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Laser-induced nanometer expansion and contraction dynamics of polystyrene films depending on its molecular weight
Applied Surface Science 197±198 (2002) 796±799
Laser-induced nanometer expansion and contraction dynamics of polystyrene ®lms depending on its molecular weight Takashi Mito, Hiroshi Masuhara* Department of Applied Physics and Frontier Research Center, Osaka University, Suita, 565-0871 Osaka, Japan
Abstract We have measured surface morphological changes of polystyrene ®lms using nanosecond time-resolved interferometry. In the case of low molecular weights (3:0 103 and 5:0 103 ), the surface contracts exponentially. In the case of high molecular weight 1:0 106 , however, two different contraction processes are observed, that is, a major contraction and a minor tail. The thermal diffusion coef®cients of major components acquired experimentally from both high and low molecular weights are almost the same as the value in the literature, which indicates thermal diffusion is dominant for a major contraction. On the other hand, we consider the minor tail from the viewpoints of rubber±glass transition and polymer entanglements. # 2002 Published by Elsevier Science B.V. Keywords: Laser ablation; Interferometry; Polystyrene; Nanosecond expansion; Tens of microsecond contraction
1. Introduction It is well known that when a pulsed laser is irradiated to a polymer ®lm below the ablation threshold, the surface layer of the polymer ®lm begins to expand within the pulse width and contracts with a time constant from several hundreds of nanoseconds to several tens of microseconds [1±6]. This nanometer± nanosecond morphological dynamics can be measured by using time-resolved interferometry. So far, we have applied this technique to several polymer ®lms and proved that each polymer shows its characteristic morphological behavior, especially for contraction dynamics [7]. For example, we have measured an oscillatory expansion in the case of PMMA [8,9], and rapid and slow contraction dynamics in the case of polyurethane. In this study, we have used polystyrene (PS) ®lms with *
Corresponding author. Tel.: 81-6-6879-7837; fax: 81-6-6876-8580. E-mail address: [email protected] (H. Masuhara).
different molecular weights to investigate effects of glass±rubber transition and entanglement of polymer chains on rapid morphological phenomena. 2. Experimental Three types of PS (Fluka) whose molecular weights were 3:0 103 , 5:0 103 and 1:0 106 were dissolved into chlorobenzene (Wako Pure Chemicals, 99.0% purity). These polymers have single peaks in Mw and Mn and their Mw/Mn are 1.05, 1.07 and 1.03, respectively. After spin-coating them on quartz substrates, PS ®lms of low molecular weights (3:0 103 and 5:0 103 ) and those of high molecular weight 1:0 106 were dried for 8 h in vacuum at 363 and 393 K, respectively. The ®lm thickness was adjusted from 0.8 to 4.1 mm for each molecular weight. Their absorption coef®cients were all about 0.3 mm 1 at 248 nm, that is, those penetration depths were about 3 mm at 248 nm.
0169-4332/02/$ ± see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 4 1 7 - 8
T. Mito, H. Masuhara / Applied Surface Science 197±198 (2002) 796±799
The nanosecond surface morphological changes have been measured by time-resolved interferometry system [1±9]. A 248 nm pulse (30 ns FWHM) from excimer laser (Lambda Physik, LEXTRA 200) was used as an excitation light source, while the second harmonic pulse (532 nm, 10 ns FWHM) of Q-switched Nd3:YAG laser (Continuum, Surelite I) was used as a probe light for the Michelson-type interferometer. The time-resolved measurement was carried out by controlling a delay time (Dt) between excitation and probe laser pulses with a digital delay/ pulse generator (Stanford Research System, DG535). The delay time was monitored shot by shot by a digital oscilloscope, and Dt 0 is de®ned to be the time when two pulse peaks coincide with each other. All data were obtained by single shot measurement to avoid effects due to excitation of photoproducts formed by the previous irradiation. A shift of one fringe spacing corresponds to 266 nm expansion/ etching, a half-wavelength of the probe pulse. The ¯uence of excimer laser pulse was here set to 200 mJ/cm2, which is below the ablation threshold of PS (400 mJ/cm2 at 248 nm). We con®rmed for each measurement that the surface of PS ®lm recovers completely to the original ¯at surface. 3. Results and discussion We measured ®lm thickness dependence of maximum expansion. For both cases of high and low molecular weights, there are two regions whose boundary is around 2.0 mm, areas A and B as shown in Fig. 1. In area A, the maximum expansion increases linearly with ®lm thickness, however, in area B it is saturated.
