Langmuir 1991, 7, 2860-2865
2860
Environmental Responses on the Surface of Polyurethane-BasedGraft Copolymers Having Uniform Size Polyether and Polyamine Segments Yasuyuki Tezuka,' Mami Yoshino, and Kiyokazu Imai Department of Material Science and Technology, Nagaoka University of Technology, Kamitomioka, Nagaoka, Niigata 940-21, J a p a n Received February 19, 1991. I n Final Form: J u n e 3, 1991 The surface of polyurethane-based graft copolymers having a defined content of uniform size polyether, Le., poly(tetrahydrofuran), and polyamine, i.e., poly(tert-butylaziridine),graft segments have been studied in both dry and wet states. XPS inspection of polyurethane-polyether graft copolymer film samples indicated the enrichment of the polyether component at the surface, while the polyurethane component dominated the surface of polyurethane-polyamine graft copolymer films. A significant surface rearrangement was detected on polyurethane-polyether graft copolymer surfaces along with the change of the contacting phase from dry to aqueous medium. The dynamics of the rearrangement process was monitored by means of a contact angle measurement with a CCld-in-water technique, in which the contact angle changed from one similar to polyether homopolymer to one close to polyurethane homopolymer within a 1-3-h period. By contrast, the polyurethane-polyamine graft copolymer surface was found to be insensitive to the change of the contacting medium from air to water.
Introduction Polymer material surfaces are characterized by their notable flexibility compared to their metallic and ceramic counterparts, and they can reorganize the interfacial structure and morphology by responding to the change of the contacting medium within a relatively short time scale.' Polyurethane-based copolymers, in particular polyurethane-polyether block-type copolymers, have been recognized as promising polymer material for cardiovascular devices due to their favorable mechanical and antithrombogenic propertiesS2 Their dry surfaces, of both commercial and their synthetic analogues,6s have been under extensive investigation to elucidate the structureproperty relationship of these biomaterials.+ll However, studies on their surfaces in aqueous medium and on their surface response behavior along with the change of the contacting medium from dry to aqueous phase are surprisingly limited12-15despite the significant importance in evaluating the polymeric biomaterial in an actual service condition. This is apparently due to the serious limitation (1) Andrade, J. D., Ed. Polymer Surface Dynamics; Plenum: New York, 1988. (2) Lelah, M. D., Cooper, S. L., Ed. Polyurethanes in Medicine; CRC Press: Boca Raton, FL, 1986. (3) Paik Sung, C. S.; Hu, C. B. J. Biomed. Mater. Res. 1987,13, 161. (4) Graham, S. W.; Hercules, D. M. J. Biomed. Mater. Res. 1981,15,
of analytical techniques applicable t o the polymer-water interface in contrast to those for the polymer-air (or vacuum) interface. We have recently described an environmentally induced surface rearrangement process occurring on polyurethanepolysiloxane block and graft copolymers having uniform size polysiloxane segments,l2J3 where the mode, i.e., block or graft, and the length of polysiloxane segment in addition to the total siloxane content in the copolymer exert a remarkable influence on the dynamics of the surface rearrangement process occurring along with the change of the contacting medium from dry to aqueous phase. As the extension of the preceding studies, we have examined in the present study the surface response behavior of two types of polyurethane-based graft copolymers having a well-defined content of uniform size polyether and polyamine segments. The objective of the present study is to elucidate how the nature of the graft segment component in the polyurethane-based copolymer can influence the response with change of the contacting medium from dry to aqueous phase.
