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Reinforcement of Silica Aerogels Using Silane-End-Capped Polyurethanes Yannan Duan, ,§ Sadhan C. Jana,* ,Bimala Lama, and Matthew P. Espe Department of Polymer Engineering, The University of Akron, Akron, Ohio 44325-0301, United States Department of Chemistry, The University of Akron, Akron, Ohio 44325-3601, United States * S Supporting Information ABSTRACT: Proper selection of silane precursors and polymer reinforcements yields more durable and stronger silica aerogels. This paper focuses on the use of silane-end-capped urethane prepolymer and chain-extended polyurethane for reinforcement of silica aerogels. The silane end groups were expected to participate in silica network formation and uniquely determine the amounts of urethanes incorporated into the aerogel network as reinforcement. The aerogels were prepared by one-step solgel process from mixed silane precursors tetraethoxysilane, aminopropyltriethoxysilane (APTES), and APTES-end-capped polyurethanes. The morphology and mechanical and surface properties of the resultant aerogels were investigated in addition to elucidation of chemical structures by solid-state 13 C and 29 Si nuclear magnetic resonance. Modication by 10 wt % APTES- end-capped chain-extended polyurethane yielded a 5-fold increase in compressive modulus and 60% increase in density. APTES- end-capped chain-extended polyurethane was found to be more eective in enhancement of mechanical properties and reduction of polarity. 1. INTRODUCTION Aerogels have been synthesized from metals or metalloid elements surrounded by various reactive ligands. 1,2 Of these, the most common and extensively studied is silica aerogel. Silica aerogels present several unique attributes, such as porosity (>95%), extremely low density (3350 mg/cm 3 ), 3 large surface area (5001200 m 2 /g), low thermal conductivity (0.0040.03 W/(m·K)), low dielectric constant (1.12.2), 4,5 low index of refraction (1.05), 6 monoliths or powder morphological forms, and optical appearance such as trans- parent, 7,8 opaque, or translucent. 911 Silica aerogels found applications in high-energy particle physics (Cherenkov emitters) 12 and as excellent thermal insulation. 13,14 The monolithic silica aerogels with transparency and low thermal conductivity are well suited for building and construction industry especially for energy savings. 1517 Additional applica- tions of silica aerogels are found as adsorbent of oils and organic liquids, as sensors, catalysts, storage media, and templates, 1820 in life sciences dealing with biocatalysis, 21 for detection of viral particle by immobilized bacteria, 22 or as host of biomaterials or drugs. 23 However, the inherent fragility of silica networks and the hygroscopic nature act as deterrents to more widespread applications. A stress of 31 kPa can easily destroy native silica aerogels of density 120 mg/cm 3 derived from tetramethox- ysilane. 24 The pearl necklacestructures and the necksformed between secondary spherical silica particles held together by a few SiOSi bonds 25 are responsible for the fragility of native silica aerogels. Leventis and co-workers 25,26 proposed methods for strengthening the neck zones between the particles in silica networks based on reactions between the residual surfaceOH groups and isocyanates. The study implied that organic molecules other than isocyanates can be used in reinforcing aerogels. Meador et al. 2729 synthesized silica aerogels from a mixture of aminopropyltriethoxysilane (APTES) and tetramethoxysilane (TMOS) and used the amine groups of APTES for cross-linking with isocyanates. Meador et al. 30 later extended the study to include cross-linking by di-, tri-, Received: February 26, 2013 Revised: April 17, 2013 Article pubs.acs.org/Langmuir © XXXX American Chemical Society A dx.doi.org/10.1021/la4007394 | Langmuir XXXX, XXX, XXXXXX

Reinforcement of Silica Aerogels Using Silane-End-Capped Polyurethanes

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Reinforcement of Silica Aerogels Using Silane-End-CappedPolyurethanesYannan Duan,†,§ Sadhan C. Jana,*,† Bimala Lama,‡ and Matthew P. Espe‡

†Department of Polymer Engineering, The University of Akron, Akron, Ohio 44325-0301, United States‡Department of Chemistry, The University of Akron, Akron, Ohio 44325-3601, United States

*S Supporting Information

ABSTRACT: Proper selection of silane precursors and polymer reinforcements yields more durable and stronger silica aerogels.This paper focuses on the use of silane-end-capped urethane prepolymer and chain-extended polyurethane for reinforcement ofsilica aerogels. The silane end groups were expected to participate in silica network formation and uniquely determine theamounts of urethanes incorporated into the aerogel network as reinforcement. The aerogels were prepared by one-step sol−gelprocess from mixed silane precursors tetraethoxysilane, aminopropyltriethoxysilane (APTES), and APTES-end-cappedpolyurethanes. The morphology and mechanical and surface properties of the resultant aerogels were investigated in additionto elucidation of chemical structures by solid-state 13C and 29Si nuclear magnetic resonance. Modification by 10 wt % APTES-end-capped chain-extended polyurethane yielded a 5-fold increase in compressive modulus and 60% increase in density. APTES-end-capped chain-extended polyurethane was found to be more effective in enhancement of mechanical properties and reductionof polarity.

