nanostructures and surface dewettability of epoxy thermosets containing hepta(3,3,3-trifluoropropyl)...

Nanostructures and Surface Dewettability of Epoxy Thermosets ContainingHepta(3,3,3-trifluoropropyl) Polyhedral Oligomeric Silsesquioxane-Capped Poly(ethyleneOxide)

Ke Zeng and Sixun Zheng*Department of Polymer Science and Engineering, Shanghai Jiao Tong UniVersity,Shanghai 200240, People’s Republic of China

ReceiVed: July 26, 2007; In Final Form: October 5, 2007

Hepta(3,3,3-trifluoropropyl) polyhedral oligomeric silsesquioxane (POSS)-capped poly(ethylene oxide) (PEO)was synthesized via the reaction of hydrosilylation between hepta(3,3,3-trifluoropropyl)hydrosilsesquioxaneand allyl-terminated PEO. The POSS-capped PEO was characterized by means of Fourier transform infrared(FTIR) and nuclear magnetic resonance (NMR) spectroscopy. The organic-inorganic amphiphile wasincorporated into epoxy resin to prepare the organic-inorganic nanostructured thermosetting composites.The morphology of the hybrid composites was characterized with field emission scanning electronic microscopy(FESEM) and transmission electronic microscopy (TEM). The formation of nanostructures was addressed onthe basis of miscibility and phase behavior of the sub-components (viz. POSS and PEO chains) of the organic-inorganic amphiphile with epoxy after and before curing reaction. The static contact angle measurementsindicate that the organic-inorganic nanocomposites displayed a significant enhancement in surfacehydrophobicity as well as reduction in surface free energy. The atomic force microscopy (AFM) showed thatthere is significant migration of the POSS moiety at the surface of the thermosets. The improvement in surfaceproperties was ascribed to the enrichment of the POSS moiety on the surface of the nanostructured thermosets,which was evidenced by X-ray photoelectron spectroscopy (XPS).

Introduction

Organic-inorganic hybrid composites have provoked con-siderable interests since they can combine the advantages ofinorganic materials with those of organic polymers1. It has beenreported that organic-inorganic nanocomposites can be preparedvia sol-gel process2, intercalation and exfoliation of layeredsilicates by organic polymers.1a-d It is well-known that thefavorable combination of properties from inorganic and organiccomponents requires the optimum dispersion of inorganicsegments in the polymeric matrix. In the sol-gel process, thefine dispersion of inorganic nanoparticles is fulfilled viahydrolysis and condensation of silanes bearing organic ligandsso that the affinity can be formed between polymers andinorganic components. For preparation of layered silicatescontaining nanocomposites, the pristine inorganic layeredsilicates must be organically modified to achieve intercalationor exfoliation of layers of silicates.

Polyhedral oligomeric silsesquioxane (POSS) reagents, mono-mers, and polymers are emerging as a new chemical technologyfor the organic-inorganic hybrid nanocomposites.1e-g A typicalPOSS molecule possesses the structure of cube-octamericframework represented by the formula (R8Si8O12) with aninorganic silica-like core (Si8O12; ∼0.53 nm in diameter)surrounded by eight organic corner groups, one or more of whichis reactive (Scheme 1). With the functional groups, POSS canbe incorporated into organic polymers via copolymerization orreactive grafting approaches. During the past years, there hasbeen the ample literature on the POSS-containing nanocom-

posites. A variety of POSS macromers have been incorporatedinto thermosetting polymer systems to prepare the nanocom-posites with enhanced thermomechanical properties.3 Lee andLichtenhan3a reported that the molecular level reinforcementprovided by a monofunctional POSS could significantly retardthe physical aging process of epoxy thermosets in the glassystate. Laine et al.3b-g investigated the modifications of epoxyby a series of octasilsesquioxanes with a variety of reactive Rgroups such as aminophenyl and dimethylsiloxypropylglycidylether groups and found that the dynamic mechanical properties,fracture toughness, and thermal stability of the epoxy hybridswere closely dependent on the types of R groups, tetherstructures between epoxy matrices and POSS cages, and thedefects in silsesquioxane cages and so forth. Williams et al.3h

observed that a primary liquid-liquid phase separation occurredat the time of adding the POSS-diamine precursors to epoxybecause of the incompatibility between epoxy and heptaisobutylglycidyl POSS. It is noted that the nature of the organic inertgroups and prereaction of a monofunctional POSS havepronounced impact on the morphology of the resulting POSS-modified polymer networks.3i Zheng et al.3j reported that thedifferent morphological structures could be formed in the POSS-containing hybrid composites depending on the types of Rgroups; moreover, the phase-separated composites and nano-composites could be prepared by adjusting the degrees ofreaction between epoxy matrix and POSS macromer. Matejkaet al.3k,l investigated the structure and properties of epoxynetworks reinforced with POSS, and the effects of POSS-POSSinteractions on the thermal properties were addressed. All ofthe previous studies reported the modification of epoxy ther-mosets using POSS macromers via the chemical reaction

* To whom all correspondence should be addressed. E-mail: [email protected]. Phone:+86-21-54743278. Fax:+86-21-54741297.

