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Synthesis of new erasable optical data storage polymers and their applications
Optical Materials 21 (2002) 621–625 www.elsevier.com/locate/optmat
Synthesis of new erasable optical data storage polymers and their applications Yang-Kyoo Han b
a,*
, Bong-Soo Ko
b
a Department of Chemistry, Hanyang University, Seoul 133-791, South Korea Korea and Center for Advanced Functional Polymers, KAIST, Taejon 305-701, South Korea
Abstract Novel liquid crystalline malonic ester monomer was synthesized from malonyl dichloride and disperse red 1, a photoresponsive group. The monomer was polymerized with 1,6-dibromohexane, 1,8-dibromooctane, 1,10-dibromodecane, 1,12-dibromododecane, a; a0 -dibromo-p-xylene, a; a0 -dibromo-m-xylene, or a; a0 -dibromo-o-xylene in the presence of sodium hydride to give seven kinds of novel poly(malonic esters) with two symmetrical photoresponsive groups in the side chain. The resulting polymer films were found to be excellent as reversible optical information recording media for data storage and retrieval through a trans–cis isomerization of azobenzene group by Ar laser irradiation. The sensitivity of data recording was dependent not only on the thickness of the polymeric thin film but also on the intensity of laser beam. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 42.70.Jk; 78.20.Fm Keywords: Optical data storage; Photoresponsive polymers; Liquid crystalline monomer; Digital bit; Optical birefringence
1. Introduction In recent years, there have been increasing interests in photoresponsive polymers for optical holography, optical information storage, and integrated optics [1]. Furthermore, since they show a strong potential as information recording media for data storage and retrieval, they have been the subjects of a number of papers [2,3]. For an example, with irradiating either linearly polarized or unpolarized light, optical information can be written, erased, and rewritten on liquid crystalline *
Corresponding author. Tel: +82-2-2290-0941; fax: +82-22299-7845. E-mail address: [email protected] (Y.-K. Han).
(LC) or amorphous polymer films containing azobenzene group sensitive to the light. The information is stored by optically induced birefringence. The mechanism of information recording involves a photochemical excitation of azobenzene group, which undergoes a photoisomerization from its trans conformation to cis one and then goes back to the trans form affecting some reorientation. By the repetition of these reorientation cycles (trans–cis– trans isomerization), a substantial portion of unoriented azobenzene groups is aligned perpendicular to the plane of polarization of the light irradiated, which results in a high birefringence. Such optical orientation is measured by monitoring the intensity of the probe beam transmitting the recording medium during a read-out process.
0925-3467/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 2 ) 0 0 2 1 1 - 2
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Y.-K. Han, B.-S. Ko / Optical Materials 21 (2002) 621–625
In this study, we synthesized seven kinds of novel poly(malonic esters) having two disperse red 1 groups in the side chain and examined the structural influence of the polymer backbone on the characteristics and stability for their optical data storage media.
2. Experimental As optical recording media, novel poly(malonic esters) were used. It was prepared by modifying the procedure we already reported [4,5], which is shown in Fig. 1. New thermotropic LC malonic ester monomer (MDR1) was synthesized by reacting in THF at 0 °C for 24 h malonyl dichloride and disperse red 1. It was then condensed with 1, 6-dibromohexane, 1,8-dibromooctane, 1,10-dibromodecane, 1,12-dibromododecane, a; a0 -dibromo-p-xylene,a; a0 -dibromo-m -xylene, or a; a0 -dibromo-o-xylene in THF in the presence of sodium hydride at 65 °C for 24 h to give seven kinds of poly(malonic esters) with two symmetrical disperse red 1, a photoresponsive group. Their thermal behaviors and LC texture were measured by DSC, TGA, and optical polarizing microscope (OPM) with a hot stage. The polymeric thin films were cast from the polymer solution (5 wt%) in CHCl3 onto a glass
plate for 30 s using a spin caster. We controlled the thickness of the films with varying the spin speed from 500 to 1500 rpm. We examined the possibility of the application of the polymeric thin films to optical data storage materials with irradiating linearly polarized argon laser (488 nm) as a writing beam. The optical anisotropy of the polymer derived from the Ar laser in information recording step was measured by using low power Ga/As laser of 847 nm, a probe beam.
