Aerogels from Chloromethyltrimethoxysilane and Their

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Aerogels from chloromethyltrimethoxysilane and their functionalizations Tomoki Kimura, Taiyo Shimizu, Kazuyoshi Kanamori, Ayaka Maeno, Hironori Kaji, and Kazuki Nakanishi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03013 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 2, 2017

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Aerogels from chloromethyltrimethoxysilane and their functionalizations Tomoki Kimura,† Taiyo Shimizu,†,§ Kazuyoshi Kanamori,*,† Ayaka Maeno,‡ Hironori Kaji,‡ Kazuki Nakanishi† †

Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa,

Sakyo-ku, Kyoto 606-8502, Japan ‡

Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan

ABSTRACT. Reactions of chloromethyltrimethoxysilane (CMTMS) and its derived colloidal network polychloromethylsilsesquioxane (PCMSQ) have been investigated to extend the material design strategy toward mechanically reinforced aerogels. In a carefully designed sol-gel system, CMTMS has afforded transparent aerogels under the presence of cationic surfactant. The surface chloromethyl groups with polarity and reactivity are shown to be useful for supporting

nanostructures,

photoluminescent

carbon

dots

(C-dots)

prepared

from

polyethyleneimine and citric acid as an example. Furthermore, since nucleophilic substitution (SN2) reactions on the surface chloromethyl groups are found to control the equilibrium of formation/dissociation of siloxane bonds, a new gelation strategy triggered by SN2 reactions in sol-gel has been developed.

Under the presence of nucleophilic organic species such as

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polyamines, a hybrid network consisting of PCMSQ crosslinked with a polyamine nucleophile can be prepared to enhance mechanical properties of aerogel.

INTRODUCTION Functionalization of materials is a crucial issue to gain desired chemical and physical properties and is usually demonstrated employing adequate attractive bonding/interactions (covalent, van der Waals, etc.) on various material interfaces such as pore surfaces in sol-gel-derived silicas1,2 and carbons3,4 and in a broad range of nanomaterials including nanoparticles and quantum dots5. In particular, it is well known that silylation using classical alkoxy-/chloro-/aminosilanes6 and more recent vinyl7/(meth)allyl8,9/hydrosilanes10,11 on porous silica gel surfaces is a basis of designing efficient separation media in the modern chromatographic technology. Loading of catalytically active molecules/nanostructures on porous media is also extensively studied to develop highly efficient heterogeneous catalysts. A mesoporous MCM-41-based solid acid was fabricated by copolymerization of mercaptopropyltriethoxysilane (MPTES) with tetramethoxysilane (TMOS), followed by oxidation of the mercapto groups to sulfo groups12. Functionalization of metal organic frameworks (MOFs)13 and porous organic polymers (POPs)14 using metal nanoparticles or coordinated metal species is widely studied for heterogeneous catalysts as well. In different examples, liquid-repellent antifouling/self-cleaning surfaces are fabricated by chemically forming a coating layer with low enough surface energy on a substrate, often through a combination with various physical and chemical roughening of surfaces primarily by lithographic and etching techniques15,16.


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Silylation for tailoring hydrophobic surface is also crucial for aerogel materials characterized with low density (high porosity)17−19, since even atmospheric moisture may destroy the delicate pore structure and cause macroscopic damage on their body. Trimethylsilylation with hexamethyldisilazane (HMDS, (CH3)3Si−NH−Si(CH3)3) has been reported to render the silanolrich hydrophilic silica gel surfaces hydrophobic20. While this hydrophobization is beneficial for stabilizing the materials, it also gives possibility in avoiding costly supercritical drying that is necessary for aerogel productions in general21. Preparations of aerogel-like xerogel granules via ambient pressure drying have thus been reported, which leads to a commercial product being applied to various thermal superinsulation purposes in housing and industry22,23. Different examples show that modification with organic crosslinkers typically on amino-functionalized polysiloxane frameworks provides opportunities to strengthen the seriously friable aerogel structures24. Thus, introductions of hydrophobic groups such as trimethylsilyl and functional groups such as amino play important roles in improving the physical/chemical stabilities, productivity and applicability of this attractive material. Employing organo-substituted alkoxysilanes (R4−nSi(OR’)n, n = 1−3)25 instead of tetraalkoxysilanes (Si(OR’)4) as a single precursor or co-precursor in the preparation of aerogels is another effective way to give hydrophobicity and other functionalities such as adsorption properties26−29. Mechanical properties have also been successfully improved by preparing aerogels solely from methyltrimethoxysilane (MTMS)30,31. Since the resultant transparent polymethylsilsesquioxane (PMSQ, CH3SiO3/2) gels show outstanding flexibility against compression, aerogel-like xerogels can be obtained via simple ambient pressure drying31. Further, polysiloxane networks bearing reactive functional groups have been recently prepared from vinyltrimethoxysilane (VTMS)32,33 and 1,2-bis(methyldiethoxysilyl)ethene (BMDEethe)34.


