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Surface modification of a biomedical poly(ether)urethane by a remote air plasma
Applied Surface Science 217 (2003) 210–222
Surface modification of a biomedical poly(ether)urethane by a remote air plasma J.E. Gray*, P.R. Norton1, K. Griffiths Department of Chemistry and Interface Science Western, University of Western Ontario, London, Ont., Canada, N6A 5B7 Received 19 September 2002; accepted 22 March 2003
Abstract Plasma modification of polymer surfaces is widely used, but the plasma/polymer interaction is very complex and still not fully understood. In this paper, the interaction of a biomedical poly(ether)urethane with a remote air plasma treatment has been studied. Atomic force microscopy studies show the domain structure of the polymer as well as the absence of any surface roughening due to plasma treatment. Contact angle goniometry shows an improved wettability of the surface after plasma treatment. X-ray photoelectron spectroscopy indicates an increase in C¼O and C¼C at the surface, as well as the presence of new functional groups such as alcohols, ketones, aldehydes and imines. There is also evidence that the energy imparted to the polymer during plasma treatment causes surface segregation of polyol segments. # 2003 Elsevier Science B.V. All rights reserved. PACS: 80, Cross-disciplinary physics and related areas of science and technology; 82, Physical chemistry; 82.40.Ra, Plasma reactions Keywords: Poly(ether)urethane; Plasma; Surface modification; Microdomain; X-ray photoelectron spectroscopy
1. Introduction Polymers are widely used as biomaterials for medical implants and devices, but despite their extensive use in medicine they tend to exhibit poor biocompatibility and poor adhesion to other materials. An understanding of the polymer surface is key to improving the function and lifetime of medical implants and devices through surface modifications that do not affect the bulk properties and function of the material. Biocompatibility of * Corresponding author. Present address: Head hall Chemical Engineering, University of New Brunswick, Fredericton, Canada, E3A 5B3. Tel.: þ1-5064473461; fax: þ1-5064533591. E-mail addresses: [email protected] (J.E. Gray), [email protected] (P.R. Norton). 1 Tel.: þ1-519-661-4180; fax: þ1-519-661-3022.
these devices can be improved through changes in wettability, incorporation of surface functional groups that help prevent thrombogenicity, and surface coatings of biologically compatible species such as albumin, proteins and antibiotics. Surface modifications can be used to enhance the adhesion of specific molecules to the surface; this can either promote or prevent the adhesion of biological molecules and cells to the polymer substrate. The development of polymer scaffolds to promote bone growth for orthopedic patients and antibacterial coatings for the prevention of device associated infection are two areas where surface modification can play an important role in improving device function and lifetime. Plasma treatment is a technique that can be used to modify the surface properties of biomaterials without also altering bulk properties that affect their function,
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00552-X
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since the depth of modification is typically limited to the first few nanometers of the surface [1]. A plasma is a gaseous mixture of electrons, radicals, ions and excited molecular states which are created by inelastic collisions between high energy electrons and ground state atoms or molecules. The high energy particles and photons of a plasma interact with the polymer surface by both chemical and physical processes. Plasma treatments are known to improve wettability, induce cross-linking and chemical functionalization of the surface and cause surface etching and ablation [1]. The type of surface modification generated by a plasma treatment is strongly dependent on the choice of reactive gas and treatment conditions. Inert gas plasmas such as argon and helium have been shown to modify the surface through crosslinking, chain scission, chain branching and surface roughening [2–6]. Reactive gas plasmas involving gases such as oxygen, nitrogen, hydrogen, ammonia and hydrogen sulfide, have been shown to introduce new functional groups onto the polymer surface as well as to induce changes similar to those observed with inert gases [5,7–11]. The high energy ions in a plasma can often cause significant surface roughening and formation of low molecular weight organic material on the surface. This can decrease adhesion of coatings to the material through the formation of weak boundary layers and hence affect the interaction of cells with the material [12,13]. Remote plasma treatments allow chemical modification of the surface without etching and ablation. In a remote plasma treatment, the sample to be modified is placed at some distance downstream from the plasma source. This limits the ion bombardment of the surface since the ions lose energy through collisions with the wall of the plasma reactor and other plasma species before they can reach the polymer surface. Only the longer lived excited species can interact with the surface. For a nitrogen plasma these are thought to be Nð4 sÞ ground state atoms and electronically and vibrationally excited nitrogen molecules, while in a remote oxygen plasma these species are oxygen atoms and singlet molecular oxygen [14]. Therefore, remote plasma treatment allows surfaces to be chemically modified while minimizing unwanted sample damage. In this paper, the effect of an air plasma treatment on the surface chemistry, wettability and morphology of a
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biomedical poly(ether)urethane have been investigated. X-ray photoelectron spectroscopy has been used to determine changes in surface chemistry while contact angle goniometry and atomic force microscopy have been used to investigate changes in wettability and surface roughening, respectively.
2. Experimental X-ray photoelectron spectroscopy studies were performed on an SSL SSX-100 XPS spectrometer using a monochromatized Al Ka X-ray source and hemispherical analyzer at a takeoff angle of 538. Elemental analysis was conducted with a 600 mm spot size and 129 eV pass energy, while high resolution spectra had a spot size of 300 mm and a pass energy of 54 eV. An electron flood gun was used for charge compensation (1 V, 0.05 mA). The binding energies were corrected for charging using the C 1s binding energy peak for CH2 in the polymer at 285.0 eV as a reference [15]. The spectra were deconvoluted after subtraction of a Shirley background using a Gaussian–Lorentzian product function with a FWHM of 1.0–1.2 eV. Contact angle measurements were performed using a Rame´ –Hart contact angle goniometer with a microsyringe attachment. The probe liquid used for all studies was 18 mO deionized water which had been distilled to remove any hydrocarbons. Static contact angle measurements were taken by measuring the angle formed between a 0.3 ml drop of water and the surface. Advancing and receding contact angles were determined by measuring the angle formed between the polymer surface and the water droplet as a small amount of water was added and removed from the drop, respectively. Atomic force microscopy images were taken with a Digital Instruments Multimode SPM Nanoscope IIIa controller operated in tapping mode. Both topography and phase information were collected. Nanoprobe silicon nitride tips with resonant frequencies 250– 340 kHz and force constants from 18 to 46 N/m were used. The ‘‘J’’ scanner head that has a maximum scan range of 150 mm was used with a scan rate of 0.5–1.0 Hz. All parameters such as roughness and heights of topographical features were evaluated with the Nanoscope IIIa 4.22 software after flattening of the raw data.
