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Full PaperReceived: 29 April 2011 Revised: 2 June 2011 Accepted: 3 June 2011 Published online in Wiley Online Library: 10 August 2011

(wileyonlinelibrary.com) DOI 10.1002/aoc.1820

Preparation and properties of polyhedraloligomeric silsesquioxane polymersTakahiro Shiodaa, Takahiro Gunjia∗, Noritaka Abea and Yoshimoto Abeb

Polyhedral oligomeric silsesquioxane (POSS) polymers were synthesized by the dehydrogenative condensation of (HSiO3/2)8with water in the presence of diethylhydroxylamine followed by trimethylsilylation. Coating films were prepared by spin-coatingof the coating solution prepared by the dehydrogenative condensation of POSS. The hardness of the coating films was evaluatedusing a pencil-hardness test and was found to increase up to 8H with increases in the curing temperature. Free-standing filmand silica gel powder were prepared by aging the coating solution at room temperature. The silica gel powder was subjectedto heat treatment under air atmosphere to show a specific surface area of 440 m2 g−1 at 100 ◦C, which showed a maximum at400 ◦C as 550 m2 g−1. Copyright c© 2011 John Wiley & Sons, Ltd.

Keywords: polyhedral oligomeric silsesquioxane; diethylhydroxylamine; dehydrogenative condensation; silica gel; free-standing film

Introduction

Polysilsesquioxanes are polysiloxanes consisting of the structureunit (RSiO3/2)n. They are classified into three groups: amorphous,cage-type and ladder-structured polysilsesquioxanes. Amongthese polymers, cage-type polysilsesquioxanes have attractedconsiderable attention from the perspective of synthesis andapplication owing to their nano-sized three-dimensional structureconsisting of a silica backbone, an angstrom-sized cavity and highthermal stability.[1 – 13]

Polyhedral oligomeric silsesquioxanes (POSS) are the best-known and most useful polysiloxanes and have the formula(RSiO3/2)8. Since POSS have a pore at the center of the molecule andare composed of 12 Si–O–Si bonds to form a rigid structure, POSSare potential candidates for providing functional nano-buildingblocks that can be used to form micro- or mesoporous silica.The organic/inorganic hybrids are prepared by introducing POSSmoiety into organic polymers. Hydrosilylation of (HSiO3/2)8 withthe corresponding olefins is the simplest and most widely usedtechnique to connect POSS moieties with organic polymer chainsby chemical bonding. Such organic/inorganic hybrids are also pre-pared by the reaction of carbo-functional groups on silicon atoms,for example the polymerization of methacryloyloxy groups for thecompound octakis(methacryloyloxypropyl)octasilsesquioxane.On the other hand, the preparation of siloxane-based hybrids hasbeen insufficiently investigated owing to the difficulty of synthe-sizing sila-functionalized POSS derivatives through conventionalmeans: mesoporous silica materials have been prepared by thehydrolytic polycondensation of {[(EtO)3−nMenSiO]SiO3/2}8 in thepresence of polymer surfactants.[14] Films with heat-resistivityand easily modulated films with high heat resistance have beenprepared by the reaction of (PhSiO3/2)8[(HO)PhSiO]2 with chloro-terminated polydimethylsiloxane.[15] Preparation of porous silicaor Si–C–O ceramic material was reported by simple pyrolysis ofPOSS without a precise investigation of its precursor polymer.[16]

We have reported the dehydrogenative reaction of alcoholwith (HSiO3/2)8 in the presence of diethylhydroxylamine toproduce octaalkoxylated octasilsesquioxanes.[17] This reactionwas applied to the synthesis of siloxane-based POSS hybrids

by the dehydrogenative reaction of (HSiO3/2)8 with silanolssuch as diphenylsilanol, tetraphenyldisiloxanediol and α, ω-dihydroxypolydimethylsiloxanes[18] to show a relatively high heatresistivity and high surface area on heating. In the same way, POSSpolymer (W-POSS) was synthesized by the reaction of (HSiO3/2)8

with water in the presence of diethylhydroxylamine, which wasbriefly reported as a short communication.[19]

In this paper, therefore, the synthesis of W-POSS according toScheme 1 and its thermal and mechanical properties are reported.In particular, the preparation and properties of W-POSS coatingfilms and silica gels are presented in detail.