Fig. 1. The ®lm thickness dependence of maximum expansion of PS ®lm whose molecular weight is 1:0 106 .
797
The ®lm thickness is smaller than the penetration depth of PS, so that the photothermal heating of the ®lm is relatively homogeneous. The result on area A corresponds well to this simple expectation. Then we examined the contraction process of PS ®lms until the expansion amplitude returned to about several tens of nanometers. In the case of low molecular weights, the expansion amplitude decreases single exponentially as shown in Fig. 2. On the other hand, in the case of high molecular weight there are two components; here we name a major contraction process for the earlier one and a minor tail for the later one, although there seems to be only a major contraction process when the ®lm thickness is 0.89 mm. Normally, the amplitude of each contraction component obtained by extrapolation to time zero is used for analysis, but the S/N value of a minor tail is not good. Instead, we introduce t1±2 as the time when the major process is taken over by the minor one. At ®rst, the major contraction process is considered. There is little difference among the relaxation times of low molecular weights and those for the major contraction process of high molecular weight. The thermal diffusion coef®cient acquired from these experimental data is in good agreement with the value in the literature [10]. Consequently, thermal diffusion is dominant in a major contraction. As shown in Fig. 2, it is obvious that the contraction process of PS shows an interesting molecular weight dependence. Now we consider why the two contraction components are observed for high molecular weight. The maximum temperature elevation of PS ®lm can be estimated as more than 400 K using the following equation [11], T T0 I0 a=gC, where T is a ®lm temperature, T0 a room temperature (293 K), I0 a ¯uence (200 mJ/cm2), a an absorption coef®cient (0.3 mm 1), g and C are the density and speci®c heat (1.06 g/cm2 and 1.25 J/g K) [10], respectively. Since Tg of PS is 373 and 340 K for high and low molecular weights, respectively, there is a possibility that t1±2 indicates the time when ®lm temperature decreases to Tg and the state of the ®lm changes from rubber to glass. Since the thermal expansion coef®cient of PS above Tg is about three times larger than that below Tg, the rate of contraction becomes smaller after t1±2. As the present excitation heats up both polymers with high and low molecular weights above their respective Tg, the difference in contraction behavior cannot be ascribed to that in Tg.
798
T. Mito, H. Masuhara / Applied Surface Science 197±198 (2002) 796±799
Fig. 2. The molecular weight dependence of transient contraction dynamics of PS ®lms. Low molecular weight PS ®lms of: (a) 3:0 103 and (b) 5:0 103 contract with a single exponential function, while high one of (c) 1:0 106 does with double exponential components.
Alternatively, the minor tail may be due to the entanglements of polymer chains. The length of PS chains between the entanglements usually corresponds to Mw of a few thousands which is named the characteristic length L. Therefore, physical properties of PS ®lms above L are almost the same. In the case of low molecular weights, polymer chains behave as liquid and
hardly show the effect of the entanglements, because the molecular chains are shorter than L. In the case of high molecular weight, however, the motion of PS chains may be restricted each other. Namely, the contraction takes longer time compared to decrease in temperature. According to the idea, t1±2 should be observed when the molecular weight is larger than L.
