Novel bridged polysilsesquioxane aerogels with great mechanical properties and hydrophobicity

Zhiyao Qiao; Xiaotian Liu; Kejie Heng; Farong Huang

doi:10.1515/epoly-2022-0077

Article Open Access

Novel bridged polysilsesquioxane aerogels with great mechanical properties and hydrophobicity

Zhiyao Qiao , Xiaotian Liu , Kejie Heng and Farong Huang

Published/Copyright: November 7, 2022

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From the journal e-Polymers Volume 22 Issue 1

Abstract

Two novel bridged silsesquioxane (BSQ) precursors, 1,4-bis(trimethoxysilylpropylthioethenylphenoxy)benzene and 1,3-bis(trimethoxysilylpropylthioethenyl)benzene, were synthesized from arylacetylenes and 3-mercaptopropyltrimethoxysilane through the thiol-yne click reaction. Then, bridged polysilsesquioxane aerogels were prepared from BSQ precursors alone or together with tetramethoxysilane (TMOS) as a co-precursor through the sol–gel method. The resulted aerogels were characterized by scanning electron microscopy, mercury intrusion porosimetry, thermogravimetric analysis, transient hot-wire method, and water contact angle tests. Their mechanical properties were evaluated by unidirectional compression tests. The properties of the aerogels are strongly affected by precursor content, mass ratio of BSQ to TMOS, and the structures of BSQ. The obtained aerogels do not break even when compressive strain is up to 55% and show high compressive modulus (≥2.46 MPa) and hydrophobicity (water contact angle ≥130°).

Graphical abstract

Keywords: thiol-yne click reaction; bridged polysilsesquioxane aerogels; hybrid aerogels; aerogels with great mechanical properties and hydrophobicity; light material with heat resistance

1 Introduction

Aerogels are known for high porosity, high specific surface area, low density, and extraordinary thermal insulation abilities, which is unique solid-state composed of three-dimensional (3D) interconnected networks filled with a huge number of air pores (1). The unique structure and outstanding performance of aerogels make them potential to be applied in many fields, such as acoustic insulation (2), optical applications (3), electrical applications (4), catalyst supports (5), thermal insulation (6), oil-spill cleanup (7), and aerospace applications (8).

Silica aerogels have widely been used owing to their extensive source of raw materials and mature preparation technology (9). However, the commercial use of silica aerogels is restricted due to their brittleness and instability toward atmospheric moisture. The silica aerogels would shatter into powder due to large capillary force developed as water permeates into the mesoporous structure (10).

Synthesizing organic–inorganic hybrid aerogels (11,12,13,14,15) is the most popular way to improve mechanical properties of silica aerogels, of which bridged polysilsesquioxane (BPSQ) aerogels (16,17,18) are the representative ones. BPSQ aerogels can be synthesized from bridged silsesquioxane (BSQ) precursors through the sol–gel process. The variable organic bridges of the precursors offer BPSQ aerogels considerable flexibility to tune the performances inherently because the covalently bonded organic parts possess more than 50% mass of BPSQ aerogels (19,20). Yun et al. (21) had introduced C–O and C–S bonds into the molecular chains of BSQ aerogels to obtain mechanically enhanced aerogels whose compressive modulus is 0.11 MPa. However, it has been found that BPSQ aerogels with more rigid bridging groups tend to exhibit better mechanical properties. For example, Boday et al. (22) prepared hexylene- and phenylene-bridged polysilsesquioxane aerogels and showed that exceptional improvements in the mechanical properties were realized in the case of phenylene-bridged polysilsesquioxane aerogels. Zou et al. (23) synthesized three kinds of thiourethane-bridged BPSQ aerogels from three diisocyanates with different rigidity, and they found that the aerogels with the most rigid bridging groups exhibit the best mechanical properties and are able to withstand at least 50% deformation under compression.

In this article, through thiol-yne click reaction, two novel vinylene-aromatic-vinylene BSQ precursors with greater rigidity than those mentioned above were synthesized and used to further prepare vinylene-aromatic-vinylene BPSQ aerogels alone or together with tetramethoxysilane (TMOS) by using the sol–gel method. The rigid vinylene-aromatic-vinylene structure in BPSQ aerogels contributes to their high resistance to compression. Meanwhile, C–S bond in the BSQ structure gives aerogels flexibility to withstand deformation under compression. The influence of precursor content, mass ratio of BSQ to TMOS, and the structure of BSQ on the properties of BPSQ aerogels was discussed.