3. Results and discussion 3.1. Phase transition behavior and thermal properties. The new malonic ester monomer MDR1 showed enantiotropic behavior in which LC phase appears at 117 and 86 °C, respectively, on both heating and cooling cycles by DSC. It, however, was observed to show a typical smectic batonnet texture only on a cooling cycle (96 °C) by means of OPM. On the other hand, the corresponding polymers (PDR1) obtained from the MDR1 had no mesophase. That is, the PDR1 having six methylene units (m ¼ 6) in the polymer backbone, which is one of the obtained polymers, exhibited only isotropization temperature (76 °C) on a
Fig. 1. Synthesis of poly(malonic esters) with disperse red 1 groups in the side chain.
Y.-K. Han, B.-S. Ko / Optical Materials 21 (2002) 621–625
heating cycle without a mesophase. Its glass transition temperature (Tg ) was about 50 °C. In addition, introducing a rigid group such as p-xylene, m-xylene, or o-xylene unit into the polymer backbone enhanced Tg of the corresponding polymers. The PDR1 with p-xylene group was observed to have the highest Tg (96 °C). From TGA thermograms, the monomer and polymer were found to have excellent thermal stabilities. The decomposition temperature of 10% weight loss was over 300 °C. The weight average molecular weights of the PDR1 were in the range of 5000 to 15,000, which were controlled by the polymerization conditions. 3.2. Photoisomerization of azobenzene UV–VIS absorption spectrum of the chloroform solution of the polymer with p-xylene group was measured to investigate a trans–cis isomerization of azobenzene moiety incorporated into the polymer. As a result, we observed that the absorbance at 466 nm (kmax ) due to the stable trans conformation of azobenzene decreased gradually as the irradiation time of Ar laser increased. 3.3. Application to optical data storage materials We examined the possibility of the application of the PDR1s to erasable optical data storage media through a trans–cis isomerization of azo dye by Ar laser irradiation. Writing process for optical data storage is as follows. The polymeric thin film (thickness: 0.26–0.54 lm) was heated above the melting temperature of the polymers and then immediately quenched below Tg at which isotropic phase was frozen. The homogeneous film with an isotropic phase was placed between two crossed polarizers and the linearly polarized Ar laser with an intensity of 0.2–8.0 mW/cm2 was irradiated onto the sample at room temperature. The writing beam was irradiated at 45° relative to the two polarized axes. In a read-out process, the probe beam through the two crossed polarizers measured the transmitted intensity, which results from the optical birefringence of the polymer, with the irradiation time of the pump beam. It is known that the transmitted intensity I can be expressed as
I ¼ I0 sin2
pDnd k
sin2 ð2hÞ
623
ð1Þ
where, I0 is the incident power of the probe laser, Dn is the birefringence induced from the film, d is the thickness of the film, k is the wavelength of the probe laser, and h is the angle between the polarization angle of the pump beam and the optic axis of a polarizer used in the read-out process [6]. The measurement was carried out at h ¼ 45° to obtain the highest transmitted intensity from the equation. On the other hand, the optical anisotropy of the polymer is derived by a trans–cis isomerization of azobenzene moiety in the side chain of the polymer. In other words, the azobenzene groups that are unoriented inside the isotropic phase (no transmission) are aligned perpendicular to the plane of polarization of the writing laser irradiated. Such an orientation gives rise to high transmission in the data storage step. As a result, Fig. 2 shows writing and relaxation profiles for the PDR1 (m ¼ 6) thin films with irradiation time: When the polarized argon laser was turned on, the transmitted intensity increased logarithmically up to about 20 min and then leveled off. This stands for that the polymer chains were aligned perpendicular to the plane of laser polarization through a trans–cis isomerization of azobenzene group incorporated into the polymer. The normalized transmitted intensity rapidly
Fig. 2. Writing and relaxation profiles for PDR1 (m ¼ 6) thin films.
