Polysiloxane Bonded Silica Aerogel with Enhanced Thermal Insulation and Strength

materials

Article Polysiloxane Bonded Silica Aerogel with Enhanced Thermal Insulation and Strength

Weilin Wang, Zongwei Tong , Ran Li, Dong Su * and Huiming Ji *

Key Laboratory for Advanced Ceramics and Machining Technology of Ministry of Education, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China; [email protected] (W.W.); [email protected] (Z.T.); [email protected] (R.L.) * Correspondence: [email protected] (D.S.); [email protected] (H.J.)

Abstract: In order to improve the mechanical properties of SiO2 aerogels, PHMS/VTES-SiO2 composite aerogels (P/V-SiO2) were prepared. Using vinyltriethoxysilane (VTES) as a coupling agent, the PHMS/VTES complex was prepared by conducting an addition reaction with polyhydromethylsiloxane (PHMS) and VTES and then reacting it with inorganic silica sol to prepare the organic–inorganic composite aerogels. The PHMS/VTES complex forms a coating structure on the aerogel particles, enhancing the network structure of the composite aerogels. The composite aerogels can maintain the high speciﬁc surface area and excellent thermal insulation properties, and they have better mechanical properties. We studied the reaction mechanism during preparation and discussed the effects of the organic components on the structure and properties of the composite aerogels. The composite aerogels we prepared have a thermal conductivity of 0.03773 W·m−1·K−1 at room temperature and a compressive strength of 1.87 MPa. The compressive strength is several times greater than that of

inorganic SiO2 aerogels. The organic–inorganic composite aerogels have excellent comprehensive properties, which helps to expand the application ﬁelds of silicon-based aerogels.

Citation: Wang, W.; Tong, Z.; Li, R.; Keywords: silicon-based aerogel; polysiloxane; organic–inorganic composite; coating; thermal Su, D.; Ji, H. Polysiloxane Bonded insulation properties; mechanical properties Silica Aerogel with Enhanced Thermal Insulation and Strength. Materials 2021, 14, 2046. https://doi.org/10.3390/ma14082046 1. Introduction

SiO2 aerogels have many advantages, including low thermal conductivity, low density Academic Editor: Csaba Balázsi and large speciﬁc surface area [1–3]. They can be used for thermal insulation [4,5], oil-water separation [6], wastewater treatment [7], sound insulation [8] and catalysts [9]. However, Received: 23 March 2021 the mechanical properties of SiO aerogels are poor. The compressive strength of the Accepted: 17 April 2021 2 Published: 19 April 2021 SiO2 aerogels prepared by ambient pressure drying is generally less than 0.6 MPa [10,11]. This is because SiO2 aerogels have a three-dimensional network structure composed of

Publisher’s Note: MDPI stays neutral nanoparticles. The network can easily collapse under the action of external forces, resulting with regard to jurisdictional claims in in the poor mechanical properties of SiO2 aerogels [12]. In order to enhance the mechanical published maps and institutional affil- properties of SiO2 aerogels, the following methods have been proposed: changing pre- iations. cursor [13,14], organic polymer reinforcement [7,15] and fiber reinforcement [16,17]. We focused on two methods: changing precursor and organic polymer reinforcement. On the one hand, some precursors are used to replace tetraethyl orthosilicate (TEOS) to prepare the silicon-based aerogels. Common precursors are low-molecular-weight silanes, including methyltrimethoxysilane (MTMS) [18] and vinyltriethoxysilane (VTES) [19]. These Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. precursors all have a non-hydrolyzable group (such as methyl and vinyl), which can This article is an open access article enhance the elasticity of aerogels and reduce the fragility of aerogels [20]. However, this distributed under the terms and method decreases the specific surface area of the silicon-based aerogels [21]. Gen et al. used conditions of the Creative Commons vinyltrimethoxysilane (VTMS) and vinylmethyldimethoxysilane (VMDMS) as precursors Attribution (CC BY) license (https:// to prepare elastic aerogels [22]. Ma et al. used VTES/TEOS and MTES/TEOS as precursors creativecommons.org/licenses/by/ to prepare the silicon-based aerogels. It was found that the aerogels prepared by using 4.0/). VTES/TEOS had better mechanical properties, and the compressive strength could reach

Materials 2021, 14, 2046. https://doi.org/10.3390/ma14082046 https://www.mdpi.com/journal/materials Materials 2021, 14, 2046 2 of 12

1.45–3.17 MPa; however, the specific surface area was only 365–488 m2/g [23]. However, in these studies, they only focused on improving the mechanical properties by changing precursors. The thermal insulation properties of the composite aerogels were not studied. On the other hand, through the condensation reaction of silica sol and organic poly- mers, the last are connected with the aerogel network to form polymer-enhanced composite aerogels. Liu et al. used polyurethane to reinforce SiO2 aerogels, which increased the compressive strength of the composite aerogels to 4 MPa. However, the specific surface area significantly decreased to only 346 m2/g [24]. With NaOH as a catalyst, Lin et al. prepared the composite aerogels using polyhydromethylsiloxane (PHMS) and TEOS. The specific surface area of the composite aerogels was still small, ranging from 284 to 576 m2/g [25]. In our study, in order to comprehensively improve the thermal insulation properties and mechanical properties of the silicon-based aerogels, we used the two methods, changing precursor and organic polymer reinforcement, at the same time. The new precursor (VTES) and polymer (PHMS) were used to prepare the PHMS/VTES complex through an addition reaction. Next, the PHMS/VTES-SiO2 composite aerogels (P/V-SiO2) were prepared by a condensation reaction of the PHMS/VTES complex and inorganic silica sol. The PHMS/VTES complex formed a coating structure on the aerogel particles, achieving chemical bonding between adjacent aerogel particles, which improved the strength of the aerogel network. Therefore, the mechanical properties of the composite aerogels could be improved, while maintaining the high specific surface area and the thermal insulation properties. We analyzed the structure of the composite aerogels and discussed the effects of PHMS and VTES on the structure and properties of the composite aerogels. It provided a new idea for preparing the silicon-based aerogels with high specific surface area, low thermal conductivity and high strength.