Experimental Section
1. Materials. A series of polyurethane-polyether and polyurethane-polyamine graft copolymers used in the present study were prepared according to the method shown in Scheme I and 465. listed in Table I, where uniform size polyether and polyamine (5) Grobe, G. L., 111; Gardella, J. A., Jr.; Hopson, W. L.; Mckenna, W. macromonomers having a diol end group were synthesized and P.; Eyring, E. M. J . Biomed. Mater. Res. 1987,21, 211. subjected to the polyaddition reaction with diphenylmethyldi(6) Takahara, A.; Tashita, J.; Kajiyama, T.; Takayanagi, M.; MacKisocyanate (MDI),followed by chain extension with 1,rl-butanenight, W. J. Polymer 1985, 26, 978. diol (BD).16 The molecular weights of graft copolymers were in (7) Hearn, M. J.; Ratner, B. D.; Briggs, D. Macromolecules 1988,21, 2950. the range of 1W1V as estimated by GPC measurements. Poly(8) Castner, D. G.; Ratner, B. D.; Hoffman, A. S. J. Biomater. Sci., urethane homopolymer was synthesized by the equimolar polyPolym. Ed. 1990, 1, 191. (9) Furusawa,K.;Shimura, Y.;Otobe, K.;Ataumi,K.;Tsuda,K.Kobun- addition reaction of MDI and BD. Polyether, poly(tetrahydrofuran) [poly(THF)],and polyamine, poly(tert-butylaziridine) shi Ronbunshu 1977,34, 317. [poly(TBA)],homopolymers were prepared by a living cationic (10) Sa Da Costa, V.; Brier-Russell, D.; Salzman, E. W.; Merrill, E. W. J . Colloid Interjace Sci. 1981,80,445. polymerization technique with methyl trifluoromethanesulfonate (11) Lelah, M. D.; Grasel, T. G.;Pierce, J. A.;Cooper,S. L. J. Biomed. as an initiator.17J8Other reagents were used after conventional Mater. Res. 1986, 20, 433. purification procedures. (12)Tezuka, Y . ; Ono, T.; Imai, K. J.Colloid Interface Sci. 1990,136, 408. (13) Tezuka, Y.; Kazama, H.; Imai, K. J . Chem. SOC.,Faraday Trans. 1991, 87, 147. (14) Takahara, A.; Jo, N. J.; Kajiyama, T. J. Biomater. Sci., Polym. Ed. 1990, 1, 17. (15) Bummer, P. M.; Knutaon, K. Macromolecules 1990,23, 4357.
(16) Kazama, H.; Hoshi, M.; Nakajima, H.; Horak, D.; Tezuka, Y.; Imai, K. Polymer 1990,31,2207. (17) Penczek, S.; Kubiea, P.; Matyjaazewski, K. Adu. Polym. Sci. 1980,
37, 1. (18) Bossaer, P. K.; Goethals, E. J. Makromol. Chem. 1977,178,2983.
0 1991 American Chemical Society
Langmuir, Vol. 7, No. 11, 1991 2861
Surface Responses of Polyurethane-Based Graft Copolymers Table I. Polyurethane-Polyether and Polyurethane-Polyamine Graft Copolymer Samples polyether polyether or po1yamine:MDI:BD M, of polyether or polyamine or polyamine molar graft copolymer content, w t % sample0 ratio in feed A- 1 A-2 A-3 A-4 A-5 A-6 A-7 A-8 A-9 A-10
1:109 1:50:49 1:100:99 1:3:2 1:5049 1:100:99 1:200:199 1:3:2 1:100:99 1:200:199
2900 2900 2900 5400 5400 5400 5400 7500 7500 7500
43.8 12.8 8.0 87.2 25.0 15.5 8.6 89.1 16.5 10.7
B-1 B-2 B-3 B-4 B-5 B-6
1:109 1:50:49 1:100:99 1:109 1:50:49 1:100:99
3200 3200 3200 4900 4900 4900
56.1 14.5 9.9 51.4 21.2 14.0
A-1-A-10, polyurethane-polyether graft copolymers; B-1-B-6, polyurethane-polyamine graft copolymers.
Scheme I CF3S03CH3
HNSCH$H?OH
I2
>
C H $ O f C H 2 ~ N f C H ~ G ~ 12 OH
(I)
-H OSC
H2.&OH
(BD)
6 0 OC
'1.
1oc
a
50
1000
900
800
700
600
500
B.E. (ev)
400
300
200
100
0
Figure 1. Full-range XPS result for a polyurethane-polyether graft copolymer (sample A-10 in Table I). nitrogen just prior to the XPS measurement to avoid any contamination from ambient atmosphere. Contact Angle Measurements. Contact angle measurements were performed with a contact angle meter CA-A manufactured by Kyowa Kagaku Co. Besidesthe conventional waterin-air and air-in-water s y ~ t e m s , a~ sessile ~ J ~ droplet technique with a variety of liquid probes, including 0.1 N HCI and 0.1 N NaOH, was employed to monitor the dry surface of sample films. Also a CCL-in-water technique was developed to sense the wet surface of sample films, where a film sample was immersed into water thermostated at 20 O C and a small CCL droplet was placed on the film surface at prescribed time intervals. The contact angle of each film sample was measured on at least seven spots and five mean values were averaged. The observed contact angle values were generally within *3O otherwise stated. Polyurethane-polyether and polyurethane-polyamine graft copolymersamplesfor contact angle measurements were prepared by spreading 2.0 wt % DMA and DMA-chloroform solution, respectively, on a clean glass plate (slide glass pretreated with hot HN03 for more than 15 h), and the samples were evacuated for 48 h at room temperature and finally for more than 48 h at 60 OC. The polyurethane, polyether, and polyamine homopolymer film samples were prepared on a glass plate by similar procedures described under XPS Measurements.
