RAIRS and TPD Study of the Direct Photopolymerization of Styrene

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Langmuir 1997, 13, 2307-2313

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RAIRS and TPD Study of the Direct Photopolymerization of Styrene Thin Films in Ultrahigh Vacuum S. R. Carlo and V. H. Grassian* Department of Chemistry, University of Iowa, Iowa City, Iowa 52242 Received October 8, 1996X Photoinduced polymerization of styrene monomer thin films condensed on a Ag(110) substrate has been studied using a combination of reflectance absorption infrared spectroscopy (RAIRS) and temperatureprogrammed desorption (TPD). At 100 K, styrene polymerizes upon ultraviolet irradiation. Postirradiation TPD indicates that several species form in the film. These species are identified as styrene dimer, trimer, and tetramer. RAIR spectra as a function of temperature provide evidence for the formation of polystyrene as well. A mechanism for the direct UV photopolymerization of styrene thin films is proposed.

Introduction Recently, there have been reports in the literature on the in-situ formation of polymer films on metal surfaces. Formation of thin polymer films on a metal substrate can be achieved by adsorbing a preformed polymer1,2 or by forming the polymer in situ.3-11 The motivation for these studies is the desire to form a polymer with a regular two-dimensional morphology that may have novel anisotropic properties. In-situ polymerization has been initiated by photon, electron, and γ-ray irradiation. Radiationinduced chemistry allows for select areas of the substrate to be polymerized, opening the way for the production of positive and negative resists. In addition to the interest in surface modification of metal substrates, in-situ methods for forming polymers in an ultrahigh vacuum allow for further study of the chemistry of the polymer surface and the metal-polymer interface using standard surface analysis techniques. There have been several studies on photoinduced polymerization of adsorbed monomers on metal surfaces. Laser-initiated (514 nm) polymerization of 1,4-dinitrobenzene on Ag colloids was studied by surface-enhanced Raman scattering (SERS).4 Thick films of 1,4- and 1,3dinitrobenzene were deposited onto Ag colloids from solution. SERS showed that 1,4-dinitrobenzene polymerized readily on the Ag surface upon CW Ar ion irradiation whereas 1,3-dinitrobenzene did not. Differences in molecular orientation of 1,4-dinitrobenzene compared to 1,3dinitrobenzene were determined to be the cause of the difference in the photoreactivity of these two isomers on the metal substrate. Ford et al. used a self-assembled monolayer approach to form ordered polymer films by adsorbing 4-(mercap* Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, March 15, 1997. (1) Lenk, T. J.; Hallmark, V. M.; Rabolt, J. F.; Haussling, L.; Ringsdorf, H. Macromolecules 1993, 26, 1230. (2) Sun, F.; Castner, D. G.; Grainger, D. W. Langmuir 1993, 9, 3200. (3) Balasubramanian, K. K.; Cammarata, V. Langmuir 1996, 12, 2035. (4) Tsai, W. H.; Boerio, F. J.; Clarson, S. J.; Montaudo, G. J. Raman Specros. 1990, 21, 311. (5) Ford, J. F.; Vickers, T. J.; Mann, C. K.; Schlenoff, J. B. Langmuir 1996, 12, 1944. (6) Batchelder, D. N.; Evans, S. D.; Freeman, T. L.; Haussling, L.; Ringsdorf, H.; Wolf, H. J. Am. Chem. Soc. 1994, 116, 1050. (7) Kwok, C. C.; Kim, T.; Schoer, K. J.; Crooks, R. M. J. Am. Chem. Soc. 1995, 117, 5875. (8) Fleck, L. E.; Feehery, W. F.; Plummer, E. W.; Ying, Z. C.; Dai, H. L. J. Phys. Chem. 1991, 95, 8428. (9) Fleck, L. E.; Ying, Z. C.; Dai, H. L. J. Vac. Sci. Technol., A 1993, 11, 1942. (10) Land, T. A.; Hemminger, J. C. Surf. Sci. 1992, 268, 179. (11) Wells, S. K.; Giergiel, J.; Land, T. A.; Lindquist, J. M.; Hemminger, J. C. Surf. Sci. 1991, 257, 129.

