Cite this: RSC Advances, 2013, 3,17048

Flexible, optically transparent, high refractive, andthermally stable polyimide–TiO2 hybrids for anti-reflection coating3

Received 2nd May 2013,Accepted 16th July 2013

DOI: 10.1039/c3ra42180e

www.rsc.org/advances

Hung-Ju Yen,{ Chia-Liang Tsai,{ Pei-Hsuan Wang, Jiang-Jen Lin and Guey-Sheng Liou*

In this study, flexible and transparent polyimide–nanocrystalline-titania hybrid optical films with a high

titania content (up to 50 wt%) and thickness (15 mm) were prepared from new soluble poly(o-hydroxy-

imide)s and fabricated for a three-layer antireflection coating. The hydroxyl groups on the backbone of the

polyimides could provide a reaction site for organic–inorganic bonding and resulted in homogeneous

hybrid solutions by controlling the mole ratio of titanium butoxide/hydroxyl groups. The well-dispersed

nanocrystalline-titania was demonstrated by XRD and TEM measurements. The flexible hybrid films with

good surface planarity, thermal dimensional stability, tunable refractive index (1.63–1.80 at 633 nm), and

high optical transparency were successfully obtained and further utilized to prepare a three-layer anti-

reflection coating, indicating its potential optical applications.

Introduction

High refractive index polymers have been widely proposed inrecent years for their potential in advanced optoelectronicapplications.1 In addition to the basic parameter of therefractive index, other ones such as birefringence, Abbe’snumber, optical transparency, processability, and thermalstability are often taken into consideration. Regarding theencapsulation and antireflection coating for organic light-emitting diodes (OLEDs),1b commercial applications requirematerials with high refractive index, low birefringence, highoptical transparency, and a long-term ultraviolet light andthermal stability. In addition, grading the high refractivecontrast between semiconductor and air could efficientlyreduce the reflectivity. Therefore, to achieve a good combina-tion of the above-mentioned parameters is a crucial and on-going issue.2

Recently, systematic work by Ueda revealed the influence ofsulfur groups and related structures on the refractive indexand optical dispersion of the resulting polyimides (PIs).2d–2i

The incorporation of the sulfur atom into polymer systemscould enhance the refractive index and optical transparencydue to its large atomic refraction.3 Moreover, the bulky andrigid fluorene which sterically hinders the intermolecular

interaction of the PI chains, reduces the packing density, thusincreasing the transparency and processability of the PIs.4 Onthe other hand, non-aromatic PIs derived from alicyclicdianhydrides or diamines5 displayed good solubility, lowdielectric constant, and high transparency, which is relatedto their relatively low molecular density and polarity, especiallythe absence of intra- and inter-molecular charge-transfer (CT)interactions.6 Therefore, the incorporation of alicyclic units inPIs is considered as one of the effective ways to enhance thetransparency in the UV-visible region.7 Organic–inorganichybrid materials have attracted considerable attention inrecent years due to their novel physical and chemical proper-ties. The nanocomposite can have specialized properties thatcannot be found in their respective single phase. Hybridmaterials with enhanced electrical, mechanical and opticalproperties have been reported.8 Chemical methods based onan in situ sol–gel polymerization approach make it possible tomanipulate the organic/inorganic interfacial interactions atvarious molecular and nanometer length scales, resulting inhomogeneous structures and thus overcoming the problem ofnanoparticle agglomeration.9 The obtained polyimide–titania(PI–TiO2) hybrid materials also could be further processed byhydrothermal treatment to induce the nanocrystalline titaniadomain.10 For such applications, the inorganic domains mustbe well controlled at less than 40 nm to avoid scattering lossand retain the optical transparency of the prepared thin films.According to Rayleigh’s law, decreasing the size of inorganicparticles is a crucial approach for obtaining thick hybrid filmswithout significant loss of the transparency. That means theparticle domains have to be 10 nm and preferably below 5 nm

Functional Polymeric Materials Laboratory, Institute of Polymer Science and

Engineering, National Taiwan University, 1 Roosevelt Road, 4th Sec., Taipei 10617,

Taiwan. E-mail: gsliou@ntu.edu.tw

3 Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra42180e{ Equal contribution to this work.

