PEO Thin Films Prepared by Plasma


Mar 8, 2016 - Andrei Choukourov†, Ivan Gordeev†‡, Jessica Ponti§, Chiara Uboldi§, Iurii Melnichuk†, Mykhailo Vaidulych†, Jaroslav Kousalâ€...
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Microphase-Separated PE/PEO Thin Films Prepared by PlasmaAssisted Vapor Phase Deposition Andrei Choukourov,*,† Ivan Gordeev,†,‡ Jessica Ponti,§ Chiara Uboldi,§ Iurii Melnichuk,† Mykhailo Vaidulych,† Jaroslav Kousal,† Daniil Nikitin,† Lenka Hanyková,† Ivan Krakovský,† Danka Slavínská,† and Hynek Biederman† †

Charles University in Prague, Faculty of Mathematics and Physics, Department of Macromolecular Physics, V Holešovičkách 2, 180 00 Prague, Czech Republic ‡ Jan Evangelista Purkyne University in Usti nad Labem, Faculty of Science, Department of Physics, Č eské mládeže 8, Usti nad Labem 400 96, Czech Republic § European Commision Joint Research Centre, Institute for Health and Consumer Protection, Nanobiosciences, via Fermi 2749, 21027 Ispra, Italy ABSTRACT: Immiscible polymer blends tend to undergo phase separation with the formation of nanoscale architecture which can be used in a variety of applications. Different wetchemistry techniques already exist to fix the resultant polymeric structure in predictable manner. In this work, an all-dry and plasma-based strategy is proposed to fabricate thin films of microphase-separated polyolefin/polyether blends. This is achieved by directing (−CH2−)100 and (−CH2−CH2− O−) 25 oligomer fluxes produced by vacuum thermal decomposition of poly(ethylene) and poly(ethylene oxide) onto silicon substrates through the zone of the glow discharge. The strategy enables mixing of thermodynamically incompatible macromolecules at the molecular level, whereas electron-impactinitiated radicals serve as cross-linkers to arrest the subsequent phase separation at the nanoscale. The mechanism of the phase separation as well as the morphology of the films is found to depend on the ratio between the oligomeric fluxes. For polyolefinrich mixtures, polyether molecules self-organize by nucleation and growth into spherical domains with average height of 22 nm and average diameter of 170 nm. For equinumerous fluxes and for mixtures with the prevalence of polyethers, spinodal decomposition is detected that results in the formation of bicontinuous structures with the characteristic domain size and spacing ranging between 5 × 101 −7 × 101 nm and 3 × 102−4 × 102 nm, respectively. The method is shown to produce films with tunable wettability and biologically nonfouling properties. KEYWORDS: plasma polymerization, copolymer, nucleation and growth, spinodal decomposition, bicontinuous structure

1. INTRODUCTION Polymeric blends are highly attractive from both fundamental and applied point of view because mixing of two (or more) polymers may provide novel properties unattainable by individual components. In many cases, mixing polymers is thermodynamically unfavorable and generally leads to phase separation at different length scales. Various methods were developed to fabricate phase-separated blends from otherwise immiscible polymers, yet rigorous control of resultant morphology still represents a challenging task. Low-temperature plasma based methods have proven to be very effective for fabrication of ultrathin, pinhole-free, polymeric and nanocomposite coatings.1 In this case, a glow discharge is initiated in the atmosphere containing organic compounds so that electrons with a mean energy of several electron volts (eV) serve as initiators that induce bond dissociation and lead to the formation of radicals. The latter subsequently participate in random recombination reactions © 2016 American Chemical Society

which, under low pressures, occur predominantly on surfaces adjacent to the plasma and result in the formation of crosslinked networks in the form of thin films. Virtually any organic substance introduced into the gas phase under rarefied conditions may serve as a precursor for plasma polymerization. Vacuum evaporation of liquid monomers or sputtering of conventional polymers2−7 (also combined with cosputtering of inorganic materials8−13) can be used to deliver organic species into the gas phase. Furthermore, several precursors of different chemical composition can be plasma copolymerized to produce thin films with tunable chemistry and with advanced properties. Besides outstanding tailoring of the chemical composition, good structural correlation of plasma polymers with convenReceived: December 18, 2015 Accepted: March 8, 2016 Published: March 8, 2016 8201

