Microporous and Mesoporous Materials 148 (2012) 152–158

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Microporous and Mesoporous Materials

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An improved method for preparing monolithic aerogels based onmethyltrimethoxysilane at ambient pressure Part II: Microstructureand performance of the aerogels

Bi Xu a,c, Jackie Y. Cai a,⇑, Zongli Xie a, Lijing Wang b, Iko Burgar a, Niall Finn a, Zaisheng Cai c, Lisa Wong a

a Division of Materials Science and Engineering, Commonwealth Scientific and Industrial Research Organisation (CSIRO), P.O. Box 21, Belmont, Victoria 3216, Australiab School of Fashion and Textiles, RMIT University, 25 Dawson Street, Brunswick, Victoria 3056, Australiac College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China

a r t i c l e i n f o

Article history:Available online 16 August 2011

Keywords:AerogelMethyltrimethoxysilaneMicrostructureAmbient pressure

1387-1811/$ - see front matter Crown Copyright � 2doi:10.1016/j.micromeso.2011.08.015

⇑ Corresponding author. Tel.: +61 3 52464000; fax:E-mail address: Jackie.cai@csiro.au (J.Y. Cai).

a b s t r a c t

The microstructures, physical properties and mechanical properties of the methyltrimethoxysilane(MTMS) derived aerogels, prepared at ambient pressure using an improved method reported in Part I,have been comprehensively characterised in this paper, as Part II. The study has revealed sophisticatedpore structures and morphologies of the aerogels produced, and demonstrated the relationships betweenthe key processing conditions and microstructures of the resultant aerogels. The results of the study alsohelp to further understand the effect of microstructure changes on the macrostructure, volume shrinkageand performances of the aerogels, and provide a general guide on how to manipulate the processing con-ditions to produce monolithic MTMS based aerogels with desirable structure and properties by ambientpressure drying.

Crown Copyright � 2011 Published by Elsevier Inc. All rights reserved.

1. Introduction

The key challenge in the aerogel production is to remove thesolvent from the wet gel, while maintaining the gel volume andnetwork structure. At present, the MTMS based aerogels are pre-dominantly prepared by a supercritical drying process. In our previ-ous report (Part I) [1], we introduced a more practical method forfabricating MTMS based aerogels at ambient pressure. The methoddeveloped is able to produce monolithic aerogels with low bulk den-sities over a range of the precursor concentrations at ambient pres-sure without the need for any solvent exchange and surfacemodification. This has been achieved through specifically tailoredgelation and ageing conditions according to the MeOH:MTMS molarratios used. The effect of the key processing parameters on alcogelformation and macrostructures of the aerogels has been systemati-cally investigated for achieving minimum volume shrinkage andbulk density.

This paper, as Part II, will focus on the characterisation of theaerogels prepared (in particular, the microstructures, physicalproperties and mechanical properties) and reveal the relationshipsbetween the processing conditions and microstructures. Theresults of the study will also help to further understand the effectof microstructure changes on the macrostructure, volume shrink-age and performances of the aerogels, and provide a general guide

011 Published by Elsevier Inc. All r

+61 3 52464057.

on how to manipulate the processing conditions to produce mono-lithic MTMS based aerogels with desirable structure and proper-ties. The characterisation of the aerogels produced at variousprocessing conditions were carried out by SEM, BET, solid stateNMR, the contact angle test and compression test.

2. Experimental

2.1. Preparation of MTMS derived aerogels

All chemicals used were purchased from Aldrich and used as re-ceived. The MTMS derived aerogels were prepared via a two-step,acid–base catalyzed sol–gel process, as described in Part I [1]. All ofthe aerogels were prepared with a MeOH:MTMS molar ratio of30:1 at various gelation temperature (Tgel) and ageing temperature(Tage) ranging from 30 �C to 60 �C.

