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Surface characterization of thin film induced by He+ ion-beam irradiation into PLLA
Surface & Coatings Technology 196 (2005) 383 – 388 www.elsevier.com/locate/surfcoat
Surface characterization of thin film induced by He+ ion-beam irradiation into PLLA Tasuku Yotoriyamaa,b,*, Yoshiaki Suzukic, Takaya Misec, Takeyo Tsukamotoa, Masaya Iwakic a
Graduate School, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo, 162-8601, Japan b Research Fellow, Japan Society for the Promotion of Science, Tokyo, Japan c RIKEN, 2-1 Hirosawa, Wako-shi, Saitama, 351-0198, Japan Available online 2 October 2004
Abstract Ion beams were irradiated into biodegradable polymer sheets to develop thin film, self-assembled cellular sheets that exfoliated spontaneously from the substrates in a water solution. We previously reported that thin film and self-assembled cellular sheet were obtained by irradiating He+ ion beams into biodegradable polymer sheets. However, the mechanisms of exfoliation from the substrates have not been clarified. Poly-l-lactic acid (PLLA) sheets were used as substrates. He+ ion-beams were irradiated at an energy of 150 keV with a fluence of 11015 ions/cm2. We investigated surface characteristics of He+ ion-beam-irradiated PLLA by means of SAICAS, SIMS, FT-IR-ATR and Raman scattering measurements. Ion-beam-irradiated exfoliated thin films had two layers, a harder layer than the original and a softer one. He+ ion-beam-irradiated exfoliated thin film included a disordered graphite structure produced by ion-beam irradiation. We concluded that thin film and self-assembled cellular sheets could be obtained by He+ ion-beam irradiation into PLLA using localized energy deposition. D 2004 Elsevier B.V. All rights reserved. Keywords: Ion implantation; Biodegradable polymer; Cell sheet
1. Introduction Bombardment with energetic ions is a unique method of modifying surface structures and properties of materials. Most research effort in the field of ion irradiation has concentrated on inorganic materials such as metals, ceramics and semiconductors. In recent years, ion beam irradiation into polymers was investigated to improve their compatibility with tissue [1–3]. The biodegradable polymer is widely used to construct cell scaffolds for tissue engineering purposes due to its excellent biocompatibility. Poly-lactic acid (PLA) is a biodegradable polymer that hydrolyzes into a low molecule when implanted in a body. The hydrolysis-generated reaction products are non-toxic to the body since they are broken down into carbon * Corresponding author. Tel.: +81 48 467 9395; fax: +81 48 462 4623. E-mail address: [email protected] (T. Yotoriyama). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.157
dioxide and water by the body’s metabolism cycle. The family of aliphatic polymers derived from lactide stereomers and other lactones, especially glycolide and Ecaprolactone, is currently considered as a source of biodegradable materials for temporary therapeutic applications, namely suture materials, bone fracture internal fixation devices in surgery, and drug delivery systems in pharmacology [4,5]. Our previous study reported that ion-beam irradiation into a biodegradable polymer produced a thin film, selfassembled cellular sheet or spheroid, which exfoliated spontaneously from the substrate in a water solution [6]. He+ ion-beam irradiation formed a thin film at an energy of 150 keV with a fluence of 11015 ions/cm2; the film thickness was about 1.2 Am after dipping in a phosphatebuffered saline solution (PBS()). The 1.2 Am thickness of the sheet was comparable to the peak value of energy deposition estimated by TRIM code (IBM, USA).
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The purpose of this work is to characterize the thin film obtained by He+ ion irradiation into PLA by Surface and Interfacial Cutting Analysis System (SAICAS), Secondary Ion Mass Spectroscopy (SIMS), Fourier Transform InfraRed spectroscopy combined with Attenuated Total Reflectance (FT-IR-ATR), and Raman scattering measurement (Raman).
