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Obtaining thin films of “reactive polymers” on metal surfaces by electrochemical polymerization
J. Electroanal. Chem., 99 (1979) 331--340 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
331
OBTAINING THIN FILMS OF "REACTIVE POLYMERS" ON METAL SURFACES BY ELECTROCHEMICAL POLYMERIZATION PART II. ALCOHOL SUBSTITUTED POLYPHENYLENE OXIDE FILMS
MINH-CHAU PHAM, JACQUES-EMILE DUBOIS * and PIERRE-CAMILLE LACAZE
Institut de Topologie et de Dynamique des Systdmes, Universitd Paris VII, associd au C.N.R.S., 1, rue Guy de la Brosse, 75005 Paris (France) (Received 25th September 1978)
ABSTRACT
The selective electrochemical oxidation of the phenol function in the case of hydroxymethyl phenol derivatives (o-, m-, p-hydroxybenzyl alcohol) leads to "reactive p o l y m e r " films of polyphenylene oxide substituted by CH2OH groups. The transformation of the hydroxyl function into an ester function by acetyl chloride indicates the reactivity of the CH2OH group. As for the family of carbonylated polyphenylene oxide films, reactivity is limited to the superficial layers of film. Average film thickness is between 50--100 nm; however with the ferrocene-ferricinium system acting as a redox catalyst, it can reach about 300 nm. This catalytic mechanism intervenes only when the oxidation potential of t h e ferrocene-ferricinium couple is very similar to that of the phenol derivative.
INTRODUCTION
In prior work [1] we showed that polyphenylene oxide (PPO) type polymer films can be deposited on iron, copper, nickel, titanium, platinum and gold anodes by electrochemical oxidation of phenol derivatives. The characteristics of these films, i.e. uniform thickness between 50--100 nm, hydrophobicity, and strong adherence to the metal surface, indicate certain applications in the field of metal protection. In order to extend the area of use of these films we have sought to deposit, onto metal, "reactive polymer" films bearing reactive functional groups in the chain that axe capable of reacting according to a well-defined chemical reaction. It should be noted that it is very difficult to obtain such a reaction by chemical polymerization [2--4], We were able, for example, to prepare polyphenylene oxide films bearing reactive carbonyl functional groups [5] by selective oxidation of the phenol function of phenol derivatives since the carbonyl function remains intact during electrochemical polymerization. Indeed, the reactivity of these carbonyl groups on the polymer chain is comparable to that of simple ketones or aldehydes. It
* To whom correspondence should be addressed.
332 thus seems likely that this new procedure for obtaining reactive films where the implantation of carbonyl functional groups can be monitored presents numerous advantages over chemical surface treatment procedures for which the functional groups are distributed at random [6]. Similar results, obtained for --CH2OH substituted phenols, are presented here so as to prove the generality of the procedure [7]. A new family of "reactive polymers", whose basic structure is that of polyphenylene oxide, b u t which have --CH2OH functional groups, is obtained. Aside from the modification of the reactive functions we show that these films can be rendered thicker b y using a redox couple (ferrocene--ferricinium) that acts as catalyst in the phenol oxidation mechanism. EXPERIMENTAL Our study was restricted to highly pure commercial products: o-, m-, p-hyd r o x y b e n z y l alcohols (Aldrich). As previously, the polymer films were studied and analyzed b y voltammetry, polaromicrotribometry (p.m.t.), i.r. and x.p.s. [ 5]. Massive or thin-layer Fe, Cu, Pt or Ni electrodes were used to deposit the films. All potentials were measured with respect to a calomel reference electrode saturated with KC1 in methanol. For multiple reflection i.r. spectra, the electrodes were plane metallic mirrors prepared by sputtering according to a previously described process [8]. --CH2OH group reactivity was tested b y dipping the film in pure acetyl chloride (Prolabo). RESULTS
Experiments were conducted with o-, m-, p - h y d r o x y b e n z y l alcohol (0.1 M) in a methanol solution with 0 . 3 M sodium hydroxide. Since the oxidation mechanism of all three alcohols are very similar, discussion of the m o d e of formation of the polyphenylene oxide (PPO) t y p e polymer films is restricted to the study of m-hydroxybenzyl alcohol which leads to the most p r o n o u n c e d passivation phenomena. The results indicate a certain number of differences which depend on the m o n o m e r and on the metal and are mostly expressed b y variations in film thickness and b y a fairly strong adherence to the substrate. Formation of these films is generally accompanied b y a decrease in current intensity that corresponds to partial passivation of the electrode surface.
Polaromicrotribometric (p.m.t.) study of polymer film formation: m-hydroxybenzyl alcohol Change in the electrode surface during electrolysis was followed b y polaromicrotribometry (p.m.t.) which is based on the simultaneous recording of current/potential curves and of variations in a dynamic friction coefficient that
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.
