Effects of UV Irradiation and Plasma Treatment on a Polystyrene

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Effects of UV Irradiation and Plasma Treatment on a Polystyrene Surface Studied by IR-Visible Sum Frequency Generation Spectroscopy D. Zhang, S. M. Dougal, and M. S. Yeganeh* Corporate Research Science Laboratories, Exxon Research and Engineering Company, Route 22 East, Annandale, New Jersey 08801 Received October 13, 1999. In Final Form: February 17, 2000 IR-visible sum frequency generation (SFG) spectroscopy was used to study surface modification of polystyrene by its exposure to a UV light source or plasma. It was found that the polystyrene surface underwent dramatic changes after exposure to these treatments, as evidenced by marked changes in the surface SFG spectra. Before the treatments, the surface spectrum showed a pronounced peak at 3068 cm-1 which is characteristic of the symmetric stretch of the aromatic C-H of polystyrene. This peak decreased markedly, and other vibrational bands associated with the CH2 and CH3 groups appeared after the treatments. The observed spectral changes provided direct evidence of surface reactions involving the aromatic ring. In addition, our data showed that the degrees of oxidation of the polystyrene surface were different with the two processes. The oxidation to a higher oxidation state, resulting in the formation of carbonyl/carboxyl species, was observed with plasma treatment but not with UV irradiation. This difference was also reflected in contact angle measurements. Before the treatments, the contact angle was 95 ( 4°. It decreased to 45 ( 4° and to less than 10° with UV irradiation and plasma treatment, respectively. The different pathways for the two treatments are discussed. In addition, the kinetics of photooxidation of the polystyrene surface was measured in situ, yielding a half-life of 15 min, which is much shorter than that of the bulk.

Introduction Modification of the chemical composition and structure of a polymer surface is important to many materials applications, ranging from adhesion, friction, and wear to biomedical utilization.1 In these applications, special surface properties, such as bondability, lubricity, or hydrophilicity, are required. Although polymers possess excellent bulk physical and chemical properties, they often do not have the surface properties needed for these applications. As a result, enormous efforts have been directed toward understanding the modification mechanism to achieve better control of the modification processes.2 In line with these efforts, we have used IR-visible SFG spectroscopy to study surface modifications of polystyrene (PS) by UV irradiation and plasma treatment. By taking advantage of the surface selectivity, submonolayer sensitivity, and molecular specificity of this technique, we have characterized the polymer surface at a molecular level and monitored the kinetics of the UV irradiation process in situ. Information of this type will shed light on the mechanism of surface modifications of polystyrene and provide an in-depth understanding of how photons and plasma interact with a polymer surface. Polystyrene has been the subject of extensive studies. Its surface chemistry has received much attention, with a growing interest in surface modification and stabilization.3-9 Various techniques have been employed to in* To whom correspondence should be addressed. E-mail: [email protected]. (1) Chan, C. M. Polymer Surface Modification and Characterization; Hanser Publishers: New York, 1993. (2) Garbassi, F.; Morra, M.; Occhiello, E. Polymer Surfaces: From Physics to Technology; John Wiley & Sons: New York, 1994. (3) Strom, G.; Fredriksson, M.; Klasson, T. J. Colloid Interface Sci. 1988, 123, 324. (4) Toshiyuki, T.; Hashida, I.; Inoue, N.; Mizuno, K. Polymer 1996, 37, 5525. (5) Otocka, E. P. J. Appl. Polym. Sci. 1983, 28, 3227.

vestigate changes in surface properties and the mechanism of surface modification processes. Contact angle measurements showed that the polystyrene surface becomes hydrophilic after UV radiation or plasma treatment.3,4 However, this method cannot provide information about changes in chemical composition of the treated surface at a molecular level. ATR-IRS (attenuated total reflectance)5 and Raman spectroscopy6 have been used to characterize polystyrene surfaces. With a sampling depth in the range of a micrometer, these analytical techniques are not suitable for probing modified layers at the polymer surface, which are typically in the range of tens of nanometers. Surface-sensitive techniques, such as X-ray photoelectron spectroscopy (XPS)7 and secondary ion mass spectrometry (SIMS),8 have provided valuable information about the surface modification of polystyrene. It has been generally accepted that UV photons and plasma interact with polystyrene, leading to carbon-carbon scissions and generating a variety of oxygen-containing functionalities, such as C-O, CdO, or COOH, at the polymer surface. However, an overlap between the carbonate carbon, a photoproduct, and the shake-up satellites from the aromatic structure of PS complicates the interpretation of the XPS data on the changes of the aromatic ring, making it inconclusive.7-9 Published papers do not give consistent results for determining whether the aromatic ring is the reaction center in the modification process and what kinds of photoproducts form at the modified polymer surfaces.10-13 The mechanism of the surface modifications (6) Parry, D. B.; Dendramis, A. L. Appl. Spectrosc. 1986, 40, 656. (7) Onyiriuka, E. C. J. Appl. Polym. Sci. 1993, 47, 2187. (8) France, R. M.; O’Toole, L.; Short, R. D. Macromol. Chem. Phys. 1995, 196, 3695. (9) Foerch, R. N.; Mcintyre, S.; Hunter, D. H. J. Polym. Sci. 1990, A28, 803. (10) Wells, R. K.; Badyal, J. P. S.; Drummond, I. W.; Robinson, K. S.; Street, F. J. Polymer 1993, 34, 3611.