2. Experiment 2.1. Sample preparation Fig. 1 shows the formula for poly-lactic acid (PLA). PLA incorporated a mirror image isomer such as poly-l-lactic acid (PLLA) and poly-d-lactic acid (PDLA). In this report, we selected and used PLLA sheets for the experiments. The substrates used were PLLA sheets (LACTY; SHIMADZU, Kyoto, Japan) fabricated on 33 cm substrates. The mass density of PLLA was 1.27 g/cm3 and the thickness of the sheet was 200 Am. We conducted He+ ion irradiation at an energy of 150 keV with a fluence of 11015 ions/cm2 at room temperature using the RIKEN 200 kV Low-Current Implanter. The beamcurrent density was kept below 0.1 AA/cm2 to prevent heating of the specimen. The pressure of the target chamber was maintained at a base pressure of 104 Pa order during ionbeam irradiation. Four samples were prepared for measurements of the depth profile of the He+ ion-beam-irradiated PLLA as shown in Fig. 2: (a) non-irradiated PLLA, (b) the substrate after exfoliation in the PBS() solution, (c) the bottom surface of the thin film and (d) the top surface of the thin film after exfoliation in the PBS() solution. All the samples were dehydrated in the desiccator with silica gel before measurements.
Fig. 2. Schematic diagram of sample preparation for measurements: (a) non-irradiated PLLA; (b) substrate after exfoliation; (c) bottom surface of thin film; (d) top surface of thin film.
[7]. The cutting knife was made of diamond and was 1 mm wide. The measurement mode was the constantvelocity shaving mode set to V Horizontal=0.1 Am/s and V Vertical=0.01 Am/s. 2.3. SIMS measurements The relation between sputtering time and concentration of H+, C+ and O+ ions was investigated by means of SIMS (IMS-6F, CAMECA, France). Primary ions used were below 1.0 kV Cs+ ions, and the beam current was 2.5 nA. Secondary ions detected were 1H+, 12C+ and 16O+ ions, and the raster size was 150150 Am. The total depth of removal by sputtering in SIMS measurements was measured by means of a Surface Texture Measuring Instrument (SURFCOM 1400D; ACCRETECH, Japan).
2.2. SAICAS measurements A Surface and Interfacial Cutting Analysis System (SAICAS CN-20; DAIPLA WINTES, Japan) was the peeling instrument used to measure mechanical parameters such as shear strength, peel strength, degree of adhesion and the force a blade receives while scraping a sample
Fig. 1. Constitutional formula of poly-lactic acid.
Fig. 3. The depth profiles of non-irradiated (a) and irradiated sample (b) as measured by SAICAS.
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TRIM code. The F H of irradiated sample is divided into the following four stages: (1) ion-beam-irradiated surface layer region from surface to 0.8 Am, (2) F H decreasing region from 0.8 to 1.2 Am, (3) F H increasing region from 1.2 to 1.5 Am, and (4) non-irradiated layer region from 1.5 Am. The point where the horizontal force suddenly decreased after reaching its maximum at F H decreasing region (from 0.8 to 1.2 Am) was observed. It probably started in the weak areas in the ion-beam-irradiated layer. Fig. 4 illustrates the shear strength of irradiated sample of stages 1, 3 and 4 above.The shear strength s at the cutting stage can be described as s ¼ FH =2wdcot/ Fig. 4. Graph of the shear strength of irradiated sample of stages 1, 3 and 4. 1: Ion-beam irradiated surface layer region from surface to 0.8 Am. 3: F H increasing region from 1.2 to 1.5 Am. 4: Non-irradiated layer region from 1.5 Am.
2.4. FT-IR-ATR spectra Functional group analyses were carried out by means of FT-IR-ATR (Nexus 470; Thermo Nicolet, USA). In these analyses, Germanium was chosen for use as an internal reflection prism and the incident angle of light emitted was 458. The absorbance was obtained as a function of wavenumber by measuring the intensity of the reflected light. Each spectrum was obtained after at least 128 scans and averaged at a resolution of 4 cm1 from 4000 to 750 cm1. Spatial resolution was calculated to be about 0.3 Am at 2000 cm1. 2.5. Raman spectra Raman spectra were obtained with a Raman microspectrometer (LabRam; Jobin-Yvon, France) at room temperature. A He–Ne laser (632.817 nm) was used as the excitation source. The exposure time was 3 s and five scans were accumulated in order to improve signal-to-noise ratio. The spectral resolution was 1 cm1.
