Preparation and applications of novel fluoroalkyl end-capped oligomers/polyimide/silica nanocomposites

Received: 8 July 2010, Revised: 26 September 2010, Accepted: 11 October 2010, Published online in Wiley Online Library: 1 February 2011

Preparation and applications of novelfluoroalkyl end-capped oligomers/polyimide/silica nanocomposites

Yuki Gotoa, Nobuyuki Otsukab and Hideo Sawadaa*

Fluoroalkyl end-capped acrylic acid, N,N-dimethylacrylamide, N-(1,1-dimethyl-3-oxobutyl)acrylamide and vinyl-trimethoxysilane oligomers reacted with polyamic acid possessing trimethoxysilyl groups under alkaline conditionsto yield the corresponding fluoroalkyl end-capped oligomers/polyamic acid/silica nanocomposites. These isolatedfluorinated composite powders were found to afford nanometer size-controlled fine particles with a good disper-sibility and stability in water and traditional organic solvents. We succeeded in preparing new fluoroalkyl end-cappedoligomers/polyimide/silica nanocomposites by the imidization of fluorinated polyamic acid silica nanocompositesthrough the stepwise heating at 110 and 270-C under air atmosphere conditions. These fluorinated polyimide/silicananocomposites thus obtained were applied to the surface modification of glass and poly(methyl methacrylate)(PMMA) to exhibit good hydro- and oleo-phobic characteristics imparted by fluoroalkyl groups in the composites ontheir surface. In addition, the surface morphology of the modified glass treated with these fluorinated nanocompo-sites were analyzed by using FE-SEM and DFM. Copyright � 2011 John Wiley & Sons, Ltd.

Keywords: polyamic acid possessing alkoxysilyl groups; fluorinated oligomer; silica nanocomposite; polyimide; surfacemodification; FE-SEM; DFM

INTRODUCTION

Organic–inorganic hybrid nanocomposites have proved to benew advanced materials due to their unique combination ofadvantages of individually organic and inorganic components.[1]

The introduction of well dispersed inorganic nanoparticles intopolymer matrices has been proved to be extremely effective inthe improvement of the performance of the polymers.[2] In avariety of traditional organic polymers, especially, aromaticpolyimides are high-performance polymeric materials character-ized by exceptional thermal stability and mechanical andelectrical properties which have been widely applied in theaerospace and electrical industries.[3] Therefore, there has beengreat interest in the preparation of polyimides/silica nanocom-posites due to not only the improvement of the mechanicalproperties, thermal stability, and adhesion property but also thedecrease of the coefficient of thermal expansion of polyimides,because the parent silica nanoparticles possess the exceptionallyhigh thermal stability and very low coefficient of thermalexpansion.[4–15] Usually, it is well known that fluorinated polymerscan exhibit various interesting properties such as high thermaland oxidative stability, and excellent resistance to the mostchemicals, which cannot be achieved by the correspondingnon-fluorinated polymers.[16–19] In these fluorinated polymers,fluoroalkyl end-capped oligomers are attractive materials,because they exhibit numerous unique properties such ashigh solubility, surface active properties, and nanometersize-controlled molecular aggregates.[20–24] Therefore, it is ofparticular interest to develop new fluorinated polyimide/silicananocomposites by the use of fluoroalkyl end-capped oligomersfrom the developmental viewpoints of new fluorinated functionalmaterials. However, studies for fluoroalkylated polyimide/silica

hybrids have hitherto been very limited except for a coupleof reports for trifluoromethylated polyimides/silica hybrids.[25–27]

During our comprehensive studies on the development of novelorganic/inorganic hybrid materials possessing unique character-istics imparted by fluorine, we have already reported thatfluoroalkyl end-capped oligomers are applicable to the prep-aration of the corresponding oligomers/silica nanocompositesthrough the sol–gel reactions with tetraethoxysilane and silicananoparticles under alkaline conditions.[28] Here we reportthat novel fluoroalkyl end-capped oligomers/polymide/silicananocomposites can be prepared by the imidzation of thecorresponding fluorinated oligomer/polyamic acid/silica nano-composites, which are prepared by the sol–gel reactions offluorinated oligomers with polyamic acids possessing trimeth-oxysilyl groups under alkaline conditions. In addition, thesefluorinated polyimide/silica nanocomposites were applied tothe surface modification of traditional organic polymers suchas poly(methyl methacrylate) (PMMA). These results will be alsodescribed herein.