Polyurethane I Polyether G r a f t Copolymer
Results
or
XPS Measurements. XPS analysis on the dry surface of a series of polyurethane-polyether and polyurethanepolyamine graft copolymers was carried out together with analysis of polyurethane, polyether, and polyamine homopolymers. A typical example of full-range XPS results for polyurethane-polyether graft copolymer samples is shownin Figure 1. All peaks in the spectrum were assigned to those of polyurethane and polyether segments in the graft copolymer and the sample was free from surface contamination either by a polycondensation catalyst residue (Sn) or by a lubricant for the joint of the glass apparatus used in the preparation procedure (Si). XPS results for the Cia, Ols, and N1, regions on polyurethanepolyether and on polyurethane-polyamine graft copolymers are summarized in Figure 2 and in Figure 3, respectively. In Figure 2, the results of angular-dependent XPS measurements were also listed, in which the escape angle of photoelectrons was changed from 90' to 15' in order to monitor the elemental ratio closer to the surface of the sample film. The relative intensity of the NI, signal from the polyurethane component in graft
Polyurethane I Polyamine Graft Copolymer
2. Measurements. XPS Measurements. XPS measurements were carried out with a Shimadzu ESCA 750 equipped with an ESCAPAC 760 data system. Typical operation conditions were as follows: Mg K q 2 radiation with 8 kV, 30 mA. The pressure in the instrumental chamber was kept below 5 x 10-5 Pa. Angular-dependent XPS measurements were carried out by using a sample holder adjusted to maintain the photoelectron escape angles at 4 5 O and 15O. Samples of polyurethane-polyether graft copolymers as well as polyurethane homopolymer for XPS measurements were prepared by casting Nfl-dimethylacetamide (DMA) solution (2.0 wt R) onto an aluminum sheet and evacuating for 48 h at room temperature and for more than 48 hat 60 OC. Samples of polyurethane-polyamine graft copolymers were prepared in the same way from DMA-chloroform (3/1,1/1, and 1/3 volume ratio with the increase of polyamine content) solutions. An air-side surface of the sample film was subjected to the XPS analysis. Polyether and polyamine homopolymer samples for the XPS measurement were prepared by casting THF solution (2.0 wt 5% and evacuating for 48 h at room temperature. The samples thus prepared were kept under dry
Tezuka et al.
2862 Langmuir, Vol. 7, No. 11, 1991
2
Ax2 A2
A'
u 405 400 N@. E .(eV 1
- u h L u L L -
290 285
535
530
405 400
%e. E .(eV 1 Figure 2. Cl,,Ol,,and N1, region XPS results for (1) polyurethane homopolymer, (2) polyether homopolymer,and (3a-c)polyurethane-polyether graft copolymer (sample A-9 in Table I). The escape angle of the photoelectron is given in 8. C6B.E.(eV1
qsB. E. (eV )
copolymer sample films was notably lower than that in the polyurethane homopolymer and decreased further a t smaller escape angles of photoelectrons, i.e., closer to the surface. Thus, the surface of a polyurethane-polyether graft copolymer with 16.5 wt 96 bulk polyether content was examined at the escape angles of 45" and 15' together with usual 90°, and the polyether component a t each surface was estimated from N1,/C1, signal intensity ratios with reference to that of the polyurethane homopolymer. The polyether component was found to be 62 wt 5% a t 90" detection and increased to 79 wt 5% a t 15' detection. The surface enrichment of the polyether component in polyurethane-polyether graft copolymers was also confirmed by inspection of the C1, signal region! where the shoulder peak due to the ether carbon a t 286.5 eV became distinct, and in turn, the carbonyl carbon signal at around 290 e\' became weaker nearer to the surface of the sample film. In contrast, the surface of polyurethanepolyamine graft copolymer samples wa9 found to he dominated bythepolyurethane component as indicated in Figure 3, where the signal intensity ratio of Cis, Ol,,and N1, of graft copolymer samples was almost identical to that of polyurethane homopolymer. Contact Angle Measurements. Contact angle measurements for the surface of polyurethane-polyether and polyurethane-polyamine graft copolymers were performed both in air and in water. In the dry state, a variety of liquid droplets, including water, ethylene glycol, n-octane,
