Trialkoxysilane-Capped Acrylic Resin/Alumina HybridMaterials Prepared by In Situ Sol–Gel Process

Mingqi Li, Shuxue Zhou, Bo You, Limin WuDepartment of Materials Science, Advanced Coatings Research Center of China EducationalMinistry, Fudan University, Shanghai 200433, People’s Republic of China

The (3-methacryloxypropyl)trimethoxysilane (MPMS)capped acrylic resin/alumina hybrid materials withhighly homogeneous structure were successfully pre-pared by in situ sol–gel process. The effects of the con-tents of alumina, ethylacetoacetate (EAA), and wateras well as the types of catalyst on the structures andproperties of the hybrid materials were studied. It wasfound that the Tg, thermal stability, microhardness, andYoung’s modulus of the hybrid materials were obviouslyimproved with increasing alumina and water contents,low EAA content, and catalyst employed. POLYM. COM-POS., 29:208–215, 2008.ª 2007 Society of Plastics Engineers

INTRODUCTION

In the past more than 10 years, organic–inorganic

hybrid materials are becoming one of the most exciting

areas since the promise of these new hybrid materials is a

tunable combination of the properties of the polymer

(flexibility, lightweight, good impact resistance, easy

processing, etc.) and the ceramic (hardness, durability,

thermal stability, optical properties, etc.) [1–12]. Many or-

ganic–inorganic hybrid materials have been prepared by

the sol–gel process at macro- or microscale, nanoscale,

even at the molecular level [13, 14]. The inorganic pre-

cursors used in the sol–gel process are usually silicon/

metal alkoxide ((M(OCnH2nþ1)z, where M ¼ Si, Ti, Al,

Sn, Zr, etc.), in which tetraethoxysilane (TEOS) is the

most frequently employed [15, 16], titanium n-butoxide

[Ti(OBu)n]4 is occasionally tried as well [17, 18], but alu-

minum alkoxide and other metal alkoxides are very seldom

adopted because of their high reactivity.

In our previous work, we prepared acrylic resin/alumina

hybrid materials by blending the carboxyl-containing

acrylic resin and the alumina sol which was first obtained

by the sol–gel process [19]. The reaction between the

��COOH groups of acrylic resin and the ��OH and/or

��OR groups from alumina sol successfully retarded the

phase separation, causing obvious improvement in the me-

chanical and thermal properties of the hybrid materials.

But the maximum embedded alumina content in the hybrid

material prepared by this approach was only 2 wt% due to

the easy gelation at high content of alumina sol during

blending stage.

In this article, (3-methacryloxypropyl)trimethoxysilane

(MPMS) was chosen to functionalize acrylic resin to pre-

pare MPMS-capped acrylic resin. The hybrid materials

were then prepared by mixing the solutions of the

MPMS-capped acrylic resin and aluminum sec-butoxide

(Al(OBus)3) with ethylacetoacetate (EAA) as the stabi-

lizer, and then performing an in situ sol–gel process under

the presence or absence of catalyst. The effects of EAA

content, the different catalysts, and water content on the

structure and properties of the hybrid materials were

investigated by 27Al nuclear magnetic resonance (27Al

NMR), small angle X-ray scattering (SAXS), atomic force

microscopy (AFM), differential scanning calorimetry

(DSC), thermogravimetric analysis (TGA), and nano in-

dentation tester.

EXPERIMENT

Materials

Methyl methacrylate (MMA), butyl acrylate (BA), (3-

methacryloxypropyl)trimethoxysilane (MPMS), butyl ace-

tate, absolute ethanol (EtOH), ethylacetoacetate (EAA),

and 2,20-azobisisobutyronitrile (AIBN) were purchased

from Sinopharm Group Chemical Reagent Co. Aluminum

sec-butoxide (Al(OBus)3,) was obtained from Alfa Aesar

Company. EAA and EtOH were dried by 12-h storage in

a desiccator with molecular sieve as a drier before use

and other chemicals were used as received.

