Polysilsesquioxane Nanosheets Synthesized in

Confined Environment

Jun Ma,*1,2Wei Su,1 Yong-Jun Zhang,1 Teng-Jiao Hu,1Hai-Yun Liu,1 Bai-Yu Li,1 Liang-He Shi,1 Jian Xu,*1 Yiu-Wing Mai2

1State Key Laboratory of Polymer Physics & Chemistry, Center for Molecular Science, Institute of Chemistry,The Chinese Academy of Sciences, Beijing 100080, P.R. ChinaFax: þ86 10 62556180; E-mail: jxu@infoc3.icas.ac.cn; jma504@pplas.icas.ac.cn

2Centre for Advanced Materials Technology (CAMT), School of Aerospace, Mechanical & Mechatronic Engineering J07,The University of Sydney, NSW, 2006, AustraliaFax: þ61 2 93513760; E-mail: jun.ma@aeromech.usyd.edu.au

Received: May 5, 2003; Revised: June 16, 2003; Accepted: June 16, 2003; DOI: 10.1002/marc.200350011

Keywords: montmorillonite; nanosheet; polymethylsilsesquioxane; templates

Introduction

For many years, polymer science has focused on linear

polymers and their derivatives. The derivatives include

nonlinear extensions, such as branched and crosslinked

polymers, and linear extensions, such as macromolecular

rings, stars, combs and ladders. In recent years, however, a

new class of polymers characterized by well-defined shape

has received intense attention. Examples of such objects

include molecular tubes, larger diameter rods, and two-

dimensional polymers (denoted as sheets). Preparation of

sheet-like polymers often requires pre-organization of

the small precursor molecules by external means, such as

confined assembly in a bilayer membrane.[1–3] However,

the sheet-like geometrymay be lost once the confinement is

isolated and the polymer dissolved. Stupp et al. proposed a

complicated pathway for chemically bonded sheet-like

polymers with thicknesses of about 10 nm, which is

prepared by catenating the oligomers by two different

stitching reactions involving the corresponding reactive

sites.[4] The transformation of polymer research from one-

to two-dimensional architectures may produce a new gene-

ration of polymers with unexpected improved properties.

Brown first proposed the sheet-like structure of poly-

silsesquioxane (PSSQ) microgel chemically bonded by a

trifunctional monomer.[5] Montmorillonite (MMT) is well

known for its high aspect ratio structure and is widely used

as a template to create various hybrid structures.[6–11]

However, no literature has been reported to date on MMT

acting as a template to prepare sheet-like polymers. We

report here a study using MMT as a template to load an

organic precursor and then allow polymerization to proceed

in the confined layer space of MMT. After extraction and

filtration, nano-scale sheets of polymethylsilsesquioxane

Communication: Polysilsesquioxane nanosheets with athickness of four nanometers and lateral dimensions ofseveral hundreds of nanometers were synthesized by poly-merization of a trifunctional monomer in the layer space ofmontmorillonite as the confined environment.

AFM image of a polymethylsilsesquioxane nanosheet.

676 Macromol. Rapid Commun. 2003, 24, 676–680

Macromol. Rapid Commun. 2003, 24, No. 11 � WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2003 1022-1336/2003/1107–676$17.50þ.50/0

(PMSQ) or polyphenylsilsesquioxane (PPSQ) were ob-

tained. Since the sheet has a thickness of �4 nm, the term

‘‘nanosheet’’ was coined to represent this class of polymers.

Experimental Part

General

The thickness and configuration of PMSQ sheets weremeasured by means of atomic force microscopy (AFM; digitalinstrumentNanoscope IIIA) in the tappingmode, and its lateralsize was recorded by an H-800 transmission electronmicroscope (TEM). The particle size and its distribution wereobserved with a light scattering spectrometer (LLS spectro-meter ALV/SP-125) equipped with multi-t correlator (ALV-5000E) and He–Ne Laser (632.8 nm, 22mw). UV spectra wereobtained on a UV-vis spectrophotometer (Shimadzu, UV-1601PC).

Materials

Naþ-MMT from Qinghe Chemical Factory (Zhangjiakou,Hebei Province) was modified by hexadecyltrimethylammo-nium bromide according to a published procedure.[12] Themodified MMT is denoted as org-MMT. Methyltrimethoxysi-lane and phenyltrimethoxysilane were provided by Shin-EtsuChemical Co. Ltd, Japan. All other agents were commercialproducts.

Preparation of Polymethylsilsesquioxane (PMSQ) Nanosheets

1 g of org-MMTwas immersed in 10 g of chloroform for 10 hand sonicated using an ultrasonic generator for 1 h. Then 5 g ofmonomer (methyltrimethoxysilane) was added to the suspen-sion followed by sonicating for 0.5 h to obtain a uniformsuspension. Then the mixture was transferred to the centrifugetubes and centrifuged at 8000 rpm for 5 min.