Figure 3. C16,Oltrand N1, region XPS results for (1) polyurethane homopolymer, (2) polyamine homopolymer, and (3,4)polyurethane-polyamine graft copolymers (samplesB-4 (3) and B-6 (4) in Table I). Table 11. Contact Angles (Degrees) on Polyurethane-Polyamine Graft Copolymers samplea
H20
polyurethane
85f 1 96 f 3 90 dz 5 83 dz 4 82 f 6 80 f 1 85 f 2 84 dz 2
polyamine B-1 B-2 B-3 B-4 B-5 B-6
probe liquid 0.1NNaOH 81fl 97 f 1 94 f 3 78f3 75f3 83f 1 79f2 72f 1
0.1 N HC1 84 f 2 62 f 5 75f4 75i6 69f2 78 f 3 80f 1 81 f 2
See footnote a in Table I. and DMSO,were placed on polyurethane and polyether homopolymer surfaces, but failed to discriminate the two segment components in the present graft copolymers, presumably due to the similar surface tension of the two components, namely39 dyn/cm for polyurethane and 38.2 dyn/cm for p01yether.l~ On the other hand, polyurethane and polyamine homopolymer surfaces were differentiated by the application of a contact angle titration techniquem using a 0.1 N HCl droplet. As summarized in Table 11, the contact angle of a 0.1 N HC1 droplet on the polyurethane homopolymer surface was identical to that by either neutral water or 0.1 N NaOH, while the polyamine homopolymer showed a significantly higher wettability only against the 0.1 N HC1 droplet due to its high acid affinity. The surface of poly(19) Brandmp, J., Immergut, E.H.; Ed. Polymer Handbook, 3rd ed.; Wiley Interscience: New York, 1989. (20) Holmes-Farley,S. R.; Bain, C. D.; Whitesides, G. M. Langmuir 1988, 4, 921.
Langmuir, Vol. 7, No. 11, 1991 2863
Surface Responses of Polyurethane-Based Graft Copolymers urethane-polyamine graft copolymers was then monitored by the above three liquids and the results were also summarized in Table 11. Contact angles on polyurethane polyamine graft copolymer surfaces with the longer polyamine graft segments were insensitive to the change of liquid probes and were almost identical to those on polyurethane homopolymer irrespective of the polyamine content of up to around 50%. On the other hand, the surface on the graft copolymer films with the shorter polyamine graft segments and particularly with a high polyamine content showed high wettability against only the 0.1 N HC1 droplet. A series of graft copolymer sample films prepared on a glass plate were then immersed into deionized water thermostated a t 20 "C. After a variety of probe liquids were examined, CC4 was found to be a remarkably sensitive probe to monitor the nature of the surface component in graft copolymers in water. The contact angle of a CC4 droplet underwent a dynamic change from one similar to that of polyether homopolymer to one close to polyurethane homopolymer, while little change was observed on both polyurethane and polyether homopolymer surfaces. This change of contact angles is believed to correspond directly to the surface rearrangement process occurring on polyurethanepolyether graft copolymer films. The dynamics of this surface rearrangement process on a series of the polyurethane-polyether graft copolymers were followed and the results given in Figure 4. The dynamic change of contact angles was observed a t the surface of the most graft copolymer samples, while those with high polyether contents were found to be insensitive or very sluggish to respond to the change of the contacting medium from air to water. The contact angle of a CC4 droplet approached that of the polyurethane homopolymer within a 1-3-h period, and the longer the graft segment or the smaller the polyether content in the graft copolymer, the more rapidly the surface responded. Thus, the present surface response behavior is closely relevant to the previous polyurethane-polysiloxane block and graft copolymer systems.12J3 In contrast, the contact angle of a CC4 droplet on polyurethane-polyamine graft copolymers in water remained almost unchanged and close to that on the polyurethane homopolymer over a 3-h period of immersion, as shown in Figure 5. The XPS and contact angle observations on the dry surface of polyurethane-polyamine graft copolymers indicated that the polyurethane component dominated the graft copolymer surface and the surface is considered to be too rigid to rearrange the surface structure by responding to the change of the contacting phase from air to water.