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remains unclear. In addition, very little information on the kinetics of these processes is available. In the past several years, IR-visible sum frequency generation has been developed into a powerful surface tool and successfully applied to study a wide variety of surfaces and interfaces,14-16 including polymer surfaces.17 As a second-order nonlinear optical process, SFG is intrinsically interface-specific: it is electric-dipole-forbidden in centrosymmetric bulk but necessarily allowed at an interface where the inversion symmetry is broken.18,19 Thus, it can selectively probe molecules at an interface between centrosymmetric media where the bulk contribution to the SF signal vanishes. The resonant enhancement of the SF signal occurs when the input IR frequency hits a vibrational mode that is both IR- and Raman-active. This feature allows one to obtain surface vibrational spectra by measuring the SF intensity as a function of IR frequency. The SFG vibrational spectra can subsequently be used to identify molecular chemical species in the interfacial region. This spectroscopic technique is particularly attractive for studying polymer surface modifications where surface selectivity and sensitivity are crucial. In this report, we present results on the chemistry and kinetics of surface modifications of polystyrene using IRvisible SFG spectroscopy. We characterized a polystyrene surface modified by exposure to a UV light source and plasma treatment and monitored the photoirradiation process at the polymer surface in situ. The SFG surface spectrum of the initial spin-coated polystyrene thin film displays an intense peak at 3068 cm-1, which is characteristic of the symmetric stretch of the aromatic C-H of polystyrene.20 After treatment, this peak decreases greatly, and other peaks, associated with the CH2 and CH3 groups, increase at the modified surface. These spectral changes, which are similar for UV irradiation and plasma treatment, clearly indicate that surface reactions involving the aromatic structures occur, resulting in a loss of aromaticity. The occurrence of such surface reactions was controversial in previous studies.7-11 From XPS measurements, the oxidation of the polystyrene surface is evident, as seen in the bands associated with oxygen and oxygencontaining species. Although, oxidation of polystyrene occurs in both processes, its degree is different in UV irradiation and plasma treatments. In the SFG spectra, oxidation of polystyrene to a higher oxidation state, forming carbonyl/carboxyl species, is observed for plasma treatment, but not for UV radiation. These findings are further confirmed by water contact angle measurements. Before the treatments, the contact angle of polystyrene is 95 ( 4°; it decreases to 45 ( 4° and to less than 10° for UV irradiation and plasma treatment, respectively. The SFG, XPS, and contact angle data suggest different pathways for the two treatments. We propose that the (11) Otocka, E. P.; Curran, S.; Porter, R. S. J. Appl. Polym. Sci. 1983, 28, 3227. (12) Duffy; D. C.; Davies, P. B. J. Phys. Chem. 1995, 99, 15241. (13) Grant, J. L.; Dunn, D. S.; McCiure, D. J. J. Vac. Sci. Technol. 1988, A6, 2213. (14) Du, Q.; Superfin, R.; Freysz, E.; Shen, Y. R. Phys. Rev. Lett. 1993, 70, 2313. (15) Johal, M. S.; Ward, R. N.; Davies, P. B. J. Phys. Chem. 1996, 100, 274. (16) Conboy, J. C.; Messmer, M. C.; Richmond, G. L. J. Phys. Chem. 1996, 100, 7617. (17) Zhang, D.; Shen, Y. R.; Somorjai, G. A. Chem. Phys. Lett. 1997, 281, 394. (18) Shen, Y. R. The Principles of Nonlinear Optics; John Wiley: New York, 1984; Chapter 25. (19) Shen, Y. R. Nature 1989, 337, 519. (20) Varsanyi, G. Vibrational Spectra of Benzene Derivatives; Academic Press: New York, 1969.