where F H is the horizontal force, w is the width of the cutting knife (1 mm), d is the cutting depth and u is the shear angle (458). Fig. 5 presents the SEM image of the He+ ion-beamirradiated PLLA at an energy of 150 keV with a fluence of 11015 ions/cm2; a cracked section was observed. The thin film was 1.2 Am thick (measured by SEM). Close agreement was obtained between cracked section formed by ion-beam irradiation and the shear strength estimated by SAICAS. The results indicated that the He+ ion-beam-irradiated region had gradient properties depending on localized energy deposition. 3.2. SIMS measurements Fig. 6 presents relation between sputtering time and concentration of H+, C+ and O+ ions obtained by SIMS measurement of the non-irradiated PLLA (A) and ion-beamirradiated PLLA (B). The measurement was carried out to a depth of 7.2 Am from the surface. This depth was estimated including the whole ion range of He+ ion-beam irradiation. These curves had peaks at 3627 s. The reduction of secondary ion counts suggested that chemical composition
3. Results and discussion 3.1. SAICAS measurements Fig. 3 depicts the force a blade receives while scraping a sample ( F H: the horizontal force) and depth profiles of the non-irradiated and irradiated PLLA. The measurement mode was the constant-velocity shaving mode. In Fig. 3(a), F H of the non-irradiated sample was proportional to depth as shown by the dotted line. In Fig. 3(b), however, F H of the irradiated sample had a peak 1.2 Am from the surface. This clearly suggested that structural change occurred 1.26 Am from the surface due to ion-beam irradiation near the peak of energy deposition estimated by
Fig. 5. SEM image of a thin film formed by He+ ion-beam irradiation at an energy of 150 keV with a fluence of 11015 ions/cm2.
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Fig. 6. Relation between sputtering time and concentration of H+ (a), C+ (b) and O+ (c) ions obtained by SIMS measurement of the non-irradiated PLLA (A) and ion irradiated PLLA (B).
was changed by ion-beam irradiation. The depth after 3627 s of sputtering was estimated to be 1.3 Am by means of a Surface Texture Measuring Instrument. This value was comparable to the peak of the F H curve by SAICAS and to the peak value of energy deposition estimated by TRIM code.
exfoliated substrate surface indicated that the main peaks at 1080, 1180 and 1760 cm1 slightly decreased, and new peaks at 3450 cm1 were also emerged. The results indicated that ion-beam irradiation penetrated the exfoliated substrate surface, and formation of a thin film occurred at a peak position of energy deposition.
3.3. FT-IR-ATR spectra
3.4. RAMAN spectra
Fig. 7 shows the FT-IR-ATR spectra of non-irradiated PLLA (a), the exfoliated substrate surface (b), the ion-beamirradiated bottom surface (c) and the top surface of the exfoliated thin film (d). The spectrum of the non-irradiated surface contains three main peaks at 1080, 1180 and 1760 cm1 assigned to ´ıs C–O–C and C–C–O stretching vibration of ether-like, ester-like or carboxylic acid functionalities (1080 and 1180 cm1) and NCMO stretching vibration (1760 cm1). The three main peaks decreased dramatically in the ionbeam-irradiated thin film, and new peaks were observed at 1640 and 3450 cm1. The bands at 3450 and 1640 cm1 are due to hydroxyl (O–H) group and CMC double bonds. Part of the carbonyl absorption shifted from 1760 cm1 to 1700 cm1, indicating that the carbonyl group was transformed to a carboxyl group as a result of ion-beam irradiation [8,9]. Considering that the proportion of hydroxyl group absorption at the top surface of the thin film was larger than at the bottom surface, the formation of radical occurred mainly at the top surface of the thin film. The spectrum of the
Fig. 8 shows the Raman spectra of the non-irradiated PLLA (a), the exfoliated substrate surface (b), the ion-beamirradiated bottom surface (c) and the top surface of the exfoliated thin film (d) in the region of 2000 to 1000 cm1. The peak at 1770 cm1 assigned to carbonyl (NCMO) decreased, and the peak at 1590 cm1 assigned to CMC double bonds was produced as a result of ion irradiation [10]. The base line level was raised at the bottom and top surface of the thin film. These results prove that the chemical bond was broken by ion-beam irradiation and that a new chemical bond was produced such as a CMC double bond. Raman spectra of the ion-beam-irradiated bottom surface of the exfoliated thin film showed two new Raman active mode such as a peak centered at 1530 cm1 and a peak centered 1360 cm1. The higher frequency band is characteristic of polycrystalline graphite (G band) or amorphous carbon with graphitic bonding. The lower frequency band seems to be in agreement with that observed in disordered graphite (D band) [11].