13919J. Phys. Chem. B2007,111,13919-13928

10.1021/jp075891c CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 11/22/2007

between polymer matrix and POSS. Relatively, the nanocom-posites prepared via direct physical blending were less reportedpossibly because of the unfavorable miscibility (or solubility)of silsesquioxanes with polymers. The immiscibility could beresponsible for the big difference in solubility parameters.Nonetheless, it is proposed that the nanostructured compositescontaining POSS can be prepared via incorporation of POSS-containing macromolecular amphiphile into organic polymersvia the self-assembly approach. To the best of our knowledge,the control of nanostructures in thermosets using POSS-cappedmacromolecular amphiphile has been scarcely reported.3n

In the present work, we designed and synthesized hepta(3,3,3-trifluoropropyl) polyhedral oligomeric silsesquioxane (POSS)-capped poly(ethylene oxide) (PEO). The POSS-capped PEO isa novel tadpole-like amphiphile composed of an organic “tail”(viz. PEO chain) and inorganic “head” (viz. silsesquioxane cage)portions. The organic-inorganic amphiphile was incorporatedinto epoxy to prepare the organic-inorganic hybrid nanocom-posites. It is expected that the formation of the nanostructureswill follow the mechanism of self-assembly since it has beenknown that (i) the PEO subchain is miscible with epoxy afterand before the curing reaction4 and (ii) the POSS is immisciblewith epoxy resin after and before the curing reaction. In addition,the POSS portion combines the features of both organosiliconand organofluorine compounds and is of low free energy;5 thus,the enrichment of the POSS portion at the surface could occurupon incorporating the organic-inorganic amphiphile intoorganic polymer, and the surface hydrophobilicity (or dewet-tability) of materials will significantly be enhanced.6 In thiswork, atomic force microscopy (AFM), transmission electronmicroscopy (TEM), and differential scanning calorimetry (DSC)were used to investigate the morphology of the thermosets, andthe surface properties of the nanostructured thermosets were

addressed on the basis of static contact angle measurements andX-ray photoelectron spectroscopy (XPS).

Experimental Section

Materials. The epoxy monomer used in this work isdiglycidyl ether of bisphenol A (DGEBA) and was obtainedfrom Shanghai Resin Co., China. Its epoxide equivalent weightwas measured to be 177 g mol-1. 4,4′-Methylenebis(2-chlo-roaniline) (MOCA) was of chemically pure grade, used as thecuring agent, and purchased from Shanghai Reagent Co., China.3,3,3-Trifluoropropyltrimethoxysilane was purchased from Zhe-jiang Chem-Technology Co., China. Trichlorosilane was ob-tained from Shanghai Lingguang Chemical Co., China, and wasused as received. Unless specially indicated, other reagents suchas sodium, calcium hydride (CaH2), and sodium hydroxide wereof chemically pure grade, purchased from Shanghai ReagentCo., China. The solvents such as tetrahydrofuran (THF), dich-loromethane, petroleum ether (distillation range: 60∼90 °C),and triethylamine (TEA) were of chemically pure grade andalso obtained from commercial resources. Before use, THF wasrefluxed above sodium and then distilled and stored in thepresence of the molecular sieve of 4 Å. Triethylamine (TEA)was refluxed over CaH2 and filtered. After that, the TEA wastreated withp-toluenesulfonyl chloride, followed by distillation.

Synthesis of Hepta(3,3,3-trifluoropropyl)propylhydrosil-sesquioxane [(CF3CH2CH2)7Si8O12H]. First, hepta(3,3,3-trif-luoropropyl)tricycloheptasiloxane trisodium silanolate [Na3O12-Si7(C3H4F3)7] was synthesized by following the method reportedby Fukuda et al.5a Typically, 50 g (0.23 mol) (3,3,3-trifluoro-propyl)trimethoxysilane, 250 mL THF, 5.25 g (0.29 mol)deionized water, and 3.95 g (0.1 mol) sodium hydroxide werecharged into a flask equipped with a condenser and a magneticstirrer. The mixture was refluxed for 5 h, then cooled down to

SCHEME 1: Synthesis of Hepta(3,3,3-trifluropropy) POSS-Capped PEO

13920 J. Phys. Chem. B, Vol. 111, No. 50, 2007 Zeng and Zheng

room temperature, and stirred for 15 h. The solvent and othervolatiles were removed by rotary evaporation; the products weredried at 40°C in a vacuum oven for 12 h, and 37.3 g of productwas obtained with the yield of 98%. Hepta (3,3,3-trifluoropro-pyl)propylhydrosilsesquioxane was synthesized via the reactionbetween Na3O12Si7(C3H4F3)7 and trichlorosilane. Typically, 10g (8.8 mmol) Na3O12Si7(C3H4F3)7 and 1.3 mL (8.8 mmol)triethylamine were charged to a 500 mL single-necked flaskequipped with a magnetic stirrer; 200 mL of anhydrous THFwere added to the flask with vigorous stirring. The flask wasimmersed into an ice-water bath, and 1.43 g (10.56 mmol)trichlorosilane (which was dissolved in 20 mL of anhydrousTHF) were slowly dropped within 30 min. The mixture wasmaintained at 0°C for 4 h and then at room temperature for 20h. The system was filtrated to remove the solid. The solventand the volatile were removed via rotary evaporation at 40°Cto obtain white powder, which was further dried at 40°C in avacuum oven for 24 h, and 7.3 g of product was obtained withthe yield of 76%. Solid state29Si NMR (Q8M8): -68.6 ppm(O-Si-O), -58.3 ppm (O-Si-H). FTIR (cm-1, KBr win-dow): 1090∼1000 (Si-O-Si), 2900∼2850 (-CH2), 1120∼1300(-CF3), 2262 (Si-H). 1H NMR (ppm, acetone-d6): 4.30 (s,1.0H, Si-H), 2.32 (m, 14.0H, SiCH2H2CF3), 1.03 (m, 14.0H,SiCH2H2CF3).

