Surface functionalization of poly(ether ether ketone) by wet-chemical modification with carboxylic acids and diamine

Abstract Polyaryletherketones (PAEK) are high-performance plastics with excellent mechanical properties and high thermal resistance used, e.g. as material for implants. Their molecular structure gives them a high level of chemical inertness but prevents bonding and adhesive interactions with other materials and integration into body tissue. Using the example of poly(ether ether ketone) (PEEK), we show novel synthetic routes for wet-chemical modification of the surface to create reactive carboxylic and amino groups. In a first approach, the ketone functionalities of the PEEK molecules were reduced to hydroxy groups and subsequently esterified with carboxylic acids. Using dicarboxylic acids, one acid group was bonded covalently to the PEEK surface while the other provided free acidic functionalities. In a second approach, the PEEK surface was modified in one step by reaction of the ketone group with a diamine, creating primary amino groups on the PEEK surface. All reaction products were analyzed by a powerful combination of surface sensitive X-ray photoelectron spectroscopy (XPS) and advanced infrared spectroscopic methods.


Introduction
Polyaryletherketones (PAEK) are high-performance thermoplastic materials.They consist of benzophenone units coupled via ether groups, are chemically inert and thermally stable up to temperatures above 300 C. Their unique properties make them suitable as substitutes for metal parts but high price and energy-intensive processing limit their use to special applications.Thanks to their thermoplastic behavior, PAEK parts can be manufactured by techniques like extrusion, injection and compression molding for mass production or by 3D printing if customized special parts CONTACT Cordelia Zimmerer zimmerer@ipfdd.deLeibniz-Institut f€ ur Polymerforschung Dresden e.V., Dresden, Germany Supplemental data for this article can be accessed online at https://doi.org/10.1080/01694243.2023.2223334.
Outside the medical field, PAEK are applied as lightweight construction materials for parts exposed to elevated temperatures, e.g. in high-voltage engineering or as cable insulation in crude oil production [16,17].Metal and carbon fiber/PAEK composites are employed as framing materials in automotive industries, railway technologies and aerospace engineering or to manufacture plain bearings and nozzle holders in 3D printers.The use as flexible circuit carrier or electromagnetic shielding of electronic or electrical devices requires metallization by chemical or physical vapor deposition.Valve seal settings [18] and mechanically movable assemblies and conveyors in ultra-high vacuum systems in semiconductor electronic and microsystem technologies are often made from PAEK due to their low outgassing.Furthermore, PAEK have a great potential for magnesium-based lightweight construction.PAEK coatings directly applied on the highly corrosive oxidized magnesium surface prevent permanently contact with other metal parts to protect magnesium alloys against corrosion also in zones with high temperature demands [19].
While the hydrophobicity and chemical inertness of PAEK are advantageous for the long-term stability of the material, they complicate the bonding and adhesion to other materials, proteins and cells, the interaction with body tissue and osseointegration [20,21].To overcome this drawback, the PAEK surfaces need to be equipped with polar, reactive functional groups that may serve as basis for further reactions or anchor for bioactive components.In contrast to unpolar mass polymers as polyethylene, polypropylene and polyvinylchloride, the phenyl rings in combination with ketone groups in PAEK provide a structural basis for the bonding of other molecules and thus allow for a chemical modification of the PAEK surface [22,23].Especially amino and carboxylic acid surface groups have been proven to promote wettability, protein adhesion and cell growth [24,25].
In their reviews, Yin et al. [20] and Zheng et al. [21] give a comprehensive survey about biological requirements of PEEK implants and strategies for biologically active surface functionalization.Simple surface coatings often suffer a lack of adhesion on the inert material.Physico-chemical surface modification techniques, such as plasma, laser or UV treatment (in combination with ozone) are able to improve wetting and adhesion [26].It is, however, difficult to control the surface reactions to produce defined functional groups, and the effect is not stable in the long term if it is not immediately followed by bonding or adsorption of a functional layer [27,28].Recently, Ates et al. [29] and Dede et al. [30,31] showed that the adhesion of PAEK to composite resin can be improved significantly by abrasion, silica-coating, sulfuric acid (H 2 SO 4 ) etching, laser and plasma treatment and a combination thereof; the treatment did always affect the surface topography of the PAEK samples, too.
The ketone or benzophenone groups of PAEK are potential reaction centers.Since their p-electrons are involved in the conjugate p-electron system of the benzophenone units, the reactivity of the ketone groups is rather limited [32].Many chemical modification procedures of PAEK are based on harsh treatment with concentrated acids (sulphuric, phosphonic, hydrofluoric or nitric acid) [21,33].The sulfonation creates hydrophilic À SO 3 H groups in a fast reaction at low cost but affects the surface topography by producing micropores [34].After sulfonation, the surface can be further modified by amino groups to improve its hydrophilicity and cell compatibility [35].Furthermore, a mixture of concentrated nitric and H 2 SO 4 was used for the nitration of PEEK.Then, amino groups can be released by the subsequent reduction of nitro groups to amino groups.Ji et al. [36] hydroxylated laser-structured PEEK surfaces with dimethyl sulfoxide (DMSO) and sodium borohydride and thus created a microsubmicro structure and hydroxy groups.Miyagaki et al. [37] reacted 10-indecanoyl chloride to the PEEK surface using the Friedel-Crafts reaction to obtain terminal carbon double bonds as basis for covalent bonding to an epoxy matrix.
Modifications based on concentrated acids are rather harsh and hardly tempting.Franchina et al. reacted the carbonyl groups of PEEK surfaces with hydroxylamine and hydrazine derivatives.More recently, Kassick et al. [38] introduced a mild surface modification: an acid-mediated addition of hydrophilic oxyamine and hydrazine nucleophiles to the ketone moiety of PEEK to improve the osteoconductive properties.The reduction of the ketone groups to hydroxyl groups breaks the p-electron conjugation between the neighboring phenyl rings and enables subsequent modification reactions.Marchand-Brynaert and coworkers used this route for the acylation of PEEK with reactive carbonic acid derivatives [24] and for the nucleophilic substitution especially with amides [25,39] to create carboxyl and amino groups on the PEEK surface.Such a functionalization improves wettability and adhesion [40].It provides a wide range of possibilities for further surface modifications and can serve, e.g. as basis for the condensation of peptide sequences for medical applications [41] or the bonding of chitosan to improve the osteogenic capability and antibacterial properties [42,43].
The aim of the present study is to develop a wet-chemical method for the surface modification of PAEK that does neither affect the surface topography and its mechanical properties nor require hazardous or corrosive chemicals and harsh reaction conditions, and to identify spectroscopic techniques able to verify the surface modification.PEEK surfaces are used as model systems for the creation of designed surface properties of PAEK polymers.In one approach, PEEK was first reduced to PEEK-OH.The PEEK-OH surfaces were subsequently acylated with carboxylic acids.The esterification with aliphatic dicarboxylic acids creates PEEK surfaces with free carboxylic groups (PEEK-COOH).A novel, promising alternative is the one-step modification of PEEK surfaces by reacting them with a lithiated diamine.Extensive characterization of the surfaces by X-ray photoelectron spectroscopy (XPS), ATR-FTIR and transmission IR (TIR) spectroscopy proved the successful modification.[4] PEEK granules were purchased from Evonik Resource Efficiency GmbH (Marl, Germany).The organic acids used to modify PEEK surfaces are summarized in Table 1.Dimethyl sulfoxide (99% p.a.), sodium hydroxide (NaOH) pellets, sodium tetrahydridoborate (NaBH 4 ) (>96%), and n-butyllithium (tetra-m3-butyl-tetralithium) dissolved in hexane (2.5 molÁL À1 ) (n-BuLi) were purchased from Sigma-Aldrich (St. Louis, MO).Dimethylformamide (DMF) (99.9%) and hydrochloric acid (HCl) (36%) were provided by Alfa Aesar (Haverhill, MA).n-Butanon (99.5%) and potassium hydroxide (KOH) pellets were obtained from Merck KGaA (Darmstadt, Germany).Before its use, EDA was dried over KOH and freshly distilled under argon.Tetrahydrofuran (THF) (99.99%) from Acros Organics B.V.B.A. (Geel, Belgium) was stored over KOH and freshly distilled before use.Sulfuric acid (>98%, p.a.) was used as received from VWR International (Leuven, Belgium).Deionized water (conductivity less than 0.055 mSÁcm À1 ) was provided by a PURELAB V R Lab Water dispenser (ELGA LabWater, High Wycombe, UK).The Eosin Y (2-(2,4,5,7-tetrabromo-6-oxido-3-oxo-3H-xanthen-9-yl)benzoate) dye for the detection of amino groups was provided by Sigma-Aldrich (St. Louis, MO).