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Figure 1. SFG surface vibrational spectra of polystyrene, with ppp-, ssp-, and sps-polarization combinations (for SF output, visible input, and IR input) . The spectra were offset for clarity of presentation.

uptake of oxygen by the phenyl group, resulting in a phenol-like species, is the main pathway for UV irradiation, while an oxidative attack of the benzene ring, followed by a ring opening to form aldehyde/carboxylate species, is the main mechanism for plasma treatment. Last, we monitored the UV irradiation process in situ and found that the kinetics of the photoreaction at the polystyrene surface (τ1/2 ) 15 min) is an order of magnitude faster than that reported in the bulk.11 This difference is attributed to UV light attenuation and limited oxygen diffusion in the polystyrene bulk. Experimental Section The experimental setup of our IR-visible SFG has been described in detail elsewhere.21,22 Briefly, the experiment was performed by overlapping a visible (532 nm, ∼7 ns duration pulse) and a tunable infrared (1300-3800 cm-1, 7 ns duration pulse) beam at the polymer surface. The visible beam was provided by a frequency-doubled Nd:YAG laser. The tunable IR radiation was generated by stimulated Raman scattering of a tunable dye laser beam in a hydrogen Raman cell.22 The reflected SF emission was detected by a photomultiplier tube. The XPS was performed on polymer surfaces using a Perkin-Elmer, PHI model 5600 series spectrometer, equipped with a Mg/Al dual-anode source. The Mg KR X-rays (253.6 eV) were used and operated at 300 W. The base pressure was lower than 5 × 10-9 Torr with an operating pressure of ∼10-8 Torr. Pass energies of 100 and 27 eV were used for acquisitions of the survey and the high-resolution spectra, respectively. The polystyrene film (Mw ) 200 000, ∼0.15 µm) was prepared by spin-coating the material from a toluene solution of 4 wt % onto silica supports at 2000 rpm. The sample was then heated to 110 °C in a vacuum oven for 4 h. The UV light source was a Hanovia medium-pressure mercury lamp. The plasma treatment was performed in a rf inductive plasma reactor.

Results and Discussion Figure 1 displays the SFG vibrational spectra of polystyrene in the C-H stretching region with three polarization combinations. In the ppp-polarization (the SFG output, visible input, and IR input were detected and set in a p-polarization configuration) spectrum, a strong peak is observed at 3068 cm-1 and assigned to the symmetric C-H stretching vibration of the phenyl ring of polystyrene. This band correlates well with the same (21) Hatch, S. R.; Polizzotti, R. S.; Dougal, S.; Rabinowitz, P. J. Vac. Sci. Technol. A 1993, 11, 2232. (22) Yeganeh, M. S.; Dougal, S. M.; Polizzotti, R. S.; Rabinowitz, P. Thin Solid Films 1995, 270, 226.

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Figure 2. Normal modes of vibration for aromatic rings.

Figure 3. SFG surface vibrational spectra of polystyrene, with ppp-polarization combination before and after the irradiation.

mode, υ2 (Figure 2), reported for the aromatic ring.20,23 In addition, we observed two other vibrational bands associated with the ring. They are centered at 3053 and 3027 cm-1 and assigned υ7a and υ7b, respectively, as described in Figure 2. To obtain a more complete physical picture of the polystyrene surface, we also measured two additional independent susceptibility components, χ(2) xxz and χ(2) xzx, with ssp (the SFG output, visible input, and IR input were detected and set in a s-, s-, and p-polarization configurations, respectively) and sps-polarization combinations, respectively. χ(2) xxz represents the ability of the air/polymer interface to generate SFG photons with their E-field perpendicular to the plane of incidence, when the E-field of the visible and IR laser beam is parallel and perpendicular to plane of incidence, respectively. A similar description for χ(2) xzx may be given. The ssp spectrum yields all three vibrational components associated with the ring, as observed in the ppp spectrum. The feature at 3027 cm-1 is a dip rather than a peak, indicating that this band has an opposite phase to those of υ2 at 3068 cm-1 and υ7a at 3053 cm-1. This finding is consistent with the calculation reported by Duffy et al., showing that υ2 and υ7b have an 12 which further opposite phase in the χ(2) xxz component, supports our assignment. In the sps spectrum, the peak at 3027 cm-1 becomes more intense. In all of these SFG spectra, the peaks due to the CH2 group are relatively weak. The low signal of CH2 in PS is probably due to orientation factors, as was found in the case of polypropylene.17 Figure 3 compiles SFG spectra of polystyrene before and after 40 min of UV irradiation. Dramatic changes were observed in these spectra, as manifested by the significant decrease in the υ2 band at 3068 cm-1. The observed spectral changes clearly demonstrate surface reactions involving the aromatic ring during photoirradiation. Previous studies showed that exposure to UV radiation led to photooxidation of polystyrene by carboncarbon bond scission.7,10,11 Peroxy and its degraded oxygen(23) Fuson, N.; Garrigou-Lagrange, C.; Josien, M. L. Spectrochim. Acta 1960, 16, 106.