2. Materials and Methods 2.1. Materials The following materials were sourced as indicated. Polyhydromethylsiloxane (PHMS, 1.55%, calculated by hydrogen) was from Kaihuasantai, Quzhou, China. Platinum divinyl- tetramethyldisiloxane complex (2%, calculated by platinum) was from Anpin Silicone Materials, Shenzhen, China. Isopropanol (99%) and formamide (99%) were from Fuchen, Tianjin, China. Triethoxyvinylsilane (VTES, 97%), tetraethyl orthosilicate (TEOS, 98%), trimethylsilylchloride (TMCS, 98%), methyltrimethoxysilane (MTMS, 98%), methyltriethoxysilane (MTES, 98%), nitric acid (63%), propylene oxide (99%) and n-hexane (97%) were all from Aladdin, Shanghai, China. All the materials were of analytical grade and used directly, without further treatment of puriﬁcation.

2.2. Experimental Procedure

The preparation of the PHMS/VTES-SiO2 composite aerogels (P/V-SiO2) was divided into three stages: (1) preparation of the PHMS/VTES complex, (2) preparation of the wet gels and (3) post-processing of the composite aerogels. All reactions were carried out in beakers.

2.2.1. Preparation of the PHMS/VTES Complex PHMS and VTES underwent an addition reaction at 45 ◦C with platinum divinylte- tramethyldisiloxane complex as the catalyst. The amount of the catalyst was 1 wt.% of PHMS and VTES. The PHMS/VTES complex was obtained in 10 min.

2.2.2. Preparation of the Wet Gels Firstly, TEOS, isopropanol and deionized water were mixed at the volume ratio of 1.0:1.6:0.5. Nitric acid was added to adjust the pH to 2. The mixture was reacted for 5 h. Then, propylene oxide was added to adjust the pH to 5 to 6, and formamide was added to optimize the network structure of gels. Calculated by volume, the amount of formamide was 0.16 times of TEOS. Next, the inorganic silica sol and organic components were mixed, Materials 2021, 14, 2046 3 of 12

and then poured into molds. The formulation is shown in Table1. The wet gels were generated in approximately 12 h, at 40 ◦C.

Table 1. Formulation of the organic–inorganic composite aerogels (mole ratio).

Samples PHMS * VTES TEOS

P/V-SiO2-1 1.0 0.3 3.5 P/V-SiO2-2 1.0 0.3 5.0 P/V-SiO2-3 1.0 0.3 6.5 P/V-SiO2-4 1.0 0.3 8.0 * The amount of PHMS is calculated based on the hydrogen content of PHMS.

2.2.3. Post-Processing of the Composite Aerogels The post-process of the composite aerogels included aging, alcohol–water exchange, surface modification and ambient pressure drying. First, the wet gels were soaked in isopropanol and aged at 45–60 ◦C for 48 h. Next, an alcohol–water exchange was performed at room temperature, and the water in the wet gels was replaced with ethanol and isopropanol. The replacement was performed every 8 h, for a total of 6 times. Third, surface modification was performed to replace the hydrophilic groups on the surface of the wet gels with hydrophobic groups to reduce the collapse of pores due to water evap- oration during the drying process. The modification process was performed under two conditions, using n-hexane as the solvent: The first time, trimethylchlorosilane was used; the second time, methyltrimethoxysilane and methyltriethoxysilane were used to obtain a better modification effect [11]. The volume ratio of solvent to modifiers was 9:1. The time for each modification was 24 h, and the temperature was 50 ◦C. Finally, drying at 80 ◦C for 3–5 h was conducted to obtain the organic–inorganic composite aerogels.

2.3. Characterizations In this study, the morphology of the composite aerogels was studied by scanning electron microscopy (SEM; Hitachi, S-4800, Tokyo, Japan) at an acceleration voltage of 3 kV. Before SEM analysis, the samples underwent the treatment of gold sputtering to enhance conductivity. The chemical composition of the composite aerogels was characterized by Fourier-transform infrared spectroscopy (FTIR; Thermo, Nicolet iS5, Waltham, MA, USA). The pore structure was analyzed by a Surface Area and Pore Size Analyzer (Quantachrome, NOVA 2200e, Boynton Beach, FL, USA) with N2 as an adsorbent. The speciﬁc surface area was calculated using Brunauer–Emmett–Teller (BET) method, and the pore size distribution was calculated by using the Barrett–Joyner–Halenda (BJH) method. Before the analysis of pore structure, the samples underwent the treatment of degassing at 130 ◦C, for 15 h. Thermal conductivity was measured by the hot-wire method, using a thermal conductivity measuring instrument (XIATECH, TC3000, Xi’an, China). During the measurement, the heating voltage was 0.9 V, and the sampling time was 2 s. Compressive strength was measured by a digital mechanical strength measurement instrument (KEBAO, DL-15, Dongguan, China) with a load application speed of 0.1 mm/min.

3. Results 3.1. Microscopic Morphology and Structural Characteristics of the Composite Aerogels The preparation of the composite aerogels can be divided into three stages: (1) The organic polymer (PHMS) and coupling agent (VTES) formed the PHMS/VTES complex by an addition reaction. (2) The PHMS/VTES complex reacted with silica sol to form composite gels. (3) The composite aerogels were obtained from the composite gels by ambient pressure drying. The process and mechanism are shown in Figure1. Materials 2021, 14, x FOR PEER REVIEW 4 of 12

Compressive strength was measured by a digital mechanical strength measurement instrument (KEBAO, DL-15, Dongguan, China) with a load application speed of 0.1 mm/min.

3. Results 3.1. Microscopic Morphology and Structural Characteristics of the Composite Aerogels The preparation of the composite aerogels can be divided into three stages: (1) The organic polymer (PHMS) and coupling agent (VTES) formed the PHMS/VTES complex by an addition reaction. (2) The PHMS/VTES complex reacted with silica sol to form com- Materials 2021, 14, 2046 posite gels. (3) The composite aerogels were obtained from the composite gels by ambient4 of 12 pressure drying. The process and mechanism are shown in Figure 1.