DOI: 10.1021/acsami.5b12382 ACS Appl. Mater. Interfaces 2016, 8, 8201−8212

Research Article

ACS Applied Materials & Interfaces

certain level. Then the crucible with PEO was heated and its temperature was adjusted to reach the required 60 Hz/min in total. Finally, the substrates were introduced via a load-lock at the distance of 10 cm above the crucibles, the 7 min deposition was performed and the samples were removed from the vacuum chamber. Films with different concentration of PE and PEO were produced by changing the ratio between the deposition rates of both components. Polished silicon, gold-mirror-covered silicon, and glass were used as substrates. Ellipsometric measurements (Woolam M-2000DI) were performed ex situ to find the thickness of the films to be 170 ± 20 nm. Their chemical composition was analyzed both by XPS and FTIR. XPS (Phoibos 100, Specs) wide and high-resolution spectra were acquired with pass energy of 40 and 10 eV to assess the elemental composition and the C 1s binding environment, respectively. The spectra were charge referenced for the position of the C−O−C groups at 286.5 eV. The FTIR measurements (Bruker Equinox 55) were performed in reflectance−absorbance mode on the gold-mirror-covered substrates with the resolution of 2 cm−1 and with 250 scans. Gel permeation chromatography (GPC) measurements were performed on single component films using chromatographs with the differential refractometric detection. For the measurements of PE (PL-GPC 220 with PL-220DRI refractive index detector, columns 3× PL gel 10 μm MIXED-B, 300 × 7.5 mm, with guard column PL gel 10 μm MIXED-B, 50 × 7.5 mm), the solutions were prepared in filtered 1,2,4-trichlorobenzene for chromatography (Scharlau) containing 0.025 wt % of an antioxidant Santonox R to prevent oxidative degradation, and the GPC analysis was performed at 160 °C. For the measurements of PEO (Labio, 600 × 7.5 mm gel column of porosity 500 Å, Polymer Laboratories), the solutions of the polymers in tetrahydrofuran were prepared and used at room temperature. In both cases, poly(styrene) standards were used for calibration. Films several micrometers thick were deposited onto glass substrates and then scratched off to collect 5−10 mg of the plasma polymer for nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC) measurements. For the NMR measurements, the samples were left in D2O for 1 day to extract the soluble (uncross-linked) component of the plasma polymer. Then the insoluble part was filtered off and the liquid-state 1 H spectra were measured by an NMR Bruker Avance 500 spectrometer operating at 500.1 MHz. The spectra were referenced to an internal standard of sodium 2, 2-dimethyl-2-silapentane-5sulfonate. DSC was performed using a calorimeter (DSC8500, PerkinElmer). Helium was used to purge the DSC cell at a flow of 20 mL/min. Nitrogen was used as a protection gas. The temperature was calibrated using n-hexane, water and indium. The samples were subjected to a first cooling from the room temperature to 173 K, kept at 173 K for 5 min, subjected to a first heating to 393 K, kept at 393 K for 1 min, and then subjected to a second cooling/heating cycle with the same parameters. The cooling/heating scans were performed with a 10 K/ min rate. Scanning probe microscopy (SPM Ntegra Prima, NT-MDT) was used to study the morphological peculiarities of the films deposited on blank silicon substrates. The samples were scanned under ambient air at a relative humidity of 23 ± 3% and a temperature of 24 ± 1 °C in an amplitude modulation intermittent contact regime with standard silicon cantilevers (NSG03, NT-MDT, spring constant k = 1.6 N/m, resonant frequency 78 kHz, tip radius is better than 10 nm). The height and the phase images with 256 × 256 data points were taken simultaneously for each of the samples. The height images were corrected for the tilt by subtracting first order polynomial from the raw data. Imaging in the amplitude modulation intermittent contact regime is very sensitive to the strength of the tip−sample interaction. Generally, smaller amplitude of the cantilever oscillations and smaller damping (a decrease of the amplitude as compared to free oscillations) are preferable for measurements of the topography because the force acting on the surface is minimized. In contrast, stronger damping enhances the influence of the surface properties on the cantilever phase lag.27 Preliminary tests with different oscillation amplitudes and

tional polymers is often considered to be highly advantageous. However, typical low-temperature plasma conditions are characterized by relatively low concentration of the precursors and relatively high concentration of the radicals which, together with ion bombardment and UV radiation from plasma, hinder chain propagation reactions and favor branching.14 As a result, highly cross-linked random networks are typically formed in plasma polymerization in which the original structure of the precursor is, to a great extent, lost. Thus, plasma polymers do not have much in common with conventional counterparts. In the case of plasma polymers prepared from mixtures of precursors, chemical functionalities of different monomers become homogeneously and randomly mixed in a single phase, and no phase separation is typically observed.15−21 Recently, Plasma-Assisted Vapor Phase Deposition was developed to deposit coatings that fill the gap between the randomness of plasma polymers and regularity of conventional polymers. It was demonstrated that certain polymers decompose when heated under vacuum and release oligomeric species into the gas phase: for example, poly(ethylene oxide) (PEO) and poly(ethylene) (PE) produce oligomeric fluxes with average molar mass Mn ∼ 103 g/mol and polydispersity of DM ∼ 1.10.22−24 Having passed through the glow discharge, these species undergo plasma polymerization transformations and build up thin films of polymeric networks. In contrast to low molar mass precursors, incorporation of longer macromolecular segments can be achieved here, especially under less energetic plasma conditions. The polymeric matrixes with at least several monomeric units between cross-links and with the cross-link density in the range of 10−4−10−2 mol/cm3 were obtained by this method.23,25 On the basis of the results of the previous research on homopolymers, we attempted here to blend immiscible PE and PEO by plasma-assisted vapor phase co-deposition of both polymers. We expected that mixing of larger molar mass species would lead to the phase separation which can be detected at nanoscale, and that the method would offer the possibility of production of binary phase-separated polymeric films by dry, technologically feasible and environmentally benign technology.

