authored by Svea Petersen1*
Abstract
The surface chemistry of different plasma-treated PEEK was characterized by contact angle measurements and X-ray photoelectron spectroscopy and the results of the different methods were compared. In addition, the physicochemical changes on the surface were documented and evaluated within the course of the aging/recovery of the surface within one week. All plasma treatments had serious effects on the aromatic basic structure of the polymer, but at the same time increase the nitrogen and oxygen content of the surface up to a maximum of 28.2 % by means of oxygen plasma. Regardless of the process gas, the surface energy increases to >63 mN/m what results in a clearly hydrophilic surface. Overall, three phases of hydrophobic recovery were observed, in two of which the degradation of functional groups was shown by a decreasing surface energy. The changes in the acid properties could be quantified by Berger’s method. The treatment in the oxygen plasma leads to an acidic surface with a Dshort value of 1.6. In nitrogen plasma, on the other hand, a slightly alkaline surface is generated with a shortened Berger parameter of -0.3. Regardless of the process gas and the initial nature of the surface, a slightly more acidic surface is formed during storage compared to the reference. The elemental composition of the plasma-treated PEEK appears to be relatively constant over the storage time, but with significant changes in the binding states. The double bonds induced during the plasma treatment are degraded and with them also the induced carboxylic acid.
Introduction
Its excellent temperature and chemical resistance, its outstanding mechanical properties and its thermoplastic processability characterize the semi-crystalline, aromatic highperformance polymer polyetheretherketone (PEEK). Because of this property profile, it has tremendous potential in many areas of the high technology industry. The fields of application range from load-bearing components to abrasion and corrosion protection coatings, ball bearings, high-pressure and vacuum components to implant applications in biomedical engineering [1-4]. Particularly in the latter field, this type of polymer offers numerous possibilities to substitute metallic materials. This is especially interesting because PEEK offers some additional benefits, such as X-ray transparency and bone-like strength, preventing stress shielding effects. In addition, the tensile strength of the resulting composite material can be adjusted within a range of 3.6 to 18 GPa by the implementation of glass or carbon fiber and thus adapted to the application [5,6]. Unfortunately, the chemical resistance results in a general inertness of the surface and a hydrophobic character of PEEK. This can be challenging not only for the joining of PEEK components by gluing or the coating on other materials, but also for the acceptance of possible PEEK implants in biological systems. However, modification of the surface would significantly improve the applicability of this promising high-performance polymer in biomedical fields as dentistry or implantology. In particular, plasma technology could play a primary role as it combines advantages such as cleaning, nanostructuring, the establishment of functional groups, depending on the process gas used, and an enormous improvement of the wetting behavior in one process step [7,8]. These three factors alone have a serious impact on the bondability as well as the biocompatibility of materials [9,10]. Due to its free availability, a frequently used process gas represents air, but pure nitrogen, oxygen, hydrogen and many more are also used for the process [11]. Especially low-pressure plasma technology offers great potential due to the easily controllable process parameters and the resulting reproducibility. Thus, Pawson et al. and Ha et al.demonstrated the formation of carboxylic acid groups on the PEEK surface by treatment in oxygen plasma using XPS and To F-SIM [12,13]. Due to its reactivity towards biomolecules present in the human body, this functional group can be used for the mediation of the biomaterials surface-biological system interaction. For the same reasons, amino groups are relevant whose formation on the PEEK surface is achieved by a nitrogen plasma treatment, as reported by Terpilowski et al. [14]. As a mixture of oxygen and nitrogen with varying composition, air is a more complex process gas in which the processes occurring in the plasma state are difficult to predict. Meanwhile, due to its free availability and associated cost savings, it is frequently used in industrial applications. However, regardless of the process gas, plasma modifications are not stable because implemented groups often show a volatile character. In addition, the rearrangement of the macromolecules leads to a recovery of the surface and thus additional time-related changes in the surface chemistry [7,15]. Taking this into account, Rymuszka et al. carried out studies on the long-term stability of air plasma modifications to PEEK, which showed that the number of functional groups is reduced during storage due to atmospheric contact, which also has a particularly negative influence on the wettability of the surface [16]. Since there is currently insufficient data for the long-term stability of other plasma-modified surfaces, an essential goal of this work is the characterization of the surface chemistry of PEEK over the storage time. Furthermore, the recovery of the surface should be described by means of contact angle measurements. From the determined contact angles, the surface chemistry of the PEEK surfaces will be characterized by using different evaluation methods and compared with XPS data.