Synthesis of Allyl-Terminated PEO. The allyl-terminatedPEO was synthesized by following the literature method withslight modification.7 In a typical experiment, 1.46 g of NaHand 50 mL of anhydrous THF were charged into 250 mL single-necked flask equipped with magnetic stirrer; 13.4 g of PEOmonomethyl ether that was dissolved in 100 mL of anhydrousTHF was added to the flask. The mixture was stirred at roomtemperature for 3 h, and then 5.000 g of allyl bromide (whichwas dissolved in 30 mL of anhydrous THF) was slowly dropped.The reaction was carried out at room temperature for 24 hto access a complete reaction. The salts (i.e., NaBr and theexcessive NaH) were removed by filtration, and the solutionwas concentrated with rotary evaporation and dropped into alarge amount of petroleum ether to precipitate the product. Theprecipitates were dissolved in THF and then reprecipitated inpetroleum ether. The procedure was repeated for three times,and 12.8 g of solids were obtained at the yield of 92%. FTIR(cm-1, KBr window): 1150∼1060 (C-O-C), 1640 (CH2dCH), 2900∼2850 (-CH2). 1H NMR (ppm, CDCl3): 5.90 (m,1.0H, CH2dCHs), 5.27, 5.16 (m, 2.0H, CH2dCHs), 4.01(d, 2.0H, CH2dCHsCH2s), 3.63 (m, 96.6H,-O-CH2CH2-O-), 3.36 (s, 3.0H, -O-CH3).

Synthesis of POSS-Capped Poly(ethylene Oxide) (POSS-Capped PEO). The POSS-capped PEO was synthesized viathe hydrosilylation reaction between hepta(3,3,3-trifluoropro-pyl)trifluoropropylhydrosilsesquioxane and allyl-terminated PEO,which was catalyzed by the Karstedt catalyst. Typically, 1.2000g (1.095 mmol) (CF3CH2CH2)7Si8O12H and 1.0000 g (0.877mmol) allyl-terminated PEO were charged into a round-bottomflask equipped with a magnetic stirrer. The flask was connectedto a standard Schlenk line. The system was dried by exhausting-refilling process using highly pure nitrogen for three times. Thereaction system was heated up to 60°C, at which the reactionwas allowed to perform for 48 h to ensure that the hydrosily-lation reaction went to completion. After that, the solvent waseliminated with rotary evaporation, and the reacted product wasdropped into 100 mL methanol and stored in a refrigerator forrecrystallization. The recrystallization was repeated at least threetimes to purify the product. After being dried in vacuo, 1.6 gof product was obtained at the yield of 82%. FTIR (cm-1, KBr

window): 1090∼1000 (Si-O-Si), 2900∼2850 (-CH2),1120∼1300 (-CF3), 1150∼1060 (C-O-C). 1H NMR (ppm,acetone-d6): 2.32 (m, 14.0H, SiCH2H2CF3); 1.03 (m, 14.0H,SiCH2H2CF3); 3.47 (m, 2.0H, SiCH2CH2CH2-PEO); 1.30 (m,2.0H, SiCH2CH2CH2-PEO); 1.00 (m, 2.0H, SiCH2CH2CH2-PEO); 3.58 (m, 96H, -O-CH2CH2-O-); 3.28 (s, 3.0H, -O-CH3).

Preparation of Epoxy Thermosets Containing POSS-Capped PEO.The desirable amount of POSS-capped PEO wasmixed with DGEBA with continuous stirring at 60°C until thehomogeneous mixture was obtained. After that, the stoichio-metric MOCA with respect to DGEBA was added to the systemwith vigorous stirring. The mixtures were degassed in a vacuumoven and poured into an aluminum foil. The mixtures were curedat 150°C for 3 h plus 180°C for 2 h. The organic-inorganichybrid epoxy thermosets were prepared with the content ofPOSS-capped PEO up to 20 wt %. The specimens for surfaceanalysis were prepared via the solution casting approach.Typically, the desired amount of POSS-capped PEO was addedto the stoichiometric mixture of DGEBA with MOCA at roomtemperature, and the smallest amount of THF was added to thesystem to obtain a homogeneous solution. The solution was spin-coated on clean silicon wafers (dimension: 1× 1 cm2) at 1500r/min, and the film specimens were obtained at the thicknessof about 20µm. After the majority of THF was evaporated, theresidual solvent was eliminated in vacuo at 40°C for 48 h. Thesamples were cured at 150°C for 3 h and 180°C for 2 h.