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Fig. 1. C u r r e n t / p o t e n t i a l a n d f r i c t i o n / p o t e n t i a l curves s i m u l t a n e o u s l y r e c o r d e d d u r i n g m - h y d r o x y b e n z y l a l c o h o l o x i d a t i o n o n a Pt e l e c t r o d e , m - H y d r o x y b e n z y l a l c o h o l c o n c e n t r a t i o n = 0.1 M, s o d i u m h y d r o x i d e c o n c e n t r a t i o n = 0.3 M, s w e e p speed = 3 0 0 m V rain -1 .
are characteristic of the metal surface [9,10]. In the oxidation of m - h y d r o x y b e n z y l alcohol on a platinum electrode, the p.m.t, curve recorded during a linear potential sweep {rate = 300 mV min -') is shown in Fig. 1. The oxidation current begins increasing at 0.2 V, reaches a m a x i m u m at around 0.35 V (im,x = 0.78 mA cm-2), decreases to imin = 0.06 mA cm -~ and remains fairly constant at this low density between 0.6--1.1 V. Beyond 1.2 V, a significant increase in current which in all likelihood corresponds to OH- ion discharge is observed. At the same time, as soon as the current reaches 0.5 V, the friction coefficient increases from 0.3 to 0.4 and thereafter remains constant. When polarization of the electrode is reduced to 0 V, the current remains at a very low positive value and the friction coefficient remains steady. In contrast, a second sweep of the anode between 0 and 1.2 V brings about a new oxidation peak at 0.35 V t h a t has the same characteristics as above. The p h e n o m e n a observed are very different when the range of polarization is restricted to 0--1.1 V. After obtaining the oxidation peak of phenol at 0.35 V, stopping the anodic sweep at 1.1 V, and returning to potential 0, a second anodic sweep carried out at 0--1.1 V no longer brings about the phenol oxidation peak (Fig. 2). This indicates t h a t film porosity varies according to the range of potential explored. Beyond 1.2 V, OH- ion discharge probably increases film porosity and the film becomes permeable to ¢ O - ions. During all experiments, after the first oxidation peak, the friction coefficient remains steady and the curve becomes very regular, thereby indicating t h a t friction occurs on a very homogeneous surface other than Pt. Examination of the Pt surface after an anodic sweep between 0 and 1.2 V, followed by rinsing in ultrasonically stirred methanol, shows t h a t a yellowish
334
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Fig. 2. Consecutive current/potential curves (a --> b ~ c) during m-hydroxybenzyl alcohol oxidation on a Pt electrode. TABLE 1 Oxidation potentials of 0-, m-, p - h y d r o x y b e n z y l alcohol on massive electrodes (Fe, Cu, Pt, Ni) in a methanol-NaOH 0.3 M medium Monomers
Formula
Oxidation potential Eox/V (SCE)a Fe
Cu
Pt
Ni
o-Hydroxybenzyl alcohol
oH [~CH~OH
0.40
0.22
0.20
0.22
m-Hydroxybenzyl alcohol
OH ~--CH20H
0.45
0.22
0.23
0.28
0.49
0.24
0.16
0.31
p-Hydroxybenzyl alcohol
oN ~oH
a Values of oxidation potentials shown in this Table correspond to the b o t t o m of the waves of oxidation.
335
film has formed. This film is very adherent and cannot be removed by peeltesting with Scotch tape. Similar results are observed with o- and p - h y d r o x y b e n z y l alcohol on different metals. The films formed on Fe, Cu and Ni are less passivating and are obtained at higher oxidation potentials than those formed on Pt (Table 1). A non-negligible oxidation current is observed after the passivation peak attributed to the phenol group has appeared. Whereas the imin/ima x ratio is close to zero in the case of Pt, it is around 0.8 for Fe, 0.7 for Cu, and 0.3 for Ni.
Identification of polymer films Multiple-reflection i.r. absorption spectroscopy and the x.p.s, technique were used to identify the alcohol substituted polyphenylene oxide films formed on the electrodes.
I.r. spectroscopy. The spectra of all of the films formed have an intense absorption band at around 1230 c m - ' which can be attributed to the C--O--C polyoxide group previously observed with PPO films [1]. Bands characteristic of the alcohol function are also found between 2900--3400 cm -1 (depending on the type of substitution): this indicates t h a t the m o n o m e r alcohol function is not attacked during electrolysis. The spectra of the polymer obtained f r o m m-hydroxybenzyl alcohol are shown in Fig. 3. It should be noted that the spectra of the polymers obtained from o° and m-hydroxybenzyl alcohol are identical. This leads to the conclusion that, in both of these cases, the electrochemical polymerization which occurs via a previously discussed radical-ion mechanism [1 ] results from a coupling of the phenyl nuclei in the para position with respect to the phenol function.
