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Monolayer formation of a novel amphiphilic polymer PAMC16S
839
Thin Solid Films, 210/211 (1992) 839-841
Monolayer T. R. Fang,
formation
of a novel amphiphilic polymer PAM&S
G. F. Xu, X. B. Zhu, Z. Tan, S. X. Xu and Z. Y. Jiang
Changchun Institute of Applied Chemistry, Academia Sinica, Changchun 330022 (People’s Republic of China)
Abstract Poly( 2-acrylamido-hexadecylsulfonic acid) (PAMC,,S) forms a stable monolayer on a pure water surface. More closely packed monolayers have been formed on gold monolayers can be obtained when the subphase contains Cd’+ or Ca*+. Self-assembled surfaces and characterized by contact angle measurement, XPS and electrochemical analysis. The results show that the monolayers are hydrophobic with the hydrophilic sulfonic acid groups adjacent to the metal surfaces and with the hydrocarbon chains extended from the surfaces. The monolayers exhibit great adsorption stability during the faradaic reactions, illustrating the advantage of polymeric LB films in potential applications.
1. Introduction A major drawback in the application of LB films made from low molecular amphiphiles is the inability to form a stable monolayer. In order for these films to exhibit good mechanical and thermal properties, they must be in a polymeric form. This goal could be reached by using either polymerizable amphiphilic monomers or prepolymers. It can be expected that the synthesis of tailored amphiphilic monomers and polymers will become more and more important [ 11. Ordered thin films can be obtained by either the Langmuir-Blodgett method (LB method) or the selfassembly technique (SA method). Organized monolayers by SA method of several organic amphiphiles have been studied [2-81, but there is little information about self-assembled monolayers of amphiphilic polymers. Poly( 2-acrylamido-hexadecylsulfonic acid) (PAMC,,S) is a new comb-type amphiphilic polymer, each repeat unit of which contains a hydrophilic sulfonic acid group, an amido group and a hydrophobic long aliphatic hydrocarbon chain. In this paper, the spreading behavior of PAMC,,S was examined, and the properties of self-assembled monolayers on gold surfaces were investigated by contact angle measurement, XPS and electrochemical analysis.
The spreading behavior of PAMC,6S was investigated on a Joyce-Loebl Langmuir trough with a constant perimeter tape at 20 “C. An Erma Anglemeter G-l (Japan) goniometer was used to measure the static contact angles. Chloroform and water were the solvents for spreading and contact angle measurement, respectively. XPS measurements were carried out on an ESCALAB Mk II (VG Company) photoelectrometer. The electrochemical analysis of gold electrodes was performed on a 175 universal programmer/l73 potentiostat/galvanstat (EG&G PARC) combined with a 3655 analyzing recorder (YEW).
3. Results and discussion 3.1. Spreading behavior of PAMC,,S Figure 1 shows the Z-A isotherms of PAMC,,S. For comparison, the 7c-A curve of the amphiphilic monomer, 2-acrylamidohexadecylsulfonic acid ( AMC, 6S), is also
z-40 ! ,, 2
‘30 ‘y :,
$20 :
2. Experimental
details
The synthesis of PAMC,,S has been described in a previous paper [ 11. The sample used here has an intrinsic viscosity of 20.3 ml/g (THF, 30 “C). The cleaning and anodization of gold plates were done in 0.1 M NaOH solution as used by Finklea et al. [3].
0040-6090/92/$5.00
IL-
p IO ‘t lZ 0
1 \
i
‘,
i
‘,
i
‘a,
i ! \ \
‘\ ‘\
‘\
\
‘\
‘.*.._
0.4
__
0.8
1.2
Arenlmonomer tnm*l
Fig. I. Surface pressure-area isotherms of PAMC,,S and AMC,,S. -, PAMC,,S, subphase pure water; - ‘- ‘- ‘, PAMC,,S, subphase I mM CdCI,; - - - -, AMC,,S, subphase 1 mM CdCI,.
0
1992 ~
Elsevier
Sequoia.