624
Y.-K. Han, B.-S. Ko / Optical Materials 21 (2002) 621–625
decreased about 30% within the first few minutes and maintained a certain value after relaxation at room temperature, when the writing laser was turned off. In addition, the sensitivity of information recording was dependent on the film thickness. The value of the normalized transmitted intensity increased almost two times as the thickness of the polymer film increased from 0.26 to 0.54 lm. This means that the transmitted intensity is proportional to the number of the azobenzene molecules oriented perpendicular to the plane of polarization of the pump beam. However, the stored information (high transmitted intensity) was very rapidly erased as the ambient temperature was increased. Thus, we examined the effect of the polymer backbone structure on the storage stability of the data with temperature. In the case of using the PDR1 film with p-xylene unit, a rigid group, we found that the optical data were stored stably up to 75 °C, as shown in Fig. 3. It indicates that the rigid group incorporated into the polymer backbone greatly affects the stability of optical data storage. To investigate how fast the optical information can be recorded in the film, we carried out the write-in process for the data storage with the power intensity of Ar laser. As a result, the higher the intensity of pump beam, the faster the recording speed of the optical data. Especially, it was
Fig. 3. Writing and erasing profiles for PDR1 (X ¼ p-xylene) thin film with recording temperature.
Fig. 4. Writing and erasing profiles for PDR1 (X ¼ p-xylene) film with different intensity of writing beam.
possible to write the data even at the intensity of 0.5 mW/cm2 , as shown in Fig. 4. Fig. 5 shows a representative photograph of the image pattern stored in the PDR1 film with p-xylene group. The analogue image was obtained through the following process: A photo mask with a line width from 50 to 2 lm was directly placed on the polymer film and argon laser was irradiated onto the sample at room temperature. In order to store an image pattern, the illuminated film was finally cooled below Tg . The resolution of the information stored in the film became better with increase in the irradiation time of pump beam, and a clear pattern with a minimum line width of 2 lm
Fig. 5. Analogue image patterns stored in PDR1 (X ¼ p-xylene) film.
Y.-K. Han, B.-S. Ko / Optical Materials 21 (2002) 621–625
625
irradiating the circularly polarized Ar laser can erase the digital and analogue data stored in the PDR1 thin films and repeating the write-in process, which is described previously, can rewrite them [8]. On the basis of the above results, we found that the poly(malonic esters) containing disperse red 1 were excellent as reversible optical information recording media for data storage and retrieval. Acknowledgements Fig. 6. Digital bits stored in PDR1 (X ¼ p-xylene) film with power intensity of Ar ion pulsed laser (5.0, 4.0, 3.0, 2.0 and 1.0 mW): pulse width, 10 ls.
(!) was obtained when exposed over 5 min. In the photograph a bright pattern results from the phase change from isotropic state to anisotropic one induced by a trans–cis isomerization of azobenzene moiety of the disperse red 1 in the side chain. For a practical application, the focused linearly and circularly polarized laser beams (pulse width: 10 lm) were irradiated to write and erase bits, digital data, respectively. For this experiment, we used the schematic optic-setup reported in our previous work [7]. Before preparing a polymeric thin film, Al reflection layer was deposited onto a substrate material using a sputtering method in order to obtain higher contrast. Fig. 6 shows the digital bits stored in the PDR1 thin film. The resolution of the bits was getting better with increase in the power intensity of the pulsed laser beam from 1.0 to 5.0 mW/cm2 . On the other hand,
The authors acknowledge financial support from the Korea Science and Engineering Foundation (96-0300-1001-3) as well as the Center for Advanced Functional Polymers, KAIST for financial support in part. References [1] D.D. Nolte, Photorefractive effects and materials, KAP (1995) 265–302. [2] A. Natanson, P. Rochon, Chem. Mater. 5 (1993) 403. [3] A. Natanson, P. Rochon, J. Gosselin, Macromolecules 25 (1992) 403. [4] Y.K. Han, H.S. Na, C.H. Oh, Mol. Cryst. Liq. Cryst. 327 (1999) 271. [5] W.J. Joo, H.D. Shin, C.H. Oh, S.H. Song, P.S. Kim, B.S. Ko, Y.K. Han, J. Chem. Phys. 113 (19) (2000) 8848. [6] A. Yariv, Optical Electronics in Modern Communications, Oxford Press, New York, 1977, p. 24. [7] J.H. Kim, K.M. Hong, H.S. Na, Y.K. Han, Jpn. J. Appl. Phys. 40 (2001) 1585. [8] Y.K. Han, B.C. Kim, B.S. Ko, J.H. Kim, K.M. Hong, H.S. Na. Korea Patent Filed #1999-46209 (1999); Korea Patent #0252948 (2000).