X-ray photoelectron spectroscopy. The nature of the films deposited on the electrodes was also confirmed by x.p.s. For Czs the spectra indicate two distinct chemical groups:
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3000
2000
1800
1600
1/-,-00
1200
1000
v/cm -1
Fig. 3. I.r. s p e c t r u m of t h e p o l y m e r film o b t a i n e d f r o m m - h y d r o x y b e n z y l a l c o h o l o n Pt a l i n e a r p o t e n t i a l s w e e p b e t w e e n 0 a n d 2 V (SCE) in a m e t h a n o l - N a O H 0.3 M m e d i u m .
by
336 a simple C--C bond blended with the reference contamination carbon at 285 eV [11]; the C--O--C polyoxide bond at 286.60 eV, previously observed for carbonylated PPO films [5].
Reactivity of polymer films: esterification of the alcohol function of polyphenylene oxide The alcohol function of the above-mentioned polymers reacts with acetyl chloride and becomes an ester function, as per the following scheme: .~..n
.~..n
CH2_O_~_CH3
CH20H
The formation of this ester function was identified by i.r.spectroscopy and x.p.s. Comparison of i.r. sample spectra of polymer films obtained from the various h y d r o x y b e n z y l alcohols with the i.r. spectra of these same polymers treated with acetyl chloride shows that there is a new very intense absorption band at 1735 cm-1 which corresponds to the ester group. The intensity of the absorption band at around 3400 c m - ' has decreased considerably, b u t retains a strong value. This is a surface reactivity effect similar to that observed for carbonylated polyphenylene oxide [5] where the chemical transformation of the film is a surface reaction and not a volume reaction. The identical spectra of the film obtained b y polymerization of o- and mh y d r o x y b e n z y l alcohol and treated with acetyl chloride (Fig. 4} confirm the fact that the ortho and meta monomers lead to the same polymer. Aside from the t w o peaks at 285 and 286.60 eV indicating respectively a
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1200
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Fig. 4. I.r. spectrum of the polymer film obtained from m-hydroxybenzyl alcohol after treatment with acetyl chloride.
337
simple C--C bond and a C--O--C polyoxide bond, the x.p.s, spectrum for Cls also shows a peak at 289.2 eV which is characteristic of the ester function. Absolutely no chlorine is detected in any of the treated films.
Film thickening The films formed b y polymerization of o-, m-, p - h y d r o x y b e n z y l alcohols after a single sweep of potential ranging from 0 to 1.2 V have a uniform thickness of a b o u t 50 nm. When the electrode is subjected to several sweep cycles between 0 and 1.5 V, the thickness can reach a b o u t 150 nm. These results confirm the fact that, b e y o n d 1.2 V, film porosity increases, thereby enhancing thickening. Film thickening can also be achieved when oxidation is carried o u t in the presence of ferrocene (5 × 10 -3 M) after four sweep cycles b e t w e e n 0 and 2 V (sweep rate 3 V min-~). In the case of m - h y d r o x y b e n z y l alcohol, film thickness surpasses 300 nm. In comparison to previous films obtained in the absence of ferrocene, its behavior in cyclic voltammetry is very different. When a cyclic potential sweep b e t w e e n 0.25 and 0.6 V is carried o u t in an aqueous solution of HC1 (0.1 M) and KC1 (0.1 M) under argon with the Pt electrode coated b y the film formed in the presence of ferrocene, a reversible redox system with an oxidation peak at 0.47 + 0.02 V (SCE) is observed (Fig. 5). The difference between the oxidation peak Epa and the reduction peak Epc is less than 30 mV. The overall results tend to prove that the Fc is partially adsorbed at the electrode surface [12,13]. After 900 sweep cycles at a rate of 200 mV s -1, there is only a single decrease in peak current of a b o u t 10% in comparison to the initial state. The anchoring of the system on the electrode is partly enhanced b y the non-solubility of the Fc/Fc ÷ system in an aqueous medium. Integration of these peaks shows that the amounts of anodic current (Qa) and
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E/V vs.SCE
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-
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0.4,. Fig. 5. Cyclic v o l t a m m o g r a m s o f a film c o a t e d Pt e l e c t r o d e o b t a i n e d by p o l y m e r i z a t i o n o f m - h y d r o x y b e n z y l alcohol in the presence of f e r r o c e n e (scan rate 200 m V s - l ). T h e e l e c t r o l y t e solution is an a q u e o u s solution of HC1 0.1 M and KCI 0.1 M under Ar at 298°K.
