Journal of Non-Crystalline Solids JNCS Mechanical properties of...Mechanical properties of polymer-modified silica aerogels dried under ambient pressure Hailong Yang, Xiangming Kong⁎,

Journal of Non-Crystalline Solids 357 (2011) 3447–3453

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Journal of Non-Crystalline Solids

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Mechanical properties of polymer-modified silica aerogels dried underambient pressure

Hailong Yang, Xiangming Kong ⁎, Yanrong Zhang, Chunchao Wu, Enxiang CaoDepartment of Civil Engineering, Tsinghua University, Beijing, 100084, PR China

⁎ Corresponding author. Tel.: +86 1062783703; fax:E-mail address: [email protected] (X. Kong

0022-3093/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.jnoncrysol.2011.06.017

a b s t r a c t

Article history:Received 7 April 2011Received in revised form 14 June 2011Available online 19 July 2011

Keywords:Silica aerogels;Polymer;Ambient pressure drying;Mechanical properties;Silica skeleton

Silica gels prepared by copolymerizing tetraethylorthosilicate with 3-aminopropyltriethoxy-silane weremodified using polymer derived from toluene diisocyanate and dried under ambient pressure. The successfulpreparation of silica aerogels depended on the effective control of shrinkage during drying. The resultingmaterial, polymer-modified silica aerogel, was then characterized by thermogravimetric analysis and uniaxialcompression tests. Results indicated that the apparent elastic modulus and compressive strength of thepolymer-modified silica aerogels decreased with increasing amounts of incorporated polymer because ofdecreasing shrinkage and density, while the strains at the surface cracking point and the final failure pointincreased significantly during compression tests. The strength and modulus of the silica skeleton could becalculated from the apparent strength and modulus of the silica aerogels respectively. It was interestinglyshown that the elastic modulus of the silica skeleton of the silica aerogels increased because of theincorporated polymers, while the polymers had no effects on the compressive strength of the silica skeleton.In addition, the relationships between the apparent elastic modulus or the apparent compressive strength ofthe polymer-modified silica aerogels and their shrinkage were quantitatively expressed.

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1. Introduction

Silica aerogels are highly porous materials with low density andthermal conductivity. Their unique physical properties make themattractive for use in various applications [1–4]. However, silica aerogelmonoliths have limited use in specialized environments, such as inCerenkov radiation detectors in certain nuclear reactors [5–7],collectors of hypervelocity particles in space, and thermal insulatorsin space vehicles [6–9]. The wider industrial and commercial use ofsilica aerogels has been difficult to implement because of their poormechanical properties coupled with the need for supercritical fluid(SCF) drying during manufacture.

The most common strategy for improving the mechanicalproperties of silica aerogels is to reinforce them using materialssuch as xonotlite [10,11], ceramic fibers [12–14], nonwoven fibers[15], and carbon or silica fiber felt [16,17]. However, no covalentbonding occurs between the two components of this kind ofcomposite material; thus, the inherent fragility and brittle nature ofsilica aerogels remain unaltered. The reinforcement of silica aerogelsusing a conformal coating of polymer on the silica skeleton of silicaaerogels was recently demonstrated to be effective in improvingmechanical properties [7,8,18–29]. Although polymer-modified silicaaerogels have good mechanical properties, SCF drying is still used in

most studies except for Ref. [18]. SCF drying of silica gels features bothhigh costs and risks, thus limiting its broader application. Moreover,few studies focus on the mechanical properties of polymer-modifiedsilica aerogels dried under ambient pressure in Ref. [18]. In the currentstudy, we report on the preparation and mechanical properties ofpolymer-modified silica aerogels dried under ambient pressure.

2. Experiment

2.1. Materials

Analytical-grade tetraethylorthosilicate (TEOS), acetone, pentane,3-aminopropyltriethoxy-silane (APTES, 98%; Nanjing Capture Chem-ical Co.), toluene diisocyanate (TDI, ≥99.5%; GCP Union Chemical Co.,Ltd., Zhengzhou), hydrochloric acid (HCl, 37% w/w), and double-distilled water were used as received without further purifications.