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In the former case, vinyl groups are reacted to form hydrocarbon chains in the preformed polysiloxane solid network through the radical polymerization as a post treatment32. The additional hydrocarbon chains significantly improve the mechanical properties such as increased compressive stress and resilience. The vinyl groups are also reactive for the thiol-ene reaction and hydrosilylation to impart different functionality, e.g., hydrophilicity, to the material33. The ethenylene groups in the latter case not only contribute to the mechanical properties but also can be reacted with a dienophile through the Diels-Alder reaction34. It is obvious that an introduction of reactive functional groups into a network gives additional chances to alter the chemical and physical properties of the preexisting network. In the present paper, we demonstrate new possibilities in reaction and modification of a different silicone (polyorganosiloxane) network to extend the possibility of organofunctionalized networks. Hydrolysis and polycondensation behaviors of chloromethyltrimethoxysilane (CMTMS, ClCH2Si(OCH3)3) as an organo-functionalized trialkoxysilane have been investigated in a sol-gel system to afford a macroscopically homogeneous polychloromethylsilsesquioxane (PCMSQ, ClCH2SiO3/2) network (Scheme 1a). Transparent aerogels have been obtained by an acid-catalyzed sol-gel under the presence of an adequate cationic surfactant. Functionalization of the PCMSQ network has been investigated using photoluminescent carbon (carbonaceous) dots (C-dots) as a model nanostructure and amine compounds through nucleophilic substitution (SN2) as a molecule. The latter SN2 reaction (Scheme 1b) has been found to be beneficial in changing the equilibrium of formation/dissociation of the siloxane bonds, by which a new sol-gel strategy toward organicinorganic hybrid networks is suggested.


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EXPERIMENTAL SECTION Materials. Chloromethyltrimethoxysilane (CMTMS) was purchased from Gelest Inc. (USA). n-Dodecyltrimethylammonium chloride (C12TAC), n-tetradecyltrimethylammonium chloride (C14TAC), n-hexadecyltrimethylammonium chloride (C16TAC), n-hexadecyltrimethylammonium bromide (C16TAB), sodium azide (NaN3) and diethylene glycol bis(3-aminopropyl) ether (DGBE) were from Tokyo Chemical Industry Co., Ltd. (Japan). Aqueous nitric acid (60 %), acetic acid, dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), N,Ndimethylacetamide (DMAc), potassium acetate (KOAc), methanol, 2-propanol and distilled water were from Kishida Chemical Ltd. (Japan). Citric acid (CA) was from Sigma-Aldrich Co. (USA). Polyethyleneimine (PEI, average molecular weight 1,800 and 600) was from Wako Pure Chemical Industries, Ltd. (Japan). Methyltrimethoxysilane (MTMS) was from Shin-Etsu Chemical Co. (Japan). Urea was from Hayashi Pure Chemical Ind., Ltd. (Japan). All reagents were used as received. Preparation of PCMSQ aerogels with surfactant. In a typical run, 0.15 g of C12TAC (or surfactant with different hydrocarbon length) was dissolved in 2 mL of diluted nitric acid with varied pH. To the solution was added 1 mL of CMTMS under stirring, and after 10 min, the reaction solution was allowed to gel at 40 °C. The obtained gels were aged for 4 d (after 1 d, the surface of some of the gels was covered with methanol or diluted nitric acid (pH = 3.5) to prevent cracks, see RESULTS AND DISCUSSION for details). The aged gels were washed with methanol and solvent-exchanged with 2-propanol at 60 °C. Each washing/solventexchange process was repeated three times. Finally, the wet gels were dried with supercritical carbon dioxide at 80 ºC and 14 MPa for 10 h to obtain aerogels.