Experimental

Reagents and Substrate

(HSiO3/2)8 was synthesized by a previously described method.[20]

Other chemicals were of reagent grade or higher and purifiedaccording to standard protocols.

Synthesis of W-POSS

In a 200 ml two-necked flask with a reflux condenser wereplaced (HSiO3/2)8 (300 mg, 710 µmol), water (0.026 g, 1.4 mmol),tetrahydrofuran (THF; 30 ml) and benzene (40 ml) under a nitrogenatmosphere. To this solution was added N,N-diethylhydroxylamine(1.0 µl, 9.7 µmol), and the mixture was stirred at 0 ◦C for 90 min.Chloro(trimethyl)silane (0.92 g, 8.5 mmol) was added, and themixture was stirred at room temperature for 30 min followed by the

∗ Correspondence to: Takahiro Gunji, Department of Pure and Applied Chemistry,Faculty of Science and Technology, Tokyo University of Science, Tokyo, Japan.E-mail: gunji@rs.noda.tus.ac.jp

a Department of Pure and Applied Chemistry, Faculty of Science and Technology,Tokyo University of Science, Tokyo, Japan

b Department of Food Science, Faculty of Health and Nutrition, Tokyo SeieiCollege, Tokyo, Japan

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Scheme 1. Schematic figure for the synthesis of W-POSS.

addition of triethylamine (0.86 g, 8.5 mmol). After the mixture wasrefluxed for 2 h, the solvents were removed on a rotary evaporator.The residue was extracted with THF (20 ml), filtered and pouredinto methanol to give a white powder (111 mg, 34%) of W-POSS.

Preparation of Coating Films

In a 200 ml two-necked flask with a reflux condenser were placed(HSiO3/2)8 (300 mg, 710 µmol), water (0.026 g, 1.4 mmol), THF(30 ml) and benzene (30 ml) under a nitrogen atmosphere. To thissolution was added N,N-diethylhydroxylamine (1.0 µl, 9.7 µmol),and the mixture was stirred at 0 ◦C for 90 min. The solvents wereremoved on a rotary evaporator and the residue was dilutedwith THF to 10 wt%. To this solution, water (10 µg) and N,N-diethylhydroxylamine (10 µl) were added and this solution wasstirred at 0 ◦C for 30 min.

Coating films were prepared by spin-coating on a silicon wafer(30 s, 2000 rpm) followed by heating in an electrical furnace for1 h under air atmosphere at 100–800 ◦C.

Preparation of Free-standing Films

In a 200 ml two-necked flask with a reflux condenser were placed(HSiO3/2)8 (300 mg, 710 µmol), water (0.026 g, 1.4 mmol), THF(30 ml) and benzene (30 ml) under a nitrogen atmosphere. To thissolution was added N,N-diethylhydroxylamine (1.0 µl, 9.7 µmol),and the mixture was stirred at 0 ◦C for 90 min. The solvents wereremoved on a rotary evaporator and the residue was dilutedwith THF to 10 wt%. To this solution, water (10 µg) and N,N-diethylhydroxylamine (10 µl) were added and this solution wasstirred at 0 ◦C for 30 min.

The process above was repeated 10 times and the solutionwas collected. This solution was poured into a sharle made frompolymethylpentane and subjected to aging at room temperaturefor several days to evaporate solvent.

Measurements

Gel permeation chromatography was carried out by ShimadzuLD-10AD with two Polymer Laboratory Mixed-D 250 × 20 mmcolumns and a refractive index detector. THF was used as aneluent. Molecular weights were calculated based on standardpolystyrene.

The 29Si NMR spectra were recorded using a Jeol ECP-500 (29Siat 99 MHz) spectrometer. Chemical shifts were reported as δ units(ppm) relative to SiMe4.

The Fourier transform infrared (FTIR) spectra were measuredusing a Jasco FT/IR-6100 IR spectrophotometer using the KBr disk

method or CCl4 solution method. Differential thermogravimetricanalysis (TG-DTA) was performed using MAC Science TG-DTA2020Sunder an air atmosphere.

BET surface area was measured using a Shimadzu Gemini 2360.Samples were degassed by heating under an nitrogen atmosphereto 100 ◦C for 1 h and then cooling to room temperature beforemeasurement.

