Article pubs.acs.org/Langmuir
Composition and Stability of Plasma Polymer Films Exhibiting Vertical Chemical Gradients Patrick Rupper,*,† Marianne Vandenbossche,† Laetitia Bernard,‡ Dirk Hegemann,†,§ and Manfred Heuberger†,§ †
Laboratory for Advanced Fibers, Empa, Swiss Federal Laboratories for Materials Science and Technology, Lerchenfeldstrasse 5, 9014 St. Gallen, Switzerland ‡ Laboratory for Nanoscale Materials Science, Empa, Swiss Federal Laboratories for Materials Science and Technology, Ü berlandstrasse 129, 8600 Dübendorf, Switzerland S Supporting Information *
ABSTRACT: Controlling the balance between stability and functional group density in grown plasma polymer films is the key to diverse applications such as drug release, tissue-engineered implants, filtration, contact lenses, microfluidics, electrodes, sensors, etc. Highly functional plasma polymer films typically show a limited stability in air or aqueous environments due to mechanisms like molecular reorganization, oxidation, and hydrolysis. Stabilization is achieved by enhancing crosslinking at the cost of the terminal functional groups such as −OH and −COOH, but also −NH2, etc. To overcome such limitations, a structural and chemical gradient was introduced perpendicular to the surface plane; this vertical gradient structure is composed of a highly cross-linked base layer, gradually changing into a more functional nanoscaled surface termination layer. This was achieved using CO2/C2H4 discharges with decreasing power input and increasing gas ratio during plasma polymer deposition. The aging behavior and stability of such oxygen-functional vertical gradient nanostructures were studied in air and in different aqueous environments (acidic pH 4, neutral pH ≈ 6.2, and basic pH 10). Complementary characterization methods were used, including angle-resolved X-ray photoelectron spectroscopy (ARXPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) as well as water contact angle (WCA) measurements. It was found that in air, the vertical gradient films are stabilized over a period of months. The same gradients also appear to be stable in neutral water over a period of at least 1 week. Changes in the oxygen depth profiles have been observed at pH 4 and pH 10 showing structural and chemical aging effects on different time scales. The use of vertical gradient plasma polymer nanofilms thus represents a novel approach providing enhanced stability, thus opening the possibility for new applications.
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INTRODUCTION Plasma treatments are often used to increase the surface energy and hydrophilicity of a sample by grafting chemically bound oxygen atoms onto the polymer structure imparting a degree of polarity. However, due to the rotation and potential mobility of the motile polymer chains, the induced hydrophilicity may not be permanent at the surface. An important contribution to this ubiquitous aging effect comes from hydrophobic recovery, which is driven by minimization of the surface energy after the nonequilibrium plasma processing. The resultant restructuring can further be driven by the compensation of surface charges.1,2 Additional known aging effects include migration, diffusion, rearrangements of chemical groups, oxidation and degradation reactions, swelling, dissolution, and mesoscale morphological changes.3 As one might expect, these effects are dependent on the surface treatment conditions, material properties, and storage conditions.4−7 Functional plasma polymer films (PPFs) denote a material that is created as a result of the passage of an organic gas through the glow discharge and is usually deposited in the form of a thin film.8 PPFs are used in diverse applications © XXXX American Chemical Society
where stability and shelf life aging are important factors, such as drug release,9 tissue engineered implants,10 (blood) filtration,11,12 contact lenses,13,14 microfluidics,15 electrodes,16 and (bio)sensors.17 The usage of ultrathin PPFs is thus often limited by their stability in aqueous environments. We develop and characterize novel PPF structures based on a highly cross-linked subsurface structure that gradually changes to a less cross-linked and more functionalized structure toward the surface to form a vertical cross-linking and chemical gradient structure. A suitable choice of analytical tools is required to investigate the aging effects of PPFs in air or aqueous environments. Water contact angle (WCA) measurements18 and X-ray photoelectron spectroscopy (XPS)19 were thus used to study structural and chemical changes after the aging period. XPS can yield elemental composition information from a sampling depth of some 2−10 nm,20 while WCA is Received: December 22, 2016 Revised: February 13, 2017 Published: February 14, 2017 A