2. EXPERIMENTAL SECTION The experimental details of plasma-assisted vapor phase deposition of homopolymers were given elsewhere.22−24,26 In this work, twocomponent blends were produced by simultaneous processing of PE (Sigma-Aldrich, Mn = 20 × 103 g/mol) and PEO (Sigma-Aldrich, Mn = 2.5 × 103 g/mol). Two copper crucibles with independent heaters were placed in a vacuum chamber at the distance of 1 cm from each other. The chamber was brought to the base pressure of 10−3 Pa by rotary and diffusion pumps and then argon was introduced under the pressure of 1 Pa and the flow rate of 5 sccm. A planar magnetron was located beneath the crucibles to create the zone of the glow discharge above them. An rf generator (Dessler Ceasar, 13.56 MHz) with an automatic matching unit (MFJ-962D) was used to deliver power to the magnetron. Since the magnetron served only as a source of plasma, special precautions were taken to minimize the sputtering from its working surface. For this purpose, a low-sputtering-yield graphite target was attached to the magnetron. All the depositions were performed at 15 W power. Quartz crystal microbalance (QCM) was used to monitor and adjust the deposition rate. The total deposition rate of both components was set constant at 60 Hz/min (in terms of the QCM frequency shift) whereas the supply of individual polymers was varied. After the adjustment of the pressure/discharge power parameters, the crucible with PE was heated and its deposition rate was stabilized at a 8202

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ACS Applied Materials & Interfaces the strength of damping were performed on the PE/PEO samples to find the optimal scanning conditions. Free oscillation amplitude of 80 nm and damped oscillation amplitude of 50 nm (set point ratio of 60%) were found to produce the maximal phase contrast without damaging or inducing modifications to the surface. All the SPM results presented here were acquired under these scanning parameters. The AFM images were analyzed in the Image Analysis software (NT-MDT, build 3.5.0.9990),28 the specific details are given further in the text where relevant. Water contact angle (WCA) measurements were performed by a sessile drop method using a home-built goniometer. Five water droplets 3 mm in diameter were dropped onto the surface and the average WCA was obtained. To study cell adhesion, Balb/3T3 immortalized mouse fibroblasts were selected as an in vitro model. Prior to seeding, the PE/PEO samples deposited on glass were put in Petri dishes and sterilized under UV light. Then, the culture medium was added to the Petri dishes to completely cover the samples and 1 × 105 Balb/3T3 cells were seeded onto each sample. At day 1 after the seeding, the culture medium was changed by removing the old one and adding 5 mL of fresh one. At day 3 after the seeding, the culture medium was removed and 200 μL of staining solution (1.2 μL of Fluoresceine DiAcetate, FDA, 5 mg/mL stock suspension in acetone; 4 μL of Ethidium Bromide, EtBr, 1% w/v water solution; 1995 μL of complete culture medium) was dropped onto the surface of the sample. The sample was then immediately removed from the Petri dish and protected with a coverslip. The analysis was performed using a fluorescent microscope (ApoTome system, Zeiss) taking merging images with FITC and DsRed filters.

Figure 1. Molar mass distribution of PE- and PEO-like plasma polymers measured by GPC. Repolymerization of the original oligomers is manifested via the peak at the high molar mass side.

3. RESULTS AND DISCUSSION 3.1. Chemical Composition. It was shown previously that the films prepared separately from PE and PEO without plasma show narrow distributions of molar mass. The GPC measurements gave Mn = 1.4 × 103 g/mol for the PE film24 and Mn = 1.1 × 103 g/mol for the PEO film.22 The chemical composition detected by XPS and FTIR was identical to original PE and PEO, and the NMR measurements showed that the oligomers have linear structure.22,29 If the molar mass of the monomer unit (14 g/mol for CH2 and 44 g/mol for CH2−CH2−O) is taken into account, the average formulas (CH2)100 and (CH2− CH2−O)25 are obtained to indicate that the oligomers consist in average of 100 and 25 monomeric units for PE and PEO, respectively. When the oligomeric flux is allowed to pass through the glow discharge, the bond dissociation is initiated via electronoligomer collisions. The radical-bearing oligomers then tend to repolymerize on the substrate surface and bimodal distribution of molar mass is now detected by GPC in the soluble (uncross-linked) phase of the polymers (Figure 1). The low-molar-mass peak is centered at 2.7 × 103 g/mol for PE, which approximately corresponds to two oligomer molecules, and at 1.2 × 103 g/mol for PEO, which corresponds to Mn of the original oligomers incident onto the substrate. These peaks are accompanied by the higher-molar-mass peaks that reach maxima at 1.5 × 105 g/mol for the PE and 1.0 × 104 g/mol for the PEO, and it points to a partial repolymerization of the oligomers into larger species. The chemical composition of the films prepared from homopolymers as well as from simultaneous deposition of PE and PEO was analyzed by XPS and the C 1s spectra are given in Figure 2. The legends on the spectra show the ratio of the deposition rates of PE and PEO in terms of the QCM frequency shift with the total deposition rate being constant at 60 Hz/min. The spectra of the films deposited from single components are very close to original PE (the bottom