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Fig. 1. The primary structure of the TecothaneTM poly(ether)urethane studied.
The polymer studied was a medical grade aromatic poly(ether)urethane block copolymer (TecothaneTM) supplied by Thermedics Inc. The primary structure of this polymer is shown in Fig. 1. Samples for plasma treatment were prepared by spin-casting from a 1% (w/w) solution in tetrahydrofuran onto polished Si(1 1 1) at 2000 rpm. The thickness of these films was measured by scratching the film with a micro-syringe and imaging over the scratch by AFM. The thickness was determined to be 0.2–0.3 mm by obtaining a section analysis of the image, using the Nanoscope IIIa software. Plasma treatments were performed ex situ in a reaction chamber equipped with a 2.45 GHz microwave generator and an Evensan Cavity. The distance between the plasma source and the sample was set at 16.5 cm. The base pressure of the system prior to treatment was typically 4:5 105 Torr. During treatment the pressure rose to 7:5 104 Torr. The pressure in the plasma tube was kept constant at 50 mTorr. After plasma treatment the samples were left in the reactor at a pressure of 50 mTorr to terminate as many of the residual reactions as possible prior to exposure to the atmosphere. Air was supplied by Praxair with a purity of 99.99%.
3. Results and discussion Polyurethanes are block copolymers made up of alternating urethane and polyol segments. The urethane segment is typically called the hard segment due to its higher glass transition temperature, Tg, while the polyol segment, which typically has a Tg well below room temperature, is termed the soft segment. These polymers are known to phase separate into microscopic hard and soft segment domains due to the difference in interfacial energy of the two components. This microphase separation is responsible for many of the characteristics that make PU’s desirable, such as elasticity, tensile strength and durability [16]. The inherent biocompatibility of polyurethanes has also been attributed to this domain morphology [17], although surface studies have shown that at the air/polymer interface, polyurethanes are enriched in the concentration of the soft segment with few or no urethane groups in the first few nanometers [18–20]. There are two proposed models for the domain morphology in polyurethanes. These are: interconnected hard domains and discrete hard
Fig. 2. 2D Schematic representation of possible domain morphologies for biomedical polyurethanes.
J.E. Gray et al. / Applied Surface Science 217 (2003) 210–222 Table 1 Comparison of expected XPS ratios to actual XPS ratios for a polyurethane surface XPS ratio
Theoretical ratio
Actual ratio
O/C N/C O/N
0.26 0.075 3.5
0.20 0.041 4.95
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domains [16,21]. A schematic representation of these two models is shown in Fig. 2. The morphology of the PU is believed to be affected by factors such as hard:soft ratio, polymer chain length and primary structure [22–25]. X-ray photoelectron spectroscopy has confirmed the existence of a soft segment overlayer for the polymer used in this study. Table 1 shows the expected O/C, N/C and O/N ratios for PU based on its primary
Fig. 3. AFM height images of polyurethane (250 mm 250 mm): (a) untreated; (b) air plasma treated (reactive gas pressure ¼ 50 mTorr, 10 min, 100 W).
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structure compared to the actual ratios measured. This shows that the amount of oxygen and nitrogen at the surface is less than would be expected for an even distribution of hard and soft segments at the surface. The low nitrogen concentration at the surface is particularly diagnostic since nitrogen is only present in the urethane blocks of the polymer. This result confirms that there is less urethane and consequently more polyol at the air/polymer interface.
3.1. Atomic force microscopy results The domain morphology at the surface of these polyurethane films before and after air plasma treatment has been examined by atomic force microscopy. The height images of untreated and air plasma treated polyurethane are shown in Fig. 3a and b. The corresponding phase images are shown in Fig. 4a and b. In tapping mode, topographical and phase information
Fig. 4. AFM phase images of polyurethane (250 nm 250 nm): (a) untreated; (b) air plasma modified (reactive gas pressure ¼ 50 mTorr, 10 min, 100 W).
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can be detected simultaneously. Phase imaging gives a map of stiffness variations and/or differences in tip– sample adhesion on the sample surface [26] and is obtained by detecting the phase shift between the driving oscillation of the cantilever and the actual oscillation of the tip as it scans the surface [27]. At moderate tapping force, stiffer materials appear higher in phase [26]. Moderate tapping forces were required in order to tap through the soft segment overlayer so that the nanoscale phase-separated domains could be imaged [28]. At light tapping forces there was no contrast in the phase image, confirming that there was indeed a uniform polyol layer at the surface of the polymer films [28]. The height image of untreated polyurethane shows a fairly flat surface with maximum height variations of less than 2 nm. There is no significant change in topography observed after plasma treatment. In fact,
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the surface appears to have slightly less variations in height, indicating that there is little or no etching or ablation of the polymer surface during plasma treatment. The phase image for the untreated polyurethane shows distinct high phase spots that are 12 1 nm in size. These high phase areas are probably due to the urethane hard segment blocks which have phase-separated in the material. However, because of uncertainties in the interpretation of phase shifts, all we can say with confidence, is that we can image the microphase segregation. The size of these domains compare well with the 9 4 nm domain size found for a similar polyurethane [27]. After plasma treatment, the high phase areas are slightly larger in size at 16 2 nm, which may indicate further phase separation of the material during plasma treatment. There is also an increase in the apparent total amount of hard phase at the surface.
Fig. 5. (a) Graph of contact angle vs. treatment time (air pressure ¼ 50 mTorr, plasma power ¼ 100 W); (b) graph of contact angle vs. plasma power (air pressure ¼ 50 mTorr, treatment time ¼ 5 min).
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3.2. Effects of treatment time and plasma power The effects of treatment time and plasma power on the surface modification of polyurethane by an air plasma treatment have been investigated. Contact angle goniometry has been used to determine changes in wettability of the polymer surface, while X-ray photoelectron spectroscopy has been used to deduce changes in oxygen and nitrogen concentration at the surface. 3.2.1. Contact angle results The angle between a probe liquid and the surface of a polymer changes as a function of the surface tension at the liquid/polymer interface. This angle is sensitive to both chemical and physical changes in the first few monolayers of the surface. The effect of power and treatment time on the contact angles of air plasma treated polymers with water are shown in Fig. 5a and b, respectively. There is no change in the contact angle for short treatment
times but as the treatment time increases, the contact angle changes significantly. There is very little difference between the static and advancing contact angles indicating that the surface is fairly homogeneous both chemically and morphologically. However, the receding contact angle is consistently lower than the static angle. It has been shown that at the polymer/air interface the polymer is structured such that the more hydrophobic groups are present at the surface. Upon exposure to an aqueous environment polymers are known to reorient with more hydrophilic groups at the surface [29,30]. This can happen at the individual chain level, i.e. rotation about the bonds and as phase segregation of blocks of hydrophilic domains to the surface [31]. This process is thought to be driven by thermodynamic minimization of the interfacial energy. Upon reorientation, the polymer surface is more easily wetted and the receding contact angle is decreased with respect to the initial contact angle. Polyurethane is also known to absorb water and this may affect the receding contact angle [32,33].