(wileyonlinelibrary.com) DOI: 10.1002/pat.1868

Research article

* Correspondence to: H. Sawada, Department of Frontier Materials Chemistry,Graduate School of Science and Technology, Hirosaki University, Hirosaki036-8561, Japan.E-mail: [email protected]

a Y. Goto, H. Sawada

Department of Frontier Materials Chemistry, Graduate School of Science and

Technology, Hirosaki University, Hirosaki 036-8561, Japan

b N. Otsuka

Chisso Corporation, Chiyoda-ku, Tokyo 100-8105, Japan

Polym. Adv. Technol. 2012, 23 290–298 Copyright � 2011 John Wiley & Sons, Ltd.

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EXPERIMENTAL

Measurements

Dynamic light-scattering (DLS) measurements were measured byusing Otsuka Electronics DLS-7000 HL (Tokyo, Japan). Fourier-transform infrared (FT-IR) spectra were measured using ShimadzuFTIR-8400 FT-IR spectrophotometer (Kyoto, Japan). Field emissionscanning electron micrographs (FE-SEM) were obtained using JEOLJSM-7000F (Tokyo, Japan). Thermal analyses were recorded onBruker axs TG-DTA2000SA differential thermobalance (Kanagawa,Japan). Dynamic force microscopy (DFM) was measured by using SIINanoTechnology Inc. E-sweep (Chiba, Japan). The zeta potential wasmeasured by the use of Microtec Nition ZEECOM/ZC-2000 (Chiba,Japan). Contact angles were measured using a Kyowa InterfaceScience Drop Master 300 (Saitama, Japan).

Materials

Acrylic acid (ACA), N,N-dimethylacrylamide (DMAA),N-(1,1-dimethyl-3-oxobutyl)acrylamide (DOBAA), and vinyltri-methoxysilane (VM) were used as received from ToagoseiCo., Ltd. (Tokyo, Japan), Kojin Co., Ltd. (Tokyo, Japan), KyowaHakko Chemical Co., Ltd. (Tokyo, Japan), and Dow Corning TorayCo., Ltd. (Tokyo, Japan), respectively. Aromatic polyamic acid[PAA-Si] dimethylacetamide (DMAc) solution was used as receivedfrom Chisso Co., Ltd. (Tokyo, Japan). Fluoroalkyl end-cappedacrylic acid oligomer(Mn¼ 1620),N,N-dimethylacrylamide oligomer(Mn¼ 1690), N-(1,1-dimethyl-3-oxobutyl)acrylamide oligomer(Mn¼ 12 160), and vinyltrimethoxysilane oligomer (Mn¼ 730) wereprepared by reaction of fluoroalkanoyl peroxide with thecorresponding monomers according to our previously reportedmethods.[29–31]

Preparation of fluoroalkyl end-capped oligomer/polyamicacid/silica nanocomposites [RF-oligomer/PAA-Si/SiO2

nanocomposites]

To a DMAc solution (2.0ml) of PAA-Si (800mg) were addeda tetrahydrofuran (THF) solution (10.0ml) of fluoroalkylend-capped N-(1,1-dimethyl-3-oxobutyl)acrylamide oligomer[RF–(DOBAA)n–RF; RF––CF(CF3)OC3F7 (120mg)] and 25% aqueousammonia solution (0.10ml). The mixture was stirred with amagnetic stirring bar at 258C for 3 hr. After the solvent wasevaporated off, methanol (25ml) was added to the obtainedcrude products. The methanol suspension was stirred withmagnetic stirring bar at room temperature for 1 day, and then

centrifuged for 30min. The expected fluorinated nanocompositeswere easily separated from the methanol suspension. Fluorinatednanoparticle powders thus obtainedwere dried in vacuo at 508C for1 day to yield purified particle powders (174mg). The obtainedRF–(DOBAA)n–RF/PAA-Si/SiO2 nanocomposites showed the follow-ing FT-IR spectra: FT-IR (cm�1) 3205 (OH), 1662, 1542 (C––O), 1500(C––C), 1245 (CF2), 1130, 1076 (Si–O–Si).Other fluoroalkyl end-capped oligomers/PAA-Si/SiO2 nano-

composites were prepared from the sol–gel reactions ofRF–(ACA)n–RF, RF–(DMAA)n–RF or RF–(VM)n–RF with PAA-Si undersimilar conditions, and the results are shown in Table 1 and Fig. 3.FT-IR spectra of these products are as follows:

RF–(ACA)n–RF/PAA-Si/SiO2 nanocomposite: IR (n/cm�1) 3209 (OH),

1670, 1542 (C––O), 1500 (C––C), 1245 (CF2), 1134, 1072 (Si–O–Si);

RF–(DMAA)n–RF/PAA-Si/SiO2 nanocomposite: IR (n/cm�1) 3197

(OH), 1666, 1542 (C––O), 1496 (C––C), 1245 (CF2), 1134, 1072

(Si–O–Si);

RF–(VM)n–RF/PAA-Si/SiO2 nanocomposite: IR (n/cm�1) 3220 (OH),

1662, 1542 (C––O), 1500 (C––C), 1245 (CF2), 1137, 1076 (Si–O–Si).

Imidization of RF-oligomers/PAA-Si/SiO2 nanocomposites

RF–(DOBAA)n–RF/PAA-Si/SiO2 nanocomposites were heated atthe rate of 108Cmin�1 from room temperature to 1108C, and keptat this temperature for 10min under air atmosphere conditions.The nanocomposites were then heated at the same rate from 110to 270 8C and kept at this temperature for 180min in airatmosphere conditions to afford RF–(DOBAA)n–RF/PI-Si/SiO2

nanocomposites.The obtained RF–(DOBAA)n–RF/PI-Si/SiO2 nanocomposites

showed the following FT-IR spectra: FT-IR (cm�1) 3490 (OH),1716 (C––O), 1500 (C––C), 1373 (C–N), 1245 (CF2), 1137, 1099[(OC)2NC, Si–O–Si], 721 (imide ring).Other fluorinated nanocomposites were prepared by the

imidization of RF–(DMAA)n–RF/PAA-Si/SiO2 nanocomposites andRF–(VM)n–RF/PAA-Si/SiO2 nanocomposites under similar con-ditions, and the results are also shown in Table 2 and Fig. 3. FT-IRspectra of these products are as follows:

RF–(DMAA)n–RF/PI-Si/SiO2 nanocomposite: RF––CF(CF3)OC3F7;

IR n/cm�1 3494 (OH), 1716 (C––O), 1500 (C––C), 1377 (C-N),

1245 (CF2), 1141, 1095 [(OC)2NC, Si–O–Si], 721 (imide ring);

Table 1. Preparation of RF-oligomers/PAA-Si/SiO2 nanocompositesa

Run RF-oligomersProductyieldb (%)

Size of compositesc

(nm)� STDZeta potentialin water (mV)

1 RF–(ACA)n–RF 69 79.6� 4.7 �39.952 RF–(DMAA)n–RF 50 133� 23.1 �23.013 RF–(DOBAA)n–RF 56 73.6� 6.9 �28.024 RF–(VM)n–RF 63 192� 28.7 �26.93

a Used RF-oligomers: 120mg, PAA-Si: 800mg, 25 wt% aq.b Isolated yield based on RF-oligomers and PAA-Si NH3: 0.10ml in each case in methanol solutions.c Determined by DLS (dynamic light scattering) measurements.

Polym. Adv. Technol. 2012, 23 290–298 Copyright � 2011 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/pat

PREPARATION AND APPLICATIONS OF NOVEL NANOCOMPOSITES

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RF–(VM)n–RF/PI-Si/SiO2 nanocomposite: RF––CF(CF3)OC3F7;

IR n/cm�1 3494 (OH), 1724 (C––O), 1500 (C––C), 1377 (C–N),

1245 (CF2), 1145, 1099 [(OC)2NC, Si–O–Si], 721 (imide ring).

Preparation of modified PMMA films treated withfluorinated oligomers/PAA-Si/SiO2 nanocomposites andfluorinated oligomers/PI-Si/SiO2 nanocomposites

The modified PMMA film was prepared by casting the1,2-dichloroethane solution (10ml) containing PMMA (990mg)and RF-oligomer/PAA-Si/SiO2 nanocomposites (or RF-oligomer/PI-Si nanocomposites (10mg)) on glass plate. The solvent wasevaporated at room temperature, and the film formed peeled offand dried at room temperature for 24 hr in vacuo to afford themodified PMMA film. The contact angles for dodecane on boththe surface and the reverse sides of this film were measured atroom temperature by use of goniometer-type contact anglemeasurements.