2 Materials and methods

2.1 Materials

1,4-Bis-(4-ethynylphenoxy)benzene (BEPB) was purchased from Beijing HWRK Chemical Co. Ltd., China. m-Diethynylbenzene (DEB) was supplied by Shanghai Record Pharmaceuticals Co. Ltd., China, which was distilled under nitrogen before use. 3-Mercaptopropyltrimethoxysilane (MTMS) was purchased from Shanghai Adamas Reagent Co. Ltd., China. Azodiisobutyronitrile (AIBN) was obtained from Sinopharm Chemical Reagent Co. Ltd., China. 1,4-Dioxane, hydrochloric acid, and tetramethoxysilane (TMOS) were provided from Shanghai Taitan Scientific Co. Ltd., China.

2.2 Characterization

The proton nuclear magnetic resonance (¹H NMR) spectrum was measured on an AVANCE III 400 spectrometer (Bruker, Massachusetts, USA) and CDCl₃ containing tetramethylsilane was used as a solvent. The FT-IR spectrum was recorded on the Nicolet iS10 infrared spectrometer (Thermo Scientific, Waltham, USA). The scanning range is from 4,000 to 400 cm⁻¹. The porosity and average pore size were measured by the mercury intrusion method using a fully automatic mercury porosimeter AutoPore V 9600 (Micromeritics, Norcross, USA). The surface morphology of the prepared aerogels was characterized by scanning electron microscopy (SEM) on the S-4800 electron microscope (Hitachi, Tokyo, Japan) at an accelerating voltage of 15 kV. The density of the samples was obtained by the measurement of the weight of aerogels per volume. The contact angle measurements were applied by using a contact angle meter JC2000D2 (Shanghai Zhongchen Digital Technology Equipment Co. Ltd., Shanghai, China) to quantify the degree of hydrophobicity of the aerogels. A drop of water was dropped on an aerogel surface located on a moveable table. The droplet was illuminated from one side by light and a camera at the opposite side recorded the image. The compression tests were performed in accordance with ASTM D695-10 standard on a universal material testing machine CRIMS DDL 100 (Changchun Research Institute for Mechanical Science Co. Ltd., Changchun, China) moved at a speed of 10 mm·min⁻¹. The bottom and top surfaces of the samples were polished to make sure they were smooth and parallel. The modulus was obtained by the initial slope from the stress vs strain curve. The thermal conductivity of the aerogels was measured using C–T meter TC3200 (Xi’an Xiaxi Electronic Technology Co. Ltd., Xi’an, China) by sandwiching the ring probe sensor between two plane aerogel sheets. Thermogravimetric analysis (TGA) was performed on the TGA/DSC 1LF analyzer (Mettler Toledo, Greifensee, Switzerland) at a heating rate of 10 K·min⁻¹ in nitrogen atmosphere from RT to 900°C. The nitrogen flowed at 60 mL·min⁻¹.

2.3 Synthesis of BSQ precursors

The synthesis route of BSQ precursors is shown in Scheme 1. All synthetic reactions were carried out under dry nitrogen. The BSQ precursors were prepared in a 500-mL four-neck flask equipped with a condenser, a mechanical stirrer, a thermometer, and a constant pressure dropping funnel. The detailed procedure for the preparation is as follows. BEPB (18.621 g, 0.06 mol), 1,4-dioxane (100 mL), and AIBN (0.3 g, 2 mmol) were fed into the flask in order. And then the solution of MTMS (0.12 mol, 23.561 g) and 1,4-dioxane (100 mL) was dropped into the flask from the constant pressure dropping funnel. Keep the reaction system at room temperature for about 15 min after the dropping addition. Thereafter, the mixture was heated to and kept at 100°C over 10 h. The solvent was evaporated under reduced pressure to obtain a viscous yellow liquid bis(trimethoxysilylpropylthioethenylphenoxy)benzene, which was named as BTPB. Bis(trimethoxysilylpropylthioethenyl)benzene (named as BTB) precursor was synthesized in the same way as above by using DEB (7.569 g, 0.06 mol) instead of BEPB (18.621 g, 0.06 mol) during the synthetic process.

Scheme 1

The synthesis route of the BSQ precursors (a) and BPSQ aerogels (b).