Discussion The XPS and contact angle inspection on the surface of the two polyurethane-based graft copolymers indicated that one of the segment components accumulated a t the surface of cast film samples, namely, the polyether component in polyurethane-polyether graft copolymers, as was observed before in the relevant block-type copolymers,e8 and the polyurethane component in polyurethane-polyamine graft copolymers. In the previous polyurethane-polysiloxane block and graft copolymer systems, a significant surface accumulation of polysiloxane component was observed and the topmost surface was covered completely with polysiloxane as thick as several tens of angstrom~.'~J3This was rationalized by the fact that the surface tension of polysiloxane is much lower than that of polyurethane and that the two components are
120
c
A
26
Y
a
40
2o 00
30
60
90 120 150 180
T i me (min.)
120
i.
2ot 3b
6b
9b 1;O O 1; Time(min.1
lA0
-
2o 0'
30
60 90
I20 150 180
Time (min .)
Figure 4. Time vs contact angle variations on polyurethanepolyether graft copolymerfilms after immersion into water at 20 OC, measured by the CCL-in-watertechnique: (m)polyurethane homopolymerand (0) polyether homopolymerand polyurethanepolyether graft copolymers in (a, top) (A)A-1, (A) A-2, and (A) A-3; in (b, middle) ( 0 )A-4, (e) A-5, ( 0 )A-6, and (0) A-7; and in (c, bottom) (v)A-8, ( 0 )A-9, and (v)A-10 in Table I.
incompatible with each other. On the other hand, the surface tension values of polyether (38.2 dyn/cm) and polyurethane (39 dyn/cm) are close to each other and a certain degree of phase mixing of the two components will be envisaged in the present system. Nevertheless, the
Tezuka et a1.
2864 Langmuir, Vol. 7,No. 11, 1991 120
1
I
?p loo= 80
2 60
2o '!I
water
t 30
6b
sb
I I;O
lb0
j k
water
Figure 6. Schematic picture of a polyurethane-polyether graft copolymer surface in the dry and wet states. PU and PTHF
indicate polyurethane and polyether domains, respectively.
I60
T ime(min .I
Figure 5. Time vs contact angle variations on polyurethanepolyamine graft copolymer films after immersion into water at
20 O C , measured by the CCb-in-water technique: (m) polyurepolyamine homopolymer and polythane homopolymer and (0) B-2, (@)B-4, and urethane-polyamine graft copolymers of (0) ( 0 )B-6in Table I.
topmost surface layer formation with one component in the block copolymers consisting of segment components having almost identical surface tension values has been recognized as a common practice,21-26provided that the segment components are incompatible with each other.27 It is considered to be the case for the present polyurethanepolyether graft copolymer system, where the topmost surface is covered by the polyether component. On the other hand, the surface accumulation of the polyurethane component in polyurethane-polyamine graft copolymers might not be directly anticipated from the contact angle measurement with neutral water, where 96" for the polyamine and 85" for the polyurethane surfaces were obtained. This is indicative of the lower surface energy of the polyamine component, which might form the topmost surface layer. The high contact angle value on the present polyamine surface may be explained by the orientation of tert-butyl groups on the nitrogen along the polymer chain toward the air-surface. On the other hand, the polar nature of the polyamine segment could be operative when the polyamine segment is confined in the graft copolymer matrix to allow the polyurethane component to dominate a t the surface of the graft copolymer. A sea-island-type bulk structure shown in Figure 6 will be envisaged for polyurethane-polyether graft copolymers, as was observed in the relevant block-type copolymers reported by 0thers.~&~0 And the polyether domain, which has the size of, presumably, a few hundred angstroms and has a certain degree of phase mixing with polyurethane domains,28can spread over the surrounding polyurethane domain a t the surface region to form a practically pure (21) Hasegawa, H.; Hashimoto, T. Macromolecules 1985,18, 589. (22) Hasegawa, H.; Tanaka, H.; Yamasaki, K.; Hashimoto, T. Macromolecules 1987,20, 1651. (23) Henkee, C. S.; Thomas, E. L.; Fetters, L. J. J. Mater. Sci. 1988, 23, 1685. (24) Coulon, G.; Russell, T. P.; Deline, V. R.; Green, P. F. Macromolecules 1989,22, 2581. (25) Russell, T. P.; Coulon, G.; Deline, V. R.; Miller, D. C. Macromolecules 1989, 22, 4600. (26) Ishizu, K.; Yamada, Y.; Fukutomi, T. Polymer 1990, 31, 2047. (27) Green,P. F.; Christensen, T. M.; Russell, T. P.; Jerome, R. J. Chem. Phys. 1990, 92, 1478. (28) Wang, C. B.; Cooper, S. L. Macromolecules 1983,16, 775. (29) Li, C.; Cooper, S. L. Polymer 1990, 31, 3. (30) Takahara, A.; Tashita, J.; Kajiyama, T.; Takayanagi, M.; MacKnight, W. J. Polymer 1985, 26, 987.
