Clay reinforced polyimide/silica hybrid aerogel

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www.rsc.org/materialsA

0959-9428(2010)20:1;1-A

ISSN 2050-7488

Materials for energy and sustainability

Journal ofMaterials Chemistry Awww.rsc.org/MaterialsA Volume 1 | Number 1 | January 2013 | Pages 0000–0000

Journal ofMaterials Chemistry A

View Article OnlineView Journal

http://dx.doi.org/10.1039/c3ta00439b

http://pubs.rsc.org/en/journals/journal/TA

CLAY REINFORCED POLYIMIDE/SILICA HYBRID AEROGEL

Jiao Guo,1 Baochau N. Nguyen,

2 Lichun Li,

1 Mary Ann B. Meador,

3 Daniel A. Scheiman

2 and Miko

Cakmak1

1 The University of Akron, Department of Polymer Engineering, Akron, Ohio 44325, USA.

2 Ohio Aerospace Institute, 22800 Cedar Point Road, Cleveland, Ohio 44111, USA

3 NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, Ohio 44135, USA.

KEYWORDS: aerogel, clay, silica, nanocomposite.

ABSTRACT: Silica aerogels are comprised of highly porous three-dimensional networks. They typically are very fragile and brit-

tle due to the inter-particle connections in the pearl-necklace-like fractal network. This behavior prevents their wider utility. The

present study aims to reinforce the silica-based gel to improve the poor mechanical strength through crosslinking the silica particles

with polyimide and incorporating Lucentite STN clay into the skeletal silica-polyimide network. 3-Aminopropyltriethoxysilane

(APTES) end-capped polyamic acid oligomers were first formed followed by gelation with TMOS at a range of clay concentrations

to generate a silica network. The incorporation of clay leads to slightly lower BET surface area with little effect on shrinkage, po-

rosity and density. Microscopy revealed that the aerogel preferentially grows from the edges of well dispersed clay particles while

minimal growth occurs from clay surfaces. The formation of covalent bonds and hydrogen bonding through the OH functionalized

clay edges is thought to enhance the connectivity with silica network and clay, leading to a substantial reinforcement effect as evi-

denced by an increase in modulus.

INTRODUCTION

Aerogels have continuous porosity and a microstructure

composed of interconnected colloidal like particles or

polymeric chains with characteristic diameters of 10 nm.

These microstructures impart the high surface areas, low-bulk

densities, and relatively large pore volume to aerogels. The

combination of these properties in an amorphous structure

gives the lowest thermal conductivity values for any coherent

solid material. Aerogels of various compositions include

inorganic aerogels (silica, transition metal oxide),1 organic

aerogels (polyisocyanates,2

polyacrylnitrile

3 and polyimide

4,5)

and inorganic/organic aerogel composites.6 Silica aerogels are

synthesized from silicon alkoxide by a sol-gel process7 and

dried under supercritical fluid (SCF) extraction/exchange

conditions.8, 9

The supercritical drying process involves solvent

exchange of gelation solvents filling the meso pores with

liquid CO2 in an autoclave with subsequent supercritical CO2

venting. By keeping the solvent phase above the critical

pressure and temperature during the entire solvent extraction

process, strong capillary forces generated by liquid

evaporation from very small pores are eliminated.

Consequently, without surface tension forces exerted on the

skeletal framework, SCF drying leaves the geometric

dimensions of the wet gel unaffected, resulting in low density

materials with high porosity. In a silica aerogel produced

through a typical base-catalyzed silicon alkoxide sol-gel

process, nano-sized primary particles of silica 2-50 nm in

diameter are agglomerated into spherical secondary particles

50 nm-2 µm (2000 nm) in diameter, which are connected

together in strands. These interconnected strings of

nanoparticles are randomly dispersed spatially forming a

nanoparticle skeletal structure, in which there is as much as

99% empty mesoporous space or void space within the

nanoparticle network.