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for preparing a much thicker film (y15 mm) instead of a thinfilm (y500 nm).11

In this study, semi-aromatic polyimide–nanocrystalline-titania hybrid materials with tunable titania content andrefractive index for both thin film (330–550 nm in thickness)on glass and thick film (10–15 mm in thickness) were obtainedby a convenient synthetic route and solution casting developedin our laboratory.9 Instead of using poly(amic acid) andpolyimide with carboxylic acid end groups,12 the high weight-average molecular weight and organo-soluble polyimide withhydroxyl groups (PHIs) on each repeating units derived fromtwo diamines, 9,9-bis(4-(4-amino-3-hydroxyphenoxy)phenyl)-fluorene (F-DA) and 4,49-bis(4-amino-3-hydroxyphenylthio)di-phenylsulfide (3S-DA), with 1,2,3,4-cyclobutanetetracarboxylicdianhydride (CBDA) or 1,2,4,5-cyclohexanetetracarboxylic dia-nhydride (CHDA) were used to prepare their titania hybridmaterials. The hydroxyl groups could react with titaniumbutoxide (Ti(OBu)4) and provide organic–inorganic bonding oneach repeating units. Three series of highly homogeneoushybrid films with different titania content were obtained, thenAFM, TEM, and XRD were used to characterize the morphol-ogies of the prepared hybrid materials. The thermal proper-ties, optical transmittance, and refractive index dispersion ofthe prepared hybrid films were also investigated and aredescribed herein.

Results and discussion

Monomer synthesis

The synthetic routes to the bis(o-aminophenol) monomers areoutlined in Scheme 1. F-DA was synthesized by the potassiumcarbonate-mediated nucleophilic substitution reaction of2-benzyloxy-4-fluoronitrobenzene13 (1) with 9,9-bis(4-hydroxy-phenyl)fluorene, followed by simultaneous deprotection and

reduction, respectively. Elemental analysis and FTIR spectro-scopic techniques were used to identify the structures of theintermediate dinitro compounds and the hydroxyl diaminemonomers, which were in good agreement with the previousliterature.14 On the other hand, the new bis(o-aminophenol)3S-DA was synthesized by the potassium carbonate-mediatednucleophilic substitution reaction of 5-fluoro-2-nitrophenolwith 4,49-thiobisbenzenethiol, and followed by reduction.Elemental analysis, FTIR, and 1H and 13C NMR spectroscopictechniques were used to identify the structures of theintermediate compounds F-DN, 3S-DN, the hydroxyl diaminemonomer F-DA and 3S-DA. The FT-IR results of all thesynthesized compounds are summarized in Fig. S1, ESI.3 Thenitro groups of compounds F-DN and 3S-DN gave twocharacteristic bands at around 1345 and 1593 cm21 (–NO2

asymmetric and symmetric stretching). After reduction, thecharacteristic absorptions of the nitro group disappeared, andthe amino group showed the typical N–H stretching absorp-tion pair in the region of 3376 and 3305 cm21. Fig. S2 and S3,ESI3 illustrate the 1H NMR and 13C NMR spectra of products.The 1H NMR spectra confirm that the nitro groups have beencompletely transformed into amino groups by the high fieldshift of the aromatic protons.

Synthesis of polyimides and hybrid materials

The one-step procedure starting from aromatic hydroxyl-diamines withalicyclic dianhydrides in the presence of a catalytic amount ofisoquinoline at room temperature for 5 h and 150–160 uC for 16 h is aconvenient method for the preparation of polyimides (as shown inScheme 2). The polymerization proceeded homogeneously throughoutthe procedure and afforded clear, viscous polymer solutions in highyields. All the polymers precipitated in white fiber-like forms whenslowly pouring the resulting polymer solutions into stirred methanol.The inherent viscosities in the range of 0.41–1.00 dL g21 (measured ata concentration of 0.5 g dL21 in N,N-dimethylacetamide (DMAc) at 30uC) and solubility properties of obtained PHIs are listed in Table 1. The

Scheme 1 Synthetic routes to diamine compounds F-DA and 3S-DA.