2.2. Preparation of polymer-modified silica aerogels

Fig. 1 illustrates the synthetic pathway for producing polymer-modified silica aerogels dried under ambient pressure. The prepara-tion process can be divided into three steps.

2.2.1. Preparation of amine-modified wet gelsAmine-modified wet gels were synthesized using a two-step

process involving the acid hydrolysis of TEOS, followed by base-catalyzed condensation with APTES. Amine-rich APTES eliminates the

Fig. 1. Synthetic pathway for polymer-modified silica aerogels.

3448 H. Yang et al. / Journal of Non-Crystalline Solids 357 (2011) 3447–3453

need for additional base catalysis because the amine group of APTESserves as the gelation catalyst. Solutions A and B were mixed togetherto form gels in a sol–gel process. For solution A, 5.30 mL of TEOS(23.75 mmol), 2.26 mL of water (125.54 mmol, based on a 1:1 moleratio of water to one ethyoxyl group of TEOS or APTES), and 0.01 mL ofconcentrated HCl (37% w/w) were dissolved in 25.03 mL of acetone(339.30 mmol) by stirring in a plastic container (100 mL) at roomtemperature. Solution B was prepared with 2.39 mL of APTES(10.18 mmol) and 25.02 mL of acetone (339.30 mmol). Solutions Aand Bwere stirred for 1 h and 5 min, respectively. Both solutions werecooled in a refrigerator at−15 °C for 1 h to slow down gelation whencombined. The cold solutions weremixed rapidly by adding solution Binto solution A. The 60-mL output was stirred for 1 min and thenpoured into 12 cylindrical plastic molds, nominally 11 mm indiameter and 50 mm in height, to form the desired cylindricalmonoliths for testing. The gels, which formed within 5–10 min,were aged for 24 h before being removed from their molds andsubsequently placed in fresh acetone. The gels were aged at roomtemperature for 72 h in fresh acetone to remove excess water beforefurther modification. The acetone-to-gel volume ratio was approxi-mately 3:1, and the acetone was changed three times at 24-hintervals.

2.2.2. Modification of amine-modified wet gels with polymerThe aged gels were placed in a solution containing TDI in acetone.

Sufficient diffusion of TDI into the body of the gels was allowed for72 h at room temperature. Four percentages of mass concentrationsfor each TDI in acetone (0%, 2%, 10%, and 20%)were used to control theamount of polymer incorporated into the silica aerogels. Sampleswere removed from the TDI solution bath and then placed in a newcontainer with fresh acetone. The modification of silica gels broughtabout by the reaction between TDI and amine groups was initiated at60 °C and continued for 72 h to complete. The reaction scheme for thismodification is shown in Fig. 2. Consequently, the gels were cooled toroom temperature and placed in fresh acetone to remove unreactedTDI. The acetone was refreshed three times at 12-h intervals. Finally,the wet gels were immersed in a pentane bath for 36 h to exchangethe solvent with pentane, and the pentane was renewed three timesat 12-h intervals.

2.2.3. Drying of polymer-modified wet gelsThe polymer-modified wet gels were removed from pentane and

slowly dried under ambient pressure first at room temperature and

then at 60 °C. The cylindrical monoliths of polymer-modified silicaaerogels were finally obtained.

2.3. Characterization

2.3.1. Compression testsThe mechanical properties of polymer-modified silica aerogels

were assessed by uniaxial compression tests. Compression tests werecarried out on a MiniMAT 2000 using a 20 N load cell at 1 mm/min.Cylindrical specimens were compressed along their axis up until theyfractured. At least three replicate tests were conducted for each set ofsamples. Before the test, the top and bottom portions of eachspecimen were sanded using grade-400 fine-silicon carbide sandpa-per and checked using an L-square to ensure that the surfaces weresmooth and parallel. According to ASTM standards, the slendernessratio of specimens was 2:1. Load and displacement data were directlyacquired. The load–displacement curve for each test was converted toa stress–strain curve by dividing the load by the original cross-sectional area of the specimen and the displacement by the height ofthe specimen.