However, the silica aerogels produced from the sol-gel

process are inherently fragile which hinders wider applications

of these materials in aerospace, industrial and commercial use.

The fragility of the aerogel framework is traced to the fragile

inter-particle connecting zones, referred to as necks, which are

formed by the coagulation and reprecipitation of silica

particles during gelation and aging.10 To improve the

mechanical properties of monolithic silica aerogels, previous

studies have been focused on generating a polymer coating on

the surface of the silica particles.11,12,13

The essential idea is to

create a wider interparticle neck to improve the specific

stiffness and strength without significantly increasing the

weight. In this perspective, the silica gelation first takes place,

followed by crosslinking with polymers on the surface of

silica particles. The type of polymer can be controlled by the

functional groups on the surface of silica particles. For

example, the silica particle with surface terminated amine and

hydroxy groups can react with diisocyanate to form

polyurea,14

and polyurethane,15,16

respectively. These polymers

then crosslink the silica particles at the surface. Polyimides are

high-performance polymers, which have remarkable

temperature resistance, excellent mechanical properties and

chemical stability, high radiation resistance and low dielectric

constant.17,18

The combination of silica aerogel with high

performance polyimide fabricated into a hybrid material

provides structural integrity to the aerogels.

Layered silicate clays are potentially well-suited for the

design of hybrid composites due to their exceptionally stable

oxide network, plate-like morphology, and high in-plane

strength, high surface area and aspect ratio of their lamellar

elements.19

Smectite clays as layered silicates are considered

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to be good candidates for the preparation of organic-inorganic

nano-composites. This is because they can be broken down

into nano-scale building blocks after exfoliation of the nano-

phase silicate sheets, upon being mixed into the host polymer.

The rich intercalation chemistry associated with smectite clays

can be used to facilitate the dispersion of the nano-layers into

a polymer matrix. Hectorite (HT) is a 2:1 type phyllosilicate

smectite clay, where the two tetrahedral silicate layers

sandwich a central magnesium oxide octahedral layer. Its

structure is shown in Figure 1.20

Figure 1. Schematic structure of 2:1 phyllosilicates smectite clay

(Hectorite).

In this work, two approaches were implemented to improve

the fragility of the aerogels: First, polyimide should function

as a crosslinker bridging the silica particles and particle

strands to improve the structural integrity and mechanical

properties. The schematic structure of the polyimide

crosslinked silica hybrid aerogel is illustrated in Figure 2. The

structure is formed by introducing alkoxysilane end group

(APTES) to polyamic acid oligomers, followed by co-gelation

of silanes (APTES and TMOS) in presence of ammonium

hydroxide and water. A silimiliar approach was recently

reported by Dong et al. 21 In this approach, polymer oligomers

terminated with silica functional groups are formed, followed

by co-gelation of the oligomer with TMOS to form silica gels.

This method creates a silica network with polymer crosslinker

incorporated into the skeleton of the hybrid gels, instead of on

the surface of silica skeleton. Second, Lucentite STN clay is

used to reinforce the silica/polyimide hybrid aerogel matrix.

Lucentite is a fully synthetic Hectorite from magnesium

silicates and alkali salts produced hydrothermally by COOP

Chemical in Japan. As shown in Figure 2 (top), the silanol

groups in the sol react with the hydroxyl groups at the edges

of the clay layers resulting in covalent and hydrogen bonding

between the two materials. The layered silicates will function

as a skeleton to reinforce and support the inherent fragile silica

network. In addition, the exfoliated silicate layers are expected

to adjust the pore sizes of the gel network, and to add strength

to the gel network.

EXPERIMENTAL

Materials. Tetramethoxysilane (TMOS), 3-

aminopropyltriethoxysilane (APTES), ammonium hydroxide

solution (28.0-30.0% NH3 basis), 1-methyl-2-pyrrolidinone

(NMP, HPLC), acetic anhydride and pyridine were purchased

from Sigma-Aldrich and used as received. (3, 3’, 4, 4’-

Benzophenonetetracarboxylic dianhydride) (BTDA), and 2,2-

bis[4-(4-aminophenoxy)phenyl]propane (BAPP) were

obtained from CHRISKEV COMPANY, INC. and used

without further purification. The Hectorite clay was provided

by Coop Chemical Co. of Japan. It consists of two tetrahedral

silicate layers that sandwich a central magnesium oxide

octahedral layer.