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PIs were readily soluble in polar aprotic organic solvents such asN-methyl-2-pyrrolidinone (NMP), DMAc, N,N-dimethylformamide(DMF), and dimethyl sulfoxide (DMSO). Furthermore, the PIs couldbe solvent cast into flexible, transparent, and tough films (Fig. 1).Therefore, these novel high performance thin films could be preparedby spin-coating or inkjet-printing processes for practical opticalapplications.

The fabrication procedure for hybrid films from PIs andtitania precursors is depicted in Scheme 3, and the reactioncompositions are also summarized in Table 2, Tables S1 andS2, ESI.3 The flexible, transparent, and homogeneous PI–nanocrystalline-titania (F-bTPX) hybrid optical films withdifferent titania contents could be successfully prepared, andthe appearance of F-bTP50 thick optical hybrid films is alsodisplayed in Fig. 2. The FTIR spectra (Fig. S4, ESI3) of F-PHI-band F-bTP50 films show a broad absorption band in the region3000–3700 cm21 (O–H stretch) and characteristic imideabsorption bands at 1779 cm21 (asym. CLO str.), 1719 cm21

(sym. CLO str.), 1388 cm21 (C–N), 748 cm21 (imide ringdeformation), respectively. The absorption peak of the poly-imide–titania thin film, F-bTP50, at 3000–3700 cm21 might beattributed to the hydroxyl groups of the titania. In addition,the inorganic Ti–O–Ti band could also be observed at 650–800cm21, which is also similar to that in the previous report.9

Thermal properties

The thermal behavior of the polyimide–nanocrystalline-titaniahybrids (F-bTPX) was evaluated by TGA and TMA, and theresults are listed in Table 3. The TGA curves (Fig. S5, ESI3) of allthese hybrid materials both in nitrogen and air revealedexcellent thermal stability and increased carbonized residue(char yield) with increasing titania content. The titaniacontents in the hybrid materials could be estimated basedon the char yields under air flow, which were in good

agreement with the theoretical content and ensured successfulincorporation of the nanocrystalline-titania. On the otherhand, the initial weight loss could not be found before 400 uCfor these PI–titania hybrid materials, which provided moreevidence of completely organic–inorganic bonding. The typicalTMA thermogram for F-PHI-b and the corresponding hybridmaterials revealed that the glass transition temperatureincreased from 352 uC to 433 uC with increasing titaniacontent (Fig. 3). Meanwhile, the coefficient of thermalexpansion (CTE) is one of the important design parametersfor the application of polymer films in the microelectronicfield, the CTE of the pure PHI film and the PHI nanocrystal-line-titania hybrid films were measured and are summarizedin Table 3. Generally, inorganic reinforced components oftenrevealed much lower CTE values than that of organic matrixes,which suppressed CTE of the resulting hybrid materials.Therefore, CTE of the organic–inorganic hybrids decreasedwith increasing the volume fraction of inorganic reinforce-ment.

Morphology analyses

The height and phase AFM images of F-bTP50 thin films areshown in Fig. 4. The results of root mean square surfaceroughness (Rq) for the hybrid films analyzed by AFM are listedin Table 2. The ratio of surface roughness to film thickness(Rq/h) was less than 0.15% implying excellent surface planarityof the hybrid films could be obtained. These results demon-strated that the hydroxyl groups attached on the polyimidebackbone played an important role for providing the bondingsites with titania and effectively improving the dispersion andmorphological stability of inorganic titania in the hybrid

Scheme 2 Synthesis and structures of PHIs.