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tomethyl)styrene on a roughened silver surface.5 The orientation of the benzene ring plane in the styrene moiety of adsorbed 4-(mercaptomethyl)styrene was deduced from the intensities of the SERS bands to be at a slight angle from the surface normal. The 514 nm output from a CW Ar ion laser was used to both initiate and probe the polymerization reaction through SERS. The resulting polymer was relatively insoluble compared to the monomer, leading to the possibility of using this system to form a negative resist. Self-assembled monolayers containing a diacetylene unit adsorbed on gold surfaces6,7 have also been used to form such resists upon UV irradiation. There have been a few polymerization studies carried out on single-crystal metal surfaces in ultrahigh vacuum. Dai and co-workers have studied the formation of a formaldehyde polymer on Ag(111) upon ultraviolet irradiation of submonolayer coverages of formaldehyde.8,9 The polymer was identified by electron energy loss spectroscopy (EELS) as polyoxymethylene. Through careful investigation, it was shown that a radical chain initiator is formed upon UV irradiation of adsorbed formaldehyde.9 Diffusion of the initiator, and therefore the degree of polymerization, could be controlled by the temperature of the substrate. Hemminger and co-workers used single-crystal surfaces in an attempt to form ordered polymer films by electron and ion beam irradiation of multilayer organic films.10,11 Electron beam irradiation of a thiophene thin film adsorbed on Pt(111) yielded a polymer which was stable upon heating to 600 K compared with 400 K for a nonirradiated thiophene film.10 Both electron and ion beam irradiation of tetracyanoquinodimethane multilayers adsorbed on Ni(111) formed a polymer layer.11 The polymer layer was impermeable to oxygen and therefore inhibited the oxidation of the Ni surface. Baumgartner et al. have studied the adsorption and subsequent polymerization of thick and thin multilayer films of thiophene adsorbed on Ag(111).12 After exposure of multilayer films to Mg KR radiation, the photoelectron spectrum showed a shift in the C 1s and S 2p peaks, indicating a change in the chemical environment for carbon and sulfur. In postirradiation temperature-programmed desorption (TPD), oligomers of up to four units were identified. There was also some indication of the formation of larger oligomers. In this paper, we describe studies of UV irradiation of thin films of styrene monomers condensed on a Ag(110) substrate to form a polymerized styrene film. The (12) Baumgartner, K. M.; Volmer-Uebing, M.; Taborskir, J.; Bauerle, P.; Umbach, E. Ber. Bunsen-ges. Phys. Chem. 1991, 11, 1488.