2.2. Characterisation of the aerogels prepared

The adsorption–desorption isotherms of the aerogels for nitro-gen at 77 K were obtained using a Micromeritics TriStar II analyzer.The samples were degassed under a high vacuum (10�6 Torr) at110 �C for overnight prior to the analysis. The specific surface areawas determined by the BET (Brunauer–Emmett–Teller) methodfrom N2 gas adsorption isotherms. Micropore volume was mea-sured using the t-plot method. The pore size distribution wasdetermined using the BJH (Barrett–Joyner–Halenda) method on

ights reserved.

Fig. 1. Nitrogen adsorption (j)–desorption ( ) isotherms taken at 77 K for selected aerogels prepared at different gelation and ageing temperatures.

B. Xu et al. / Microporous and Mesoporous Materials 148 (2012) 152–158 153

desorption data. The average pore size was calculated from the BETsurface area. The microstructure and morphology of the aerogelswere also examined by a scanning electron microscopy (SEM, FEIPhenom, Hitachi S4300 SE/N), and the samples for SEM examina-tion were coated with a thin layer of Cr to improve their conductiv-ity. The chemical structure of the aerogel was analyzed using solidstate 13C NMR 29Si NMR spectroscopy. The hydrophobicity of theaerogel was evaluated by the contact angle of a water droplet onthe aerogel surface using a KS CAM 101 contact angle and surfacetension tester. The compression test was carried out on Instron5567 with a compression speed of 20 mm/min to assess themechanical property of the aerogels produced.

3. Results and discussion

As discussed in Part I [1], the macrostructure and volume shrink-age of the MTMS derived aerogels were highly dependent on theaerogel processing conditions used. A higher gelation temperature(Tgel) resulted in a higher volume retention and lower density ofthe aerogels, but the aerogel structure became weaker with a coarsersurface. Ageing generally improved aerogels volume retention, andthe effect was more profound for the gels formed at a lower Tgel.The aerogel produced at Tgel 40 �C and Tage 60 �C showed the bestmacrostructure and volume retention. These observations are cer-tainly determined by their microstructures formed. It is known thatthe degree of volume shrinkage is determined by capillary forcesgenerated during drying; and capillary forces are affected by poresize and pore size distribution. To further understand these phe-nomena reported in Part I, a comprehensive microstructure charac-terisation was carried out in this investigation.

3.1. The nitrogen adsorption–desorption isotherms of the aerogelsprepared at different conditions

The N2 adsorption–desorption isotherms of selected aerogelsamples prepared at different gelation and ageing temperaturesare shown in Fig. 1, and their pore size distributions are displayedin Fig. 2. The total pore volumes, specific surface areas and averagepore size derived from the corresponding isotherms are listed inTable 1.

It can be seen from Figs. 1 and 2 that these aerogels generallyshow combined features of type-I, type-II and type-IV N2 adsorp-tion–desorption isotherms according to IUPAC classification [2,3],indicating a hierarchical pore structure of the aerogels. It is alsoapparent that gelation temperature (Tgel) has a great impact onthe pore structure. For instance, at the ageing temperature (Tage)of 30 �C, when the Tgel was increased from 30 �C to 60 �C, micro-pore surface area increased from 5.26 m2/g to 80.2 m2/g and theaverage pore size reduced from 55.9 Å to 31.4 Å (Table 1), indicat-ing the increase of micropore and mesopore formed in the struc-ture. On the other hand, the total pore volume of pores atp/p� = 0.99 decreased from 0.568 cm3/g at Tgel 30 �C to 0.294 cm3/g at Tgel 60 �C (Table 1). Furthermore, at a relatively low gelationtemperature (Tgel 30 �C), the pore size distribution is very broadranging from micropores to macropores with average pore size of55.9 Å (at Tage 30 �C) and 53.4 Å (at Tage 60 �C), respectively. Incomparison, at a high gelation temperature (Tgel 60 �C), the poresize distribution is a much narrower with predominant mesoporescentered at about 31.4 Å (at Tage 30 �C) and 33.8 Å (at Tage 60 �C),respectively (Fig. 2 and Table 1).