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Preparation of C-dots35. In 10 mL of distilled water, 0.5 g of PEI (average molecular weight 1800) and 1.0 g of CA were dissolved, and the solution was stored at 200 °C in an open vessel. After water was evaporated, 1 mL of distilled water was added. The same procedure was repeated ten times over a period of 3 h. Finally, 20 mL of distilled water was added, and the supernatant solution was used as the C-dot sol. Preparation of C-dots-supported PCMSQ aerogels. In a sample tube, 1 mL of C-dot sol and 1 mL of dilute nitric acid were mixed and pH of the solution was adjusted to 3.5. Then 0.15 g of C12TAC was dissolved in the solution, and 1 mL of CMTMS was added under stirring. After stirred for 10 min, the reaction solution was allowed to gel at 40 °C. The obtained gels were aged for 6 d (after 1 d, the surface of the gels was covered with diluted nitric acid (pH = 3.5)). After aging, gels were washed with methanol and solvent-exchanged with 2-propanol at 60 °C, three times for each. Finally, the wet gels were dried with supercritical carbon dioxide at 80 ºC and 14 MPa for 10 h to obtain aerogels. Preparation of C-dots-supported PMSQ aerogels. In a sample tube, 5 mL of C-dot sol and 5 mL of 10 mM acetic acid were mixed. Then 0.40 g of C16TAB and 3.0 g of urea were dissolved in the solution. To the solution was added 5 mL of MTMS under stirring, and after 30 min, the reaction solution was allowed to gel at 60 °C. After aging for 4 d, the obtained gel was soaked in distilled water at 60 °C and further aged for 1 d. The gel was then washed with methanol and solvent-exchanged with 2-propanol at 60 °C, each for three times. Finally, the wet gel was dried with supercritical carbon dioxide at 80 ºC and 14 MPa for 10 h to obtain an aerogel. Modification of PCMSQ gel with nucleophiles. A nucleophile (kinds and amounts specified in RESULTS AND DISCUSSION) was dissolved in 10 mL of solvent. A wet PCMSQ gel solvent-


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exchanged with 2-propanol (one-sixth the size of the gel prepared in the above way) was immersed in the solution under various reaction conditions. After the reaction, the gel was solvent-exchanged with 2-propanol at 60 °C for three times. Finally, the wet gel was dried with supercritical carbon dioxide at 80 ºC and 14 MPa for 10 h to obtain an aerogel. Preparation of PCMSQ aerogels in the presence of nucleophile. In 1 mL of DMAc, 0.5 mL of DGBE and 0.2 mL of distilled water were dissolved. To the solution was added 0.4 mL of CMTMS under stirring, and after 10 min, the reaction solution was transferred in an oven at 80 °C to allow gelation. After aged for 4 d, the obtained gel was washed with methanol and solvent-exchanged with 2-propanol at 60 °C, each for three times. Finally, the wet gel was dried with supercritical carbon dioxide at 80 ºC and 14 MPa for 10 h to obtain an aerogel. Measurements. A field emission scanning electron microscope (FE-SEM, JSM-6700F, JEOL (Japan)) was employed to observe the porous morphology. For visible light transmittance measurements, a UV-Vis-NIR spectrometer V-670 (JASCO Corp., Japan) equipped with an integrating sphere ISN-732 was employed. The total light transmittance at 550 nm (T550nm) was normalized to the value of 10-mm thickness by the Lambert–Beer equation. Fourier transform infrared spectroscopy (FT-IR, IR Affinity-1, Shimadzu Co., Japan) was employed to obtain the molecular-level information. Mechanical properties of aerogels were investigated with a material tester EZGraph (Shimadzu, Japan). The measurements were performed from the strain of 0 to 50 % by uniaxial compression at a crosshead speed of 0.5 mm min−1, and then decompressed back to 0 N at the same speed. Solid-state 29Si nuclear magnetic resonance (NMR) measurements were performed on a Bruker Avance III 800US Plus NMR system operating under a static magnetic field of 18.8 T. Dipolar decoupling/magic angle spinning (DD/MAS) 29Si nuclear magnetic resonance (NMR) spectra were obtained with 29Si 45° single


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pulse excitation of 2.33 µs. The spin-lattice relaxation times of 29Si resonance lines, T1Si, were 45.5 and ~35 s for the resonance lines at −79.53 and −68.56 ppm, respectively. The recycle delay we used, 137 s, was three times of the longer T1Si, thus the spectra obtained in this work were fully-relaxed and quantitative. A 4-mm probe was used with MAS frequency at 12 kHz. Hexamethylcyclotrisiloxane was used as an external reference material referring to the signal at −9.66 ppm.