Correspondence to: Limin Wu; e-mail: lxw@fudan.ac.cn

Contract grant sponsor: National ‘‘863’’ Foundation; Shanghai Special

Nano Foundation; the Key Project of China Educational Ministry; the

Doctoral Foundation of University; and Trans-Century Outstanding Tal-

ented Person Foundation of China Educational Ministry.

DOI 10.1002/pc.20343

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2007 Society of Plastics Engineers

POLYMER COMPOSITES—-2008

Synthesis of MPMS-Capped Acrylic Resins

A 500-mL round-bottom flask equipped with a me-

chanical stirrer, a thermometer with a temperature control-

ler, an N2 inlet, a Graham condenser, and a heating man-

tle was charged with butyl acetate, and heated to 758Cunder a slow stream of N2. The mixture of MMA, BA,

MPMS (MMA/BA/MPMS ¼ 66.5/28.5.0/5.0 for weight

ratio), and 0.4% initiator (AIBN) based on the total

monomer mass were then added into the flask using an

addition funnel over a period of 1 h. After addition of all

ingredients, the reaction was maintained at 758C for addi-

tional 6 h to complete the reaction. The solid content of

the pure MPMS-capped acrylic resin was 50 wt%.

Preparation of Acrylic Resin/Alumina Hybrid Materials

The solid content of the pure MPMS-capped acrylic

resin was diluted into 30 wt% by butyl acetate, and then

a homogeneous solution of Al(OBus)3 and EAA dissolved

in EtOH was added, followed by addition of water and

EtOH without or with hydrochloric acid (HCl) or ammo-

nia (NH3�H2O). The whole mixture was vigorously stirred

at ambient temperature to obtain acrylic/alumina hybrid

resin solution. Assuming the precursor reacted completely

to form Al2O3, the theoretical alumina content in the

hybrid materials was ranged from 2 to 8 wt% based on

the solid content weight of the pure acrylic resin.

The acrylic/alumina hybrid resin solution was cast onto glass

substrates or tinplates and dried at room temperature for 30 days

to prepare hybrid films for measurements. The sample codes

and their preparation conditions are summarized in Table 1.

Characterization

27Al Nuclear Magnetic Resonance. 27Al NMR spectra

of the hybrid materials were measured using a DSX-300

(Bruker Company, Germany) with the crosspolarization

combined with magic angle spinning.

Small-Angle X-ray Scattering. SAXS measurements

were performed with a PW 1700 X-ray scattering instru-

ment (Philips Company, Holland), using copper radiation

with l ¼ 0.154 nm and pinhole collimation of the inci-

dent beam. Background collection was performed under

the same conditions as the sample data collection prior to

the data collection. The background counts were scaled

and removed from the scattering.

Atomic Force Microscopy. AFM images of the hybrid

films were recorded by a Multimode Nanoscope III

Instrument (Digital Instrument, USA) in tapping mode

with a silica probe (NSC 11) and a frequency of 65 kHz.

The scan dimension and the roughness analysis were per-

formed on 2 � 2 mm2 images. The average surface rough-

ness (Ra) is the mean vertical deviation of the surface rel-

ative to the center plane. The root mean square roughness

(RMS roughness) is the standard deviation of the Z values

(the height) calculated within the given area as:

RMS ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPðZi � ZaveÞ2

N

s

where Zi is the current Z value, Zave is the average of the

Z values, and N is the number of data points with the

given area [20].

Differential Scanning Calorimetry. DSC analysis was

performed on a DuPont 2000-910 differential scanning

calorimeter. The samples were run between �1008C and

1308C under a N2 flow at a heating rate of 108C/min. Tg

was determined by the half-point method.

Thermogravimetric Analysis. TGA was carried out

using a Perkin Elmer thermogravimetric apparatus (USA).

The temperature was scanned from room temperature to

6008C with a heating rate of 108C/min in the air flow.