Synthesis of new erasable optical data storage polymers and their applications Yang-Kyoo Han b
a,*
, Bong-Soo Ko
b
a Department of Chemistry, Hanyang University, Seoul 133-791, South Korea Korea and Center for Advanced Functional Polymers, KAIST, Taejon 305-701, South Korea
Abstract Novel liquid crystalline malonic ester monomer was synthesized from malonyl dichloride and disperse red 1, a photoresponsive group. The monomer was polymerized with 1,6-dibromohexane, 1,8-dibromooctane, 1,10-dibromodecane, 1,12-dibromododecane, a; a0 -dibromo-p-xylene, a; a0 -dibromo-m-xylene, or a; a0 -dibromo-o-xylene in the presence of sodium hydride to give seven kinds of novel poly(malonic esters) with two symmetrical photoresponsive groups in the side chain. The resulting polymer films were found to be excellent as reversible optical information recording media for data storage and retrieval through a trans–cis isomerization of azobenzene group by Ar laser irradiation. The sensitivity of data recording was dependent not only on the thickness of the polymeric thin film but also on the intensity of laser beam. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 42.70.Jk; 78.20.Fm Keywords: Optical data storage; Photoresponsive polymers; Liquid crystalline monomer; Digital bit; Optical birefringence
1. Introduction In recent years, there have been increasing interests in photoresponsive polymers for optical holography, optical information storage, and integrated optics [1]. Furthermore, since they show a strong potential as information recording media for data storage and retrieval, they have been the subjects of a number of papers [2,3]. For an example, with irradiating either linearly polarized or unpolarized light, optical information can be written, erased, and rewritten on liquid crystalline *
Corresponding author. Tel: +82-2-2290-0941; fax: +82-22299-7845. E-mail address: [email protected] (Y.-K. Han).
(LC) or amorphous polymer films containing azobenzene group sensitive to the light. The information is stored by optically induced birefringence. The mechanism of information recording involves a photochemical excitation of azobenzene group, which undergoes a photoisomerization from its trans conformation to cis one and then goes back to the trans form affecting some reorientation. By the repetition of these reorientation cycles (trans–cis– trans isomerization), a substantial portion of unoriented azobenzene groups is aligned perpendicular to the plane of polarization of the light irradiated, which results in a high birefringence. Such optical orientation is measured by monitoring the intensity of the probe beam transmitting the recording medium during a read-out process.
0925-3467/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 2 ) 0 0 2 1 1 - 2
622
Y.-K. Han, B.-S. Ko / Optical Materials 21 (2002) 621–625
In this study, we synthesized seven kinds of novel poly(malonic esters) having two disperse red 1 groups in the side chain and examined the structural influence of the polymer backbone on the characteristics and stability for their optical data storage media.
2. Experimental As optical recording media, novel poly(malonic esters) were used. It was prepared by modifying the procedure we already reported [4,5], which is shown in Fig. 1. New thermotropic LC malonic ester monomer (MDR1) was synthesized by reacting in THF at 0 °C for 24 h malonyl dichloride and disperse red 1. It was then condensed with 1, 6-dibromohexane, 1,8-dibromooctane, 1,10-dibromodecane, 1,12-dibromododecane, a; a0 -dibromo-p-xylene,a; a0 -dibromo-m -xylene, or a; a0 -dibromo-o-xylene in THF in the presence of sodium hydride at 65 °C for 24 h to give seven kinds of poly(malonic esters) with two symmetrical disperse red 1, a photoresponsive group. Their thermal behaviors and LC texture were measured by DSC, TGA, and optical polarizing microscope (OPM) with a hot stage. The polymeric thin films were cast from the polymer solution (5 wt%) in CHCl3 onto a glass
plate for 30 s using a spin caster. We controlled the thickness of the films with varying the spin speed from 500 to 1500 rpm. We examined the possibility of the application of the polymeric thin films to optical data storage materials with irradiating linearly polarized argon laser (488 nm) as a writing beam. The optical anisotropy of the polymer derived from the Ar laser in information recording step was measured by using low power Ga/As laser of 847 nm, a probe beam.
