Reactive & Functional Polymers 83 (2014) 70–75

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Reactive & Functional Polymers

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UV-light assisted single step route to functional PEEK surfaces

http://dx.doi.org/10.1016/j.reactfunctpolym.2014.07.0111381-5148/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Laboratory of Nanomedicine and Biomaterials,MIT-Harvard Center for Cancer Nanotechnology Excellence, BWH, 75 Francis Street,Boston, MA 02115, USA. Tel.: +1 617 525 7617; fax: +1 617 730 2801.

E-mail addresses: yameen@mit.edu, basityameen@gmail.com (B. Yameen).

Ahmed Yousaf a, Aleeza Farrukh a, Zehra Oluz b, Eylül Tuncel b, Hatice Duran b, Sema Yiyit Dogan c,Turgay Tekinay c, Habib ur Rehman a, Basit Yameen a,d,⇑a Department of Chemistry, SBA School of Science and Engineering, Lahore University of Management Sciences, Lahore 54792, Pakistanb Department of Materials Science & Nanotechnology Engineering, TOBB University of Economics and Technology, Sögütözü Cad. 43, 06560 Ankara, Turkeyc Gazi University, Life Sciences Application and Research Center, Gölbas�ıi, 06830 Ankara, Turkeyd Laboratory of Nanomedicine and Biomaterials, MIT-Harvard Center for Cancer Nanotechnology Excellence, BWH, 75 Francis Street, Boston, MA 02115, USA

a r t i c l e i n f o

Article history:Received 13 February 2014Received in revised form 26 May 2014Accepted 5 July 2014Available online 12 July 2014

Keywords:PEEKPhotograftingSurface functionalizationpH-responsiveAntifouling

a b s t r a c t

Polyether ether ketone (PEEK) is a thermoplastic polymer of high technological relevance and is com-posed of repeating phenyl ether and benzophenone units. In the present work we will demonstratethe potential of UV irradiation assisted generation of free radicals on the surface benzophenone unitsto graft a variety of polymer chains on the PEEK surface. Both ‘‘grafting-to’’ and ‘‘grafting-from’’ approacheswere explored by using different monomers and polymers. Styrene, butyl acrylate (BA), vinyl phosphonicacid (VPA), acrylic acid (AA), polyacrylic acid (PAA) and monomethoxy terminated oligo(ethylene glycol)methacrylate (MeOEGMA) were successfully utilized for this purpose. The functionalized membraneswere characterized by X-ray photoelectron spectroscopy (XPS), attenuated total reflectance Fourier-transformed Infrared (ATR-FTIR) spectroscopy, atomic force microscopy (AFM), and contact angle (CA)goniometry. PAA and PVPA functionalized PEEK surfaces exhibited pH responsive wettability behavior.PAA functionalized PEEK surfaces were further modified with lysine, which led to the controlled surfacewettability over a broader pH range as compared to the simple PAA functionalized surface. The graftingwith polyMeOEGMA rendered PEEK surface with nonfouling properties against bacterial growth. Employ-ing this highly economical and simple method, the surface properties of PEEK can be modulated andtuned according to a specific application.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Polyether ether ketone (PEEK), which belongs to a class of highperformance polymers (namely poly aryl ether ketones), is a chem-ically and mechanically stable, high temperature, semi crystallinethermoplastic polymer [1,2]. The repeat units in PEEK are com-posed of alternating aryl ether and benzophenone moieties. Ithas high hydrolysis resistance, low flammability, non-toxicityand biological inertness [3,4]. PEEK is compatible with glass andcarbon fiber and is employed in a variety of aerospace and engi-neering applications [5]. The application range of PEEK include,but are not limited to, microfiltration membranes, and directmethanol fuel cell membranes, surgical implants in orthopedicsand spinal surgeries [6,7]. Properties and applications of polymericmaterials are highly dependent on their surface characteristics and

