Materials Letters 64 (2010) 337–340

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Materials Letters

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Synthesis and characterization of novel photoactive poly(arylene ether ketone) withazobenzene pendants

Jingjing Zhang a, Haibo Zhang a, Xingbo Chen a, Yunhe Zhang a, Xuefeng Li a, Qidai Chen b, Zhenhua Jiang a,⁎a Alan G. MacDiarmid Institute, Jilin University, Changchun 130012, Chinab State Key Lab on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China

⁎ Corresponding author. Tel./fax: +86 431 85168886E-mail address: jiangzhenhua@mail.jlu.edu.cn (Z. Jia

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.11.009

a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 July 2009Accepted 5 November 2009Available online 10 November 2009

Keywords:PolymersPoly(arylene ether ketone)Surface-relief-gratingsOptical materials and properties

A novel azobenzene-functionalized poly(arylene ether ketone) (azo-PAEK) with azomoieties on the pendantswas prepared by post-esterification reaction of acid-containing poly(arylene ether ketone) (acid-PAEK) with4-((4-nitrophenyl)diazenyl)phenol (NAP). DSC and TGA measurements indicate the azo-PAEK has high glasstransition temperature of 152 °C and good thermal stabilitywith 5%weight loss at 319 °C. Irradiated by 360 nmUV light, the azo-PAEK shows a significant photoisomerization effect. Under the illumination of linearlypolarized laser beam, significant surface-relief-gratings (SRGs) with the surface modulation depth of 37 nmwere fabricated rapidly on the azo-PAEK film.

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1. Introduction

In recent years, azo polymers have been extensively investigateddue to their potential application in optical data storage, opticalswitch and electro-optical modulator [1–5]. Many of these applica-tions are based on the efficient photoisomerization and photoinducedanisotropy of the azobenzene chromophores. When azo polymer filmis irradiated with a linearly polarized laser beam, the azobenzenegroups become oriented perpendicular to the polarization direction ofthe incident light through trans-to-cis photoisomerization cycles. Thisreversible photoisomerization can initiate polymer chains migrationand form surface-relief-gratings (SRGs) on the azo polymer film [6].Compared with other technologies of producing topographic gratings,this photo-fabrication approach is a facile single-step processing,which could easily control the modulation depth by adjusting theexposure energy [7].

An important requirement of the SRGs is the shape stability in termsof long-term storage and durability at higher temperature. The stabilitycan be improved by using polymers with high glass transitiontemperature (Tg) [8]. It's well known that poly(arylene ether)s are afamily of high-performance engineering thermoplastics with excellentthermal, mechanical and electrical properties. Functionalized poly(arylene ether)s have been widely studied as proton-exchangemembranes, light-emitting materials and optical materials, some ofthem exhibited many attractive properties [5,9–12]. However, fewazobenzene-functionalized poly(arylene ether)s as optical storage

materials have been reported so far. Recently, our group reported anazo-poly(arylene ether) via direct polymerization from the azo-monomer [13]. Although the amount of the azo chromophores in thepolymer could be well controlled by adjusting the feed ratio of the azo-monomer, this approach requires tedious procedures for the synthesisof the azo-monomer. In this paper, we developed a facile route andobtained a novel azobenzene-functionalized poly(arylene ether ketone)(azo-PAEK) by post-esterification of acid-containing poly(arylene etherketone) (acid-PAEK) with 4-((4-nitrophenyl)diazenyl)phenol (NAP).The photoisomerization behavior of this azo-PAEK and the formation ofthe SRGs are also investigated.

2. Experimental section

2.1. Materials

4,4′-Bis(4-hydroxyphenyl)pentanoic acid, 4-(N,N-dimethylamino)pyridine (DMAP), dicyclohexylcarbodiimide (DCC) and 4-nitroanilinewere purchased from Alfa Aesar and used without further purification.4-((4-Nitrophenyl)diazenyl)phenol (NAP) and acid-containing poly(arylene ether ketone) (acid-PAEK) were synthesized according to theliteratures [14,15]. All of other reagents were obtained commerciallyand purified according to the standard procedures.

2.2. Synthesis of azo-PAEK

DMAP (0.12 g, 0.6 mmol), DCC (1.24 g, 6 mmol) and NAP (1.48 g,6 mmol) were added to a solution of the acid-PAEK (1.84 g, 4 mmol)in 50 mL of anhydrous tetrahydrofuran (THF) (Scheme 1), and thenthe mixture was stirred at room temperature for 72 h under nitrogen.

Scheme 1. Synthesis route of azo-PAEK.

Fig. 1. 1H NMR spectra of acid-PAEK and azo-PAEK.