Table 1 Viscosities and solubility behavior of PHIs

Polymerginh

a

(dL g21)

Solubility in various solventsb

NMP DMAc DMF DMSO m-Cresol THF CHCl3

F-PHI-a 1.00 ++ ++ ++ ++ — — —F-PHI-b 0.77 ++ ++ ++ ++ — — —3S-PHI-a 0.41 ++ ++ ++ ++ — — —

a Measured at a polymer concentration of 0.5 g dL21 in DMAc at 30uC. b The solubility was determined with a 10 mg sample in 1 mL ofsolvent. ++, soluble at room temperature; —, insoluble even onheating.

Fig. 1 Representative flexible and highly transparent PI film F-PHI-b (thicknessy 15 mm).

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materials. Furthermore, the TEM images of the hybrid filmsshown in Fig. 5 and Fig. S6, ESI3 show the titaniananocrystallites as well dispersed in the hybrid material withthe average domain size in the range of 3–5 nm. The XRDpatterns of the hybrid films depicted in Fig. 6 and Fig. S7, ESI3revealed that the matrix PHI hybrid materials were amorphousand the intensity of the titania crystalline peak graduallyincreased in the range 2h = 23–27u with increasing titaniacontent suggesting that the titania clusters were well dispersedin PIs due to the hydrolysis–condensation reactions occurringbetween Ti(OBu)4 and pendant hydroxyl groups of PIs. Theenhanced titania crystallization could be obviously observed inF-bTP50 with four peaks, 25.5u, 38.4u, 48.3u, and 54.8u,corresponding to the (101), (112), (200), and (211) crystallineplanes of the anatase titania phase, respectively.15 The broad

widths of the peaks were due to the scattering of X-raysresulting from the small size of the titania nanocrystallinegrains.

Optical properties

The UV-Visible (UV-Vis) spectra of the hybrid thick and thinfilms were also investigated for comparison, and the resultsare summarized in Fig. 7 and Fig. S8, ESI.3 These thin hybridfilms also revealed much higher optical transparency andlower cutoff wavelengths in the UV region, indicating thehighly homogeneous dispersed PI–nanocrystalline-titaniahybrid materials have been obtained. The cutoff wavelengthincreased and the corresponding band edge also red-shiftedwith increasing the titania content, such phenomena usuallyalso could be observed for titania sizes less than 10 nm.7

The refractive index dispersion of the obtained hybrid filmsat wavelengths of 300–800 nm is depicted in Fig. 8 and Fig. S9,ESI,3 and the inset figure shows the variation of refractiveindex at 633 nm with titania content. The refractive indexincreased linearly with increasing titania contents, implyingthat the Ti–OH groups of the hydrolyzed precursors condensedprogressively to form the Ti–O–Ti domain structures whichresulted in an enhanced refractive index. These results alsodemonstrate that using a soluble PI with hydroxyl groups ateach repeating units is a facile and successful approach forpreparing titania hybrid materials. Furthermore, the refractiveindex of the hybrid films enhanced obviously owing to the

Scheme 3 Preparation of PHIs–nanocrystalline-titania hybrids.

Table 2 Reaction composition and properties of the F-PHI-b hybrid films

Polymer

Reactant composition (wt%) Hybrid film TiO2 content (wt%)

hb (nm) Rqc (nm) ndF-PHI-b Ti(OBu)4 Theoretical Experimentala

F-PHI-b 100 0 0 0 383 2.103 1.63F-bTP10 67.8 32.2 10 9.9 271 1.872 1.68F-bTP30 35.4 64.6 30 28.7 281 1.368 1.73F-bTP50 19.0 81.0 50 48.6 295 1.021 1.80

a Experimental titania content estimated from TGA curves. b h: film thickness. c Rq: root mean square roughness. d n: refractive index at 633nm by ellipsometer.

Fig. 2 Representative flexible and highly transparent F-bTP50 hybrid films(thickness: 15 ¡ 3 mm).

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TiO2 in the hybrid film which can be crystallized viahydrothermal treatment. The hybrid films prepared both fromfluorene- (F-PHI-a) and sulfur-containing polyimides (3S-PHI-a) reveal high transparency and refractive index, which wereconsistent with the previous reports.3,4 Combining the issuesof thickness, flexibility, and optical transparency, the PHI–titania hybrid optical thick film F-bTP50 (15 ¡ 3 mm inthickness) showed the best optical transparency with highrefractive index up to 1.80 at 633 nm.

