Langmuir 1997, 13, 2603-2605
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Nonlinear Laser Intensity Dependence of the Formation of Carboxylic Acid Groups at the Surface of Polymer Films: The Effect of Coupling of Radical Intermediates Nobuyuki Ichinose,*,†,‡ Toshiyuki Tamai,§ Shunichi Kawanishi,†,‡ Isao Hashida,§ and Kazuhiko Mizuno| Osaka Laboratory for Radiation Chemistry, Japan Atomic Energy Research Institute, 25-1 Mii-minamimachi, Neyagawa, Osaka 572, Japan, Osaka Municipal Technical Research Institute, 1-6-50 Morinomiya, Joto-ku, Osaka 536, Japan, and Department of Applied Chemistry, College of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 593, Japan Received October 1, 1996. In Final Form: March 17, 1997X Carboxylic acid groups formed at the surface of poly(4-((trimethylsilyl)methyl)styrene) films upon irradiation with KrF laser pulses at various intensities were analyzed quantitatively. The yield of the acid groups per pulse [acid groups cm-2 pulse-1] as a function of the laser intensity (I, mJ cm-2 pulse-1) was shown to follow a nonlinear relationship; [acid groups cm-2 pulse-1] ) AI - BI2 (A and B are constant). The second term, an apparent two-photon process, was attributed to an effect of a coupling reaction of the radical intermediates.
Introduction
Scheme 1
Laser processing of polymer surfaces has attracted much interest from both basic and practical points of view during the past decade.1-4 Laser irradiation of polymer surfaces induces, in general, some degradative chemical reactions such as oxygenation at low laser intensities4 and physical deformation well-known as laser ablation at high laser intensities. Such laser treatments enhance the adhesivity or printing ability of the surface and have some advantages of a short-time operation, a spatial controllability, and so forth. In the schematic diagram for the laser ablation proposed by Srinivasan,2 bond scission taking place in a high density is induced by an intense laser pulse. Though dense formation of radicals seems favorable for their coupling, such a behavior has been less observed experimentally for polymer solids because of the difficulties in the estimation of cross-linking. In order to estimate the effect of the coupling of radical intermediates, we have studied the formation of carboxylic acid groups quantitatively at the surface of polymers, poly(4-((trimethylsilyl)methyl)styrene) (PTMSMS) and poly(4-methylstyrene) (P4MS), which show different reactivities in photochemical cross-linking, upon KrF laser irradiation (248 nm) at various intensities. Results and Discussion The films of both polymers undergo photooxygenation to form carboxylic acid groups at the surfaces through the formation of a benzyl-type radical derived from the homolytic cleavage of the C-Si bond of 4-(trimethylsilyl)†
Osaka Laboratory for Radiation Chemistry. Present address: Kansai Research Establishment, Japan Atomic Energy Research Institute, 25-1 Mii-minamimachi, Neyagawa, Osaka 572, Japan. § Osaka Municipal Technical Research Institute. | Osaka Prefecture University. X Abstract published in Advance ACS Abstracts, April 15, 1997. ‡
(1) Srinivasan, R.; Mayne-banton, V. Appl. Phys. Lett. 1982, 41, 576. (2) Srinivasan, R. J. Vac. Sci. Technol. 1983, B1, 923. (3) Srinivasan, R.; Braren, B. Chem. Rev. 1989, 89, 1303. (4) (a) Lazare, S.; Srinivasan, R. J. Phys. Chem. 1986, 90, 2124. Hiraoka, H.; Lazare, S. Appl. Surf. Sci. 1990, 46, 264. (b) Niino, H.; Yabe, A. Appl. Phys. Lett. 1992, 60, 2697. (c) Uchida, T.; Sugimura, H.; Kemnitz, K.; Shimo, N.; Masuhara, H. Appl. Phys. Lett. 1991, 59, 3189. Uchida, T.; Shimo, N.; Sugimura, H.; Kemnitz, K.; Masuhara, H. J. Appl. Phys. 1994, 76, 4872.
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methyl or the C-H bond of the 4-methyl group (Scheme 1).5 The surface concentrations of carboxylic acid groups at early stages of the photooxygenation were measured for various laser intensities of 1-60 mJ cm-2 pulse-1.6,7 (5) Tamai, T.; Ichinose, N.; Kawanishi, S.; Hashida, I.; Inoue, H.; Mizuno, K. Polymer 1996, 37, 5525. (6) Thin films (1-1.5 µm thickness) of PTMSMS (Mn ) 3.4 × 104)7 and P4MS (Aldrich, Mn ) 7.2 × 104) were prepared by spin-coating on quartz plates (29 mm diameter, 1 mm thickness) from its toluene solution (80 g/L). The polymer films were dried to remove the solvent and were irradiated with 100-1000 pulses (pulse duration, 30 ns fwhm) by use of an excimer laser (Lumonics PM-884) operated with a KrF gas mixture at 10 Hz through a beam homogenizer (Leonix EWO-FI-60-248, beam size, 12 × 12 mm2). (7) (a) Nakanishi, K. Ph.D. thesis, University of Osaka Prefecture, 1993. b) Mizuno, K.; Kobata, T.; Maeda, R.; Otsuji, Y. Chem. Lett. 1990, 1821.