Fig. 6. X-ray photoelectron spectroscopy ratios vs. treatment time: (a) O/N ratio; (b) N/C ratio; (c) O/C ratio (air pressure ¼ 50 mTorr, plasma power ¼ 100 W).
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The effect of plasma power was also investigated and the results are plotted in Fig. 5b. As the power level increases to 25 W the contact angle decreases rapidly after which there is only a small decrease. The advancing angle is again very similar to the static angle, indicating a homogenous surface. The receding angle is much lower than the initial contact angle. The possible reasons for this hysteresis have been discussed above. 3.2.2. X-ray photoelectron spectroscopy The effect of treatment time on oxygen/carbon and nitrogen/carbon ratios are shown in Fig. 6c and b, respectively. As the treatment time is increased the O/C ratio increases dramatically to 30 s then continues to increase slightly with increasing treatment time. This implies that the initial effect of the plasma is to incorporate oxygen into the polymer surface but as the reaction proceeds, an equilibrium is reached between the surface and the plasma in which the primary surface reactions can be attributed to conversion of species already present. Some of these include oxidation of C–O bonds to carbonyls, decomposition
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of peroxides to form functional groups such as aldehydes, ketones and alcohols, and interchain reactions due to chain scission. The N/C ratios show an interesting trend where at low treatment times nitrogen is removed from the surface and as the reaction continues nitrogen becomes reincorporated into the polymer film, until at 10 min it is at a higher level than untreated polyurethane. One possible explanation for this is the ablation of nitrogen from the sample surface. This is unlikely since there is no evidence of surface etching or ablation from the atomic force microscopy studies. It is more likely that heating of the sample during plasma treatment causes further phase segregation of the lower Tg soft segment at the surface. Fig. 7 shows graphs of percentage of the O 1s envelope due to carbonyl and polyol type oxygen versus plasma treatment time. It can be seen that the amount of polyol type oxygen initially decreases and then levels off as the reaction proceeds, while the amount of carbonyl continues to increase to about 5 min, and then levels off. The initial decrease in polyol type oxygen is due to the oxidation of these
Fig. 7. Percentage of O 1s envelope due to carbonyl and polyol following air plasma treatment (reactive gas pressure ¼ 50 mTorr, plasma power ¼ 100 W): (a) % polyol oxygen vs. treatment time; (b) % carbonyl oxygen vs. treatment time.
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species. As the reaction proceeds, an equilibrium is reached between plasma/polymer reactions and diffusion of polyol segments to the surface. If there was no diffusion of polyol to the surface, a continued decrease in polyol type oxygen should be observed coupled with the increase in carbonyl. As the reaction proceeds, the amount of carbonyl continues to increase but the polyol remains the same. This must be due to an increase in polyol at the surface. At higher treatment times nitrogen begins to be reincorporated into the surface due to the preferred reaction of nitrogen species with carbons that are already bonded to oxygen [34]. 3.2.3. Effect of power The effect of power on the oxygen/carbon and nitrogen/carbon ratios are shown in Fig. 8c and b. These show that as the power is increased the O/C ratio also increases initially and then levels off at about 25 W. This implies that either the concentration of excited oxygen species to reach the surface is independent of plasma power above 25 W or that both
polyol diffusion and oxidation of the surface are occurring. The N/C ratio decreases up to 50 W and then levels off. This adds further weight to the argument that plasma induced surface segregation of polyol is occurring. As the plasma power is increased up to 50 W, the amount of energy imparted to the surface is also increased and this increase in energy causes diffusion of polyol segments to the surface. Above 50 W an equilibrium is reached between polyol diffusion and incorporation of nitrogen groups onto the surface, and the N/C ratio levels off. The AFM results clearly show that the decrease in nitrogen at the surface cannot be due to surface etching or ablation. Without removal of nitrogen by surface etching or ablation we would expect to see an increase in nitrogen at the surface. The evidence presented clearly supports plasma induced surface reorientation of polyol segments. 3.2.4. High resolution XPS spectra High resolution XPS spectra were collected of modified and unmodified polyurethane. The C 1s
Fig. 8. X-ray photoelectron spectroscopy ratios vs. plasma power: (a) O/N ratio; (b) N/C ratio; (c) O/C ratio (air pressure ¼ 50 mTorr, treatment time ¼ 5 min).
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Fig. 9. C 1s spectra following air plasma treatment of the polyurethane surface: (a) untreated polyurethane; (b) air plasma treated polyurethane (reactive gas pressure ¼ 50 mTorr, plasma power ¼ 100 W, treatment time ¼ 5 min).