Techniques and Measurement.Nuclear Magnetic Reso-nance Spectroscopy (NMR). The 1H and 29Si NMR measure-ments were carried out on a Varian Mercury Plus 400 MHzNMR spectrometer. For the1H NMR measurement, the sampleswere dissolved with deuterated acetone (acetone-d6), and thesolutions were measured with tetramethylsilane (TMS) as theinternal reference. The high-resolution solid-state29Si NMRspectra were obtained using cross polarization (CP)/magic anglespinning (MAS) together with the high-power dipolar decou-pling (DD) technique. The 90° pulse width of 4.1µs wasemployed for free induction decay (FID) signal accumulation,and the cross polarization (CP) Hartmann-Hahn contact timewas set at 3.5 ms for all of the measurements. The rate of MASwas 4.0 kHz for measuring the spectra. The Hartmann-HahnCP matching and dipolar decoupling field was 57 kHz. Thechemical shifts of29Si CP/MAS NMR spectra were determinedby taking the silicon of solid Q8M8 relative to TMS as anexternal reference standard.

Fourier Transform Infrared Spectroscopy (FTIR). The FTIRmeasurements were conducted on a Perkin-Elmer Paragon 1000Fourier transform spectrometer at room temperature (25°C).The sample film was prepared by casting the THF solution ofthe polymer (5 wt %) onto KBr windows. After the majority ofsolvent was evaporated, the residual solvent was removed in avacuum oven at 60°C for 2 h. The ground samples of thermosetswere mixed with the powder of KBr and then pressed into thesmall flakes. All of the specimens were sufficiently thin to bewithin a range where the Beer-Lambert law is obeyed. In allcases, 64 scans at a resolution of 2 cm-1 were used to recordthe spectra.

X-ray Photoelectron Spectroscopy (XPS).The XPS wasmeasured with VG ESCALAB MK II at room temperature byusing an Mg KR X-ray source (hν ) 1253.6 eV) at 14 kV and20 mA. The sample analysis chamber of the XPS instrumentwas maintained at a pressure of 1× 10-7 Pa. To determine thecomposition on the top layer of the film, the tilting angle is 5°.The binding energies were calibrated by using the containment

Nanostructures and Surface Dewettability J. Phys. Chem. B, Vol. 111, No. 50, 200713921

carbon (C 1s) 284.6 eV). A linear background methodremoved the XPS background, and the peaks analysis wascarried out by using the curve-fitting software.

Surface Contact Angle Analysis.The flat free surfaces of thethermosets were used for the measurement of contact angle.The static contact angle measurements with the probe liquids(i.e., ultrapure water and ethylene glycol) were carried out ona KH-01-2 contact angle measurement instrument (BeijingKangsente Scinetific Instruments Co., China) at room temper-ature. The samples were dried at 60°C in a vacuum oven for24 h prior to measurement.

Atomic Force Microscopy (AFM).The AFM experimentswere performed with a Nanoscope IIIa scanning probe micro-scope (Digital Instruments, Santa Barbara, CA). Contactingmode was employed in air using a tip fabricated from silicon(125µm in length with ca. 500 kHz resonant frequency). Typicalscan speeds during recording were 0.3∼1 lines× s-1 using scanheads with a maximum range of 5µm × 5 µm.

Transmission Electronic Microscopy (TEM). The TEM wasperformed on a JEM 2010 high-resolution transmission electronmicroscope at the accelerating voltage of 200 kV. The sampleswere trimmed using an ultramicrotome, and the specimensections (ca. 70 nm in thickness) were placed in 200 meshcopper grids for observation.

Scanning Electron Microscopy (SEM). In order to observethe morphological structures, the thermosets were fracturedunder cryogenic condition using liquid nitrogen. The etchedspecimens were dried to remove the solvents. The fracturesurfaces were coated with thin layers of gold of about 100 Å.The thermosets were observed by means of a JEOL JSM 7401Ffield emission scanning electron microscope (FESEM) at anactivation voltage of 5 kV.

Differential Scanning Calorimetry (DSC). The DSC measure-ment was performed on a Perkin-Elmer Pyris-1 thermal analysisapparatus in a dry nitrogen atmosphere. The instrument wascalibrated with standard indium. All samples (about 10 mg inweight) were heated from 0 to 200°C, and the thermogramswere recorded using a heating rate of 20°C/min-1. The meltingpoint (Tm) and glass transition temperature (Tg) were taken asthe temperatures at the maximum of the endothermic transitionand the midpoint of the capacity change, respectively.

ThermograVimetric Analysis (TGA). A Perkin-Elmer TGA-7thermal gravimetric analyzer was used to investigate the thermalstability of the nanocomposites. All of the thermal analyses wereconducted in nitrogen atmosphere from ambient temperatureto 800 °C at the heating rate of 20°C/min-1. The thermaldegradation temperature was taken as the onset temperature atwhich 5 wt % of weight loss occurs.