Change in Tage also alters the surface area and pore structures,but the effect of Tage is less significant, compared to Tgel. The BET

Fig. 2. Pore size distributions of selected silica aerogels prepared at different gelation and ageing temperatures.

Table 1BET surface area, pore volume and pore size of selected MTMS derived aerogelsprepared at different gelation and ageing temperatures.

Samples Tgel 30 �C Tgel 60 �C

Tage

30 �CTage

60 �CTage

30 �CTage

60 �C

BET surface area (m2/g) 406.4 470.4 374.3 402.0t-Plot micropore area (m2/g) 5.26 6.20 80.2 65.1t-Plot micropore volume (cm3/g) 0 0.006 0.036 0.037Total pore volume at p/p0 = 0.99

(cm3/g)0.568 0.628 0.294 0.320

Average pore size (Å) 55.9 53.4 31.4 33.8

154 B. Xu et al. / Microporous and Mesoporous Materials 148 (2012) 152–158

surface area generally increases with increasing Tage. However, theeffect of Tage on pore structure is quite different at different gela-tion temperatures. At a low gelation temperature (Tgel 30 �C),increasing ageing temperature tends to favour the retention ofmicropores. The t-plot micropore area increases from 5.26 m2/g(at Tage 30 �C) to 6.20 m2/g (at Tage 60 �C) and the average pore sizereduces from 55.9 Å to 53.4 Å. This is presumably attributed to thestrengthened network structure by a higher temperature ageing,which enables small pores to withstand and survive the relativelyhigh capillary force generated during drying at ambient pressure[4,5]. On the other hand, at a high gelation temperature(Tgel 60 �C), increasing ageing temperature tends to have a negativeimpact on micropore retention. The t-plot micropore areadecreases from 80.2 m2/g (at Tage 30 �C) to 65.1 m2/g (at Tage

60 �C). Consequently, the average pore size increases (Table 1).It is more evident from Table 1 that the surface area and pore

volume of the aerogel reduce as the Tgel increases, but increase asthe Tage increases. However, the ageing-induced increase in thesurface area and pore volume is more significant at a lower Tgel.

It must be noted that the aerogels prepared in this investigationcontain complex hierarchy pore structures, and the analysis usingN2 adsorption porosimetry only partially reveals the pore structurewith the pore sizes within 100 nm. Pore sizes higher than 100 nmcannot be accurately reflected by this method. In order to revealmore comprehensively the morphologies and microstructures,the aerogels were also examined using SEM.

3.2. Morphological characteristics of the aerogels prepared

Figs. 3 and 4 are SEM images of the aerogels prepared at variousprocessing conditions. All samples examined exhibit a globular-aggregated structure, in which silica aggregates interconnect with

each other to form a porous network. The images in Fig. 3(obtained with a FEI Phenom at a low magnification) clearly dem-onstrate the effect of Tgel and Tage on the formation of the aerogelnetwork structure at a micrometer level. It is obvious that the aero-gel produced at the lowest Tgel and Tage examined (both at 30 �C)showed the densest network structure. Increasing the Tgel from30 �C to 60 �C under the same Tage led to the formation of largervoids (a few microns to over 30 microns). A closer examinationof these aerogel structures at a much higher magnification(Fig. 4, obtained using SEM Hitachi S4300 SE/N) reveals that theaerogels’ finer structures at high Tgel of 50 �C and 60 �C are actuallydenser than those gelled at 30 �C and 40 �C. This is sharply con-trasting to that observed at a lower magnification, as shown inFig. 3, but consistent with the BET results shown in Table 1 andFig. 2, which reflects the pore structure at the submicron levels.These observations suggest that gelation at a low temperature re-sults in a slow aggregation and formation of a relatively small andbulky aggregates or clusters, and small voids between the clusters.

Fig. 3. SEM images of the MTMS derived aerogels prepared at different gelation and ageing temperatures.