T. Mito, H. Masuhara / Applied Surface Science 197±198 (2002) 796±799
Now we are extending a more detailed study to understand the nature of contraction dynamics of polymer ®lms. 4. Summary By measuring the surface morphological changes with nanosecond time-resolved interferometry, an interesting contraction dynamics depending on molecular weight was observed. Thermal diffusion is dominant in the earlier contraction, polymer entanglement may result in the later contraction. We consider that the present novel behavior is important to understand laser-induced ablation and related phenomena of polymer ®lms. References [1] H. Furutani, H. Fukumura, H. Masuhara, Appl. Phys. Lett. 65 (1994) 3413.
799
[2] H. Furutani, H. Fukumura, H. Masuhara, J. Phys. Chem. 100 (1996) 6871. [3] H. Furutani, H. Fukumura, H. Masuhara, T. Lippert, A. Yabe, J. Phys. Chem. A 101 (1997) 5742. [4] H. Furutani, H. Fukumura, H. Masuhara, S. Kambara, T. Kitaguchi, H. Tsukada, T. Ozawa, J. Phys. Chem. B 102 (1998) 3395. [5] H. Masuhara, K. Sasaki, H. Fukumura, H. Furutani, Analyst 123 (1998) 531. [6] T. Lippert, J.T. Dickson, S.C. Langford, H. Furutani, H. Fukumura, H. Masuhara, T. Kunz, A. Wokaun, Appl. Surf. Sci. 127±129 (1998) 117. [7] T. Mito, T. Masubuchi, T. Tada, H. Fukumura, H. Masuhara, J. Photosci. 6 (1999) 109. [8] T. Masubuchi, H. Furutani, H. Fukumura, H. Masuhara, Chem. Phys. Chem. 3 (2000) 137. [9] T. Masubuchi, H. Furutani, H. Fukumura, H. Masuhara, J. Phys. Chem. B 105 (2001) 2518. [10] J. Brandrup, E.H. Immergut, Polymer Handbook, 3rd Edition, Wiley, New York, 1989. [11] H. Fukumura, N. Mibuka, S. Eura, H. Masuhara, Appl. Phys. A 53 (1991) 255.
Laser-induced nanometer expansion and contraction dynamics of polystyrene ®lms depending on its molecular weight Takashi Mito, Hiroshi Masuhara* Department of Applied Physics and Frontier Research Center, Osaka University, Suita, 565-0871 Osaka, Japan
Abstract We have measured surface morphological changes of polystyrene ®lms using nanosecond time-resolved interferometry. In the case of low molecular weights (3:0 103 and 5:0 103 ), the surface contracts exponentially. In the case of high molecular weight 1:0 106 , however, two different contraction processes are observed, that is, a major contraction and a minor tail. The thermal diffusion coef®cients of major components acquired experimentally from both high and low molecular weights are almost the same as the value in the literature, which indicates thermal diffusion is dominant for a major contraction. On the other hand, we consider the minor tail from the viewpoints of rubber±glass transition and polymer entanglements. # 2002 Published by Elsevier Science B.V. Keywords: Laser ablation; Interferometry; Polystyrene; Nanosecond expansion; Tens of microsecond contraction
1. Introduction It is well known that when a pulsed laser is irradiated to a polymer ®lm below the ablation threshold, the surface layer of the polymer ®lm begins to expand within the pulse width and contracts with a time constant from several hundreds of nanoseconds to several tens of microseconds [1±6]. This nanometer± nanosecond morphological dynamics can be measured by using time-resolved interferometry. So far, we have applied this technique to several polymer ®lms and proved that each polymer shows its characteristic morphological behavior, especially for contraction dynamics [7]. For example, we have measured an oscillatory expansion in the case of PMMA [8,9], and rapid and slow contraction dynamics in the case of polyurethane. In this study, we have used polystyrene (PS) ®lms with *
Corresponding author. Tel.: 81-6-6879-7837; fax: 81-6-6876-8580. E-mail address: [email protected] (H. Masuhara).