338 J. Zhang et al. / Materials Letters 64 (2010) 337–340

The product was filtered to remove dicyclohexyl urea and put intoalcohol to precipitate. The obtained polymer powders were furtherpurified by reprecipitation in alcohol and dried at 110 °C undervacuum for 24 h. Yield: 83%.

2.3. Measurements

FT-IR spectra (KBr pellet)were recorded on aNicolet Impact 410 FT-IRspectrophotometer. Gel permeation chromatography (GPC) analysiswas carried out using a Waters 410 instrument with N,N-dimethyl-formamide as an eluent. 1H (500 MHz) spectra were recorded using aBruker 510 instrument. Glass transition temperatures (Tgs) weredetermined by DSC (Model Mettler DSC821e) instrument at a heatingrate of 20 °C/min under nitrogen. Thermo-gravimetric analysis wasperformed on a Perkin Elmer Pryis 1 TGA analyzer at a heating rate of10 °C/min under nitrogen. UV–visible absorption spectra wererecorded on a UV2501-PC spectrophotometer.

2.4. Polymer film preparation and surface-relief-gratings fabrication

The homogeneous solution of azo-PAEK in cyclohexanone (10 wt.%)was filtered through 0.8 µmmembranes. The filtered solutionwas spunonto glass substrate and then heated to remove residual solvent. Thefilm thickness was about 1 µm. Experimental arrangement for the SRGsinscription has already been described by our group previously [13].Holographic gratings were written on the spin-coating film withpolarized interfering laser beams, where the beam was split by abeam splitter into two beams of equal intensity. Surface topology of thefilm was probed with an atomic force microscope (AFM) in the tappingmode.

3. Results and discussion

3.1. Synthesis and characterization of azo-PAEK

Acid-PAEK was synthesized by a typical nucleophilic substitutionpolycondensation from 4,4′-bis(4-hydroxyphenyl)pentanoic acid and4,4′-difluorobenzophenone. The number-average molecular weight ofthe acid-PAEK is 5.4×104 with a polydispersity index of 1.51determined by GPC. The azo chromophore NAP was attached to the

acid-PAEK backbones by classical esterification with DCC and DMAP ascatalyst at room temperature [14]. Low reaction temperature and weakbasic reactionmediummake synthesis conditionsmild enough to obtainfunctional polymers from the sensitive monomer [9]. The number-averagemolecular weight of azo-PAEK is 6.0×104 with a polydispersityindex of 2.20. The IR spectrum of azo-PAEK shows characteristicabsorption bands of ester-carbonyl groups at 1763 cm−1, and NO2

groups at 1522 cm−1 and 1342 cm−1 indicating successful introductionof azo chromophores into the polymer chains. Comparedwith the acid-PAEK (Fig. 1), the azo-PAEK shows three new peaks at 7.9–8.4 ppmcorresponding to the chemical shifts of the hydrogen in azo chromo-phore. On the basis of 1H NMR spectrum, the molar percentage of thechromophore in azo-PAEK is about 65% calculated from the integralratio of Hb,c and H1,1′.

The UV–vis spectra of NAP, acid-PAEK and azo-PAEK are shown inFig. 2. After incorporation of NAP into acid-PAEK, a new strongabsorption band with the absorption maximum (λmax) at around340 nm can be observed due to the intramolecular charge-transfer of

Fig. 2. UV–vis spectra of polymers and NAP in THF solution.

339J. Zhang et al. / Materials Letters 64 (2010) 337–340

azobenzene chromophores. Compared with NAP, azo-PAEK shows λmax

with a 38 nm blue shift, that may be attributed to the fact thatesterification of phenolic group decreases its electron-donating ability,which further weakens the charge–transfer interaction [16]. The azo-PAEK is well soluble in common organic solvents such as chloroform,tetrahydrofuran, N,N-dimethylformamide, N-methyl-2-pyrrolidoneand cyclohexanone, but insoluble in acetone and ethanol.

3.2. Thermal property of azo-PAEK

The thermal stability of azo-PAEK was investigated by TGA andDSC. 5% and 10% weight loss temperature of azo-PAEK are 319 °C and343 °C respectively, indicating its good thermal stability. In the DSCcurve, the azo-PAEK shows a high glass transition temperature (Tg) at152 °C, which is lower than that of acid-PAEK (165 °C), because the

Fig. 3. Changes in the UV–vis absorption spectra of azo-PAEK in THF solution: (a)

Fig. 4. AFM images of the SRGs formed on azo-PAEK film. (a) The plane view of th

introduction of azo chromophores by esterification decreases thenumber of carbonyl acid groups and weakens the hydrogen bondinginteraction of polymer chains.