1. INTRODUCTION

Aerogels have been synthesized from metals or metalloidelements surrounded by various reactive ligands.1,2 Of these,the most common and extensively studied is silica aerogel.Silica aerogels present several unique attributes, such asporosity (>95%), extremely low density (3−350 mg/cm3),3

large surface area (500−1200 m2/g), low thermal conductivity(0.004−0.03 W/(m·K)), low dielectric constant (1.1−2.2),4,5low index of refraction (∼1.05),6 monoliths or powdermorphological forms, and optical appearance such as trans-parent,7,8 opaque, or translucent.9−11 Silica aerogels foundapplications in high-energy particle physics (Cherenkovemitters)12 and as excellent thermal insulation.13,14 Themonolithic silica aerogels with transparency and low thermalconductivity are well suited for building and constructionindustry especially for energy savings.15−17 Additional applica-tions of silica aerogels are found as adsorbent of oils andorganic liquids, as sensors, catalysts, storage media, andtemplates,18−20 in life sciences dealing with biocatalysis,21 fordetection of viral particle by immobilized bacteria,22 or as hostof biomaterials or drugs.23

However, the inherent fragility of silica networks and thehygroscopic nature act as deterrents to more widespreadapplications. A stress of 31 kPa can easily destroy native silicaaerogels of density 120 mg/cm3 derived from tetramethox-ysilane.24 The “pearl necklace” structures and the “necks”formed between secondary spherical silica particles heldtogether by a few Si−O−Si bonds25 are responsible for thefragility of native silica aerogels. Leventis and co-workers25,26

proposed methods for strengthening the neck zones betweenthe particles in silica networks based on reactions between theresidual surface−OH groups and isocyanates. The studyimplied that organic molecules other than isocyanates can beused in reinforcing aerogels. Meador et al.27−29 synthesizedsilica aerogels from a mixture of aminopropyltriethoxysilane(APTES) and tetramethoxysilane (TMOS) and used the aminegroups of APTES for cross-linking with isocyanates. Meador etal.30 later extended the study to include cross-linking by di-, tri-,

Received: February 26, 2013Revised: April 17, 2013

Article

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© XXXX American Chemical Society A dx.doi.org/10.1021/la4007394 | Langmuir XXXX, XXX, XXX−XXX

and tetrafunctional epoxies and reported an increase of strengthby 2 orders of magnitude and an increase of density by a factorof 2−3. Nguyen et al.31 incorporated an organic linking group,1,6-bis(trimethoxysilyl)hexane (BTMSH), in silanes andproduced a styrene cross-linked silica aerogel offering bothstrength and elasticity. Ilhan et al.32 also reported three-dimensional core−shell structures by forming polystyrene layeron amine-modified silica networks. Randall et al.33 presentedseveral methods for tailoring of mechanical properties of silicaaerogels, including silane precursors with flexible molecularstructures, amine functionality, and cross-linking possibilitiesusing urethane and epoxy molecules. Duan et al.34 recentlymodified silica networks by reacting polyhedral oligomericsilsesquioxane (POSS) molecules carrying silanol groups withthe residual Si−OH linkages in the silica networks. With lessthan 5 wt % POSS molecules reacted within the silica networks,the aerogel structures offered significant reduction of polarityand a 3-fold increase in compressive modulus compared tonative silica aerogels, with negligible changes in bulk density.Research work on polymer reinforcement of silica aerogelsreported to date involves a postgelation step where thereinforcing molecules are introduced after the gels are formed.This also accounts for multiple solvent exchange stepsespecially if the reinforcing polymer is not soluble in mixturesof ethanol and waterthe solvents commonly used in synthesisof silica aerogels. Such means of reinforcement may also resultin nonuniform coating of the silica network by the polymer dueto an imbalance of the rate of cross-linking reactions and therate of molecular diffusion of polymer chains inside the gelnetwork. The present work was motivated by the followingneeds. First, it was felt that gelation and reinforcement ifachieved in one-pot synthesis could potentially reduce the totaltime of the process. Second, it was anticipated that anynonuniformity of reinforcement induced by slow polymer chaindiffusion in postgelation reinforcement steps could be avoidedif polymer chains were made a part of the gel forming silaneprecursor. Third, the gel forming silane precursor wouldautomatically determine the amount of reinforcing polymerchains introduced into the gel network.Polyurethanes with high hard-segment content are often

selected for reinforcement of aerogels due to their stiffness andhigh glass transition temperatures. However, phase separationand slow diffusion of polymer chains into small pores are two

major issues when reinforcement in postgelation step is sought.In view of the above, in this work, polyurethane molecules end-capped with aminopropyltriethoxysilane (APTES) weresynthesized and included with other silane precursors. Thisallowed hydrolysis and condensation reactions of APTES-end-capped polyurethanes simultaneously with the primary silaneprecursor molecules. In addition, the amount of polymer chainsintroduced in the aerogel structures was uniquely determinedby the amount of APTES moieties reacted to the gel network.In this regard, both polyurethane prepolymer and chain-extended polyurethanes were end-capped with APTES, and theeffects of polymer chain length on potential for reinforcementwere studied.

2. EXPERIMENTAL SECTION2.1. Materials. All solvents and reagents were used as received.

Tetraethoxysilane (TEOS), aminopropyltriethoxysilane (APTES), andnitric acid (70%) were purchased from Sigma-Aldrich. Reagent gradeN,N-dimethylformamide (DMF) and ethanol were purchased fromFisher Scientific. DMF was dehydrated with molecular sieves and usedas a solvent. Poly(tetramethylene glycol) (PTMG) with molecularweight of 1000 g/mol was obtained from Aldrich (Milwaukee, WI).4,4′-Diphenylmethane diisocyanate (MDI, Mondur M) with molecularweight 250 g/mol and a melting point of 39 °C was supplied by BayerMaterialScience (Pittsburgh, PA). 1,4-Butanediol (BD) from Avocado(Heysham, Lancs, UK) with molecular weight of 90 g/mol was used asa chain extender. The chain extension reactions were catalyzed bydibutyltinlaurate (DABCO T120, denoted as DBTDL) from AirProducts (Allentown, PA) and diluted to 1 wt % using anhydroustetrahydrofuran (THF).

2.2. Preparation of End-Capped Polyurethane Prepolymerand Chain-Extended Polymer. Chain-extended polyurethane wasprepared by a two-step method.35,36 First, the prepolymer wassynthesized from MDI and PTMG. Second, the prepolymer was chainextended with BD to obtain polyurethanes. Note that thestoichiometry of MDI, PTMG, and BD was maintained such thatthe polyurethane chains contained isocyanate groups on both ends.These isocyanate end groups then reacted with the amino groups ofAPTES to produce polymer chains end-capped with APTES.

