was appropriate for preparing heat-resistant RF-SP composite xerogels.

Thermal oxidation property was further demonstrated by treating RF-SP composite xerogel monoliths at 600 °C under air condition. The photographs of macroscale observation in insets of Fig. 5a–c revealed that the treated RF-SP composites still retained the pieces of monolithic structure. However, without residue was left from the heated RF-2. SEM images in Fig. 5a–c revealed that the heated sample of RF-SP-1 existed broken spherical shells, the heated RF-SP-2 melted into a continuous skeleton, and the large spheres and smaller ones coexisted in the heated RF-SP-3. Oxidative degradation of RF xerogels tended to finish at 600 °C in accordance with TGA in Fig. 4b. After the calcined samples were grinded, hollow shells with thickness in tens of nanometers were observed in Fig. S3a, b. EDS analysis (Fig. 5e) of the calcined RF-SP-2 (SEM image of large scope in Fig. 5d) indicated that content of carbon element decreased, and contents of silicon and oxygen elements increased when compared with pristine RF-SP-2, implying organic components were degraded and formed volatiles. The remains of calcined RF-SP composite xerogels were mainly composed by derivatives of polysiloxane from Fig. S4, in which a broad band of Si–O–Si absorption existed around 1100 cm−1 [45].

Fig. 5
figure 5

SEM images of RF-SP composite xerogels after being heated at 600 °C under air condition and inset of macroscale photographs, a RF-SP-1, b RF-SP-2, c RF-SP-3. d SEM image and e EDS spectrum and silicon element mapping image of RF-SP-2 after being heated at 600 °C under air condition

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The analysis on microstructure of RF-SP composite xerogels was further verified by the results in Fig. 5. When SP content was low in RF-SP-1, a thin layer of SP coated along RF skeleton. After being heated at 600 °C in air, RF was mostly removed and SP transformed into SiO2 shell. With increase of SP ratio, SP grew on surface of RF and filled the gaps among RF microspheres in RF-SP-2. The continuous skeleton of RF-SP-2 was favored for hydrophobic property and structure stability under high temperature. But, individual SP microspheres were formed due to higher concentration of SP in RF-SP-3.

3.3 Carbonized derivatives of RF-SP composite xerogels

Carbonization treatment is favorable for improving thermal stability of xerogels because unstable structure will be removed during this process. The typical sample RF-SP-2 was carbonized at 500 and 800 °C under nitrogen atmosphere. With increase of carbonization temperature, the shrinkage of derived xerogels enlarged while the intact monoliths were obtained (insets of Fig. 6). As shown in inset of Fig. 6a, hydrophobic property of xerogel was remained after 500 °C carbonization. SEM image in Fig. 6a indicated that 500 °C-RF-SP-2 was collapsed during SEM sample preparation. After raising carbonization temperature to 800 °C, 800 °C-RF-SP-2 monoliths were not easily fragile compared with RF-SP-2 and 500 °C-RF-SP-2. Particles of 800 °C-RF-SP-2 fused together to form network with interpenetrated macropores as shown in Fig. 6b. The density of 800 °C-RF-SP-2 slightly increased to 0.256 g/cm3. Mercury intrusion porosimetry (MIP) in Fig. 7 showed macropores with pore size distribution in the range from 10.0 to 20.0 μm were tested, which was coincident with SEM observation. The measured average pore diameter was about 17.1 μm.

Fig. 6
figure 6

SEM images of carbonized RF-SP-2. a 500 °C-RF-SP-2 (insets of sample photograph and contact angle test). b 800 °C-RF-SP-2 (inset of sample photograph)

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Fig. 7
figure 7

a Cumulative intrusion curve of 800 °C-RF-SP-2. b Pore size distribution curve of 800 °C-RF-SP-2

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The thermal oxidation resistance of 500 °C-RF-SP-2 and 800 °C-RF-SP-2 was evaluated by TGA and DTG under air atmosphere in Fig. 8. The degradation temperature with weight loss reaching 5% (T5%) was increased from 286 °C of RF-SP-2 to 410 °C of 500 °C-RF-SP-2 and 542 °C of 800 °C-RF-SP-2, respectively. With increasing carbonization temperature, the residual mass was enhanced from 500 °C-RF-SP-0.5 of 43.5 wt.% to 800 °C-RF-SP-0.5 of 57.1 wt.%. The temperature of maximum weight loss raised from 490 °C of RF-SP-2 to 523 °C of 500 °C-RF-SP-2 and 621 °C of 800 °C-RF-SP-2. The results indicated that carbonized samples had better thermal oxidation resistance than the pristine xerogels.