Multilayer antireflection coatings

The structure of the three layer anti-reflective coating on theglass substrate and the reflectance spectra are depicted inFig. 9. The glass substrate exhibited a refractive index (n = 1.52)higher than air (n = 1.0) and revealed an average reflectance ofabout 4.5% in the visible light range. The reflectance could bereduced significantly via the three-layer antireflection coatingconsisting of colloid SiO2, F-bTP50, and F-bTP10 for the first,second, and third layer, respectively. In order to reducereflection through adjusting phase of light, the thicknessand refractive index of the resulting films of colloid SiO2, F-

bTP50, and F-bTP10 were controlled to be 53 nm (1.29), 132nm (1.82), and 75 nm (1.70), respectively, for the three-layerstructure. As shown in Fig. 9, the reflectance of the preparedanti-reflection coatings was less than 0.5% in the visible range(400 nm to 700 nm), which was significantly reduced andmuch smaller than that of the glass with 4.5%. It suggested thepotential application of the prepared polyimide–titania hybridfilms in optical devices.

Conclusions

Three series of novel transparent and soluble polyimides withhydroxyl groups were synthesized by a one-step method from9,9-bis(4-(4-amino-3-hydroxyphenoxy)phenyl)fluorene (F-DA)and 4,49-bis(4-amino-3-hydroxyphenylthio)diphenylsulfide (3S-DA). In addition, high refractive index polyimide–titania hybridoptical films were successfully prepared from the solublepolyimide with hydroxyl groups with titanium butoxide bycontrolling the organic/inorganic mole ratio. The introductionof fluorene and alicyclic groups enhance the transparency andsolubility, while sulfur-containing PI revealed a higher value ofrefractive index. The refractive index could be tunable withtitania content (1.63–1.80 of F-bTP50 and 1.70–1.84 of 3S-aTP50)both in the thin films (100–500 nm) and the thick films (10–15mm) systems, thus a relatively high refractive index of 1.80 and1.84 could be achieved with titania content as high as 50 wt%.The hybrid thick films also possessed flexible and excellentthermal properties with high optical transparency in the visibleregion. Three-layer anti-reflective coating based on the hybridfilms exhibited reflectance of less than 0.5% in the visiblerange, suggesting a great potential of the novel PI–titania hybridfilms for optical applications.

Experimental section

Materials

2-Benzyloxy-4-fluoronitrobenzene (1) (lit.12 55–57 uC) wassynthesized by condensation of 5-fluoro-2-nitrophenol with

Table 3 Thermal and optical properties of F-PHI-b hybrid films

Index

Thermal properties Optical properties

Tga (uC) CTEb (ppm K21)

Td5 c (uC) Td

10 c (uC)

Rw800d (%) l0

e (nm) nf DngN2 Air N2 Air

F-PHI-b 352 71 470 450 495 480 57 312 1.637 0.0063F-bTP10 400 66 480 455 525 490 71 315 1.685 0.0078F-bTP30 415 50 490 465 540 505 76 320 1.733 0.0099F-bTP50 433 38 500 485 595 530 80 331 1.801 0.0113

a Glass transition temperature measured by TMA with a constant applied load of 10 mN at a heating rate of 10 uC min21 by tension mode.b The CTE data was determined over a 50–200 uC range by expansion mode. c Temperature at which 5% and 10% weight loss occurred,respectively, recorded by TGA at a heating rate of 20 uC min21 and a gas flow rate of 30 cm3 min21. d Residual weight percentages at 800 uCunder nitrogen flow. e The cutoff wavelength (l0) from the UV-vis transmission spectra of polymer films (thickness y 15 mm). f Refractiveindex at 633 nm by ellipsometer. g The in-plane/out-of-plane birefringence (Dn) was calculated as Dn = nTE 2 nTM and was measured using aprism coupler.