RESULTS AND DISCUSSION Effects of surfactant on properties of PCMSQ aerogel. Surfactant plays an important role in the formation of fine homogeneous networks with a hydrophobic character. In the case of PMSQ, cationic surfactants such as C16TAC or C16TAB and Pluronic-type triblock copolymers (typically F127, EO106PO70EO106, EO and PO denote ethylene oxide and propylene oxide, respectively) effectively interact with hydrophobic oligomers and polymers, maintaining the miscibility among constituents in the aqueous solution. In different cases, polyoxyethylene 2ethylhexyl ether has been used simultaneously acting as surfactant and solvent for suppressing phase separation in the polyvinylsilsesquioxane (PVSQ, CH2=CHSiO3/2)32, ethylene-bridged polymethylsiloxane (Ethy-BPMS, O2/2(CH3)Si−CH2CH2−Si(CH3)O2/2)36 and ethenylene-bridged polymethylsiloxane (Ethe-BPMS, O2/2(CH3)Si−CH=CH−Si(CH3)O2/2, Ethe-BPMS)34 systems. Since both Pluronic F127 and polyoxyethylene 2-ethylhexyl ether did not lead to low-density aerogels with a good reproducibility in the PCMSQ system, cationic surfactants with different chain lengths have been employed.


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Aerogels based on PMSQ and PCMSQ were prepared in the presence of C12TAC, C14TAC, or C16TAC, and light transmittance, which is a good guide for homogeneity of the porous structure, was compared. With these surfactants in the above order, T550nm of PCMSQ aerogels are 63 %, 56 % and 45 %, while those of PMSQ are 39 %, 86 % and 90 %. This result suggests that the PCMSQ network with a higher polarity substituent (ClCH2− vs. CH3−) shows higher affinity toward more hydrophilic surfactant to form a more homogeneous network, while PMSQ shows the opposite tendency, which is in line with our previous observations in the gel formation; hydrophobic interaction between a hydrophobic polysiloxane network and surfactant plays the key role in the formation of fine, homogeneous networks37. Transmittance of the aerogels prepared with varied amounts of C12TAC shows a maximum (63 %) at 0.15 g in the starting solution. Effects of pH on gelation. Gelation behavior of CMTMS in basic conditions accompanies macroscopic phase separation and is almost uncontrollable because of too rapid polycondensation. The phase separation of CMTMS-derived condensates from the aqueous solution is due to the hydrophobic chloromethyl groups, and forms a coarsened multiphase (macroporous after drying) structure in the course of gelation. In highly basic conditions, a once formed gel tends to dissolve due to hydrolysis/alcoholysis of the siloxane bonds. This behavior must be enhanced by the highly electron withdrawing chloromethyl groups; the electron density on the silicon is decreased, which makes the condensates more susceptible to the nucleophilic attack by hydroxyl anions and alcohols at high pH. Monolithic gels were thus prepared via acidcatalyzed hydrolysis and polycondensation in this study. Figure 1 shows the dependences of light transmittance T550nm, gelation time, shrinkage, and bulk density on pH of the aqueous nitric acid. The increasing gelation time at lower pH suggests


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that the isoelectric point of the CMTMS-derived condensates is lower than pH = 1.5, which is even lower than those of silica system (pH ~ 2) presumably due to the more electron withdrawing nature of chloromethyl groups. At pH lower than 1.5, completely opaque monolithic gels result, because phase separation of the hydrophobic CMTMS-derived condensates in an aqueous solution form a coarsened structure before gelation. At moderate pH between 1.5 and 3.5, monolithic gels with visible-light transparency are obtained. Transparency is higher for those obtained at higher pH in this region, since gelation takes place earlier before the phase separation develops into a well-coarsened structure that causes the Mie scattering of the visible light. Shrinkage and bulk density show a similar tendency, and aerogels with lower density are resulted at higher pH due to an enhanced development of the network. Aerogels prepared with nitric acid at pH = 1.5 (ρb = 0.27 g cm−3 and T550nm = 29 %) and 3.5 (0.26 g cm−3 and 63 %) shown in Figure 2 demonstrate differences in appearance and the size of the pore structure, which is finer in the aerogel sample prepared at pH = 3.5. While PCMSQ xerogels with less controlled meso- and macropore structures were synthesized in solvents such as ethanol, DMF, diethyl ether and acetone38, the aqueous system containing an appropriate surfactant is found to be effective to suppress the phase separation and form a well-defied mesoporous structure in this study. Effects of gelation and aging conditions. The PCMSQ gels prepared at room temperature shows significant shrinkage during supercritical drying due to highly incomplete crosslinks, and this enhanced shrinkage can be mainly attributed to additional crosslinking during supercritical drying at 80 °C. Gelation and aging at higher temperature allows a formation of more crosslinked gels that show less shrinkage during supercritical drying, leading to aerogels with