spectra are shown in Fig. 9a and b. The unmodified polyurethane spectrum can be deconvoluted into five peaks. Their peak positions and relative intensities are tabulated in Table 2. These peaks correspond to the aromatic C¼C bonds, aliphatic C–C, C–N, C–O and C¼O carbons in the polyurethane chain [15]. The spectra observed for this particular polyurethane do not exactly match those found in the database for organic polymers [15]. The polyurethane reported in the database is a homopolymer consisting of the hard segment section of the polymer studied here. The block copolymer used in this study (Fig. 1) has a slightly different spectrum due to its soft segment
blocks and its spectrum is consistent with data for similar polymers in the literature [18,34,35]. Plasma modification induces some significant changes in this spectrum. There are changes in the relative intensities of all the peaks. There is a decrease in relative intensity of C–O accompanied by an increase in C¼O and a shift of this peak by 0.4 eV to lower binding energy. This is due to reaction of excited oxygen species with the polymer surface. It is well known that ethers react with oxygen to form hydroperoxides that readily decompose to give functional groups such as alcohols, ketones, aldehydes and carboxylic acids [36]. The shift to lower binding energy
Table 2 Peak-fitted components of the C 1s spectra for untreated and air plasma treated polyurethane Peak component
Untreated poyurethane (binding energy (eV), FWHM (eV), area%)
Air plasma treated polyurethane (binding energy (eV), FWHM (eV), area%)
C¼C C–C C–N C–O C¼N C¼O (carbamate)
284.6, 285.1, 285.8, 286.6, N/A 289.6,
284.5, 285.1, 286.0, 286.7, 287.6, 289.2,
1.04, 13.8 0.98, 34.3 1.0, 14.8 1.15, 34.2 1.11, 3.0
1.03, 0.93, 0.95, 0.94, 1.27, 1.30,
24.1 22.5 23.3 13.6 6.7 9.8
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is indicative of the change from only urethane carbonyl to a mixture of carbonyl containing functionalities. There is an increase in the relative intensity of C¼C and C–N. In a previous paper on plasma treatment of PU [18], increases in the C¼O, C¼C and C–N species have been taken to mean an increase in hard segment at the surface. In these studies the increase was also accompanied by an overall increase in nitrogen concentration at the surface and was attributed to etching of the soft segment overlayer. In our study, the amount of nitrogen at the surface is less than in the untreated polymer so preferential reorientation of urethane groups to the surface is unlikely and the AFM shows no evidence of surface etching or ablation. The increase in C¼C at the surface can be attributed to double bond formation in the polymer film due to radical reactions and chain scission. In the unmodified polymer film the C¼C bonds can be solely attributed to aromatic carbon; since there is little change in binding energy shift between aromatic and other p bonds, it is likely that this peak for the modified polyurethane is a combination of the two. An increase in the C–N peak is also observed, as well as a 0.2 eV shift to higher binding energy; since there is a large decrease in the amount of
nitrogen at the surface it is unlikely that this peak is due solely to C–N. Attempts to fit this peak as two separate peaks led to a similar fit to the data. It is likely that this peak is due to the C–N in the urethane group as well as oxygen functionalities such as alcohols. A new peak that is consistent with the formation of imine groups (C¼N) is also observed at 287.6 eV. This peak is consistent with the results seen for N/C ratios with plasma treatment time, at this stage of the plasma treatment, nitrogen is beginning to be reincorporated into the polymer film and this is observed as a new nitrogen species at low relative concentration. The O 1s spectrum for unmodified polyurethane (Fig. 10a) can be deconvoluted into three peaks (Table 3). The urethane group of the hard segment contains two types of oxygen, the carbonyl oxygen at 532.1 eV and the C–O oxygen at 533.7 eV. The third peak is observed at 532.8 eV and is due to the ether oxygen in the polyol soft segment of the polymer. There are changes in the relative intensities of each of the peak components after air plasma treatment. The C¼O peak increases dramatically which is consistent with the results from the C 1s spectrum. The C–O peaks from both the hard and soft segments decrease
Fig. 10. O 1s spectra following air plasma treatment of the polyurethane surface: (a) untreated polyurethane; (b) air plasma treated polyurethane (reactive gas pressure ¼ 50 mTorr, plasma power ¼ 100 W, treatment time ¼ 5 min).
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Table 3 Peak-fitted components of the O 1s spectra for untreated and air plasma treated polyurethane Peak component
Untreated polurethane (binding energy (eV), FWHM (eV), area%)
Air plasma treated polyurethane (binding energy (eV), FWHM (eV), area%)
O¼C O–C (soft segment) O–C (hard segment)
532.1, 1.3, 11.9 532.8, 1.3, 53.4 533.7, 1.3, 34.7
532.1, 1.3, 47.3 532.8, 1.3, 32.7 533.7, 1.3, 19.9
by approximately 15 and 20%, respectively. This result is also consistent with the C 1s spectrum. There are no new peaks present which confirms that any peroxide intermediates decompose to give functional groups in which oxygen is not in a significantly different chemical environment. Functional groups such as alcohols, aldehydes and carboxylic acids would have very little
binding energy shift compared to the C¼O and C–O groups present in the unmodified polyurethane. The N 1s spectra for unmodified and plasma modified polyurethane are shown in Fig. 11a and b, respectively (Table 4). Fitting of the N 1s peak is difficult due to its low intensity. There should be only one nitrogen peak in the unmodified spectrum due to
Fig. 11. N 1s spectra following air plasma treatment of the polyurethane surface: (a) untreated polyurethane; (b) air plasma treated polyurethane (reactive gas pressure ¼ 50 mTorr, plasma power ¼ 100 W, treatment time ¼ 5 min).
Table 4 Peak-fitted components of the N 1s spectra for untreated and air plasma treated polyurethane Peak component
Untreated polyurethane (binding energy (eV), FWHM (eV), area%)
Air plasma treated polyurethane (binding energy (eV), FWHM (eV), area%)
N–C N¼C
400.8, 1.51, 100 N/A
401.1, 1.47, 26.8 400.0, 1.22, 73.2
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C–N in the urethane group. After plasma modification there is a shift of the peak to lower binding energy with tailing at the high binding energy side. This has been interpreted as a decrease in C–N at approximately 401 eV and a new peak due to C¼N at 400.0 eV. This result is consistent with the C 1s spectrum.
4. Conclusions Surface studies of the biomedical poly(ether)urethane, TecothaneTM, have demonstrated the phaseseparated structure of the polymer. The polymer film consists of a thin layer of polyol covering a phaseseparated structure consisting of hard and soft segment domains. Remote air plasma treatment has been shown to increase the wettability of the polymer as well as inducing some complex chemical and physical changes at the surface without any surface etching or ablation. High resolution XPS spectra indicate an increase in C¼O and C¼C at the surface, as well as the presence of new functional groups such as alcohols, ketones, aldehydes and imines. There is also evidence that the energy imparted to the polymer during plasma treatment causes surface segregation/ diffusion of polyol. This work demonstrates the complex nature of the plasma/polymer interaction. The complex surface chemistry and induced morphological changes that can occur upon plasma treatment have been known. However, this paper also demonstrates the more subtle effects that plasma treatment can have on the physical structure of a polymer surface. References [1] C.-M. Chan, T.-M. Ko, H. Hiraoka, Surf. Sci. Rep. 24 (1996) 1–54. [2] Q. Zhao, H. Yi Lu, D.W. Hess, J. Electrochem. Soc. 143 (1996) 2896–2905. [3] B.D. Beake, J.S.G. Ling, G.J. Leggett, J. Mater. Chem. 8 (1998) 1735–1742. [4] J. Hopkins, J.P.S. Badyal, J. Polym. Sci. A 34 (1996) 1385– 1393. [5] P. Groning, M. Collaud, G. Dietler, L. Schlapbach, J. Appl. Phys. 76 (1994) 887–892. [6] M. Keil, C.S. Rastomjee, A. Rajagopal, et al., Appl. Surf. Sci. 125 (1998) 273–286. [7] H. Muguruma, I. Karube, M. Saito, Chem. Lett. (1996) 283– 284. [8] J.P. Badey, E. Espuche, D. Sage, Polymer 37 (1996) 1337–1386.