Fig. 3 TMA curves of F-PHI-b hybrid materials with a heating rate of 10 uCmin21.

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benzyl bromide in the presence of potassium carbonateaccording to a previously reported procedure. Commerciallyavailable alicyclic tetracarboxylic dianhydrides such as 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA) and 1,2,4,5-cyclohexanetetracarboxylic dianhydride (CHDA) were purified

by vacuum sublimation. All other reagents were used asreceived from commercial sources.

Monomer synthesis

9,9-Bis(4-(4-nitro-3-hydroxyphenoxy)phenyl)fluorene (F-DN).To a solution of 24.72 g (0.10 mol) of 2-benzyloxy-4-fluoronitrobenzene (1) and 17.17 g (0.049 mol) of 9,9-bis(4-hydroxypheyl)fluorene in 100 mL of DMF, 15.20 g (0.110 mol)of potassium carbonate was added with stirring all at once,and the mixture was heated at 120 uC for 12 h under nitrogenatmosphere. The mixture was poured into 400 mL ofmethanol–water (1 : 1 by volume) to give an orange-redpowder. The crude product was recrystallized from acetonitrile300 mL to afford 33.58 g (85% in yield) of yellow crystals. Mp:174–175 uC by OptiMelt at 1 uC min21. IR (KBr): 1578, 1347cm21 (–NO2 stretch), 1277 cm21 (C–O stretch). Anal. calcd forC51H36N2O8 (804.84): C, 76.11%; H, 4.51%; N, 3.48%. Found:C, 75.85%; H, 4.53%; N, 3.53%.

9,9-Bis(4-(4-amino-3-hydroxyphenoxy)phenyl)fluorene (F-DA). In a 500 mL round-bottom flask equipped with a stirringbar, 5.00 g (0.006 mol) of compound F-DN and 0.30 g of 10%Pd/C were dissolved/suspended in 100 mL of ethanol. Thesuspension solution was heated to reflux, and 2.00 mL ofhydrazine monohydrate was added slowly to the mixture, thenthe solution was stirred at reflux temperature. After a further 6h of reflux, the solution was filtered hot to remove Pd/C. Theprecipitated product from ethanol was collected by filtrationand dried in vacuo at 80 uC to give 9.22 g (82% in yield) of whitecrystals. Mp: 215–224 uC by OptiMelt at 1 uC min21. IR (KBr):2500–3500 cm21 (broad –OH stretch), 3305, 3376 cm21 (N–Hstretch), 1286 cm21 (C–O stretch). Anal. calcd for C37H28N2O4

(564.63): C, 78.71%; H, 5.00%; N, 4.96%. Found: C, 78.32%; H,5.16%; N, 4.80%.

4,49-Bis(3-hydroxy-4-nitrophenylthio)diphenylsulfide (3S-DN). To a solution of 2.50 g (0.01 mol) of 4,49-thiobisbenze-nethiol and 3.22 g (0.021 mol) of 5-fluoro-2-nitrophenol in 20mL of DMF, 2.76 g (0.021 mol) of potassium carbonate wasadded with stirring all at once, and the mixture was stirred atroom temperature for 2 h under a nitrogen atmosphere. Themixture was poured into 500 mL of MeOH–water (1 : 4 byvolume), and washed two times by water. The product wascollected by filtration and dried to give 4.89 g (93% in yield) ofyellow solid. Mp: 170.0–177.1 uC by OptiMelt at 1 uC min21. IRFig. 6 XRD patterns of F-PHI-b and the hybrid materials.

Fig. 4 AFM images of the F-bTP50 hybrid films coated on glass: (a) height image, (b) phase image.

Fig. 5 TEM image of the hybrid material F-bTP50.

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(KBr): 3000–3500 cm21 (O–H stretch), 1339, 1572 cm21 (–NO2

stretch), 1248 cm21 (Ar–S–Ar stretch). Anal. calcd forC24H16N2O6S3 (524.59): C, 54.95%; H, 3.07%; N, 5.34%; S,18.34%. Found: C, 53.31%; H, 3.04%; N, 5.01%; S, 18.04%.