2.4 Preparation of BPSQ aerogels

Scheme 1 illustrates the synthetic way to BPSQ aerogels. The BPSQ aerogels were produced in a one-step acid-catalyzed sol–gel synthesis by hydrolysis and condensation of the BSQ precursors (or with TMOS) diluted by 1,4-dioxane, using hydrochloric acid as a catalyst. The following procedure was used for the preparation process. A certain amount of BTPB, 1,4-dioxane or BTPB, 1,4-dioxane, and TMOS were added into a beaker (50 mL, 45 mm in diameter) at a room temperature and stirred till clear solution was formed. Afterwards, the hydrolysis and condensation reactions or co-hydrolysis and co-condensation reactions were triggered by the addition of hydrochloric acid and deionized water to the beaker. The mixture in the beaker was stirred for a while and kept to stand at room temperature till gelation. The fresh wet gel was formed and aged at room temperature in the beaker for some time to strengthen the skeleton. And then the organic–inorganic hybrid wet gel was frozen before it was put into the freeze dryer. A BTPB aerogel was acquired after being freeze-dried for 24 h in a freeze dryer (FD-1A 50, Shanghai Yuming Instrument Co. Ltd., China). The freeze temperature is −50°C and the vacuum pressure is less than 20 MPa. A series of hybrid BPSQ aerogels were designed and prepared from BSQ precursors (BTPB, BTB) and TMOS listed in Table 1. As shown in Table 1, the contents of precursors (BSQ or BSQ and TMOS) in the gels were 10, 15, and 20 wt%, and the mass ratio of BSQ to TMOS was chosen as 1:0, 3:1, and 1:1. The concentration of hydrochloric acid was 0.01 mol‧L⁻¹. And the molar quantity of water added was equal to the molar quantity of Si-OCH₃ groups in the precursors.

Table 1

Formulations of the BPSQ aerogels

BPSQ aerogels	Precursor content [M _p ^a/(M _p + M _s ^b)] (wt%)	Mass ratio (M _p1 ^c/M _p2 ^d)	TMOS content (M _p2/M _p)
BTPB-10-0	10	1:0	0
BTPB-10-0.25	10	3:1	0.25
BTPB-10-0.50	10	1:1	0.50
BTPB-15-0	15	1:0	0
BTPB-15-0.25	15	3:1	0.25
BTPB-15-0.50	15	1:1	0.50
BTPB-20-0	20	1:0	0
BTPB-20-0.25	20	3:1	0.25
BTPB-20-0.50	20	1:1	0.50
BTB-10-0	10	1:0	0
BTB-15-0	15	1:0	0
BTB-20-0	20	1:0	0

^aThe mass of the precursors. ^bThe mass of the solvent. ^cThe mass of BSQ. ^dThe mass of TMOS, M _p = M _p1 + M _p2. Aerogel name: precursor-precursor content-TMOS content.

3 Results and discussion

3.1 The structure characterization of BSQ precursors

The ¹H NMR spectra of BSQ precursors are shown in Figure 1. The peaks at 0.8, 1.8, and 2.8 ppm belong to the protons of Si–C H ₂–CH₂–CH₂–S, Si–CH₂–C H ₂–CH₂–S, and Si–CH₂–CH₂–C H ₂–S, respectively. The peak at 3.56 ppm is assigned to the vibration of methoxy protons. The peaks in the range from 6.14 to 6.63 ppm are attributed to the protons of ethenylene. And the peaks from 6.81 to 7.47 ppm are ascribed to the protons on the phenylene. It is worth mentioning that there are four doublets to the protons of ethenylene, which belong to the two protons on the cis structure and the other two protons on the trans structure, respectively. As known from the spectra, the two doublets from 6.14 to 6.42 ppm with the coupling constant of 15.6 J·Hz⁻¹ and the two doublets from 6.45 to 6.63 ppm with the coupling constant of 10.8 J·Hz⁻¹ are, respectively, contributed to the protons of cis structure and those of the trans structure, since the coupling constant of the cis structure is larger than that of the trans structure. It can be known from Figure 1 that the integral area ratio of peaks assigned to different structures a:b:c:d:(e + f + g + h):i is 4.02:4.00:4.10:18.04:4.08:12.07 for BTPB (Figure 1a) and the integral area ratio of the peaks a:b:c:d:(e + f + g + h):i is 4.03:4.00:4.06:18.02:4:08:3.92 for BTB (Figure 1b), corresponding with theoretical value 4.00:4.00:4.00:18.00:4.00:12.00 and 4.00:4.00:4.00:18.00: 4.00:4.00, respectively. Therefore, the desired products have been synthesized.