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We conclude that He+ ion-beam-irradiated exfoliated thin film included amorphous carbons and disordered graphite structures produced by ion-beam irradiation. Raman spectra of the ion-beam-irradiated bottom surface of the exfoliated thin film exhibited an increasing tendency of producing disordered graphite structures because the peak of energy deposition existed below the surface in PLLA.
4. Conclusion We investigated surface characteristics of ion-beamirradiated PLLA by means of SAICAS, SIMS, FT-IR-ATR and Raman scattering measurements. The result of SAICAS measurement indicated that the He+ ion-irradiated region had gradient properties. The reduction of secondary ion counts in SIMS measurements suggested that the chemical composition was changed by He+ ion irradiation. FT-IRATR measurement showed that C–O–C and C–C–O stretching vibration of ether-like, ester-like or carboxylic acid functionalities and NCMO stretching vibration peaks decreased dramatically in the ion-beam-irradiated thin film, and new peaks of hydroxyl group and CMC double bonds as
Fig. 8. Raman spectra: (a) non-irradiated PLLA; (b) substrate after exfoliation; (c) bottom surface of thin film; (d) top surface of thin film.
well as some carboxyl absorption were observed. Raman scattering measurement showed that He+ ion-irradiated exfoliated thin film included disordered graphite structures produced by ion irradiation. We conclude that structural change occurred near the maximum of energy deposition estimated by TRIM code and ion-irradiated region had gradient properties depending on localized energy deposition.
Acknowledgements The authors are deeply indebted to Mr. M. Noda and Mr. A. Sugiyama of CAMECA INSTRUMENTS JAPAN for SIMS measurement and also to Mr. I. Nishiyama of DAIPLA WINTES for SAICAS measurement.
References
Fig. 7. FT-IR-ATR spectra: (a) non-irradiated PLLA; (b) substrate after exfoliation; (c) bottom surface of thin film; (d) top surface of thin film.
[1] Y. Suzuki, M. Kusakabe, J.-S. Lee, M. Kaibara, M. Iwaki, H. Sasabe, Nucl. Instrum. Methods Phys. Res., B 65 (1992) 142. [2] Y. Suzuki, H. Iwata, A. Nakao, M. Iwaki, H. Sasabe, S. Kaneko, H. Nakajima, M. Kusakabe, Nucl. Instrum. Methods Phys. Res., B 127/ 128 (1997) 1019. [3] K. Kurotobi, M. Kaibara, Y. Suzuki, M. Iwaki, H. Nakajima, Nucl. Instrum. Methods Phys. Res., B 175/177 (2001) 791.
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[4] Y. Wan, W. Chen, J. Yang, J. Bei, S. Wang, Biomaterials 24 (2003) 2195. [5] H. Kang, Y. Tabata, Y. Ikada, Biomaterials 20 (1999) 1339. [6] T. Yotoriyama, Y. Suzuki, T. Tsukamoto, M. Iwaki, Nucl. Instrum. Methods Phys. Res., B 206 (2003) 527. [7] N. Nagai, T. Imai, K. Terada, H. Seki, H. Okumura, H. Fujino, T. Yamamoto, I. Nishiyama, A. Hatta, Surf. Interface Anal. 34 (2002) 545.
[8] ON. Tretinnikov, Y. Ikada, J. Polym. Sci., Part B, Polym. Phys. 36 (1998) 715. [9] G. Kister, G. Cassanas, M. Vert, Polymer 39 (2) (1998) 267. [10] T. Yotoriyama, Y. Suzuki, M. Iwaki, T. Tsukamoto, Trans. Mater. Res. Soc. Jpn. 28 (2) (2003) 503. [11] E.H. Lee, D.M. Hembree Jr., G.R. Rao, L.K. Mansur, Phys. Rev., B 48 (1993) 540.