3. Results and discussion 3.1. Phase transition behavior and thermal properties. The new malonic ester monomer MDR1 showed enantiotropic behavior in which LC phase appears at 117 and 86 °C, respectively, on both heating and cooling cycles by DSC. It, however, was observed to show a typical smectic batonnet texture only on a cooling cycle (96 °C) by means of OPM. On the other hand, the corresponding polymers (PDR1) obtained from the MDR1 had no mesophase. That is, the PDR1 having six methylene units (m ¼ 6) in the polymer backbone, which is one of the obtained polymers, exhibited only isotropization temperature (76 °C) on a
Fig. 1. Synthesis of poly(malonic esters) with disperse red 1 groups in the side chain.
Y.-K. Han, B.-S. Ko / Optical Materials 21 (2002) 621–625
heating cycle without a mesophase. Its glass transition temperature (Tg ) was about 50 °C. In addition, introducing a rigid group such as p-xylene, m-xylene, or o-xylene unit into the polymer backbone enhanced Tg of the corresponding polymers. The PDR1 with p-xylene group was observed to have the highest Tg (96 °C). From TGA thermograms, the monomer and polymer were found to have excellent thermal stabilities. The decomposition temperature of 10% weight loss was over 300 °C. The weight average molecular weights of the PDR1 were in the range of 5000 to 15,000, which were controlled by the polymerization conditions. 3.2. Photoisomerization of azobenzene UV–VIS absorption spectrum of the chloroform solution of the polymer with p-xylene group was measured to investigate a trans–cis isomerization of azobenzene moiety incorporated into the polymer. As a result, we observed that the absorbance at 466 nm (kmax ) due to the stable trans conformation of azobenzene decreased gradually as the irradiation time of Ar laser increased. 3.3. Application to optical data storage materials We examined the possibility of the application of the PDR1s to erasable optical data storage media through a trans–cis isomerization of azo dye by Ar laser irradiation. Writing process for optical data storage is as follows. The polymeric thin film (thickness: 0.26–0.54 lm) was heated above the melting temperature of the polymers and then immediately quenched below Tg at which isotropic phase was frozen. The homogeneous film with an isotropic phase was placed between two crossed polarizers and the linearly polarized Ar laser with an intensity of 0.2–8.0 mW/cm2 was irradiated onto the sample at room temperature. The writing beam was irradiated at 45° relative to the two polarized axes. In a read-out process, the probe beam through the two crossed polarizers measured the transmitted intensity, which results from the optical birefringence of the polymer, with the irradiation time of the pump beam. It is known that the transmitted intensity I can be expressed as
I ¼ I0 sin2
pDnd k
sin2 ð2hÞ
623
ð1Þ
where, I0 is the incident power of the probe laser, Dn is the birefringence induced from the film, d is the thickness of the film, k is the wavelength of the probe laser, and h is the angle between the polarization angle of the pump beam and the optic axis of a polarizer used in the read-out process [6]. The measurement was carried out at h ¼ 45° to obtain the highest transmitted intensity from the equation. On the other hand, the optical anisotropy of the polymer is derived by a trans–cis isomerization of azobenzene moiety in the side chain of the polymer. In other words, the azobenzene groups that are unoriented inside the isotropic phase (no transmission) are aligned perpendicular to the plane of polarization of the writing laser irradiated. Such an orientation gives rise to high transmission in the data storage step. As a result, Fig. 2 shows writing and relaxation profiles for the PDR1 (m ¼ 6) thin films with irradiation time: When the polarized argon laser was turned on, the transmitted intensity increased logarithmically up to about 20 min and then leveled off. This stands for that the polymer chains were aligned perpendicular to the plane of laser polarization through a trans–cis isomerization of azobenzene group incorporated into the polymer. The normalized transmitted intensity rapidly
Fig. 2. Writing and relaxation profiles for PDR1 (m ¼ 6) thin films.