Figure 2. Schematic structure of polyimide crosslinked silica par-

ticles.

Sample Preparation. The reaction chemistry is illustrated in

Scheme 1. BTDA (1.0 g, 3.0 mmol) was slowly added to the

solution of BAPP (0.63 g, 1.5 mmol) in 10 ml NMP. The

mixture was stirred for 30 min at room temperature to form

polyamic acid in NMP. Then APTES (0.72 ml, 3.0 mmol) was

added and stirred for 60 min, yielding APTES end-capped

polyamic acid. The molar ratio of APTES: BTDA: BAPP =

2:2:1. TMOS (93.7 ml, 25 mmol) was then added and stirred

for 15 min to form Solution A. Solution B was prepared by

adding the desired amount of clay (based on the wt % of the

total weight of reactants in solution A excluding NMP) into

the mixture of NMP (7 ml), H2O (0.5 ml) and aqueous

ammonia (28% in H2O, 28%-30% NH3 basis)(2 ml). Solution

B was sonicated for 30 min to achieve full exfoliation and

dispersion of clay. After mixing solution B with solution A,

the mixture was cast in glass vials with 20 mm diameter. The

silica-polyamic acid-hybrid-clay gel was formed within 2

minutes. The glass vials were covered with Teflon liner along

the wall and bottom for ease of removal of aerogel following

gelation. After aging for 24 hours, the gel was removed from

the glass vial, and placed in the solution of acetic anhydride

(0.6 ml) and pyridine (1.0 ml) (1:2 molar ratio) in 100 ml

NMP at 115 °C overnight. The obtained hybrid gel was

exchanged with acetone before supercritical drying as

followed: 75/25 vol% NMP/acetone, 25/75 vol%

NMP/acetone, and 4 time of 100 vol% acetone. The gels were

soaked for one day between washes. APTES-capped

polyimide oligomer was also synthesized using acetic

anhydride and pyridine to chemically imidize the polyamic

acid. The resulting oligomer was precipitated in water, washed

with acetone to remove NMP and dried in an oven at 85°C for

2 hours.

Characterization

The bulk density, ρb, was calculated by dividing the weight

of the sample by the volume. The shrinkage was determined

by the difference in diameter between the gel sample and the

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Scheme 1. Synthesis of Clay Reinforced Polyimide/Silica

Hybrid Aerogel

glass vial mold. The skeletal density, ρs, was obtained from

Micromeritics Accupyc 1340 Helium pycnometry. The %

porosity was calculated based on the bulk and skeletal

densities as shown in equation. 1

Porosity = (1-ρb/ρs) × 100% (1)

Nitrogen sorption measurements were analyzed by

Brunauer-Emmett-Teller (BET) method, performed on a

Micromeritics ASAP2020 chemisorption system with relative

pressure range (P/P0) from 0.14 – 0.995. All samples were

degased at 80 °C for 8 hr under vacuum prior to analysis.

X-ray WAXD data were obtained using a Bruker X-ray

generator equipped with a Cu target tube and two dimensional

detectors. The generator was operated at 40 KV and 40 mA

and beam was monochromatized to Cu Kα1 radiation with

Scheme 2. Process for Fabricating Clay Reinforced Polyi-

mide/Silica Aerogel

tube side mounted monochromator. The samples were scanned

for 120 s with a 2 theta step incremental of 0.02 degree.

Solid 29

Si and solid 13

C NMR spectra were performed on a

Bruker Avance 300 spectrometer with 4-mm solids probe

using cross polarization and magic angle spinning at 11 kHz.

Approximately 16000 scans were collected per sample.