Surface characterization of thin film induced by He+ ion-beam irradiation into PLLA Tasuku Yotoriyamaa,b,*, Yoshiaki Suzukic, Takaya Misec, Takeyo Tsukamotoa, Masaya Iwakic a
Graduate School, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo, 162-8601, Japan b Research Fellow, Japan Society for the Promotion of Science, Tokyo, Japan c RIKEN, 2-1 Hirosawa, Wako-shi, Saitama, 351-0198, Japan Available online 2 October 2004
Abstract Ion beams were irradiated into biodegradable polymer sheets to develop thin film, self-assembled cellular sheets that exfoliated spontaneously from the substrates in a water solution. We previously reported that thin film and self-assembled cellular sheet were obtained by irradiating He+ ion beams into biodegradable polymer sheets. However, the mechanisms of exfoliation from the substrates have not been clarified. Poly-l-lactic acid (PLLA) sheets were used as substrates. He+ ion-beams were irradiated at an energy of 150 keV with a fluence of 11015 ions/cm2. We investigated surface characteristics of He+ ion-beam-irradiated PLLA by means of SAICAS, SIMS, FT-IR-ATR and Raman scattering measurements. Ion-beam-irradiated exfoliated thin films had two layers, a harder layer than the original and a softer one. He+ ion-beam-irradiated exfoliated thin film included a disordered graphite structure produced by ion-beam irradiation. We concluded that thin film and self-assembled cellular sheets could be obtained by He+ ion-beam irradiation into PLLA using localized energy deposition. D 2004 Elsevier B.V. All rights reserved. Keywords: Ion implantation; Biodegradable polymer; Cell sheet
1. Introduction Bombardment with energetic ions is a unique method of modifying surface structures and properties of materials. Most research effort in the field of ion irradiation has concentrated on inorganic materials such as metals, ceramics and semiconductors. In recent years, ion beam irradiation into polymers was investigated to improve their compatibility with tissue [1–3]. The biodegradable polymer is widely used to construct cell scaffolds for tissue engineering purposes due to its excellent biocompatibility. Poly-lactic acid (PLA) is a biodegradable polymer that hydrolyzes into a low molecule when implanted in a body. The hydrolysis-generated reaction products are non-toxic to the body since they are broken down into carbon * Corresponding author. Tel.: +81 48 467 9395; fax: +81 48 462 4623. E-mail address: [email protected] (T. Yotoriyama). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.157
dioxide and water by the body’s metabolism cycle. The family of aliphatic polymers derived from lactide stereomers and other lactones, especially glycolide and Ecaprolactone, is currently considered as a source of biodegradable materials for temporary therapeutic applications, namely suture materials, bone fracture internal fixation devices in surgery, and drug delivery systems in pharmacology [4,5]. Our previous study reported that ion-beam irradiation into a biodegradable polymer produced a thin film, selfassembled cellular sheet or spheroid, which exfoliated spontaneously from the substrate in a water solution [6]. He+ ion-beam irradiation formed a thin film at an energy of 150 keV with a fluence of 11015 ions/cm2; the film thickness was about 1.2 Am after dipping in a phosphatebuffered saline solution (PBS()). The 1.2 Am thickness of the sheet was comparable to the peak value of energy deposition estimated by TRIM code (IBM, USA).
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The purpose of this work is to characterize the thin film obtained by He+ ion irradiation into PLA by Surface and Interfacial Cutting Analysis System (SAICAS), Secondary Ion Mass Spectroscopy (SIMS), Fourier Transform InfraRed spectroscopy combined with Attenuated Total Reflectance (FT-IR-ATR), and Raman scattering measurement (Raman).