Figure 2. XPS C 1s spectra of the plasma copolymers prepared at different PE/PEO deposition rate ratio given in terms of the QCM frequency shift. The total deposition rate was held constant at 60 Hz/ min. The spectra gradually change from PE to PEO with the decreasing PE/PEO deposition rate ratio. The dashed lines are guides to the eye.

spectrum) and PEO (the top spectrum). For PE, the spectrum is dominant at 285.0 eV, which corresponds to the aliphatic 8203

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ACS Applied Materials & Interfaces carbon of the CH2 groups.30 A small shoulder at the higher binding energy side points at the postdeposition oxidation effects that occur due to the interaction of molecular oxygen with unreacted radicals after the extraction of the samples to the ambient atmosphere. The spectrum of PEO shows a maximum at 286.5 eV which is assigned to the ether bonds. The smaller peak at 285.0 eV indicates the occurrence of crosslinking via the formation of the deoxygenated C−C and C−H bonding environments. Remarkably, the spectra of the films fabricated with the intermediate values of the PE/PEO ratio gradually transform from the PE-rich to the PEO-rich shape. The FTIR analysis supports the findings of XPS (Figure 3). The bottom spectrum corresponds to the PE film and it is

kind of block copolymer structure. The DSC measurements were performed on the plasma polymers prepared from homopolymers of PE and PEO as well as on the polymeric blend of PE/PEO = 30/30. Figure 4a−c shows the

Figure 4. DSC curves of the plasma polymers prepared from a) PE; b) PEO; c) mixture of PE/PEO = 30/30. Only the second cooling run is shown. The total deposition rate was held constant at 60 Hz/min. The dashed lines are guides to the eye.

corresponding DSC curves obtained during the second cooling scan. The samples of homopolymers exhibit strong crystallization peaks located at Tc = 90−100 °C for PE and at Tc= −3 to +10 °C for PEO (where Tc stands for the temperature of the crystallization peak). This is a direct evidence that both plasma polymers have semicrystalline structure. For the PE/PEO blend, it can be expected that if the significant amount of the copolymerized PE/PEO chains were present with randomly interleaving PE and PEO blocks it would degenerate both of the peaks or at least it would change their shape and position. Remarkably, the DSC calorigram of the PE/PEO blend consists of the same peaks at the same positions and this suggests that both components are present predominantly in separate phases. Further evidence of the inefficacy of block copolymerization can be found when performing the NMR measurements on the aqueous solutions of the uncross-linked phase of these films. It was previously shown that the films prepared by PlasmaAssisted Vapor Phase Deposition are partially cross-linked and that the cross-link density increases with the discharge power.23,25 For example, the PEO plasma polymer fabricated under the conditions close to those reported here showed 89 vol % of the cross-linked phase and 11 vol % of the uncrosslinked phase.25 Two samples prepared from homopolymer of PEO and from the mixture of PE/PEO = 30/30 were incubated in D2O to extract their soluble (uncross-linked) components which were further analyzed by NMR spectroscopy. The liquid state 1H NMR spectra of both solutes overlap perfectly, as seen

Figure 3. FTIR spectra of the plasma copolymers prepared at different PE/PEO deposition rate ratio given in terms of the QCM frequency shift. The total deposition rate was held constant at 60 Hz/min. The ether band gradually increases with the decreasing PE/PEO deposition rate ratio. The dashed lines are guides to the eye.

typical for polyolefins. The CH2 groups are manifested through the asymmetric and symmetric stretching vibrations at 2918 and 2851 cm−1 and through the deformation vibrations at 1464 cm−1.31 With the increasing contribution from PEO, a new band gradually develops at 1142 cm−1 which is attributed to the asymmetric stretching of the C−O−C groups. Eventually, the spectrum reaches the shape typical for PEO plasma polymers.22 In contrast to highly surface sensitive XPS, the reflectance− absorbance mode of FTIR analyses the specimens through their thickness. The agreement between the two methods indicates that the blending of PE and PEO occurs all throughout the film thickness. XPS and FTIR distinctly show the variation of the chemical composition of the polymeric blends, but they cannot provide an insight into their structural peculiarities. Because both polymers are deposited simultaneously, the question arises as to whether the PE and the PEO oligomers become mixed in a 8204

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ACS Applied Materials & Interfaces in Figure 5. The position of the dominating peak at 3.69 ppm allows its assignment to the O−CH2 groups. Hence, the

Figure 5. Liquid state 1H NMR spectra of the uncross-linked component of the plasma polymers prepared from PEO without the addition of PE and from the mixture of PE/PEO = 30/30. The dominant peak at 3.69 ppm corresponds to the O−CH2 groups. Overlapping spectra indicate identical chemical composition of the soluble phase and evidence the inefficacy of block copolymerization between PE and PEO.