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Surface modification of a biomedical poly(ether)urethane by a remote air plasma J.E. Gray*, P.R. Norton1, K. Griffiths Department of Chemistry and Interface Science Western, University of Western Ontario, London, Ont., Canada, N6A 5B7 Received 19 September 2002; accepted 22 March 2003
Abstract Plasma modification of polymer surfaces is widely used, but the plasma/polymer interaction is very complex and still not fully understood. In this paper, the interaction of a biomedical poly(ether)urethane with a remote air plasma treatment has been studied. Atomic force microscopy studies show the domain structure of the polymer as well as the absence of any surface roughening due to plasma treatment. Contact angle goniometry shows an improved wettability of the surface after plasma treatment. X-ray photoelectron spectroscopy indicates an increase in C¼O and C¼C at the surface, as well as the presence of new functional groups such as alcohols, ketones, aldehydes and imines. There is also evidence that the energy imparted to the polymer during plasma treatment causes surface segregation of polyol segments. # 2003 Elsevier Science B.V. All rights reserved. PACS: 80, Cross-disciplinary physics and related areas of science and technology; 82, Physical chemistry; 82.40.Ra, Plasma reactions Keywords: Poly(ether)urethane; Plasma; Surface modification; Microdomain; X-ray photoelectron spectroscopy
1. Introduction Polymers are widely used as biomaterials for medical implants and devices, but despite their extensive use in medicine they tend to exhibit poor biocompatibility and poor adhesion to other materials. An understanding of the polymer surface is key to improving the function and lifetime of medical implants and devices through surface modifications that do not affect the bulk properties and function of the material. Biocompatibility of * Corresponding author. Present address: Head hall Chemical Engineering, University of New Brunswick, Fredericton, Canada, E3A 5B3. Tel.: þ1-5064473461; fax: þ1-5064533591. E-mail addresses: [email protected] (J.E. Gray), [email protected] (P.R. Norton). 1 Tel.: þ1-519-661-4180; fax: þ1-519-661-3022.
these devices can be improved through changes in wettability, incorporation of surface functional groups that help prevent thrombogenicity, and surface coatings of biologically compatible species such as albumin, proteins and antibiotics. Surface modifications can be used to enhance the adhesion of specific molecules to the surface; this can either promote or prevent the adhesion of biological molecules and cells to the polymer substrate. The development of polymer scaffolds to promote bone growth for orthopedic patients and antibacterial coatings for the prevention of device associated infection are two areas where surface modification can play an important role in improving device function and lifetime. Plasma treatment is a technique that can be used to modify the surface properties of biomaterials without also altering bulk properties that affect their function,
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00552-X
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since the depth of modification is typically limited to the first few nanometers of the surface [1]. A plasma is a gaseous mixture of electrons, radicals, ions and excited molecular states which are created by inelastic collisions between high energy electrons and ground state atoms or molecules. The high energy particles and photons of a plasma interact with the polymer surface by both chemical and physical processes. Plasma treatments are known to improve wettability, induce cross-linking and chemical functionalization of the surface and cause surface etching and ablation [1]. The type of surface modification generated by a plasma treatment is strongly dependent on the choice of reactive gas and treatment conditions. Inert gas plasmas such as argon and helium have been shown to modify the surface through crosslinking, chain scission, chain branching and surface roughening [2–6]. Reactive gas plasmas involving gases such as oxygen, nitrogen, hydrogen, ammonia and hydrogen sulfide, have been shown to introduce new functional groups onto the polymer surface as well as to induce changes similar to those observed with inert gases [5,7–11]. The high energy ions in a plasma can often cause significant surface roughening and formation of low molecular weight organic material on the surface. This can decrease adhesion of coatings to the material through the formation of weak boundary layers and hence affect the interaction of cells with the material [12,13]. Remote plasma treatments allow chemical modification of the surface without etching and ablation. In a remote plasma treatment, the sample to be modified is placed at some distance downstream from the plasma source. This limits the ion bombardment of the surface since the ions lose energy through collisions with the wall of the plasma reactor and other plasma species before they can reach the polymer surface. Only the longer lived excited species can interact with the surface. For a nitrogen plasma these are thought to be Nð4 sÞ ground state atoms and electronically and vibrationally excited nitrogen molecules, while in a remote oxygen plasma these species are oxygen atoms and singlet molecular oxygen [14]. Therefore, remote plasma treatment allows surfaces to be chemically modified while minimizing unwanted sample damage. In this paper, the effect of an air plasma treatment on the surface chemistry, wettability and morphology of a
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biomedical poly(ether)urethane have been investigated. X-ray photoelectron spectroscopy has been used to determine changes in surface chemistry while contact angle goniometry and atomic force microscopy have been used to investigate changes in wettability and surface roughening, respectively.
2. Experimental X-ray photoelectron spectroscopy studies were performed on an SSL SSX-100 XPS spectrometer using a monochromatized Al Ka X-ray source and hemispherical analyzer at a takeoff angle of 538. Elemental analysis was conducted with a 600 mm spot size and 129 eV pass energy, while high resolution spectra had a spot size of 300 mm and a pass energy of 54 eV. An electron flood gun was used for charge compensation (1 V, 0.05 mA). The binding energies were corrected for charging using the C 1s binding energy peak for CH2 in the polymer at 285.0 eV as a reference [15]. The spectra were deconvoluted after subtraction of a Shirley background using a Gaussian–Lorentzian product function with a FWHM of 1.0–1.2 eV. Contact angle measurements were performed using a Rame´ –Hart contact angle goniometer with a microsyringe attachment. The probe liquid used for all studies was 18 mO deionized water which had been distilled to remove any hydrocarbons. Static contact angle measurements were taken by measuring the angle formed between a 0.3 ml drop of water and the surface. Advancing and receding contact angles were determined by measuring the angle formed between the polymer surface and the water droplet as a small amount of water was added and removed from the drop, respectively. Atomic force microscopy images were taken with a Digital Instruments Multimode SPM Nanoscope IIIa controller operated in tapping mode. Both topography and phase information were collected. Nanoprobe silicon nitride tips with resonant frequencies 250– 340 kHz and force constants from 18 to 46 N/m were used. The ‘‘J’’ scanner head that has a maximum scan range of 150 mm was used with a scan rate of 0.5–1.0 Hz. All parameters such as roughness and heights of topographical features were evaluated with the Nanoscope IIIa 4.22 software after flattening of the raw data.
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Fig. 1. The primary structure of the TecothaneTM poly(ether)urethane studied.