FigureFigure 1. 1. PreparationPreparation process process and and preparation preparation me mechanismchanism of the composite aerogels.

3.1.1. Chemical Composition and Reaction Mechanism 3.1.1. Chemical Composition and Reaction Mechanism During the addition reaction between PHMS and VTES, the samples in the reaction During the addition reaction between PHMS and VTES, the samples in the reaction −1 werewere analyzed analyzed by by infrared spectroscopy, as shown in FigureFigure2 2A.A. The The peak peak at at 2980 2980 cm cm−1 waswas attributable attributable to to the the stretching stretching vibration vibration of ofmethyl methyl group. group. The Thepeaks peaks at 2926 at 2926and 2884 and −1 cm2884 were cm− 1attributablewere attributable to the stretching to the stretching vibration vibration of methylene of methylene group [26]. group There [26 were]. There two sources of methylene: the ethoxy (-OCH2CH3) of VTES and Si-CH2-CH2-Si generated in were two sources of methylene: the ethoxy (-OCH2CH3) of VTES and Si-CH2-CH2-Si thegenerated addition in reaction the addition between reaction PHMS between and VTES. PHMS By and quantitative VTES. Byquantitative analysis of the analysis methylene of the content,methylene the content, progress the of progressthe addition of the reaction addition could reaction be estimated. could be estimated. In quantitative In quantitative analysis, theanalysis, stronger the absorption stronger absorption peak at 2926 peak cm at−1 2926represented cm−1 represented methylene methylene(Figure 2B). (Figure The peak2B). intensityThe peak was intensity normalized was normalized according according to the absorption to the absorption peak of the peak methyl of the group methyl at group 2980 cmat 2980−1. As cm shown−1. As in shown Figure in 2C, Figure as the2C, reaction as the reactionprogressed, progressed, the stretching the stretching vibration vibration peak of 2 2 methylenepeak of methylene increased. increased. This was due This to was the dueformation to the of formation Si-CH -CH of -SiSi-CH by 2the-CH addition2-Si by there- actionaddition between reaction Si-H between bond and Si-H C=C bond bond, and which C=C bond,indicated which that indicated the addition that reaction the addition pro- ceededreaction gradually. proceeded The gradually. absorption The peak absorption of C=C bond peak at of 1620 C=C cm bond−1 and at the 1620 absorption cm−1 and peak the ofabsorption Si-H bond peak at 2200 of Si-H cm bond−1 decreased at 2200cm as− the1 decreased reaction asprogressed, the reaction which progressed, also proved which that also proved that the addition reaction was proceeded gradually. After 10 min, the absorption peak of C=C at 1620 cm−1 tended to disappear, indicating that PHMS and VTES had sufﬁciently reacted. Materials 2021, 14, Materialsx FOR PEER 2021 REVIEW, 14, x FOR PEER REVIEW 5 of 12 5 of 12

Materials 2021, 14, 2046 5 of 12 the addition reactionthe addition was proceeded reaction was gradually. proceeded After gradually. 10 min, the After absorption 10 min, thepeak absorption of C=C peak of C=C −1 at 1620 cm tendedat 1620 to cmdisappear,−1 tended indicating to disappear, that indicatingPHMS and that VTES PHMS had sufficientlyand VTES had reacted. sufficiently reacted.

Figure 2. InfraredFigureFigure spectra 2.2.Infrared Infraredof the addition spectraspectra reaction ofof thethe additionaddition of PHMS reactionreaction and VTES: ofof PHMSPHMS (A) complete andand VTES:VTES: infrared ((AA)) completecomplete spectrum; infraredinfrared (B) enlarged spectrum;spectrum; view ((BB )) enlargedenlarged viewview −1 −1 of part of the spectraofof partpart shown ofof thethe in spectraspectra (A), in shown shownthe wavenumber in in ( A(A),), in in the the range wavenumber wavenumber of 2800–3000 range range cm of of; 2800–3000 (2800–3000C) quantitative cm cm−1;;( (analysisC) quantitative of methylene. analysis of methylene.