Nanoindentation Tests. The nanoindentation tests were

performed on Nano Indenter1

XP (MTS cooperation,

USA) instrument. A three-side pyramid (Berkovich) dia-

mond indenter was employed. The nanoindentation experi-

ments were carried out on the specimens with 2 � 2 cm2

dimensions at ambient conditions and constant strain rate

of 0.05 mN s�1. At least eight indents were performed on

each sample and the interval of each two indents was 50

mm to avoid interaction [21].The microhardness (H) and

Young’s modulus (S) were calculated as:

H ¼ P=A

S ¼ b2ffiffiffip

p Er

ffiffiffiA

p

TABLE 1. Reaction conditions of the hybrid materials.

Sample

code

Alumina

content

(wt%)a

EAA/Al

(OBus)3

(molar ratio)b

Water/Al

(OBus)3

(molar ratio)b Catalystb

A0 (pure

resin) 0 – – –

A2 2.0 2.0 2.0 None

A4 4.0 2.0 2.0 None

A8 8.0 2.0 2.0 None

E1.5 2.0 1.5 2.0 None

E2.5 2.0 2.5 2.0 None

AA 2.0 2.0 2.0 HCl

AB 2.0 2.0 2.0 NH3�H2O

H5 2.0 2.0 5.0 None

H8 2.0 2.0 8.0 None

a The alumina content in the hybrid materials, 50 g MPMS-capped

acrylic resin in 30 wt% solid content.b Preparation conditions during in situ sol–gel process.

DOI 10.1002/pc POLYMER COMPOSITES—-2008 209

where P is the load, A is the contact area, b is a constant

that depends on the geometry of the indenter and Er is the

reduced modulus.

RESULTS AND DISCUSSION

Structure of Acrylic Resin/Alumina Hybrid Materials

27Al NMR. The 27Al NMR spectra of A2 and A4 were

shown in Fig. 1. Both A2 and A4 profiles showed an

intense resonance at around �1 ppm from octahedral alu-

minum (AlVI). The resonance at 66 ppm was assigned to

aluminum in four- fold (AlIV) coordination [22]. The up-

field resonance of AlIV in A2 and A4 at about 50 and

55 ppm, respectively, indicated that Al��O��Si bonds

were formed in the hybrid materials [23 24].

SAXS Analysis. Figure 2 displayed the SAXS profiles

of the pure MPMS-capped acrylic resin and hybrid mate-

rials with different alumina content. The hybrid materials

exhibited higher intensity compared with pure acrylic

resin, resulting from the higher electron density difference

between organic and inorganic phase, and the intensity

increased with increasing alumina content. But all sam-

ples showed rather low intensity (below 10), and no inter-

ference peak was observed, indicating alumina-rich

domains with short-range periodicity did not exist and the

acrylic resin/alumina hybrid materials were highly homo-

geneous. The high uniformity should be attributed to the

simultaneous reaction between the MPMS-capped acrylic

resin and alumina cluster once the hydrolysis and conden-

sation of aluminum sec-alkoxide took place, which greatly

hampers the growth of alumina clusters and thus leading

to the formation of oligomer or chain-like structure rather

than compact alumina phase.

Figure 3 illustrated the SAXS profiles of the hybrid

materials prepared with various ratios of EAA/Al(OBus)3.

Lower intensity was observed for the sample prepared at

higher ratio of EAA/Al(OBus)3. Since EAA was chelated

with Al(OBus)3 by occupying a site for condensation

[25], the higher the EAA dosage was added, the smaller

the condensation degree of alumina phase and thus the

lower electron density difference between organic and

inorganic phase. Also, no interference peak was observed

even for the hybrid materials prepared at 1.5 molar ratio

for EAA/Al(OBus)3, further confirming the high homoge-

neity of the hybrid materials.