2.2.1. Preparation of APTES-End-Capped Prepolymer. Thesynthesis route is shown in Scheme 1. The reaction was conductedin a three-neck round-bottom flask equipped with a mechanical stirrerat 80 °C under nitrogen. 10.00 g of PTMG was reacted with 5.13 g ofMDI at a molar ratio 1:2.1 for 2 h to ensure a completely end-cappedprepolymer I. 2.00 mL of APTES in 50 mL of dry DMF was addeddropwise into the prepolymer I in a period of 30 min after the flask

Scheme 1. Synthesis of Urethane Prepolymer (Prepolymer I) and APTES-End-Capped Prepolymer (Prepolymer II)

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was cooled to 25 °C. All chemical agents were kept in dry conditionprior to their use in experiments. The prepolymer II thus synthesizedwas kept in solution in a sealed bottle to avoid contact with moisture.The structure was examined by Fourier transform infrared (FTIR)spectroscopy.2.2.2. Preparation of APTES-End-Capped Chain-Extended Polyur-

ethane. The synthesis route of APTES-end-capped polyurethane(polymer II) is shown in Scheme 2. The prepolymer I was synthesizedfollowing the procedure described in section 2.2.1. Then 9.86 g ofMDI was added into 15.13 g of prepolymer I at 80 °C along with a fewdrops of DBTDL/THF solution as catalyst. 4.51 g of BD in 100 mL ofdried DMF was added dropwise for 1 h under a dry nitrogenatmosphere and then kept for another 24 h for complete reaction. Themolar ratio of PTMG/MDI/BD was kept at 1/6/5 up to this point. Anadditional 0.50 g of MDI was added under stirring for another 2 h toobtain NCO end groups (polymer I). 1.50 mL of APTES was addedand kept for 24 h at room temperature for the formation of polymer II.The completion of the reaction was confirmed from the absence ofisocyanate peaks in FT-IR spectra. Polymer II was kept in solution insealed bottles to avoid contacts with moisture. The amount of polymerII was determined by evaporating the solvent followed by overnightvacuum drying.

2.3. Preparation of Silica Aerogels Reinforced withPrepolymer and Chain-Extended Polyurethane. The wet gelswere prepared by a one-step sol−gel synthesis process. TEOS (0.75mol/L), APTES (0.25 mol/L), water (4 mol/L), and different weightpercentage (over total silane) of APTES-end-capped polyurethane inDMF or APTES-end-capped prepolymer in DMF were dissolved inDMF. The mixture was poured into 16 mm diameter polypropylenemolds and allowed to sit for 24 h for complete gelation. After the gelswere formed, DMF was exchanged four times with ethanol andethanol was exchanged with liquid carbon dioxide. The resultant gelswere supercritically dried in an autoclave at 55 °C under a pressure of10 MPa, which is above the critical point of CO2 (31 °C, 7.4 MPa).Note that exchange of DMF with liquid CO2 was not possible due topoor solubility between these two liquids.

In this work, a series of aerogels were prepared with APTES-end-capped polyurethane (denoted as PU) and APTES-end-cappedprepolymer (denoted as prePU) with a weight ratio of 1, 3, 5, 7, 10,15, and 50 wt % relative to total silane weight in the formulation.These series of aerogels are denoted as “TA-polymer type-wt %”; e.g.,TA-prePU-1 represents aerogels reinforced with 1 wt % prepolymer IIand TA-PU-1 represents aerogels reinforced with 1 wt % polymer II.

Scheme 2. Synthesis of MDI-End-Capped Polyurethane (Polymer I) and APTES-End-Capped Chain-Extended Polyurethane(Polymer II)

Table 1. Density, Shrinkage, and Weight Loss at 600 °C of Silica Aerogels Modified with Polymer II

sample polymer II wt % added initially residual wt % weight loss % from PU density (g/cm3) shrinkage (%)

TA-0 0 76.1 n/a 0.096 ± 0.004 10.3 ± 1.3TA-PU-3 3.0 73.0 3.1 0.119 ± 0.003 10.1 ± 0.9TA-PU-5 5.0 71.1 5.0 0.132 ± 0.006 12.3 ± 1.6TA-PU-7 7.0 69.2 6.9 0.142 ± 0.009 12.5 ± 1.4TA-PU-10 10.0 66.3 9.8 0.155 ± 0.009 12.9 ± 2.1

Table 2. Density, Shrinkage, and Weight Loss at 600 °C of Silica Aerogels Modified with Prepolymer II

sample prepolymer II wt % added initially residual wt % weight loss % from prepolymer density (g/cm3) shrinkage (%)

TA-0 0 79.4 n/a 0.096 ± 0.004 10.3 ± 1.3TA-prePU-3 3.0 76.3 3.1 0.106 ± 0.004 11.1 ± 1.6TA-prePU-5 5.0 74.3 5.1 0.119 ± 0.005 11.9 ± 1.3TA-prePU-7 7.0 72.1 7.3 0.124 ± 0.004 12.1 ± 0.8TA-prePU-10 10.0 69.5 9.9 0.126 ± 0.009 12.4 ± 2.1