The infrared spectrumThe infrared of the spectrum composite of theaero compositegels is shown aerogels in Figure is shown 3A. The in Figure absorp-3A. The absorp- The infrared spectrum of the composite aero− gels is shown in Figure 3A. The absorption peaks of Si-O-Si at 1030,−1 778 and 442 cm 1 are derived from the network of the tion peaks of Si-O-Sition peaks at 1030, of Si-O-Si 778 and at 442 1030, cm 778 are and derived 442 cm from−1 are the derived network from of thethe networkaerogels of the aerogels −1 −1 and the molecularaerogels chain and of PHMS the molecular [27]. The chain peak of at PHMS 1030 cm [27]. is The the peakasymmetric at− 10301 stretching cm is the asymmetric and the molecular chain of PHMS [27]. The peak at 1030− cm is the asymmetric stretching −1 1 vibration absorptionstretching peak vibration of Si-O-Si, absorption 778 cm peak is attributable of Si-O-Si,−1 to 778 the cm symmetricis attributable stretching to the symmetric vibration absorption peak of Si-O-Si, 778 cm is attributable−1 to the symmetric stretching stretching vibration absorption peak−1 of Si-O-Si and 442 cm is attributable to the bending vibration absorptionvibration peak absorption of Si-O-Si peak and 442of Si-O-Si cm is and attributable 442 cm−1 tois attributablethe bending tovibration the bending vibration −1 absorption peakvibration of Si-O-Si. absorption The absorption peak ofpeaks Si-O-Si. of Si-CH The3 absorptionare at 1270 and peaks 840of cm Si-CH [28,29].3 are at 1270−1 and absorption−1 peak of Si-O-Si. The absorption peaks of Si-CH3 are at 1270 and 840 cm [28,29]. The Si-CH3 comes840 cmfrom PHMS[28,29]. and The the Si-CH modifiers3 comes trimethylchlorosilane, from PHMS and the modifiersmethyltrimethox- trimethylchlorosilane, The Si-CH3 comes from PHMS and the modifiers trimethylchlorosilane, methyltrimethox- methyltrimethoxysilane and methyltriethoxysilane introduced in the surface−1 modification. ysilane and methyltriethoxysilaneysilane and methyltriethoxysilane introduced in introducedthe surface inmodification. the surface Atmodification. 960 cm , At 960 cm−1, At 960 cm−1, there is a symmetrical stretching vibration absorption peak of Si-O-C [30]. there is a symmetricalthere is astretching symmetrical vibration stretching absorption vibration peak absorption of Si-O-C peak [30]. ofSi-O-C Si-O-C comes [30]. Si-O-C comes Si-O-C comes from the modifiers methyltrimethoxysilane and methyltriethoxysilane. The from the modifiersfrom methyltrimethoxysilane the modifiers methyltrimethoxysilane and methyltriethoxysilane. and methyltriethoxysilane. The in-plane rock- The in-plane rock- in-plane rocking absorption peak of CH is at 735 cm−1 [29], which comes from the ing absorption peak of CH2 is at 735 cm−1 [29], which comes2 from the CH2-CH2 group ing absorption peak of CH2 is at 735 cm−1 [29], which comes from the CH2-CH2 group formed in the CHaddition2-CH 2reaction.group formed No absorption in the addition peak of reaction.the Si-H bond No absorption is observed peak at 2160 of the Si-H bond formed in the addition −reaction.1 No absorption peak of the Si-H bond is observed at 2160 −1 is observed at 2160 cm , which is due to the reaction between PHMS and formamide cm , which is due− 1to the reaction between PHMS and formamide in isopropanol solvent, incm isopropanol, which is due solvent, to the causing reaction the between Si-H bond PHMS to beand consumed. formamide As in shownisopropanol in Figure solvent,3B, causing the Si-Hcausing bond tothe be Si-H consumed. bond to Asbe consumed.shown in Figure As show 3B, nwith in Figure the increase 3B, with in theTEOS, increase in TEOS, with the increase in TEOS, the symmetric stretching vibration−1 absorption peak of Si-O-Si the symmetric stretching vibration absorption peak of Si-O-Si at 778 cm increases grad-−1 atthe 778 symmetric cm−1 increases stretching gradually, vibration which absorption indicates peak that of there Si-O-Si are at more 778 cm TEOSs increases undergoing gradually, which indicates that there are more TEOSs undergoing condensation reaction with condensationually, which indicates reaction that with there VTES. are This more reaction TEOSs undergoing can improve condensation the symmetry reaction of Si-O-Si with VTES. This reaction can improve the symmetry of Si-O-Si network and enhance the ab- networkVTES. This and reaction enhance can the improve absorption the peak. symmetry of Si-O-Si network and enhance the absorption peak. sorption peak.

Figure 3. Infrared spectra of the composite aerogels: (A) samples P/V-SiO -1, P/V-SiO -2, P/V-SiO -3 and P/V-SiO -4; Figure 3. Infrared spectra of the composite aerogels: (A) samples2 P/V-SiO2-1,2 P/V-SiO2-2, P/V-SiO2 2- 2 Figure 3. Infrared spectra of the composite aerogels: (A) samples P/V-SiO−1 2-1, P/V-SiO2-2, P/V-SiO2- (B) enlarged3 viewand P/V-SiO of part of2-4; the (B spectra) enlarged shown view in of (A part), in of the the wavenumber spectra shown range in ( ofA), 600–1000 in the wavenumber cm . range 3 and P/V-SiO2-4; (B) enlarged view of part of the spectra shown in (A), in the wavenumber range −1 of 600–1000 cm of. 600–1000 cm−1. 3.1.2. Microscopic Morphology The microscopic morphology of the composite aerogels is shown in Figure4. With the decrease in PHMS and VTES, the aerogel particles gradually become smaller, and the Materials 2021, 14, x FOR PEER REVIEW 6 of 12

Materials 2021, 14, 2046 6 of 12 3.1.2. Microscopic Morphology The microscopic morphology of the composite aerogels is shown in Figure 4. With the decrease in PHMS and VTES, the aerogel particles gradually become smaller, and the particle size decreases from the range of 80–100 nm (the sample P/V-SiO2-1 is 85–100 nm, particle size decreases from the range of 80–100 nm (the sample P/V-SiO2-1 is 85–100 nm, and the sample P/V-SiO2-2 is 80–95 nm) to 30–50 nm (the sample P/V-SiO2-3 and the and the sample P/V-SiO2-2 is 80–95 nm) to 30–50 nm (the sample P/V-SiO2-3 and the sam- sample P/V-SiO2-4 are both 30–50 nm). This shows that, when the proportion of PHMS and ple P/V-SiO2-4 are both 30–50 nm). This shows that, when the proportion of PHMS and VTES is high, the coating formed by PHMS and VTES wraps the inorganic aerogel particles, VTES is high, the coating formed by PHMS and VTES wraps the inorganic aerogel parti- making the aerogel particles large. At this time, the agglomeration phenomenon of the cles, making the aerogel particles large. At this time, the agglomeration phenomenon of aerogel particles becomes clear (Figure4A). This is because the PHMS/VTES complex the aerogel particles becomes clear (Figure 4A). This is because the PHMS/VTES complex coated on aerogel particles continues to react with the adjacent aerogel particles, resulting coated on aerogel particles continues to react with the adjacent aerogel particles, resulting in chemical bonding between aerogel particles. It causes the agglomeration of aerogel in chemical bonding between aerogel particles. It causes the agglomeration of aerogel par- particles. Therefore, the proportion of PHMS and VTES determines the thickness of the ticles. Therefore, the proportion of PHMS and VTES determines the thickness of the coat- coating, which affects the morphology and particle size of the composite aerogels. ing, which affects the morphology and particle size of the composite aerogels.