DOI: 10.1021/acs.langmuir.6b04600 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Plasma physical effects influence the ion-assisted surface processes and can be adjusted by changing the available energy density (i.e., the energy that dissipates within the growing film through ion bombardment).54 Oxygen functionalities can be incorporated into the growing hydrocarbon film (in this work: formed from the precursor gas ethylene, C2H4) using admixtures of oxygen-containing molecules (in this work: CO2). An increase of the gas ratio (CO2 to C2H4) causes an enhanced oxygen incorporation at a decreased mass deposition rate (due to chemical etching effects) and leads to a low-density polymer film with increased oxygen functionality (high oxygento-carbon ratio, O/C). A higher plasma power input has an effect on the film-forming species and etching rate and leads to polymer films with increased film densification (indicating a higher degree of cross-linking) with concomitant reduced oxygen incorporation.55 Hence, there is an inherent trade-off between stability (cross-linking, density, modulus) and functionality (oxygen functional group density); that is, the more stable is a layer, the less functional it can be,55−57 and the stability of highly functional PPFs is often limited for long-term use or harsh application conditions.58−60 Stability indicators for a plasma polymer film include (i) loss of film material in aqueous environments, (ii) changes in water contact angle over time, and (iii) detection of oxidation reactions. We have recently shown that by depositing a stable highly cross-linked subsurface followed by a less cross-linked, yet highly functional surface termination, aging effects in air can be inhibited. This tactic works particularly well for ultrathin functional surface terminations, which then exist as few nanometers vertical chemical gradient structures.61,62 The cross-linked subsurface zone effectively hinders reorientation of the less cross-linked surface functional groups, thus restricting hydrophobic recovery and minimizing oxidation effects. Figure 1 schematically shows the type of vertical
sensitive to the hydrophilicity and surface energy of a sample at the very surface to an estimated maximum depth of ca. 0.5 nm.21 Often, XPS analysis alone remains ambiguous regarding the chemical composition of PPFs due to their rather complex chemistry. Therefore, time-of-flight secondary ion mass spectrometry (ToF-SIMS) with a surface sensitivity of ca. 1 nm22 served as complementary analysis technique to XPS.22,23 To also distinguish subsurface from surface, angle-resolved Xray photoelectron spectroscopy (ARXPS) was used here together with XPS signal modeling with chemical composition profiles. In a review article, Cumpson24 gives a useful overview on how to interpret ARXPS data. Paynter and co-workers7,25−28 ̈ as well as Haidopoulos et al.29 have previously applied ARXPS to study aging effects in polystyrene exposed to an oxygen/ argon plasma. Because of the complex nature of plasmaprocessed surfaces, only few other studies have been published using ARXPS data.30−34 Other researchers35−38 have employed information from the inelastic electron background in XPS (Tougaard’s method), which is related to the depth distribution of elements in the surface region.39 Unlike ARXPS, the inelastic background method also works for the study of rough surfaces. Recently, the method of excitation energy-resolved XPS (synchrotron) was applied for the study of aged plasma polymer films, and compositional depth profiles were obtained.40−42 The use of fixed angle XPS for the characterization of plasma-modified polymer surfaces is nevertheless very popular. Vandencasteele and Reniers18 give numerous examples in their review article. In addition, XPS has been applied for the study of the aging behavior of neat polymers.43 Unger and coworkers44,45 investigated plasma-deposited films prepared from different organic monomers. They observed pronounced aging in air dependent on the deposition conditions that affected the degree of cross-linking. Ruiz et al.46 studied oxygen-rich thin organic films and could influence the stability of the layers in air and water over 1 day by adapting the process conditions (gas mixture ratio and plasma input power). König et al.47 investigated the stability and aging of plasma-treated poly(tetrafluoroethylene) (PTFE) surfaces and found significant hydrophobic recovery in air storage depending on the plasma treatment. López-Santos et al.38 observed a significant decrease in the nitrogen concentration from the topmost surface layers in polyethylene terephthalate (PET) treated with different nitrogen plasmas. Larrieu et al.48 described the reorientation of polar groups for ambient air aged polystyrene, in which the change in WCA over the first couple of hours was less pronounced for the sample having the highest rate of crystallinity. Paynter and co-workers26,27 observed in polystyrene, exposed to an oxygen plasma, an oxygen loss at the surface with a significant increase of the water contact angle after only a few days of storage in ambient air. Bosso et al.49 used an atmospheric pressure cold plasma jet and obtained water-stable coatings by reducing the content of carboxylic groups in the coating. Recently, our group has investigated amorphous oxygen-containing functional plasma polymer films (a-C:H:O).50−52 By changing the deposition parameters in a well-defined manner, the properties of the resulting layers can be controlled. For micrometer thick plasma polymerized films, gradient structures are deposited even without changing the deposition parameters due to an inhomogeneous growth, as shown by Förch and co-workers.53 Plasma chemical effects are responsible for the film-forming species in the gas phase and are determined by the parameter W/F, which corresponds to the power input per monomer gas flow rate in the plasma zone.