Preparation of modified glasses treated with fluorinatedoligomers/PAA-Si/SiO2 nanocomposites

The glass plate (18� 18mm2 pieces) was dipped into THFsolutions of RF-oligomer/PAA-Si/SiO2 nanocomposites at roomtemperature and left for 1min. This was lifted from the solution atconstant rate and dried at 508C for 1 hr under vacuum to afford

the modified glass. The contact angles of water and dodecane forthis glass plate were measured.

Preparation of surface modified silicon wafer treated withRF–(DOBAA)n–RF/PI-Si/SiO2 nanocomposites

The surface modified silicon wafer treated with RF–(DOBAA)n–RF/PAA-Si/SiO2 nanocomposites was prepared by spin-coating of

Table 2. Size and zeta potential of RF-oligomers/P/-S//SiO2 nanocomposites

RunRF-oligomers/P/-S//SiO2

nanocompositesSize of compositesa

(nm)� STDZeta potential in

water (mV)

RF-oligomersin nanocomposites1 RF–(ACA)n–RF

b b

2 RF–(DMAA)n–RF 578� 169 �19.363 RF–(DOBAA)n–RF 79.8� 4.8 �17.384 RF(VM)n–RF 187� 18.3 �18.60

a Determined by DLS (dynamic light scattering) measurements in methanol solutions.b Not dispersed in methanol.

Scheme 1.

Figure 1. FE-SEM (field emission scanning electron microscopy) image

of RF–(DOBAA)n–RF/ PAA-Si/SiO2 nanocomposites in methanol solution.

wileyonlinelibrary.com/journal/pat Copyright � 2011 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2012, 23 290–298

Y. GOTO, N. OTSUKA AND H. SAWADA

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the PAA-Si (24 wt%) DMAc solution containing the fluorinatednanocomposites (content of the composites based on PAA-Si: 0.1wt%) on the silicon wafer. The expected RF–(DOBAA)n–RF/PI-Si/SiO2 nanocomposites film was prepared by stepwise heating ofthe RF–(DOBAA)n–RF/PAA-Si/SiO2 nanocomposites film at 908Cfor 30 sec, 1008C for 30min, and then 2408C for 60min. Thedielectric constants of the obtained modified silicon wafersurface were measured.

RESULTS AND DISCUSSION

Fluoroalkyl end-capped acrylic acid oligomer [RF–(ACA)n–RF], N,N-dimethylacrylamide oligomer [RF–(DMAA)n–RF], N -(1,1-dimethyl-3-oxobutyl)acrylamide oligomer [RF–(DOBAA)n–RF], and vinyl-trimethoxysilane oligomer [RF–(VM)n–RF] reacted with aromaticpolyamic acid possessing trimethoxysilyl groups (PAA-Si) underalkaline conditions to yield the corresponding fluorinated

Scheme 2.

Figure 2. FT-IR spectra of RF–(DOBAA)n–RF/PAA-Si/SiO2 nanocomposites (A) and RF–(DOBAA)n–RF/P/-S//SiO2 nanocomposites (B). ’This figure is

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oligomers/PAA-Si/SiO2 nanocomposites in 50–69% isolatedyields. These results are shown in Scheme 1 and Table 1.Fluoroalkyl end-capped oligomers/PAA-Si/SiO2 composites in

Table 1 were shown to exhibit a good dispersibility and stabilityin traditional organic solvents such as methanol, ethanol,isopropyl alcohol, THF, 1,2-dichloroethane, dimethyl sulfoxide,N,N-dimethylformaide, and fluorinated aliphatic solvents (1:1mixedsolvents (AK-225) of 1,1-dichloro-2,2,3,3,3-pentafluoropropaneand 1,3-dichloro-1,2,2,3,3-pentafluoropropane). RF–(VM)n–RF/PAA-Si/SiO2 composites had a poor dispersibility in water;however, other fluorinated composites exhibited a gooddispersibility and stability toward water. Thus, we havemeasured the size of fluorinated oligomers/PAA-Si/SiO2 nano-composites in methanol by DLS measurements at 208C.These results are also shown in Table 1. The size of fluorinatedoligomers/PAA-Si/SiO2 composites was nanometer size-controlled fine particles (74–192 nm).FE-SEM photograph of RF–(DOBAA)n–RF/PAA-Si/SiO2 nano-

