Preparation and properties of polyhedral oligomeric silsesquioxane polymers

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  • 661

    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)8withwater in thepresenceofdiethylhydroxylaminefollowedbytrimethylsilylation.Coatingfilmswerepreparedbyspin-coatingof thecoatingsolutionpreparedby thedehydrogenative condensationofPOSS.Thehardnessof thecoatingfilmswasevaluatedusing 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 440m2 g1 at 100 C, which showed a maximum at400 C as 550m2 g1. Copyright c 2011 JohnWiley & 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.[113]

    Polyhedral oligomeric silsesquioxanes (POSS) are the best-known and most useful polysiloxanes and have the formula(RSiO3/2)8. SincePOSShaveaporeat thecenterof themoleculeandare composed of 12 SiOSi 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)3nMenSiO]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 SiCO 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 , -dihydroxypolydimethylsiloxanes[18] to show a relatively high heatresistivity and high surface area on heating. In the sameway, POSSpolymer (W-POSS) was synthesized by the reaction of (HSiO3/2)8with 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 andmechanical 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 mol), water (0.026 g, 1.4 mmol),tetrahydrofuran (THF; 30 ml) andbenzene (40 ml) under anitrogenatmosphere. To this solutionwasaddedN,N-diethylhydroxylamine(1.0 l, 9.7 mol), and the mixture was stirred at 0 C for 90 min.Chloro(trimethyl)silane (0.92 g, 8.5 mmol) was added, and themixturewasstirredat roomtemperature for30 min followedbythe

    Correspondenceto:Takahiro Gunji,DepartmentofPureandAppliedChemistry,Faculty of Science and Technology, Tokyo University of Science, Tokyo, Japan.E-mail: gunji@rs.noda.tus.ac.jp

    a Department of PureandAppliedChemistry, Faculty of ScienceandTechnology,Tokyo University of Science, Tokyo, Japan

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

    Appl. Organometal. Chem. 2011, 25, 661664 Copyright c 2011 John Wiley & Sons, Ltd.

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    T. Shioda et al.

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

    addition of triethylamine (0.86 g, 8.5 mmol). After themixture wasrefluxed for 2 h, the solventswere removedon 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 mol), 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 l, 9.7 mol),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 g) and N,N-diethylhydroxylamine (10 l) 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 100800 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 mol), 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 l, 9.7 mol),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 g) and N,N-diethylhydroxylamine (10 l) 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 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)wasperformedusingMACScienceTG-DTA2020Sunder an air atmosphere.

    BET surface area was measured using a Shimadzu Gemini 2360.Samplesweredegassedbyheatingunder annitrogenatmosphereto 100 C for 1 h and then cooling to room temperature beforemeasurement.

    The pencil-hardnesswas 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)8was 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 Td5c)

    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 L, 80 mol). 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.

    wileyonlinelibrary.com/journal/aoc Copyright c 2011 John Wiley & Sons, Ltd. Appl. Organometal. Chem. 2011, 25, 661664

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    Polyhedral oligomeric silsesquioxane polymers

    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% andMw 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 Cwas 90%; theywere 539 C and 90%, respectively, whenthemolar ratio of water to (HSiO3/2)8 was 4. The relatively high Td5shows thehigh thermal stability ofW-POSS. Theweight lossmainlystems from the combustion of the trimethylsilyl group in W-POSS.

    The FTIR spectrum of W-POSS is shown in Fig. 1. Signals fromCH (ca. 3000 cm1), SiH (2300 cm1), SiOSi (ca. 1100 cm1)and OSiO (ca. 450 cm1) were observed, while the absorptionpeak owing to the hydroxy group was not observed. Theappearance of CH supports the formation of hydroxy group andthe following trimethylsilylation. The remaining SiH suggeststhat all of the hydrosilyl groups are not reacted with water.

    The 29Si NMR spectrumofW-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 decreaseMw.

    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

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    T. Shioda et al.

    Figure 4. Nitrogen adsorptiondesorption 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 g1 at 100 C,470 m2 g1 at 200 C, 550 m2 g1 at 400 C, 510 m2 g1 at 650 C,and 280 m2 g1 sintered at 800 C. Themaximumwas observed at400 C.Onheatingat400 C,astiff silicanetworkwas formedbytheoxidative condensation of hydrosilyl group, which resulted in theformationofporous silicagels.When the silicagelswere sinteredat800 C, this probably allowed the newly formed siloxane bridgesto be densified and form a dense silica network. The nitrogenadsorptiondesorption 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 g1, 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.

    References

    [1] J. D. Lichtenhan, Comm. Inorg....

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