Figure 1

¹H NMR spectra of BTPB (a) and BTB (b).

The FT-IR spectra of MTMS, BSQ, and BPEP or DEB are shown in Figure 2. The FT-IR spectrum for MTMS shows primary absorption peaks at 2,560, 2,840, and 2,940 cm⁻¹ for –SH, –CH₃, and –CH₂, and C–Si, C–O, and C–O–Si bonds vibrate at 1,200, 1,080, and 812 cm⁻¹, respectively. It can be seen in Figure 2a, in the case of BEPB, the primary absorptions of ≡C–H, Ar–H, and C–O–C are observed at 3,280, 3,040, and 1,010 cm⁻¹, respectively. The absorption peaks at 1,600 and 1,500 cm⁻¹ are attributed to the vibration of the skeleton of the benzene ring. The situation is similar in Figure 2b for DEB, the absorptions of ≡C–H, Ar–H, and the skeleton of the benzene ring can also be observed at 3,280 and 3,060 cm⁻¹ and 1,600 and 1,500 cm⁻¹. Besides, the observation peak of 2,100 cm⁻¹ is attributed to C≡C bond. It is worth noticing that the peak of ≡C–H at 3,280 cm⁻¹ is not observed in the FT-IR spectra of BTPB and BTB, and the peak for C≡C bond vibrating at 2,100 cm⁻¹ in DEB spectrum could also not be observed in the BTB spectrum. This fact with the presence of the peak for –SH in MTMS at 2,560 cm⁻¹ and the absence of the peak in BTPB and BTB represents the terminal alkynyl of BEPB and DEB at 3,280 cm⁻¹ does undergo an addition reaction with sulfhydryl of MTMS, which further corroborates that the desired product has been obtained.

Figure 2

FT-IR spectra of MTMS, BTPB, and BEPB (a) and MTMS, BTB, and DEB (b).

3.2 Formation and properties of BPSQ aerogels

3.2.1 Formation and morphology of BPSQ aerogels

The wet gels were formed in the beaker (50 mL, 45 mm in diameter) and then freeze-dried to obtain aerogels. A macroscopic view of some selected BTPB(/TMOS) aerogels is shown in Figure 3. It can be seen that the obtained aerogels are complete cylinders without obvious cracks. As seen in Figure 3, upon drying, the wet gels become opaque solids, and the obtained aerogels are yellow with a flat surface, which all show shrinkage to some extent. With increasing TMOS content in the precursor (the mass ratio of BTPB to TMOS changes from 1:0 to 1:1) and decreasing total precursor content from 20 to 10 wt%, obvious reduction is observed in the shrinkage of the aerogels.

Figure 3

The photos of a BTPB(/TMOS) wet gel and some aerogels with different shrinkage (a) BTPB-20-0, (b) BTPB-20-0.25, (c) BTPB-20-0.50, (d) BTPB-15-0, and (e) BTPB-10-0.

Figure 4 presents the SEM images of BPSQ aerogels made with different formulations. The influence of precursor content, TMOS content in precursors, and precursor structure on the nanostructural properties of aerogels was studied. BTB aerogels were fabricated by replacing flexible BTPB with rigid BTB to study the effect of chain rigidity on structures and properties. In general, the framework of the obtained aerogels all shows a 3D network structure like honeycomb. The pore size decreases with the increase in the precursor content, which is consistent with previous reports (19,23). Besides, the pore size becomes larger with TMOS content increasing. In addition, the pore size of BTB aerogels is larger than that of BTPB aerogels with the same formulations, which is consistent with the result of the mercury intrusion porosimetry presented in Figure 5.

Figure 4

SEM images of BPSQ aerogels: (a) BTPB-10-0, (b) BTPB-15-0, (c) BTPB-15-0.50, and (d) BTB-15-0.

Figure 5

Pore diameter distributions of selected BPSQ aerogels: (a) BTPB (/TMOS) aerogels and (b) BTB aerogels. Average pore diameter of (c) BTPB (/TMOS) aerogels and (d) BTPB and BTB aerogels. Porosity of (e) BTPB (/TMOS) aerogels and (f) BTPB and BTB aerogels.