polyether layer a t the topmost surface of the graft copolymer sample films. In contact with water, the surface of polyurethane-polyether graft copolymers responded to the change of the contacting medium within a few hours. The plausible paths of the surface rearrangement process are postulated in Figure 6, to be as follows: (a) the rearrangement of polyether domains a t the surface takes place to form a coagulated structure from the spreading one; or (b) the polyurethane domain coversthe polyether domains to form a practically pure polyurethane surface. The flexible nature of the polyether segments spreading over the polyurethane domain will be rationalized by the low Tgof polyether (-84 "C)rather than the measurement temperature of 20 "C. I t should be noted that the polyether segment in the graft copolymer matrix fails to crystallize due to the interaction with the polyurethane component through hydrogen bonding between the ether and urethane groups.16t2* The relevant flexible response of polyether graft segments was also noticed a t the surface of poly(vinyl alcohol)-poly(THF) graft copolymer systems.31 In the two plausible surface rearrangement processes depicted in Figure 6, we favor (a) since the sufficiently long polyether segments inside the domain are considered to retain their flexibility even though one end of the segments is bonded to the rigid polyurethane domains (Tg = 109 OC).I9 Thus, the polyurethane domain emerges through the coagulation of the polyether domain a t the surface, resulting in a contact angle in the CCb-in-water system close to that of the polyurethane homopolymer surface. The surface rearrangement process is believed to begin with the penetration of water through the surface covering layer of polyether component to form a polyurethanewater interface, and this should be more energetically favorable than the polyether-water interface. Thus, the principal driving force of this surface rearrangement process is considered to be the interfacial energy gap between the initial and final states of the interfaces. Hence, one may expect that the poly(viny1 alcohol)-poly(THF) graft copolymer, which possesses more hydrophilic segment components than polyurethane together with common polyether segments, can respond much more rapidly due to the greater energy gap between the initial and the final states of the interfaces. Indeed, the dry surface of this graft copolymer was covered with polyether component, and the rearrangement with the immersion into water was completed within 15-30 min as (31) Tezuka, Y.; Okabayashi, A.; Imai, K. J. Colloid Interface Sci. 1991,141, 586.
Surface Responses of Polyurethane-Based Graft Copolymers estimated by contact angle measurement with the air-inwater t e ~ h n i q u e . ~ ~ The surface of polyurethane-polyamine graft copolymer, on the other hand, was covered by the polyurethane component and was found to be insensitive to the change of the contacting medium from air to water. The rigid nature of the polyurethane component with high T,and/ or the formation of the thick polyurethane surface layer over the polyamine domain may account for the insensitivity of the surface region. In conclusion, the present study revealed, first, that the nature of the segment components in graft copolymers could exert a significant influence on the surface formation process during the film preparation and, second, that the surface response behavior could be controlled by the
Langmuir, Vol. 7, No. 11, 1991 2865 appropriate choice of the type of segment components and the structural parameters of the graft component, i.e., the graft segment length and the total graft content. Hence, the present results provide a new insight for the interpretation of the interaction between biological components and polymer material surfaces and, consequently, for the molecular design of polymer materials for a wide variety of biomedical applications.
Acknowledgment. We thank Prof. Y. Yoshida, Toyo University, Saitama, for kind cooperation in the XPS measurements. This work was supported partly by grants from the Ministry of Education, Science and Culture, Japan (02205050 and 03205051).