composites is spherical particles with a mean diameter of 24 nm

(refer Fig. 1), and we have obtained the slightly smaller size thanthat (74 nm) of DLS measurements in Table 1 (Run 3).In order to clarify the surface morphology of the obtained

fluorinated nanocomposite particles, we have measured themean zeta potential of well-dispersed aqueous solutionscontaining these fluorinated nanocomposites. The results arealso shown in Table 1. The mean zeta potential of fluorinatedoligomers/PAA-Si/SiO2 nanocomposites was negativelyenhanced charge: �23.01 to �39.95mV, compared to that(�3.67mV) of the parent silica nanoparticles (particle size:11 nm),[32] indicating that negatively charged carboxyl groups orresidual silanol groups in the composites should be arranged onthe composite particle surface. Therefore, the formation offluoroalkyl end-capped oligomers/PAA-Si/SiO2 nanocompositeswould be due to the molecular-level combination between thecarboxyl, amide or residual silanol groups in PAA-Si and carboxyl,amido or silanol groups in fluorinated oligomers through theintermolecular hydrogen bonding.We have studied the imidization of fluorinated oligomers/

PAA-Si/SiO2 nanocomposites, and the results were shown inScheme 2.Fluorinated oligomers/PAA-Si/SiO2 nanocomposites were

heated at 1108C for 10min, and then 2708C for 180min toafford the expected fluorinated oligomers/PI-Si/SiO2 nanocom-posites (refer Scheme 2). FT-IR spectra of the obtained productalso showed the formation of the fluorinated oligomers/polyimide/SiO2 nanocomposites as shown in Fig. 2. Thecharacteristic absorption bands of fluorinated oligomericpolyamic acid/SiO2 nanocomposites were measured around3200 cm�1 (–OH in carboxyl group or hydroxyl group of residualsilanol group), 1662, 1542 cm�1 (–C––O in amide bond),1500 cm�1 (–C––C in C6H5), 1245 cm�1 (–C–F in fluoroalkylgroup), and 1130, 1076 cm�1 (–Si–O–Si in siloxane bond).[25–27,33]

After the imidization of the composites, we have observed thecharacteristic absorption bands related to the fluorinatedoligomeric polyimide/SiO2 nanocomposites around 3490 cm�1

(residual silanol group), 1716 cm�1 (-C––O in imide), 1500 cm�1

(–C––C in C6H5), 1373 cm�1 (-C-N in imide), 1245 cm�1 (–C–F in

fluoroalkyl group), 1137, 1099 cm�1 (–Si–O–Si in siloxane bond),

Figure 3. FE-SEM (field emission scanning electron microscopy) image

of Rp–(DOBAA)n–RF/PI-Si/SiO2 nanocomposites in methanol solution.

Figure 4. Thermogravimetric analyses (TGA, (A)), differential thermal analyses (DTA, (B)) and Tdec (defined by 10% mass loss) of RF-oligomers/PI-Si/SiO2

nanocomposites.

RF-oligomers in nanocomposites: (a) RF–(DMAA)n–RF; (b) RF–(DOBAA)n–RF; (c) RF–(VM)n–RF; (d) parent PI-Si/SiO2 composites. This figure is available in color

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1099 cm�1 (–(OC)2NC in imide ring) and 721 cm�1 (–C–N in imidering).[25–27,33]

We have tested RF–(DOBAA)n–RF/PI-Si/SiO2 nanocompositesand RF–(VM)n–RF/PI-Si/SiO2 nanocomposites for their dispersi-bility in a variety of solvents including water. These fluorinatednanocomposites exhibited a poor dispersibility in water andcommon organic solvents such as THF, methanol, ethanol,and isopropyl alcohol, compared to that of the correspondingfluorinated oligomers/PAA-Si/SiO2 nanocomposites; however,these fluorinated oligomers/PI-Si/SiO2 nanocomposites werefound to have a good dispersibility in organic solvents such as1,2-dichloroethane, N,N-dimethylformamide, dimethyl sulfoxideand fluorinated aliphatic solvents (AK-225).We have measured the size of fluorinated oligomers/PI-Si/SiO2

nanocomposites in methanol by DLS at 208C. We have alsomeasured the mean zeta potential of well-dispersed aqueoussolutions containing fluorinated nanocomposites in order to