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lower density. An aerogel with ρb = 0.20 g cm−3 and T550nm = 58 % has been obtained at 60 °C, while one with ρb = 0.23 g cm−3 and T550nm = 66 % at 40 °C. Covering the free surface of a PCMSQ gel with a different solvent during aging is found to change the properties of obtained aerogels. In particular, aerogels with light transmittance T550nm = 71 % and bulk density ρb = 0.20 g cm−3 are obtained with good reproducibility when protected the gel surface with aqueous nitric acid (pH = 3.5) during aging. Further, duration of aging casts additional influences; since aging for 6 d is found to give the best results as low-density (0.17 g cm−3), transparent (73 %) aerogels, we employed this aging condition for the following experiments. Figure 3a exhibits the 29Si DD/MAS NMR spectrum of a sample gelled at 40 °C and aged with a coverage layer of diluted nitric acid. The major peak at higher magnetic field (−79.53 ppm) corresponds to the T3 (ClCH2Si(OSi)3), and one at the lower field (−68.56 ppm) to the T2 (ClCH2Si(OSi)2(OCH3/H))25. The T3/T2 peak area ratio is ~ 93/7, showing a high degree of condensation. The formation of PCMSQ network with negligible silanol groups can also be confirmed in the FTIR spectrum of the same sample shown in Figure 3b. The sample gelled at 60 °C shows a slightly higher degree of condensation in NMR. Modification of PCMSQ aerogels with C-dots. Providing a luminescent function to porous materials will further extend the applicability to (bio)sensors and displays39-42. Among typical fluorescent materials, C-dots are emerging metal-free, less-toxic fluorescent with a size dimension below 10 nm43. An embedment of polyamine-functionalized C-dots with a high fluorescent quantum yield (40 %) onto silica aerogels was demonstrated, and the resultant materials were used for NO2 gas sensors41.


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In the present PCMSQ system, we have replaced the aqueous nitric acid in the starting solution with the C-dot sol prepared separately35, and obtained aerogel via supercritical drying. Comparing PCMSQ aerogels without/with the C-dots, bulk density shows an increase from 0.17 g cm−3 to 0.21 g cm−3, and light transmittance does not change. Although the FTIR spectra shown in Figure S1 does not prove the presence of C-dots possibly due to a low loading, the aerogel shows luminescent of blue light under the irradiation of UV (Figure 4). The same procedure was applied to the PMSQ system, which resulted in significantly weak luminescence. While the polar chloromethyl groups in the PCMSQ network contribute to anchoring the C-dots with residual amino (and carboxy) groups by covalent and/or van der Waals interaction, the methyl groups in PMSQ do not show strong interactions with the C-dots, which allows leaching of the C-dots during washing and supercritical drying processes. Modification of PCMSQ aerogels with nucleophilic compounds. The chloromethyl groups on silicon are highly reactive toward the attack by nucleophiles due to the α effect44, in which the intermediate α carbanions are relatively stabilized and the substitution reaction is promoted. This enhanced reactivity of SN2 is useful for organic synthesis45 and material designs46,47. We also investigated SN2 reactions on PCMSQ using several nucleophiles. Wet PCMSQ gels were immersed in several nucleophile-solvent pairs and it has been found that the gel dissolves under the presence of nucleophiles such as −N3 and −OAc in various solvents such as DMF, DMSO and alcohols. It has also been found that in some cases the dissolved gels turn into gel again in DMF or DMSO with a higher concentration of a nucleophile; e.g. 3 equiv. (against chloromethyl groups) of KOAc. Keppeler et al. prepared a polysiloxane network from a mixed precursor of CMTMS and tetrakis(2-hydroxyethyl)orthosilicate (EGMS) and did not report such dissolution when they conducted SN2 on the chloromethyl groups with NaN3 for the following surface click