Solid 29

Si uses 3-trimethoxysilylpropionic acid as an external

reference at 0 ppm. Solid 13

C NMR Spectra were externally

referenced to the carbonyl of glycine (176.1 ppm, relative to

tetramethylsilane (TMS)).

FTIR spectra were obtained on a Nicolet 380, using ATR

Crystal (Attenuated Total Reflection). 64 scans were collected

and dry air was used as a base line.

Atomic Force Microscopy was performed using a digital

instruments Nanoscope III multimode scanning probe

microscope in the tapping mode. The aerogel specimen was

prepared by cutting a thin and smooth slice with a razor blade.

Transmission Electron Microscopy (TEM) measurements

were performed to characterize the behavior of clay dispersion

in the aerogel. The 100% clay specimens were prepared by

dispersing the ground clay powder in toluene followed by

sonication to form a 1% solution. One drop of the solution was

introduced on the TEM sample grid and vacuum dried over

night at 60°C before characterization.

The morphology of the hybrid gels was also analyzed by a

scanning electron microscope (JSM 7401F-7401). Drops of

the same STN clay powder solution in toluene were placed on

the SEM holder, and vacuum dried overnight at 60°C. The

samples were sputter coated by Emitech sputter coater K575X

for 1 min with silver before observation.

The thermal stability of aerogel samples was measured by

thermo gravimetric analysis. The tests were carried out by

using a TA model 2950 Hi-Res TGA instrument, with a

temperature ramp rate of 10°C /min under nitrogen.

Compression tests were conducted according to ASTM

standard D695. The specimens were installed between two

parallel compression platens on the Instron 5567 load frame

controlled with Bluehill software. The top platen was held

stationary whereas the bottom one was raised at a speed of

1.27 mm s-1

. Compression tests were run using 1 kN load. The

load-displacement curve for each test was converted to a

stress-strain curve by dividing the load by the original cross-

O

O O O

NH NH

O

O O O

NHNH

O O

OO

HO OH

OHHO

O

O

Si

O

O

Si

O

O

Si

O

O

O

O

O O

NH2H2N

OO

O O

O

OO

Si

O

O

O

H2N

BAPP BTDA

APTES

TMOS

APTES end capped polyamic acid

O

O O O

NH NH

O

O O O

NHNH

Si

Si

O

O

O

O

O

O

Si

Si

Si

O

OSi

O

O

O

O

Si

SiO

Si

OSi

SiSi O

OSi

Si

O O

OO

HO OH

OHHO

Polyamic acid crosslinked silica gel

O

O

O

O O

O

N N

O

O

O O O

O

NN

Si

Si

O

O

O

O

O

O

Si

Si

Si

O

OSi

O

O

O

O

Si

SiO

Si

OSi

SiSi O

OSi

Si

Polyimide crosslinked silica gel

NH3.H

2O in NMP

Pyridine and Acetic

anhydride


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sectional area of the specimen and the displacement by the

thickness of the specimens, respectively. The modulus was

taken as the initial slope from the stress-strain curve of the

first compression.

DISCUSSION

The PI/silica hybrid aerogels are obtained as light yellow

cylinders cast from glass vials with 20 mm inner diameter.

The bottom and sidewall of the vial were covered with Kapton

liner to aid in removal of the sample after gelation.

The chemical composition of the aerogel samples during the

sol-gel process is listed in Table 1. The composition of

polyimide/silanes was maintained at 36.16% by weight in the

precursors. The samples were prepared by varying the

concentration of Lucentite STN clay. The bulk density,

skeletal density, porosity, and shrinkage, along with the results

from BET measurements are also listed in Table 1. The BET

surface area slightly decreases after incorporation of clay into

the aerogel. As clay content increases from 1% to 6%, the

BET surface area varies in the rang of 400 to 500 m2/g.

Discussion of other individual properties follows.

Table 1. Chemical Composition of clay reinforced polyi-

mide/silica hybrid aerogel.