© 1997 American Chemical Society

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Carlo and Grassian

polymerization reaction was monitored by reflectance absorbance infrared spectroscopy (RAIRS) and temperature-programmed desorption. This study represents a first step in our investigation of thin polymer film formation and characterization on metal substrates. Future studies will be directed toward investigating the surface chemistry of thin polymer films. Experimental Section Experiments were carried out in an ultrahigh-vacuum (UHV) chamber with a base pressure of 5 × 10-10 Torr. The UHV chamber is equipped with a cylindrical mirror analyzer for Auger electron spectroscopy, a quadrupole mass spectrometer for TPD and residual gas analysis, an ion sputtering gun for sample cleaning, and three variable leak valves for introducing gases into the chamber. A Ag(110) substrate was mounted in the UHV chamber on a tantalum cup attached to a liquid nitrogen-cooled Cu block. The temperature of the Ag(110) substrate was measured by a chromel-alumel thermocouple. A Mattson 6021 Galaxy FT-IR spectrometer equipped with external beam capabilities and a narrow band mercurycadmium-telluride (MCT) detector was used for reflectance infrared measurements. Transmission spectra of several IR standards were collected using the internal compartment and the internal optics of the spectrometer. The lower limit of the spectral range of the instrument was limited to ∼750 cm-1 by the MCT detector. Each absorbance spectrum is acquired by summing 1000 sample scans at an instrument resolution of 4 cm-1 and referencing to background scans of the clean Ag(110) substrate acquired under the same conditions. For TPD experiments, the Ag(110) substrate was resistively heated at a rate of 1.5 K/s. A UTI-100C quadrupole mass spectrometer (QMS) with a mass range from m/e ) 1 to 300 was interfaced to a 486 PC through an AD/DA board (National Instruments AT-MIO-16H-9). TPD was used to determine the exposure at which monolayer saturation of styrene was achieved. The monolayer peak has a desorption rate maximum near 370 K in TPD. It was found that at an exposure of 2 langmuir, the monolayer peak saturated and a desorption peak at lower temperatures near 188 K began to grow in. The 188 K peak did not saturate as a function of styrene exposure and is assigned to multilayer styrene desorption. The 2 langmuir exposure that produced a saturated surface, is defined as one monolayer (1 ML); thus, the 200 langmuir exposure used in most of these experiments corresponds to 100 ML. Styrene (99+%, inhibited with 4-tert-butylcatechol) was obtained from Aldrich Chemical Co. The inhibitor was removed from the styrene liquid using an inhibitor removal column purchased from Aldrich. The liquid was freeze-pumped-thawed several times prior to use. Fresh styrene samples were prepared in this manner each day. A liquid polystyrene standard sample of nominal molecular weight of 500 g/mol was obtained from Goodfellow Corp. It predominantly consisted of pentamers with smaller amounts of lower order oligomers. Broad band irradiation was accomplished with a 500 W Hg arc lamp (Oriel Corporation). A water filter was used to remove the infrared radiation. In all of the experiments, the light was incident at 30° with respect to the crystal normal. The measured power at the sample was approximately 400 mW/cm2.

Results and Discussion RAIRS and TPD were used to monitor changes upon UV irradiation of styrene thin films deposited on a Ag(110) substrate. Infrared and mass spectral standards were employed to aid in the characterization of the photoproducts formed upon irradiation of the styrene monomer thin film. Infrared spectra of liquid styrene, polystyrene liquid, and a polystyrene IR standard film were obtained in order to identify characteristic vibrational bands for these species. Mass spectral fragmentation patterns from the literature were used to characterize the desorption products in postirradiation TPD. I. Infrared Spectra of Styrene Monomer, Polystyrene Liquid, and Polystyrene Film. IR data were

Figure 1. Transmission IR spectra of standard samples: styrene monomer liquid (bottom); polystyrene liquid with a nominal molecular weight of 500 g/mol (middle); and polystyrene IR standard film (top).

collected in transmission mode for styrene liquid, polystyrene liquid (molecular weight of approximately 500 g/mol), and a polystyrene IR standard film. The spectra for these species are shown in Figure 1. There are marked differences between the styrene liquid sample and the two polystyrene samples. The four major differences between the styrene liquid sample and the two polystyrene samples occur in the following spectral regions: (i) the CsH stretching region, between 2800 and 3200 cm-1; (ii) the CdC stretching region near 1631 cm-1; (iii) the 14501500 cm-1 region; and (iv) the 750-1050 cm-1 region. These differences are discussed in more detail below. For styrene monomer, there are three distinct bands in the CsH stretching region of nearly equal intensity with frequencies above 3000 cm-1. The frequencies of these three bands are very similar for the two polystyrene samples; however, the intensity pattern of the three bands above 3000 cm-1 changes with the lowest frequency band being of greatest intensity and the highest frequency band being of lowest intensity. In addition, for the polystyrene samples, there are two intense bands below 3000 cm-1 that are not present in the monomer spectrum. These bands are associated with the vibrational modes of aliphatic CsH bonds that are present in the polymer. Another diagnostic band in the styrene spectrum is at 1631 cm-1, which is assigned to the CdC stretch of the vinyl group in styrene.13 In the polymer sample, this band is obviously not present. Additional differences in the intensity patterns of some of the bands can also be used to distinguish styrene monomer from polystyrene. For example, the relative intensities of the pair of bands at 1496 and 1451 cm-1 which correspond to in-plane vibrational modes are quite different for styrene and polystyrene. For styrene, the band at 1451 cm-1 is associated with an in-plane vibration of the aromatic ring only whereas the band at 1496 cm-1 (13) Marchand, A.; Quintard, J.-P. Spectrochim. Acta 1980, 36A, 941.