338
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339 cathodic current (Qc) are equal, i.e. Qa = Qc = 0.25 pC cm -2. The a m o u n t of ferrocene involved at the electrode is very small (0.25 × 10 -~' mol cm -2) and represents about 0.005 of the ferrocene monolayer (assuming that a monolayer corresponds to a concentration of 5 × 10 -~° mol cm -2) [14]. We believe that, under these conditions, polymerization is initiated by two mechanisms: direct oxidation of the phenol group and catalytic initiation by the ferricinium ion, in accordance with the following scheme: Fc -- e- -+ Fc ÷ Fc ÷ + cPO- -~ Fc + cPO" ncPO" -+ polymer For both processes to occur simultaneously, the phenol and ferrocene oxidation potentials must be very similar to each other. This is verified for the three h y d r o x y b e n z y l alcohols (o-, m- and p-) as well as for 3,5-xylenol and 4-(phydroxyphenyl)-2-butanone. When the potential for phenol oxidation is less than that for ferrocene, polymer film formation is the result of direct oxidation of the phenol group and prevents oxidation of the ferrocene. In this case, the system has the same electrochemical behavior as a phenol solution in the absence of ferrocene; this is, for example, the case of 2,6-xylenol whose oxidation potential is --0.02 V, whereas for Fc it is 0.22 V (Table 2). When the Fc/Fc ÷ system participates in the catalytic initiation of the phenol group, the polymer film thickens and the redox couple characterized by an oxidation peak potential of 0.47 + 0.02 V (SCE) is f o u n d in the film. CONCLUSION
The overall results described in this work show t h a t the electrochemical oxidation of polyfunctional phenol-alcohol compounds is selective and leads to polyphenylene oxide type polymer films with regularly distributed CH2OH groups on the polymer chain. Their reactivity is comparable to t h a t of the CH2OH groups of the monomer; however, as is the case for carbonyl substituted polyphenylene oxide films, surface reactivity is restricted to the initial layers of film. Elaboration of this new series of polymers, together with the possibility of thickening t h e m through use of the F c / F c + system, affords a glimpse o f new applications for reactive polymer films. Their good adherence on metal surfaces also makes it possible to conceive a more general approach to functionalized electrodes. A preliminary study of the electrochemical properties of redox couples on an electrode coated with h y d r o x y l a t e d polyphenylene oxide films has yielded interesting results which will be reported in a forthcoming article. REFERENCES 1 F. Bruno, M.C. Pham and J.E. Dubois, Electrochim. Acta, 22 (1977) 451. 2 P. Ferru ti in J.A. Moore (Ed.), Reactions on polymers, NATO Advanced S t u d y I n s t i t u t e s Series, 1973~ p p . 73--77.
340 3 T. Alfrey, Jr. in E.M. Fettes (Ed.), Chemical Reactions on Polymers, Interscience, N e w York, 1964, p. 2 4 8 . 4 G.D. J o n e s in E.M. F e t t e s (Ed.), Chemical R e a c t i o n s o n P o l y m e r s , I n t e r s c i e n c e , New Y o r k , 1 9 6 4 , pp. 2 5 0 - - 2 5 5 . 5 M.C. P h a m , P.C. Lacaze a n d J.E. Dubois, J. E l e c t r o a n a l . Chem., 8 6 ( 1 9 7 8 ) 147. 6 K o i e h i r o K a t o , J. Applied P o l y m e r Sci., 19 ( 1 9 7 5 ) 9 5 1 . 7 A E u r o p e a n p a t e n t (No. 7 8 - 4 - 0 0 0 0 1 - 0 ) a n d an A m e r i c a n p a t e n t (No. 9 1 2 - 7 4 0 ) have b e e n applied f o r G. T o t ~ i l l o n , J.E. D u b o i s a n d P.C. Laeaze, J. Chim. Phys., 7 4 ( 1 9 7 7 ) 685. 8 J.E. D u b o i s , P.C. Laeaze, R. Courtel, C.C. H e r m a n n a n d D. Maugis, J. E l e c t r o c h e m . Soc., 1 2 2 ( 1 9 7 5 ) 9 1454. M. Delamaz, P.C. Laeaze a n d J.E. Dubois, J. Chim. Phys., 75 ( 1 9 7 8 ) 182. 10 J.P. C o n t o u r a n d G. Mouvier, J. E l e c t r o n S p e c t r o s c . , 7 ( 1 9 7 5 ) 85. 11 E. Laviron, J. E l e c t r o a n a l . C h e m . , 39 ( 1 9 7 2 ) 1. 12 R . F . L a n e a n d A.T. H u b b a r d , J. Phys. C h e m . , 77 ( 1 9 7 3 ) 1 4 0 1 ; 81 ( 1 9 7 7 ) 734. 13 M.S. W r i g h t o n , R.G. Austin, A.B. B o c a r s l y , J.M. Bolts, O. Haas, K.D. Legg, L. Nadjo a n d M.C. 14 P a l a z z o t t o , J. E l e c t r o a n a L Chem., 87 ( 1 9 7 8 ) 4 2 9 .