The polymer studied was a medical grade aromatic poly(ether)urethane block copolymer (TecothaneTM) supplied by Thermedics Inc. The primary structure of this polymer is shown in Fig. 1. Samples for plasma treatment were prepared by spin-casting from a 1% (w/w) solution in tetrahydrofuran onto polished Si(1 1 1) at 2000 rpm. The thickness of these films was measured by scratching the film with a micro-syringe and imaging over the scratch by AFM. The thickness was determined to be 0.2–0.3 mm by obtaining a section analysis of the image, using the Nanoscope IIIa software. Plasma treatments were performed ex situ in a reaction chamber equipped with a 2.45 GHz microwave generator and an Evensan Cavity. The distance between the plasma source and the sample was set at 16.5 cm. The base pressure of the system prior to treatment was typically 4:5 105 Torr. During treatment the pressure rose to 7:5 104 Torr. The pressure in the plasma tube was kept constant at 50 mTorr. After plasma treatment the samples were left in the reactor at a pressure of 50 mTorr to terminate as many of the residual reactions as possible prior to exposure to the atmosphere. Air was supplied by Praxair with a purity of 99.99%.
3. Results and discussion Polyurethanes are block copolymers made up of alternating urethane and polyol segments. The urethane segment is typically called the hard segment due to its higher glass transition temperature, Tg, while the polyol segment, which typically has a Tg well below room temperature, is termed the soft segment. These polymers are known to phase separate into microscopic hard and soft segment domains due to the difference in interfacial energy of the two components. This microphase separation is responsible for many of the characteristics that make PU’s desirable, such as elasticity, tensile strength and durability [16]. The inherent biocompatibility of polyurethanes has also been attributed to this domain morphology [17], although surface studies have shown that at the air/polymer interface, polyurethanes are enriched in the concentration of the soft segment with few or no urethane groups in the first few nanometers [18–20]. There are two proposed models for the domain morphology in polyurethanes. These are: interconnected hard domains and discrete hard
Fig. 2. 2D Schematic representation of possible domain morphologies for biomedical polyurethanes.
J.E. Gray et al. / Applied Surface Science 217 (2003) 210–222 Table 1 Comparison of expected XPS ratios to actual XPS ratios for a polyurethane surface XPS ratio
Theoretical ratio
Actual ratio
O/C N/C O/N
0.26 0.075 3.5
0.20 0.041 4.95
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domains [16,21]. A schematic representation of these two models is shown in Fig. 2. The morphology of the PU is believed to be affected by factors such as hard:soft ratio, polymer chain length and primary structure [22–25]. X-ray photoelectron spectroscopy has confirmed the existence of a soft segment overlayer for the polymer used in this study. Table 1 shows the expected O/C, N/C and O/N ratios for PU based on its primary
Fig. 3. AFM height images of polyurethane (250 mm 250 mm): (a) untreated; (b) air plasma treated (reactive gas pressure ¼ 50 mTorr, 10 min, 100 W).
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structure compared to the actual ratios measured. This shows that the amount of oxygen and nitrogen at the surface is less than would be expected for an even distribution of hard and soft segments at the surface. The low nitrogen concentration at the surface is particularly diagnostic since nitrogen is only present in the urethane blocks of the polymer. This result confirms that there is less urethane and consequently more polyol at the air/polymer interface.
3.1. Atomic force microscopy results The domain morphology at the surface of these polyurethane films before and after air plasma treatment has been examined by atomic force microscopy. The height images of untreated and air plasma treated polyurethane are shown in Fig. 3a and b. The corresponding phase images are shown in Fig. 4a and b. In tapping mode, topographical and phase information
Fig. 4. AFM phase images of polyurethane (250 nm 250 nm): (a) untreated; (b) air plasma modified (reactive gas pressure ¼ 50 mTorr, 10 min, 100 W).
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can be detected simultaneously. Phase imaging gives a map of stiffness variations and/or differences in tip– sample adhesion on the sample surface [26] and is obtained by detecting the phase shift between the driving oscillation of the cantilever and the actual oscillation of the tip as it scans the surface [27]. At moderate tapping force, stiffer materials appear higher in phase [26]. Moderate tapping forces were required in order to tap through the soft segment overlayer so that the nanoscale phase-separated domains could be imaged [28]. At light tapping forces there was no contrast in the phase image, confirming that there was indeed a uniform polyol layer at the surface of the polymer films [28]. The height image of untreated polyurethane shows a fairly flat surface with maximum height variations of less than 2 nm. There is no significant change in topography observed after plasma treatment. In fact,
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the surface appears to have slightly less variations in height, indicating that there is little or no etching or ablation of the polymer surface during plasma treatment. The phase image for the untreated polyurethane shows distinct high phase spots that are 12 1 nm in size. These high phase areas are probably due to the urethane hard segment blocks which have phase-separated in the material. However, because of uncertainties in the interpretation of phase shifts, all we can say with confidence, is that we can image the microphase segregation. The size of these domains compare well with the 9 4 nm domain size found for a similar polyurethane [27]. After plasma treatment, the high phase areas are slightly larger in size at 16 2 nm, which may indicate further phase separation of the material during plasma treatment. There is also an increase in the apparent total amount of hard phase at the surface.
Fig. 5. (a) Graph of contact angle vs. treatment time (air pressure ¼ 50 mTorr, plasma power ¼ 100 W); (b) graph of contact angle vs. plasma power (air pressure ¼ 50 mTorr, treatment time ¼ 5 min).
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3.2. Effects of treatment time and plasma power The effects of treatment time and plasma power on the surface modification of polyurethane by an air plasma treatment have been investigated. Contact angle goniometry has been used to determine changes in wettability of the polymer surface, while X-ray photoelectron spectroscopy has been used to deduce changes in oxygen and nitrogen concentration at the surface. 3.2.1. Contact angle results The angle between a probe liquid and the surface of a polymer changes as a function of the surface tension at the liquid/polymer interface. This angle is sensitive to both chemical and physical changes in the first few monolayers of the surface. The effect of power and treatment time on the contact angles of air plasma treated polymers with water are shown in Fig. 5a and b, respectively. There is no change in the contact angle for short treatment
times but as the treatment time increases, the contact angle changes significantly. There is very little difference between the static and advancing contact angles indicating that the surface is fairly homogeneous both chemically and morphologically. However, the receding contact angle is consistently lower than the static angle. It has been shown that at the polymer/air interface the polymer is structured such that the more hydrophobic groups are present at the surface. Upon exposure to an aqueous environment polymers are known to reorient with more hydrophilic groups at the surface [29,30]. This can happen at the individual chain level, i.e. rotation about the bonds and as phase segregation of blocks of hydrophilic domains to the surface [31]. This process is thought to be driven by thermodynamic minimization of the interfacial energy. Upon reorientation, the polymer surface is more easily wetted and the receding contact angle is decreased with respect to the initial contact angle. Polyurethane is also known to absorb water and this may affect the receding contact angle [32,33].