Preparation of polymer surfaces
Three types of surfaces were investigated in this study: injection-molded PEEK plates, PEEK nanoparticle films and spin-coated films of reduced PEEK (PEEK-OH).
Sheets of 80 mm Â 80 mm Â 2 mm size were injection-molded from PEEK granules using a mass temperature of 370-380 C and a tool temperature of 140-160 C. For the modification experiments, these sheets were cut into pieces of 15 mm Â 10 mm Â 2 mm.Processing aids were removed by rinsing with ethanol abs.(VWR International, LLC, Radnor, PA).Pieces (10 mm Â 15 mm Â 0.7 mm) of silicon wafers (provided by Si-Mat, Kaufering, Germany) were used as substrates for spin-coated films.They were cleaned 5 min with dichloromethane (Acros Organics B.V.B.A., Geel, Belgium) and 5 min with ethanol in an ultrasonic bath at room temperature.
PEEK particle films were prepared as follows: 25 mg (0.086 mmol) of granular PEEK was dissolved in 5 mL (70 mmol) of methanesulfonic acid (CH 3 SO 3 H).The resulting dark yellow solution was precipitated dropwise in 50 mL of deionized water and 2.8 g (70 mmol) NaOH were added to neutralize the acid.The precipitate was filtered over a glass frit, washed six times with 300 mL deionized water and dispersed PEEK-OH was prepared as described in Section 2.3.1.For the spin-coating, a 20 gÁL À1 PEEK-OH solution was prepared with freshly distilled THF.Insoluble particles were removed with a syringe filter (pore size 0.45 mm, WICOM Germany GmbH, Heppenheim, Germany).The filtered solution was spin-coated on pieces of cleaned silicon wafers (POLOS SPIN150i spin coater, Putten, The Netherlands) in four steps (100 rpm for 3 s, 1600 rpm for 6 s, 1800 rpm for 10 s, and 3000 rpm for 15 s).The sample was dried for 2 h in an oil pump vacuum at 60 C.

Modification of the PEEK surfaces
Scheme 1 shows an overview of the synthetic routes for the modification of PEEK surfaces.The first approach is based on the reduction of the ketone carbonyl group creating a hydroxy group onto which various organic acids are bound subsequently by esterification.In a second approach, a diamine is bound in a one-step procedure to the carbonyl group forming an imine and a free amine functionality.