624
Y.-K. Han, B.-S. Ko / Optical Materials 21 (2002) 621–625
decreased about 30% within the first few minutes and maintained a certain value after relaxation at room temperature, when the writing laser was turned off. In addition, the sensitivity of information recording was dependent on the film thickness. The value of the normalized transmitted intensity increased almost two times as the thickness of the polymer film increased from 0.26 to 0.54 lm. This means that the transmitted intensity is proportional to the number of the azobenzene molecules oriented perpendicular to the plane of polarization of the pump beam. However, the stored information (high transmitted intensity) was very rapidly erased as the ambient temperature was increased. Thus, we examined the effect of the polymer backbone structure on the storage stability of the data with temperature. In the case of using the PDR1 film with p-xylene unit, a rigid group, we found that the optical data were stored stably up to 75 °C, as shown in Fig. 3. It indicates that the rigid group incorporated into the polymer backbone greatly affects the stability of optical data storage. To investigate how fast the optical information can be recorded in the film, we carried out the write-in process for the data storage with the power intensity of Ar laser. As a result, the higher the intensity of pump beam, the faster the recording speed of the optical data. Especially, it was
Fig. 3. Writing and erasing profiles for PDR1 (X ¼ p-xylene) thin film with recording temperature.
Fig. 4. Writing and erasing profiles for PDR1 (X ¼ p-xylene) film with different intensity of writing beam.
possible to write the data even at the intensity of 0.5 mW/cm2 , as shown in Fig. 4. Fig. 5 shows a representative photograph of the image pattern stored in the PDR1 film with p-xylene group. The analogue image was obtained through the following process: A photo mask with a line width from 50 to 2 lm was directly placed on the polymer film and argon laser was irradiated onto the sample at room temperature. In order to store an image pattern, the illuminated film was finally cooled below Tg . The resolution of the information stored in the film became better with increase in the irradiation time of pump beam, and a clear pattern with a minimum line width of 2 lm
Fig. 5. Analogue image patterns stored in PDR1 (X ¼ p-xylene) film.
Y.-K. Han, B.-S. Ko / Optical Materials 21 (2002) 621–625
625
irradiating the circularly polarized Ar laser can erase the digital and analogue data stored in the PDR1 thin films and repeating the write-in process, which is described previously, can rewrite them [8]. On the basis of the above results, we found that the poly(malonic esters) containing disperse red 1 were excellent as reversible optical information recording media for data storage and retrieval. Acknowledgements Fig. 6. Digital bits stored in PDR1 (X ¼ p-xylene) film with power intensity of Ar ion pulsed laser (5.0, 4.0, 3.0, 2.0 and 1.0 mW): pulse width, 10 ls.
(!) was obtained when exposed over 5 min. In the photograph a bright pattern results from the phase change from isotropic state to anisotropic one induced by a trans–cis isomerization of azobenzene moiety of the disperse red 1 in the side chain. For a practical application, the focused linearly and circularly polarized laser beams (pulse width: 10 lm) were irradiated to write and erase bits, digital data, respectively. For this experiment, we used the schematic optic-setup reported in our previous work [7]. Before preparing a polymeric thin film, Al reflection layer was deposited onto a substrate material using a sputtering method in order to obtain higher contrast. Fig. 6 shows the digital bits stored in the PDR1 thin film. The resolution of the bits was getting better with increase in the power intensity of the pulsed laser beam from 1.0 to 5.0 mW/cm2 . On the other hand,
The authors acknowledge financial support from the Korea Science and Engineering Foundation (96-0300-1001-3) as well as the Center for Advanced Functional Polymers, KAIST for financial support in part. References [1] D.D. Nolte, Photorefractive effects and materials, KAP (1995) 265–302. [2] A. Natanson, P. Rochon, Chem. Mater. 5 (1993) 403. [3] A. Natanson, P. Rochon, J. Gosselin, Macromolecules 25 (1992) 403. [4] Y.K. Han, H.S. Na, C.H. Oh, Mol. Cryst. Liq. Cryst. 327 (1999) 271. [5] W.J. Joo, H.D. Shin, C.H. Oh, S.H. Song, P.S. Kim, B.S. Ko, Y.K. Han, J. Chem. Phys. 113 (19) (2000) 8848. [6] A. Yariv, Optical Electronics in Modern Communications, Oxford Press, New York, 1977, p. 24. [7] J.H. Kim, K.M. Hong, H.S. Na, Y.K. Han, Jpn. J. Appl. Phys. 40 (2001) 1585. [8] Y.K. Han, B.C. Kim, B.S. Ko, J.H. Kim, K.M. Hong, H.S. Na. Korea Patent Filed #1999-46209 (1999); Korea Patent #0252948 (2000).