Fig. 8
figure 8

a TGA and b DTG curves of 500 °C-RF-SP-2 and 800 °C-RF-SP-2 under air atmosphere

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The chemical structure was analyzed for exploring the thermal oxidation stability of carbonized sample by FTIR and XPS. Structure transformation of RF-SP-2 was analyzed in Fig. 9 by FTIR spectra. The spectrum of pristine RF-2 xerogels existed C–O–C absorptions at 1224 and 1108 cm−1, aromatic structure at 1602 cm−1, and C–H stretching vibration at 1442 and 2971 cm−1 [2]. FTIR spectrum of SP xerogels (Fig. 9b) had some difference with that of SP prepolymer (Fig. 1a). The decreased Si–OH absorption at around 3500 cm−1 and the splitting peaks in the range from 1000 to 1100 cm−1 in Fig. 9b indicated the condensation of SP prepolymer and formation of SP polymer in xerogel. In RF-SP-2, the characteristic peaks of polysiloxane and RF were observed. Comparison of RF-SP-2 with 500 °C-RF-SP-2 showed that RF absorption was weakened and polysiloxane characteristics were apparently observed by C–H peak of Si–CH3 at 1271 cm−1 after 500 °C carbonization [37, 38]. The existence of Si–CH3 groups in 500 °C-RF-SP-2 provided the hydrophobic property. After 800 °C carbonization, C–H structure was detected by the small peaks around 2853 and 2923 cm−1. The bands around 1000 to 1100 cm−1 of pristine RF-SP-2 merged into a broad one, indicating transformation of polysiloxane into SiO2 [45].

Fig. 9
figure 9

FTIR spectra. a RF-2, b SP, c RF-SP-2, d 500 °C-RF-SP-2, and e 800 °C-RF-SP-2

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XPS was applied for testing the surface chemical structure of carbonized samples in Fig. 10. The spectrum of C1s of 500 °C-RF-SP-2 in Fig. 10a presented the detailed chemical environment of C atom. It was shown that the carbon states were composed by C=O (288.7 eV), C–O (286.1 eV), C–C (284.9 eV) of aliphatic and aromatic structures, and Si–C (284.2 eV) structures [27]. Compared with 500 °C-RF-SP-2, Fig. 10b showed that C=O structure was not detected in 800 °C-RF-SP-2. C–C ratio increased due to the carbonization reaction of RF. The analyses indicated that oxygen atoms were gradually removed during carbonization procedure. Si2p spectra in Fig. 10c, d certified that SiO2 (103.6/103.5 eV) existed in the carbonized derivatives. 102.9/102.8 and 102.3/102.1 eV were attributed to organic group-substituted siloxane structure [47].

Fig. 10
figure 10

a, b C1s spectra of 500 °C-RF-SP-2 and 800 °C-RF-SP-2. c, d Si2p spectra of 500 °C-RF-SP-2 and 800 °C-RF-SP-2

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XRD patterns of carbonized samples of 500 °C-RF-SP-2 and 800 °C-RF-SP-2 were shown in Fig. 11. 500 °C-RF-SP-2 had a broad band at around 22°, reflecting the amorphous characteristics. Broad diffractions centered at 22° and 44° in 800 °C-RF-SP-2 were ascribed to weakly ordered graphitic structure of (002) and (100) [27, 48]. The results indicated that carbonized derivatives gradually transformed into regular structure with enhancing treatment temperature.

Fig. 11
figure 11

XRD patterns of 500 °C-RF-SP-2 and 800 °C-RF-SP-2

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The structure analyses of carbonized derivatives were investigated in detail by above methods. The results revealed the structure transformation of xerogels after carbonization process. As reported, oxidation reactivity of RF-carbonized derivatives decreased with increase of carbonization temperature in that carbonization process removed heteroatoms and reduced defects of RF [49]. In addition, FTIR and XPS analyses indicated that after carbonization, SiO2 as protection layer was benefit for preventing permeation of oxygen. The structural transformation of polysilxoane by breakage of Si–C and Si–O reconstruction was accordant with the reported pyrolysis mechanism [27]. The thermal transformation of RF-SP composite xerogels improved the antioxidation degradation of carbonized samples.

As potential application in thermal insulation, thermal conductivity of as-prepared RF-SP-2 and carbonized derivatives of 500 °C-RF-SP-2 and 800 °C-RF-SP-2 were tested to be 0.152, 0.193, and 0.186 W/(m•K), respectively. The results were similar to samples prepared by ambient drying [50]. Herein, hydrophobic and thermal-resistant RF-SP composite xerogels were prepared by incorporation of polysiloxane. The monolithic structure was not collapsed up to 800 °C under nitrogen condition. Especially, 500 °C-carbonized RF-SP xerogels still preserved hydrophobicity.

4 Conclusion

In this work, poly(methylphenylsiloxane) was synthesized to modify RF xerogels. The results showed that polysiloxane-modified RF composite xerogels formed microscale phase separation texture. The lightweight and hydrophobic RF-SP composite xerogels were obtained with density lower than 0.300 g/cm3 and water contact angle about 130°. TGA results with a heating rate 10 °C/min indicated that the residual yield at 1000 °C of typical RF-SP-2 xerogels was 12.0 wt% larger in argon and 41.0 wt% larger in air than those of pristine RF xerogels. After carbonization treatment at 500 and 800 °C in nitrogen atmosphere, thermal degradation temperature (T5%) was enhanced by 124 and 256 °C compared with as-prepared RF-SP composites. Poly(methylphenylsiloxane)-modified RF xerogels were hydrophobic, and the hydrophobicity was maintained up to 500 °C after carbonization under nitrogen. XRD patterns indicated that weakly ordered carbon was formed after carbonization. FTIR and XPS analyses indicated that structure transformation of polysiloxane at high temperature resulted in further condensation of residual Si–OH and formation of heat-resisted SiO2 structure. The as-prepared composite xerogels and carbonized derivatives with low density and low thermal conductivity could be potentially used in the field of thermal insulation under moisture condition and oxidation condition.