Reduction of the carbonyl groups of PEEK
The reduction of carbonyl surface groups was performed on pieces of PEEK injection-molded sheets and PEEK particle films.The samples were placed in a Schlenk tube equipped with a gas bubbler and stir bar.Under constant stirring, 200 mL ethanol (abs.), 2.9 g (76.7 mmol) NaBH 4 and 0.8 g (20 mmol) ground NaOH were added subsequently.The reaction was carried out at room temperature (23 C).After 48 h, the modified PEEK samples (now named PEEK-OH) were removed from the Scheme 1. Synthetic routes to endow PEEK surfaces with aliphatic functionalization (PEEK-Ac), and carboxylic end groups (PEEK-COOH) or primary amino groups (PEEK-EDA).
reaction mixture and washed three times 15 min with 100 mL ethanol, 100 mL deionized water, and 100 mL ethanol.The PEEK-OH sheets were dried at 60 C in an oil pump vacuum overnight.
To prepare bulk PEEK-OH, 10 g (34.4 mmol) of ground and sieved PEEK (particle size < 500 mm) were dispersed in 250 mL of DMSO in a two-necked flask with a reflux condenser.After adding 3 g (79.3 mmol) NaBH 4 , the reaction was started at 120 C. The work done to optimize the reaction time is described in the Supporting Information.After a reaction time of six days, the soluble PEEK-OH educts were separated from the insoluble fraction by a B€ uchner funnel; DMSO was evaporated in a rotary evaporator at 90 C under reduced pressure.After adding 100 mL diluted HCl (10%) to the soluble fraction, the polymer was separated by a B€ uchner funnel, washed with 150 mL deionized water, 150 mL ethanol and again with 150 mL deionized water and dried at 80 C in a vacuum overnight.

Oxidation of PEEK-OH films
Sixty milligrams (0.2 mmol) K 2 Cr 2 O 7 were dissolved in 2.5 mL aqueous 2 N H 2 SO 4 solution and a wafer piece carrying a PEEK-OH film was added.After 67 h, the sample was taken off the mixture.Since the film detached from the wafer, it was prewashed for 15 min in ethanol and dried 5 min at 60 C in an oil pump vacuum to improve the adhesion to the wafer.Then, the sample was washed twice for 15 min in ultrapure water and dried for 30 min at 60 C in an oil pump vacuum.

Esterification of PEEK-OH films with acetic acid
In a rolled rim vial (; 15 mm) mounted on a laboratory shaker, 20 mg (0.2 mmol) H 2 SO 4 were dissolved in 2 mL acetic acid (AcOH).Then, a piece of a silicon wafer coated with a PEEK-OH film was added to the reaction mixture.After a reaction time of 15.5 h at room temperature (23 C), the sample (now named PEEK-Ac) was removed, washed three times for 15 min with ethanol and dried at 60 C in an oil pump vacuum overnight.

Esterification of PEEK-OH films with dicarboxylic acids
In separate rolled rim vials (; 30 mm, height 46 mm) mounted on a laboratory shaker, 380 mg (3.65 mmol) of malonic acid (MaA), 290 mg (2.46 mmol) of succinic acid (SuA), 860 mg (6.51 mmol) glutaric acid (GlA), and 280 mg (1.92 mmol) adipic acid (AdA), respectively, were dissolved in a mixture of 1 g H 2 SO 4 and 4 mL DMF.Then, a piece of a silicon wafer coated with a PEEK-OH film was added to each of the reaction mixtures.After a reaction time of 64.5 h at room temperature (23 C), the supports with the PEEK-COOH modified films (PEEK-MaA, PEEK-SuA, PEEK-GlA, and PEEK-AdA) were removed, washed three times for 15 min with ethanol and dried at 60 C in an oil pump vacuum overnight.

Amine functionalization of PEEK
A baked 25 mL Schlenk tube equipped with dropping funnel and stir bar was filled with 5 mL (75 mmol) of carefully dried EDA.Under stirring, 5 mL (12.5 mmol) of 2.5 molÁL À1 n-BuLi dissolved in n-hexane were slowly added.After 4 h the solution of the activated diamine was transferred with a syringe to a baked 50 mL Schlenk tube containing a silicon wafer piece with a PEEK particle film.The reaction mixture was gently shaken over 42 h with a laboratory shaker.Then, the solution was withdrawn with a syringe and separately deactivated in n-butanol.The wafer with the modified polymer film (now named with PEEK-NH 2 ) was removed, washed three times for 15 min with ethanol and dried at 60 C in an oil pump vacuum overnight.Amine functionalization of PEEK powder was performed in the same way but with 1 g of ground and sieved PEEK granulate (particle size <500 mm) instead of the coated wafer piece, a diamine activation time of 6 h and a reaction time of 142 h.

Sample imaging and profilometry
Scanning force microscopy (SEM) images of the surfaces were taken after sputtering a 3 nm Pt conductive layer using a Gemini Ultra plus (Zeiss, Deutschland, Oberkochen, Germany) SEM with an Everhart Thornley type detector, an acceleration voltage of 3 kV and a working distance of 4.2 mm.Atomic force microscopy (AFM) was performed with a Dimension FastScan AFM (Bruker, Billerica, MA) in PeakForce tapping mode with a FastScan B probe (spring constant 1.8 N/m, tip radius 5 nm).The arithmetic mean roughness was determined from several 10 mm Â 10 mm images of each sample and averaged.To determine the thickness of the particle and PEEK-OH films, they were scratched with a steel cannula; the thickness was determined from the height profiles of AFM images or by means of the profilometer DektakXT (Bruker, Billerica, MA) equipped with a diamond needle (radius: 2 mm; load 3 mg).