Fig. 6. X-ray photoelectron spectroscopy ratios vs. treatment time: (a) O/N ratio; (b) N/C ratio; (c) O/C ratio (air pressure ¼ 50 mTorr, plasma power ¼ 100 W).
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The effect of plasma power was also investigated and the results are plotted in Fig. 5b. As the power level increases to 25 W the contact angle decreases rapidly after which there is only a small decrease. The advancing angle is again very similar to the static angle, indicating a homogenous surface. The receding angle is much lower than the initial contact angle. The possible reasons for this hysteresis have been discussed above. 3.2.2. X-ray photoelectron spectroscopy The effect of treatment time on oxygen/carbon and nitrogen/carbon ratios are shown in Fig. 6c and b, respectively. As the treatment time is increased the O/C ratio increases dramatically to 30 s then continues to increase slightly with increasing treatment time. This implies that the initial effect of the plasma is to incorporate oxygen into the polymer surface but as the reaction proceeds, an equilibrium is reached between the surface and the plasma in which the primary surface reactions can be attributed to conversion of species already present. Some of these include oxidation of C–O bonds to carbonyls, decomposition
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of peroxides to form functional groups such as aldehydes, ketones and alcohols, and interchain reactions due to chain scission. The N/C ratios show an interesting trend where at low treatment times nitrogen is removed from the surface and as the reaction continues nitrogen becomes reincorporated into the polymer film, until at 10 min it is at a higher level than untreated polyurethane. One possible explanation for this is the ablation of nitrogen from the sample surface. This is unlikely since there is no evidence of surface etching or ablation from the atomic force microscopy studies. It is more likely that heating of the sample during plasma treatment causes further phase segregation of the lower Tg soft segment at the surface. Fig. 7 shows graphs of percentage of the O 1s envelope due to carbonyl and polyol type oxygen versus plasma treatment time. It can be seen that the amount of polyol type oxygen initially decreases and then levels off as the reaction proceeds, while the amount of carbonyl continues to increase to about 5 min, and then levels off. The initial decrease in polyol type oxygen is due to the oxidation of these
Fig. 7. Percentage of O 1s envelope due to carbonyl and polyol following air plasma treatment (reactive gas pressure ¼ 50 mTorr, plasma power ¼ 100 W): (a) % polyol oxygen vs. treatment time; (b) % carbonyl oxygen vs. treatment time.
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species. As the reaction proceeds, an equilibrium is reached between plasma/polymer reactions and diffusion of polyol segments to the surface. If there was no diffusion of polyol to the surface, a continued decrease in polyol type oxygen should be observed coupled with the increase in carbonyl. As the reaction proceeds, the amount of carbonyl continues to increase but the polyol remains the same. This must be due to an increase in polyol at the surface. At higher treatment times nitrogen begins to be reincorporated into the surface due to the preferred reaction of nitrogen species with carbons that are already bonded to oxygen [34]. 3.2.3. Effect of power The effect of power on the oxygen/carbon and nitrogen/carbon ratios are shown in Fig. 8c and b. These show that as the power is increased the O/C ratio also increases initially and then levels off at about 25 W. This implies that either the concentration of excited oxygen species to reach the surface is independent of plasma power above 25 W or that both
polyol diffusion and oxidation of the surface are occurring. The N/C ratio decreases up to 50 W and then levels off. This adds further weight to the argument that plasma induced surface segregation of polyol is occurring. As the plasma power is increased up to 50 W, the amount of energy imparted to the surface is also increased and this increase in energy causes diffusion of polyol segments to the surface. Above 50 W an equilibrium is reached between polyol diffusion and incorporation of nitrogen groups onto the surface, and the N/C ratio levels off. The AFM results clearly show that the decrease in nitrogen at the surface cannot be due to surface etching or ablation. Without removal of nitrogen by surface etching or ablation we would expect to see an increase in nitrogen at the surface. The evidence presented clearly supports plasma induced surface reorientation of polyol segments. 3.2.4. High resolution XPS spectra High resolution XPS spectra were collected of modified and unmodified polyurethane. The C 1s
Fig. 8. X-ray photoelectron spectroscopy ratios vs. plasma power: (a) O/N ratio; (b) N/C ratio; (c) O/C ratio (air pressure ¼ 50 mTorr, treatment time ¼ 5 min).
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Fig. 9. C 1s spectra following air plasma treatment of the polyurethane surface: (a) untreated polyurethane; (b) air plasma treated polyurethane (reactive gas pressure ¼ 50 mTorr, plasma power ¼ 100 W, treatment time ¼ 5 min).