2.3.2. Thermogravimetric analysis (TGA)The amount of polymer incorporated into the silica aerogels was

measured by TGA. TGA was performed using a Q500 thermalgravimetric analyzer from TA Instruments under a nitrogen gasatmosphere. The mass of each sample was approximately 2 mg. TGAtests were run at a temperature ramp rate of 10 °C/min, and thenitrogen gas flow was 100 mL/min.

2.3.3. Bulk density (ρb)Bulk density (ρb) was determined by measuring the mass and

volume of a sample. The mass was measured on an analytical balanceup to four significant figures. The volume was obtained by measuringthe diameter and the length three times at different positions using avernier caliper (resolution, 0.02 mm). Three samples from each setwere tested for reproducibility to obtain the average value.

2.3.4. Linear shrinkage (τ, %)The linear shrinkage (τ, %) of the silica aerogels throughout the

process was taken as the difference between the diameters of thesilica aerogel monoliths and that of the mold divided by the diameterof the mold. Twelve samples from each set were tested forreproducibility to obtain the average value.

Fig. 2. Reaction–modification scheme for amine-modified aerogel with TDI.

3449H. Yang et al. / Journal of Non-Crystalline Solids 357 (2011) 3447–3453

3. Results

3.1. Shrinkage and density

The physical properties of the polymer-modified silica aerogelsand their respective preparation conditions are listed in Table 1. Theamount of polymer incorporated into the silica aerogels in Table 1 wasobtained through themass loss between 300 and 400 °C from the TGAcurves in Fig. 3. Blank sample 0TDI had a significantly highermass lossbelow 200 °C compared with the other samples (about 10% w/w versus 4% w/w) because of the presence of organic solvents such aspentane and adsorbed water. The amount of organic material lostfrom 0TDI at higher temperatures (N400 °C) could be due to mainly

Table 1Preparation conditions and properties of polymer-modified silica aerogels.

Sample TDIconcentration (%)

Linearshrinkage (%)

Bulk density(kg/m3)

Polymercontent (%)

0TDI 0 61.05 954 02TDI 2 46.16 666 38.7210TDI 10 37.60 544 45.0620TDI 20 15.73 392 65.46

the loss of propyl groups from APTES, which accounts for approx-imately 11% w/w of the total mass of the silica aerogels. Samples 2TDI,10TDI, and 20TDI lost 53.06%, 57.32%, and 74.33% w/w of their weight

Fig. 3. TGA curves of polymer-modified silica aerogels under nitrogen atmosphere.

Fig. 4. Typical stress–strain curves for each set of samples with different polymercontents. The inset is the stress–strain curve of 20TDI. (A–C) schematic diagramillustrating three states of the sample during compression. (A) Regions I and II. (B)Region III. (C) Final failure.

3450 H. Yang et al. / Journal of Non-Crystalline Solids 357 (2011) 3447–3453

at high temperatures (N200 °C), respectively, indicating polymercounts of 38.72%, 45.06%, and 65.46% w/w in the polymer-modifiedsilica aerogels, respectively, after correction of the data obtained fromthe organic matter from APTES.

Table 1 shows that the linear shrinkage for polymer-modifiedsilica aerogels decreases with increasing TDI concentrations inacetone or polymer in the silica aerogels. The reduction of the bulkdensity of these samples is as pronounced as the increase in TDIconcentration.

3.2. Mechanical properties

The typical stress–strain curves of polymer-modified silicaaerogels for each set of samples with different polymer contentsare shown in Fig. 4. Table 2 summarizes the mechanical propertiesof the individual samples measured during the compression tests,as well as the average values of at least three samples for each set ofsamples.

As an example, the stress–strain curve obtained from thecompression of 20TDI is presented in the inset of Fig. 4. The curvecan be divided into three distinct regions. Region I shows a lineartrend corresponding to the elastic behavior of the material. Region IIshows that the stress became nearly constant, with only a slightincrease. Region III shows that although the stress was constant, a fewsmall surface cracks began to appear on the samples, which graduallyled to small fragments as the strain increased (Fig. 4, B in inset). Finalfailure occurs at the end of the test (Fig. 4, C in inset), together with a

Table 2Mechanical properties of polymer-modified silica aerogels.