331
OBTAINING THIN FILMS OF "REACTIVE POLYMERS" ON METAL SURFACES BY ELECTROCHEMICAL POLYMERIZATION PART II. ALCOHOL SUBSTITUTED POLYPHENYLENE OXIDE FILMS
MINH-CHAU PHAM, JACQUES-EMILE DUBOIS * and PIERRE-CAMILLE LACAZE
Institut de Topologie et de Dynamique des Systdmes, Universitd Paris VII, associd au C.N.R.S., 1, rue Guy de la Brosse, 75005 Paris (France) (Received 25th September 1978)
ABSTRACT
The selective electrochemical oxidation of the phenol function in the case of hydroxymethyl phenol derivatives (o-, m-, p-hydroxybenzyl alcohol) leads to "reactive p o l y m e r " films of polyphenylene oxide substituted by CH2OH groups. The transformation of the hydroxyl function into an ester function by acetyl chloride indicates the reactivity of the CH2OH group. As for the family of carbonylated polyphenylene oxide films, reactivity is limited to the superficial layers of film. Average film thickness is between 50--100 nm; however with the ferrocene-ferricinium system acting as a redox catalyst, it can reach about 300 nm. This catalytic mechanism intervenes only when the oxidation potential of t h e ferrocene-ferricinium couple is very similar to that of the phenol derivative.
INTRODUCTION
In prior work [1] we showed that polyphenylene oxide (PPO) type polymer films can be deposited on iron, copper, nickel, titanium, platinum and gold anodes by electrochemical oxidation of phenol derivatives. The characteristics of these films, i.e. uniform thickness between 50--100 nm, hydrophobicity, and strong adherence to the metal surface, indicate certain applications in the field of metal protection. In order to extend the area of use of these films we have sought to deposit, onto metal, "reactive polymer" films bearing reactive functional groups in the chain that axe capable of reacting according to a well-defined chemical reaction. It should be noted that it is very difficult to obtain such a reaction by chemical polymerization [2--4], We were able, for example, to prepare polyphenylene oxide films bearing reactive carbonyl functional groups [5] by selective oxidation of the phenol function of phenol derivatives since the carbonyl function remains intact during electrochemical polymerization. Indeed, the reactivity of these carbonyl groups on the polymer chain is comparable to that of simple ketones or aldehydes. It
* To whom correspondence should be addressed.
332 thus seems likely that this new procedure for obtaining reactive films where the implantation of carbonyl functional groups can be monitored presents numerous advantages over chemical surface treatment procedures for which the functional groups are distributed at random [6]. Similar results, obtained for --CH2OH substituted phenols, are presented here so as to prove the generality of the procedure [7]. A new family of "reactive polymers", whose basic structure is that of polyphenylene oxide, b u t which have --CH2OH functional groups, is obtained. Aside from the modification of the reactive functions we show that these films can be rendered thicker b y using a redox couple (ferrocene--ferricinium) that acts as catalyst in the phenol oxidation mechanism. EXPERIMENTAL Our study was restricted to highly pure commercial products: o-, m-, p-hyd r o x y b e n z y l alcohols (Aldrich). As previously, the polymer films were studied and analyzed b y voltammetry, polaromicrotribometry (p.m.t.), i.r. and x.p.s. [ 5]. Massive or thin-layer Fe, Cu, Pt or Ni electrodes were used to deposit the films. All potentials were measured with respect to a calomel reference electrode saturated with KC1 in methanol. For multiple reflection i.r. spectra, the electrodes were plane metallic mirrors prepared by sputtering according to a previously described process [8]. --CH2OH group reactivity was tested b y dipping the film in pure acetyl chloride (Prolabo). RESULTS
Experiments were conducted with o-, m-, p - h y d r o x y b e n z y l alcohol (0.1 M) in a methanol solution with 0 . 3 M sodium hydroxide. Since the oxidation mechanism of all three alcohols are very similar, discussion of the m o d e of formation of the polyphenylene oxide (PPO) t y p e polymer films is restricted to the study of m-hydroxybenzyl alcohol which leads to the most p r o n o u n c e d passivation phenomena. The results indicate a certain number of differences which depend on the m o n o m e r and on the metal and are mostly expressed b y variations in film thickness and b y a fairly strong adherence to the substrate. Formation of these films is generally accompanied b y a decrease in current intensity that corresponds to partial passivation of the electrode surface.
Polaromicrotribometric (p.m.t.) study of polymer film formation: m-hydroxybenzyl alcohol Change in the electrode surface during electrolysis was followed b y polaromicrotribometry (p.m.t.) which is based on the simultaneous recording of current/potential curves and of variations in a dynamic friction coefficient that
333 f
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.
1
.
.