All rights
reserved
840
T. R. Fang et al. / Monolayer jormation of a novel amphiphilic polymer PAMCI6S
depicted. AMC16S is water-soluble, its monolayer cannot be formed on pure water. PAMCj6S is insoluble in water, indicating that the hydrophile-lipophile balance of PAMCI6S is shifted to less hydrophilicity relative to that of AMC16S, which may be a common rule between amphiphilic polymers and monomers. Clearly, PAMCj6S can form rather stable condensed solid films. The limiting area of PAMC,6S is strongly dependent on the nature of the subphase. Pure water results in a larger area/monomer (0.43 nm2). When the water subphase contains CdCl2, the limiting area/monomer is seen to decrease.
3.2. Organized films of PAMCI6 S by SA method The adsorption isotherms from chloroform solutions are monitored by measuring the static contact angles. In the case of clean or anodized gold surfaces, the adsorption plateau occurs when the concentration is beyond 0 . 4 m M of monomer units. Water does not wet the monolayer-covered gold surfaces, indicating that the films are hydrophobic with the hydrophilic sulfonic acid groups adjacent to the metal surfaces and with the hydrocarbon chains extended from the surfaces. This conclusion is supported by XPS measurements. Finklea et al. [3] have found that the Au20 thin layer on an anodized gold surface is unstable and will be stripped completely during the adsorption process. In our case, however, the Au4f XPS spectrum of the polymer-coated anodized gold surface (Fig. 2) shows the presence of Au +, although its intensity is decreased (intensity ratio of Au(O)/Au is 0.26 and 0.13 before and after the adsorption process, respectively, where Au(O) denotes the gold atoms bonded with oxygen). Also, the Oi5 peak of A u 2 0 (530.9 eV) can hardly be seen after the polymer monolayer is coated. This means that the gold oxide layer has not been stripped completely, because a part of it has reacted with the sulfonic acid groups of PAMCI6 S to form the gold sulfonate during the assembling process,
~
b
Binding energy ( eV I
Fig. 2. XPS spectra of Au4r of gold surfaces before and after adsorption (polymer concentration 0.5 mM monomer units; adsorption time 5 min). (a) Clean gold; (b) anodized gold; (c) polymer-coated gold; (d) polymer-coated anodized gold.
lO.O "E
8.0
~ 6.o -~4.o ~140
ZC
,bm ~b 4b sb 6b Scan number
Fig. 3. Plots of peak currents iv and peak potentials Ep versus scan number (0.1 M K4Fe(CN)6 and 0.1 M K3Fe(CN)6 in 1 M NazSO4; sweep rate, 50 mV/s; the right curves are the results of the experiments performed next day). (3, polymer-coated gold electrode; O, polymer-coated anodized gold electrode.
and that the sulfonic acid groups of PAMCI6S are close to the gold surfaces.
3.3. Electrochemical characterization Electrochemical analysis is a sensitive probe for a monolayer coated on an electrode. For the polymercoated electrodes, the peak currents decrease markedly for both forward and reverse scans, and the peak potentials are shifted to slightly larger overpotentials. The great peak current attenuation occurs on the first scan, just as in the case of octadecyltrichlorosilane [3]. Finklea et al. found that a second potential scan increased the peak currents and the peak potentials shifted to that of the uncoated gold electrodes, concluding that the octadecyltrichlorosilane monolayer was unstable and was destroyed rapidly during the faradaic process [3]. Thus, they performed only two potential scans and did not carry out any sweep-rate study. Because we believe that PAMC16S will show adsorption stability, at least to a certain degree, the cyclic voltammetric experiments have been continued. It is found that a plateau value of peak currents, which is about half that of the clean or anodized gold electrode, is reached (Fig. 3). The stability of PAMCI6S monolayers during the redox reaction makes possible further sweeprate study. After sweep-rate study, the polymer-coated gold electrode was washed with water and kept in a desiccator overnight, and then potential scan experiments were performed again (right curves in Fig. 3). The results reveal that the self-assembled monolayers of PAMCI6S exhibit great adsorption stability during the faradaic reactions. The results of the sweep-rate study show that the peak currents are proportional to the square root of the sweep-rate (S) for both clean and polymer-coated gold electrodes. Peak potentials are independent of S in the range of ! - 5 mV/s, and increase linearly with log S when S is in the range of 5-100mV/s. These data indicate that the polymer monolayer has not changed the pattern of the redox reaction, except for the reduced
T. R. Fang et al. / Monolayer formation of a novel amphiphilic polymer PAMC16S
peak currents a n d shifted peak potentials. Therefore, the resulting m o n o l a y e r - c o a t e d gold surfaces behave as stable blocked electrodes c o n t a i n i n g pinhole defects. The great a d s o r p t i o n stability of P A M C I 6 S m o n o l a y e r s d u r i n g the faradaic reactions illustrates the a d v a n t a g e of polymeric LB films in potential applications.