Sample E (MPa) Average E (MPa) εb (%) Average εb (%) εc (%) Ave

0TDI 50.27 52.71 5.79 5.48 5.79 6.51.67 5.88 5.9456.20 4.77 7.35

2TDI 40.89 36.16 4.82 4.98 7.36 7.35.84 5.72 9.8437.10 4.68 7.3530.82 4.69 6.85

10TDI 24.83 22.15 8.23 7.81 18.23 16.21.66 7.27 16.3419.95 7.93 15.00

20TDI 16.19 14.57 24.25 23.36 39.16 45.15.37 19.49 42.3012.15 26.33 56.33

sharp decrease in stress. The elastic modulus of sample E can becalculated from the initial slope of the stress–strain curve in Region I,as described in the inset of Fig. 4. Point “a” corresponds to the elasticlimit of the silica aerogels. Point “b” is the surface cracking point ofsamples at the end of Region II and at the start of Region III. Similarly,the point “c” corresponds to the final failure point of the samples. Thestrains and stress at point “b” and “c” are defined as εb, σb and εc, σc,respectively.

Tables 1 and 2 show that the elastic modulus and compressivestrength, σb and σc, respectively, of the polymer-modified silicaaerogels gradually decrease with increasing amounts of incorporatedpolymer, whereas the strains of εb and εc increase significantly. Inaddition, σb and σc appear identical.

4. Discussion

4.1. Shrinkage and density

The shrinkage in silica aerogels consists of two parts: the shrinkagein the aging process and the shrinkage in the drying process. Theshrinkage during the aging process, which is caused by thecondensation of Si–OH groups on the inner surface of the silica gels,was low (~2%). This should be identical for all samples because thesame preparation conditions were followed for all the wet gels beforemodification. Therefore, the difference in shrinkage for polymer-modified silica aerogels is due to the different shrinkage during thedrying process. When silica wet gels were dried under ambientpressure, they were subjected to very large capillary forces exerted bythe meniscus of the pores liquid as liquid evaporated from the wetgels [1,2]. Fig. 5A and B shows that the unmodified wet gels did notwithstand the capillary forces and shrunk significantly during dryingbecause of their filigree structure and the low mechanical propertiesof the silica skeleton. Hence, the linear shrinkage of blank sample 0TDIin Table 1 was large.

The shrinkage of wet gels could be reduced if they are modifiedwith the polymer. During the modification process for wet gels, TDImonomers in acetone diffused from the solution into the interior ofthe body of the gels, which was driven by the concentration differencebetween the interior of the body of the gels and the TDI solutionsurrounding them. As described in Fig. 5C and D, a conformal polymercoating was formed on the silica skeleton because of the reaction ofTDI monomers with amine functional groups located on the surface ofsilica particles. On the one hand, the polarity of the inner surface waslowered by polymer coating; thus, the capillary forces exerted by themeniscus of the pore liquid during ambient drying were reduced. Onthe other hand, the silica skeleton of the silica gels was reinforced bypolymer coating. Therefore, polymer-modified silica gels tended toshrink less than unmodified silica gels during ambient pressuredrying. In addition, the shrinkage of polymer-modified silica aerogels

rage εc (%) σb (MPa) Average σb (MPa) σc (MPa) Average σc (MPa)

36 1.24 1.20 1.24 1.201.19 1.191.16 1.17

85 0.61 0.67 0.61 0.670.73 0.730.65 0.650.69 0.69

52 0.48 0.47 0.48 0.480.44 0.450.50 0.50

93 0.27 0.26 0.29 0.260.25 0.250.25 0.25

Fig. 5. Schematic drawing of the origin of shrinking during drying in (A, B) unmodified silica gels and (C, D) modified silica gels under ambient pressure.