Photopolymerization of Styrene Thin Films in UHV

is associated with an in-plane vibration of the aromatic ring and the vinyl substituent.13 For styrene liquid, the band at 1496 cm-1 is of greater intensity than the band at 1451 cm-1 whereas, for polystyrene, these bands are of nearly equal intensity. The spectral region between 750 and 1050 cm-1 also shows differences in the spectrum of styrene liquid compared to that for the polystyrene samples. For styrene liquid, there is a fairly intense band in the spectrum at 998 cm-1 which is not present in the spectra of the polystyrene samples. This band has been assigned to the out-of-plane bend of the CsH bond of the R-carbon of the vinyl group in styrene. The band at 908 cm-1, associated with an out-of-plane ring breathing mode,13 is present in both the styrene and polystyrene spectra but is of weaker intensity relative to the other bands in the spectrum in the two polystyrene spectra. The band at 784 cm-1 in the styrene spectrum, associated with the out-of-plane aromatic CsH bending mode in styrene is shifted to lower frequencies,13 near the narrow band MCT detector cutoff for the polystyrene film and just below the detector cutoff for the polystyrene liquid. These spectral differences between styrene monomer and polystyrene are used in the interpretation of the infrared spectra recorded of styrene thin films condensed on a Ag(110) substrate as a function of UV irradiation. II. RAIRS of a Styrene Thin Film Condensed on Ag(110). The characteristic features of styrene and polystyrene discussed in Section I include differences in the frequencies of the infrared absorption bands and in the band intensities. It is important to note that the intensities of the bands can change if there is preferential orientation of the molecules in the styrene film when adsorbed on a metal substrate. According to the wellknown metal surface selection rule, only modes with a perpendicular dipole component will be observed in the RAIR spectrum. The RAIR spectrum of a thin film of styrene, estimated to be 100 ML, deposited on a Ag(110) substrate is shown in Figure 2 (bottom spectrum). The frequencies of the infrared absorption bands in the transmission spectrum of liquid styrene and the RAIR spectrum of the thin styrene film are almost identical with only a few bands shifted by up to 10 cm-1; however, there are differences in the relative intensities of some of the modes. Although it may be impossible to unambiguously determine the orientation of the molecules in the thin film relative to the Ag(110) substrate, as there are probably several different orientations in the film, it may be possible to make some comments concerning the average orientation on the basis of the differences between the transmission IR spectrum of styrene liquid, which is not preferentially oriented, and the RAIR spectrum of a styrene thin film. There are two sets of bands that have a different intensity pattern in the transmission spectrum of styrene liquid and the reflectance absorption spectrum of an adsorbed styrene film. One set of bands is at 1029 and 990 cm-1. These two bands at 1029 and 990 cm-1 are assigned to in-plane and out-of-plane bending modes,13 respectively, that are of different symmetries. The inplane bending mode is of A′′ symmetry, and the out-ofplane bending mode is of A′ symmetry. In the transmission spectrum of styrene liquid, the 1029 cm-1 band is approximately one-fourth the intensity of the band at 998 cm-1. In the RAIR spectrum of an adsorbed styrene film, the 1029 cm-1 band is of slightly greater intensity relative to the 998 cm-1 band. A ring breathing mode is clearly seen in the spectrum of condensed styrene at 1601 cm-1 which would have little intensity if the styrene molecules were adsorbed flat with respect to the metal substrate. In