The upper transparent solution was removed and the opaquewhitish layer accumulated at the bottomwas dissolved in 60mlof petroleumether by stirring and sonicating in order to achievea uniform suspension. Since petroleum ether is a good solventfor themonomer but a poor solvent for org-MMT, themonomeroutside the layer space could be removed while that inside thelayer space remains by washing the intercalated blend withpetroleum ether. Then the suspension was centrifuged at4000 rpm for 5 min and the upper transparent solution wasremoved. Dissolving, centrifuging and separating were repea-ted 12 times until all monomer leaking out of the interlayerspacewas removed. At last, a uniform petroleum ether suspen-sion with the monomer dwelled between the MMT layers wasobtained.

10ml of 0.1M aqueousH3PO4, 36ml ofCH3OHand 72ml ofCHCl3 were mixed in a three-neck flask. The sonicatedpetroleum ether suspension obtained was then added into thereactor dropwise at 60 8C with vigorous stirring. The resultantmixture was washed several times with deionized water untilthe pH was between 6.5 and 7.

The solution was condensed to about 15 ml and then mixedwith 15 ml of THF under stirring for 5 min. The mixture was

centrifuged at 7000 rpm for 10 min. The upper transparentsolution was collected and filtered over films with 4.5-microndiameter holes. The resultant solution is called PMSQnanosheet solution.

Results and Discussion

In this work, multi-functional methyltrimethoxysilane was

loaded into the layer spaces ofMMTand then the monomer

was polymerized in the confined space. After extraction and

filtration, PMSQ nanosheets were obtained. However,

two questions arose during processing. Could MMT be

removed from the PMSQ nanosheet solution just by

extraction and filtration? How could it be proved that

polymerization took placewithin the confined layers but not

outside the layers? To answer these questions, a solution

in which exfoliated MMT was prepared was used for

comparison. AFM, TEM, light scattering and UV-vis

spectroscopy provided indirect but solid proofs for confined

polymerization.

Modified MMT Solution

In previous research,[13] a solution of modified MMT was

obtained by treating MMT, modified with ammonium ions,

with polydimethylsiloxane.When the solution was blended

with polar polymers, exfoliated nanocomposites formed.

However, when blended with some nonpolar polymers,

intercalated nanocomposites were obtained. The details for

preparation of the solution are given in the literature.[13]

An amount of an MMT solution (10�5–10�6M) obtained

such was spin coated (4000 rpm) on mica for AFM

observation. In Figure 1a, single exfoliated MMT layers

with a smooth surface are shown, which proved that the

MMT in the MMT solution is exfoliated. The thickness of

MMT is 1.1 nm.

A chosen quantity of theMMT solution was filtered over

filmswith holes of 4.5 mm in diameter and then investigated

by means of light scattering spectroscopy. The experiment

was repeated three times and each time the spectrum

showed no signal, thus indicating that even exfoliatedMMT

could not pass through the films. This means that no MMT

layers exist in the PMSQnanosheet solution (as the result of

filtration) and that MMT plays no role for characterizations

described below.

PMSQ Microgel Prepared in Non-ConfinedEnvironment and PMSQ Nanosheets

PMSQ synthesized in non-confined environment was pre-

pared according to a literature procedure.[14] As shown in

Figure 1b, these non-confined products appear spherical in

shape. The PMSQ nanosheet solution (10�5–10�6M)

was dropped on mica for AFM observation. In Figure 1c,

sheet-like particles were observed the thickness of which

was about 7 nm. Although the center of the particles is

Polysilsesquioxane Nanosheets Synthesized in Confined Environment 677

apparently flat, the brim tends to rise up, which could be

explained in light of the PMSQ nanosheet preparation

procedure. As mentioned above, there are two possible

polymerization locations, either inside or outside the layer

space. If exfoliation occurs before polymerization or the

monomer leaks out of the layer space, polymerization takes

place outside the layer and irregular or spherical microgel

particles are formed. As shown in Figure 1b, the config-

uration of the microgel synthesized in non-confined

environment is totally different from that of the nanosheet

(Figure 1c). This suggests that the nanosheets are located

inside the confined layer space due to polymerization.

Since stress exists in the molecular chains as a result of

polymerization in confined environment, chain deforma-

tion in the form of brim rising will occur as the environment

is removed. The nanosheets show smoother surfaces and are

thinner when spin-coated on mica at 4000 rpm. Many

button-like particles (Figure 1d) with thicknesses of 4 nm

and lateral sizes of 100–200 nm were observed. Although

the same nanosheet solution was used for all AFM

characterizations (except those displayed in Figure 1a and

1b), the button-like particles show only 4 nm thickness in

Figure 1d in comparison with a thickness of 7 nm in

Figure 1c. The decreased sheet thickness caused by

different AFM specimen preparation might be explained

by the flexible siloxane chains of the sheet. At high spin-

coating speed the siloxane chains were fully stretched and

attached to the mica surface firmly. Hence, a reduced

thickness resulted.