chemical composition of the soluble phase in both cases is identical and corresponds to uncross-linked PEO. It can be concluded that the (−CH2−CH2−O−) oligomers repolymerize preferentially with like species but do not tend to undergo block copolymerization with PE. Both the DSC and the NMR results do not allow ruling out the existence of block copolymerized macromolecules; however, even if present, their contribution to the overall behavior of the PE/PEO polymeric blends is minimal. 3.2. Phase Separation as Witnessed by SPM. An interesting effect was observed when measuring the surface of the PE/PEO = 30/30 plasma copolymer by SPM. The height image (Figure 6a) shows the random distribution of the surface asperities which is common for plasma polymers. The surface morphology does not bear distinct signs of phase separation as it is sometimes evidenced in SPM studies of conventional copolymers.32−35 Nevertheless, the phase image acquired simultaneously (Figure 6b) showed a very strong and reproducible contrast. It is known that under scanning in the amplitude modulation intermittent contact mode, the phase shift between the driving voltage and the cantilever response depends on the quality factor and on the strength of damping of the cantilever oscillations.27 For scanning homogeneous materials, the phase shift is invariant if all the scanning parameters are held constant because the cantilever-sample interaction is constant throughout the scanned area. However, for heterogeneous surfaces, the tip−sample force gradients may significantly change depending on the local properties of the sample and this effect leads to the appearance of the contrast in the phase image. For example, phase AFM imaging has been extensively applied to distinguish the areas of different viscoelastic properties in multicomponent polymeric surfaces.36−39 As mentioned earlier, phase separation was expected for the PE/PEO binary blend. Since all the measurements were performed at the same tip oscillation amplitude and damping,

Figure 6. Procedure used for the calculation of the size distribution and the fraction of the surface area occupied by the PEO domains in the PE/PEO = 30/30 sample: (a) the height and (b) the corresponding phase images are measured simultaneously by SPM with the scan size of 5 × 5 μm (the insets show the higher resolution 1 × 1 μm images); (c) the advanced watershed method is applied to the height image to distinguish all local maxima at the surface; (d) the grain analysis is performed on the phase image to discriminate between the PE and PEO domains; (e) those surface asperities in the height image are discarded that do not coincide with the grains obtained from the phase image domains; the surface asperities left in the height image correspond to the PEO domains.

we attribute the spatial contrast in phase image to drastically different properties of the phase-separated PE and PEO domains. At closer look at the higher resolution images (the insets in Figure 6a,b), one can notice that not all of the surface asperities in the height image find their counterparts in the phase image; however, all the domains in the phase image correspond to certain local height maxima in the height image. This complicates the statistical analysis of the phase separation because the surface height fluctuations cannot be unambiguously assigned to the PE (or PEO) domains. To overcome this complication, the following strategy was used: (1) The height image was processed by an advanced watershed method28 to establish all local maxima at the surface and to find the boundaries of the areas occupied by them. The advanced watershed method is capable of finding boundaries between closely sitting objects as well as between the objects 8205

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ACS Applied Materials & Interfaces and the background. The result is shown in Figure 6c where the same height image as in the inset of Figure 6a is given with the boundaries between the local maxima emphasized with the white color. (2) The grain analysis was performed on the phase image by defining a section plane that is parallel to the base plane of the image and lies above it at the threshold level of 50% between the maximal and the minimal phase values. For example, the threshold value was set at 27° in Figure 6b to be in the middle of the maximal 55° and minimal 0° phase values. The image was then binarized by considering all the image points with the values larger than the threshold value as belonging to the same local object (white color). By analogy, all the points below the threshold value were considered as belonging to the background of the second polymer component and were colored black (Figure 6d). (3) Finally, both binary phase and advanced watershed height images were compared and the nonmatching objects were discarded. Figure 6e shows the same height image as in Figure 6c but with the final result of this procedure where the areas limited within white boundaries correspond to the domains of one polymeric phase (PEO, as it will be shown later) whereas the surface in between corresponds to another polymeric phase (PE). Hence, quantitative topographical data of thus-identified PEO domains are obtained and their statistical analysis becomes possible. 3.2.1. PE-Rich Binary Blends. The SPM images of films deposited with dominant contribution from PE are shown in Figure 7. The left column corresponds to the height and the right column−to the phase images acquired simultaneously. The height images demonstrate that the surface topography changes upon addition of more PEO to the total evaporation flux. However, it is only phase images that are of use for unambiguous detection of the phase separation. For the film prepared from homopolymer of PE, the phase image is characterized by random fluctuations which are significantly smaller than in the case of binary polymeric blends. The phase contrast may be given by differences in local adhesion, elastic deformation as well as by inhomogeneous adsorption of water but it is not given by the PEO component because none was added. For the PE/PEO blends, distinct areas of high contrast appear in the phase images. The area they occupy increases with the increasing deposition rate of PEO and therefore these objects can be assigned to the PEO domains. Following the procedure described in section 3.2, the height asperities corresponding to the PEO domains were distinguished and their size distributions were calculated. Figure 8 shows the height and diameter histograms of the PEO domains in the PE/PEO plasma copolymers. The height was calculated as the difference between the domain maximum and the local base level whereas the diameter was calculated as D = 2 A /π , where A is the section area of the PEO domain taken at the corresponding section level. Note that for the PE/PEO = 30/ 30 sample (Figure 6) the shape of the domains deviates from the spherical symmetry observed for the films with lower concentration of PEO, and the diameter in this case is taken as the effective diameter of a circle of area equal to that of the PEO domain. The number of the PEO domains markedly increases with the deposition rate of PEO, although their mean size does not change. The mean height and the mean diameter remain at 22 and 170 nm, respectively. This is characteristic for phase separation by nucleation and growth, which is often observed in

Figure 7. Height (left column) and the corresponding phase (right column) SPM images of the PE/PEO plasma copolymers with a prevalence of the PE phase. Legends on the height images indicate different PE/PEO deposition rate ratio.