4,49-Bis(4-amino-3-hydroxyphenylthio)diphenylsulfide (3S-DA). In a 100 mL round-bottom flask equipped with a stirringbar, 3.00 g (6.0 mol) of 3S-DN and 0.18 g of 10% Pd/C weredissolved/suspended in 70 mL of THF–EtOH (1 : 1 by volume).The suspension solution was heated to reflux, and 2 mL ofhydrazine monohydrate was added slowly to the mixture, thenthe solution was stirred at reflux temperature. After 12 h ofreflux, the solution was filtered hot to remove Pd/C. The crudeproduct was purified by recrystallization from ethanol anddried in vacuo at 80 uC to give 1.61 g (61% in yield) of pale-yellow crystals. Mp: 202–216 uC by OptiMelt at 1 uC min21. IR(KBr): 2500–3500 cm21 (O–H stretch), 3352, 3440 cm21 (N–Hstretch), 1213 cm21 (Ar–S–Ar stretch). Anal. calcd forC24H20N2O2S3 (464.62): C, 62.04%; H, 4.34%; N, 6.03%; S,20.70%. Found: C, 61.29%; H, 4.44%; N, 6.00%; S, 20.73%.

Polymer synthesis

Poly(o-hydroxy-imide)s PHI-a–PHI-b. The synthesis of poly-imide F-PHI-b is used as an example to illustrate the generalsynthetic route used to produce the hydroxyl-containingpolyimide. The stoichiometric mixture of diamine F-DA (1.13g, 2.00 mmol), dianhydride CHDA (0.45 g, 2.00 mmol), and afew drops of isoquinoline in NMP (5 mL) were stirred atambient temperature under nitrogen. After stirring for 5 h, 1.5mL of toluene was added and heated to 150–160 uC andmaintained at that temperature for 16 h. During this time, thewater of imidization was allowed to distil from the reactionmixture along with toluene. The toluene was continuallyreplaced so as to keep the total volume of the solutionconstant. After the solution was allowed to cool to ambienttemperature, the viscous solution then was poured slowly into300 mL of methanol with stirring. The precipitated fiber-likepolymer was collected by filtration, washed thoroughly withhot methanol, and dried under reduced pressure at 150 uC for15 h. The inherent viscosity of the obtained polyimide F-PHI-bwas 0.77 dL g21 (measured at a concentration of 0.5 g dL21 inDMAc at 30 uC). The IR spectrum of F-PHI-b (film) exhibited

Fig. 7 Transmittance UV-visible spectra of F-PHI-b hybrid (a) thick films (y15 mm) and (b) thin films (330–550 nm).

Fig. 8 Variation of the refractive index of the F-PHI-b hybrid materials withwavelength. The inset figure shows the variation of refractive index at 633 nmwith titania content.

Fig. 9 Variation of the reflectance of the glass slide and three-layer antireflec-tion coating with wavelength.

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broad absorption bands in the region of 2500 to 3700 cm21

(O–H stretch) and characteristic imide absorption bands at1779 (asymmetrical CLO), 1719 (symmetrical CLO), 1388 (C–N), and 748 cm21 (imide ring deformation); Yield: 99%. Anal.calcd for (C47H32N2O8)n (752.77)n: C, 74.99%; H, 4.28%; N,3.72%. Found: C, 72.95%; H, 4.76%; N, 3.68%.

Preparation of the polymer film

DMAc solutions of the PIs were drop-coated onto fused silica(amorphous SiO2) or glass substrates and dried at 80 uC for 6h, and then 150 uC for 8 h under vacuum. PI films withthicknesses around 15 mm could be prepared and used forsolubility tests, refractive index, transmittance, and thermalanalyses.