The pencil-hardness was tested using a Yasuda Seiki Seisakushoelectric system pencil hardness tester no. 533-M1 according toJapanese Industrial Standard JIS-K5400. The hardness was evalu-ated in the increasing order of 6B, 5B, 4B, 3B, 2B, B, HB, F, H, 2H, 3H,4H, 5H, 6H, 7H, 8H and 9H using the Mitsubishi Pencil Uni series.

Results and Discussion

Results of the Synthesis of W-POSS

The results of the synthesis of W-POSS are summarized inTable 1. W-POSS was synthesized by the same procedure bychanging the molar ratio of water to (HSiO3/2)8 to 1, 2 or 4followed by the end-capping of the terminal hydroxy groupswith chloro(trimethyl)silane. The progress of the dehydrogenativereaction was monitored by the evolution of hydrogen gas whendiethylhydroxylamine was added to the system. W-POSS wasisolated as a white gel or solid by reprecipitation from methanol.White gel was recovered when the molar ratio of water to(HSiO3/2)8 was 1. When the molar ratio of water to (HSiO3/2)8

was 2, the yield of W-POSS was 34% and the weight-averaged

Table 1. Results of the preparation of W-POSSa)

Molecular weight by GPCb)

Molar ratio ofwater/POSS Yield/% Mw Mw/Mn Td5

c)

1 – d) – – –

2 34 29,000 2.0 512

4 42 15,000 1.9 539

a) Scale in operation: POSS 0.30 g (0.71 mmol), THF 30 mL, benzene40 mL, diethylhydroxylamine (8 µL, 80 µmol). Time: 2 h. Temp.: r.t.Silylation: chloro(trimethyl)silane (7.1 mmol, 14 mmol), triethylamine(7.1 mmol, 14 mmol). Time: 1 h. Temp: r.t.b) Calculated based on standard polystyrene.c) Temperature of the 5% weight loss. Measured by thermogravimetry;10 ◦C/min, under air atmosphere.d) State: gel.

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Figure 1. FTIR spectrum of W-POSS.

Figure 2. 29Si NMR spectrum of W-POSS.

molecular weight (Mw) was 29 000. The yield was 42% and Mw was15 000 when the molar ratio was 4. W-POSS solids were solublein THF, diethyl ether, chloroform, carbon tetrachloride, benzene,acetone and hexane, and insoluble in methanol.

The 5% mass loss temperatures (Td5) and ceramic yield weredetermined by thermogravimetric analysis. When the molar ratioof water to (HSiO3/2)8 was 2, Td5 was 512 ◦C and the ceramic yieldat 1000 ◦C was 90%; they were 539 ◦C and 90%, respectively, whenthe molar ratio of water to (HSiO3/2)8 was 4. The relatively high Td5

shows the high thermal stability of W-POSS. The weight loss mainlystems from the combustion of the trimethylsilyl group in W-POSS.

The FTIR spectrum of W-POSS is shown in Fig. 1. Signals fromνC – H (ca. 3000 cm−1), νSi – H (2300 cm−1), νSi – O – Si (ca. 1100 cm−1)and νO – Si – O (ca. 450 cm−1) were observed, while the absorptionpeak owing to the hydroxy group was not observed. Theappearance of νC – H supports the formation of hydroxy group andthe following trimethylsilylation. The remaining νSi – H suggeststhat all of the hydrosilyl groups are not reacted with water.

The 29Si NMR spectrum of W-POSS is shown in Fig. 2. The signalsat around 12, −83 and −109 ppm were assigned to the Me3SiO(M), HSiO3 (T) and SiO4 (Q) units, respectively. The appearanceof the signal owing to the Q unit supports the progress of thedehydrogenative reaction to form a siloxane network, while thesignal owing to the T unit suggests the presence of a remaininghydrosilyl group in W-POSS. The peak areas of M, T and Q signalswere calculated to be 22, 33 and 45%, respectively, when themolar ratio of water to (HSiO3/2)8 was 2. The composition of(HSiO3/2)8 and water in W-POSS was calculated to be 1 : 1.18,which suggests that (HSiO3/2)8 reacts as difunctional monomerto form a pseudo-linear polymer of W-POSS. The composition of(HSiO3/2)8 and water was increased to 1 : 1.53 when the molarratio of water to (HSiO3/2)8 was increased to 4. Although weexpected there to be complete consumption of the hydrosilyl

Figure 3. FTIR spectra of W-POSS silica gels on heat treatment.

groups in (HSiO3/2)8 by water, the Mw decreased with increasingthe molar ratio of water. The reaction between the hydrosilylgroups and water probably became less favorable in responseto increasing steric hindrance owing to the (HSiO3/2)8 moiety. Inaddition, the reaction between two silanols is not favored in thepresence of diethylhydroxylamine to decrease Mw.