X-ray photoelectron spectroscopy
XPS spectra were recorded with an Axis Ultra photoelectron spectrometer (Kratos Analytical, Manchester, UK) equipped with a monochromatic Al Ka (hÁ ¼ 1486.6 eV) X-ray source of 300 W at 15 kV.The kinetic energy of photoelectrons was determined with a hemispheric analyzer set to pass energy of 160 eV for wide-scan spectra and 20 eV for high-resolution spectra.Electrostatic charging of the sample was avoided by means of a low-energy electron source working in combination with a magnetic immersion lens.All recorded peaks were shifted by the value necessary to set the C 1s component peak showing the carbon atoms involved in conjugated p-electron systems of the phenyl rings to 284.70 eV [44].Quantitative elemental compositions were determined from peak areas using experimentally determined sensitivity factors and the spectrometer transmission function.Spectrum background was subtracted according to Shirley [45].The high-resolution spectra were deconvoluted by means of the Kratos spectra deconvolution software.Free parameters of component peaks were their binding energy (BE), height, full width at half maximum and the Gaussian-Lorentzian ratio.

Infrared spectroscopy
Attenuated total reflection Fourier transform infrared spectroscopy (ATR FTIR) was applied to study the injection-molded PEEK plates; TIR spectroscopy for the investigation of PEEK nanoparticle films and spin-coated films of reduced PEEK (PEEK-OH).For both techniques, a Vertex 70x spectrometer (Bruker Optics, Billerica, MA) equipped with a Bruker RT-DTGS-FIR detector and a Bruker Platinum ATR Diamond (H ¼ 45 ) was used.In order to prevent water adsorption, all measurements were carried out under reduced pressure (<1 hPa).Fifty scans in the wavenumber range between 4000 and 400 cm À1 at a spectral resolution of 4 cm À1 were co-added for one spectrum.The aperture was 3 mm.All spectra were baseline corrected by OPUS software (Bruker Optics, Ettlingen, Germany).

Contact angle measurements
Advancing (h adv ) and receding contact angles (h rec ) of ultrapure water were measured by sessile drop experiments using an optical contact angle instrument OCA35 XL (DataPhysics Instruments GmbH, Filderstadt, Germany).A water droplet of 5 mL was placed with a motor-driven syringe onto the sample surface.To measure the advancing contact angle, the droplet volume was increased to 15 mL at a rate of 0.1 mLÁs À1 .For the determination of the receding contact angle, the droplet volume was decreased at the same rate.During the contact angle measurements, the syringe was kept in the droplet.The software option SCA 20 (DataPhysics Instruments GmbH, Filderstadt, Germany) was used to analyze the shape of the droplet and calculate the corresponding contact angles.The contact angle values given here are mean values of three individual measurements carried out at different positions on the sample surfaces.
2.4.5.Dye adsorption PEEK-NH 2 samples were exposed to a weakly basic aqueous solution of Eosin Y (ca.1ÁmmolÁL À1 ) for 5-10 min.Then, the samples were carefully washed off with deionized water.A remaining pink color indicated the presence of amino groups on the sample surface.

Reduction of PEEK
This study aims for a chemical functionalization of technically relevant PEEK surfaces by very thin surface coatings not affecting the surface morphology, and the identification of techniques to verify these modifications.The first modification step, the reduction of the ketone groups to obtain hydroxy groups, was used to test the suitability of various types of PEEK surfaces for surface characterization by two fundamental spectroscopic techniques: XPS and IR spectroscopy.With an information depth of few nanometers, XPS is very surface-sensitive.It reveals elemental compositions and, by deconvolution of high-resolution spectra, binding states of single elements.In contrast, IR spectroscopy allows the identification of molecules and functional groups but has a 100 times higher information depth than XPS [46].A higher surface sensitivity is obtained by attenuated total reflection (ATR) IR spectroscopywhich was applied to the injection-molded PEEK platesor by studying systems with a high surface-to-volume ratioin our case films of PEEK nanoparticleswith TIR.
The injection-molded PEEK sheets (Figure 1(a)) had an intrinsic arithmetic mean roughness of Sa ¼ 14 ± 6 nm but mirrored also the topography and defects of the molding tool.The PEEK nanoparticle film (Figure 1(b)) had a much higher roughness, especially in the central part of the wafer.The thickness of this film varied between 300 nm in the outer regions and 2 mm in the inner region of the wafer.AFM and SEM images reveal a particle size in the order of 30 nm, a rather porous structure of the film and full coverage of the substrate (Figure 1(d)).
These two types of PEEK surfaces were subjected to the reduction procedure.Before and after modification, they were characterized by XPS, the PEEK sheet additionally by ATR-FTIR, the nanoparticle film by TIR spectroscopy.It would have been desirable to use smooth PEEK films but we did not succeed to prepare such films due to the high melting point and poor solubility of PEEK in nonhazardous and non-corrosive solvents.Smooth films could only be prepared from the reduced polymer PEEK-OH (Figure 1(c)) as described in more detail in Section 3.2.Photoelectrons of these electronically excited states contribute the shake-up peaks found at BE values higher than 288 eV (in contrast, the photoelectrons of the component peak Ph originate from the electronic ground state of the sp 2 -hybridized carbon atoms).After the reduction to PEEK-OH, the component peak D completely disappears in the C 1s spectra (Figure 2(b,c)).Regardless of the shape of the samples, the reduction converted all carbonyl groups on the sample surface into hydroxy groups.Photoelectrons from these alcoholic groups are counted as component peak C ( C C-OH), like the photoelectrons from the ether groups ( C C-O-C C).As a result, the component peaks C in the spectra of the reduced samples appear slightly more intense than those in the pristine PEEK material.The areas of the shake-up peaks were not significantly affected by the reduction ([shake-up]:[C 1s] % 0.07).This indicates that the conjugated p-systems and thus the triphenyl diether blocks of the PEEK molecules are not damaged during the reduction.The findings ensure that the sample surfaces are endowed with a maximum number of reactive OH groups that can be used for subsequent functionalization reactions.