Figure 4. The microscopic morphology of the composite aerogels: (A) P/V-SiO2-1, (B) P/V-SiO2-2, (C) P/V-SiO2-3 and (D) Figure 4. The microscopic morphology of the composite aerogels: (A) P/V-SiO2-1, (B) P/V-SiO2-2, (C) P/V-SiO2-3 and P/V-SiO2-4. (D) P/V-SiO2-4. 3.1.3. Pore Structure

3.1.3.The Pore N Structure2 adsorption and desorption curves of the composite aerogels are shown in FigureThe 5A. N The2 adsorption adsorption and and desorption desorption curves curves of are the all composite type IV curves aerogels according are shown to the in IUPACFigure5 A.classification The adsorption [31,32], and indicating desorption that curves the composite are all type aerogels IV curves are accordingtypical mesopo- to the rousIUPAC materials. classiﬁcation There [ 31is ,a32 hysteresis], indicating loop that in the the composite adsorption aerogels and desorption are typical curves mesoporous that is duematerials. to capillary There iscondensation a hysteresis loopin the in meso the adsorptionporous materials and desorption [13]. The curves sample that P/V-SiO is due2 to-1 showscapillary a type condensation H2(b) hysteresis in the mesoporousloop, and the materials samples [P/V-SiO13]. The2-2, sample P/V-SiO P/V-SiO2-3 and2-1 P/V-SiO shows2 a- 4type show H2(b) a type hysteresis H1 hysteresis loop, and loop the [33]. samples These P/V-SiO two types2-2, of P/V-SiO hysteresis2-3 andloops P/V-SiO both show2-4 show that thea type pore H1 size hysteresis distribution loop [is33 uniform.]. These twoThe typespore size of hysteresis distribution loops of boththe composite show that aerogels the pore issize shown distribution in Figure is 5B. uniform. The pore The size pore is sizemainly distribution in the range of the of composite5–40 nm, which aerogels is consistent is shown within Figure the 5B.characteristics The pore size of is mainlymesoporous in the materials range of 5–40[13]. nm, With which the is increase consistent in with the PHMS/VTESthe characteristics complex, of mesoporous the pore size materials gradually [13 decreases.]. With the increase in the PHMS/VTES complex, the pore size gradually decreases.

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Figure 5. N2 adsorption and desorption analysis of the composite aerogels (samples P/V-SiO2-1, P/V-SiO2-2, P/V-SiO2-3 Figure 5. N2 adsorption and desorption analysis of the composite aerogels (samples P/V-SiO2-1, P/V-SiO2-2, P/V-SiO2-3 and P/V-SiO -4): (A) adsorption and desorption curves and (B) pore size distribution. and P/V-SiO22-4): (A) adsorption and desorption curves and (B) pore size distribution.

The analysis results of the N2 adsorption and desorption of the composite aerogels The analysis results of the N2 adsorption and desorption of the composite aerogels are shown in Table2. The most probable pore sizes of the composite aerogels are between are shown in Table 2. The most probable pore sizes of the composite aerogels are between 7 and 20 nm. As the proportion of PHMS/VTES increases, the most probable pore size 7 and 20 nm. As the proportion of PHMS/VTES increases, the most probable pore size of of the composite aerogels decreases. This is because the PHMS/VTES complex can bond the composite aerogels decreases. This is because the PHMS/VTES complex can bond with with the inorganic Si-O-Si network. A coating structure forms on the aerogel particles, the inorganic Si-O-Si network. A coating structure forms on the aerogel particles, which which makes the aerogel particles larger and the pore size smaller. This is consistent withmakes the the morphological aerogel particles characteristics larger and shown the pore in Figuresize smaller.4. The analysisThis is consistent results show with that the themorphological composite aerogels characteristics have high shown specific in Figure surface 4. The area. analysis When theresults pore show volume that does the com- not significantlyposite aerogels change, have the high smaller specific the poresurface size, area. the largerWhen thethe specific pore volume surface does area. not Therefore, signifi- whencantly the change, proportion the smaller of PHMS/VTES the pore size, increases, the larger the porethe specific size decreases surfaceand area. the Therefore, specific when the proportion of PHMS/VTES increases, the pore size decreases and the specific surface area of the composite aerogels increases (sample P/V-SiO2-2). However, when thesurface size ofarea the of aerogel the composite particles ae furtherrogels increases increases, (sample the pore P/V-SiO volume2-2). of However, aerogels decreases when the obviously,size of the thus aerogel making particles the specific further surface increases, area the of thepore composite volume of aerogels aerogels decrease decreases (sample obviously, thus making the specific surface area of the composite aerogels decrease (sample P/V-SiO2-1). P/V-SiO2-1).

Table 2. N2 adsorption and desorption analysis results of the composite aerogels. Table 2. N2 adsorption and desorption analysis results of the composite aerogels. Samples BET Speciﬁc Surface Area (m2/g) Pore Volume (cm3/g) Pore Size (nm) Samples BET Specific Surface Area (m2/g) Pore Volume (cm3/g) Pore Size (nm) P/V-SiO2-1 622.8 1.205 7.288 P/V-SiO2-1 622.8 1.205 7.288 P/V-SiO2-2 673.1 1.922 11.43 2 P/V-SiOP/V-SiO-22-3 673.1 631.3 2.903 1.922 16.1211.43 P/V-SiOP/V-SiO2-32-4 631.3 599.7 2.691 2.903 20.8216.12 P/V-SiO2-4 599.7 2.691 20.82