Results of the Preparation of Films and Silica Gels,and Free-standing Film

Coating films were prepared using the reaction mixture of(HSiO3/2)8 with water. Therefore, unreacted or remaining waterwould contribute to the formation of the films. Starting fromthe (HSiO3/2)8 –water systems, transparent coating films wereprepared with a sub-micrometer thickness.

The pencil-hardness of coating films was evaluated by pencil-hardness tests. The pencil-hardness changed in the order of <6B,<6B, 4B, HB, 5H, 7H, and 8H by heating at 100, 200, 300, 400,500, 600 and 700 ◦C, respectively. The FTIR spectra of coating filmare shown in Fig. 3. After heating at 100 ◦C, some characteristicabsorption bands were observed owing to νCH, νSiH and νSi – O – Si .The absorption band owing to νSiOH disappeared, and the intensityof the absorption band owing to νSiH was decreased after heatingat 300 ◦C. The absorption bands from νCH and νSiH disappeared,and a weak absorption band from δSi – O – Si appeared at 500 ◦C.The absorption band from νSiOH appeared again, and the intensityof the absorption band owing to δSiOSi was increased at 600 ◦C.These spectral changes correspond to the pencil-hardness of thecoating films, which is based on the thermal behavior of functionalgroups: the increase in the pencil hardness can be ascribed to theoxidation and condensation of hydrosilyl groups to form siloxanenetworks, consistent with the formation of silanol groups andsiloxane bondings between 400 and 500 ◦C.

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Figure 4. Nitrogen adsorption–desorption isotherm of W-POSS, calcinedat 400 ◦C.

Figure 5. Pore size distribution of W-POSS, calcined at 400 ◦C.

Figure 6. Photograph of the free-standing film of W-POSS.

Silica gels were prepared by drying and heating the coatingsolutions. The BET surface areas were 470 m2 g−1 at 100 ◦C,470 m2 g−1 at 200 ◦C, 550 m2 g−1 at 400 ◦C, 510 m2 g−1 at 650 ◦C,and 280 m2 g−1 sintered at 800 ◦C. The maximum was observed at400 ◦C. On heating at 400 ◦C, a stiff silica network was formed by theoxidative condensation of hydrosilyl group, which resulted in theformation of porous silica gels. When the silica gels were sintered at800 ◦C, this probably allowed the newly formed siloxane bridgesto be densified and form a dense silica network. The nitrogenadsorption–desorption isotherm and pore size distribution of W-POSS, calcined at 400 ◦C, are shown in Figs 4 and 5, respectively.

The isotherm was type I. The pore size was mainly distributed lessthan 2 nm, indicating that W-POSS is a microporous material.

Free-standing films of W-POSS were prepared successfully asshown in Fig. 6. The films were highly transparent and rigid. Thefilm had a large BET surface area of 480 m2 g−1, comparable tothose of calcined powder of W-POSS.

Conclusions

POSS polymer, W-POSS, was synthesized by the dehydrogenativecondensation reaction of (HSiO3/2)8 with water in the presenceof diethylhydroxylamine followed by trimethylsilylation. Theprogress of the dehydrogenative reaction was confirmed byinfrared spectroscopy and 29Si nuclear magnetic resonance of thepolymers.

Coating films were prepared by spin-coating of the polymersolutions, which were prepared by the dehydrogenative conden-sation of (HSiO3/2)8 with water. The hardness of the coating filmswas evaluated by a scratch test, with the hardness increasing to 8Hwith increased sintering temperature. In addition, silica gels wereprepared by sintering the products prepared by concentrating thecoating solution. These silica gels showed a relatively high surfacearea even at 100 ◦C, then a maximum surface area at 400 ◦C. Thesurface area trend upon sintering showed good agreement withthe formation of siloxane networks in response to oxidation ofhydrosilyl groups and the formation of a dense silica network onheating.

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