B. Xu et al. / Microporous and Mesoporous Materials 148 (2012) 152–158 155

On the other hand, gelation at a high temperature tends to cause afast aggregation and to form relatively large and dense clusters, butalso large voids between the clusters. Moreover, the pore size dis-tribution within the clusters is more uniform at a high Tgel thanthat obtained at a low Tgel.

The effect of Tage on the gel structure of the MTMS based gels isalso demonstrated in Fig. 3. Increasing the Tage significantly in-creases the porosity of the aerogels prepared at Tgel 30 �C, but ata higher Tgel, the effect of ageing on the pore structure of this levelis less profound. However, ageing does impart significant changesat submicron levels at all ageing temperatures examined, asreflected by the BET results.

3.3. Effect of the microstructure on the macrostructure and volumeshrinkage of the aerogels

These microstructure details have provided better understand-ing of the macrostructures and volume shrinkages of the aerogelsobserved in Part I.

At a low Tgel (30 �C), a relatively dense structure and muchbroader pore size distribution formed would generate high capil-lary force and unbalanced force gradient throughout the gel bodyduring ambient pressure drying, leading to a high degree of volumeshrinkage and/or cracks [6]. Ageing generally promoted the

condensation reactions and strengthened the alcogel networkstructure [3,4]. The stronger wet gel obtained by high temperatureageing would allow the gel to be more resistant to the capillarypressure generated during the ambient pressure drying, resultingin an increased pore volume and specific area of the aerogel, whichwere reflected by a lower volume shrinkage and bulk densityobserved.

At a high Tgel (e.g. 50 �C and 60 �C), large clusters and large voidsformed contributed to the coarse surface of the gels, and the pres-ence of a sufficient amount of the large voids between the clustersand relatively narrow pore size distributions within the clusterswould substantially alleviate and balance the capillary force inthe gel structure during the ambient pressure drying. Therefore,crack-free and low volume shrinkage aerogels were readilyobtained at a high Tgel (50 �C or 60 �C) independent of ageing tem-perature. However, too large voids in the aerogels will decrease theoverall connectivity of the clusters at the same time, resulting in aweakened structure and reduced mechanical properties of theaerogel. Therefore, the aerogel produced at a Tgel lower than50 �C and aged at a high temperature would provide an optimalstructure and volume retention.

It is noted that the amount of large voids (in micrometer level)present in the aerogel play predominant role in alleviating capil-lary forces during ambient pressure drying and correlate well to

Fig. 4. FESEM images of the MTMS derived aerogels prepared under the same Tage (30 �C), but at different Tgel: (a) Tgel 30 �C; (b) Tgel 40 �C; (c) Tgel 50 �C; (d) Tgel 60 �C.

Fig. 5. 13C (a) and 29Si (b) CP/MAS NMR spectra of a MTMS based aerogel prepared at Tgel 40 �C and Tage 60 �C.

156 B. Xu et al. / Microporous and Mesoporous Materials 148 (2012) 152–158

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the volume retention of the gels dried at ambient pressure, as re-ported in Part I.

3.4. Chemical structure of the MTMS based aerogel

Solid state NMR is used to investigate the chemical structure ofthe MTMS based aerogel prepared at Tgel 40 �C and Tage 60 �C. Fig. 5(a) and (b) display the 13C and 29Si CP/MAS NMR spectra of theaerogel respectively. In the 13C spectrum (Fig. 5(a)), an outstandingpeak observed at �3 ppm represents an abundant amount of SiCH3

groups in the aerogel, which determine the hydrophobicity of theaerogel [7]. The extremely weak peak at 50 ppm indicates thatthe content of unreacted methoxyl group (SiOCH3) is extremelylow. 29Si CP/MAS NMR spectrum (Fig. 5(b)) shows a predominantresonance at �112 ppm, which is ascribed to T3 (CH3SiO3/2), repre-senting the specific siloxane backbone structure of the aerogel. It isalso noted that few residual silanol groups are present.