Results and Discussion

Synthesis of POSS-Capped Poly(ethylene Oxide).The routeof synthesis for hepta(3,3,3-trifluoropropyl)POSS-capped PEOis described in Scheme 1. To prepare hepta(3,3,3-trifluoropro-pyl)hydrosilsesquioxane (Figure 1), hepta(3,3,3-trifluoropropyl)-tricycloheptasiloxane trisodium silanolate [Na3O12Si7(C3H4F3)7]was prepared by following the method reported by Fukuda etal.6a Na3O12Si7(C3H4F3)7 was used to react with trichlorosilane,to afford hepta(3,3,3-trifluoropropyl)hydrosilsesquioxane. Theresonance signals at-68.6 and-58.3 ppm in the29Si NMRspectrum (See Figure 2) indicates that the cubic silsesquioxanewas successfully obtained.6a The characteristic bands of hepta-(3,3,3-trifluoropropyl)hydrosilsesquioxane at 2262 and 1119cm-1 assignable to the stretching vibration of Si-H and Si-O-Si bonds were clearly exhibited in the FTIR spectrum (not

shown here for brevity). In terms of the silicon resonance inthe29Si NMR spectrum and the ratio of integration intensity ofSi-H proton to those of 3,3,3-trifluropropyl groups in1H NMRspectrum (see Figure 1), it is judged that hepta(3,3,3-trifluoro-propyl)hydrosilsesquioxane was successfully prepared. The allyl-terminated PEO was prepared via the reaction between ahydroxyl-terminated PEO monomethyl ether and allyl bromidein the presence of sodium hydride. In order to promote thecomplete conversion of the terminal hydroxyl groups of PEOinto allyl groups, excessive allyl bromide was used. The1HNMR spectrum shows that the allyl-terminated PEO is suc-cessfully obtained (see Figure 3). The allyl-terminated PEO wasfurther employed to prepare the POSS-capped PEO via itsreaction of hydrosilylation with hepta(3,3,3-trifluoropropyl)-hydrosilsesquioxane as depicted in Scheme 1. The reaction ofhydrosilylation between allyl-terminated PEO and hepta(3,3,3-trifluoropropyl)hydrosilsesquioxane was carried out in thepresence of Karstedt catalyst.7b According to the difference insolubility, the excessive hepta(3,3,3-trifluoropropyl)hydrosils-esquioxane was easily isolated from the reacted mixtures. Shownin Figure 4 is the1H NMR spectrum of the POSS-capped PEO.It is seen that the resonance of methylene protons (CH2) of PEOchains and 3,3,3-trifluropropyl groups appeared at 3.59, 2.32,and 1.04 ppm, respectively. According to the ratio of integration

Figure 1. 1H NMR spectrum of hepta(3,3,3-trifluropropyl)hydrosils-esquioxane.

Figure 2. 29Si NMR spectrum of hepta(3,3,3-trifluropropyl)hydrosils-esquioxane. SSB stands for spin side bands.


intensity of PEO methylene protons to those of 3,3,3-trifluro-propyl groups, it is judged that hepta(3,3,3-trifluoropropyl)POSS-capped PEO was successfully prepared.

Morphology of Epoxy Containing POSS-Capped PEO.The organic-inorganic amphiphile, POSS-capped PEO wasincorporated into the precursors of epoxy (viz., DGEBA andMOCA), and the mixtures were cured at elevated temperature.Before curing, all of the ternary mixtures composed of DGEBA,POSS-capped PEO, and MOCA were homogeneous, suggestingthat no macroscopic phase separation occurred. After beingcured at the elevated temperatures, the epoxy thermosets withthe POSS-capped PEO contents up to 20 wt % were obtained.It is seen that all of the cured products investigated arehomogeneous and translucent, suggesting that no macroscopicphase separation occurred at the scale exceeding the wavelengthof visible lights with the occurrence of curing reaction. Themorphologies of the thermosets were examined by means offield emission scanning electronic microscopy (FESEM) andtransmission electronic microscopy (TEM).

The fracture ends of the epoxy thermosets containing POSS-capped PEO were observed by means of field emission scanningelectronic microscopy (FESEM). At lower magnifications (e.g.,

5000×), the featureless morphologies (not shown here forbrevity) were observed in all cases, suggesting that no macro-scopic phase separation occurred. Nonetheless, the FESEMmicrographs at the higher magnifications (e.g.,>10 000×)exhibited the morphology of microdomains. Representativelyshown in Figure 5 is a FESEM micrograph of the epoxycontaining 10 wt % POSS-capped PEO. It is seen that theparticles at the size of 10∼100 nm were displayed. It is proposedthat the nanoparticles were formed because of the aggregationof POSS portion since the PEO chains of the organic-inorganicamphiphilic macromolecules are miscible with epoxy matrix.4

The ultrathin sections of the organic-inorganic hybrid thermo-sets were subjected to transmission electronic microscopy(TEM). Shown in the Figure 6 was the TEM micrograph of theepoxy containing 10% POSS-capped PEO. In term of thedifference in transmitted electronic density between organic andinorganic portion (viz., POSS moiety), the dark area is assign-able to the POSS portion. It is seen that the spherical POSSdomains were dispersed in the continuous epoxy matrix withthe average size of approximately 60 nm. The sphericalnanoparticles exhibited a “core-shell” structure. It is proposed

Figure 3. 1H NMR spectrum of ally terminated PEO monomethylether.

Figure 4. 1H NMR spectrum of POSS-capped PEO.