IR spectroscopy
The injection-molded PEEK sheets were characterized by ATR-FTIR spectroscopy before and after reduction.Although the reduction was verified clearly by XPS, no change of the IR spectra could be observed with different experimental setups and angles of incidence (see Supporting Information).The spectra are dominated by the ketone bands from the PEEK bulk.Obviously, the information depth of the ATR-FTIR spectroscopy is still too high to detect any signal from the very thin PEEK-OH surface layer.This is the main reason that the effect of the reduction was studied with the PEEK particle film, too.Due to the high surface-to-volume ratio, there are obvious differences in the TIR spectra before and after reduction.
Figure 3 shows the infrared spectrum of PEEK and the reduction product PEEK-OH.In Table 2, typical IR spectral bands of PEEK, PEEK-OH, and -esters used for the analysis are summarized.The wave numbers and their assignments are referenced by literature data.The successful reduction of PEEK to PEEK-OH is evident from the disappearance of the v C¼O band of PEEK at 1653 cm À1 , and the bands at 1305 cm À1 and at 927 cm À1 .In the spectrum of PEEK-OH, the O-H stretching vibration is very weakly detected because the hydroxy functionalities are associated and can therefore only be perceived as a very broad band.No analytical significance is attributed to it in the following.
The IR spectrum of the reduced particle film proves that the material of the nanoparticles has been fully converted to PEEK-OH.The grain size in the particle films leads, however, to a broadening of the bands towards higher wavenumbers due to the Christiansen effect [46], which leads to a reduced quality of the spectrum and makes the detection of smaller bands difficult.

Preparation of PEEK-OH films and re-oxidation to PEEK
A better quality of IR spectra is expected for smooth flat films.Such films are usually prepared from melt or solution by spin-coating.Unmodified PEEK cannot be spin- coated for two reasons: first, it has a high melting point (343 C) and is insoluble in most organic solvents.PEEK can only be dissolved in concentrated strong acids as e.g.CH 3 SO 3 H [49] but the corrosivity of the solvent and its high boiling point (167 C) makes it not suitable for spin-coating.Second, the reduction would only affect the ketone groups at the very surface of the film.Even if the film is very thin, IR spectroscopy would detect the bulk material, too, and it would be hard to verify the success of the chemical reaction.
Therefore, model surfaces for analytical studies and further modification were prepared from soluble PEEK-OH obtained by bulk reduction of PEEK powder.Despite a reaction time of six days, only 33% of the PEEK powder could be converted to a soluble PEEK-OH educt.A large fraction of the material remained insoluble even after longer reaction times although IR spectroscopy showed the formation of -OH functionalities on this fraction, too (see Supporting Information).
The soluble PEEK-OH was spin-coated on silicon wafer pieces.The resulting film was very smooth and homogeneous with an arithmetic mean roughness Ra $ 0.3 nm and a film thickness of 130 ± 6 nm obtained by AFM measurements.Transmission IR spectroscopy of these films (Figure 4) proved the full conversion of the ketone carboxyl groups to -OH functionalities.These findings correspond to the XPS results.As known from organic chemistry, the hydroxy functionality can be oxidized and thereby converted back into a keto group.For their re-oxidation, the PEEK-OH film was treated with chromic acid, which is an extremely strong oxidizing agent.During this rather harsh procedure, the film was detached from the silicon substrate and damaged.Nonetheless, after rinsing and drying it could be investigated by TIR spectroscopy.Figure 4 [50].Considering the damage of wafer and polymer film during the oxidation reaction, the PEEK films obtained in this way were not used for further experiments.Nonetheless, oxidation reactions have the potential to convert remaining hydroxy groups after their subsequent functionalization reactions into a non-reactive form, thus restoring the chemical inertness of the modified PEEK sample.This offers the opportunity to carry out a targeted surface activation and modification after which the remaining -OH functionalities are converted back into the non-reactive initial state while the modification is restored.

Esterification on PEEK-OH surfaces
The hydroxy groups of the PEEK-OH surface provide the basis for further functionalization via esterification with carboxylic acids.This can be done with the reduced injection-molded sheets as well as with the reduced PEEK nanoparticle films.For better analysis of the reactions, all further modifications were done, though, using the spin-coated films of PEEK-OH because the smooth surface facilitates spectroscopic characterization and contact angle measurements.In this film, the ketone groups are fully converted into hydroxy groups.The esterification with acidic acid was performed to demonstrate the success of the reaction on solid surfaces.Supposed almost all reactive hydroxy groups are esterified with AcOH on the PEEK-Ac surface, the reactive potential of the substrate surface decreases such that subsequent modification reactions on the substrate become more difficult.This disadvantage can be overcome by using dicarboxylic acids.If only one carboxylic acid group is esterified, the second one is available for subsequent reactions on the substrate surface.Thus, the esterification was transferred to dicarboxylic acids whose chain length was varied to evaluate the scope to crosslink the polymer chains inter and intra molecularly.