different molecular weights to investigate effects of glass±rubber transition and entanglement of polymer chains on rapid morphological phenomena. 2. Experimental Three types of PS (Fluka) whose molecular weights were 3:0 103 , 5:0 103 and 1:0 106 were dissolved into chlorobenzene (Wako Pure Chemicals, 99.0% purity). These polymers have single peaks in Mw and Mn and their Mw/Mn are 1.05, 1.07 and 1.03, respectively. After spin-coating them on quartz substrates, PS ®lms of low molecular weights (3:0 103 and 5:0 103 ) and those of high molecular weight 1:0 106 were dried for 8 h in vacuum at 363 and 393 K, respectively. The ®lm thickness was adjusted from 0.8 to 4.1 mm for each molecular weight. Their absorption coef®cients were all about 0.3 mm 1 at 248 nm, that is, those penetration depths were about 3 mm at 248 nm.
0169-4332/02/$ ± see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 4 1 7 - 8
T. Mito, H. Masuhara / Applied Surface Science 197±198 (2002) 796±799
The nanosecond surface morphological changes have been measured by time-resolved interferometry system [1±9]. A 248 nm pulse (30 ns FWHM) from excimer laser (Lambda Physik, LEXTRA 200) was used as an excitation light source, while the second harmonic pulse (532 nm, 10 ns FWHM) of Q-switched Nd3:YAG laser (Continuum, Surelite I) was used as a probe light for the Michelson-type interferometer. The time-resolved measurement was carried out by controlling a delay time (Dt) between excitation and probe laser pulses with a digital delay/ pulse generator (Stanford Research System, DG535). The delay time was monitored shot by shot by a digital oscilloscope, and Dt 0 is de®ned to be the time when two pulse peaks coincide with each other. All data were obtained by single shot measurement to avoid effects due to excitation of photoproducts formed by the previous irradiation. A shift of one fringe spacing corresponds to 266 nm expansion/ etching, a half-wavelength of the probe pulse. The ¯uence of excimer laser pulse was here set to 200 mJ/cm2, which is below the ablation threshold of PS (400 mJ/cm2 at 248 nm). We con®rmed for each measurement that the surface of PS ®lm recovers completely to the original ¯at surface. 3. Results and discussion We measured ®lm thickness dependence of maximum expansion. For both cases of high and low molecular weights, there are two regions whose boundary is around 2.0 mm, areas A and B as shown in Fig. 1. In area A, the maximum expansion increases linearly with ®lm thickness, however, in area B it is saturated.
Fig. 1. The ®lm thickness dependence of maximum expansion of PS ®lm whose molecular weight is 1:0 106 .
797
The ®lm thickness is smaller than the penetration depth of PS, so that the photothermal heating of the ®lm is relatively homogeneous. The result on area A corresponds well to this simple expectation. Then we examined the contraction process of PS ®lms until the expansion amplitude returned to about several tens of nanometers. In the case of low molecular weights, the expansion amplitude decreases single exponentially as shown in Fig. 2. On the other hand, in the case of high molecular weight there are two components; here we name a major contraction process for the earlier one and a minor tail for the later one, although there seems to be only a major contraction process when the ®lm thickness is 0.89 mm. Normally, the amplitude of each contraction component obtained by extrapolation to time zero is used for analysis, but the S/N value of a minor tail is not good. Instead, we introduce t1±2 as the time when the major process is taken over by the minor one. At ®rst, the major contraction process is considered. There is little difference among the relaxation times of low molecular weights and those for the major contraction process of high molecular weight. The thermal diffusion coef®cient acquired from these experimental data is in good agreement with the value in the literature [10]. Consequently, thermal diffusion is dominant in a major contraction. As shown in Fig. 2, it is obvious that the contraction process of PS shows an interesting molecular weight dependence. Now we consider why the two contraction components are observed for high molecular weight. The maximum temperature elevation of PS ®lm can be estimated as more than 400 K using the following equation [11], T T0 I0 a=gC, where T is a ®lm temperature, T0 a room temperature (293 K), I0 a ¯uence (200 mJ/cm2), a an absorption coef®cient (0.3 mm 1), g and C are the density and speci®c heat (1.06 g/cm2 and 1.25 J/g K) [10], respectively. Since Tg of PS is 373 and 340 K for high and low molecular weights, respectively, there is a possibility that t1±2 indicates the time when ®lm temperature decreases to Tg and the state of the ®lm changes from rubber to glass. Since the thermal expansion coef®cient of PS above Tg is about three times larger than that below Tg, the rate of contraction becomes smaller after t1±2. As the present excitation heats up both polymers with high and low molecular weights above their respective Tg, the difference in contraction behavior cannot be ascribed to that in Tg.