the way they interact with the environment. PEEK exhibitshydrophobic surface characteristics mainly due to the non-polararomatic backbone, which limits its usage in biocompatible appli-cations because of the low compatibility and biofouling. Thus theability to modulate PEEK surface characteristics is an essential toolin establishing and expanding its field of applications. Surfacemodification of thin films can be accomplished by both non-cova-lent means such as by deposition or spraying a polymer coatingfrom a solution or by covalently tethering polymeric chains suchas polymer brushes. Plasma polymerization [8], UV-assisted graftpolymerization [9,10], reversible addition-fragmentation chaintransfer (RAFT) polymerization [11], nitroxide-mediated polymer-ization (NMP) and atom transfer radical polymerization (ATRP)are among the leading strategies employed for covalent anchoringof polymer chains onto a surface [12–14]. The polymers can be sur-face grafted by both ‘‘grafting-to’’, approach involving covalentgrafting of pre-synthesized polymer chains on an appropriatelyfunctionalized surface via a coupling reaction, or by ‘‘grafting-from’’ pathway, which involves initiation of polymerization froman initiator moiety covalently tethered onto a surface [14].

A. Yousaf et al. / Reactive & Functional Polymers 83 (2014) 70–75 71

The surface functionalization of PEEK has been carried outmainly by the treatment with high energy species (such as plasmaand ozone), by the wet chemical methods, and by a simple twostep method to graft an ATRP initiator followed by surface initiatedATRP [15,16]. UV assisted surface photograft polymerization isanother simple and cost effective approach to surface functionali-zation that provides lower reaction time and is effective for a vari-ety of monomers [9]. Various UV assisted photoinitiation methodshave been developed to achieve surface graft polymerization withthe aid of a photoinitiator such as benzophenone. Kyomoto andIshihara have reported a ‘‘self-initiated’’ surface graft polymeriza-tion whereby PEEK surface was modified by employing UV inducedpolymerization while heating the reaction externally at 60 �C using2-methacryloyloxyethyl phosphorylcholine monomer [17]. How-ever, heating to 60 �C may itself result in polymerization or crosslinking of the monomer which may cause substantial solutionpolymerization in addition to the surface initiated (SI) polymeriza-tion. The present work deals with this problem and exhibits a prac-tically simple and general route to polymer grafted functional PEEKsurfaces by simply capitalizing on the reactive nature of the freeradical species generated at the PEEK surface via UV irradiationwithout any external heating. Both the ‘‘grafting-to’’ and ‘‘graft-ing-from’’ approaches are demonstrated in this work. Styrene,vinyl phosphonic acid (VPA), butyl acrylate (BA), acrylic acid(AA), polyacrylic acid (PAA) and monomethoxy terminated oligo(ethylene glycol) methacrylate (MeOEGMA) were successfullyemployed for PEEK surface functionalization. These modified PEEKmembranes were characterized using ATR-FTIR spectroscopy,water contact angle measurements, atomic force microscopy(AFM) and X-ray photoelectron spectroscopy (XPS). Furthermore,polyMeOEGMA grafted PEEK membranes were evaluated for theirnonfouling characteristic against the bacterial growth. Negativelycharged PAA modified PEEK membrane was dyed with positivelycharged Rhodamine 6G dye to demonstrate the electrostatic inter-action of the functionalized PEEK surface. PAA modified PEEKmembrane was further functionalized with lysine that, along withVPA grafted membranes, exhibiting pH tunable surface wettabilitybehavior.

2. Experimental section

2.1. Materials

PEEK membranes (thickness �50 microns) were obtained fromVictrex (Lancashire, England). Monomethoxy oligo (ethylene gly-col) methacrylate methacrylate (MeOEGMA, average Mn 300),acrylic acid (AA, 99%), polyacrylic acid (PAA, 25% solution in water),styrene (99%), n-butyl acrylate (BA, 99%), vinyl phosphonic acid(VPA, 98%), copper(II) chloride (CuCl2, 99.9%), ethylenediaminetet-raacetic acid (EDTA, 99%), tetrahydrofuran (THF, 99%), L-lysine(98%), and ethanol (98%) were used as obtained from Sigma–Aldrich, Germany. Pentafluorophenol (99%) and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC, 98%) were obtainedfrom Alpha Aesar, UK, while Rhodamine 6G was acquired fromMP Biomedicals, Germany. PEEK membrane was cleaned prior touse by refluxing in acetone for 48 h at 60 �C with continuous stir-ring, and it was subsequently dried in an oven at 60 �C for 24 h andstored under nitrogen [16].