Figure 5. FESEM micrograph of epoxy thermosets containing 10 wt% POSS-capped PEO.

Figure 6. TEM micrograph of epoxy thermosets containing 10 wt %POSS-capped PEO.


that the “shell” is the epoxy-rich layer that is mixed with PEOchains whereas the “core” results from the spherical aggregatesof POSS cages. The results of FESEM and TEM indicate thatthe nanostructured epoxy thermosets containing POSS-cappedPEO were obtained.

Depending on the miscibility of subchains of block copoly-mers with thermosets after and before curing, the formation ofnanostructures in thermosets can be accounted for with self-assembly8 or reaction-induced microphase separation9 mecha-nisms. The self-assembly route is a templating technique whereamphiphilic block copolymers in their mixture with precursorsof thermosets is self-assembled into a variety of micellestructures. The morphology at the nanometer can be fixed viathe subsequent curing reaction of precursor of thermosets. In1997, Bates et al.8a,b proposed this strategy of creating thenanostructures in thermosets using amphiphilic block copoly-mers. In this protocol, precursors of thermosets act as selectivesolvents of block copolymers and some self-assembled mor-phology such as spherical, lamellar, hexagonally packed cylin-drical or cubic structures are formed depending on the compo-sitions of the blends. The micelle or vesicle structures can furtherbe fixed with adding hardeners and subsequent curing. Morerecently, it is proposed that the nanostructured thermosets canalternatively be able to prepare via the approach of reaction-induced microphase separation; that is, the nanostructures canbe accessed by controlling the reaction-induced microphaseseparation of a part of subchain of block copolymers fromcompletely homogeneous solutions of block copolymers withprecursors of thermosets.9 The difference between the aboveapproaches is quite dependent on miscibility and phase behaviorof subchains of block copolymers with thermosets after andbefore curing reaction. In the present case, it is important toknow the miscibility and phase behavior of the sub-components(viz., POSS and PEO chains) of the organic-inorganic am-

phiphilic macromolecule with epoxy after and before curingreaction. In order to examine the miscibility of epoxy withPOSS, hepta(3,3,3-trifluoropropyl)hydrosilsesquioxane was se-lected as the model compound for the POSS terminal group ofthe POSS-capped PEO. It was seen that at room and elevatedtemperature (e.g., 150°C) all of the mixtures composed ofDGEBA, MOCA, and POSS model compound were heteroge-neous and cloudy at room temperature and elevated temperature(e.g., 150 °C); that is, the macroscopic phase separationoccurred. This observation suggests that the POSS is immiscible(or insoluble) with the precursors of epoxy (i.e., DGEBA andMOCA). In contrast, all of the mixtures of epoxy resin withthe model PEO are transparent and homogeneous at 80°C(above the melting point of PEO) and at the curing temperature(i.e., 150°C), implying that no phase separation occurred atthe scale exceeding the wavelength of visible lights. The single,composition-dependent glass transition temperatures (Tg’s) ofthe blends indicate that the aromatic amine-cured epoxy wasfully miscible with the PEO after and before curing.4 Thedifference in miscibility of the sub-constituents of the organic-inorganic amphiphile implies that the formation of nanostruc-tures in the present system will follow a mechanism of self-assembly. It is plausible to propose that, in the mixtures of theprecursors of epoxy with POSS-capped PEO, the precursors ofepoxy act as the selective solvent of POSS-capped PEO andthus the self-organized vesicle (or micelle) nanostructures willbe formed. The vesicle (or micelle) structures were further fixedwith the curing reaction at elevated temperature as depicted inScheme 2.

Thermal Properties of Nanostructured Thermosets.Theabove nanostructured epoxy thermosets were subjected tothermal analyses, and the DSC thermograms are shown in Figure7. In the DSC curve of POSS-capped PEO, an endothermic peakat 30 °C was displayed, which is assignable to the melting

SCHEME 2: Formation of Nanostructures and Surface POSS Enrichment in Epoxy Thermosets ContainingPOSS-Capped PEO


transition of PEO chain of the organic-inorganic amphiphile.It is noted that all of the thermosets investigated did not exhibit

the melting transition of PEO subchains, suggesting that thePEO subchains were not crystallizable in the nanostructuredthermosets. It is plausible to propose that the PEO subchainsare miscible with the epoxy thermosets and could interpenetratewith the crosslinked epoxy networks. The miscibility was furtherevidenced by the depression in glass transition temperatures(Tg’s) for the epoxy-rich phases as shown in Figure 7. It is seenthat each thermoset containing POSS-capped PEO displayed

Figure 7. DSC curves of control epoxy, POSS-capped PEO, and thenanocomposites containing POSS-capped PEO.

Figure 8. TGA curves of control epoxy, POSS-capped PEO, and thenanocomposites containing POSS-capped PEO.