Fig. 6. Relationships between log[1−τ], log[ρb], log[Ea], and log[σa]. (A) log[ρb] versuslog[1−τ]. (B) log[Ea] and log[σa] versus log[1−τ].

3451H. Yang et al. / Journal of Non-Crystalline Solids 357 (2011) 3447–3453

decreased more when more polymers were incorporated into thesilica aerogels.

The variation of the density of polymer-modified silica aerogelsdepends on the shrinkage and the amount of incorporated polymers.In our experiments, the effect of polymer amount on density increasewas offset by the reduction of the shrinkage, thus monotonicallydecreasing the density.

For normal silica aerogels, the correlation between the bulkdensity (ρb) of cylindrical silica aerogels and the linear shrinkage (τ,%)during preparation follows the equation

ρb =4m= πϕ 2l1−τð Þ3 ; ð1Þ

wherem,Φ and l are the mass of silica skeleton, the diameter, and thelength of a wet silica gel, respectively.

Eq. (1) shows a linear relationship between log[ρb] and log[1−τ],and the slope of the line is −3. Although Eq. (1) is only valid fornormal silica aerogels, we plotted log[ρb] versus log[1−τ] forpolymer-modified silica aerogels, as shown in Fig. 6A to determinethe relationship between bulk density and linear shrinkage forpolymer-modified aerogels. A linear relationship with a slope of−1.16 was clearly observed, obviously larger than the theoreticalslope value of −3 in Eq. (1). The increased slope of the polymer-modified aerogels is believed to be contributed to the polymersincorporated into the silica aerogels. The polymers in the silicaaerogels effectively reduced shrinkage during drying and contributedto the increase in bulk density. In general, the bulk density continuedto decrease with increasing polymer content in the silica aerogels.

4.2. Mechanical properties

4.2.1. Elastic modulusThe decrease in elastic modulus with increasing amounts of

incorporated polymer is caused by the decrease in density orshrinkage of the silica aerogels brought about by polymermodification.

Similar to the calculation of density as a function of shrinkage inEq. (1), the elastic modulus of normal silica aerogels as a function ofshrinkage can also be calculated. If we define the elastic modulus ofthe silica skeleton of a silica aerogel as Es and the porosity as α, thenthe apparent elastic modulus of the silica aerogel Ea is

Ea = Es 1−αð Þ: ð2Þ

The porosity α of normal silica aerogels depends on the shrinkageduring preparation, as described by Eq. (3):

1−αð Þ = 1−τð Þ−2 1−α 0ð Þ; ð3Þ

where 1−α0 is the fraction of the silica skeleton at the radial sectionof the silica gel.

Fig. 7. Structural deformation of (A–C) normal silica aerogels and (D–F) polymer-modified silica aerogels under compression. (A, D) At the beginning of the compression test. (B, E)Deformation of the silica skeleton, which corresponds to Region I in the stress–strain curve. (C, F) Crack initiation and propagation until final failure, which corresponds to Regions IIand III in the stress–strain curve.

3452 H. Yang et al. / Journal of Non-Crystalline Solids 357 (2011) 3447–3453

One can easily deduce that the Ea and the linear shrinkage complywith Eq. (4), specifically, log[Ea] is proportional to log[1−τ] with aslope of −2:

Ea = Es 1−α 0ð Þ 1−τð Þ−2: ð4Þ

For a normal silica aerogel, the apparent modulus is related to onlythe shrinkage because the elastic modulus of silica skeleton Es isbelieved to be constant regardless of the shrinkage. For polymer-modified silica aerogels, Es may change because of the polymercoating on the silica skeleton. For polymer-modified silica aerogels,the log[Ea] as a function of log[1−τ], plotted as shown in Fig. 6B,appears to be a perfect linear curve, although the slope is −1.72,which is close but not equal to the theoretical value of−2 in Eq. (4). Ifwe assume that the function of the polymer incorporated into thesilica aerogels is to reduce only the shrinkage during preparation, thenthe slope should be around −2 when we plot log[Ea] as a function oflog[1−τ]. The increased slope suggests that the function of thepolymer in aerogels was not only to reduce the shrinkage duringpreparation but also provide stiffening effects on the silica skeleton insilica aerogels.