The effects of different catalysts and water/Al(OBus)3

ratios on the SAXS profiles of the hybrid materials are

demonstrated in Fig. 4. The sample prepared under no

catalyst presented the lowest intensity, while the sample

prepared under the base-catalyzed condition exhibited the

highest intensity, as shown in Fig. 4a, implying the most

compact inorganic phase, which was consistent with the

hydrolysis and condensation of silicone/titanium alkoxide

FIG. 1. The 27Al NMR profiles of A2 and A4. FIG. 2. SAXS profiles of the hybrid materials with various alumina

content.

FIG. 3. SAXS profiles of the hybrid materials prepared with various

ratios of EAA/Al(OBus)3.

210 POLYMER COMPOSITES—-2008 DOI 10.1002/pc

[26, 27], namely, more compact inorganic phase inclined

to form under base-catalyzed condition. Comparison of

sample A2 with samples H5 and H8 (as seen in Fig. 4b)

showed that the intensity of the hybrid materials increased

as the molar ratio of water/Al(OBus)3 increased, indicat-

ing that more compact structure had formed with higher

water content. But a quite low intensity and no interfer-

ence peak were observed for all samples, also suggesting

that the hybrid materials were highly homogeneous at all

preparing condition as discussed in Figs. 2 and 3.

Surface Morphology of Acrylic Resin/AluminaHybrid Coats

The typical three-dimensional AFM images of the

hybrid films and MPMS-capped acrylic resin are illustrated

in Fig. 5, and the surface roughness values of all hybrid

films are summarized in Table 2. Both Fig. 5 and Table 2

showed that the surface roughness values slightly increased

with increasing alumina content, possibly because more

alumina resided on the surfaces of the hybrid films as

more alumina was embedded. In addition, a rougher sur-

face could be seen for the hybrid films prepared at less

EAA, base-catalyzed and higher molar ratio of water/Al(O-

Bus)3 based on the data in Table 2. In comparison with the

SAXS results, it could be seen that the more compact the

alumina phase was, the rougher the surface of the hybrid

films was. However, all these hybrid films exhibited rather

smooth surfaces with the surface roughness lower than 1

nm, indicating excellent surface planarity and high homo-

geneity, just as shown by SAXS analysis.

Thermal Properties of the Hybrid Materials

DSC Analysis. Figure 6 displayed the typical DSC

curves of the pure MPMS-capped acrylic resin and hybrid

materials containing various alumina content. The glass-

transition region shifted to higher temperature range at

higher alumina loading, and the corresponding Tg signifi-

cantly increased from 12.38C to 34.78C as the alumina

FIG. 4. SAXS profiles of the hybrid materials prepared (a) with different catalysts and (b) at different

water/Al(OBus)3 ratios.

FIG. 5. The three-dimensional AFM images of the films: (a) A0; (b) A2; and (c) A8. [Color figure can be

viewed in the online issue, which is available at www.interscience.wiley.com.]

DOI 10.1002/pc POLYMER COMPOSITES—-2008 211

content increased from 0 to 8 wt% (Fig. 7 and Table 3).

The higher Tg suggested that the segmental chain mobility

was much more restricted because of the more crosslink-

ing reactions between acrylic macromolecular chains and

alumina phases. Although some silane self-condensation

possibly occurred, the crosslinking reaction between

MPMS and alumina should play a dominating role in

influencing the properties of the hybrids due to higher

reactivity of Si��O��Al than that of Si��O��Si [23].