Acknowledgment F i n a n c i a l s u p p o r t by A c a d e m i a Sinica is acknowledged.
841
References 1 T. R. Fang and X. P. Zhu, Polymer Bull., 25(1991) 467. 2 R. Maoz and J. Sagiv, J. Colloid Interface Sci., lO0 (1984) 465. 3 H. O. Finklea, L. R. Robinson, A. Blackburn, B. Richter, D. Allara and T. Bright, Langmuir, 2 (1986) 239. 4 E. B. Troughton, C. D. Bain, G. M. Whitesides, R. G. Nuzzo, D. L. Allara and M. D. Porter, Langmuir, 4 (1988) 365. 5 S. H. Chen and C. W. Frank, Langmuir, 5 (1989) 978. 6 C. A. Widrig and M. Majda, Langmuir, 5 (1989) 689. 7 Th. Arndt, H. Schupp and W. Schrepp, Thin Solid Films, 178 (1989) 319. 8 T. M. Putvinski, M. L. Schilling, H. E. Katz, C. E. D. Chidsey, A. M. Mujsce and A. B. Emerson, Langmuir, 6 (1990) 1567.
Thin Solid Films, 210/211 (1992) 839-841
Monolayer T. R. Fang,
formation
of a novel amphiphilic polymer PAM&S
G. F. Xu, X. B. Zhu, Z. Tan, S. X. Xu and Z. Y. Jiang
Changchun Institute of Applied Chemistry, Academia Sinica, Changchun 330022 (People’s Republic of China)
Abstract Poly( 2-acrylamido-hexadecylsulfonic acid) (PAMC,,S) forms a stable monolayer on a pure water surface. More closely packed monolayers have been formed on gold monolayers can be obtained when the subphase contains Cd’+ or Ca*+. Self-assembled surfaces and characterized by contact angle measurement, XPS and electrochemical analysis. The results show that the monolayers are hydrophobic with the hydrophilic sulfonic acid groups adjacent to the metal surfaces and with the hydrocarbon chains extended from the surfaces. The monolayers exhibit great adsorption stability during the faradaic reactions, illustrating the advantage of polymeric LB films in potential applications.
1. Introduction A major drawback in the application of LB films made from low molecular amphiphiles is the inability to form a stable monolayer. In order for these films to exhibit good mechanical and thermal properties, they must be in a polymeric form. This goal could be reached by using either polymerizable amphiphilic monomers or prepolymers. It can be expected that the synthesis of tailored amphiphilic monomers and polymers will become more and more important [ 11. Ordered thin films can be obtained by either the Langmuir-Blodgett method (LB method) or the selfassembly technique (SA method). Organized monolayers by SA method of several organic amphiphiles have been studied [2-81, but there is little information about self-assembled monolayers of amphiphilic polymers. Poly( 2-acrylamido-hexadecylsulfonic acid) (PAMC,,S) is a new comb-type amphiphilic polymer, each repeat unit of which contains a hydrophilic sulfonic acid group, an amido group and a hydrophobic long aliphatic hydrocarbon chain. In this paper, the spreading behavior of PAMC,,S was examined, and the properties of self-assembled monolayers on gold surfaces were investigated by contact angle measurement, XPS and electrochemical analysis.