Figure 4. XPS survey spectra for polystyrene (top), plasmatreated polystyrene (middle), and UV-irradiated polystyrene (bottom).

containing species appeared to be the major products of the photoreactions. Although XPS provided valuable information regarding the effect of UV irradiation on a polystyrene surface, the interpretation of the XPS data was complicated by an overlap of the core level carbon of carbonate, a photoproduct, and the shake-up satellites due to the aromatic structures of polystyrene. The loss of aromaticity remains a debated issue. Our XPS measurements suffered similar drawbacks. As shown in Figures 4 and 5, although the oxidation of the polystyrene surface was clearly demonstrated by the emergence of bands associated with oxygen and oxygen-containing species, changes in shake-up bands are not evident. Thus, it is not clear whether the phenyl rings engage in the surface reactions. However, the SFG spectra have shown a dramatic decrease in the bands associated with the aromatic group. This observation offers direct support to the hypothesis that the reactions, caused by the UV irradiation, involve the ring structure. Two possibilities are considered for the decrease in the 3068 cm-1 feature: (1) The phenyl centers undergo fragmentation during the treatment. The disappearance of the phenyl ring results in the decrease of the C-H vibration at 3068 cm-1 band. (2) There is an uptake of oxygen by the ring, forming phenol-like species at the polymer surface. The attachment of functional groups at the para position of the ring increases the symmetry of the chromophore, thus decreasing the intensity of the aromatic C-H vibration in the SFG spectrum. We will return to this point after we present more data regarding photoproducts that resulted from the modification processes. Accompanying the decrease of the aromatic C-H band is an increase in the bands associated with the CH2 (∼2854 cm-1) and CH3 (∼2875 cm-1) groups after photoirradiation. The broadness of these features suggests a variety of C-H environments. The oxidation of the polystyrene is evidenced by the XPS spectrum in which new bands associ-

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Figure 7. SFG surface vibrational spectra polystyrene, with ppp-polarization combination before and after the plasma treatment.

Figure 5. XPS high-resolution spectra for polystyrene, plasmatreated polystyrene, and UV-irradiated polystyrene.

Figure 6. Square root of the SF intensity at 3068 cm-1 changes with the irradiation time. The data were fitted to an exponential decay curve with a half-life of 15 min.

ated with oxygen and oxygen-containing species emerge at the modified polymer surface (Figures 4 and 5). The photooxidation of the polymer surface also results in changes in the surface properties of polystyrene, as measured by contact angle goniometry. The contact angle of the untreated film is 96 ( 4°, and it decreases to 45 ( 4° after photoirradiation. These changes reflect the transformation of the polystyrene surface from a hydrophobic surface to a hydrophilic one, caused by the polar species generated in the UV irradiation process. To understand the kinetic effect of UV irradiation on the polystyrene surface, we monitored the photooxidation process by measuring the decrease in the 3068 cm-1 band in real time (Figure 6). We found that the square root of the intensity of the 3068 cm-1 band, which relates to surface density of the aromatic ring, decays exponentially with a half-life of 15 min. Earlier FTIR studies indicated that photooxidation of polystyrene bulk occurred over a time scale of hours.11 There was an induction period during which hydroperoxide species were formed, which subse-