Figure 5a and b presents the pore distribution of the selected BPSQ aerogels. It could be seen that the pore size distributes from a few nanometers to a few micrometers. The pore size distribution gradually widens with increasing TMOS content in precursor and decreasing precursor content, which is the same as SEM observation results. In Figure 5a, when the mass ratio of BTPB to TMOS is 1:0, there are mainly mesopores (2–50 nm) in the aerogels, and as TMOS content increasing, many larger-diameter pores (about 200 nm) could be observed. When the mass ratio is 1:1, the aerogels show macropores with the peak pore size of about 1 μm. It could probably be explained that BSQ precursors easily react with TMOS and the hybrid polymers with higher molecular weight are produced during the formation of the aerogels. In Figure 5b, when the precursor content is up to 20 wt%, the main pore size is 5–100 nm, when precursor content is 15 wt%, the aerogels contain many larger pores with the pore size about 200 nm, and when precursor content is 10 wt%, there are larger pores with the pore size about 400 nm. It could be explained that there are fewer crosslinking points among the gel but more solvent with precursor content decreasing, resulting in larger-diameter pores. The result is consistent with the shrinkage mentioned above. Larger-diameter pores are beneficial to decrease the capillary pressure and inhibit the volume shrinkage of the aerogels.

Figure 5c and d presents the pore size of BPSQ aerogels with different formulations and compares the average pore size of BTPB and BTB aerogels. The pore size of BPSQ aerogels decreases with precursor content increasing and TMOS content decreasing. For the aerogels with same formulation but different structure, the average pore diameter of BTB aerogels is larger than that of BTPB aerogels. It is possible that BTB with stronger rigidity could resist the pore shrinkage.

Figure 5e and f presents the porosity of BPSQ aerogels made with different formulations. The obtained porosity is from 56.00% to 84.81%. The porosity shows a downward trend with decreasing TMOS content in the precursor and increasing precursor content. BTPB-derived aerogels show relatively lower porosity than BTB-derived ones.

3.2.2 Physical properties of BPSQ aerogels

The density of the aerogels is tabulated in Table 2. The density ranges from 0.102 to 0.321 g·cm⁻³. Obviously, the density increases with increasing precursor content and decreasing TMOS content in the precursor. Besides, BTB-derived aerogels present lower density than BTPB-derived ones. It could be observed that the density follows the opposite trend with porosity in Figure 5 as expected, high porosity contributes to low density.

Table 2

Density, water contact, compressive modulus, and thermal conductivity of BPSQ aerogels

BSQ/TMOS aerogels	Density (g·cm⁻³)	Water contact angle (°)	Compressive modulus (MPa)	Thermal conductivity (W·m⁻¹·K⁻¹)
BTPB-10-0	0.181	134	5.55 ± 0.14	0.065
BTPB-10-0.25	0.177	132	4.83 ± 0.09	0.062
BTPB-10-0.50	0.102	130	2.46 ± 0.04	0.040
BTPB-15-0	0.240	143	6.25 ± 0.07	0.076
BTPB-15-0.25	0.201	142	5.26 ± 0.13	0.067
BTPB-15-0.50	0.169	134	4.47 ± 0.09	0.055
BTPB-20-0	0.321	146	8.38 ± 0.09	0.084
BTPB-20-0.25	0.205	144	5.46 ± 0.11	0.072
BTPB-20-0.50	0.179	138	4.62 ± 0.14	0.065
BTB-10-0	0.114	133	5.98 ± 0.14	0.049
BTB-15-0	0.172	137	7.76 ± 0.17	0.057
BTB-20-0	0.306	138	13.65 ± 0.09	0.072

The contact angles of water droplet on the surface of BPSQ aerogels are shown in Table 2. The water contact angles could demonstrate the hydrophobicity of BPSQ aerogels. The water contact angles for the aerogels are not lower than 130°, which is much higher than that of silica aerogels with the water contact angle between 20° and 30° (24). It is clear that the obtained aerogels show good hydrophobic behavior relating to the existence of the bridging groups in BPSQ aerogels. Wettability of a material depends on both surface microscopic properties and the chemical functional groups of the surface. It is worth mentioning that when water drops on the surface of the aerogels, the air entering the aerogel pores will not be squeezed out by water but will be surrounded by water and stay in the nanopores, filling the air space of the microstructure, thus forming a composite surface where the air and the solid surface intersect. The presence of air will further increase the value of the apparent water contact angle. That contributes to the extremely high hydrophobicity of the obtained aerogels.