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The unmodified aerogel is denoted as TA-0 (see Tables 1 and 2 forother aerogel specimens).2.4. Characterization. Fourier transform infrared spectroscopy

was used to confirm the completion of isocyanate/alcohol andisocyanate/amine reactions. Size exclusion chromatography (SEC) wasused to obtain molecular weight of APTES-end-capped polyurethanesand its prepolymer. Spectrophotometric grade THF and spectropho-tometric grade DMF were used as solvents respectively forpolyurethane prepolymer and chain-extended polyurethane.Bulk density was obtained from mass and volume of aerogel

specimens. The diameter and length of cylindrical aerogel specimenwere measured by a caliper with a precision of 0.01 mm. Threemeasurements for each specimen and five specimens for each aerogeltype were used to obtain reproducible data. Shrinkage in diameter wascalculated based on diameter of the gel and the aerogel.Solid-state 13C and 29Si nuclear magnetic resonance (SSNMR)

spectra were used to determine the extent of hydrolysis andcondensation reactions of the silanes and the reactions between thesilanes and the silane-modified polymers.37 The data were collected ona Varian INOVA 400 MHz (9.4T) spectrometer using a Varian 4 mmDR-T3 probe. Samples were packed into 4 mm ceramic rotors, and allspectra were collected with magic-angle spinning (MAS) and a samplespinning speed of 12 kHz. Cross-polarization (CP) data were acquiredusing 1H 90° pulse widths of 4.0 and 5.0 μs, recycle delay of 2 s, CPspin-lock time of 3 ms for 1H−29Si experiments, and 1 ms for the1H−13C data. The 13C chemical shifts were referenced tohexamethylbenzene (17.3 ppm, methyl), and 29Si chemical shiftswere referenced to 2,2-dimethyl-2-silapentanesulfonate (1.46 ppm).Small-angle X-ray scattering (SAXS) is a choice method for studying

the structure of porous materials in the 1−100 nm range.38 SAXS wasused as a tool for determining the fractal features of silicaaerogels.39−41 The fractal dimension is measured from the powerdecay of the static structure in the Porod region.40 For this type ofsurfaces, Porod’s law is given in eq 1

=→∞

−I q q Plim [ ( ) ]q

Ds(6 )

(1)

where I(q) is the SAXS intensity at low angles as a function of themodulus of the scattering vector q = 4π sin θ/λ, 2θ being the scatteringangle, λ is the X-ray wavelength, Ds is the dimensionality of the fractalsurface, and P is a constant. A value of Ds = 2 corresponds to classical(two-dimensional) surfaces. Several humid gels were shown to exhibitinterfaces having the fractal properties with 2 < Ds < 3, which has aslope between −3 and −4,42 describing the elementary units as 3Dparticles with fractally rough surfaces.43

The SAXS intensity was collected using an X-ray generator ofRigaku MicroMax-002+ and Cu as the X-ray source with a wavelengthof 1.54 Å. The voltage was set at 45 kV with a current of 0.88 mA. Thescattering signal was detected by SAXS legacy 120 mm detector. Therange of angles was selected by setting scattering vector q in the range0.017−0.32 Å−1.The compressive modulus of aerogels was determined from the

stress−strain data obtained using a tensile tester, Instron 5567,Canton, MA, following the ASTM D695-85 method for compressivetest. The ASTM standard calls for a slenderness ratio of 11:1 to 16:1;however, using this sample size would have caused some of the lowerdensity specimens to buckle. Because of the difficulty of molding astandard “dog bone” tensile bar, cylindrical samples of a diameter ∼15mm and height ∼20 mm were used instead. Each aerogel cylinder wassanded at the top and bottom to ensure that they were smooth andlevel. The cross-head speed for the test is chosen 1.27 mm/min.Differential scanning calorimetry (DSC) was used to determine the

thermal transitions in modified silica aerogels. The samples wereanalyzed using a TA Instruments model DSC Q50. The samples wereexposed to a heat−cool−heat cycle in the analysis. The temperaturerange of each segment was from −20 to 220 °C at a heating/coolingrate of 10 °C/min. A nitrogen gas purge of 50 mL/min was used.Thermogravimetric analysis (TGA) was used to determine the thermalstability of silica aerogels. The samples were analyzed using a TAInstruments model TGA Q50. The temperature range was from room

temperature to 700 °C at a heating rate of 10 °C/min in air with a flowrate of 60 mL/min.

The morphology of aerogels was characterized by scanning electronmicroscopy (SEM). For this purpose, the aerogels were fractured atroom temperature and SEM images were taken after sputter-coatingthe fractured surface with silver. SEM images were taken using ascanning electron microscope (SEM, JEOL JSM5310).

The surface area, average pore size, and pore size distribution weremeasured via gas adsorption measurement by studying the adsorptionand desorption isotherms.44 The surface area, pore size, and pore sizedistribution were measured using TriStar II from Micromeritics. Eachsample was cut and placed in the chamber followed by outgassingeither at 40 °C or at room temperature until the mass attained aconstant value. Adsorption/desorption isotherms were collected byusing nitrogen as the adsorbent at −196 °C using liquid nitrogen bathto achieve this temperature.

The surface properties were studied by measuring the contact angleof water and diiodomethane. Aerogel specimens were converted intodisks to eliminate the effect of pore structures following a test methoddescribed elsewhere.34 The contact angle values were calculated fromthe image taken by Rame-Hart contact angle goniometer (model 100-00) on both edges of the drop using the software ImageJ 1.42. Thecalculation of surface energy for silica aerogels followed Wu’s theorydescribed elsewhere.34