798
T. Mito, H. Masuhara / Applied Surface Science 197±198 (2002) 796±799
Fig. 2. The molecular weight dependence of transient contraction dynamics of PS ®lms. Low molecular weight PS ®lms of: (a) 3:0 103 and (b) 5:0 103 contract with a single exponential function, while high one of (c) 1:0 106 does with double exponential components.
Alternatively, the minor tail may be due to the entanglements of polymer chains. The length of PS chains between the entanglements usually corresponds to Mw of a few thousands which is named the characteristic length L. Therefore, physical properties of PS ®lms above L are almost the same. In the case of low molecular weights, polymer chains behave as liquid and
hardly show the effect of the entanglements, because the molecular chains are shorter than L. In the case of high molecular weight, however, the motion of PS chains may be restricted each other. Namely, the contraction takes longer time compared to decrease in temperature. According to the idea, t1±2 should be observed when the molecular weight is larger than L.
T. Mito, H. Masuhara / Applied Surface Science 197±198 (2002) 796±799
Now we are extending a more detailed study to understand the nature of contraction dynamics of polymer ®lms. 4. Summary By measuring the surface morphological changes with nanosecond time-resolved interferometry, an interesting contraction dynamics depending on molecular weight was observed. Thermal diffusion is dominant in the earlier contraction, polymer entanglement may result in the later contraction. We consider that the present novel behavior is important to understand laser-induced ablation and related phenomena of polymer ®lms. References [1] H. Furutani, H. Fukumura, H. Masuhara, Appl. Phys. Lett. 65 (1994) 3413.
799
[2] H. Furutani, H. Fukumura, H. Masuhara, J. Phys. Chem. 100 (1996) 6871. [3] H. Furutani, H. Fukumura, H. Masuhara, T. Lippert, A. Yabe, J. Phys. Chem. A 101 (1997) 5742. [4] H. Furutani, H. Fukumura, H. Masuhara, S. Kambara, T. Kitaguchi, H. Tsukada, T. Ozawa, J. Phys. Chem. B 102 (1998) 3395. [5] H. Masuhara, K. Sasaki, H. Fukumura, H. Furutani, Analyst 123 (1998) 531. [6] T. Lippert, J.T. Dickson, S.C. Langford, H. Furutani, H. Fukumura, H. Masuhara, T. Kunz, A. Wokaun, Appl. Surf. Sci. 127±129 (1998) 117. [7] T. Mito, T. Masubuchi, T. Tada, H. Fukumura, H. Masuhara, J. Photosci. 6 (1999) 109. [8] T. Masubuchi, H. Furutani, H. Fukumura, H. Masuhara, Chem. Phys. Chem. 3 (2000) 137. [9] T. Masubuchi, H. Furutani, H. Fukumura, H. Masuhara, J. Phys. Chem. B 105 (2001) 2518. [10] J. Brandrup, E.H. Immergut, Polymer Handbook, 3rd Edition, Wiley, New York, 1989. [11] H. Fukumura, N. Mibuka, S. Eura, H. Masuhara, Appl. Phys. A 53 (1991) 255.