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Figure 2. RAIR spectra of styrene condensed on Ag(110) at 100 K after 0, 5, 15, 30, 60, 90, and 120 min of broad band UV irradiation (λ > 237 nm).

general, an increase in the intensity of in-plane modes relative to out-of-plane modes in benzene and benzene derivatives adsorbed on a metal surface is indicative of an orientation of the ring plane tilted away from the surface plane toward the surface normal.14 For a thin film of benzene derivatives condensed on a metal substrate, the enhancement of in-plane modes relative to out-of-plane modes indicates formation of a film with molecules whose ring planes are tilted toward the surface normal and not parallel to the surface. In the ground state, styrene is a planar molecule with Cs symmetry. If styrene had an orientation with the ring plane parallel to the surface plane, then only modes of A′′ symmetry would be observed in the RAIR spectrum. If styrene was oriented with the ring plane parallel to the surface normal, then only modes of A′ symmetry would be observed. Since both A′ and A′′ modes are observed in the styrene thin film RAIR spectrum, it is clear that the ring plane of the styrene molecules is tilted at some angle with respect to the surface normal. Comparing the relative integrated areas of the two bands at 1029 and 998 cm-1 in the RAIR spectrum to the transmission IR spectrum of the liquid, a tilt angle can be calculated. This angle is determined to be 26°; that is, the styrene molecules are oriented nearly perpendicular to the surface normal. This observation is consistent with that of Kubono,15 who observed that adsorbate multilayers on a cold substrate were predominantly in the normal orientation (perpendicular to the surface plane). The other set of bands that shows different intensity patterns in the RAIR spectra of the thin film and the liquid styrene are the set at 1496 and 1451 cm-1. There is an enhancement of the mode at 1496 cm-1 relative to the (14) Demuth, J. E.; Christmann, K.; Sanda, P. N. Chem. Phys. Lett. 1980, 76, 201.

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mode at 1451 cm-1 in the RAIR spectrum, which may provide further information about the orientation of the styrene molecules in the film relative to the surface. Both of these bands are due to in-plane vibrational modes that have a significant component of in-plane motion of the aromatic ring.13 As noted previously, the band at 1496 cm-1 has an additional component associated with the in-plane CsH bending motion of the vinylic CsH bonds.13 The enhancement of intensity for the 1496 cm-1 band suggests that the vinyl CdC bond axis may be directed toward the surface normal. III. RAIRS of a Styrene Thin Film Condensed as a Function of UV Irradiation. Styrene has two absorption bands in the near UV. The longer wavelength band, designated as the 1Lb band, is of weak intensity and has an absorption maximum near 280 nm.16 The shorter wavelength band, designated as the 1La band, has an absorption maximum near 240 nm.16 Therefore, direct excitation of the styrene molecules in the film can be accomplished with broad band Hg arc irradiation. The RAIR spectra of a 100 ML condensed styrene film as a function of broad band UV irradiation (λ > 237 nm) are shown in Figure 2. It can be seen in Figure 2 that as the sample is irradiated, the IR spectrum begins to change. Specifically, (i) there is a change in the relative intensities of the bands above 3000 cm-1 in the CsH stretching region and the growth of new bands below 3000 cm-1, associated with the stretching motion of aliphatic CsH bonds, (ii) the band due to the CdC stretching mode at 1631 cm-1 decreases in intensity, (iii) the relative intensities of the pair of bands at 1496 and 1451 cm-1 change and become nearly equal after 120 min of irradiation, and (iv) there is also a change in the intensities and frequencies of the bands in the 750-1050 cm-1 region. In the 750-1050 cm-1 region, there is a significant loss of intensity of the bands at 998 and 908 cm-1, and the band at 784 cm-1 decreases in intensity and shifts to lower frequencies toward the cutoff of the narrow band MCT detector. These changes are consistent with the photopolymerization of styrene. As the film polymerizes, there is a loss of intensity of the characteristic band due to the CdC stretching mode of the vinyl group in the styrene monomer and the growth of bands due to aliphatic CsH stretching modes that are characteristic of polystyrene. The decrease in intensity of the 1631 cm-1 band (υCdC) and the growth of the absorption band at 2932 cm-1 (υaliphatic C-H) are plotted as a function of irradiation time in Figure 3A. The plot in Figure 3B shows the change in the relative ratio of the pair of bands at 1496 and 1451 cm-1 as a function of irradiation time; again these changes in the RAIR spectra are characteristic of polymerization of the styrene film upon UV irradiation. The RAIRS data allow for the identification of the species adsorbed on the surface; however, as has been previously noted, there were no discernible differences between the transmission spectra of polystyrene film and polystyrene liquid. These two polystyrene samples differ of course in their degree of polymerization, average chain length, and molecular weight. Therefore, RAIRS cannot be used alone to determine the chain length of the formed polymer. As discussed below, TPD provides evidence for the presence of small oligomers in the film. IV. Postirradiation TPD of Styrene Thin Films. The standard mass spectrum of styrene contains major signals corresponding to m/e ) 27, 51, 78, and 104,17 which (15) Kubono. Prog. Polym. Sci. 1988, 19, 402. (16) Steer, R. P.; Swords, M. D.; Crosby, P. M.; Phillips, D.; Salisbury, K. Chem. Phys. Lett. 1976, 43, 461.