Figure 1. Schematic illustration of the gradient samples and ARXPS setup. Vertical chemical gradient structures, composed of a highly cross-linked base layer followed by a functional termination at the surface, are deposited onto Si wafer substrates. In ARXPS measurements, the different photoemission takeoff angles, θ, θ′, determine the XPS information depth (ID).
gradient structures investigated in this work. It is composed of a highly cross-linked base layer with a nominal thickness of ca. 18 nm, terminated by a less cross-linked but highly oxygenfunctional film at the top 1−2 nm. We note that Li et al.63 followed along similar lines as they produced more stable amino-functional plasma polymers, which consist of a distinctive bilayer structure based on a strongly cross-linked first layer and a less cross-linked, highly functional top layer; however, each layer was ca. 20 nm thick. In this work, new functional possibilities for nanometer-thick gradient top layers are shown as a further optimization of B
DOI: 10.1021/acs.langmuir.6b04600 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
changed every day. After immersion, samples were dried under a nitrogen flow and stored in a desiccator before being analyzed; one sample was used for WCA measurements and a duplicate sample for XPS. Care was taken to limit air exposure before transfer into the XPS chamber to less than 30 min. All aging studies were carried out with fresh samples exposed for the full aging period indicated. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were performed with a Scanning XPS Microprobe (PHI VersaProbe II spectrometer, Physical Electronics) using monochromatic Al Kα radiation (1486.6 eV). The operating pressure of the XPS analysis chamber was below 5 × 10−7 Pa for all data presented here. Survey scan spectra, higher resolution narrow region spectra, and angleresolved X-ray photoelectron spectra (ARXPS) were acquired. The former two spectrum types were collected at a photoemission takeoff angle θ of 45° with respect to the surface plane. For ARXPS, takeoff angles of 15°, 30°, 45°, 60°, and 75° have been selected. In the lens mode used for ARXPS measurements, the analyzer acceptance angle is determined to be ca. ±5°, whereas in the normal mode for survey and region scans, it is about ±20°. Survey scan spectra (0−1100 eV) were acquired with an energy step width of 0.8 eV, an acquisition time of 160 ms per data point, and an analyzer pass energy of 187.85 eV. Higher resolution narrow spectra over carbon C 1s (278−298 eV binding energy range) and oxygen O 1s (523−543 eV binding energy range) regions were acquired with an energy step width of 0.125 eV, acquisition time of 1.44 s per data point, and analyzer pass energy of 29.35 eV. Because of the longer total measurement time for ARXPS due to the sequential recording of the spectra at five different takeoff angles at the same sample location, the step width and pass energy were slightly increased to 0.2 and 46.95 eV, respectively, leading to 400 ms acquisition time per data point (C and O). The energy resolution (fwhm, full width at half-maximum height) measured on the silver Ag 3d5/2 photoemission line is 2.3 eV (for a pass energy of 187.85 eV), 0.85 eV (for a pass energy of 46.95 eV), and 0.7 eV (for a pass energy of 29.35 eV). Total acquisition times were approximately 5 min for survey scans, 8 min together for the two higher energy resolution elemental scans, and 45 min for the ARXPS measurements. No significant progressive sample damage was observed after 45 min, as inferred from reproducible spectra that have been obtained if the measurement is repeated at the same sample position. The measurement position was randomly chosen on each sample and analyzed using a microfocused X-ray beam with a diameter of 100 μm (operated at a power of 25 W at 15 kV). The 180° spherical capacitor energy analyzer was operated in the fixed analyzer transmission (FAT) mode. Samples were placed on a 5 cm × 1.5 cm stainless steel sample holder suitable for survey and region scans as well as ARXPS measurements. Sample charging of the plasma polymer films was compensated using dual beam charge neutralization with a flux of low energy electrons (∼1 eV) combined with very low energy positive Ar ions (10 eV). The binding energy is referenced to