3. RESULTS AND DISCUSSION

3.1. Elucidation of Structures of APTES-End-CappedPolymers. The structures of prepolymer I, prepolymer II,polymer I, and polymer II were identified using FT-IR and 1HNMR spectroscopy data. Detailed data are presented asSupporting Informationonly the peak locations are discussedhere. FT-IR spectra (Figure S1) of prepolymer, APTES-end-capped prepolymer, and APTES-end-capped chain-extendedpolymer showed the following peaks: a wide peak at 3400 cm−1

due to hydrogen-bonded −OH or −NH groups, a strong peakat 1740 cm−1 due to urethane carbonyl group, and a mediumstrong peak at 2250 cm−1, indicating the formation ofisocyanate-end-capped prepolymer I. The absence of isocyanatepeak in the FT-IR spectra of prepolymer II indicated completeAPTES-end-capping of prepolymer I. The number-averagemolecular weight (Mn) and polydispersity index (PDI) wereobtained from SEC data as follows: Mn ∼ 4200, PDI ∼ 1.68 forprepolymer I; Mn ∼ 5200 and PDI ∼ 1.73 for prepolymer II;Mn ∼ 28.7K and PDI ∼ 2.2 for polymer I; and Mn ∼ 28.5K andPDI ∼ 2.2 for polymer II. It can be inferred from the closelymatched values of Mn and PDI that both polymer I andpolymer II were linear molecules.The 1H NMR spectra (Figure S2) showed identical

resonance peaks for polymer I and polymer II due to smallerfractions of different end groups compared to long polyur-ethane chain. The peaks at 1.45 and 1.66 ppm are due tomethylene groups from PTMG and the group of peaks at 3.27ppm is due to methylene groups connected to oxygen inPTMG. The methylene group between two aromatic rings inMDI is inferred from the peak at 3.74 ppm. The methylenegroup connected to oxygen in butanediol shows peaks at 4.05and 4.00 ppm. The peaks at 7.05 and 7.32 ppm are from thearomatic rings of MDI respectively due to −CH closer (ortho-position) to methylene group substitution and −CH at themeta-position of the methylene substitution. The peak at 9.49ppm is from the hydrogen atom of the urea linkage. Thecombination of FT-IR and 1H NMR spectra discussed aboveconfirms that the expected urethane structures, as shown inSchemes 1 and 2, were obtained.

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3.2. Structure of Silica−Polyurethane Hybrid Aero-gels. The chemical makeup of aerogels was determined fromanalysis of materials using 13C and 29Si solid-state NMR. The13C NMR spectrum of APTES-end-capped polyurethane isshown in Figure 1a. The peak at 27.2 ppm arises from the

central CH2 groups of the butane diol addition product or thePTMG segments of the polymer. The CH2 groups bonded toan ether carbon have a peak at 65.9 ppm and those CH2 groupsnext to the carbamate groups give rise to a peak at 71.2 ppm.The lower intensity peak at 41.0 ppm is from the CH2 groupbetween the phenyl rings. The peaks from aromatic carbonsoccur in the region 120−140 ppm, and the carbonyl carbon ofthe carbamate group has a chemical shift of 154.6 ppm. The 13CSSNMR data is completely consistent with the 1H solutionNMR spectrum from this polymer and confirms the structureof the polymer included in Scheme 2.The APTES portion of the polymer will give rise to peaks at

58.2 and 15.9 ppm from the ethoxy groups and three peaks at43.5, 23, and 10 ppm from the aminopropoyl group. In the 13Cspectrum from the polymer alone (Figure 1a), these peaks arenot observed as the chain ends concentration is too low.However, the lower concentration of APTES end groupscompared to the urethane units of the polymer can be inferredfrom 1H NMR data. After hydrolysis and formation of theaerogel, any unreacted ethoxy groups will yield peaks at 58.2and 15.9 ppm. As these peaks are not observed in the 13CSSNMR spectrum from the aerogel (Figure 1b), completehydrolysis of the ethoxy groups from TEOS has occurred. Thesmall peak at 165 ppm observed in unmodified aerogels isattributed to imine formation in silica aerogel when DMF isused in the synthetic procedure.45

The 13C NMR spectrum from TA-PU-0.5, i.e., aerogelreinforced with 0.5 wt % polymer II, is shown in Figure 1c.Only the peaks from the propyl group of APTES are observedin the 13C spectrum as the concentration of polymer in thissample is very low. The lack of peaks from ethoxy groupsindicates that all the ethoxy groups from TEOS and APTEShave been hydrolyzed during aerogel formation. In 13C NMRspectrum of TA-PU-15 aerogel specimen (Figure 1d), the peaksfrom APTES as well as from the polyurethane are clearlyobserved. The lack of the signature peaks from the ethoxygroups in this spectrum again shows that all ethoxy groups,even those on the APTES end groups of polymer II, have beenhydrolyzed.

The 29Si NMR spectrum of silica materials typically showseveral distinct peaks arising from Si(−OSi)2(−OH)2, Si-(−OSi)3(−OH), and Si(−OSi)4 linkages.45 These species arelabeled respectively Q2, Q3, and Q4, with the peaks occurringmore upfield with higher Q value. With the use of cross-polarization, higher Q3 peak than Q4 peak maybe be displayeddespite being almost fully condensed. This is attributed to theproximity of hydrogen atoms in hydroxyl groups to the siliconatoms in incompletely condensed silanes.The 29Si NMR spectrum from the unmodified aerogel

(Figure 2b) has the expected resonances in the silica region of

the spectrum, with peaks at −92 ppm (Q2), −101 ppm (Q3),and −110 ppm (Q4). The larger population of Q3 sites relativeto Q4 sites results from the very high surface area of thematerial and the large number of surface hydroxyl groupspresent. The additional peak around −66 ppm is due to the R−Si(−OSi)3 group from APTES. In the 29Si NMR spectrum fromthe polymer, the peaks near 66 ppm due to APTES end groupsare observed (Figure 2a). The signal intensity was low,indicating that the APTES end group concentration was verylow in comparison to the polymer. Similar to the 13C results,the 29Si SSNMR data shows that polymer II has undergone thetypical hydrolysis reactions during the formation of the aerogeland is likely incorporated into the aerogel matrix. Since thepeaks in the 29Si NMR data from the APTES groups of polymerII and the APTES used to make the aerogel overlap, it is notpossible to determine the amount of polymer II present in theaerogel from the 29Si NMR data. Also, for those systems withlow levels of polymer II, the 13C NMR data did not providesufficient signal-to-noise to measure this value accurately.In view of the above results, we used thermogravimetric

analysis to determine the amount of polymer in the samplefrom residue left by the aerogel specimens. The weight loss ofdifferent samples in air was measured using a TA Instrumentsmodel TGA Q50 by heating sample specimens from roomtemperature to 700 °C at a heating rate of 10 °C/min in airwith a flow rate of 60 mL/min. These data are presented inFigure S3. The unmodified aerogel showed a residue of 78.6 wt% at 600 °C. At this temperature, the APTES-end-cappedpolyurethane decomposed almost completely and left 10.0 wt% residue. This residue came from the APTES end groups andthe excess of APTES added during the end-capping reaction.The difference of weight loss between unmodified aerogels withand without APTES-end-capped polyurethane should indicatethe polymer contents in each sample. As is evident from the