Carlo and Grassian

Figure 3. Changes in the integrated areas of some of the diagnostic bands in the RAIR spectra as a function of UV irradiation: (A) there is a decrease of the vinyl CdC stretch at 1631 cm-1 concomitant with a growth of the aliphatic CsH stretch at 2932 cm-1 upon irradiation; (B) the ratio of the two diagnostic peaks at 1496 and 1451 cm-1 decreases as a function of irradiation. These changes in the RAIR spectra are consistent with styrene polymerization.

are the major peaks observed in the residual gas analysis scan taken after backfilling the UHV chamber with styrene. Initially a 100 ML film was deposited on the Ag(110) substrate, and the film was then irradiated with a 500 W Hg arc lamp. TPD spectra of a styrene film were taken after 0, 5, 15, 30, 60, and 120 mins of irradiation. Representative TPD profiles for m/e ) 91, 128, and 208 as a function of irradiation time are shown in Figure 4. The TPD spectrum prior to irradiation shows only one major desorption peak in the m/e ) 91 channel at 188 K. The peak at 188 K is due to desorption of the styrene monomer multilayer. The m/e ) 91 channel is a small mass spectral fragment of styrene.17 There is also a small peak in the m/e ) 128 ion channel at short irradiation times attributed to reactions of styrene in the ionizer of the QMS.18 The parent ion, m/e ) 104, although not shown, was used to determine the loss of styrene. A plot of the integrated TPD area of the multilayer peak of the parent ion as a function of irradiation time is shown in Figure 5. Also shown in Figure 5 is the loss in the integrated area of the absorption band due to the CdC stretching mode in the RAIR spectrum of the styrene film as a function of irradiation (the same data points as plotted in Figure 3A). Both the IR and TPD data show that approximately 90% of the styrene monomer has reacted after 120 min of irradiation. Several new bands appear in the m/e ) 91, 128, and 208 ion channels as a function of irradiation. After 5 min of broad band irradiation, there is a small peak in the desorption curve near 245 K for these three ions. The intensity of this peak in all three ion channels increases after 15 min of irradiation and even more after 30 min. The peak also shifts to slightly higher temperatures near 252 K. There are two more desorption peaks that become apparent in these three ion channels after 15 and 30 min of irradiation with desorption maxima near 303 and 340 K. The 303 K peak is less intense than the 245 K peak and of greater intensity than the higher temperature peak at 340 K. Upon further irradiation for 60 min, the intensity of the desorption peak near 245 K diminishes and the two peaks at 303 and 340 K increase and become more distinct (17) Wiley/NBS Mass Spectral Catalogue Vol. 2. (18) There is a small peak in TPD near 188 K in the m/e ) 128 channel. This peak may be due to reactions in the ionizer of the QMS from the large flux of styrene molecules desorbing from the surface at these very high coverages.