Compared with the typical inorganic SiO2 aerogels prepared with ethanol as the solvent,Compared the composite with the aerogels typical we inorganic prepared SiO have2 aerogels a larger poreprepared size whenwith ethanol the proportion as the sol- of PHMS/VTESvent, the composite is low. Thisaerogels is related we prepared to changing have in a solvent larger pore from size ethanol when to the isopropanol proportion [34 of]. However,PHMS/VTES the is use low. of This isopropanol is related as to a changing solvent is in a necessarysolvent from condition ethanol toto achieveisopropanol the mixing[34]. However, of the organic the use and of isopropanol inorganic components. as a solvent Itisis a difﬁcultnecessary to condition achieve similar to achieve effects the withmixing ethanol. of the organic and inorganic components. It is difficult to achieve similar effects with ethanol. 3.2. Properties of the Composite Aerogels 3.2.1.3.2. Properties Thermal of Properties the Composite Aerogels 3.2.1.The Thermal heat-transfer Properties process of aerogels can be divided into two aspects: solid-phase heat transferThe heat-transfer and gas-phase process heat of transfer. aerogels The can solid-phase be divided heatinto transfertwo aspects: depends solid-phase on the aerogelheat transfer framework, and gas-phase and the gas-phase heat transfer. heat transferThe solid-phase is closely heat related transfer to the depends pore structure on the ofaerogel aerogels. framework, When theand pore the gas-phase size of the heat aerogel transfer is smalleris closely than related the to mean the pore free structure path of airof molecules,aerogels. When the gas-phase the pore size heat of transfer the aerogel can be is signiﬁcantly smaller than reduced, the mean and free the path thermal of air insulation properties of aerogels can be improved [35].

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molecules, the gas-phase heat transfer can be significantly reduced, and the thermal insulation properties of aerogels can be improved [35]. The thermal conductivity of the composite aerogels is shown in Figure 6. With the increase in PHMS and VTES, the thermal conductivity of the composite aerogels first decreases and then increases. The PHMS/VTES complex forms a coating on the aerogel particles. With the increase in PHMS and VTES, the thickness of the coating is increased (Fig- ure 4). On the one hand, the thickening of the coating helps to improve the specific surface area of the composite aerogels (Table 2), reducing the gas-phase heat transfer and reduc- Materials 2021, 14, 2046 ing the thermal conductivity of the composite aerogels. On the other hand, when the coat-8 of 12 ing is too thick, the solid-phase heat transfer of the composite aerogels will increase and the thermal conductivity will also increase. Therefore, there is an optimal proportion of PHMS and VTES. The thermal conductivity of the composite aerogels is shown in Figure6. With the As shown in Figure 6, when the raw-material ratio of the composite aerogels is increase in PHMS and VTES, the thermal conductivity of the composite aerogels ﬁrst PHMS:VTES:TEOS=1:0.3:5 (sample P/V-SiO2-2), the best heat insulation properties are decreases and then increases. The PHMS/VTES complex forms a coating on the aerogel achieved. Thermal conductivity is as low as 0.03773 W·m−1·K−1. Under the same conditions, particles. With the increase in PHMS and VTES, the thickness of the coating is increased we prepared inorganic SiO2 aerogels with only TEOS as the raw material, and its thermal (Figure4). On the one hand, the thickening of the coating helps to improve the speciﬁc −1 −1 conductivitysurface area is of 0.03424 the composite W·m ·K aerogels. Compared (Table with2), reducing the inorganic the gas-phase SiO2 aerogels, heat transferthe thermal and conductivityreducing the of thermal the composite conductivity aerogels of thehas composite not significantly aerogels. increased On the other[36]. It hand, is shown when that the thecoating method is too of thick,simultaneously the solid-phase introducing heat transfer long-chain of the compositemolecules aerogelsand small-molecule will increase silaneand theto prepare thermal the conductivity compositewill aerogels also increase.does not Therefore,significantly there decrease is an optimal the thermal proportion insu- lationof PHMS properties and VTES. of aerogels.

Figure 6. Thermal conductivity of the composite aerogels (samples P/V-SiO2-1, P/V-SiO2-2, Figure 6. Thermal conductivity of the composite aerogels (samples P/V-SiO2-1, P/V-SiO2-2, P/V- P/V-SiO -3 and P/V-SiO -4). SiO2-3 and2 P/V-SiO2-4). 2 As shown in Figure6, when the raw-material ratio of the composite aerogels is 3.2.2. Mechanical Properties PHMS:VTES:TEOS=1:0.3:5 (sample P/V-SiO2-2), the best heat insulation properties are achieved.The stress–strain Thermal conductivity curves during is as compressio low as 0.03773n and W the·m compressive−1·K−1. Under properties the same of condi- the composite aerogels are shown in Figure 7. The compressive strength of the composite aer- tions, we prepared inorganic SiO2 aerogels with only TEOS as the raw material, and its ogels increases with the increase in the− prop1 −ortion1 of the organic components, PHMS and thermal conductivity is 0.03424 W·m ·K . Compared with the inorganic SiO2 aerogels, VTES.the thermal In our study, conductivity the compressive of the composite strength of aerogels the composite has not aerogels significantly was 1.87–2.28 increased MPa [36 ]. (samplesIt is shown P/V-SiO that2-1, the P/V-SiO method2-2), of simultaneouslywhile the compressive introducing strength long-chain of the inorganic molecules SiO and2 aerogelssmall-molecule was usually silane less to than prepare 0.6 theMPa composite [10,11]. At aerogels the same does time, not we significantly have tested decrease the inor- the ganicthermal SiO insulation2 aerogels prepared properties with of aerogels. isopropanol as a solvent under the same conditions, and the compressive strength was also only 0.740 MPa. Therefore, the mechanical prop- erties3.2.2. of Mechanical the composite Properties aerogels with the PHMS/VTES complex are significantly better than thoseThe stress–strainof the inorganic curves SiO2 during aerogels. compression and the compressive properties of the composite aerogels are shown in Figure7. The compressive strength of the composite aerogels increases with the increase in the proportion of the organic components, PHMS and VTES. In our study, the compressive strength of the composite aerogels was 1.87–2.28 MPa (samples P/V-SiO2-1, P/V-SiO2-2), while the compressive strength of the inorganic SiO2 aerogels was usually less than 0.6 MPa [10,11]. At the same time, we have tested the inorganic SiO2 aerogels prepared with isopropanol as a solvent under the same conditions, and the compressive strength was also only 0.740 MPa. Therefore, the mechanical properties of the composite aerogels with the PHMS/VTES complex are significantly better than those of the inorganic SiO2 aerogels. Materials 2021, 14,Materials x FOR PEER2021 ,REVIEW14, 2046 9 of 12 9 of 12

Figure 7. Compressive performance of the composite aerogels (samples P/V-SiO -1, P/V-SiO -2, P/V-SiO -3 and Figure 7. Compressive performance of the composite aerogels (samples P/V-SiO2 2-1, P/V-SiO2 2-2, 2 P/V-SiO2-4):P/V-SiO (A) stress–strain2-3 and P/V-SiO curves2-4): and (A (B) )stress–strain compressive curves strength and and (B) density.compressive strength and density.