The water contact angle slightly decreases with decreasing mass ratio of BSQ to TMOS owing to the increase of hydrophilic Si–O–Si groups and the decrease of non-polar organic groups. Besides, there are more macropores with TMOS content increasing, which is beneficial to make waterdrop into the surface of the aerogels, leading to lower hydrophobicity. The hydrophobicity of the aerogels is improved a little with precursor increasing. This might also result from decreasing average pore sizes. Moreover, BTPB-derived aerogels are a little more hydrophobic than BTB-derived aerogels with the same formulation. It could be explained that there are less hydrophobic organic groups and more hydrophilic Si–O–Si in BTB-derived aerogels comparing to BTPB-derived ones with the same formulation, on the one hand. And on the other hand, the average pore size of BTB-derived aerogels is larger than BTPB-derived ones.

3.2.3 Mechanical properties of BPSQ aerogels

The compressive stress–strain curves for the aerogels are presented in Figure 6. The compressive modulus is defined as the slope of the initial linear portion of stress–strain curves. As we know, a native silica aerogel derived from TMOS with a density of 0.12 g·cm⁻³ can be completely crushed into dust under a small stress of 31 kPa (25). However, as seen in Figure 6a, BTPB, BTPB/TMOS, and BTB aerogels will not shatter under compressive load of higher than 500 kPa even at 55% strain. The modulus of the obtained aerogels is from 2.46 to 13.65 MPa, which is much higher than that of silica aerogels (102 kPa) with the density of 0.071 g·cm⁻³ (26). As seen in Table 2, the compressive modulus has a strong dependence on precursor content and the mass ratio of BSQ to TMOS. The modulus increases with increasing of precursor content and decreasing of TMOS content in precursor. Generally, extensive experimental results demonstrate that the modulus of aerogels increases with increasing density (27). As shown in Figure 6b, the modulus of the aerogels does have a tight correlation with density (R ² = 0.95). And the compressive modulus changes follow the same trend with density as expected. However, BTB-derived aerogels show a higher compressive modulus comparing with BTPB-derived aerogels with the same formulation. It could be explained by the greater rigidity of the BTB backbone. The modulus (13.65 MPa) is highest for aerogels when precursor content is 20 wt% and mass ratio of BTB to TMOS is 1:0.

Figure 6

(a) Stress–strain curves for selected BPSQ aerogels. (b) Modulus versus density plot for various BTPB(/TMOS) aerogels.

3.2.4 Thermal properties of BPSQ aerogels

The thermal conductivity evaluates the suitability of the BPSQ aerogels for thermal insulation purposes, which is presented in Table 2. The obtained aerogels show a relatively low thermal conductivity, ranging from 0.040 to 0.084 W·m⁻¹·K⁻¹. The low thermal conductivity originates in high porosity and small pore size. The very little fraction of solid structure exhibits extremely low solid conductivity, transmitting low thermal energy hence. Thermal energy could also be transported through gases in the open pores of aerogels. If the pore size is lower than the free path of air (69 nm, in standard state), it could lead to a very low gaseous thermal conductivity (28). The porosity of BPSQ aerogels (56.00–84.81%) is lower than that of silica aerogels (95%), and the average pore size of the obtained aerogels is up to tens of micrometers, which is greater than that of silica aerogels (2–5 nm). Thereby, the main thermal conductivity is higher than that of silica aerogels (0.02 W·m⁻¹·K⁻¹) (9).