XPS
The success of the esterification of PEEK-OH is proven by the appearance of the component peaks E in the high-resolution C 1s spectra at ca. 289 eV (Figure 5, right   2) sample, the shape of the spectrum has changed significantly.Although photoelectrons from sp 2 -hybridized carbon atoms of the phenyl rings contribute to the component peak A at 285.00 eV (their proportion is 14Á[C']), the polymer surface appears to be saturated with hydrocarbons.This is also evident from the low-intensity component peak C' summarizing the alcohol-sited carbon atoms of the ester groups and the remaining hydroxy groups of the PEEK-OH film.Due to the layer formed on the PEEK substrate, the shake-up peaks disappear completely.The corresponding wide-scan spectrum recorded from the PEEK-Ac sample shows only traces of surface contaminations accompanied by hetero-elements.
If PEEK-OH is esterified with MaA, only a very small component peak E identifying the carbonyl carbon atoms of the ester groups appears in the C 1s spectrum (Figure 5(b)).This shows that due to the decarboxylation of MaA the esterification was not successful.Dicarboxylic acids with several methylene groups in their chains, such as SuA, GlA, and AdA, are more suitable to create free acid groups by esterification with PEEK-OH.The corresponding C 1s spectra are characterized by the clear presence of the component peaks E representing the carbonyl carbon atoms of the carboxylate ester and the non-reacted carboxylic acid groups (Figure 5(c-e)).In the case of ester formation with SuA, the high degree of esterification leads to the formation of a layer covering the substrate surface homogeneously (Figure 5(c)).This can be concluded from the strongly reduced intensity of the component peak C (the designation C' in the C 1s spectrum combines the alcohol-side carbon atoms of the ester groups and the ether groups of the PEEK material whose photoelectrons still contribute to the spectral information) and the disappearance of the shake-up peaks.The formation of the hydrocarbon-rich layer (identified by the intense component peak A) on the sample surface suggests that both of the carboxylic acid groups of SuA underwent esterification and are thus preferably intermolecularly crosslinked the polymer surface.The tendency to esterification decreases for longer-chain dicarboxylic acids.In the case of GlA and AdA, not only do the intensities of the component peaks E appear smaller; the intensities and positions of the component peaks C, too, are essentially determined by photoelectrons escaped from the ether groups of PEEK (Figure 5(d,e)).The shake-up peaks are also clearly visible.Analogously, the chemical coupling of various dicarboxylic acids via esterification was characterized using IR spectroscopy.The esterification of the reduced PEEK form (PEEK-OH) with aliphatic dicarboxylic acids is expected to result not only in ester functionalities but also acid grouping in the vicinity of the carbonyl stretching vibration.Figure 7 shows the IR spectra of the esterification products of PEEK-OH with the aliphatic dicarboxylic acids: MaA (PEEK-MaA), SuA (PEEK-SuA), GlA (PEEK-GlA), and AdA (PEEK-AdA).Except for MaA, the IR spectra of the reaction products with the dicarboxylic acids show a typical double band.This results from the different energetic states of the stretching vibrations of the two different carbonyl groups (ester and carboxylic acid).

Transmission IR spectroscopy
Two results can be discussed in the spectra of the reaction products in the region of the stretching vibration of the carbonyl band.
On the one hand, MaA has a significant tendency for decarboxylation due to the acidic hydrogen in its methylene group.Therefore, the esterification reaction results in a mixture, the formation of AcOH ester and possibly one-side-reacted MaA.The esterified product possesses a broad absorption band of the carbonyl stretching vibration, which can no longer be explained by a conjugated structure of two functional groupsester and acidthat are positioned close to each other.This result is supported in particular by the XPS results.Only at a distance of at least two CH 2 groups, as in the case of SuA and all longer-chain esters of dicarboxylic acids, two separate bands 1740 cm À1 (stretching vibration of ester carbonyl group) and 1712 cm À1 (stretching vibration of acid carbonyl group) are detectable.The absence of the usually typical carbonyl double band in the series of aliphatic dicarboxylic acid half-esters is apparently a peculiarity of MaA half-esters, as suggested by a comparison with the IR spectrum of MaA mono-t-butyl ester [51].
Both carbonyl stretching vibration bands of the esters or acid groups are perceived in different absorption intensity (see Table 3).This result can be explained either by different absorption coefficients or by partial double esterification, where aliphatic bridging of two OH groups of PEEK-OH occurs.
The proportioning of v C¼O (ester) to an absorption band that is assigned to the aromatic system of PEEK makes the degree of esterification related to polymer subunits accessible in quantitative measurements.The sole quantitative intensity comparison of the IR bands without reference methodology, e.g.solid-state NMR spectroscopy, certainly does not give absolute stoichiometric ratios in the measurement volume.Since the stretching vibration bands of the C¼O groups of free carboxylic acids tend to be broader than those of the esters, band areas according to the band separation method are rather to be used for the determination of the degrees of esterification.Double esterification cannot be ruled out in principle, but for steric reasons it is considered unlikely with the dicarboxylic acids used.The band at 1106 cm À1 is assigned to the substrate material of the wafer, which is characterized by the very strong stretching vibration band Si-O.Its different height results from shading of the substrate by the modified PEEK-OH films.
Both XPS and TIR spectroscopy show that starting from PEEK-OH, it was possible to obtain the corresponding half esters by esterification of the hydroxy groups with dicarboxylic acids.Thus, PEEK surfaces functionalized with carboxyl groups can be obtained under mild reaction conditions.