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Fig. 1. C u r r e n t / p o t e n t i a l a n d f r i c t i o n / p o t e n t i a l curves s i m u l t a n e o u s l y r e c o r d e d d u r i n g m - h y d r o x y b e n z y l a l c o h o l o x i d a t i o n o n a Pt e l e c t r o d e , m - H y d r o x y b e n z y l a l c o h o l c o n c e n t r a t i o n = 0.1 M, s o d i u m h y d r o x i d e c o n c e n t r a t i o n = 0.3 M, s w e e p speed = 3 0 0 m V rain -1 .
are characteristic of the metal surface [9,10]. In the oxidation of m - h y d r o x y b e n z y l alcohol on a platinum electrode, the p.m.t, curve recorded during a linear potential sweep {rate = 300 mV min -') is shown in Fig. 1. The oxidation current begins increasing at 0.2 V, reaches a m a x i m u m at around 0.35 V (im,x = 0.78 mA cm-2), decreases to imin = 0.06 mA cm -~ and remains fairly constant at this low density between 0.6--1.1 V. Beyond 1.2 V, a significant increase in current which in all likelihood corresponds to OH- ion discharge is observed. At the same time, as soon as the current reaches 0.5 V, the friction coefficient increases from 0.3 to 0.4 and thereafter remains constant. When polarization of the electrode is reduced to 0 V, the current remains at a very low positive value and the friction coefficient remains steady. In contrast, a second sweep of the anode between 0 and 1.2 V brings about a new oxidation peak at 0.35 V t h a t has the same characteristics as above. The p h e n o m e n a observed are very different when the range of polarization is restricted to 0--1.1 V. After obtaining the oxidation peak of phenol at 0.35 V, stopping the anodic sweep at 1.1 V, and returning to potential 0, a second anodic sweep carried out at 0--1.1 V no longer brings about the phenol oxidation peak (Fig. 2). This indicates t h a t film porosity varies according to the range of potential explored. Beyond 1.2 V, OH- ion discharge probably increases film porosity and the film becomes permeable to ¢ O - ions. During all experiments, after the first oxidation peak, the friction coefficient remains steady and the curve becomes very regular, thereby indicating t h a t friction occurs on a very homogeneous surface other than Pt. Examination of the Pt surface after an anodic sweep between 0 and 1.2 V, followed by rinsing in ultrasonically stirred methanol, shows t h a t a yellowish
334
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0.5
500
1000
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1500
Fig. 2. Consecutive current/potential curves (a --> b ~ c) during m-hydroxybenzyl alcohol oxidation on a Pt electrode. TABLE 1 Oxidation potentials of 0-, m-, p - h y d r o x y b e n z y l alcohol on massive electrodes (Fe, Cu, Pt, Ni) in a methanol-NaOH 0.3 M medium Monomers
Formula
Oxidation potential Eox/V (SCE)a Fe
Cu
Pt
Ni
o-Hydroxybenzyl alcohol
oH [~CH~OH
0.40
0.22
0.20
0.22
m-Hydroxybenzyl alcohol
OH ~--CH20H
0.45
0.22
0.23
0.28
0.49
0.24
0.16
0.31
p-Hydroxybenzyl alcohol
oN ~oH
a Values of oxidation potentials shown in this Table correspond to the b o t t o m of the waves of oxidation.
335
film has formed. This film is very adherent and cannot be removed by peeltesting with Scotch tape. Similar results are observed with o- and p - h y d r o x y b e n z y l alcohol on different metals. The films formed on Fe, Cu and Ni are less passivating and are obtained at higher oxidation potentials than those formed on Pt (Table 1). A non-negligible oxidation current is observed after the passivation peak attributed to the phenol group has appeared. Whereas the imin/ima x ratio is close to zero in the case of Pt, it is around 0.8 for Fe, 0.7 for Cu, and 0.3 for Ni.
Identification of polymer films Multiple-reflection i.r. absorption spectroscopy and the x.p.s, technique were used to identify the alcohol substituted polyphenylene oxide films formed on the electrodes.
I.r. spectroscopy. The spectra of all of the films formed have an intense absorption band at around 1230 c m - ' which can be attributed to the C--O--C polyoxide group previously observed with PPO films [1]. Bands characteristic of the alcohol function are also found between 2900--3400 cm -1 (depending on the type of substitution): this indicates t h a t the m o n o m e r alcohol function is not attacked during electrolysis. The spectra of the polymer obtained f r o m m-hydroxybenzyl alcohol are shown in Fig. 3. It should be noted that the spectra of the polymers obtained from o° and m-hydroxybenzyl alcohol are identical. This leads to the conclusion that, in both of these cases, the electrochemical polymerization which occurs via a previously discussed radical-ion mechanism [1 ] results from a coupling of the phenyl nuclei in the para position with respect to the phenol function.