According to FigureAccording 7A, when to Figure the proporti7A, whenon theof PHMS/VTES proportion ofis small PHMS/VTES (samples isP/V- small (samples P/V-SiO2-3, P/V-SiO2-4), the stress–strain curves of the composite aerogels show the SiO2-3, P/V-SiO2-4), the stress–strain curves of the composite aerogels show the character- characteristics of typical brittle materials [37]. When the proportion of PHMS/VTES is high istics of typical brittle materials [37]. When the proportion of PHMS/VTES is high (sam- (samples P/V-SiO2-1 and P/V-SiO2-2), the composite aerogels have a large compression ples P/V-SiO2-1 and P/V-SiO2-2), the composite aerogels have a large compression shape shape in the initial stage of the load application. At this stage, the PHMS/VTES coating is in the initial stage of the load application. At this stage, the PHMS/VTES coating is the the main reason for the deformation. Figure7B also shows the density of the composite main reason for the deformation. Figure 7B also shows the density of the composite aero- aerogels. With the increase in PHMS and VTES, the density of the composite aerogels gels. With the increase in PHMS and VTES, the density of the composite aerogels gradu- gradually increases. This is because the PHMS/VTES complex is coated on the inorganic ally increases. This is because the PHMS/VTES complex is coated on the inorganic aerogel aerogel particles, resulting in a higher density. The density of the composite aerogels has a particles, resulting in a higher density. The density of the composite aerogels has a posi- positive correlation with mechanical properties, and both of them increase with the increase tive correlation with mechanical properties, and both of them increase with the increase of PHMS/VTES. Moreover, the PHMS/VTES complex is not only the factor that increases of PHMS/VTES. Moreover, the PHMS/VTES complex is not only the factor that increases the density of the aerogels but also the factor that enhances the mechanical properties of the density of thethe aerogels.aerogels but also the factor that enhances the mechanical properties of the aerogels. 4. Discussion 4. Discussion We use linear polymer (PHMS) and low-molecular-weight silane (VTES) to form a We use linearcoating polymer structure (PHMS) on the and aerogel low-molecular-weight particles. The coating silane affects (VTES) the structure to form of a the composite coating structureaerogels. on the aerogel It is the particles. reason that The thecoating composite affects aerogelsthe structure have of high the compo- specific surface area, site aerogels. Itexcellent is the reason thermal that insulation the composite properties aerogels and have mechanical high specific properties. surface area, In order to obtain excellent thermalthis insulation coating structure, properties we and improved mechanical the properties. one-pot synthesis In order method.to obtain Firstly,this the liquid coating structure,PHMS/VTES we improved complex the was one-po preparedt synthesis by the additionmethod. reaction Firstly, between the liquid PHMS and VTES. PHMS/VTES complexNext, the was liquid prepared PHMS/VTES by the addition complex reaction was mixed between with PHMS the inorganic and VTES. silica sol, and the Next, the liquidcomposite PHMS/VTES aerogels complex were was prepared mixed by with the the one-pot inorganic synthesis silica method.sol, and the In this way, the composite aerogelscarbon–carbon were prepared double by bond the one-pot of VTES synthesis avoids being method. destroyed In this by way, strong the acidcar- added during bon–carbon doublethe hydrolysis bond of ofVTES silica avoids sol, which being can destroyed ensure theby formationstrong acid of added the PHMS/VTES during complex. the hydrolysis of silicaFigure sol,8 A–Cwhich shows can ensure the effect the formation of different of proportions the PHMS/VTES of PHMS/VTES complex. on the mor- Figure 8A–Cphology shows of the aerogeleffect of particles. different Firstly, proportions the PHMS/VTES of PHMS/VTES complex on the forms mor- a coating on the phology of theaerogel aerogel particles particles. that Firstly, makes the the PHMS/VTES aerogel particles complex larger. forms The a coating organic on components the have a aerogel particleshigher that density.makes the With aerogel an increase particles in thelarger. particle The size,organic the components density of the have composite a aerogels higher density.increase. With an Secondly,increase in when the partic the proportionle size, the of density PHMS/VTES of the composite is higher, theaerogels PHMS/VTES com- increase. Secondly,plexaccumulated when the proportion between theof PHMS/VTES aerogel particles is higher, is increased, the PHMS/VTES as shown incom- Figure8A, which plex accumulatedresults between in the the agglomeration aerogel particles of the is increased, aerogel particles. as shown The in Figure SEM images 8A, which in Figure 4 prove results in the agglomerationthis. At the same of the time, aerogel the PHMS/VTES particles. The complex SEM images does not in Figure change 4 theprove porous structure this. At the sameof thetime, aerogels. the PHMS/VTES The composite complex aerogels does cannot maintainchange the a highporous specific structure surface of area of more 2 the aerogels. Thethan composite 600 m /g (Tableaerogels2). Therefore,can maintain the preparationa high specific method surface we area proposed of more can maintain the than 600 m2/g (Tableadvantages 2). Therefore, of the high the specific preparation surface method area of we the proposed aerogels. can maintain the advantages of the high specific surface area of the aerogels.