The spreading behavior of PAMC,6S was investigated on a Joyce-Loebl Langmuir trough with a constant perimeter tape at 20 “C. An Erma Anglemeter G-l (Japan) goniometer was used to measure the static contact angles. Chloroform and water were the solvents for spreading and contact angle measurement, respectively. XPS measurements were carried out on an ESCALAB Mk II (VG Company) photoelectrometer. The electrochemical analysis of gold electrodes was performed on a 175 universal programmer/l73 potentiostat/galvanstat (EG&G PARC) combined with a 3655 analyzing recorder (YEW).
3. Results and discussion 3.1. Spreading behavior of PAMC,,S Figure 1 shows the Z-A isotherms of PAMC,,S. For comparison, the 7c-A curve of the amphiphilic monomer, 2-acrylamidohexadecylsulfonic acid ( AMC, 6S), is also
z-40 ! ,, 2
‘30 ‘y :,
$20 :
2. Experimental
details
The synthesis of PAMC,,S has been described in a previous paper [ 11. The sample used here has an intrinsic viscosity of 20.3 ml/g (THF, 30 “C). The cleaning and anodization of gold plates were done in 0.1 M NaOH solution as used by Finklea et al. [3].
0040-6090/92/$5.00
IL-
p IO ‘t lZ 0
1 \
i
‘,
i
‘,
i
‘a,
i ! \ \
‘\ ‘\
‘\
\
‘\
‘.*.._
0.4
__
0.8
1.2
Arenlmonomer tnm*l
Fig. I. Surface pressure-area isotherms of PAMC,,S and AMC,,S. -, PAMC,,S, subphase pure water; - ‘- ‘- ‘, PAMC,,S, subphase I mM CdCI,; - - - -, AMC,,S, subphase 1 mM CdCI,.
0
1992 ~
Elsevier
Sequoia.
All rights
reserved
840
T. R. Fang et al. / Monolayer jormation of a novel amphiphilic polymer PAMCI6S
depicted. AMC16S is water-soluble, its monolayer cannot be formed on pure water. PAMCj6S is insoluble in water, indicating that the hydrophile-lipophile balance of PAMCI6S is shifted to less hydrophilicity relative to that of AMC16S, which may be a common rule between amphiphilic polymers and monomers. Clearly, PAMCj6S can form rather stable condensed solid films. The limiting area of PAMC,6S is strongly dependent on the nature of the subphase. Pure water results in a larger area/monomer (0.43 nm2). When the water subphase contains CdCl2, the limiting area/monomer is seen to decrease.
3.2. Organized films of PAMCI6 S by SA method The adsorption isotherms from chloroform solutions are monitored by measuring the static contact angles. In the case of clean or anodized gold surfaces, the adsorption plateau occurs when the concentration is beyond 0 . 4 m M of monomer units. Water does not wet the monolayer-covered gold surfaces, indicating that the films are hydrophobic with the hydrophilic sulfonic acid groups adjacent to the metal surfaces and with the hydrocarbon chains extended from the surfaces. This conclusion is supported by XPS measurements. Finklea et al. [3] have found that the Au20 thin layer on an anodized gold surface is unstable and will be stripped completely during the adsorption process. In our case, however, the Au4f XPS spectrum of the polymer-coated anodized gold surface (Fig. 2) shows the presence of Au +, although its intensity is decreased (intensity ratio of Au(O)/Au is 0.26 and 0.13 before and after the adsorption process, respectively, where Au(O) denotes the gold atoms bonded with oxygen). Also, the Oi5 peak of A u 2 0 (530.9 eV) can hardly be seen after the polymer monolayer is coated. This means that the gold oxide layer has not been stripped completely, because a part of it has reacted with the sulfonic acid groups of PAMCI6 S to form the gold sulfonate during the assembling process,
~
b
Binding energy ( eV I
Fig. 2. XPS spectra of Au4r of gold surfaces before and after adsorption (polymer concentration 0.5 mM monomer units; adsorption time 5 min). (a) Clean gold; (b) anodized gold; (c) polymer-coated gold; (d) polymer-coated anodized gold.