Preparation of the polyimide–titania hybrid films

The preparation of PI–titania hybrid F-bTP50 is used as anexample to illustrate the general synthesis route used toproduce the hybrid F-bTPX. Firstly, 0.12 g (0.15 mmole) of F-PHI-b was dissolved in 6.0 ml of DMAc, and then 0.20 ml ofHCl (37 wt%) was added very slowly into the PI solution andfurther stirred at room temperature for 30 min. Then, 0.50 ml(1.46 mmole) of Ti(OBu)4 dissolved in 0.50 ml of butanol wasadded drop-wise into the above solution by a syringe, and thenstirred at room temperature for 30 min. Finally, the resultingprecursor solution of F-bTP50 was filtered through a 0.45 mmPTFE filter and poured into a 6 cm glass Petri dish. The hybridoptical thick film could be obtained by a subsequent heatingprogram at 60 uC for 10 h, 110 uC for 5 h under vacuum. Inaddition, the above prepared solution was also spin-coatedonto a glass plate or silicon wafer at 1000–3000 rpm for 30 s.The obtained film was then treated by the multi-step heatingprocess of 60, 80 uC for 30 min and 110 uC for 60 min,respectively, to afford hybrid thin film. Then, these resultingPI–titania hybrid thick and thin films were further treated via ahydrothermal process by immersing them into the water vaporat 100 uC for 12 h. After the above process, the films were driedat 100 uC and the hybrid thick films and thin films could beobtained with thickness of around 15 mm and 330–550 nm,respectively. Thus, a series of flexible, transparent, and nano-scale well dispersed PI–TiO2 hybrid optical films with differenttitania contents could be successfully prepared.

Measurements

Fourier transform infrared (FT-IR) spectra were recorded on aPerkinElmer Spectrum 100 Model FT-IR spectrometer.Elemental analyses were run in a Heraeus VarioEL-III CHNSelemental analyzer. 1H spectra were measured on a JEOL JNM-AL 300 MHz spectrometer in DMSO-d6, using tetramethylsi-lane as an internal reference, and peak multiplicity wasreported as follows: d, doublet; m, multiplet. The inherentviscosities were determined at 0.5 g dL21 concentration usinga Tamson TV-2000 viscometer at 30 uC. Thermogravimetricanalysis (TGA) was conducted with a PerkinElmer Pyris 1 TGA.Experiments were carried out on approximately 6–8 mg filmsamples heated in flowing nitrogen or air (flow rate = 20 cm3

min21) at a heating rate of 20 uC min21. Coefficient of thermalexpansion (CTE) and glass transition temperatures (Tg) weremeasured on a dilatometer (TA instrument TMA Q400EM). The

TMA experiments were conducted from 50 to 470 uC at a scanrate of 10 uC min21 with a tensile probe under an appliedconstant load of 50 mN. Tg was taken as the onset temperatureof probe displacement on the TMA traces. The CTE data weredetermined in the range of 50–200 uC by film-fiber probe withexpansion mode. Ultraviolet-visible (UV-vis) spectra of thepolymer films were recorded on a Hitachi U-4100 UV-vis-NIRspectrophotometer. An ellipsometer (SOPRA, GES-5E) wasused to measure the refractive index (n) of the prepared filmsin the wavelength range of 300–800 nm. The thickness (h) ofthe prepared film was also determined simultaneously. In-plane (nTE), and out-of-plane (nTM) refractive indices of thefilms formed on the silica substrates were measured using aprism coupler (Metricon, PC-2000) at wavelengths of 632.8 nmat room temperature. The in-plane/out-of-plane birefringence(Dn) was calculated as Dn = nTE 2 nTM. Wide-angle X-raydiffraction (WAXD) measurements were performed at roomtemperature (25 uC) on a Shimadzu XRD-7000 X-ray diffract-ometer (40 kV, 20 mA), using graphite-monochromatized Cu-Ka radiation. An atomic force microscope (AFM, NanoscopeInc., Model DI 5000) was used to examine the surfacemorphology of the coated films. The microstructure of theprepared films was examined using a JEOL JEM-1230transmission electron microscope (TEM).

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This journal is � The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 17048–17056 | 17055

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17056 | RSC Adv., 2013, 3, 17048–17056 This journal is � The Royal Society of Chemistry 2013

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