Solution A 0.99 g BTDA (3.0 mmol); 0.63 g BAPP (1.5

mmol); 0.68 g APTES (3.0 mmol); 3.8 g TMOS (25.0

mmol) and 10 ml NMP; Solution B 2.0 ml NH

4OH; 0.5 ml H

2O; 7.0 ml NMP

Clay

(wt%) 0% 1% 2% 3% 4% 5% 6%

Bulk den-

sity

(g/cm3)

0.28 0.22 0.27 0.22 0.27 0.26 0.30

Porosity

(%) 82.0 84.5 87.0 88.1 84.2 86.6 84.4

Shrinkage

(%) 30.67 25.34 29.17 23.84 28.35 24.10 26.75

BET sur-

face area

(m2/g)

504 446 376 378 501 428 406

Based on the the Brunauer-Emmett-Teller (BET)

method, the N2 adsorption-desorption isotherms of hybird

gels without and with clay are presented in Figure 3. The

plot corresponds to IUPAC classification type IV for

mesoporous materials with a characteristic hysteresis

loop.22

The loop is associated with the occurrence of

capillary condensation, while the plateau at the range of

high relative pressure indicates complete pore filling. From

Figure 3, the position of the loop for 0% clay aerogels is at

a lower relative pressure compared with aerogels with clay,

suggesting a smaller size of the mesopores for 0%

clay.Error! Bookmark not defined.

The pore size distribution of aerogels with various clay%

is shown in Figure 4. The 0% clay has a sharp peak at 18

nm, indicating a narrow distribution at relatively small pore

size. The pore size distribution curves of 2% and 5% clay


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Figure 3. N2 adsorption-desorption isotherms of hybrid aerogels

with and without clay.

loading samples exhibit a wider pore size distribution. The

predominant pores are located in a higher diameter range

with a peak observed at 22 nm. The 3% clay sample has the

widest pore size distribution in the plot. The pore volume

peak is at 32 nm diameter. These observations indicate that

introduction of clay into the silica/polyimide aerogel results

in a wider distribution of the pore size and an increase in

the average pore diameter.

Figure 4. Pore size distribution of hybrid aerogels.

The porosity and shrinkage of aerogels with various clay%

were compared in Figure 5. The shrinkage was calculated

based on diameter difference between the mold and the

aerogel. The aerogels with clay exhibit a similar range of

shrinkage and porosity compared to the aerogel without clay.

Figure 5. Aerogel shrinkage and porosity (%) as a function of

clay%.

The gels were also air dried under ambient pressure to

check the effects of clay and drying method on the final

structure. Pictures of air dried and SCF dried samples are

shown in Figure 6. Prior to being air dried, the gels also

underwent similar solvent exchange as those being

supercritically dried with liquid CO2. The air dried gel is dark

brown in color and smaller in size due to shrinkage, though it

did not crack and maintained its shape during the drying.

Figure 6. Pictures of clay reinforced monoliths after ambient dry-

ing (left) and supercritical drying (right).

The densities of monoliths with different clay concentration

after air drying and supercritical drying are shown in Figure 7.

As expected, the air dried monoliths resulted in drastically

higher density compared to the supercritically dried ones,

indicating an extensive shrinkage and collapse of pores under

capillary pressure.23 The density of supercritically dried

aerogels are in the range of 0.2 to 0.30 g/cm3, which is similar

to the aerogel without clay. However, the density of aerogel

does not show a clear relationship with the clay concentration

over and above random error.

Figure 7. Aerogel density as a function of clay%.

Wide angle X-ray diffraction is a useful technique to

examine the dispersion of clay in composites. Figure 8 shows

the diffractogram of aerogels vs. clay%. The original

Lucentite STN exhibits a (001) peak around 2Ө = 4.8. The

space between planes of silica clay at 2Ө = 4.8 is calculated to

be 1.839 nm based on Bragg’s law. This peak disappears in all

PI/Si composites containing clay% indicating that clays

become well exfoliated in the aerogel composites.