4.2.2. Compressive strengthThe decrease in the compressive strength of polymer-modified

silica aerogels is caused by the decrease in shrinkage and density.Analogous to the calculation of elastic modulus as a function of

shrinkage for normal silica aerogels, the compressive strength as afunction of shrinkage can also be calculated. If the compressivestrength of the silica skeleton of a normal silica aerogel is σs, theapparent compressive strength σa of the normal silica aerogel is

σa = σs 1−αð Þ: ð5Þ

According to Eqs. (3) and (5), the relationship between σa and τ is

σa = σs 1−α 0ð Þ 1−τð Þ−2: ð6Þ

Therefore, the plot of log[σa] versus log[1−τ] should be a straightlinewith a slope of−2. For polymer-modified silica aerogels, the log[σa]as a function of log[1−τ], plotted as shown in Fig. 6B, appears to be aperfect linear curve with a slope of −2, which is exactly equal to thetheoretical value in Eq. (6). This demonstrates that the apparentcompressive strength of polymer-modified silica aerogels is mainlyinfluenced by shrinkage. The incorporated polymer in the modifiedsilica aerogels does not contribute to the strength.

4.2.3. StrainsIn previous studies [7,28–30], the fragility of normal silica aerogels

subjected to SCF drying can be traced through interparticle necks.Because the silica skeleton is an aggregate of spherical particles, itsmechanical properties are limited by the narrow necks of silica thatinterconnect the neighboring secondary silica particles. Fig. 7A–Cshows that during the compression test, the bending of the silicaskeleton at the axial section took place, and deformation increasedgradually with increasing strain. The failure process sequentiallyincludes skeleton deformation, bond-breaking between secondaryparticles, crack propagation, and final failure.

Blank samples of 0TDI unmodified with polymer showed inherentstress and strain in the skeleton framework before the compressiontest because the deformation of silica skeleton took place during thepreparation. Hence, they were brittle and fragmented at low strain oncompression, which resulted in small breaking strains. One should benoted that the failure breaking of the silica aerogels is mainly due tothe disconnection of secondary particles. Therefore, the strength andthe deformation ability of neck region is the dominant factor in themechanical behavior of silica aerogels.

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Once silica wet gels are modified with polymer, their shrinkageduring drying can be reduced as previously discussed. Therefore, theinherent stress and strain were smaller in the skeleton framework.More importantly, as shown in Fig. 7D–F, the polymer coating on thesilica skeleton reinforced the neck region between neighboringsecondary particles, and the macropores did not link into macroscopiccracks. In summary, the polymer-modified silica aerogels were lessbrittle andmore ductile than the blank sample 0TDI. Thus, they can bedeformed at high strains.

5. Conclusions

(1) Polymer-modified silica aerogels with significantly improvedmechanical properties can be successfully prepared throughambient pressure drying. In this approach, the process of SCFdrying can be eliminated.

(2) The successful preparation of polymer-modified silica aerogelsdepends on the control of shrinkage during drying. Whenincorporating polymers into the structure of silica aerogels, theshrinkage decreases with increasing amounts of incorporatedpolymers.

(3) With increasing amounts of incorporated polymers, theapparent elastic modulus and compressive strength of poly-mer-modified silica aerogels decrease because of the decreas-ing shrinkage and density. In contrast, the strains at the surfacecrack point and the final failure point significantly increaseduring compression.

(4) The elastic modulus of the silica aerogel skeleton increasesbecause of the incorporated polymers. The polymers have noeffects on the compressive strength of silica skeleton.

(5) The variations in the apparent elastic modulus and compressivestrength of polymer-modified silica aerogels are closely relatedto the shrinkage during preparation, and this relationship canbe quantitatively expressed. log[Ea] and log[σa] are linearlyproportional to log[1−τ] with slopes of −1.72 and −2,respectively.

Acknowledgment

The authors acknowledge Daikin Industries, Ltd for financialsupport.

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