Fig. 6. Water contact angle measuring process of the MTMS based aerogel (prepar

3.5. Superhydrophobicity of the MTMS based aerogel

Static contact angle measurement was used to assess the wetta-bility of the aerogels prepared at Tgel 40 �C and Tage 60 �C (Fig. 6). Itwas interesting to observe that when a 4 lL water droplet wasused, the water droplet, after being triggered to release and reach-ing the aerogel surface, adhered back to the dripper and could notbe released/placed onto the aerogel surface. A similar phenomenonwas observed when the volume of the water droplet increased to6 lL. Further increasing the water droplet to 8 lL finally lead tosuccessful release of the water droplet from the syringe, but thewater droplet often rolled off from the surface of the aerogelimmediately, indicating the superhydrophobicity of the MTMS de-rived aerogels. The contact angle measured was over 155�. It isknown that geometrical structure and chemical composition arethe two key factors determining the wettability of a solid surface.It is believed that the high concentration of methyl groups

ed at Tgel 40 �C and Tage 60 �C) by increasing the volume of the water droplet.

Str

ess

(KP

a)

Strain

Fig. 7. A multi-cycle compression test of the MTMS based aerogel prepared at Tgel

40 �C and Tage 60 �C.

158 B. Xu et al. / Microporous and Mesoporous Materials 148 (2012) 152–158

attached to network skeleton and the extremely porous structureof the areogel render the superhydrophobic property of the MTMSderived aerogel.

This is sharply contrasting to the conventional silica aerogelsderived from tetraalkoxysilane or water glass which are hydro-philic in nature due to the abundant silanol groups present onthe surface of the aerogels [5].

3.6. Compression testing of the MTMS based aerogel

To assess the mechanical property of the aerogel, the Uniaxialcompression test was performed on the aerogel prepared at Tgel

40 �C and Tage 60 �C. The aerogel sample was repeatedly com-pressed and released for four consecutive cycles on an Instron,and the compressive stress–strain curves of the sample are shownin Fig. 7. It can be seen that in the first compression cycle, the aero-gel was compressed to 60% of its original length and returned backto about 95% of its original length immediately upon the release ofthe compression force. During the subsequent 3 compressioncycles, only very minor reduction in linear size occurred, demon-strating the high compressibility and excellent elasticity of theMTMS derived aerogel.

4. Conclusions

The microstructures of the aerogels prepared in this investiga-tion exhibit complex hierarchical pore structures, and the porestructures formed are highly dependent on the gelation and ageing

conditions used. Increasing the gelation temperature results in lar-ger aggregates (or clusters), and more voids (at a micrometer level)between the clusters, but, within the clusters, a higher gelationtemperature produces a smaller average pore size (submicronlevel) and more uniform pore size distribution. Furthermore, thepore volume and the specific surface area of the aerogel reduceas gelation temperature increases, but increase as ageing tempera-ture increases, and this ageing-induced effect is more evident at alow gelation temperature.

These results have provided good explanations on the macro-structure changes observed in Part I. Most of the aerogels preparedat a low gelation temperature (Tgel 30 �C) shrink considerably andreadily crack because of the huge capillary force gradient, resultingfrom the dense structure and the broad pore size distribution. Theaerogels produced at a higher gelation temperature shrink less andare more readily made crack-free due to the presence of morevoids and more uniformly distributed pore sizes within the aggre-gates, resulting in a more balanced and largely reduced capillaryforce. However, too large and too many voids at a high gelationtemperature also lead to decreased connectivity of the aggregates,resulting in fragility and weakness of the aerogels. Ageing at ahigher temperature provides more profound reinforcement in thenetwork structure of the gel. Therefore, the combination of a prop-er gelation temperature and a high ageing temperature would pro-duce a good gel structure with low volume shrinkage. It has alsobeen found in this investigation that the amount of large voidspresent in the aerogel play a predominant role in alleviating capil-lary forces during ambient pressure drying and correlate well tothe volume retention of the gels, as reported in Part I.

The results of this study have also demonstrated that the aero-gel produced is superhydrophobic and highly elastic, attributed toits polymethylsiloxane structure.

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