Figure 7 shows the TGA analysis for BPSQ aerogels with different formulations conducted in N₂ from ambient temperature to 900°C. And the corresponding data are tabulated in Table 3. The degradation temperature at the weight loss of 5% (T _d5) is around 350°C. The degradation temperature at the weight loss of 5% of silica aerogels is above 400°C (22), which is a little higher than that of the BPSQ aerogels (around 350°C), which is inevitable due to the introduction of organic groups in the aerogels. However, the BPSQ aerogels absorb less water than silica aerogels because of existence of organic groups, which makes the BPSQ aerogels more stable during use (22). Compared with other bridged polymethylsiloxane aerogels, the BPSQ aerogels exhibit good heat resistance. For instance, as for the thiourethane BPSQ aerogels prepared by Zou et al. (23), the T _d5 is around 300°C. At below 350°C, the weight loss of these BPSQ aerogels is small and possibly from the removal of solvent residuals, the small molecules’ release of unreacted active groups like the dehydration of some Si–OH groups. At the temperatures above 500°C, the thermal decomposition of S–CH₂, Si–CH₂, Si–O bonds, and so on possibly occurs (29). As expected, the residual rate at 800°C increases with decreasing of the mass ratio of BSQ to TMOS. And the residual rate at 800°C of BTB-derived aerogels is a little higher than BTPB-derived ones.

Figure 7

TGA curves of selected BPSQ aerogels.

Table 3

TGA analysis results of the BPSQ aerogels

BPSQ aerogels	T _d5 (°C)	Y _800°C (%)
BTPB-10-0	346.7	54.5
BTPB-10-0.25	349.3	61.6
BTPB-10-0.5	353.3	71.0
BTPB-15-0	343.3	54.2
BTPB-15-0.25	353.0	63.9
BTPB-15-0.5	345.5	69.1
BTPB-20-0	346.3	54.9
BTPB-20-0.25	349.3	63.0
BTPB-20-0.5	325.8	71.0
BTB-10-0	343.8	57.0
BTB-15-0	345.2	57.7
BTB-20-0	341.5	57.7

4 Conclusions

BSQ (BTPB and BTB) precursors were synthesized through thiol-yne reaction and then used as a precursor alone or co-precursor with TMOS to obtain BPSQ that are monolithic without crack by sol–gel method, which show high water contact angle (≥130°), compressive modulus (2.46–13.65 MPa), and low-thermal conductivity (0.040–0.084 W·m⁻¹·K⁻¹). The properties of the obtained aerogels show strong independence on precursor content, mass ratio of BSQ to TMOS, and the structure of BSQ.

With precursor content increasing, the density of aerogels increases, which contributes to higher compressive modulus and thermal conductivity. However, the density, compressive modulus, and thermal conductivity show a downtrend with mass ratio of BSQ to TMOS decreasing. Besides, BTB aerogels show lower density, thermal conductivity but higher compressive modulus than BTPB aerogels with the same formulations because of their more rigid bridging groups. The BPSQ aerogels with high hydrophobicity and compressive modulus would provide a promising application as a light heat insulation material in the field of aerospace industry.

Funding information: The authors gratefully acknowledge the support of the Fundamental Research Funds for the Central Universities (no. JKD 01221701).
Author contributions: Zhiyao Qiao: investigation, writing – original draft, writing – review and editing, visualization, methodology, formal analysis, project administration; Xiaotian Liu: methodology, formal analysis; Kejie Heng: investigation, methodology; Farong Huang: conceptualization, resources, funding acquisition, writing – review and editing, project administration and supervision.
Conflict of interest: Authors state no conflict of interest.
Data availability statement: All data generated or analyzed during this study are included in this published article.

Appendix

Figure A1 shows the water contact angle of the obtained BPSQ aerogels.

Figure A1

Water contact angle of BPSQ aerogels: (a) BTPB-20-0, (b) BTPB-20-0.25, (c) BTPB-20-0.50, (d) BTPB-15-0, (e) BTPB-15-0.25, (f) BTPB-15-0.50, (g) BTPB-10-0, (h) BTPB-10-0.25, (i) BTPB-10-0.50, (j) BTB-20-0, (k) BTB-15-0, and (l) BTB-10-0.

Figure A2 shows the pore size distributions of BTPB(/TMOS) aerogels, whose precursor content are 15 and 10 wt%.

Figure A2

Pore size distributions of selected BTPB (/TMOS) aerogels: (a) the precursor content of the aerogels is 15 wt% and (b) the precursor content of the aerogels is 10 wt%.

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Received: 2022-07-17

Revised: 2022-09-22

Accepted: 2022-09-30

Published Online: 2022-11-07

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

Articles in the same Issue

https://doi.org/10.1515/epoly-2022-0077

Keywords for this article

thiol-yne click reaction; bridged polysilsesquioxane aerogels; hybrid aerogels; aerogels with great mechanical properties and hydrophobicity; light material with heat resistance

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