© 1997 American Chemical Society
2604 Langmuir, Vol. 13, No. 10, 1997
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Laser ablation, however, occurred at laser intensities of >55 mJ cm-2 pulse-1 for PTMSMS and >47 mJ cm-2 pulse-1 for P4MS. After irradiation, each film was stained with an aqueous solution (1 × 10-4 M) of a cationic basic dye, rhodamine 6G (Rh6G) for 10 min and washed with water. The dye was observed to be introduced selectively onto the irradiated area almost irreversibly but not onto the unirradiated area.5,8 The adsorption of cationic dyes will proceed by the ion exchange reaction:9
RCOOH + D+A- f RCOO-D+ + H+A- (Dye: D+A-) The surface concentration of the carboxylic acid group was estimated from the film absorbance at 536 nm (OD) due to the dye adsorption, assuming 1:1 adsorption to the acid group and the extinction coefficient of Rh6G ( ) 1 × 105 M-1 cm-1). The surface concentration of the acid groups is given by OD/1000 (mol cm-2). The surface oxygenated layer of P4MS became soluble in an ethanol-benzene mixture (6:1, v/v), a poor solvent of the polymer, owing to a decrease of the molecular weight by the oxidative scission of the main chain.5 The thickness of the surface oxygenated layer was measured by atomic force microscopy (AFM) to be ≈100 nm after the treatment of the irradiated film with the ethanol-benzene mixture.10 Though the cross-linking prevented the dissolution of the oxygenated layer for PTMSMS, the oxygenation must take place to a depth of ≈100 nm. As shown in Figure 1, the surface concentrations of the acid groups formed per pulse (molecules cm-2 pulse-1) are proportional (slope ) 1) to the laser intensity (mJ cm-2 pulse-1) at low irradiation levels for PTMSMS and P4MS, indicating that the reaction is a one-photon process. Assuming a thickness of the surface layer being analyzed by this method to be ≈100 nm, the quantum yield for the acid formation at the surface for PTMSMS was estimated to be ≈6.5 × 10-4 from an absorption coefficient of the PTMSMS films (R ) 1.94 × 103 cm-1) and the photon flux of the laser in a unit of photons cm-2 pulse-1 (for 248-nm light, 1 mJ ) 1.25 × 1015 photons). In the same way, the quantum yield for the P4MS films (R ) 1.11 × 103 cm-1) was obtained to be ≈4.1 × 10-4. As the laser intensity increased, the slope of the plot for PTMSMS decreased gradually and eventually deviated completely from a linear relationship. Furthermore, the slope became negative above ≈30 mJ cm-2 pulse-1 and the yield of the acid group per pulse decreased further with increasing the laser intensity. On the other hand, the plot for P4MS was almost linear with a slope of ≈1. The nonlinear relationship for the acid group formation for PTMSMS could be explained by the effect of the coupling of the benzyl-type radical intermediates.11 The (8) Anionic or acidic dyes such as sulforhodamine were not adsorbed onto the irradiated surface. See also ref 4c. (9) In the strict sense, this adsorption must be in a Langmuir-type equilibrium and take place in a ratio of less than 1:1 to the carboxylic acid groups. However, adsorbed Rh6G was not removed by washing with water indicating that the reaction proceeded almost irreversibly. A quantitative fluorometric analysis by the similar labeling of carboxylic groups with cationic dyes on polymer surfaces has been reported very recently: Ivanov, V. B.; Behnish, J.; Holla¨nder, A.; Mehdorn, F.; Zimmerman, H. Surf. Interface Anal. 1996, 24, 257. (10) To determine the depth where the oxygenation occurs, a P4MS film was irradiated through a photomask with slits of 5-200 µm.5 The oxygenated layer was removed by the treatment with the ethanolbenzene mixture. The profile of the positive image of the mask was obtained by atomic force microscopy and the depth was estimated from its cross section. (11) Contributions of other two-photonic processes in the present power dependence such as singlet-singlet (S-S) or triplet-triplet (T-T) anihilation or two-photonic absorption can be ruled out because these processes produce at least one excited state of the chromophore to lead to the radical formation. The linear relationship observed for P4MS also supprots the absence of these processes.