Figure 8. Height histograms and diameter distributions of the PEO domains in the PE/PEO plasma copolymers with a prevalence of the PE phase.

binary mixtures with a small fraction of one of the phases.40 We have already shown that PE oligomeric species incident on the surface are sufficiently mobile to self-organize into the 8206

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ACS Applied Materials & Interfaces nanoislands.24 Apparently in simultaneous PE and PEO deposition, high chain mobility leads to the self-organization of like−like species into separate domains. For small concentration of PEO, the domains are of spherical symmetry, which can be attributed to the minimization of free energy of the polar polyether chains surrounded by nonpolar polymethylene environment. 3.2.2. PEO-Rich Binary Blends. As shown in Figure 6, the morphology of the PE/PEO = 30/30 blend is characterized by closely located PEO domains, part of them deviating from the regular spherical shape. Further increase of the deposition rate of PEO leads to the percolation of the PEO domains into a single interconnected phase (Figure 9). PE becomes the minor

is therefore facilitated. For PE/PEO = 10/50 the phase contrast represents the dark maze of the PE phase on the light background of more abundant PEO. Logically, no phase contrast is obtained for the film deposited from homopolymer of PEO. The morphological difference of the PE domains is intriguing. It can be explained by the significant difference in the surface mobility of the PE and PEO macromolecules. The results of the DSC analysis indicate that the PE oligomers deposit on the surface which is well below their crystallization temperature whereas the situation is opposite for PEO. The hindered mobility of the PE chains may lead to their selforganization in a diffusion-limited regime. Indeed, the SPM analysis of plasma-assisted vapor deposited plasma polymers revealed that in the initial stage of growth PEO completely wets the surface whereas PE grow via the formation of dendrite islands45 by the mechanism that resembles (but not identical to) Diffusion Limited Aggregation (DLA).46 A prerequisite for the DLA mechanism is that the number of the diffusing species is low and that they attach to an already existing domain with the probability close to unity. It can be hypothesized that the analogous mechanism holds here for the PE-deficient blends and it results in the formation of the PE domains of the irregular shape. The lack of isolated PE domains makes the grain size analysis used in previous sections impossible. Earlier, quantitative structural analysis of bicontinuous microemulsions was suggested to be performed via neutron or X-ray scattering experiments at which the scattering intensity distribution is measured in dependence on the wave vector.44,47,48 The scattering function of the bicontinuous structure goes through a maximum which is characterized by two characteristic length scales: the domain spacing, d, and the correlation length, ξ. It was also shown that AFM data may be processed by Fast Fourier Transformation algorithms to derive the structure factor in dependence on the spatial frequency that correlates well with the scattering function49 and a number of works appeared that took advantage of such analysis for structural characterization of bicontinuous polymeric blends.39,42,50 Here, the height AFM images do not represent unambiguously the PE/PEO phase separation. To derive quantitative information, we performed the Fourier transformation on the phase AFM images and the resultant isotropic power spectral density (PSD) functions are shown in Figure 10a. The insets in this figure demonstrate the surface structure of the PEO-rich plasma copolymers which was emphasized by taking threshold sectioning of the phase images with the subsequent binarization. The PSD functions show distinct wave selection λ which allows the estimation of the average domain spacing. The average domain size can be related with the correlation length which is inversely proportional to the full width at halfmaximum of the PSD peak.51 For better accuracy, however, ξ is usually calculated from the height−height correlation functions that are more sensitive to smaller length scale fluctuations of the surface height. By analogy with the PSD, the correlation functions were calculated from the phase AFM images of the PE−PEO plasma copolymers. Consequently, we call them phase−phase correlation functions because they show the distance beyond which the phase signal becomes uncorrelated (Figure 10b). The phase correlation length is then calculated as the distance at which the phase−phase correlation functions decrease by 1/e of their plateau values. The typical PE domain size and spacing between them are shown in the insets of

Figure 9. Height (left column) and corresponding phase (right column) SPM images of the PE/PEO plasma copolymers with a prevalence of the PEO phase. Legends on the height images indicate different PE/PEO deposition rate ratio.

component now but, in contrast to the PEO-deficient blends, PE does not self-organize by the nucleation and growth mechanism. Instead, interconnected morphology of two coexisting phases can be observed in the phase image of the PE/PEO = 20/40 sample. Formation of bicontinuous polymeric structures is typical for the spinodal mechanism of phase separation at which small fluctuations in the concentration result in spontaneous segregation of immiscible components.40−44 Random spatial and temporal fluctuations of depositing flux are inherent to physical vapor deposition and spinodal phase separation of polyolefin and polyether moieties 8207