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FigureFigure 8. The 8. Theschematic schematic diagram diagram of the of composite the composite aeroge aerogell particles particles with different with different proportion proportion of of PHMS/VTES:PHMS/VTES: (A) sample (A) sample P/V-SiO P/V-SiO2-1, (B2-1,) sample (B) sample P/V-SiO P/V-SiO2-2 and2-2 (C and) sample (C) sample P/V-SiO P/V-SiO2-3. (D2) -3.The (D ) The connectionconnection method method of the of theadjacent adjacent aerogel aerogel particles. particles.

FigureFigure 8A–C8A–C shows shows the the two two modes modes of of heat conductionconduction ofof the the aerogels: aerogels: solid-phase solid-phase heat heattransfer transfer by by the the aerogel aerogel particles, particles, and and gas-phase gas-phase heat heat transfer transfer by pores.by pores. When When the the proportion pro- portionof PHMS/VTES of PHMS/VTES is too is too high high (Figure (Figure8A), 8A), the the PHMS/VTES PHMS/VTES coating coating is is too too thick, thick, and and the the particleparticle sizesize andand densitydensityincrease, increase, resulting resulting in in an an increase increase in in solid-phase solid-phase heat heat transfer transfer [38 ]. [38].When When the the proportion proportion of of PHMS/VTES PHMS/VTES is is too too small small (Figure (Figure8C), 8C), the the aerogels aerogels have have a larger a largerpore pore size, size, which which makes makes porosity porosity increase increase and and density density decrease, decrease, resulting resulting in an in increase an in- in creasegas-phase in gas-phase heat conduction heat conduction [39]. Both[39]. ofBo theseth of these conditions conditions cause cause the thermal the thermal conductivity con- ductivityof the aerogelsof the aerogels to increase. to Therefore,increase. Ther thereefore, is an optimalthere is proportionan optimal of proportion PHMS/VTES, of at PHMS/VTES,which the negativeat which impactthe nega oftive PHMS/VTES impact of onPHMS/VTES the thermal on insulation the thermal properties insulation of the propertiescomposite of the aerogels composite is minimized aerogels is (Figure minimi6).zed This (Figure is the 6). reason This is that the samplereason that P/V-SiO sam-2-2 exhibits the best thermal insulation properties. ple P/V-SiO2-2 exhibits the best thermal insulation properties. FigureFigure 8D8 showsD shows the thechemical chemical bond bond between between the composite the composite aerogel aerogel particles. particles. Due to Due the toPHMS/VTES the PHMS/VTES complex, complex, chemical chemical bonds can bonds form can between form the between aerogel the particles. aerogel There- particles. Therefore, the adjacent aerogel particles are connected by the PHMS/VTES complex. As the fore, the adjacent aerogel particles are connected by the PHMS/VTES complex. As the pro- proportion of PHMS/VTES increases, the agglomeration of the aerogel particles becomes portion of PHMS/VTES increases, the agglomeration of the aerogel particles becomes clear clear (Figure4). The connection strength of the secondary particles is improved [ 40], which (Figure 4). The connection strength of the secondary particles is improved [40], which en- enhances the compressive strength of the composite aerogels (Figure7). hances the compressive strength of the composite aerogels (Figure 7). Considering the thermal insulation properties (Figure6) and the mechanical properties Considering the thermal insulation properties (Figure 6) and the mechanical proper- (Figure7), the sample P/V-SiO 2-2 has the best thermal insulation properties and better ties (Figure 7), the sample P/V-SiO2-2 has the best thermal insulation properties and better mechanical properties, achieving a balance between thermal insulation properties and mechanical properties, achieving a balance between thermal insulation properties and mechanical properties. It proves that the composite aerogels we prepared can achieve an mechanical properties. It proves that the composite aerogels we prepared can achieve an improvement in comprehensive properties. improvement in comprehensive properties. 5. Conclusions 5. Conclusions In this study, we prepared the PHMS/VTES-SiO2 composite aerogels with low thermal conductivityIn this study, and we high prepared compressive the PHMS/VTES-SiO strength. It is proved2 composite that aerogels the PHMS/VTES with low complexther- malcan conductivity be compounded and high with compressive SiO2 aerogel stre particlesngth. It is by proved wrapping. that the Through PHMS/VTES this compound com- plexmethod, can be thecompounded mechanical with properties SiO2 aerogel of aerogels particles can by be improved.wrapping. Through At the same this time,com- the poundnetwork method, structure the mechanical of aerogels properties is maintained, of aerogels and can the be thermal improved. insulation At the same properties time, is the notnetwork signiﬁcantly structure reduced. of aerogels We isdiscuss maintain theed, effect and the of thethermal PHMS/VTES insulation complexproperties on is the notnetwork significantly structure reduced. of the We composite discuss aerogels,the effect thermal of the PHMS/VTES insulation properties complex and on mechanicalthe net- workproperties. structure Whenof the thecomposite composite aerogels, aerogel thermal ratio ofinsulation PHMS:VTES:TEOS properties and is 1:0.3:5, mechanical the best thermal insulation properties and better mechanical properties are obtained. At this ratio,

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the thermal conductivity is 0.03773 W·m−1·K−1 and the compressive strength is 1.87 MPa, which achieves a balance between thermal insulation and mechanical properties. This study expands the research platform for composite aerogels and proposes a combination approach in which long-chain molecules and low-molecular-weight silanes are introduced. This concept should provide a basis for improving the mechanical properties and network stability of silica aerogels.

Author Contributions: Conceptualization, D.S. and H.J.; methodology, D.S.; software, W.W.; vali- dation, W.W. and R.L.; formal analysis, W.W. and Z.T; investigation, W.W.; resources, D.S. and H.J.; data curation, W.W.; writing—original draft preparation, W.W.; writing—review and editing, Z.T., D.S. and H.J.; visualization, W.W.; supervision, D.S. and H.J.; project administration, D.S. and H.J.; funding acquisition, D.S. and H.J. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Natural Science Foundation of China (grant number 51972222) and Natural Science Foundation of Tianjin City (No. 19JCYBJC18200). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data are contained within the article. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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