2. Experiment 2.1. Sample preparation Fig. 1 shows the formula for poly-lactic acid (PLA). PLA incorporated a mirror image isomer such as poly-l-lactic acid (PLLA) and poly-d-lactic acid (PDLA). In this report, we selected and used PLLA sheets for the experiments. The substrates used were PLLA sheets (LACTY; SHIMADZU, Kyoto, Japan) fabricated on 33 cm substrates. The mass density of PLLA was 1.27 g/cm3 and the thickness of the sheet was 200 Am. We conducted He+ ion irradiation at an energy of 150 keV with a fluence of 11015 ions/cm2 at room temperature using the RIKEN 200 kV Low-Current Implanter. The beamcurrent density was kept below 0.1 AA/cm2 to prevent heating of the specimen. The pressure of the target chamber was maintained at a base pressure of 104 Pa order during ionbeam irradiation. Four samples were prepared for measurements of the depth profile of the He+ ion-beam-irradiated PLLA as shown in Fig. 2: (a) non-irradiated PLLA, (b) the substrate after exfoliation in the PBS() solution, (c) the bottom surface of the thin film and (d) the top surface of the thin film after exfoliation in the PBS() solution. All the samples were dehydrated in the desiccator with silica gel before measurements.
Fig. 2. Schematic diagram of sample preparation for measurements: (a) non-irradiated PLLA; (b) substrate after exfoliation; (c) bottom surface of thin film; (d) top surface of thin film.
[7]. The cutting knife was made of diamond and was 1 mm wide. The measurement mode was the constantvelocity shaving mode set to V Horizontal=0.1 Am/s and V Vertical=0.01 Am/s. 2.3. SIMS measurements The relation between sputtering time and concentration of H+, C+ and O+ ions was investigated by means of SIMS (IMS-6F, CAMECA, France). Primary ions used were below 1.0 kV Cs+ ions, and the beam current was 2.5 nA. Secondary ions detected were 1H+, 12C+ and 16O+ ions, and the raster size was 150150 Am. The total depth of removal by sputtering in SIMS measurements was measured by means of a Surface Texture Measuring Instrument (SURFCOM 1400D; ACCRETECH, Japan).
2.2. SAICAS measurements A Surface and Interfacial Cutting Analysis System (SAICAS CN-20; DAIPLA WINTES, Japan) was the peeling instrument used to measure mechanical parameters such as shear strength, peel strength, degree of adhesion and the force a blade receives while scraping a sample
Fig. 1. Constitutional formula of poly-lactic acid.
Fig. 3. The depth profiles of non-irradiated (a) and irradiated sample (b) as measured by SAICAS.
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TRIM code. The F H of irradiated sample is divided into the following four stages: (1) ion-beam-irradiated surface layer region from surface to 0.8 Am, (2) F H decreasing region from 0.8 to 1.2 Am, (3) F H increasing region from 1.2 to 1.5 Am, and (4) non-irradiated layer region from 1.5 Am. The point where the horizontal force suddenly decreased after reaching its maximum at F H decreasing region (from 0.8 to 1.2 Am) was observed. It probably started in the weak areas in the ion-beam-irradiated layer. Fig. 4 illustrates the shear strength of irradiated sample of stages 1, 3 and 4 above.The shear strength s at the cutting stage can be described as s ¼ FH =2wdcot/ Fig. 4. Graph of the shear strength of irradiated sample of stages 1, 3 and 4. 1: Ion-beam irradiated surface layer region from surface to 0.8 Am. 3: F H increasing region from 1.2 to 1.5 Am. 4: Non-irradiated layer region from 1.5 Am.
2.4. FT-IR-ATR spectra Functional group analyses were carried out by means of FT-IR-ATR (Nexus 470; Thermo Nicolet, USA). In these analyses, Germanium was chosen for use as an internal reflection prism and the incident angle of light emitted was 458. The absorbance was obtained as a function of wavenumber by measuring the intensity of the reflected light. Each spectrum was obtained after at least 128 scans and averaged at a resolution of 4 cm1 from 4000 to 750 cm1. Spatial resolution was calculated to be about 0.3 Am at 2000 cm1. 2.5. Raman spectra Raman spectra were obtained with a Raman microspectrometer (LabRam; Jobin-Yvon, France) at room temperature. A He–Ne laser (632.817 nm) was used as the excitation source. The exposure time was 3 s and five scans were accumulated in order to improve signal-to-noise ratio. The spectral resolution was 1 cm1.