Contact angle measurements
The wettability of the plain and modified PEEK surfaces was investigated by measuring the advancing and receding contact angle of water.The results are summarized in Table 4.It has to be noted that most contact angles were measured on spin-coated films; the contact angles of unmodified PEEK had to be determined on rougher injection-molded plates because PEEK cannot be spin-coated (see Section 3.2).With an advancing contact angle (h adv ) of 85 , the PEEK surface appears weakly hydrophilic.The low receding contact angle (h rec ) of 35 is very likely an effect of the intrinsic surface roughness of the injection-molded plates.The spin-coated PEEK-OH films exhibit a slightly lower advancing contact angle (79 ).The effect is, however, weakobviously the hydroxy groups are difficult to access due to the shielding by the sterically demanding phenyl rings.The receding contact angle is higher compared to the unmodified PEEK sample, probably a result of the lower roughness of the spin-coated films.The esterification with AcOH decreases both advancing and receding contact angle further.The increased sterical demand of the carboxylic acid ester groups (compared to the hydroxy groups) makes it easier for water to access the polar surface sites.Since the esterification with MaA was unsuccessful, the wetting behavior of the sample surface is not significantly changed.The esterification with SuA, GlA and AdA results in nearly equal wetting and dewetting properties regardless of the dicarboxylic acid used.The advancing contact angles are similar to those of the unmodified PEEK surface.The receding contact angle values are highervery likely an effect of the lower roughness of the spin-coated surfaces.Summarizing, the contact angle measurements show that neither the conversion of the ketone groups into hydroxy groups nor the esterification with AcOH and the creation of acidic groups by bonding bifunctional acids affect the wetting behavior strongly.It is assumed that the sterically demanding environment allowed only a marginal change in surface polarity.Nevertheless, deeper investigations need to be carried out.

Imine formation by reaction with ethylenediamine
The previous section described the surface functionalization of PEEK in a time-consuming two-step procedure.For practical purposes, a one-step procedure is desirable that binds molecules with the desired functionalities directly to functional groups at PEEK surfaces.Soper et al. [52] proposed the direct aminolysis of the ester functionality of PMMA with n-BuLi-activated ethylene diamine (EDA).In this study, an analogous reaction (see Scheme 2) was performed to bond EDA to the keto function of PEEK in a PEEK particle film.For comparison, the film was treated in one case only with EDA and in a second experiment with n-BuLi without addition of EDA.

Transmission IR spectroscopy
Figure 8 compares the TIR spectra of the particle film treated with n-BuLi alone (PEEK-Bu) and n-BuLi-activated EDA (PEEK-EDA).
In the case of alkylated PEEK-Bu, aliphatic (C-H) bands of the bonded alkyl chain appear in addition to the aromatic (C-H) vibrations of PEEK in the spectral Scheme 2. Proposed reaction mechanism of imine formation by a one pot approach using prior amine activation with n-BuLi.range of 3000-2850 cm À1 .Furthermore, a weak (O-H) vibration of the tertiary alcohol formed during the alkylation is visible at about 3500 cm À1 .In contrary, the PEEK-EDA film shows only very few intense bands in the aliphatic (C-H) range with a poor signal-to-noise ratio.We could not identify any bands indicating the presence of amino groups.
The particle film was modified with an activation time of 4 h and a reaction time of 42 h.Longer reaction times led to a detachment of the particles.Therefore, the reaction was repeated with ground and sieved PEEK granulate (particle size <500 mm) with an activation time of 6 h and a reaction time of 142 h.For these samples, the Eosin-Y color test indicated the presence of amino groups.It does, however, not distinguish between chemically bound and physically adsorbed amino groups.

XPS
Better results were obtained with XPS.The carbonyl reaction of PEEK with EDA led to the incorporation of a considerable amount of nitrogen in the PEEK-EDA sample ([N]:[C] ¼ 0.037).This seems remarkable because the carbonyl carbon atoms of PEEK are particularly stabilized due to their involvement in the conjugated p-electron system of the phenyl rings.The limited accessibility of the ketone groups on the polymer surface also lowers the degree of reaction.Beside the N 1s peak in the wide-scan spectrum, the carbon-nitrogen bonds produced during the reaction with EDA are identified by the component peak B at 285.62 eV in the C 1s spectrum (Figure 9(a), center).The appearance of the component peak D (at 287.84 eV) shows that not all of the ketone groups were converted into ketimine groups (Schiff base).The shape of the high-resolution N 1s spectrum is characterized by its tailing on the high-energy side requiring a deconvolution into two component peaks.The component peak L represents the photoelectrons from the nitrogen atoms of the bonded EDA.Due to the contribution of the electron-rich p-bonds of the ketimine groups, the BE value of component peak L appears low for organically bound nitrogen at 399.21 eV.The small component peak M at 411.2 eV results from protonated amino groups (-N H 3 ).The intensity ratio [M]:([L] þ [M]) reflects the state of the protonation/deprotonation equilibrium.
While the success of this modification could not be definitely verified by IR spectroscopy and color test, XPS gives clear evidence.