Figure 1. 13C CP/MAS SSNMR spectra of (a) polymer II, (b) TA-0,(c) TA-PU-0.5 (aerogel modified with 0.5 wt % polymer II), and (d)TA-PU-15 (aerogel modified with 15 wt % polymer II).

Figure 2. 29Si CP/MAS SSNMR spectra of (a) polymer II, (b) TA-0,(c) TA-PU-0.5, (d) TA-PU-10, and (e) TA-PU-15.

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data presented in Table 1, the percentage of weight loss agreeswell with the amount of polymer added in the initial silanemixtures. The combination of results from TGA and SSNMRshows that almost all the added polymers remained in theaerogel. Similar results are also observed for aerogels modifiedwith prepolymer II, as shown in Table 2.The thermal transitions of aerogels with and without

polymer II were characterized via DSC (Figure S3). PolymerII (Scheme 2) shows a melting temperature at ∼185 °C and aTg around 86 °C. Aerogels with polymers do not show thermaltransitions during the first heating scan due to an amorphousnetwork and due to good dispersion of polymer II within theaerogel network. Thermal transitions were observed only athigh weight percentage of the polymer during the secondcooling scan, e.g., at 50 wt %. Sample TA-PU-50 showed a verybroad exothermic peak at around 166 °C, suggesting that phaseseparated polymer could form during the second cooling scanat this concentration of polyurethane.3.3. Properties of Silica/Polyurethane Hybrid Aero-

gels. 3.3.1. Density and Shrinkage. The data on density andshrinkage of aerogels reinforced with polymer II andprepolymer II are presented respectively in Tables 1 and 2.Several trends are apparent from this data. First, the density andshrinkage increased due to modification with prepolymer II andpolymer II, although density increased is more in the case ofpolymer II. For example, 10 wt % polymer caused an increaseof aerogel bulk density by 61% with polymer II vs 31% withprepolymer II. Second, in view of the magnitude of error bars,we can infer that the shrinkage beyond 3 wt % polymer contentremained almost insensitive to the amount of polymer in theaerogel.3.3.2. Morphology and Surface Properties. 3.3.2.1. Mor-

phology. A representative set of SEM images is presented inFigure S4 and Figure 3. The unmodified aerogel contained

secondary particles of relatively uniform size (Figure S4a). Thesize of secondary particles did not change much with theaddition of APTES-end-capped prepolymer, as shown in FigureS4b. In the presence of polymer II, the particles showed somedegree of aggregation although the pearl-necklace structure waspreserved. The size of aggregated particles appeared uniform inthe range of 90−110 nm, as shown in Figure S4c,d. At highconcentration of polyurethane, e.g. 50 wt %, most particles are

seen with sizes in the range of 90−110 nm. However, somelarger particles in the range of 300−400 nm are seen clearly inFigure 3. The larger particles have regular round shapes incontrast to fractal aggregates of smaller particles. One suchlarge particle marked with a white circle in Figure 3 is presentedat high magnification.The particle aggregation in the presence of polymer II can be

attributed to the formation of a coating layer of polymer chainsaround the particles as reported by Leventis and co-workers.26

Such coating layers or conformal coating can widen the neckregions between the adjacent secondary particles. This will befurther explored while discussing SAXS and contact angle data.It is possible that the polymer chains encapsulated some of thesilica particles during phase separation, as evident from theinset image in Figure 3. In this context, polymer coatingsformation on silica particles can alter the fractal morphology ofthe pores in aerogels as discussed below.The SAXS intensity I(q) plotted as a function of scattering

vector q (Figure S5) did not show distinct peaks, indicating thatthe aerogels contained disordered structures. A plot of log I(q)versus log q was used to obtain the value of fractal dimensionsDs following the method described by Duan et al.34 Theintensity followed a power-law behavior in the interval 0.06 ≤ q≤ 0.18 Å−1 with exponents of −3.78 to −3.31. Thesecorrespond to fractal dimensions Ds in the range of 2.22−2.69, as listed in Table 3. The power law was not satisfied for q

≥ 0.18 Å−1 because q became so large that the scatteringprocess could resolve the structures of the size of individualatoms and the two-phase approximation was no longersatisfied.43 The values of the slope of the Porod’s plot in therange of −4 to −3 imply fractal surfaces.39 A reduction of fractaldimension (Table 3) implies that the curved surfaces of thepores in unmodified aerogel turned into smooth cylindricalpores after modification with prepolymer II and polymer II.This also corroborates that the pores became smooth due toformation of polymer coatings on the secondary silica particles,as is inferred from the SEM images in Figure 4. The fractaldimension of aerogels modified with prepolymer II is larger

Figure 3. SEM image of TA-PU-50 aerogel (modified with 50 wt %polymer II) showing polymer-coated silica particles.

Table 3. Slopes and Fractal Dimension from Porod’s Plot ofAerogels Modified with Polymer II

slope Ds slope Ds

TA-0 −3.31 2.69 TA-PU-50 −3.78 2.22TA-PU-3 −3.46 2.54 TA-prePU-3 −3.40 2.60TA-PU-7 −3.62 2.38 TA-prePU-5 −3.51 2.49TA-PU-10 −3.64 2.36 TA-prePU-7 −3.58 2.42TA-PU-15 −3.75 2.25

Figure 4. Images of water droplets sitting on polymer films andcompressed aerogel disks. The values represent water contact angle.