2.2. Surface initiated UV assisted radical polymerization

2.2.1. General procedurePolystyrene (PEEK-PS), polyMeOEGMA (PEEK-PolyMeOEGMA),

polyBA (PEEK-PBA), PolyVPA (PEEK-PVPA), PAA (PEEK-PAA), andPAA (via grafted-to, PEEK-PolyAA) functionalized PEEK surfaces

were obtained by UV initiated photografting. PEEK membrane1 � 1 cm2 was placed in a Schlenk flask (Witeg LabortechnikGmbH, Borosilicate glass) and degassed by applying four vac-uum/N2(g) cycles. Pure monomers and polymer solution of 25%PAA were degassed by purging N2(g) for 1 h and transferred tothe degassed PEEK membrane, adding enough to completelyimmerse the membrane, with continuous stirring under inertatmosphere. The flask was exposed to UV lamp (Osram Ultra Vitalux230 V, at 315–400 nm) for 3 h at a distance of 15 cm. The polymer-ization was quenched by exposing the content to air and mem-branes were rinsed with the excess of an appropriate solvent togive polymer functionalized PEEK membrane. Toluene was usedfor the rinsing step of PEEK-PS, and THF was used for PEEK-PBA.Water was used to rinse PEEK-PVPA, PEEK-PolyAA, PEEK-PAA andPEEK-PolyMeOEGMA.

2.3. pH responsive tuning of surface wettability (Lysine grafted PAA,PEEK-LAA)

PEEK-PAA membrane was functionalized with lysine to demon-strate change in surface wettability with pH by tuning surfacecharge. Carboxylic acid and a-amino group of lysine were blockedby forming its complex with copper, by employing a previouslyreported method [18]. Briefly; 0.05 M lysine solution (73 mg) wasprepared in 60% ethanol:water (5 mL) followed by the gradualaddition of CuCl2 (21.5 mg, 0.025 M) to form a blue coloredlysine–Cu complex. In a separate flask, the carboxylic acid moietyof PEEK-PAA membrane was activated by immersing it for 1 h in0.1 M ethanolic solution of EDC (1.55 mg/10 mL) and pentafluor-ophenol (20 mM, 368 mg/10 mL). The amine reactive pentafluor-ophenyl ester functionalized membrane was then immersed intothe lysine–Cu complex solution for overnight at room temperature.The functionalized PEEK membrane was washed with distilledwater and immersed in 100 mM EDTA solution to remove copperions. The membrane was thoroughly washed with deionized waterand air dried. The water contact angles were measured by immers-ing membrane in acidic, basic and neutral pH solutions to exhibitswitching of surface wettability. One part of PEEK-LAA membranewas immersed for overnight in 1 M NaOH solution while anotherpart was immersed for overnight into the 1 M HCl solution beforemeasuring its surface water contact angles.

2.4. Demonstration of surface charge on PEEK-PAA

To display the differential electrostatic interaction of the sur-faces of the pristine PEEK and PEEK-PAA membranes with chargeddye molecules, pristine PEEK and PEEK-PAA membranes wereimmersed in 25 ppm solution of positively charged Rhodamine6G for 2 days at room temperature, followed by thoroughly rinsingof the membranes with deionized water [16].

2.5. Antifouling assessment of PEEK-polyMeOEGMA membrane

The antifouling activity of PEEK membranes was determined byexposing the surfaces to the growth of Gram-negative bacterium,(Escherichia coli) E. coli. A 100 lL of the overnight grown E. coli(0.5 McFarland) was spread onto (Luria–Bertani) LB agar. PEEKmembranes were sterilized by UV irradiation and placed into thetest flask. Test and control samples were incubated at 37 �C for24 h. After the incubation, membranes were washed with PBS(phosphate buffer saline, pH 7.5) buffer and stained with the Gramstain, and bacteria adhesion was examined by light microscopy(Leica DM 750, Las EZ V3).