Conclusions
The covalent bonding of dicarboxylic acids in a two-step reaction is a facile and costeffective way to create acidic functionalities on primarily non-reactive PEEK surfaces without affecting the surface topography and the bulk properties.In this study, various dicarboxylic acids were chemically bound to model PEEK surfaces to produce tailored surface properties.First, the ketone carbonyl groups of the PEEK are reduced to hydroxy groups onto which, in a second step, dicarboxylic acids are bound via esterification.With regard to application and life time demands, e.g. for implants, it has to be considered that ester bonds can be hydrolyzed in acidic or basic environment.
A second modification approach comprised the covalent bonding of n-BuLi-activated EDA in a single step directly to the PEEK surface via accelerated carbonyl reaction.In this way, amino groups were created on the PEEK surface and a stable cross-link in basic environmental conditions is generated.
The modification results were evaluated using a powerful combination of XPS and IR spectroscopy.IR spectroscopy provides the means for a fast and cost-effective analysis of the chemical structure and gives detailed information about the presence of functional groups.In the literature, usually IR methods based on ATR are used for the characterization of PEEK molded parts or foils.In this study, it was shown that TIR spectroscopy is a promising technique to investigate the chemical structure of surfaces in thin spin-coated or particle films on suitable carriers.By adjusting the film thickness, this technique additionally allows for a study of the modification depth.Not all groups can be detected with high sensitivity, though.Especially weakly absorbing functionalities as, e.g.amino groups, can be concealed in the spectra dominated by intense aromatics and ether absorptions.In this case, XPS complements the study by elemental analysis and detection of binding states.
More research is needed to transfer the reactions from the model surfaces to application-oriented systems and to find ways to reduce the reaction times.Due to the similar chemistry, the results obtained for PEEK can be transferred easily to other polymers from the PAEK group, as, e.g.PEKK.

Figure 2
summarizes XPS spectra recorded from unmodified (a) and reduced (b, c) PEEK samples.Beside the expected elements carbon (C 1s peak) and oxygen (O 1s, O 2s peaks and O KLL Auger series), the wide-scan spectrum of the injection-molded PEEK sheet shows the presence of traces of fluorine (F 1s and F KLL Auger series) and silicon (Si 2p and Si 2s peaks) resulting from surface contaminations by processing aids.The shape of the corresponding C 1s peak in the high resolution XPS spectrum is a characteristic for PEEK.Deconvolution into three component peaks (Ph, C and D) reveals the binding states of the carbon atoms.Their assignment to the chemical structure of the PEEK molecule is presented on the right-hand side of Figure 2(a).The component peak Ph (at 284.70 eV) results from sp 2 -hybridized carbon atoms without heteroatoms in their immediate neighborhood.The second maximum of the C 1s spectrum is composed of the two component peaks C (at 286.25 eV) and D (at 287.01 eV), with C showing ether groups ( C C-O-C C) and D resulting from the carbonyl carbon atoms ( D C¼O).

Figure 1 .
Figure 1.Photographs of the PEEK surfaces: (a) injection-molded PEEK plate, (b) PEEK particle film, (c) spin-coated PEEK-OH film, and (d) SEM image of the PEEK particle film.

Figure 2 .
Figure 2. Wide-scan and C 1s high-resolution spectra recorded from an injection-molded PEEK sheet (a), from a PEEK sheet after its reduction to PEEK-OH (b), and from a PEEK particle film reduced to PEEK-OH (c).The right column shows the assignment of the component peaks to the chemical structure of the PEEK and PEEK-OH units.

Figure 3 .
Figure 3. Normalized transmission IR spectra of the PEEK particle film before and after reduction to PEEK-OH.
shows that the hydroxy groups of the PEEK-OH sample are completely converted into the ketone groups of the original PEEK.The characteristic PEEK-OH bands ((C-O), (C-C-O)) have disappeared.Instead the keto bands ((C¼O), d(C¼O), as (C-CO-C), s (C-CO-C)) are visible.The oxidizing by chromic acid affects the silicon wafer, too, which results in an intensive signal of the (Si-O-Si) vibration at 1104 cm À1

Figure 4 .
Figure 4. Normalized transmission IR spectra of the PEEK-OH film as prepared and after oxidation with chromic acid.
column).These component peaks result from the carbonyl carbon atoms of the carboxylate ester groups (C-O-E C[¼O]-C).The alcohol-sited carbon atoms ( C' C-O-C[¼O]-C) provided by the PEEK-OH sample are labelled C'.These component peaks cannot be clearly separated from the component peaks caused by photoelectrons escaped from the non-reacted hydroxy groups.According to the stoichiometry of the carboxylate ester group, the area of component peak C' must equal the area of component peak E. The differences j[C'] -[E]j correspond to the number of nonreacted hydroxy groups localized on the surfaces of the PEEK-OH films.Photoelectrons dislodged from the carbon atoms in the a-position of the carbonyl carbon atoms (C-O-C[¼O]-B C) are the origin of component peak B at ca. 285.46 eV. Figure 5(a) shows the XPS spectra after esterification of the PEEK-OH film with AcOH (PEEK-Ac).Compared to the C 1s spectrum from the PEEK-OH (Figure

Figure 6
compares the TIR spectra of the PEEK-OH film before and after reaction with AcOH.The newly formed ester group can be clearly detected in the spectrum of PEEK-Ac.The stretching vibration of the carbonyl group in the ester unit -C-(C¼O)-O-appears at 1740 cm À1 .Deformation vibrations of C-H bonds assigned to the methyl group of AcOH appears in the range of 1350-1380 cm À1 and 1450-1490 cm À1 .

Figure 6 .
Figure 6.Normalized transmission IR spectra of PEEK-OH films as prepared and after esterification with acetic acid (PEEK-Ac).

Table 1 .
Compounds used for PEEK functionalization.

Table 2 .
Assignment of IR spectral bands of PEEK, PEEK-OH, and -esters.

Table 3 .
Evaluation of stretching vibration bands of carbonyl group in esters and acids after esterification of PEEK-OH with acetic acid and aliphatic dicarbonic acids.

Table 4 .
Contact angles of ultrapure water on plain and modified PEEK surfaces.
a Measured on injection-molded plates.b Measured on spin-coated films.