clarify the surface morphology of these nanocomposites. Theseresults are shown in Table 2.As shown in Table 2, the size of RF–(DOBAA)n–RF/PI-Si/SiO2

nanocomposites and RF–(VM)n–RF/PI-Si/SiO2 nanocompositeswas 80 and 187 nm, respectively, and the size of these compositeswas almost the same as that (74 and 192 nm) of thecorresponding fluorinated oligomers/PAA-Si/SiO2 nanocompo-sites in Table 1 (Runs 3 and 4). However, the size (578 nm) ofRF–(DMAA)n–RF/PI-Si/SiO2 nanocomposites was found toincrease extremely, compared to that (133 nm) of the corre-sponding fluorinated DMAA oligomer/PAA-Si/SiO2 nanocompo-sites (Run 2 in Table 1). In addition, we were unable to disperseRF–(ACA)n–RF/PAA-Si/SiO2 nanocomposites in methanol or waterto have DLS and zeta potential measurements. These findingswould be due to the poor solubility of RF–(DMAA)n–RF andRF–(ACA)n–RF oligomers toward the sol–gel reaction media inScheme 1. In contrast, RF–(DOBAA)n–RF and RF–(VM)n–RF

Figure 5. Photographs of modified PMMA film treated with RF–(DOBAA)n–RF/PAA-Si/SiO2 nanocomposites (A), modified PMMA film treated withRF–(DOBAA)n–RF/PI-Si/SiO2 nanocomposites (B), and modified glass treated with RF–(DOBAA)n–RF/PAA-Si/SiO2 nanocomposites (C). This figure is

available in color online at wileyonlinelibrary.com/journal/pat

Table 3. Contact angles of dodecane on the modified PMMA films treated with R-oligomers/PAA-Si/SiO and R-oligomers/PI-Si/SiOnanocompositesa

Run Fluorinated nanocomposites

Contact angle (Degree)

Film thickness (mm)Dodecane

Surface side Reverse side

RF-oligomers in RF-oligomers/PAA-Si/SiO2 nanocomposites1 RF–(ACA)n–RF 16 0 2282 RF–(DMAA)n–RF 9 0 2223 RF–(DOBAA)n–RF 29 0 2174 RF–(VM)n–RF 41 0 2445 Original PAA-Si/SiO2 composites 0 0 246RF-oligomers in RF-oligomers/PI-Si/SiO2 nanocomposites6 RF–(DOBAA)n–RF 11 0 2227 RF–(VM)n–RF 18 0 2468 Original PI-Si/SiO2 composites 0 0 204

a Concentration of fluorinated nanocomposites (original PAA-Si/SiO2 composites or original PAA-Si/SiO2 composites) based onPMMA: 1%.



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oligomers were found to have a good solubility toward thesol–gel reaction media, indicating that such reaction conditionsfor RF–(DOBAA)n–RF and RF–(VM)n–RF oligomers should affordnanometer size-controlled fine composite particles possessing agood dispersibility in a variety of solvents.FE-SEM photograph of RF–(DOBAA)n–RF/PI-Si/SiO2 nanocom-

posites shows that these composite particles are sphericalparticles with a mean diameter of 51 nm as shown in Fig. 3,and the size of the composites was almost the same as that(80 nm) of DLS measurements in Table 2 (Run 3).The mean zeta potential of fluorinated oligomers/PI-Si/SiO2

nanocomposites was positively enhanced charge: �17.38 to�19.36mV, compared to those (�23.01 to �28.02mV) of thecorresponding fluorinated oligomers/PAA-Si/SiO2 nanocompo-

sites (refer Table 1), indicating that negatively charged carboxylgroups in the composites have been completely disappearedthrough the imidization process.Thermal stability of RF-oligomers/PI-Si/SiO2 nanocomposites

was studied by thermogravimetric analyses (TGA), in which theweight loss of these nanocomposites was measured by raisingthe temperature around 8008C (the heating rate: 108Cmin�1) inair atmosphere conditions, and the results are shown in Fig. 4.The weight of parent fluoroalkyl end-capped oligomers