spectra are shown in Fig. 9a and b. The unmodified polyurethane spectrum can be deconvoluted into five peaks. Their peak positions and relative intensities are tabulated in Table 2. These peaks correspond to the aromatic C¼C bonds, aliphatic C–C, C–N, C–O and C¼O carbons in the polyurethane chain [15]. The spectra observed for this particular polyurethane do not exactly match those found in the database for organic polymers [15]. The polyurethane reported in the database is a homopolymer consisting of the hard segment section of the polymer studied here. The block copolymer used in this study (Fig. 1) has a slightly different spectrum due to its soft segment
blocks and its spectrum is consistent with data for similar polymers in the literature [18,34,35]. Plasma modification induces some significant changes in this spectrum. There are changes in the relative intensities of all the peaks. There is a decrease in relative intensity of C–O accompanied by an increase in C¼O and a shift of this peak by 0.4 eV to lower binding energy. This is due to reaction of excited oxygen species with the polymer surface. It is well known that ethers react with oxygen to form hydroperoxides that readily decompose to give functional groups such as alcohols, ketones, aldehydes and carboxylic acids [36]. The shift to lower binding energy
Table 2 Peak-fitted components of the C 1s spectra for untreated and air plasma treated polyurethane Peak component
Untreated poyurethane (binding energy (eV), FWHM (eV), area%)
Air plasma treated polyurethane (binding energy (eV), FWHM (eV), area%)
C¼C C–C C–N C–O C¼N C¼O (carbamate)
284.6, 285.1, 285.8, 286.6, N/A 289.6,
284.5, 285.1, 286.0, 286.7, 287.6, 289.2,
1.04, 13.8 0.98, 34.3 1.0, 14.8 1.15, 34.2 1.11, 3.0
1.03, 0.93, 0.95, 0.94, 1.27, 1.30,
24.1 22.5 23.3 13.6 6.7 9.8
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is indicative of the change from only urethane carbonyl to a mixture of carbonyl containing functionalities. There is an increase in the relative intensity of C¼C and C–N. In a previous paper on plasma treatment of PU [18], increases in the C¼O, C¼C and C–N species have been taken to mean an increase in hard segment at the surface. In these studies the increase was also accompanied by an overall increase in nitrogen concentration at the surface and was attributed to etching of the soft segment overlayer. In our study, the amount of nitrogen at the surface is less than in the untreated polymer so preferential reorientation of urethane groups to the surface is unlikely and the AFM shows no evidence of surface etching or ablation. The increase in C¼C at the surface can be attributed to double bond formation in the polymer film due to radical reactions and chain scission. In the unmodified polymer film the C¼C bonds can be solely attributed to aromatic carbon; since there is little change in binding energy shift between aromatic and other p bonds, it is likely that this peak for the modified polyurethane is a combination of the two. An increase in the C–N peak is also observed, as well as a 0.2 eV shift to higher binding energy; since there is a large decrease in the amount of
nitrogen at the surface it is unlikely that this peak is due solely to C–N. Attempts to fit this peak as two separate peaks led to a similar fit to the data. It is likely that this peak is due to the C–N in the urethane group as well as oxygen functionalities such as alcohols. A new peak that is consistent with the formation of imine groups (C¼N) is also observed at 287.6 eV. This peak is consistent with the results seen for N/C ratios with plasma treatment time, at this stage of the plasma treatment, nitrogen is beginning to be reincorporated into the polymer film and this is observed as a new nitrogen species at low relative concentration. The O 1s spectrum for unmodified polyurethane (Fig. 10a) can be deconvoluted into three peaks (Table 3). The urethane group of the hard segment contains two types of oxygen, the carbonyl oxygen at 532.1 eV and the C–O oxygen at 533.7 eV. The third peak is observed at 532.8 eV and is due to the ether oxygen in the polyol soft segment of the polymer. There are changes in the relative intensities of each of the peak components after air plasma treatment. The C¼O peak increases dramatically which is consistent with the results from the C 1s spectrum. The C–O peaks from both the hard and soft segments decrease
Fig. 10. O 1s spectra following air plasma treatment of the polyurethane surface: (a) untreated polyurethane; (b) air plasma treated polyurethane (reactive gas pressure ¼ 50 mTorr, plasma power ¼ 100 W, treatment time ¼ 5 min).
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Table 3 Peak-fitted components of the O 1s spectra for untreated and air plasma treated polyurethane Peak component
Untreated polurethane (binding energy (eV), FWHM (eV), area%)
Air plasma treated polyurethane (binding energy (eV), FWHM (eV), area%)
O¼C O–C (soft segment) O–C (hard segment)
532.1, 1.3, 11.9 532.8, 1.3, 53.4 533.7, 1.3, 34.7
532.1, 1.3, 47.3 532.8, 1.3, 32.7 533.7, 1.3, 19.9
by approximately 15 and 20%, respectively. This result is also consistent with the C 1s spectrum. There are no new peaks present which confirms that any peroxide intermediates decompose to give functional groups in which oxygen is not in a significantly different chemical environment. Functional groups such as alcohols, aldehydes and carboxylic acids would have very little
binding energy shift compared to the C¼O and C–O groups present in the unmodified polyurethane. The N 1s spectra for unmodified and plasma modified polyurethane are shown in Fig. 11a and b, respectively (Table 4). Fitting of the N 1s peak is difficult due to its low intensity. There should be only one nitrogen peak in the unmodified spectrum due to
Fig. 11. N 1s spectra following air plasma treatment of the polyurethane surface: (a) untreated polyurethane; (b) air plasma treated polyurethane (reactive gas pressure ¼ 50 mTorr, plasma power ¼ 100 W, treatment time ¼ 5 min).
Table 4 Peak-fitted components of the N 1s spectra for untreated and air plasma treated polyurethane Peak component
Untreated polyurethane (binding energy (eV), FWHM (eV), area%)
Air plasma treated polyurethane (binding energy (eV), FWHM (eV), area%)
N–C N¼C
400.8, 1.51, 100 N/A
401.1, 1.47, 26.8 400.0, 1.22, 73.2
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C–N in the urethane group. After plasma modification there is a shift of the peak to lower binding energy with tailing at the high binding energy side. This has been interpreted as a decrease in C–N at approximately 401 eV and a new peak due to C¼N at 400.0 eV. This result is consistent with the C 1s spectrum.
4. Conclusions Surface studies of the biomedical poly(ether)urethane, TecothaneTM, have demonstrated the phaseseparated structure of the polymer. The polymer film consists of a thin layer of polyol covering a phaseseparated structure consisting of hard and soft segment domains. Remote air plasma treatment has been shown to increase the wettability of the polymer as well as inducing some complex chemical and physical changes at the surface without any surface etching or ablation. High resolution XPS spectra indicate an increase in C¼O and C¼C at the surface, as well as the presence of new functional groups such as alcohols, ketones, aldehydes and imines. There is also evidence that the energy imparted to the polymer during plasma treatment causes surface segregation/ diffusion of polyol. This work demonstrates the complex nature of the plasma/polymer interaction. The complex surface chemistry and induced morphological changes that can occur upon plasma treatment have been known. However, this paper also demonstrates the more subtle effects that plasma treatment can have on the physical structure of a polymer surface. References [1] C.-M. Chan, T.-M. Ko, H. Hiraoka, Surf. Sci. Rep. 24 (1996) 1–54. [2] Q. Zhao, H. Yi Lu, D.W. Hess, J. Electrochem. Soc. 143 (1996) 2896–2905. [3] B.D. Beake, J.S.G. Ling, G.J. Leggett, J. Mater. Chem. 8 (1998) 1735–1742. [4] J. Hopkins, J.P.S. Badyal, J. Polym. Sci. A 34 (1996) 1385– 1393. [5] P. Groning, M. Collaud, G. Dietler, L. Schlapbach, J. Appl. Phys. 76 (1994) 887–892. [6] M. Keil, C.S. Rastomjee, A. Rajagopal, et al., Appl. Surf. Sci. 125 (1998) 273–286. [7] H. Muguruma, I. Karube, M. Saito, Chem. Lett. (1996) 283– 284. [8] J.P. Badey, E. Espuche, D. Sage, Polymer 37 (1996) 1337–1386.
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