where F H is the horizontal force, w is the width of the cutting knife (1 mm), d is the cutting depth and u is the shear angle (458). Fig. 5 presents the SEM image of the He+ ion-beamirradiated PLLA at an energy of 150 keV with a fluence of 11015 ions/cm2; a cracked section was observed. The thin film was 1.2 Am thick (measured by SEM). Close agreement was obtained between cracked section formed by ion-beam irradiation and the shear strength estimated by SAICAS. The results indicated that the He+ ion-beam-irradiated region had gradient properties depending on localized energy deposition. 3.2. SIMS measurements Fig. 6 presents relation between sputtering time and concentration of H+, C+ and O+ ions obtained by SIMS measurement of the non-irradiated PLLA (A) and ion-beamirradiated PLLA (B). The measurement was carried out to a depth of 7.2 Am from the surface. This depth was estimated including the whole ion range of He+ ion-beam irradiation. These curves had peaks at 3627 s. The reduction of secondary ion counts suggested that chemical composition
3. Results and discussion 3.1. SAICAS measurements Fig. 3 depicts the force a blade receives while scraping a sample ( F H: the horizontal force) and depth profiles of the non-irradiated and irradiated PLLA. The measurement mode was the constant-velocity shaving mode. In Fig. 3(a), F H of the non-irradiated sample was proportional to depth as shown by the dotted line. In Fig. 3(b), however, F H of the irradiated sample had a peak 1.2 Am from the surface. This clearly suggested that structural change occurred 1.26 Am from the surface due to ion-beam irradiation near the peak of energy deposition estimated by
Fig. 5. SEM image of a thin film formed by He+ ion-beam irradiation at an energy of 150 keV with a fluence of 11015 ions/cm2.
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Fig. 6. Relation between sputtering time and concentration of H+ (a), C+ (b) and O+ (c) ions obtained by SIMS measurement of the non-irradiated PLLA (A) and ion irradiated PLLA (B).
was changed by ion-beam irradiation. The depth after 3627 s of sputtering was estimated to be 1.3 Am by means of a Surface Texture Measuring Instrument. This value was comparable to the peak of the F H curve by SAICAS and to the peak value of energy deposition estimated by TRIM code.
exfoliated substrate surface indicated that the main peaks at 1080, 1180 and 1760 cm1 slightly decreased, and new peaks at 3450 cm1 were also emerged. The results indicated that ion-beam irradiation penetrated the exfoliated substrate surface, and formation of a thin film occurred at a peak position of energy deposition.
3.3. FT-IR-ATR spectra
3.4. RAMAN spectra
Fig. 7 shows the FT-IR-ATR spectra of non-irradiated PLLA (a), the exfoliated substrate surface (b), the ion-beamirradiated bottom surface (c) and the top surface of the exfoliated thin film (d). The spectrum of the non-irradiated surface contains three main peaks at 1080, 1180 and 1760 cm1 assigned to ´ıs C–O–C and C–C–O stretching vibration of ether-like, ester-like or carboxylic acid functionalities (1080 and 1180 cm1) and NCMO stretching vibration (1760 cm1). The three main peaks decreased dramatically in the ionbeam-irradiated thin film, and new peaks were observed at 1640 and 3450 cm1. The bands at 3450 and 1640 cm1 are due to hydroxyl (O–H) group and CMC double bonds. Part of the carbonyl absorption shifted from 1760 cm1 to 1700 cm1, indicating that the carbonyl group was transformed to a carboxyl group as a result of ion-beam irradiation [8,9]. Considering that the proportion of hydroxyl group absorption at the top surface of the thin film was larger than at the bottom surface, the formation of radical occurred mainly at the top surface of the thin film. The spectrum of the
Fig. 8 shows the Raman spectra of the non-irradiated PLLA (a), the exfoliated substrate surface (b), the ion-beamirradiated bottom surface (c) and the top surface of the exfoliated thin film (d) in the region of 2000 to 1000 cm1. The peak at 1770 cm1 assigned to carbonyl (NCMO) decreased, and the peak at 1590 cm1 assigned to CMC double bonds was produced as a result of ion irradiation [10]. The base line level was raised at the bottom and top surface of the thin film. These results prove that the chemical bond was broken by ion-beam irradiation and that a new chemical bond was produced such as a CMC double bond. Raman spectra of the ion-beam-irradiated bottom surface of the exfoliated thin film showed two new Raman active mode such as a peak centered at 1530 cm1 and a peak centered 1360 cm1. The higher frequency band is characteristic of polycrystalline graphite (G band) or amorphous carbon with graphitic bonding. The lower frequency band seems to be in agreement with that observed in disordered graphite (D band) [11].