Experimental
Materials
The PEEK sample material was obtained by Evonik Industries AG (Essen, Germany) in the form of a 100 μm thin, translucent foil which is marketed as VESTAKEEP 4000G. For the contact angle measurements, samples in the size of 5x10 cm were cut out of the foil. After being washed with distilled water, it was cleaned with isopropanol in an ultrasonic bath for 3 minutes. The drying was carried out by evaporation of the isopropanol at room temperature for at least 15 minutes. As sample for the XPS-measurements discs of 12 mm in diameter were prepared the same way. The plasma treatment of the samples was carried out with air, as well as with nitrogen (N2, 99.999 %) and oxygen (O2, 99.995 %) from the manufacturer Westfalen AG (Münster, Germany). To minimize contact with the atmosphere prior to XPS measurements, the plasma chamber was vented with argon (Ar, 99.999 %) after the treatment. One part of the samples was directly transferred to the XPS under argon atmosphere for the initial measurements while the other part was stored for a defined time within covered Petri dishes, which prevented the contamination with particles from the environment and at the same time allowed an exchange of air and thus the direct influence of the atmosphere.
Low-pressure plasma treatment
The plasma treatment was carried out by using a commercial low-pressure plasma system MiniFlecto® from plasma technology GmbH (Herrenberg-Gültstein, Germany) which has a variable frequency of 20-50 kHz and a maximum power of 80 W. The experiments were carried out at maximum power. The process was performed pressure-controlled with a fixed pressure of 0.2 mbar and a resulting gas flow of about 2 sccm. Before the plasma was ignited, the chamber was purged with the process gas for 90 seconds to ensure stable atmospheric conditions. The parameters were previously optimized with regard to the greatest influence on the water contact angle, since this parameter is crucial for subsequent coating or use in biological systems. The pressure inside the vacuum chamber was measured by a Pirani sensor, the gas flow control via mass-flow-controller. The surface treatment was performed by direct exposure of the sample to the plasma for 180 seconds, the distance between the sample and the electrode was 40 mm. The sample transfer into the XPS was performed under argon atmosphere with minimal atmospheric contact, the transfer from the process chamber into the transport vessel was realized under constant argon flow. XPS measurements were performed initially and after seven days of storage while the water contact angle was monitored daily.
Surface characterization methods
Contact angle measurement
The contact angles of the differently treated PEEK samples were measured using an OCA20 goniometer by dataphysics (Filderstadt, Germany). In addition, the manufacturer’s SCA20 software was used to perform drop contour analysis, define a baseline, and determine the resulting angles. The test liquids used were water, formamide, diiodomethane, n-dodecane; ethylene glycol and glycerol. To determine the contact angle, the sessile drop method was used. For this purpose, a droplet of 5μL was dosed onto the surface and after 10 seconds the contact angle at the three-phase point was measured. The surface tensions (γltotal) taken from the literature, as well as their polar (γlp) and disperse (γld) respectively acidic (γla) and alkaline (γlb) components as well as the lewis-component (γlLW), needed for the subsequent calculation of the surface energy, are listed in (Table 1).
Surface energy determination according to Owens, Wendt, Rabel and Kaelble
The method by Owens, Wendt, Rabel, and Kaelble (OWRK) was used for determining surface free energy and is based on the Young equation (1).
γ_sv=γsl+γlv*cosϴ (1)
This equation establishes a relationship between the surface tension of the liquid, the interfacial tension between the solid and the liquid, the surface free energy of the solid and the resulting measurable contact angle and is fundamental to many approaches in surface free energy calculation. In the OWRK method, the solidvapor and liquid-vapor-interactions are split into a polar and a dispersive component (2) and the interfacial tension is interpreted as a geometric mean of the disperse and polar component, see (3).
γ_sv=γ_sv^d+γ_sv^p and γ_sv=γ_lv^d+γ_lv^p (2)
γ_sl=γ_sv+γ_lv-2√(γ_sv^d*γ_lv^d )-2√(γ_sv^p*γ_lv^p ) (3)
Thus, in addition to the calculation of the surface free energy, the division into polar, including hydrogen-bonding, and disperse interactions is also possible by using in minimum two different liquids 17,18. For the experiments carried out, the surface energy was calculated taking into account the contact angles of all 6 test liquids (see above).