markedly dropped around 2508C and decomposed gradually,dropped 0% with no residue around 5508C (data not shown).However, we were not able to observe an effective weight loss offluorinated oligomers/PI-Si/SiO2 nanocomposites correspondingto the presence of the parent fluorinated oligomers from roomtemperature to 8008C as shown in Fig. 4 (A), compared to that ofthe corresponding PI-Si/SiO2 composites possessingno fluorinated oligomer (refer Fig. 4 (A-d)). The thermal stability:Tdec (defined by a 10% mass loss 108Cmin�1 heating rate underair atmosphere conditions) of these fluorinated nanocompositesdecreased effectively by the nanocomposite reactions withfluorinated oligomers from 546 to 4788C, indicating that not onlythe sol–gel reactions with fluorinated oligomers in Scheme 1 butalso imidization with fluorinated oligomers in Scheme 2 can besmoothly proceeded to afford the expected fluorinated oligo-meric polyimide silica nanocomposites.As shown in Fig. 4 (B), difference thermal analyses (DTA)

for fluorinated oligomers/PI-Si/SiO2 nanocomposites in airatmosphere conditions (heating rate: 108Cmin�1) showed aclear exothermic peak around 589�5978C. This exothermictemperature is slightly higher than that (5838C) of thecorresponding PI-Si/SiO2 nanocomposite possessing no fluori-

Table 4. Contact angles of dodecane and water on themodified glass treated with RF-oligomers/PAA-Si/SiO2

nanocomposites

RunRF-oligomers/PAA-Si/SiO2

nanocomposites

Contact angle(Degree)

Dodecane Water

RF-oligomers in nanocomposites1 RF–(DOBAA)n–RF 47 1062 RF–(VM)n–RF 52 1073 Original PAA-Si/SiO2

nanocomposites4 63

Figure 6. FE-SEM (field emission scanning electron microscopy) images of the modified glass surface glass treated with RF-oligomers/PAA-Si/SiO2

nanocomposites.



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nated oligomer (refer Fig. 5 (B–C)). This finding would be dueto the effective interaction between fluorinated oligomersand polyimides in the silica gel matrices.Our present fluorinated oligomers/PAA-Si/SiO2 nanocompo-

sites and fluorinated oligomers/PI-Si/SiO2 nanocomposites arenanometer size-controlled very fine particles, and have a gooddispersibility in a variety of traditional organic solvents. Therefore,it is very interesting to apply these nanoparticles to the surfacemodification of traditional organic polymeric materials suchas poly(methyl methacrylate) [PMMA]. The contact angles ofdodecane on the modified PMMA films treated with thesenanocomposites were measured, and the results are shown inTable 3.As shown in Table 3, contact angles of dodecane on the

modified PMMA film surface treated with fluorinated nanocom-posites were found to exhibit significantly large values (9–418)compared to that of the corresponding PAA-Si/SiO2 or PI-Si/SiO2

composites possessing no fluorinated oligomers. Interestingly,contact angles of dodecane on the modified PMMA film surfacetreated with RF–(VM)n–RF/PAA-Si/SiO2 nanocomposites showedan effectively larger value: 418 compared to that of othernanocomposites. On the other hand, each dodecane contactangle on the reverse side was 08, whose value is the same as thatof non-treated PMMA film (08), indicating that the fluorinatednanocomposites should be effectively dispersed above thepolymer surface during the cast film formation.We have prepared themodified glasses treated with fluoroalkyl

end-capped oligomers/PAA-Si/SiO2 nanocomposites, and con-

tact angles of dodecane and water on these modified glasseswere measured. These results are shown in Table 4.As shown in Table 4, the contact angles of dodecane and water

on the modified glass surface treated with fluoroalkyl end-capped oligomers/PAA-Si/SiO2 nanocomposites showed signifi-cantly large values: 47–528 and 106–1078, respectively, whichexhibit good oleo- and hydro-phobicities imparted by fluoroalkylsegments in nanocomposites on the modified glass surfacecompared to those (48 and 638) of the original PAA-Si/SiO2

composites. These findings suggest that fluoroalkyl groups in thenanocomposites should be regularly arranged above the modifiedglass surface to exhibit a high surface active characteristic impartedby fluorine. Especially, a good adhesion ability of the presentfluoroalkyl end-capped oligomers/PAA-Si/ or PI-Si/SiO2 nanocom-posites toward PMMA and glass would be due to the presence ofthe residual hydroxysilyl moieties in the nanocomposites, which arepartly obtained by the sol–gel reactions illustrated in Scheme 1.RF–(DOBAA)n–RF/PAA-Si/SiO2 nanocomposites and RF–(DOBAA