the C−C, C−H hydrocarbon signal of the C 1s peak at 285.0 eV. Intensity determination and curve fitting were carried out with CasaXPS software version 2.3.16 (Casa Software Ltd., Teignmouth, UK) using a fixed Gaussian−Lorentzian product function of 70% Gaussian and 30% Lorentzian to fit the XP spectra. In the detailed spectral fitting of the C 1s and O 1s envelopes, the component peak energy was restrained to within ±0.5 eV of the corresponding literature value,46,65 and the full width at half-maximum (fwhm) was fixed as is customary for the analysis of plasma polymer films.66 Atomic concentrations were calculated from XPS peak areas after subtracting a Shirley-type background. Thereby, tabulated PHI sensitivity factors67 corrected for our system’s transmission function and analyzer asymmetry parameter (correction due to a different angle between X-ray source and analyzer) have been used for quantification. Relative uncertainties in the measured concentration are estimated to be approximately ±10%, which includes uncertainties in the background determination from the energy window setting and transmission function correction. Detection limits under our experimental conditions (for light elements like N and Si in the plasma polymer films) are estimated on the basis of the procedure given by Shard68 as ca. 0.1 atomic %.
previous vertical gradient structures. Aging studies have been extended to include ambient air as well as aqueous environments at three different pH values (pH 4, pH ≈ 6.2, and pH 10). Simple models for the oxygen depth distributions were studied and fit to the ARXPS data. By using such unique thin oxygen-functional vertical chemical gradient structures, an improved surface hydrophilicity with significantly reduced aging effects can be achieved.
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EXPERIMENTAL SECTION
Deposition of Thin Plasma Polymer Films with a Vertical Gradient Structure. Plasma polymer films (PPFs) were deposited onto precleaned (Ar plasma, 50 W, 10 Pa, 10 min) silicon wafers in a symmetric plasma reactor with capacitive coupling, enabling precise control of the deposition conditions.64 To prepare the PPFs, a discharge was carried out in a mixture of ethylene (C2H4) and carbon dioxide (CO2) at low pressure (10 Pa). We switched between two different plasma deposition conditions to obtain the vertical gradient structures: the highly cross-linked base layer, nominal thickness ca. 18 nm, which was prepared at a power input of 70 W and a gas flow ratio of 2:1 of CO2/C2H4 (deposition rate ca. 6 nm/min), followed by the second, less cross-linked and more functional layer, which was deposited at a power input of 30 W and a gas flow ratio of 6:1 (deposition rate ca. 1 nm/min). The monomer (C2H4) gas flow rate was held constant at 4 sccm. Plasma condition switching was done while keeping the plasma power on. Two types of vertical gradient PPFs were prepared in this study. The first vertical gradient structure type was prepared after deposition of the base layer of ∼18 nm thickness by quickly changing (within ≲10 s) power input and gas flow ratio and further depositing 1 nm of the top layer. The finite penetration of plasma reactive species effectively generates a transition between the two layers; this coating type shall be named “1 nm vertical gradient”. The second vertical gradient structure type was prepared by slowly changing power input and gas flow ratio over a period of ∼30 s to generate a more gradual transition from base to functional layer and further depositing 2 nm of the top layer; this coating shall be named “2 nm vertical gradient”. Reference samples (ca. 20 nm thick) consisting of either a highly cross-linked layer (corresponding to the base layer in the vertical gradient structures) or oxygen-rich layer (corresponding to the top layer in the vertical gradient structures) were also deposited. The higher degree of cross-linking for the reference base layer is represented by an increased measured film density of 1.5 g/cm3 as compared to 1.1 g/cm3 for the reference top layer.61 In addition, PCA (Principal Component Analysis) on the measured ToF-SIMS data has confirmed the higher degree of cross-linking for the base layer (see the Supporting Information). All plasma coatings were found to be smooth (average roughness