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than those modified with the same amount of polymer II. Thissuggests that chain-extended polyurethane was more effectivein coating the surfaces of secondary particles and for wideningthe neck regions than the prepolymer.3.3.3. Surface Properties from Contact Angle Measure-

ments. The polymer modification of aerogels was expected toexert influence on surface energy and hydrophobicity ofmodified silica in aerogels. This was inferred from the valuesof contact angle of deionized water and methylene iodide. Forthis purpose, aerogel specimens were compressed into disks toremove the pores. In conjunction, polymers II and I were castinto flat films from their solutions and dried under vacuum. Theimages of droplets of deionized water on polymer films andcompressed disks of aerogels are presented in Figure 4. Thecontact angle of water droplets on films of polymers I and II are100.4° and 69.9°, respectively. A smaller contact angle on filmsof polymer II is attributed to the hydrophilic APTES endgroups. A representative set of images of water droplets oncompressed disks of aerogels modified with polymer II andprepolymer II are also shown in Figure 4. The correspondingvalues of polar and dispersion components of surface energy arepresented in Table 4. The native aerogels show a contact angleof only 36.7°, while polymer modification rendered the aerogelsmore hydrophobic as is apparent from the higher values ofcontact angle.The data presented in Table 4 also show that polymer II is

more effective in rendering the surfaces of aerogels hydro-phobic than prepolymer II. This is a reflection of higheramounts of polymer in chain-extended polyurethane than theprepolymer for every APTES end group. The higher contactangle on aerogels modified with polymer II than on films ofpolymer II is attributed to the loss of polarity of APTES endgroups in aerogels due to reactions with silica network.Prepolymers I and II were liquids at room temperature.The data in Table 4 indicate that both polymer modifications

effectively reduced the polar components of the surface energy,but only polymer II lowered the values of both dispersion andpolar components causing an overall reduction of total surfaceenergy. In other words, APTES-end-capped polyurethane canimprove hydrophobicity and reduce the total surface energy,while APTES-end-capped prepolymer can improve only thevalue of hydrophobicity. A larger value of contact angle (∼91°)for aerogel TA-PU-10 over polymer II (∼70°) in Table 4 canbe attributed to complete conversion of the hydrophilicethoxysilane groups in aerogel TA-PU-10 as per solid stateNMR data discussed earlier.

3.3.4. Surface Area and Pore Size Distribution. Arepresentative set of adsorption/desorption isotherms ofaerogels are shown in Figure 5 and Figure S6. The initial

increase of the quantity of gas absorbed indicates the presenceof micropores. The adsorption and desorption isotherms in theregion 0.05 ≤ P/P0 ≤ 0.3 overlapped on top of each other dueto the formation of monolayers and multilayer adsorption. Thehysteresis loops suggest the presence of mesopores in allsamples. These isotherms are typical for a combination of typeII and type IV isotherms. The hysteresis loop at P/P0 > 0.5 istypical for slit-shaped pores rather than the more commonspherical pores.46 Similar behavior was observed for otheraerogels.The pore size distributions (Figure S7) of the unmodified

aerogel and the aerogels modified with up to 7 wt % polymer IIand prepolymer II did not reveal dominant mesopores.Aerogels modified with 10 and 50 wt % polymer II showedbroad peak at a mesopore diameter of around 22 nm. It isconceivable that some smaller size mesopores were covered bythe polymer at higher concentration. A visual inspection ofSEM images in Figure 3 reveals large fractions of macropores inthe range 80−100 nm, but their size distribution was notcharacterized.The values of total surface area, micropore surface areas from

t-plot, average pore size from the BJH method, and dominantpore size of several aerogel samples are listed in Table 5. No

Table 4. Contact Angle and Surface Energy Components of Aerogels Modified with Polymer II (TA-PU) and Prepolymer II(TA-prePU)

contact angle (deg) surface energy and its components (dyn/cm)

sample ID H2O CH2I2 dispersion polar total polarity

polymer I 100.4 ± 0.4 55.3 ± 0.5 31.4 2.1 33.5 0.06polymer II 69.9 ± 0.2 38.1 ± 0.6 30.5 15.1 45.6 0.33TA-0 36.7 ± 0.6 37.2 ± 0.5 30.5 32.9 63.4 0.52TA-PU-3 71.2 ± 0.4 60.5 ± 0.3 20.6 18.1 38.8 0.47TA-PU-5 75.7 ± 0.3 61.7 ± 0.7 20.5 15.8 36.3 0.43TA-PU-7 85.5 ± 0.7 62.6 ± 0.5 21.5 10.5 32.0 0.33TA-PU-10 90.8 ± 0.6 68.4 ± 0.7 19.6 8.9 28.4 0.31TA-prePU-3 37.3 ± 0.3 27.2 ± 0.3 31.7 32.0 63.7 0.50TA-prePU-5 53.9 ± 0.5 29.0 ± 0.6 32.2 22.9 55.2 0.42TA-prePU-7 72.5 ± 0.5 31.6 ± 0.6 33.7 13.0 46.7 0.28TA-prePU-10 79.2 ± 0.5 32.1 ± 0.4 35.0 9.6 44.6 0.22

Figure 5. Adsorption and desorption isotherms of aerogels modifiedwith polymer II.