)n–RF/PI-Si/SiO2 nanocomposites were found to afford paleyellow-colored transparent PMMA films and glass as shown inFig. 5, indicating that these nanocomposites should be regularlyarranged above the modified film and glass surface.The surface morphology of the modified glasses treated with

RF–(DOBAA)n–RF/PAA-Si/SiO2 nanocomposites and RF–(VM)n–RF/PAA-Si/SiO2 nanocomposites were analyzed by FE-SEM and DFM,and the results are shown in Figs 6–8.Previously, we reported that RF–(VM)n–RF oligomeric nano-

particles, which were prepared by the sol–gel reactions of

Figure 7. DFM (dynamic force microscopy) topographic images of the modified glass surface treated with RF–(DOBAA)n–RF/PAA-Si/SiO2nanocompo-sites. This figure is available in color online at wileyonlinelibrary.com/journal/pat

Figure 8. DFM (dynamic force microscopy) topographic images of the modified glass surface treated with RF–(VM)n–RF/PAA-Si/SiO2nanocomposites.

This figure is available in color online at wileyonlinelibrary.com/journal/pat



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RF–(VM)n–RF oligomer under alkaline conditions, were applied tothe surface modification to exhibit a superhydrophobic charac-teristic (water contact angle: 180o).[34] In this case, fluoroalkylgroups in RF–(VM)n–RF oligomeric nanoparticles are located at afractal surface to exhibit a superhydrophobic property.[34]

However, as shown in Figs 6–8, we succeeded in observingthe relatively smooth surface on each modified glass by FE-SEMand DFM measurements. This relatively smooth surface canafford usual oleo- and hydro-phobic characteristics in Table 4.Polyimides have been widely used as high performance

polymeric materials in advanced technologies because of theirlow water absorption, low coefficient of thermal expansion, lowdielectric constant values, and thermal stability.[25–27,35] Thus, wehave measured the dielectric constants on the modified siliconwafer treated with RF–(DOBAA)n–RF/PI-Si/SiO2 nanocomposites,and the results are shown in Table 5.As shown in Table 5, RF–(DOBAA)n–RF/PI-Si/SiO2 nanocompo-

sites were effective to decrease the dielectric constant of themodified PI-Si/SiO2 composites. Therefore, our present fluori-nated oligomers/PI-Si/SiO2 nanocomposites have high potentialfor the applications to electrical fields.

CONCLUSION

We succeeded in preparing novel fluoroalkyl end-cappedoligomers/polyamic acid/SiO2 nanocomposites by the sol–gelreactions of the polyamic acid possessing trimethoxysilyl groupswith the corresponding fluorinated oligomers under alkalineconditions. These fluorinated composites have a good disper-sibility and stability in not only water but also a variety oftraditional organic solvents. DLS and FE-SEM measurementsshowed that these fluorinated composites were nanometersize-controlled very fine nanoparticles. Imidization of fluoroalkylend-capped oligomers/PAA-Si/SiO2 nanocomposites was foundto proceed smoothly by heating these composites around 2708Cto afford the corresponding fluorinated oligomeric polyimidesilica nanocomposites. These fluorinated oligomeric polyamicacid and polyimide silica nanocomposites were applied to thesurface modification of PMMA and glass. The modified PMMAand glass surface treated with these fluorinated nanocompositeswere shown to exhibit a good oleo- and hydro-phobicityimparted by fluoroalkyl groups in the nanocomposites. Thesefluorinated nanocomposites were also effective to decrease thedielectric constant. Therefore, our present fluorinated nanocom-posites are applicable to a wide variety of fields such as surfacemodification and electrical industries.

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Table 5. Dielectric constant (e) of RF–(DOBAA)n–RF/PI-Si/SiO2 nanocomposites

Run Fluorinated nanocompositesContents of

composites (wt%) 1 kHz 1MHz

Applied voltage: 10 V1 RF–(DOBAA)n–RF/PI-Si nanocomposites 0.1 3.4 3.12 Original PI-Si/SiO2 composites 0 3.5 3.3Applied voltage: 5 V3 RF–(DOBAA)n–RF/PI-Si nanocomposites 0.1 3.4 2.74 Original PI-Si/SiO2 composites 0 3.5 3.0



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Preparation and applications of novel fluoroalkyl end-capped oligomers/polyimide/silica nanocomposites