The effects of the EAA amount, the types of catalyst,

and the molar ratios of water/Al(OBus)3 on the glass tran-

sition of the hybrid materials are also listed in Table 3. As

the EAA amount increased, the hybrid materials exhibited

lower Tg. Since EAA was chelated with Al(OBus)3, EAA

not only occupied a site for condensation, but also syn-

chronously prevented the interaction between the polymer

chains and the inorganic phase, leading to less crosslinking

density. Therefore, when more EAA was employed, lower

Tg was observed. When catalysts were employed, the Tg

shifted to higher temperature (see samples A2, AA, and

AB), and the sample prepared under the acid-catalyzed

condition had a little higher Tg than that prepared under

base condition. This was because the catalysts promoted

not only the hydrolysis-condensation of the precursor itself

but also the crosslinking reaction between inorganic and

organic phases through the ��OH and/or ��OR groups of

alumina phase and the ��Si(OC2H5)3 groups of acrylic

polymer chains. In comparison with acid-catalyzed condi-

tion, the hybrid material from base condition had more

compact inorganic phase, which led to a decreasing cross-

linking reaction between inorganic and organic phases,

and thus lower Tg. Table 3 also displayed that the Tg

increased with increasing water content, and even no

obvious glass-transition region was observed for the sam-

ple H8; this is probably because more water could acceler-

ate not only the hydrolysis-condensation of Al(OBus)3 but

also the interhydrolysis-condensation between Al(OBus)3

and MPMS-capped acrylic resin.

TGA Analysis. Figure 7 presented TGA and differential

thermogravimetry (DTG) curves of pure acrylic resin and

the hybrid materials with different alumina content. For

the pure acrylic resin, the weight loss below 2008C should

be attributed to the evaporation of physically adsorbed

water and residual solvent while the broad stage of the

weight loss within 200–4008C might be ascribed to the

random decomposition of the polymer chains. For the

hybrid materials, as seen from DTG curves, the upper

limit temperature of the first weight loss stage extended

to around 2408C, which should be due to the physically

adsorbed water by alumina phase. The largest weight loss

TABLE 2. Surface roughness of the pure MPMS-capped acrylic resin

and the hybrid coats.

Sample code Ra (nm) RMS (nm)

A0 0.23 0.29

A2 0.25 0.32

A4 0.28 0.35

A8 0.46 0.57

E1.5 0.29 0.37

E2.5 0.24 0.31

AA 0.29 0.37

AB 0.33 0.43

H5 0.28 0.35

H8 0.30 0.38

FIG. 6. Typical DSC curves of the pure acrylic resin and the hybrid

materials with various alumina content.

FIG. 7. TGA and DTG thermograms of the pure acrylic resin and the

hybrid materials with different alumina content.

TABLE 3. Tg of the pure MPMS-capped acrylic resin and the hybrid

materials.

Sample code A0 A2 A4 A8 E1.5 E2.5 AA AB H5 H8

Tg (8C) 12.3 20.7 26.5 34.7 30.4 16.8 26.1 24.3 27.2 –a

a No obvious glass-transition region was observed.

212 POLYMER COMPOSITES—-2008 DOI 10.1002/pc

stage in the temperature range of 310–4308C was attrib-

uted to the same reason as that of pure acrylic resin.

Besides these stages, a new weight loss stage occurred

within 240–3108C for the hybrid materials especially

visible for the sample A8. The new weight loss stage

should be caused by the decomposition of Al(OBu-s)

3(EAA)2 complex [24]. Since the higher the alumina

content was embedded, the higher the Al(OBus)3(EAA)2

complex formed, and thus the more obvious the new

weight loss stage was. In addition, the slight weight loss

for the hybrid materials above 4308C, which was not

observed for the pure acrylic resin, might be caused by

the decomposition of some groups such as ��OH and/or

unhydrolyzed ��OR bonded on alumina.

The peak temperature of the largest weight loss in the

DTG curves was adopted as the decomposition tempera-

ture (Td) as summarized in Table 4. Td increased from

2978C to 3888C when 2 wt% embedded, indicating that

the thermal stability of the hybrid materials was improved

significantly in comparison with the pure acrylic resin

because of the chemical bonds between polymer and alu-

mina phases. However, as alumina content further

increased, the Td enhanced slightly, that is, the alumina

content had little influence on the thermal stability of the

hybrid materials [24].