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ACS Applied Materials & Interfaces

kinetic trapping of the network morphology was reached by chemical cross-linking (polymerization-induced phase separation)53−55 or by photo-cross-linking.33 In these cases, crosslinks arrest further coarsening of the microphase morphology at a scale of several nanometers. By analogy, we attribute the nanoscale trapping of the bicontinuous structure in the PEO-rich PE/PEO plasma copolymer blends to the formation of cross-links. However, in this case, the cross-linking agents are the same oligomers but bear radicals that are initiated in the plasma due to bond scission. Cross-linking occurs in the solid state due to radical− radical recombination and it competes with the surface mass transport of the oligomeric species to result in the formation of the bicontinuous structure. 3.3. Wettability and Cell Adhesion. Mixing of hydrophobic and hydrophilic counterparts in one polymeric blend suggests that the wettability of such coating can be tailored by tuning the concentration ratio between the components. This can be useful in various applications where surface wettability is of primary concern. The apparent water contact angle of a composite surface is given by the Wenzel56 and Cassie57 laws (eq 1): cos θa = r(fPE cos θPE + fPEO cos θPEO)

where θa is apparent water contact angle, r is the area ratio defined as the ratio of the total sample surface to the projected area, θPE and θPEO are the water contact angle on flat PE and on flat PEO films, respectively, and f PE and f PEO are area fractions occupied by the PE and PEO domains, respectively. Because the surface morphology may also significantly affect the wettability, we first analyzed the film roughness. The rootmean-square (RMS) roughness and the area ratio, r, were calculated from the 5 × 5 μm height SPM images. The results are shown in Figure 11 and are dependent on the concentration of the ether groups, which is proportional to the amount of the PEO phase captured in the binary blend. The roughness of the film deposited from homopolymer of PEO is very close to the roughness of the underlying silicon substrate and it reflects good wetting of hydrophilic silicon with hydrophilic PEO. For the film deposited from homopolymer of PE, the roughness is about 5 times higher reflecting higher cohesion of hydrophobic polyolefins on hydrophilic surface. For the PE/PEO plasma copolymer blends, the values of both the RMS roughness and the area ratio do not fall within the range between PE and PEO but, remarkably, go through the maximum at about 40 atom % of the C−O−C groups, which corresponds to the range of PE/PEO ratios between 30/30 and 20/40. Note that this is the region in which the PEO phase becomes spinoidally unstable and forms the bicontinuous structure with PE. Nevertheless, the extent of the surface instability is not very large: the roughness and the area ratio do not go beyond 12 nm and 1.06, respectively. Soft polymer films on rigid surfaces often reveal surface instabilities with the formation of various mesoscale structures;58−61 however, in our case, the development of the surface undulations is subdued by the stabilizing action of radicals that improve the interfacial film/substrate adhesion and create intermolecular cross-links trapping the phase separation at nanoscale. The area fraction occupied by the PE and PEO domains was calculated from the phase SPM images after the threshold sectioning procedure has been applied. Figure 12a shows the dependence of the XPS binding environment of the PE/PEO plasma copolymers on the area fraction of the PEO phase.

Figure 10. (a) PSD and (b) phase−phase correlation functions obtained from the phase SPM images of the PE/PEO plasma copolymers; the standard deviation error is smaller than the symbol size. The insets show the binary pictures obtained from the phase images after the threshold sectioning has been performed. The vertical lines designate the positions of the domain spacing d = 1/λ and the correlation length ξ.

Figure 10a,b by the red arrows, and the values of d and ξ are summarized in Table 1. With increasing contribution from Table 1. Correlation Length and the Domain Spacing Obtained from the Phase−phase Correlation and PSD Functions for the PEO-Rich Plasma Co-polymers deposition rate PE/PEO (Hz/min)

ξ (nm)

d = 1/λ (nm)

30/30 20/40 10/50

74 ± 2 67 ± 2 52 ± 2

330 ± 50 360 ± 30 420 ± 30

(1)

PEO, the average domain size decreases from 74 to 52 nm, which is accompanied by an increase of the average separation between them from 330 to 420 nm. Thus, trapping of the PE/ PEO phase separation in the PEO-rich plasma copolymers was also achieved at the nanoscale; however, the segregation mechanism is different. It has been recognized that spinodal phase separation of polymeric blends evolves with time and for highly immiscible polymers, such as PE and PEO, it may lead to macrophase separation.52 Different methods, each with a different degree of complexity, were suggested to trap the microphase-separated morphology and to tune the final structure of the polymeric blend. For example, introduction of an amphiphilic diblock copolymer to the binary polymeric blend helps in fixing the structure at the length scale below 100 nm.47 Alternatively, 8208