Photopolymerization of Styrene Thin Films in UHV

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Figure 4. TPD spectra (m/e ) 91, 128, 208) as a function of irradiation: 0, 5, 15, 30, 60, and 120 min.

Figure 5. Conversion of styrene upon irradiation determined from the TPD and RAIRS data by integrating the areas of the peaks due to the styrene monomer. The m/e ) 104 channel was used to determine the loss of styrene from TPD, and the 1631 cm-1 band due to the vinylic CdC stretch in styrene was used to determine the loss of styrene from RAIRS. From these two techniques, it can be seen that approximately 90% of the styrene reacts after 120 min of UV irradiation.

relative to those of the TPD spectra taken after 15 and 30 min of irradiation. After 120 min of irradiation, the intensities of all three desorption peaks decrease and there is a shoulder near 225 K which is only apparent in the m/e ) 91 ion channel and the m/e ) 104 ion channel (not shown). Parent ion peaks for the trimer and tetramer could not be monitored, as they have masses above the upper limit of the UTI-100C QMS used in these studies. The m/e ) 208 channel, corresponding to the dimer mass, is the largest oligomer parent ion that can be monitored. Larger molecular species containing the dimer can fragment in the ionizer of the QMS and produce a TPD peak in the dimer channel. On the basis of the appearance of a signal for the dimer mass at m/e ) 208 for all three desorption peaks near 252, 303, and 340 K and the fact that these temperatures are very low compared to the decomposition temperature of polystyrene, the TPD data are interpreted in the following way. The three peaks with desorption

maxima near 252, 303, and 340 K are attributed to desorption of dimer, trimer, and tetramer from the film. The higher temperatures for desorption of the larger species correspond to the higher sublimation temperatures for these molecules. The dimer can exist as either diphenylcyclobutane or a linear dimer. An examination of the standard mass spectrum of diphenylcyclobutane17 shows that there is a very small parent ion peak at m/e ) 208 and very little mass fragmentation, especially into the m/e ) 91 and 128 channels. In contrast, the linear dimer has a fairly intense parent ion peak and many ion fragments, with the largest being seen at m/e ) 91 and 128.17 It is probable that both cyclic and linear styrene dimers are formed, with the majority of dimers formed being linear as higher order oligomers, namely trimer and tetramer, are also formed. The loss of intensity in the desorption peaks near 252, 303, and 340 K due to loss of the dimer, trimer, and tetramer, respectively, after more than 60 min of irradiation suggests that these species can undergo further photochemistry to form higher order oligomers and polystyrene. A small peak at 225 K in the m/e ) 91 and 104 channels which appeared after irradiation for 120 min and was not observed in any of the other channels analyzed may be due to styrene occluded in the polystyrene film possibly due to unreacted styrene or the UV photodegradation of polystyrene. V. RAIRS of an Irradiated Styrene Film as a Function of Temperature. Following 120 min of irradiation of the styrene film, the sample was successively heated from 205 to 700 K. A RAIR spectrum was taken following heating to the indicated temperatures given in Figure 6 and subsequent cooling to 100 K. Heating to 205 K removed some unreacted styrene and caused some loss of intensity in the bands at 998 and 908 cm-1 that is associated with the styrene molecule. Heating to 290 K removed the dimer. There was a slight sharpening of all the absorption bands and a complete loss of the bands at 998 and 908 cm-1 after heating to 290 K. Heating to 330 and 365 K removed trimer and tetramer species, respectively, causing a decrease in intensity of all the bands.