We also observed aggregates of PMSQ sheets (Figure 2a)

prepared by the same spin-coating method. In Figure 2a, a

line was drawn across the bottom particle to measure its

thickness, which is about 8 nm (Figure 2b). Compared to the

sheet thickness of 4 nm shown in Figure 1d, the increased

thickness suggests that the particle chosen for Figure 2a

could consist of several sheets. Indeed, the peaks seen in the

height curve (Figure 2b) also support the existence of at

least two sheets. Recently, PMSQ was found to be an

excellent dielectric, owing to its intrinsic properties, but

the problem for this application is that the branches or

microgels originating from the synthesis of the trifunctional

monomer makes it difficult to obtain flat spinning coating

surfaces.[15,16] Since the PMSQ aggregates prepared by

spin-coating in this work showflat surfaces (Figure 2a) they

might find possible dielectric applications.

The configuration of the PMSQ nanosheet synthesized in

confined environment was also studied by means of TEM.

The PMSQ sheet solutionwas dipped on a coppermicrogrid

coated with a thin film of amorphous carbon (less than

10 nm) and then examined by TEM. As shown in Figure 3,

sheet-like particles (marked with circles) in the form of

rectangles, squares, triangles, and irregular shapes were

observed. Lateral dimensions of these particles are several

hundreds of nanometers, corresponding to the lateral size of

the exfoliated MMT shown in Figure 1a.

Figure 1. AFMconfiguration: (a) singleMMT layers, (b) PMSQsynthesized in non-confined environment, (c) PMSQnanosheet bydropping, and (d) PMSQ nanosheet by spin coating.

678 J. Ma et al.

The dimension of the PMSQ nanosheet in solution was

determined by means of LLS to have an average hydro-

dynamic radius of �168 nm (Figure 4). The radius

measurements reveal a wide distribution varying from

65 nm to 470 nm. There are two reasons: (i) TheMMTused

in this work is from nature and its lateral dimensions may

vary from 30 nm to several microns and even larger

dependingon the particular silicate.[17] Trifunctionalmono-

mer was intercalated inside the MMT layer space and

polymerized. Since the sheet dimensions are determined by

the confined environment, the resulting nanosheets show a

wide radius distribution. (ii) PMSQ nanosheets form aggre-

gates in solution easily (Figure 2a), which will contribute to

the large distribution of hydrodynamic radius of the

nanosheets.

The above discussions have focused on the thickness and

configuration of the PMSQ nanosheets. However, char-

acterizations of their molecular information are also

needed. Since solid-state NMR spectroscopy and X-ray

diffraction require large amounts of PMSQ sheets, which

cannot be produced at this stage, UV-vis spectroscopy was

run instead to study the molecular interactions in poly-

phenylsilsesquioxane (PPSQ) nanosheets, which were

prepared from phenyltrimethoxysilane in confined envir-

onment by a similar method. As a reference, non-sheet-

like PPSQ was prepared from phenyltrimethoxysilane

in non-confined environment similarly to a published

literature.[13] Figure 5 shows the UV spectrum of phenyl-

trimethoxysilane with obvious bands at 259, 264 and

270 nm corresponding to the fine structure of the phenyl

group. The non-sheet-like PPSQ also shows bands that can

be assigned to the fine structure of the phenyl moiety, but

there are two new bands at 235 and 282 nm. This change

could be ascribed essentially to the p–p* transition due to

the interaction among the phenyl rings. For sheet-like

PPSQ, there are no phenyl bands (fine structure) and two

newdistinctive bands at 243 and 293 nm appear. This can be

explained by the strong conjugation due to the short

distance between phenyl rings in the 4 nm thick nanosheet

synthesized in the confined environment. It should be noted

that all samples used for characterization were filtered over

films with holes of 4.5 mm in diameter. As discussed above

exfoliatedMMT could not pass through these films. Hence,

MMT did not influence the UV-vis spectroscopic results.

The difference in the UV spectra of PPSQ nanosheets

Figure 2. AFM configuration: (a) aggregate of PMSQnanosheet, and (b) height curve of chosen area.

Figure 3. TEM image of PMSQ nanosheets synthesized inconfined environment, revealing the existence of sheet-likeparticles (marked with circles) in the form of rectangles, squares,triangles, and irregular shapes.

Figure 4. Light scattering results of PMSQ nanosheet.

Polysilsesquioxane Nanosheets Synthesized in Confined Environment 679

and non-sheet-like PPSQ polymerized in non-confined

environment confirms that, for the nanosheet, the monomer

is indeed polymerized inside the layer space of MMT.

Acknowledgement: This project was supported jointly by theNatural Science Foundation of China (No. 20074039), the JointLaboratory of Polymer Science and Materials of ICCAS and theAustralian Research Council on polymer nanocomposites.

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Figure 5. UV-vis spectra of phenyltrimethoxysilane, PPSQ, andPPSQ nanosheet.

680 J. Ma et al.