Figure 1. (a) A log-log plot of the yield of the carboxylic acid groups per pulse vs laser pulse intensity for PTMSMS with the best fitted parabolic curve together with lines showing the first and second terms of eq 1 (see text) and (b) that for P4MS with the best fitted line with a slope of 1.
benzyl-type radicals formed in the bulk layer will undergo recombination with the silyl radical in a cage giving a starting material or bimolecular coupling (cross-linking) leading to insolubilization of the bulk as observed for PTMSMS even upon irradiation with a mercury lamp.5 On the other hand, molecular oxygen in air diffuses inside the polymer near the surface and reacts with the radicals within their lifetime, competing with the coupling. If a pair of radicals is closely formed (paired radicals) at the polymer surface, it would disappear rapidly by bimolecular coupling without any reaction with oxygen. Since the mobility of the polymeric radicals is expected to be small, it can safely be assumed that (a) the coupling proceeds only from the paired radicals12 and (b) surface concentrations of carboxylic acid groups formed per pulse are
Letters
proportional to initial concentrations of isolated radicals ([isolated radicals]initial).
[carboxylic acid groups] ∝ [isolated radicals]initial (molecules cm-2 pulse-1) The initial concentration of isolated radicals will be equal to the difference between those of the total and paired radicals ([total radicals]initial and [paired radicals]initial, respectively).
[isolated radicals]initial ) [total radicals]initial - [paired radicals]initial Since the probability for the photochemical formation of a radical from a 4-((trimethylsilyl)methyl)phenyl group upon pulsed laser irradiation is proportional to the laser intensity (I), the probability for the formation of a radical pair from a pair of precursors is expected to be proportional to the square of laser intensity (I2). The initial concentration of total radicals and that of radical pairs, therefore, will be proportional to I and I2, respectively. Thus, the laser power dependence of carboxylic group formation is expressed by eq 1
[carboxylic acid groups] ) AI - BI2 (molecules cm-2 pulse-1) (1) where A and B are intensity independent constants. The above parabolic function fitted fairly well with the plot shown in Figure 1a with the parameters A ) 3.55 × 1010 (molecules mJ-1) and B ) 6.20 × 108 (molecules mJ-2 cm2 pulse) for PTMSMS. On the other hand, the linear relationship in the case of P4MS can be explained by assuming an almost negligibe contribution of the second term in eq 1. Since the contributions of the second term in the oxygenation of the polymer surfaces are qualitatively in good accordance with cross-linking behaviors of the bulk films, the attribution of the second term to the coupling of radical pairs seems reasonable. Though PTMSMS (12) This was supported by the fact that films of copolymer of styrene and 1-((trimethylsilyl)methyl)-4-vinylbenzene (19:1), in which much fewer radical pairs will be formed than in PTMSMS film, did not show insolubilization even with 1000 pulses of 50 mJ cm-2 pulse-1, whereas PTMSMS film became insoluble with 10 pulses of the same intensity. This also rules out the incorporation of an excited benzylic radical as a mechanism for the two-photon cross-linking process.
Langmuir, Vol. 13, No. 10, 1997 2605
shows a high reactivity in the photochemical cross-linking, P4MS is less reactive. The insolubilization of P4MS films through a cross-linking required a high laser intensity of more than ≈20 mJ cm-2 pulse-1, and no insolubilization was observed by irradiating with a mercury lamp.5 The ratio of the efficiencies for the radical formation of P4MS to PTMSMS can be estimated to be 0.63 from the quantum yields for the formation of the acid group. Therefore, the efficiency of the radical coupling for P4MS will be approximately 0.632 ≈ 0.40 times of that for PTMSMS. Furthermore, selectivity of coupling at a given laser intensity can be easily derived from a ratio of the two terms in eq 1, [paired radicals]initial/[total radicals]initial ) BI2/AI ) 0.017I for PTMSMS. The calculated selectivity for the coupling is proportional to the laser intensity, indicating that the coupling is more favorable at high laser intensities. Since the selectivity of oxygenation for PTMSMS, on the other hand, is given by 1 - BI2/AI ) 1 - 0.017I, the oxygenation is more favorable at low laser intensities. These power dependence values of the selectivities were in harmony with our observations that the use of KrF laser was more effective compared with that of a mercury lamp in a lithographic negative pattern formation of PTMSMS film and the irradiation at 254 nm with a low-pressure mercury lamp (≈1016 photons cm-2 s-1) gave more hydrophilic surfaces than those obtained with the laser pulses under the present conditions.5 In conclusion, we have demonstrated that the surface oxygenation of polymer films giving carboxylic acid competes with the coupling reaction of the radical intermediates and the latter reaction is observed as an apparent two-photon process through an analysis of the laser power dependence of the acid formation when a photo-cross-linkable polymer is irradiated at high laser intensities. However, the effect of the radical coupling is negligibe when a less reactive polymer is irradiated even at intensites near the ablation threshold. Acknowledgment. We thank Professor H. Masuhara, Osaka University, and Dr. D. Karnakis, Japan Atomic Energy Research Institute, for their helpful discussions. K.M. is grateful for a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science and Culture of Japan, to the Shorai Foundation for Science and Technology, and to the Nippon Cable System Inc. for partial support of this research. LA960949S