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AFM was performed to investigate the existence and shape

of clay in hybrid gels as shown in Figure 9. 6% clay sample

Figure 8. X-ray diffractogram of aerogel with various clay %.

was cut into pieces by razor blade. A relatively smooth and

flat piece was affixed to the AFM pad for characterization. A

120° angle along the fractured edge of a clay platelet is

observed. This is consistent with the approximate hexagonal

symmetry of the silicon oxide tetrahedron, where the

tetrahedron unit shares three of its four oxygen atoms to form

a sheet.24

Figure 9. AFM micrograph of hybrid aerogel with 6% clay.

TEM of clay powder is shown in Figure 10. The specimen

particles are so thick that only clay aggregates can be

observed. The clay particles with their edges oriented along

the observation direction absorbs the electron beam the most

and they preferentially become more visible when observed in

TEM and appear as dark lines. The clay planes in other

orientations with respect to the e-beam direction are also

visible but with lower contrast. TEM images of aerogels

1% clay in toluene (B) 100% clay powder

Figure 10. TEM micrographs of bulk STN clay.

containing various clay weight percentages are shown in

Figure 11. Those clay particles whose planes orient along the

e-beam direction (normal to observation plane) appear to have

dark edges and the aerogel nodules are also visible. In Figure

11(B), the dark dots along the clay edge indicate that the silica

particles grown during aerogel formation are mostly in the

plane of the clay platelets. The morphology of clay in

silica/polyimide aerogels are shown in Figure 12. The porous

network of silica has a particle size around 50 nm. The layered

120°


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(A) 1% clay

(B) 3% clay

Figure 11. TEM micrographs of hybrid aerogels with various clay

loadings.

(A)

(B)

(C)

(D)

Figure 12. SEM micrographs of the silica/polyimide aerogel with

various wt% of clay. Clay wt% from top to Bottom: 4% (A), 3%

(B), 6% (C), 100% (D)

Dark lines represent clay with its plane

oriented along electronon beam.

Dark lines with dots

denote silica nodules

grow along clay edges.

Fold

Edge


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silicate clays (denote by white circle) were observed to

connect and become part of the silica network through the

edge growth mechanism illustrated in (A). As shown in

micrographs (B) and (C), the rough area along the edge of clay

plate is due to the preferential growth of silica particles from

the edges, while the surface of the plate is relatively smooth

indicating minimal growth has taken place on the faces of clay

particles. It is clear to see in the SEM micrographs that the

silica particles grow from the edges of the clay platelets. The

100% clay from 1% toluene solution was shown in (D). The

clay is bent most likely due to shrinkage during toluene

evaporation. The clay layers bend or curl due to the external

mechanical forces and the inherent structural features such as

misfits between the components. Curved edges in the

aforementioned TEM micrographs support this conclusion.

The 29

Si NMR spectrums of neat polyimide oligomer and a

representative sample of aerogel are compared in Figure 13.

Figure 13. 29Si NMR spectra of silica/polyimide aerogel com-

pared to neat polyimide oligomers. Tn denotes the number of or-

ganic species attached on the silica surface from APTES in PI

oligomer and Qn denotes the number of (Si-O-Si) formed from

silica gel.

For polyimide oligomers with APTES end caps, the peaks at -

47 (T1), -59 (T2), and -67 ppm (T3) show the extent of reaction

of silane groups in APTES. For the hybrid aerogel spectrum,

the signals at -90, -100 and -109 ppm in the spectrum are

assigned to silicon atoms derived from TMOS which are

partially condensed, [(RO)2SiO2 (Q2) and (RO)SiO3 (Q3)] and

Figure 14. 13C NMR spectra of silica/polyimide aerogel compared

to neat polyimide oligomers.

fully condensed [SiO4 (Q4)].25,26

The disappearance of T1

polyimide oligomer is due to the reaction of APTES with

TMOS. The small shoulders in the range of -53 to -65 ppm are

due to the bidentate (T2) and tridentate (T3) bonding of APTES

Figure 15. FTIR spectra as a function of the clay weight percent.