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significant difference is observed in the values of average porediameter in the range of 9−13 nm for all aerogel samples. Theunmodified aerogel had a BET surface area of 485.5 m2/g. Incomparison, the BET surface area of modified aerogel TA-PU-10 dropped to 226.8 m2/g and that of TA-prePU-10 droppedto 242.9 m2/g. A similar trend is observed for microporesurface area. Such reduction of micropore surface area and thetotal surface area in polymer-modified specimens wouldindicate that the micropores residing inside the secondaryparticles were not accessible due to polymer coating layersformed on the surfaces of secondary silica particles. It is alreadylearned from the SAXS data that the fractal dimension of thepores decreased due to polymer modification.3.3.4. Mechanical Properties. The unmodified silica aerogels

failed at 5−10% compressive strain and showed compressivestrength of 0.04 MPa. The strain at break and compressivestrength are higher for aerogels modified with greater than 3 wt% polymer II or prepolymer II (Figure S8). The aerogelsmodified with higher than 7 wt % polymer II and 10 wt %prepolymer II showed buckling at high compressive strains.The data presented in Figure 6 reveal that at 5 wt % or higherpolymer content polymer II is more effective in enhancing themodulus of aerogels than prepolymer II. A 5-fold increase incompressive moduli compared to unmodified aerogel isobtained for aerogels modified with 10 wt % polymer II. The

increase of bulk density of the aerogel may be a contributingfactor to higher modulus. Recall from Tables 1 and 2 that thebulk density of aerogel modified polymer II and prepolymer IIboth increased with the amount of polymer. Specifically, theaerogel with 10 wt % polymer II shows a 61% increase in bulkdensity compared to unmodified aerogel. However, thecompressive modulus data of aerogels presented in Figure 6show the following scaling with bulk density (ρbulk) ∼ ρbulk

0.17

for polymer II and ρbulk0.09 for prepolymer II. As is evident,

these dependences are relatively weak and cannot account forlarge increases reported in Figure 6. The widening of the neckregions of secondary particles as supported by the SAXS andcontact angle data discussed earlier accounts for such largeincreases in compressive modulus. Figure 7 shows that TA-PU-

15 and TA-PU-50 could stand a strain of higher than 80% incomparison with TA-0 and TA-PU-7 and behave like acompressed solid material at high strain as all the pores wereremoved.We now reflect on if the aerogel reinforcement process

described in this paper is able to reduce the total timecompared to postgelation reinforcement processes. In thismethod, the gels were prepared in the presence of reinforcingpolymers which also participated in the formation of silicanetworks. The steps of solvent exchange and supercriticaldrying are similar to postgelation reinforcement process. Thetypical time for production of one batch of aerogels in thismethod required 4−5 days. In comparison, postgelationmodification by the polymer required ∼10 days, as the gelswere prepared, solvent exchanged with a good solvent for thecross-linkers, reacted at a certain temperature for a few days,then solvent exchanged with a good solvent that is misciblewith liquid carbon dioxide, and finally dried under supercriticalconditions.A 5-fold improvement in compressive modulus was observed

for aerogels with 10 wt % of APTES-end-capped polyurethane.A similar extent of reinforcement was seen in other pregelationmodification.47 It is believed that a polymer coating layer wasformed on the surfaces of secondary particles, which alsostrengthened the neck regions between the secondary particles.The reduction of BET surface area and micropore surface areaseen in Table 5 and the SAXS data supported the aboveargument. The coating layer was also responsible for the largervalues of contact angle with deionized water and improvedcompressive modulus. The presence of these polymer coatingsin the aerogel structure was assumed as the origin of the high

Table 5. BET Surface Area, Micropore Surface Area, AveragePore Diameter, and the Dominant Pore Size of SilicaAerogel Modified with Prepolymer II (TA-prepPU) andPolymer II (TA-PU)

sample

BETsurfacearea

(m2/g)

microporesurface area(m2/g)

BJH desorptionaverage porediameter (nm)

dominantpore

diameter(nm)

TA-PU-0 485.5 22.9 10.2 n/aTA-PU-3 322.1 18.7 12.1 n/aTA-PU-5 287.9 16.5 11.4 n/aTA-PU-7 241.1 12.9 11.9 n/aTA-PU-10 226.8 10.5 11.4 21.4TA-PU-50 192.6 9.9 13.1 21.6TA-prePU-1 303.2 19.7 9.2 n/aTA-prePU-3 268.7 15.5 9.2 n/aTA-prePU-5 250.6 15.3 9.5 n/aTA-prePU-7 239.4 11.9 9.90 n/aTA-prePU-10 242.9 8.9 10.8 n/a

Figure 6. Compressive modulus of modified aerogels as a function ofpolymer content.

Figure 7. Stress−strain diagrams of modified aerogels.

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contact angle and their stress−strain curve similar to “soft”materials. The polymer helped to maintain the structure anddissipated energy at relatively high load and high strain duringcompressive tests.

4. CONCLUSIONSA new reinforcement method was studied for strengthening ofsilica aerogels and for chemical modification of their hydro-philic surfaces. The one-pot synthesis and reinforcementmodification method presented in this work provided scopefor shorter production time. The APTES end-capping ofpolymer chains served two purposes. First, it allowedintegration of polymer chains into gel networks via APTES-end-group participation in condensation reactions. Second, theamount of polymer chains incorporated into aerogels wasuniquely tied to the amount of APTES participating in gelnetwork formation. The reduction of fractal dimensions of thesilica networks and the reduction of polarity confirmedcoverage of silica particles by polymer chains. Of the twoAPTES-end-capped polymers, chain-extended polyurethaneproduced much better results due to higher coverage of silicaparticles, which led to larger enhancement of mechanicalproperties. At high polymer contents, the reinforced aerogelswere able to withstand above 70% compressive strain withoutfailure.

■ ASSOCIATED CONTENT*S Supporting InformationAdditional figures. This material is available free of charge viathe Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] (S.C.J.).

Present Address§Y.D.: Currently at PolyOne Corporation.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe express special thanks to Dr. Robert A. Weiss and Dr.Stephen Z. D. Cheng for allowing us to use the contact anglegoniometer and SAXS equipment. This work was partiallyfunded by National Science Foundation under Award CMMI-1200484.

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