72 A. Yousaf et al. / Reactive & Functional Polymers 83 (2014) 70–75

2.6. Surface characterization

The CA measurements were performed on KRUSS DSA 30,Germany. Ultrasonic cleaner (JPL model Ultra 8050D-H) was usedfor membrane cleaning. OSRAM ULTRA VITALUX 230 V E27/ESUV Lamp with the wavelength of 315–400 nm and intensity of300 W was used for photografting. FTIR spectra were recorded onATR-IR spectrometer (Alpha Bruker, Germany). XPS analyses werecarried out using Thermo Scientific K-Alpha. The Mg Ka(1253.6 eV) X-ray source was operated at 300 W. A pass energyof 117.40 eV was used for the survey spectra. The spectra wererecorded using a 60� take-off angle relative to the surface normal.AFM measurements were performed with PSIA XE-100E in tappingmode equipped with a microfabricated Silicon cantilever coveredwith Au (spring constant is 40 N/mm).

3. Results and discussion

3.1. Grafting and characterization of UV assisted polymer grafting onPEEK surface

Capitalizing on the tendency of benzophenone moieties to gen-erate free radicals on exposure to the UV radiations (Scheme 1),polymer chains of wide range of polymers were covalently graftedon the surface of PEEK membranes. The surface radical speciesunderwent free radical polymerization on interaction with mono-mers or insertion to the CAH bonds available in the already syn-thesized polymers, leading to polymer chains grafted PEEKsurfaces. Grafting of PAA chains on the PEEK surface by employingpolyacrylic acid exhibits the applicability of the proposed strategy

Scheme 1. Schematic demonstration of the generation of free radical and

as a ‘‘grafting-to’’ approach, whereas the rest of examples demon-strated in this study highlight its applicability as ‘‘grafting-from’’approach.

In order to estimate any detrimental effects of UV irradiation onthe PEEK, we have tested the thermal stability of pristine PEEKbefore and after exposure to the UV radiation using TGA andDSC. The untreated membrane and membrane treated with UVradiations, in the absence of any reactive monomer or polymer,showed almost similar thermal behavior. Both the membranesshowed the onset of degradation at around 510 �C, the glass tran-sition at 193 ± 2 �C and melting transition at 339–340 �C (Fig. S2).Thermal analysis clearly shows that PEEK film retains its stabilityeven after UV irradiation.

The surface functionalization of PEEK membrane was corrobo-rated by ATR-FTIR spectroscopy. Absorption peaks were observedat 1596, 1485, 1278 and 1183 cm�1 for bare PEEK membrane.The stretching vibration of C@O bond appearing in the FTIR spectraof the PEEK-PolyMeOEGMA, PEEK-PAA and PEEK-PBA at 1720,1731 and 1724 cm�1 established incorporation of the respectivepolymer chains on the surface of PEEK membrane (Fig. 1). No addi-tional peaks were observed in FTIR spectra of PEEK-PS and PEEK-PVPA, which is due to overlapping of IR absorption range of PSand PVPA with that of PEEK. In order to quantify PEEK surface mod-ification, the FTIR spectra were normalized according to the peakintensity of phenyl rings (at 1485 cm�1) of bare PEEK, which areconsidered to be invariant during the transformation. The estima-tion based on the relative stretching vibration of C@O bond inPEEK-PolyMeOEGMA, PEEK-PAA and PEEK-PBA at 1720, 1731 and1724 cm�1 revealed 25.10%, 15.28% and 10.85% conversion of thecarbonyl groups for the respective SI polymerization on the PEEKsurface.

subsequent functionalization of polymer chains at the PEEK surface.

3000 2500 2000 1500

(f)

(e)

(d)

(c)

(b)

Tra

nsm

ittan

ce (

%)

Wavenumber (cm-1)

(a)

1720 cm-1

1731 cm-1

1724 cm-1

1647 cm-1

2867 cm-1

Fig. 1. ATR-FTIR spectra of pristine PEEK (a), PEEK-PAA (b), PEEK-PBA (c), PEEK-PolyMeOEGMA (d), PEEK-PS (e) and PEEK-PVPA (f).