lO.O "E
8.0
~ 6.o -~4.o ~140
ZC
,bm ~b 4b sb 6b Scan number
Fig. 3. Plots of peak currents iv and peak potentials Ep versus scan number (0.1 M K4Fe(CN)6 and 0.1 M K3Fe(CN)6 in 1 M NazSO4; sweep rate, 50 mV/s; the right curves are the results of the experiments performed next day). (3, polymer-coated gold electrode; O, polymer-coated anodized gold electrode.
and that the sulfonic acid groups of PAMCI6S are close to the gold surfaces.
3.3. Electrochemical characterization Electrochemical analysis is a sensitive probe for a monolayer coated on an electrode. For the polymercoated electrodes, the peak currents decrease markedly for both forward and reverse scans, and the peak potentials are shifted to slightly larger overpotentials. The great peak current attenuation occurs on the first scan, just as in the case of octadecyltrichlorosilane [3]. Finklea et al. found that a second potential scan increased the peak currents and the peak potentials shifted to that of the uncoated gold electrodes, concluding that the octadecyltrichlorosilane monolayer was unstable and was destroyed rapidly during the faradaic process [3]. Thus, they performed only two potential scans and did not carry out any sweep-rate study. Because we believe that PAMC16S will show adsorption stability, at least to a certain degree, the cyclic voltammetric experiments have been continued. It is found that a plateau value of peak currents, which is about half that of the clean or anodized gold electrode, is reached (Fig. 3). The stability of PAMCI6S monolayers during the redox reaction makes possible further sweeprate study. After sweep-rate study, the polymer-coated gold electrode was washed with water and kept in a desiccator overnight, and then potential scan experiments were performed again (right curves in Fig. 3). The results reveal that the self-assembled monolayers of PAMCI6S exhibit great adsorption stability during the faradaic reactions. The results of the sweep-rate study show that the peak currents are proportional to the square root of the sweep-rate (S) for both clean and polymer-coated gold electrodes. Peak potentials are independent of S in the range of ! - 5 mV/s, and increase linearly with log S when S is in the range of 5-100mV/s. These data indicate that the polymer monolayer has not changed the pattern of the redox reaction, except for the reduced
T. R. Fang et al. / Monolayer formation of a novel amphiphilic polymer PAMC16S
peak currents a n d shifted peak potentials. Therefore, the resulting m o n o l a y e r - c o a t e d gold surfaces behave as stable blocked electrodes c o n t a i n i n g pinhole defects. The great a d s o r p t i o n stability of P A M C I 6 S m o n o l a y e r s d u r i n g the faradaic reactions illustrates the a d v a n t a g e of polymeric LB films in potential applications.
Acknowledgment F i n a n c i a l s u p p o r t by A c a d e m i a Sinica is acknowledged.
841
References 1 T. R. Fang and X. P. Zhu, Polymer Bull., 25(1991) 467. 2 R. Maoz and J. Sagiv, J. Colloid Interface Sci., lO0 (1984) 465. 3 H. O. Finklea, L. R. Robinson, A. Blackburn, B. Richter, D. Allara and T. Bright, Langmuir, 2 (1986) 239. 4 E. B. Troughton, C. D. Bain, G. M. Whitesides, R. G. Nuzzo, D. L. Allara and M. D. Porter, Langmuir, 4 (1988) 365. 5 S. H. Chen and C. W. Frank, Langmuir, 5 (1989) 978. 6 C. A. Widrig and M. Majda, Langmuir, 5 (1989) 689. 7 Th. Arndt, H. Schupp and W. Schrepp, Thin Solid Films, 178 (1989) 319. 8 T. M. Putvinski, M. L. Schilling, H. E. Katz, C. E. D. Chidsey, A. M. Mujsce and A. B. Emerson, Langmuir, 6 (1990) 1567.