(A) Illustration of clay reinforced polyimide/silica clay

network structure.

(B) Illustration of the silica grwoth behavior on a clay

platelet.

Figure 16. Conceptual illustration of the polyimide/silica hybrid

aerogel with clay reinforcement.


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with silica (TMOS).6 The size of T3 peak reveals a higher

degree of reaction for APTES derived silicon atoms. These

observations confirm the covalent bonding of polyimide

oligomer to silica gel through APTES groups.

Shown in Figure 14 are 13

C NMR spectra of polyimide

silica hybrid aerogel with no clay compared to an APTES-

capped polyimide oligomer prepared independent of the

aerogel. The spectrum of the oligomer exhibits characteristic

peaks from the methylenes of APTES at 8.7 ppm (C1, directly

bonded to Si),13

21.1 ppm (C2, adjacent to first methylene)

and 40.4 ppm (C3, directly bonded to nitrogen); the peak at

28.6 assigned to the CH3 from BAPP. The oligomer also has

two peaks at 17.4 (f) and 58.9 ppm (e) which are from the

ethoxy groups on unhydrolyzed APTES end cap. The peaks

between 123 ppm and 155 ppm are assigned to aromatic

ringsfrom BAPP and BTDA.4 The peak at 167 ppm is the

C=O from the imides, and the peak at 192 ppm is the

benzophenone C=O from BTDA. The spectrum of the aerogel

contains similiar peaks arising from the imide oligomer

confirming that it is incorporated into the aerogel. Peaks at

49.5 ppm and 177 ppm also present in the spectrum of the

(A)

(B)

Figure 17. Compression test of selected aerogels as a function of

clay%. (A) Stress-strain curve from 40 % clay; (B) Compressive

modulus from the initial slope of the stress strain curve.

aerogel are most likely due to CH3 and C=O from residual

NMP.27

FTIR for the aerogels with different clay composition were

also examined. From the FTIR in

Figure 15, the peaks of Si-O and N-CH3 of NMP overlap in

the range between 1000 and 1200 cm-1

. Two signals at 1720

and 1780 cm-1

are assigned to symmetrical and asymmetrical

stretch of C=O group in imide. A slight signal at 3300 cm-1

is

most likely due to adsorbed water.

A conceptual model of the hybrid aerogel structure is

proposed and shown in Figure 16. The silica matrix bonds to

layered Lucentite STN clay through covalent and hydrogen

bonding mostly from the edge of the clay platelet as shown

through SEM and TEM observations.

Figure 18. TGA curves as a function of the clay weight percent in

nitrogen.


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Figure 19. TGA thermogram of 6% clay hybrid aerogel as func-

tions of time and temperature.

The compression tests were conducted based on ASTM

D695. (B)

Figure 17 (A) shows a typical stress-strain curve of

polyimide/silica hybrid aerogels with 4% clay. The curve can

be divided into three zones. Zone I represents small strain

linear elastic behavior. The modulus of each aerogel specimen

was calculated from the initial slope of this zone I. This zone

is primarily due to the deformation of aerogel and clay. Zone

II is the yielding behavior where plastic hardening occurs

during compaction. This zone is attributed by the effects of

buckling and collapse of pores in silica particles. Zone III is

where the densification and inelastic stress hardening develops

until the sample fails. The ultimate failure of the sample is a

complicated procedure combining many factors, such as the

distribution of pores and the pore sizes. The specimen may

gradually shred from the outer surface until ultimate collapse.

This can result in inconsistent trend of stress-strain

relationship as the clay% increases. The modulus as a function

of aerogel clay concentration is plotted in (B)

Figure 17 (B). Aerogel with clay content from 2% to 6%

has higher modulus in the range of 8~21 MPa compared with

the aerogel without clay. Modulus increases with increasing

clay%. This may be due to the bonding of clay and silica

particles leading to improvement in the ultimate strength of

the skeletal structure.