Characterization of Acid/Base Properties
For the characterization of the acid/base properties two different methods were applied, namely the method by van Oss, Chaudhury and Good (vOCG) and the Berger method.
Van Oss, chaudhury and good
The method by van Oss, Chaudhury and Good is also based on Young`s equation (1) but divides the surface tensions into an apolar component and an acid/base component (4).
γ_sv=γ_sv^LW+γ_sv^AB and γ_lv=γ_lv^LW+γ_lv^AB (4)
This is further subdivided into electron acceptors or donors according to the acid/base definition of Lewis under use of the geometric mean, see (5). Thereby it is assumed that hydrogen bonds are also included into σAB:
γ_sv^AB=2√(γ_sv^+*γ_sv^- ) and γ_lv^AB=2√(γ_lv^+*γ_lv^- ) (5)
γ_sl=γ_sv+γ_lv-2√(γ_sv^LW*γ_lv^LW)-2√(γ_sv^+*γ_lv^- )-2√(γ_sv^-*γ_lv^+ ) ( 6 )
Following the theory of Lewis, an interaction takes place exclusively between the acid/base pairs with each other, as well as between the apolar Lifshitz-van der Waals fractions of solids and liquids (6). Due to the sometimes-critical reviews regarding this method, Volpe et al published additional instructions for using the method. Accordingly, the set of test liquids used in this work consisted of water, diiodomethane and formamide [17–19].
Berger-Method
Furthermore, the Berger method was used in order to characterize the influence of the plasma treatment on the PEEK surface chemistry. Due to the different contact angles of test liquids with similar disperse and polar components, e.g. formamide and glycerol, Berger extended the usual graphical method by Fowkes. The deviations between the contact angles and therefore in the performed work of adhesion (7) of the liquids led back to the different acid-base properties of the liquids.
W_a=γ_lv*(〖1+cos〗ϴ ) (7)
Assuming that all non-disperse interactions are covered by acid-base interactions, their contribution to adhesion work is calculated by subtracting the liquids’ disperse component from the total adhesion work, see Equation 8. In the case that no acidbase interactions take place, 2√(γ_lv^p ) determined by Equation 9 would be identical for all test liquids.
W_a^AB=W_a-W_a^d=W_a-(2*√(γ_lv^d*γ_s^d )) (8)
2√(γ_s^p )=(W_a^AB)/√(γ_lv^p ) (9)
Since this is not the case, the difference between the base pairs of equal surface tensions can be considered as a measure of the acidity of the solid surface. The acidity parameter D introduced by Berger, originally calculated according to equation (10), indicates the character of the surface. Positive values stand for an acidic character, whereas negative values indicate an alkaline character of the surface. Due to the extreme spreading of phenol and aniline on high energetic substrates with surface energies beyond 40 mN/m and the associated limitation of this method, Kraus et al. introduced the shortened acidity parameter Dshort (11) which also found application in this work [10,20,21].
D=2[√(γ_s^p (aniline) )+√(γ_s^p (formamide))]-2[√(γ_s^p (phenol) )+√(γ_s^p (glycerol))]
D_short=2√(γ_s^p (formamide))-2√(γ_s^p (glycerol)) (11)
X-ray photoelectron spectroscopy
The analysis of the PEEK surfaces was carried out with an XPS system from the manufacturer Physical Electronics GmbH (Ismaning, Germany) with the type designation “PHI 5600-CI”. Monochromatic aluminum Kα radiation with an energy of 1486 eV was used. In order to counteract the insulating effect of the PEEK-polymer samples, the electron deficiency of the surface during the measurement was compensated by using an electron emitter. The measurement was carried out under UHV conditions at a pressure of <10-8 Pa, at a 45° angle, the analyzed area was 800μm2. The initial measurements were carried out immediately after the plasma treatment, the transfer of the samples was carried out under an argon atmosphere. Data acquisition and evaluation of the resulting survey spectra were performed using the device manufacturer’s software. Fitting the detail peaks was done using Raymund Kwok’s freeware software XPSPeak 4.1 using a Tougaard background.
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