X-ray photoelectron spectroscopy. The nature of the films deposited on the electrodes was also confirmed by x.p.s. For Czs the spectra indicate two distinct chemical groups:
I
8O
60
I
3000
2000
1800
1600
1/-,-00
1200
1000
v/cm -1
Fig. 3. I.r. s p e c t r u m of t h e p o l y m e r film o b t a i n e d f r o m m - h y d r o x y b e n z y l a l c o h o l o n Pt a l i n e a r p o t e n t i a l s w e e p b e t w e e n 0 a n d 2 V (SCE) in a m e t h a n o l - N a O H 0.3 M m e d i u m .
by
336 a simple C--C bond blended with the reference contamination carbon at 285 eV [11]; the C--O--C polyoxide bond at 286.60 eV, previously observed for carbonylated PPO films [5].
Reactivity of polymer films: esterification of the alcohol function of polyphenylene oxide The alcohol function of the above-mentioned polymers reacts with acetyl chloride and becomes an ester function, as per the following scheme: .~..n
.~..n
CH2_O_~_CH3
CH20H
The formation of this ester function was identified by i.r.spectroscopy and x.p.s. Comparison of i.r. sample spectra of polymer films obtained from the various h y d r o x y b e n z y l alcohols with the i.r. spectra of these same polymers treated with acetyl chloride shows that there is a new very intense absorption band at 1735 cm-1 which corresponds to the ester group. The intensity of the absorption band at around 3400 c m - ' has decreased considerably, b u t retains a strong value. This is a surface reactivity effect similar to that observed for carbonylated polyphenylene oxide [5] where the chemical transformation of the film is a surface reaction and not a volume reaction. The identical spectra of the film obtained b y polymerization of o- and mh y d r o x y b e n z y l alcohol and treated with acetyl chloride (Fig. 4} confirm the fact that the ortho and meta monomers lead to the same polymer. Aside from the t w o peaks at 285 and 286.60 eV indicating respectively a
I
$0
4
0 3 000
2000
1B00
-C-~C1600
1400
1200
1000
1)//C rn -1
Fig. 4. I.r. spectrum of the polymer film obtained from m-hydroxybenzyl alcohol after treatment with acetyl chloride.
337
simple C--C bond and a C--O--C polyoxide bond, the x.p.s, spectrum for Cls also shows a peak at 289.2 eV which is characteristic of the ester function. Absolutely no chlorine is detected in any of the treated films.
Film thickening The films formed b y polymerization of o-, m-, p - h y d r o x y b e n z y l alcohols after a single sweep of potential ranging from 0 to 1.2 V have a uniform thickness of a b o u t 50 nm. When the electrode is subjected to several sweep cycles between 0 and 1.5 V, the thickness can reach a b o u t 150 nm. These results confirm the fact that, b e y o n d 1.2 V, film porosity increases, thereby enhancing thickening. Film thickening can also be achieved when oxidation is carried o u t in the presence of ferrocene (5 × 10 -3 M) after four sweep cycles b e t w e e n 0 and 2 V (sweep rate 3 V min-~). In the case of m - h y d r o x y b e n z y l alcohol, film thickness surpasses 300 nm. In comparison to previous films obtained in the absence of ferrocene, its behavior in cyclic voltammetry is very different. When a cyclic potential sweep b e t w e e n 0.25 and 0.6 V is carried o u t in an aqueous solution of HC1 (0.1 M) and KC1 (0.1 M) under argon with the Pt electrode coated b y the film formed in the presence of ferrocene, a reversible redox system with an oxidation peak at 0.47 + 0.02 V (SCE) is observed (Fig. 5). The difference between the oxidation peak Epa and the reduction peak Epc is less than 30 mV. The overall results tend to prove that the Fc is partially adsorbed at the electrode surface [12,13]. After 900 sweep cycles at a rate of 200 mV s -1, there is only a single decrease in peak current of a b o u t 10% in comparison to the initial state. The anchoring of the system on the electrode is partly enhanced b y the non-solubility of the Fc/Fc ÷ system in an aqueous medium. Integration of these peaks shows that the amounts of anodic current (Qa) and
~, Ok. '-~"~ 0.2. O. 02,
E/V vs.SCE
~,
V
/
-
,, :l~cyc[e
. . . . : 900fhcqcte
0.4,. Fig. 5. Cyclic v o l t a m m o g r a m s o f a film c o a t e d Pt e l e c t r o d e o b t a i n e d by p o l y m e r i z a t i o n o f m - h y d r o x y b e n z y l alcohol in the presence of f e r r o c e n e (scan rate 200 m V s - l ). T h e e l e c t r o l y t e solution is an a q u e o u s solution of HC1 0.1 M and KCI 0.1 M under Ar at 298°K.
338
~
0
0
0
0
T
0
co
0
V
e~
o e~
M
o
0
0 e~
0
0 t~
'r. u 0
~ U
e~
I 0
0
~
~
o
U
U
U
.~
I 0
r~
U
.