A. Yousaf et al. / Reactive & Functional Polymers 83 (2014) 70–75 73

The surface modification of PEEK was successfully evaluated byemploying XPS analysis (refer to Supplementary informationSection). The XPS of pristine PEEK membrane showed signals at533 and 286 eV corresponding to O1s and C1s orbitals, respectively(Fig. S1a) [16]. The examination of high resolution C1s scanrevealed peaks for CAC and C@O at 287.5 and 285.6 eV respec-tively, corresponding to the benzophenone and aryl ether moietieson the PEEK surface (Fig. S1b). The change in the surface C/Oatomic ratio from 2.75 for pristine PEEK to 4.41 for PEEK-PBA(XPS Fig. S1c), supports the functionalization of PEEK surface withPBA chains [19]. Similarly, change in the C/O ratio to 6.21 for PSsurface and appearance of signals at 284.2, 284.4, 286.1 and287.1 eV for CAO, CAC, C@O and OAC@O confirmed the function-alization of PEEK surface with styrene chain [20]. The XPS surveyscan of PEEK-PVPA displayed signals for P2s and P2p orbitals at190 and 133 eV in addition to O1s and C1s at 531 and 284 eV,which substantiated the surface functionalization with VPA chains(Fig. S1i). The signals at 288.3, 286.0 and 284.3 eV for C@O, CACAOand CAC in the C1s high resolution scan of PEEK-PolyMeOEGMAverified the grafting of polyMeOEGMA chains onto the PEEK sur-face [16]. The high resolution XPS analysis of PEEK-PAA surface

Fig. 2. Topographical 3D AFM height images of pristine PEEK (a), PEEK-PolyM

showed signals for the COOH and aliphatic carbons at 288.39 and2841.18 eV respectively, proving the effective surface modificationwith PAA [21]. In the XPS survey scan of PEEK-LAA, besides thechange in intensity of O1s and C1s signals, the signal for N1s at400 eV verified the surface modification with lysine moieties[22]. The signal for NHACAO at 399.3 eV in the high resolutionN1s scan further authenticated the existence of lysine moieties atthe surface (Fig. S1n).

The pristine PEEK films were smooth and displayed an averageroughness of only Rms 0.60 ± 0.05 nm over 1 lm2 area (Fig. 2a).MeOEGMA brush layer covering film (PEEK- MeOEGMA) surfaceexhibited large features of up to 20 nm with a higher roughness(Rms = 3.37 ± 0.6 nm) and a Rmax of 5.20 ± 0.8 nm compared to thepristine PEEK (Fig. 2b). The brush grown using AA monomer(PEEK-PAA) is rougher and more uniformly distributed over thePEEK substrate with Rms of 3.70 ± 1.60 nm compared to pristinePEEK film (Fig. 2c). On the other hand, PEEK-polyBA, PEEK-PS andPEEK-polyVPA (Fig. 2d–1f) showed more waviness in the surfacetexture with the hills and valleys on the surface leading to anincreased roughness of 2.35 ± 0.6 nm (Rmax = 3.60 ± 0.7 nm),2.20 ± 0.3 nm (Rmax = 6.26 ± 1.0 nm) and 4.13 ± 1.2(Rmax = 9.60 ± 1.7 nm), respectively. As can be seen from Fig. 2a–f,the surface roughness of all polymer brush covering films are muchhigher compared to untreated PEEK films. The roughness of surfacecorresponds to the difference in the polydispersity of polymerchains tethered to the surface, which is a general consequence ofSI conventional radical polymerization.

3.2. Changes in PEEK surface characteristics by surface graftedpolymer chains

The grafting of polymer chains of different polymers rangingfrom PAA to PS imparts enormous variation in surface behaviorof PEEK. The accomplished surface functionalization assisted intuning surface characteristic from changes in wettability extendedto antifouling, modulating of surface charges, and pH responsiveproperties. PEEK displays hydrophobic surface while on functional-ization with hydrophilic PolyMeOEGMA chains, a drop in surfacetension as reflected by the drop in surface water CA was observed(Fig. 3c). The functionalization of PEEK with PolyAA, PAA and PVPA,showed pH stimulated change in the surface water CAs on varia-tion of external pH due to the (de)protonation of the acidic func-tionalities at the surface. Because of the hydrophobic nature ofthe PS and PBA, no substantial change in the surface water of

eOEGMA (b), PEEK-PAA (c), PEEK-PBA (d), PEEK-PS (e) and PEEK-PVPA (f).