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function of the concentration of the ether groups. Depending on the chemical composition, the surface changes from hydrophobic (WCA = 101° for the PE surface) to hydrophilic (WCA = 43° for the PEO surface). The WCA calculated by eq 1 is also given here for comparison. Both dependencies show reasonable agreement, especially in the region of high PEO concentration. For the region with smaller PEO contribution, the deviation is larger, probably due to higher inaccuracy in estimation of f PE and f PEO from the phase images. Thus, simple variation of the PE and PEO deposition rates provides a versatile tool for fabrication of microphase-separated films that possess amphiphilic properties, and the resultant wettability of these surfaces is given by the area occupied by hydrophobic and hydrophilic domains. The availability of the PEO domains in our PE/PEO plasma copolymers also allows tuning the manner with which cells interact with these surfaces. Polyether macromolecular chains are well-known to provide the surface with biologically nonfouling properties. The ability to resist the accumulation of biomolecules and cells stems from specific structuring of water molecules by hydrogen bonding as well as from an entropic barrier that prevents biomolecules from anchoring to the surface.62 Nonfouling is extremely important in the development of biomedical instruments that are intended for use in contact with biological fluids (vascular grafts, stents, catheters, etc.) and PEO surfaces have been extensively studied from that point of view.63−66 In our case, the test experiments were performed with the Balb/3T3 immortalized mouse fibroblast cells that were seeded onto the films prepared from homopolymers of PEO and PE, as well as onto the plasma copolymer film with PE/PEO = 40/20 (Figure 13). For the homopolymer PEO film, a very low number of cells was observed. For example, the upper photo of Figure 13 was deliberately taken to include a single cell occasionally found on the surface and to show that it appears with abnormal roundish morphology, apparently due to poor cell-adhesive properties of PEO. This finding is consistent with the numerous earlier works showing that plasma polymers of PEO resist the adhesion of cells.65,67−69 For the binary PE/PEO film, the number of adhering cells increases, but their unfavorable spherical shape persists. Furthermore, the cells are partially dead, as manifested by the red color on the photo (EtBr staining). The homopolymer PE film, in contrast, allows cell adhesion with typical Balb/3T3 polygonal morphology, although the cells are not able to become subconfluent. We see the main outcome of the biological tests in the demonstration of the ability of the PE/PEO plasma copolymers to affect the cell response in a controllable manner which can be of use in diverse biological studies.

Figure 11. (a) RMS roughness and (b) area ratio of the PE/PEO plasma copolymers in dependence on the concentration of the C−O− C groups. The retention of the ethers is proportional to the amount of PEO in the binary mixture.

4. CONCLUSIONS Application of plasma-based technology for fabrication of microphase-separated polymeric blends was demonstrated on the example of PE/PEO thin films. Under vacuum conditions, both PE and PEO undergo thermal decomposition with the release of oligomeric species with an average molar mass of about Mn ∼ 103 g/mol and the composition of (−CH2−)100 and (−CH2−CH2−O−)25. Immiscible polyolefin and polyether entities produce thermodynamically unstable blends that tend to segregate when deposited on solid supports. Phase separation is, however, trapped at the nanoscale by the stabilizing action of radicals that are formed via electron/

Figure 12. (a) The dependence of C−O−C and C−C/C−H groups on the fraction area of the PEO phase as detected by XPS; (b) apparent WCA measured on the PE/PEO plasma copolymers of different composition and WCA calculated by eq 1.

Linearly reciprocal behavior is detected for the ethers and aliphatic carbon with an increasing amount of PEO. The apparent WCA was measured on the PE/PEO plasma copolymers and the results are shown in Figure 12b as a 8209

DOI: 10.1021/acsami.5b12382 ACS Appl. Mater. Interfaces 2016, 8, 8201−8212

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ACS Applied Materials & Interfaces Author Contributions

The manuscript was written through contributions of all authors. Funding

The study was supported by the Charles University in Prague, project GA UK 1926314 and the grant SVV-2016−260215. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS The authors acknowledge the support from the Charles University in Prague through the project GA UK 1926314 and the grant SVV-2016-260215. The authors also thank Ing. Radek Matuška for the GPC measurements.



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Figure 13. Fluorescent microscope images of Balb/3T3 immortalized mouse fibroblasts on plasma polymers from homopolymer of PEO and homopolymer of PE, as well as on PE/PEO = 40/20 plasma copolymer after 3-day seeding. The cells are identified as alive (green color, stained with FDA) or dead (red color, stained with EtBr).

macromolecular interaction in the plasma and that form crosslinks as a result of radical/radical recombination on the surface. The morphology of the PE/PEO plasma copolymers is found to be compositionally controlled. In PE-rich films, spherical domains of PEO are observed that increase in number with increasing contribution from PEO but maintain their average height at 22 nm and average diameter at 170 nm. This peculiarity allows attributing the mechanism of their formation to nucleation and growth. With the further increase of the PEO component, the polymeric blends become spinoidally unstable and form bicontinuous structures with the characteristic domain size and spacing being in the range of 5 × 101 to 7 × 101 nm and 3 × 102 to 4 × 102 nm, respectively. Despite the dissimilarity with conventional polymers, the relative ease of processing and the compatibility with vacuum technologies allow this strategy to find applications in various fields. For example, the PE/PEO plasma copolymer films exhibit amphiphilic properties with compositionally tuned wettability ranging from hydrophobic (WCA = 101°) to hydrophilic (WCA = 43°). Precise control of the concentration of PEO opens the possibility to tailor the nonfouling properties and the results of the Balb/3T3 immortalized mouse fibroblast cell seeding indicate that the films can be perspective in terms of controllable cell response in protein- and cell-mediated applications.



REFERENCES

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