TABLE 1: Static Contact Angles and Surface Free Energya

static contact anglesurface free energy

(mN × m-1)POSS-cappedPEO (wt %) θH2O θethylene glycol γS

d γSp γS

0 74.3( 0.3 60.7( 0.4 9.0 20.2 29.25 89.8( 0.6 70.1( 0.3 15.5 6.5 22.0

10 93.9( 0.5 75.2( 0.6 13.8 5.4 19.215 96.8( 0.8 78.2( 0.5 13.7 4.3 18.020 102.4( 0.6 83.8( 0.6 13.1 2.7 15.8

a Water: γLd ) 21.8 mN/m,γL

p ) 51.0 mN/m. Ethylene glycol:γLd

) 48.3 mN/m,γLp ) 29.3 mN/m.10c

Figure 9. Surface water contact angle analyses of control andnanocomposites: (A) control epoxy and (B) nanocomposites containing5, (C) 10, and (D) 20 wt % POSS-capped PEO.


single Tg, which decreased with increasing the concentrationof POSS-capped PEO. The decreasedTg’s resulted from theplasticization effect of miscible PEO on epoxy matrix. Ther-mogravimetric analysis (TGA) was applied to evaluate thethermal stability of the POSS-containing nanocomposites.Shown in Figure 8 are the TGA curves of the control epoxyand the nanocomposites, recorded in nitrogen atmosphere at20 °C/min. Within the experimental temperature range, allsamples displayed similar degradation profiles, suggesting thatthe existence of POSS-capped PEO did not significantly alterthe degradation mechanism of the matrix polymers. Comparedwith the control epoxy, the initial decomposition temperatureof the hybrid nanocomposites slightly decreased with increasingthe concentration of POSS-capped PEO. The decreased initial

decomposition temperature could result from the inclusion ofPEO portion, which possesses the lower thermal stability thanthe control epoxy. However, it is noted that the nanocompositesdisplayed increased the yield of degradation residues becauseof the inclusion of organic-inorganic amphiphile. The sum-mation of char and ceramic yields increased with increasingthe content of POSS-capped PEO.

Surface Properties of Nanocomposites.The POSS-cappedPEO is an organic-inorganic amphiphile. The POSS portioncombines the features of both organosilicon and organofluorinecompounds and is of low free energy5. The POSS cages werecovalently connected with PEO chains, which are miscible withepoxy resin. It is expected that the enrichment of the POSSportion at the surface will occur upon incorporating the organic-

Figure 10. AFM micrographs of the surfaces of the nanostructured thermosets: (A) control epoxy and (B) nanocomposites containing 5, (C) 10,and (D) 20 wt % POSS-capped PEO.


inorganic amphiphile into organic polymer and thus the surfacehydrophobicity (or dewettability) of materials will be enhanced.The surface properties of the organic-inorganic nanocompositeswere investigated in terms of the measurement of static contactangle. The contact angles were measured with ultrapure waterand ethylene glycol as probe liquids, respectively, and the resultsare summarized in Table 1. The contact angle of the controlepoxy was measured with water to be approximately 74.3°.Upon adding POSS-capped PEO to the system, the contactangles are significantly enhanced, and the contact angle for theorganic-inorganic nanocomposites containing 20 wt % POSS-capped PEO was increased up to 102.4° as shown in Figure 9.This observation indicates that the hydrophobilicity of thematerials was significantly enhanced; that is, the surface freeenergy of the materials was reduced. The surface free energiesof the hybrid nanocomposites containing a different percentageof POSS-capped PEO were calculated according to the geo-metric mean model:10a,b

whereθ is contact angle andγL is the liquid surface tension;γL

p and γLd are the polar and dispersive components ofγL,

respectively. According to the data of surface contact anglesfrom water and ethylene glycol,10c the calculated results ofsurface free energy were also incorporated into Table 1. Thesurface free energy of the control epoxy is about 29.2 mN×m-1. It is noted that the total surface free energies of thenanocomposites were diminished to 15.8 mN× m-1 while theconcentration of POSS-capped PEO is 20 wt %. The nonpolarcomponent (i.e., γs

d) seems to be more sensitive than the polarcomponent (i.e., γs

p) to the concentration of the POSS-cappedPEO, suggesting that the inclusion of POSS-capped PEOincreased the distribution of the nonpolar groups (i.e., silses-quioxane cage) on the surface energy of materials whereas thewater-soluble portion of the organic-inorganic amphiphile (i.e.,PEO chains) did not significantly enrich at the surface of thethermosets. The silsesquioxane portion of the organic-inorganicamphiphile on the surface could behave as a screening agentand reduce the surface energy of the nanostructured thermosets.It should be pointed out that the formation of micro- ornanostructures at the surface also contributed to the enhancementof the surface hydrophobicity of the materials.11

Atomic force microscopy (AFM) was employed to investigatethe surface morphologies of the nanostructured thermosets. TheAFM micrographs with contact mode can give the informationabout the topological structure of the materials. Shown in Figure10 are the surface AFM micrographs of the nanostructuredthermosets. For comparison, the micrograph of the control epoxywas also obtained with the identical mode condition. Accordingto the viscoelasticity and volume fraction of POSS-capped PEOin the hybrid thermosets, it is plausible to propose that the lightregion in the micrographs was ascribed to the POSS portionwhereas the dark region was ascribed to the epoxy matrix. Whilethe content of POSS-capped PEO is 5 wt %, a part of POSSmigrated to the surfaces of the thermosets and the sphericalnanoparticles at the size of approximately 60 nm were formed.It is seen that these nanoparticles were highly interconnectedat the surface of the thermosets (Figure 10A). With increasingthe concentration of POSS-capped PEO, the interconnectedPOSS nanoparticles were gradually converted into the continu-

ous matrix whereas the epoxy component became dispersed(Figure 10B-D). It is noted that the size of the dispersed epoxydomains deceased with increasing the concentration of POSS-capped PEO. The AFM shows that there is significant migrationof POSS moiety at the surface of the thermosets.