Fig. 3. Images of water drops on different membranes with their corresponding CAs. Pristine PEEK (a), PEEK-PS (b), PEEK-PolyMeOEGMA (c), PEEK-PBA (d), PEEK-PVPA atpH = 7 (e), PEEK-PVPA at pH > 7 (f), PEEK-PolyAA at pH = 7 (g), PEEK-PolyAA at pH > 7 (h), PEEK-PAA at pH = 7 (i), PEEK-PAA at pH > 7 (j) and PEEK-PAA at pH < 7 (k).

Fig. 4. Exploiting the change in the surface properties imparted due to the surface modification (a) pristine PEEK (I) and PEEK-PAA dyed in positively charged Rhodamine 6G(II), pH responsive wettability of PEEK-LAA (b).

Fig. 5. Antifouling characteristic of PEEK-PolyMeOEGMA, light microscope images of pristine PEEK (a), and PEEK-PolyMeOEGMA (b). (The scale bars are 2 lm).

74 A. Yousaf et al. / Reactive & Functional Polymers 83 (2014) 70–75

CAs was observed after grafting these polymers onto the PEEKsurface.

PEEK-PAA has an inherent negative charge on its surface, thisproperty was exploited to stain the membrane with a 25 ppm solu-tion of positively charged Rhodamine 6G dye. The dye showedexcellent adhesion with PAA modified PEEK while unmodified

PEEK exhibited no coloration on its surface because of the electro-static neutrality of its surface (Fig. 4a).

Furthermore, to extend the scope of pH-responsive behavior,PEEK-PAA was further derivatized with lysine. The lysine function-alized membrane presented zwitterionic groups, which showedthe modulation of surface water CA with a hydrophilic surface

A. Yousaf et al. / Reactive & Functional Polymers 83 (2014) 70–75 75

nature at pH > 7 (CA = 49�) that turned into a hydrophobic surfacewith a CV of 85� at neutral pH and turned again into a hydrophilicsurface (CA = 50�) at pH < 7. This change in surface wettability is ingood correlation with the change in the surface electrostatic chargedensity imparted by the change in the environmental pH (Fig. 4b).

The surface functionalization with PEEK Poly-MeOEGMA notmerely enhanced surface hydrophilicity, but also displayed its anti-fouling characteristics by preventing bioadhesion. The polyethyl-ene glycol moieties are known to have hygroscopic andhydrophilic properties and imparting bio-repellent properties tothe surface. The antifouling characteristic of PEEK-PolyMeOEGMAmembranes was demonstrated (Fig. 5) by treating the membranewith bacterial solution of Gram-negative bacterium, E. coli. A largenumber of bacteria were found to attach to the pristine PEEK mem-brane, while PEEK-PolyMeOEGMA membrane was resistanttowards the bioadhesion.

4. Conclusions

The PEEK surface properties were conveniently tailored byusing a simple strategy based on covalently surface tethering thepolymer chains by employing UV assisted free radical generationon the surface benzophenone moieties. The strategy is demon-strated to work under both settings i.e. ‘‘grafting-to’’ by usingpresynthesized polymers and ‘‘grafting-from’’ by using monomericprecursors. The strategy is further established to be toleranttowards a variety of monomers and polymers with completely dif-ferent chemical natures. Exploiting this one step approach, a rangeof technologically relevant surface characteristics PEEK membrane,such as surface charge (PEK-PAA), wettability (PEKK-LAA) andantifouling (PEEK-PolyMeOGMA) properties, were successfullymodulated. As such, this approach holds an excellent potential tobecome a simple single step and low cost route to functional PEEKsurfaces for industrial applications.

Acknowledgments

B.Y. acknowledges The Higher Education Commission (HEC) ofPakistan for Funding (Project No. 20-1799/R&D/10-5302 and

20-1740/R&D/10-3368) and LUMS for the Startup Grant. H.D.gratefully acknowledges The Scientific and Technological ResearchCouncil of Turkey (TUBITAK) for the financial support of Project No.112M804.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.reactfunctpolym.2014.07.011.

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