3.3. Photoisomerization behavior of azo-PAEK

The photoisomerization behavior of azo-PAEK was investigated byUV–vis spectroscopy in THF solution after irradiation with 360 nm UVlight. As shown in Fig. 3a, the UV–vis spectra were recorded overdifferent time intervals until photostationary state was reached(irradiated for 7 min). Under UV irradiation, the intensity of the π−π⁎transition band at 340 nm decreased and the intensity of the n−π⁎transition band at 444 nm increased gradually. The spectrum variationsindicate the trans-to-cis isomerization of the azo chromophores [17].What's more, when the irradiated sample was exposed to visible light,the cis form slowly relaxed to the trans form, and the spectra graduallyrecovered to the original curve (Fig. 3b), confirming the reversibility ofthe photoisomerization process.

3.4. Photoinduced surface-relief-gratings

SRGs writing experiment was performed using a ∼1 µm azo-PAEKfilm. After exposing the azo-PAEK film to two polarized interfering laserbeams of equal intensity (30 mW/cm2) for 30 s, the SRGs structureswere sufficiently formed on it. Fig. 4 shows the typical AFM plane andthree-dimensional images of the surface-relief-structures on the azo-PAEK film. Obvious sinusoidal surface-relief-gratings with regularspaces could also be observed. The surface modulation depth is about37 nm and the grating spacing is about 1.6 µm. In addition, themodulation depth could be adjusted by the irradiation time and theirradiation energy. The grating spacing is equal to the period of theinterference pattern (Λ) and can be controlled by adjusting the angle θ

induced by UV light irradiation and (b) induced by visible light irradiation.

e SRGs (the image size is 10×10 µm) and (b) Typical 3-D view of the SRGs.

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between the two interfering beams, according to the equation Λ=λ/2sin (θ/2), where λ is the wavelength of the writing beams [18].

4. Conclusion

A novel azobenzene-functionalized poly(arylene ether ketone)(azo-PAEK) has been facilely synthesized via amild post-esterificationreaction and exhibits good solubility and high thermal stability. Dueto the existence of azo chromophores, the polymer shows an obviousphotoisomerization effect upon 360 nm UV light irradiation. Afterexposed to an interference pattern of laser beams, the azo-PAEK filmcan generate obvious surface-relief-gratings rapidly, displaying itspotential applications in optical data storage.

Acknowledgements

We thank the National Science Foundation of China (50803025)for the financial support.

References

[1] Rochon P, Batalla E, Natansohn A. Appl Phys Lett 1995;66:136–8.[2] Kim DY, Tripathy SK, Li L, Kumar J. Appl Phys Lett 1995;66:1166–8.[3] Yu YL, Nakano M, Ikeda T. Nature 2003;425:145.[4] Cha SW, Choi DH, Oh DK, Han DY, Lee CE, Jin JI. Adv Funct Mater 2002;12:670–8.[5] Bai YW, Song NH, Gao JP, Sun X, Wang XM, Wang ZY, et al. J Am Chem Soc

2005;127:2060–1.[6] Yager KG, Barrett CJ. J Photochem Photobiol A Chem 2006;182:250–61.[7] Viswanathan NK, Balasubramanian S, Li L, Kumar J, Tripathy SK. J Phys Chem B

1998;102:6064–70.[8] Fukuda T,MatsudaH,ViswanathanNK, Tripathy SK, Kumar J, ShiragaT, et al. SynthMet

1999;102:1435–6.[9] Lu ZQ, Li J, Hua JL, Li X, Qin JG, Qin AJ, et al. Synth Met 2005;152:217–20.[10] Ma XY, Liu BJ, Wang D,Wang GB, Guan SW, Jiang ZH.Mater Lett 2006;60:1369–73.[11] Pang JH, Zhang HB, Li XF, Jiang ZH. Macromolecules 2007;40:9435–42.[12] Zhang YH, Chen XB, Guo MM, Zhang SL, Jiang ZH. Mater Lett 2008;62:3453–5.[13] Chen XB, Zhang YH, Liu BJ, Zhang JJ, Wang H, Jiang ZH, et al. J Mater Chem 2008;18:

5019–26.[14] Yin SC, Xu HY, Su XY, Gao YC, Song YL, Lam JWY, et al. Polymer 2005;46:10592–600.[15] Koch T, Ritter H. Macromolecules 1995;28:4806–9.[16] Jiang HW, Kakkar AK. Macromolecules 1998;31:4170–6.[17] Wu LF, Tuo XL, Cheng H, Chen Z, Wang XG. Macromolecules 2001;34:8005–13.[18] Kumar J, Li L, Jiang XL, Kim DY, Lee TS, Tripathy SK. Appl Phys Lett 1998;72:2096–8.