In order to confirm the enrichment of POSS on the surfaceof the nanostructured thermosets, X-ray photoelectron spec-troscopy (XPS) was used to examine the surface elementalcompositions. Representatively shown in Figure 11 are the XPSspectra of the control epoxy and the nanocomposites containing20 wt % POSS-capped PEO. The C 1s peak was observed atabout 286.8 eV, which was responsible for the contribution ofseveral kinds of carbon atoms from the epoxy thermoset andPOSS-capped PEO. The O 1s peak was approximately at 535eV. The Si atoms from the silsesquioxane moiety of POSS-capped PEO contributed the Si 2s and Si 2p signals at 156 and105 eV, respectively. Also observed is a F 1speak at 687.9eV, attributable to 3,3,3-trifluropropyl groups. It should bepointed out that the signals of Cl and N are too weak to beshown in this spectrum. The relative concentrations of C, O,N, Cl, F, and Si on the surfaces of the organic-inorganic hybridcomposites, calculated from the corresponding photoelectronpeak areas, are listed in Table 2. It is seen that the carbon andoxygen were dominant on the detected surfaces of all nano-structured thermosets, and a small amount of nitrogen andchlorine from the MOCA moiety was also detected. It is noticedthat the concentrations of elements Si and F on the surface ofthe nanostructured thermosets are significantly higher than the

Figure 11. XPS of control epoxy and the nanocomposites containing10 wt % POSS-capped PEO.

TABLE 2: Elemental Compositions of the Surfaces of theEpoxy Thermosets Containing POSS-Capped PEODetermined by Means of XPS

theoretical (mol %) experimental (mol %)

POSS-cappedPEO

(wt %) C O Si F N Cl C O Si F N Cl

5 81.8 12.2 0.2 0.5 2.6 2.6 70.0 19.7 1.8 4.7 2.0 1.810 81.4 12.4 0.3 0.9 2.5 2.5 58.7 20.3 4.7 12.5 1.9 1.815 80.8 12.5 0.5 1.4 2.4 2.4 58.6 20.1 4.8 12.7 1.9 1.920 80.0 12.8 0.7 1.9 2.3 2.3 55.3 21.5 5.3 13.8 2.0 2.0

cosθ ) 2γL

[(γLdγs

d)1/2 + (γLpγs

p)1/2] - 1 (1)

γs ) γsd + γs

p (2)


theoretical values calculated in terms of feed ratios. The surfacecontents of elements Si and F increased with increasing POSS-capped PEO in the nanostructured thermosets. For instance, thesurface content of elements Si and F is 19.1 mol % while theconcentration of POSS-capped PEO is 20 wt %. This value ismuch higher than 2.6 mol % of the theoretical value. Thisobservation suggests that there exists the enrichment of a Si(and/or F)-containing moiety on the surfaces of the nanostruc-tured thermosets. This result is important to understand theimprovement in surface hydrophobicity for the nanocomposites.The AFM showed that there is significant migration of POSSmoiety at the surface of the thermosets. It is worth noticingthat the enrichment of Si and F elements on the surfaces of thenanocomposites does not monotonously increase with increasingthe concentration of POSS-capped PEO in the thermosets, whichcould be related to the nanostructures of the thermosets.

Conclusions

Hepta(3,3,3-trifluoropropyl)polyhedral oligomeric silsesqui-oxane (POSS)-capped poly(ethylene oxide) (PEO) was synthe-sized via the hydrosilylation reaction between hepta(3,3,3-trifluoropropyl)hydrosilsesquioxane and allyl-terminated PEO.The inorganic-inorganic amphiphile was incorporated intoepoxy resin to prepare organic-inorganic hybrid thermosets.The nanostructures of the hybrids were investigated by meansof field emission scanning electronic microscopy (FESEM) andtransmission electronic microscopy (TEM). The formation ofnanostructures was addressed on the basis of miscibility andphase behavior of the sub-components (viz., POSS and PEOchains) of the organic-inorganic amphiphile with epoxy afterand before curing reaction. The static contact angle measure-ments indicate that the organic-inorganic nanocompositesdisplayed a significant enhancement in surface hydrophobicity(or dewettability) as well as reduction in surface free energy.AFM showed that there is significant migration of the POSSmoiety at the surface of the thermosets. The improvement insurface properties was ascribed to the enrichment of the POSSmoiety on the surface of the nanostructured thermosets, whichwas evidenced by X-ray photoelectron spectroscopy (XPS).

Acknowledgment. The financial supports from the NaturalScience foundation of China (Project No. 20474038) andShanghai Science and Technology Commission, China under akey project (No. 02DJ14048) are acknowledged. Sixun Zhengthanks the Shanghai Educational Development Foundation,China, under an award (2004-SG-18) to the “Shuguang Scholar”.

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