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Figure 6. Temperature-dependent RAIR spectra of a UVirradiated styrene thin film on a Ag(110) substrate (total irradiation time of 120 min). The sample was successively heated to 205, 290, 330, 365, 400, 500, 600, and 700 K. The sample was cooled to 100 K prior to recording each spectrum.

The spectrum did not change much after heating to 400 K. Further heating to 500 and 600 K resulted in a decrease in intensity of all the bands. By 600 K, the bands present in the RAIR spectrum are associated with the most intense polystyrene bands. All bands disappear after heating to 700 K. The presence of infrared absorption bands after heating past 360 K, which correlate with those observed for polystyrene liquid and the polystyrene standard film, indicates that the species remaining on the surface at temperatures above 360 K is most probably polystyrene. Although, it is impossible to make any definitive conclusions regarding the polymer chain length beyond the fact that presumably longer chains with more than four monomer units are present on the surface at higher temperatures. As the sample is further heated, all bands in the spectrum decrease in intensity, indicating that polystyrene begins to decompose. There are no distinct peaks observed in TPD after heating to 360 K; however, there is a broad desorption feature from approximately 370 to 700 K for m/e ) 91 and 104 (see Figure 7). This broad desorption feature is consistent with the thermal degradation of the adsorbed polymerized film. VI. Mechanism for the Direct UV Photopolymerization of Styrene Thin Films. Osmanow et al. have proposed a mechanism for the direct UV laser polymerization of styrene monomers at 248 nm.19 As this is close to the major wavelength output from the Hg arc lamp, it would be reasonable to assume that a similar mechanism would be followed. The mechanism proposed for styrene polymerization is shown in Scheme 1. Upon excitation of the styrene monomer, a biradical is formed. The biradical is partially stabilized by delocalization of the secondary radical in the ring. Dimerization can form both diphenylcyclobutanes and linear dimers. The formation of the (19) Osmanow, Von R. R.; Winkelmann, G.; Linke, E. Z. Phys. Chem. Liepzig 1988, 269, 76.

Carlo and Grassian

Figure 7. TPD spectra for m/e ) 91 and 104 following heating to 350 K a UV-irradiated 100 ML styrene film and subsequent cooling to 100 K. There is a very broad desorption feature from approximately 370 to 700 K. Scheme 1. Proposed Mechanism for the Direct Photopolymerization of Styrene by UV Irradiation

two saturated diphenylcyclobutanes results in termination of the reaction. The unsaturated linear dimer can go on to react with other radical species or styrene to form higher order oligomers and eventually polystyrene. The fact that the styrene monomer has on average a preferred orientation suggests that polymerization may occur in a specific direction. It has been recently found that styrene film thickness plays a role in the degree of polymerization, with less polymerization occurring in thinner films.20 This suggests that the polymer may be forming in a direction along the surface normal. The decrease in intensity of the aliphatic CsH bands relative to the aromatic CsH band in the RAIR spectrum of the adsorbed styrene film compared to the transmission spectra of the two polystyrene samples supports this conclusion. Further studies on the mechanism of polymerization in styrene thin films are currently underway. (20) Carlo, S. R.; Grassian, V. H. Unpublished results.

Photopolymerization of Styrene Thin Films in UHV

Conclusions In summary, RAIRS and TPD have been used to investigate the direct UV photopolymerization of styrene thin films condensed on a Ag(110) substrate. The enhancement of in-plane versus out-of-plane bending modes of styrene in the RAIR spectrum suggests that styrene molecules in the film are predominantly oriented nearly perpendicular to the surface. A tilt angle of 26° is calculated between the ring plane and the surface normal.

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UV irradiation of a styrene film results in the formation of dimers, trimers, tetramers, and polystyrene. There is some evidence that the polymerization reaction occurs preferentially in the direction along the surface normal. Acknowledgment. The authors would like to acknowledge support from the National Science Foundation (Grant CHE-9309731). LA960979V