Mechanical Properties by Nanoindentation Measurements

The mechanical properties of the hybrid materials were

determined by nanoidentation technique with a continuous

stiffness measurement. Figure 8 manifested the typical

load–displacement curves of the pure acrylic resin and the

hybrid materials with different alumina content. The

curves steadily shifted upwards with increasing alumina

content, suggesting the better resistance to indentation for

the polymer embedding with higher alumina content.

Figure 9 depicted the microhardness and Young’s mod-

ulus profiles as a function of the indentation depth for the

pure polymer and the hybrid materials with different alu-

mina content. It could be seen that both the microhard-

ness and Young’s modulus enhanced after alumina phase

incorporated and increased with increasing alumina con-

TABLE 4. Td of the pure acrylic resin and the hybrid materials with

increasing alumina content.

Sample A0 A2 A4 A8

Tda (8C) 297 388 394 397

a The peak temperature of the largest weight loss peak in the DTG

curve.

FIG. 8. Typical loading–displacement curves of the pure acrylic resin

and hybrid materials with increasing alumina content.

FIG. 9. (a) Microhardness and (b) Young’s modulus as a function of displacement into sample surface for

the pure acrylic resin and hybrid materials with different alumina content.

DOI 10.1002/pc POLYMER COMPOSITES—-2008 213

centration. For all the samples, the hardness and the elas-

tic modulus curves displayed two regions: an initial

decrease and a subsequent rise. The dramatic drops of the

hardness and the modulus at the initial region were prob-

ably due to the indentation size effect [28 29]. There are

several possible origins for the indentation size effect,

such as: (1) the blunting of the tip of the apex of the

leads to unavoidable over-estimate of the hardness and

modulus at the near-surface area; (2) the inaccuracy of

the indenter area function determination at low depth; and

(3) the inaccuracy in sample surface finding segment at

the initial loading portion. The subsequent increase in the

hardness and the elastic modulus might be attributed the

gradual increase of inorganic phase as contact depths

increased [27, 29].

Table 5 summarized the microhardness and Young’s

modulus for all samples with indentation depth of 250–

350 nm. The effects of the EAA amount, the types of cat-

alyst, and the ratios of water/Al(OBus)3 on the hardness

and elastic modulus are also listed in Table 5. When

more EAA was added, the microhardness and Young’s

modulus exhibited a decreasing trend (see samples E1.5,

A2, and E2.5) because EAA would restrict both the hy-

drolysis and condensation of Al(OBus)3 and the reaction

between organic and inorganic phases. The samples from

acid-catalyzed and base-catalyzed conditions showed bet-

ter mechanical properties than that prepared without any

catalyst, also because of more chemical reactions between

organic and inorganic phases under catalyst employed

(see samples A2, AA, and AB). As more water was used,

the hybrid materials exhibited enhanced mechanical prop-

erties since higher water concentration meant higher hy-

drolysis and condensation degree of inorganic precursor

which led to both a compact alumina phase and a stronger

interaction between organic and inorganic phases (see sam-

ples A2, H5, and H8). In general, the samples prepared at

different EAA content, water content, and catalysts all indi-

cated that stronger chemical interaction between organic

and inorganic phases availed increasing microhardness and

Young’s modulus.

CONCLUSION

MPMS-capped acrylic resin/alumina hybrid materials

were prepared by in situ sol–gel process. The crosslinking

reaction between organic and alumina phases could occur

through the reaction of ��OH and/or ��OR groups of alu-

mina phase with ��Si(OC2H5)3 groups of the polymer

side chains to form Al��O��Si bonds. All hybrid materi-

als exhibited high homogeneity proved by SAXS analyses

and AFM observation. In comparison with pure resin, the

thermal properties and mechanical properties of the poly-

mers were improved significantly when alumina was

incorporated. As the contents of alumina or water

increased or catalyst (hydrochloric acid or ammonia) was

employed, the hybrid materials showed increases in Tg,

thermal stability, microhardness, and Young’s modulus,

but more EAA resulted in decreases in these properties.

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E1.5 0.188 4.42

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