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N
M 0
339 cathodic current (Qc) are equal, i.e. Qa = Qc = 0.25 pC cm -2. The a m o u n t of ferrocene involved at the electrode is very small (0.25 × 10 -~' mol cm -2) and represents about 0.005 of the ferrocene monolayer (assuming that a monolayer corresponds to a concentration of 5 × 10 -~° mol cm -2) [14]. We believe that, under these conditions, polymerization is initiated by two mechanisms: direct oxidation of the phenol group and catalytic initiation by the ferricinium ion, in accordance with the following scheme: Fc -- e- -+ Fc ÷ Fc ÷ + cPO- -~ Fc + cPO" ncPO" -+ polymer For both processes to occur simultaneously, the phenol and ferrocene oxidation potentials must be very similar to each other. This is verified for the three h y d r o x y b e n z y l alcohols (o-, m- and p-) as well as for 3,5-xylenol and 4-(phydroxyphenyl)-2-butanone. When the potential for phenol oxidation is less than that for ferrocene, polymer film formation is the result of direct oxidation of the phenol group and prevents oxidation of the ferrocene. In this case, the system has the same electrochemical behavior as a phenol solution in the absence of ferrocene; this is, for example, the case of 2,6-xylenol whose oxidation potential is --0.02 V, whereas for Fc it is 0.22 V (Table 2). When the Fc/Fc ÷ system participates in the catalytic initiation of the phenol group, the polymer film thickens and the redox couple characterized by an oxidation peak potential of 0.47 + 0.02 V (SCE) is f o u n d in the film. CONCLUSION
The overall results described in this work show t h a t the electrochemical oxidation of polyfunctional phenol-alcohol compounds is selective and leads to polyphenylene oxide type polymer films with regularly distributed CH2OH groups on the polymer chain. Their reactivity is comparable to t h a t of the CH2OH groups of the monomer; however, as is the case for carbonyl substituted polyphenylene oxide films, surface reactivity is restricted to the initial layers of film. Elaboration of this new series of polymers, together with the possibility of thickening t h e m through use of the F c / F c + system, affords a glimpse o f new applications for reactive polymer films. Their good adherence on metal surfaces also makes it possible to conceive a more general approach to functionalized electrodes. A preliminary study of the electrochemical properties of redox couples on an electrode coated with h y d r o x y l a t e d polyphenylene oxide films has yielded interesting results which will be reported in a forthcoming article. REFERENCES 1 F. Bruno, M.C. Pham and J.E. Dubois, Electrochim. Acta, 22 (1977) 451. 2 P. Ferru ti in J.A. Moore (Ed.), Reactions on polymers, NATO Advanced S t u d y I n s t i t u t e s Series, 1973~ p p . 73--77.
340 3 T. Alfrey, Jr. in E.M. Fettes (Ed.), Chemical Reactions on Polymers, Interscience, N e w York, 1964, p. 2 4 8 . 4 G.D. J o n e s in E.M. F e t t e s (Ed.), Chemical R e a c t i o n s o n P o l y m e r s , I n t e r s c i e n c e , New Y o r k , 1 9 6 4 , pp. 2 5 0 - - 2 5 5 . 5 M.C. P h a m , P.C. Lacaze a n d J.E. Dubois, J. E l e c t r o a n a l . Chem., 8 6 ( 1 9 7 8 ) 147. 6 K o i e h i r o K a t o , J. Applied P o l y m e r Sci., 19 ( 1 9 7 5 ) 9 5 1 . 7 A E u r o p e a n p a t e n t (No. 7 8 - 4 - 0 0 0 0 1 - 0 ) a n d an A m e r i c a n p a t e n t (No. 9 1 2 - 7 4 0 ) have b e e n applied f o r G. T o t ~ i l l o n , J.E. D u b o i s a n d P.C. Laeaze, J. Chim. Phys., 7 4 ( 1 9 7 7 ) 685. 8 J.E. D u b o i s , P.C. Laeaze, R. Courtel, C.C. H e r m a n n a n d D. Maugis, J. E l e c t r o c h e m . Soc., 1 2 2 ( 1 9 7 5 ) 9 1454. M. Delamaz, P.C. Laeaze a n d J.E. Dubois, J. Chim. Phys., 75 ( 1 9 7 8 ) 182. 10 J.P. C o n t o u r a n d G. Mouvier, J. E l e c t r o n S p e c t r o s c . , 7 ( 1 9 7 5 ) 85. 11 E. Laviron, J. E l e c t r o a n a l . C h e m . , 39 ( 1 9 7 2 ) 1. 12 R . F . L a n e a n d A.T. H u b b a r d , J. Phys. C h e m . , 77 ( 1 9 7 3 ) 1 4 0 1 ; 81 ( 1 9 7 7 ) 734. 13 M.S. W r i g h t o n , R.G. Austin, A.B. B o c a r s l y , J.M. Bolts, O. Haas, K.D. Legg, L. Nadjo a n d M.C. 14 P a l a z z o t t o , J. E l e c t r o a n a L Chem., 87 ( 1 9 7 8 ) 4 2 9 .