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reactions46, probably because the copolymer network from these precursors was relatively stable due to the tetrafunctional polysiloxane moieties (Q species). This observation of dissolution and following re-gelation can be explained as follows. Since the silicons in the network in fact are electron-deficient because of the induction effect by chloromethyl groups, the siloxane bonds are susceptible to bond breaking due to the nucleophilic attack to the central silicon (state A to B, Figure 5), and eventually the gel dissolves to give a sol especially when multiple bond breaking on the silicon occurs. After the nucleophilic substitution on chloromethyl groups occurs in the sol (state C), the electron-enriched central silicon again tends to form siloxane bonds (state D), because the silicon-nucleophile bonds (such as Si−OAc) are less stable. This series of reaction seems to take place primarily when the nucleophile is negatively charged (such as –N3 and –OAc) so that the breakage of siloxane bond leaves –OSi≡ that can react in the next step; the –OSi≡ anions in turn attack to the silicon after the SN2 reaction on the chloromethyl has occurred. This speculation is plausible only when the kinetics of nucleophilic attack to silicon is well-faster than that to the chloromethyl. Both SN2 reactions can occur in a competing way in practice, but the reaction on the silicon is preferred because the silicon is less electronegative. The molecular structure of re-gelled network (state D, aerogel) by FTIR shows reduced absorption bands of Si−CH2Cl and an appearance of C=O band (Figure S2). Although further evidencing is necessary, the above speculation well explains the experimental results. Gelation mediated by nucleophilic substitution on chloromethyl groups. The above mechanism experimentally demonstrates that the SN2 reaction on chloromethyl induces gelation, even when there is no water in the system. Since gelation may thus be controlled in sol-gel systems containing CMTMS and a nucleophile at least, the following experiment has been


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performed. Moreover, an introduction of nucleophile molecules to the PCMSQ network may be beneficial to alter chemical and physical nature of the network. A sol-gel system consisting of CMTMS (0.4 mL), DGBE (0.5 mL) and DMAc (1 mL) has been reacted at 80 °C to investigate the progress of reaction by FTIR (Figure S3a). The absorption band at 690 cm−1, which corresponds to the stretching vibration of CH2−Cl, decreases in the course of the reaction. In addition, the decreasing rate becomes slower at lower temperatures such as 60 °C and 40 °C (Figure S3b). At the onset of gelation, 35−45 % of the chloromethyl groups are found to be reacted, which leads to an enhanced formation of polysiloxane bonds as depicted in state D, Figure 5. Although gelation has been confirmed to occur under the absence of water, in the next experiment water is added to develop the more crosslinked polysiloxane network and strengthen the gels as described in Experimental Section. Among the aprotic solvents DMF, DMSO and DMAc, DMAc gave the best result on the mechanical property of aerogels, i.e., higher stress and resilience as shown in Figure S4, in uniaxial compression tests on the resultant aerogel (ρb = 0.09 g cm−3 and T550nm = 25 %). Dependence of bulk density and light transmittance on the amount of DGBE is shown in Figure 6. Light transmittance reaches the maximum (T550nm = 39 %) with an increasing amount of DGBE (0.75 mL), presumably because the polarity in the network and solvent containing excess DGBE becomes closer in the course of nucleophilic reaction and phase separation is most suppressed. Above 0.75 mL, transmittance decreases due to coarsening of the pore structure. Bulk density decreases with an increasing amount of DGBE. The same process can be applied to a system containing polyethyleneimine (PEI) instead of DGBE, but resilience against uniaxial compression becomes lower presumably because unreacted amino groups hinder the spring-back behavior through attractive interactions.


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Finally, the PCMSQ gels strengthened with DGBE (0.5 mL) were employed to prepare xerogels via ambient pressure drying. The wet gels washed with methanol underwent solvent exchange with n-hexane, which was then allowed to slowly evaporate at 40 °C. Figure 7 is a picture of the xerogel and stainless reaction vessel. A transparent, porous xerogel was obtained, but bulk density of the xerogel was higher (ρb = 0.27 g cm−3) as compared to the corresponding aerogel (ρb = 0.09 g cm−3), due to irreversible shrinkage resulting from the presence of silanol groups as well as the residual amino groups. Minimization and/or sterically hindering of such polar groups left in the network by, for example, more rigorous aging and/or silylation with trimethylchlorosilane or hexamethyldisilazane should be beneficial to prepare xerogels with lower irreversible shrinkage.