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We conclude that He+ ion-beam-irradiated exfoliated thin film included amorphous carbons and disordered graphite structures produced by ion-beam irradiation. Raman spectra of the ion-beam-irradiated bottom surface of the exfoliated thin film exhibited an increasing tendency of producing disordered graphite structures because the peak of energy deposition existed below the surface in PLLA.
4. Conclusion We investigated surface characteristics of ion-beamirradiated PLLA by means of SAICAS, SIMS, FT-IR-ATR and Raman scattering measurements. The result of SAICAS measurement indicated that the He+ ion-irradiated region had gradient properties. The reduction of secondary ion counts in SIMS measurements suggested that the chemical composition was changed by He+ ion irradiation. FT-IRATR measurement showed that C–O–C and C–C–O stretching vibration of ether-like, ester-like or carboxylic acid functionalities and NCMO stretching vibration peaks decreased dramatically in the ion-beam-irradiated thin film, and new peaks of hydroxyl group and CMC double bonds as
Fig. 8. Raman spectra: (a) non-irradiated PLLA; (b) substrate after exfoliation; (c) bottom surface of thin film; (d) top surface of thin film.
well as some carboxyl absorption were observed. Raman scattering measurement showed that He+ ion-irradiated exfoliated thin film included disordered graphite structures produced by ion irradiation. We conclude that structural change occurred near the maximum of energy deposition estimated by TRIM code and ion-irradiated region had gradient properties depending on localized energy deposition.
Acknowledgements The authors are deeply indebted to Mr. M. Noda and Mr. A. Sugiyama of CAMECA INSTRUMENTS JAPAN for SIMS measurement and also to Mr. I. Nishiyama of DAIPLA WINTES for SAICAS measurement.
References
Fig. 7. FT-IR-ATR spectra: (a) non-irradiated PLLA; (b) substrate after exfoliation; (c) bottom surface of thin film; (d) top surface of thin film.
[1] Y. Suzuki, M. Kusakabe, J.-S. Lee, M. Kaibara, M. Iwaki, H. Sasabe, Nucl. Instrum. Methods Phys. Res., B 65 (1992) 142. [2] Y. Suzuki, H. Iwata, A. Nakao, M. Iwaki, H. Sasabe, S. Kaneko, H. Nakajima, M. Kusakabe, Nucl. Instrum. Methods Phys. Res., B 127/ 128 (1997) 1019. [3] K. Kurotobi, M. Kaibara, Y. Suzuki, M. Iwaki, H. Nakajima, Nucl. Instrum. Methods Phys. Res., B 175/177 (2001) 791.
388
T. Yotoriyama et al. / Surface & Coatings Technology 196 (2005) 383–388
[4] Y. Wan, W. Chen, J. Yang, J. Bei, S. Wang, Biomaterials 24 (2003) 2195. [5] H. Kang, Y. Tabata, Y. Ikada, Biomaterials 20 (1999) 1339. [6] T. Yotoriyama, Y. Suzuki, T. Tsukamoto, M. Iwaki, Nucl. Instrum. Methods Phys. Res., B 206 (2003) 527. [7] N. Nagai, T. Imai, K. Terada, H. Seki, H. Okumura, H. Fujino, T. Yamamoto, I. Nishiyama, A. Hatta, Surf. Interface Anal. 34 (2002) 545.
[8] ON. Tretinnikov, Y. Ikada, J. Polym. Sci., Part B, Polym. Phys. 36 (1998) 715. [9] G. Kister, G. Cassanas, M. Vert, Polymer 39 (2) (1998) 267. [10] T. Yotoriyama, Y. Suzuki, M. Iwaki, T. Tsukamoto, Trans. Mater. Res. Soc. Jpn. 28 (2) (2003) 503. [11] E.H. Lee, D.M. Hembree Jr., G.R. Rao, L.K. Mansur, Phys. Rev., B 48 (1993) 540.