quently reacted to give the other oxygenic functional groups. From the SFG experiments, we found the kinetics of photooxidation at the polystyrene surface is much faster than that in the bulk. This is probably due to the short penetration depth of UV light and limited oxygen diffusion in the bulk. Our data also show that there is no induction period for the ring oxidation within the experimental limits and suggest that the photons attack the phenyl centers to initiate the surface reactions. In addition to the surface modification by UV irradiation, the effect of plasma on the polystyrene surface has been investigated. Figure 7 compares the SFG spectra of polystyrene before and after plasma treatment for 1 s. Dramatic spectral changes are detected. They are similar to those observed in the photoirradiation process, exhibiting a decrease in the υ2 band at 3068 cm-1 and an increase in the bands associated with the CH2 and CH3 groups. It is known that plasma consists of ions, electrons, atoms, radicals, and molecules. Interaction of plasma with a hydrocarbon polymer mainly involved the active sites of the polymer being attacked by oxygen atoms, which have the greatest reactivity toward the polymer. The reactions generate new chemical functionalities, mainly oxygencontaining species, as shown in the XPS spectra. In addition to the aforementioned surface reactions, the oxygen plasma is also known for its etching effect with prolonged treatment (>10 min), where atomic oxygen reacts with the surface carbon atoms to give volatile reaction products.1 However, in our case this effect is negligible because of the short plasma treatment time of 1 s. This was further confirmed by a thickness measurement, where no significant change in the polymer thickness was observed after treatment. To determine the species formed at the polystyrene surface, SFG spectra of polystyrene in the CdO spectral region were also examined before and after UV irradiation and plasma treatment. We found that the oxygencontaining species formed in these two processes are different. As displayed in Figure 8, the surface vibrational spectra of polystyrene before and after UV irradiation in the carbonyl region are essentially the same, and no vibrational band is observed. This shows that there are no significant carbonyl groups at the polystyrene surface before or after the UV irradiation. However, this is not true for the plasma-treated surface. The SFG spectrum of plasma-treated polystyrene exhibits a pronounced band at 1675 cm-1, which is characteristic of a carbonyl group, indicating the generation of the carbonyl moiety at the polystyrene surface. This finding reveals that the degrees of oxidization of polystyrene by UV irradiation and plasma

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esses. To account for the surface reactions involving the aromatic ring, we have mainly considered the following two possibilities: ring opening, followed by the formation of carbonyl/carboxyl species, and uptake of oxygen by the aromatic ring, forming phenol-like species. On the basis of our data, we propose that the former is the main pathway in the plasma treatment and the latter is the main mechanism in the UV irradiation process.

Figure 8. SFG surface vibrational spectra polystyrene before and after UV radiation (a) and plasma treatment (b) in the CdO spectral region.

are different. The oxidation of polystyrene to a higher oxidation state, forming carbonyl/carboxyl species, occurs with plasma treatment, but not with UV irradiation. This difference can also be seen from the XPS data, which exhibit stronger intensities in bands associated with oxygen and oxygen-containing species for the plasmatreated polystyrene surface. In addition, a higher degree of oxidation of polystyrene by plasma is further confirmed by contact angle measurements. We have found that the contact angle decreases from 95 ( 4° for the untreated film to less than 10° for the plasma-treated film and to 45° for UV-irradiated film. Combining the SFG, XPS, and contact angle data shows that, with UV irradiation and plasma treatment, polystyrene undergoes surface oxidation reactions involving the phenyl ring. However, the difference in the degree of oxidation suggests that the reaction pathways could be different in these two proc-

Conclusions We have investigated the effects of UV light and plasma treatment on a polystyrene surface using SFG spectroscopy. The observed spectral changes provide direct support to the surface reactions involving aromatic group, manifested by the dramatic decrease of the vibrations associated with the ring structure. More interestingly, we also found that the level of oxidation is different between UV irradiation and plasma treatment. The oxidation of polystyrene to a higher oxidation state, forming carbonyl/ carboxyl species, was observed for plasma treatment but not for UV radiation. This difference was also shown by XPS data and contact angle measurements. The contact angle of polystyrene decreased from 95° for the untreated film to 45° for the UV irradiated film and to less than 10° for the plasma-treated film. In the XPS spectra, the intensities of bands associated with oxygen and oxygencontaining species were stronger for the plasma-treated polystyrene surface as compared to the UV-irradiated PS surface, indicating a higher level of oxidation. These findings suggest that the two modification processes may take different pathways. We propose that ring opening, which results in the formation of aldehyde/carboxylate species, is the main pathway for plasma treatment. Oxygen uptake, forming species of lower oxidation states, is the main mechanism in UV irradiation. Furthermore, we also found that the photooxidation process at the polystyrene surface is much faster than a similar process in the bulk. This increased speed of reaction is attributed to UV light attenuation and limited oxygen diffusion in the bulk. Acknowledgment. The authors acknowledge M. Bernardo for the help in the SFG data acquisition and S. Cameron for helpful discussions of XPS data. LA991353I