(a) FTIR spectra at 4 minutes (100 °C)

(b) FTIR spectra at 15 min ( 200 °C)

(c) FTIR spectra at 24 min

Figure 20. FTIR spectra at different time corresponding to the

weight loss at different stages in TGA.

Thermogravimetric analysis (TGA) of the hybrid aerogels

as a function of clay% is shown in Figure 18. The test was

carried out in N2 to observe the weight loss behavior. Overall,

the weight loss profile of each composition does not seem to

be affected by the weight percent of clay in the aerogel

samplein the range of experiment. The aerogels at all

compositions experience several weight loss regimes. Weight

loss up to 100 °C corresponds to the absorbed water, which

accounts for about 1~2% weight loss. This is confirmed by

tandem TGA-FTIR, shown in Figure 19 and Figure 20 for the

6% clay formulation. The FTIR spectrum in Figure 20 (a)

shows that the species coming off at 5 minutes into the TGA

run corresponding to a temperature of 100 °C, is mostly water.

In the weight loss regime with an onset of 200 °C, the

primary species evolved is again water and a small amount of

NMP, as shown in the FTIR spectrum from Figure 20 (b). The

8.83min99.10%

15.02min96.26%

19.97min93.98%

25.03min91.25%

0.0

0.5

1.0

1.5

2.0

Deriv. W

eig

ht (%

/min

)

0

200

400

600

800

1000

[ ] Tem

pera

ture

(°C

) –

– –

–

70

80

90

100

110

Weig

ht (%

)

0 10 20 30 40 50

Time (min) Universal V4.5A TA Instruments -0.014

-0.013

-0.012

-0.011

-0.010

-0.009

-0.008

-0.007

-0.006

-0.005

-0.004

-0.003

-0.002

-0.001

0.000

Ab

so

rba

nc

e

1000 1500 2000 2500 3000 3500

Wavenumbers (cm-1)

-0.012

-0.011

-0.010

-0.009

-0.008

-0.007

-0.006

-0.005

-0.004

-0.003

-0.002

-0.001

0.000

0.001

Ab

so

rba

nc

e

1000 1500 2000 2500 3000 3500

Wavenumbers (cm-1)

-0.014

-0.012

-0.010

-0.008

-0.006

-0.004

-0.002

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.020

0.022

0.024

0.026

Ab

so

rba

nc

e

1000 1500 2000 2500 3000 3500

Wavenumbers (cm-1)


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water coming off at this temperature is due to incomplete

imidization. This weight loss accounts for about 2 to 6% of the

total weight loss depending on formulation. Another weight

loss regime with an onset of about 360 °C is thought to mainly

correspond to degradation of propyl groups from APTES.14

The weight loss regime with onset occurring around 500 °C is

attributed to the decomposition of polyimide.5,28

CONCLUSION

A hybrid clay-polyimide reinforcement was developed to

attempt to improve the mechanical strength and structural

integrity of silica aerogels after supercritical drying. NMR

and FTIR show that the polyimide oligomers are incorporated

in the silica matrix through APTES amines. The BET results

show the pore size distribution is affected by the incorporation

of the clay, with wider pore size distributions and larger

average pore diameters in clay containing aerogels. The

aerogel matrix bonds to layered Lucentite STN clay mostly

from the edge of the clay platelet as shown in SEM and TEM

observations. Though the density of aerogels with various clay

concentration is not changed, modulus significantly increased

with increasing clay%, indicating that the clay reinforces the

aerogel skeletal structure.

AUTHOR INFORMATION

Corresponding Author

* Phone: 1-330-972-6928. E-mail: [email protected].

AKNOWLEDGEMENT

Financial support from the NASA Fundamental Aeronautics Pro-

gram (Hypersonics) is gratefully acknowledged.


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Clay reinforcement of aerogel.


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Clay reinforced polyimide/silica hybrid aerogel

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