CONCLUSIONS Polychloromethylsilsesquioxane (PCMSQ) aerogels have been prepared solely from chloromethyltrimethoxysilane (CMTMS) for the first time. Highly crosslinked aerogels that possess well-defined mesoporous structure and visible-light transparency have been obtained in an acid-catalyzed sol-gel system containing a surfactant n-dodecyltrimethylammonium chloride (C12TAC). The chloromethyl groups in the monomer/network are found to be useful for functionalization and improving mechanical properties. Photoluminescent carbon dots (C-dots) prepared from citric acid and polyethyleneimine (PEI) can be loaded to the PCMSQ aerogels due to their attractive interaction and/or covalent bond formations. Also, nucleophilic substitution (SN2) reactions between nucleophiles such as −OAc and −N3 and the chloromethyl groups occur


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in the network. The wet PCMSQ gel dissolves even in a non-aqueous solvent under the presence of a nucleophile, followed by re-gelation triggered by the SN2 reaction. This phenomenon, SN2triggered gelation, has been used to prepare aerogels reinforced by multifunctional nucleophiles such as polyamines. The rich chemistry of CMTMS and PCMSQ reported here due to the chloromethyl groups on silicon may be beneficial to extend the preparation strategy of organic-inorganic hybrids not only in the form of monolithic aerogels and xerogels, but also particles and films with desired physicochemical properties by hybridizing with adequate materials.

50 100 40 50

30 20 1

1.5

2

2.5 pH

3

3.5

0.32

0 4

7 0.3 6 0.28 5 0.26 4 0.24 3 1

1.5

2

2.5 pH

3

3.5

Bulk density / g cm–3

150

Shrinkage /%

60

Gelation time / min

8

Transmittance / %

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4

Figure 1. Dependence of light transmittance, gelation time, shrinkage and bulk density of the PCMSQ aerogels on pH of aqueous nitric acid in the starting solution (1 mL CMTMS, 0.15 g C12TAC and 2 mL aqueous nitric acid (varied pH)).


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Figure 2. Appearance and pore structures of the PCMSQ aerogels prepared with C12TAC and aqueous nitric acid with different pH.

Figure 3. (a) 29Si DDMAS NMR and (b) FTIR spectra of the PCMSQ aerogel sample gelled at 40 °C and aged with a coverage layer of diluted nitric acid.


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Figure 4. PCMSQ aerogel loaded with C-dots, showing fluorescence (~445 nm) under UV irradiation at 365 nm, while PCMSQ without C-dots and PMSQ with C-dots show no and only weak fluorescence properties, respectively.


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Figure 5. PCMSQ gel dissolves under the presence of a nucleophile, and re-gels when the concentration of the nucleophile is high enough. This phenomenon is speculated to be based on a series of nucleophilic reaction as demonstrated in A to D.

Figure 6. Dependence of light transmittance and bulk density on the amount of DGBE. Starting composition: CMTMS 0.4 mL, DGBE varied, DMAc 1 mL, water 0.2 mL. Inset shows a photograph of aerogel prepared with 0.75 mL of DGBE.


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Figure 7. A xerogel obtained by ambient pressure drying of a strengthened PCMSQ gel with DGBE using n-hexane as the drying solvent.

Scheme 1. Hydrolytic sol-gel reaction toward PCMSQ aerogels (a) and SN2 reaction for modification of network (b)

ASSOCIATED CONTENT Supporting Information


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The Supporting Information is available free of charge on the ACS Publications website. The following files are available free of charge. FTIR spectra and mechanical test results (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (K.K.) Present Addresses §

National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba,

Ibaraki 305-8561 Japan

ACKNOWLEDGMENT The solid-state NMR measurements were carried out in the JURC at the Institute for Chemical Research, Kyoto University. The present study has been performed under financial supports from Advanced Low Carbon Technology Research and Development Program (ALCA, JST Japan) and JSPS KAKENHI Grant Number 17K06015.

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