Langmuir 1985,1,533-537
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Articles Hydrolysis of Vinyl Stearate and Poly(viny1 stearate) Monolayers Kevin C. O'Brienf and Jerome B. Lando* Department of Macromolecular Science, Case Western Reserve University, Cleveland, Ohio 44106 Received October 4, 1984. I n Final Form: February 25, 1985 Monolayers of vinyl stearate and bulk poly(viny1stearate) were maintained at constant pressures of 15 dyn/cm and spread on subphases of pH ranging from 2 to 12 in order to determine the extent of ester hydrolysis under these experimental conditions. Monomer films exposed to basic subphases for 6 h appeared to undergo base hydrolysis, while those spread on acidic subphases for the identical length of time underwent cationic polymerization as well as acid hydrolysis of the ester. Hydrolysis of polymer films did not appear to occur over the entire surface pressure and pH range employed in this study. The resistance of the polymer film to chemical attack was attributed to the steric hindrance of the polymer backbone to attacking ions. The relation of chemical resistivity of the monolayer at the gas-water interface to device fabrication was also discussed. 1.0. Introduction Esters have been shown to undergo hydrolysis when exposed to a basic or acidic environment.' This same reaction has been observed in monomolecular films formed from ester-containing compounds.2-s Previous studies have examined the hydrolysis of methyl stearate and ethyl stearate68on 1-2 N NaOH and 1-5 N HCL subphases. On the basis of hydrolysis of esters in the bulk, the hydrolysis of vinyl stearate would result in the formation of a fatty acid. The salt of the fatty acid could also be present depending on the pH of the subphase. The hydrolysis of vinyl stearate and poly(viny1stearate) monolayers could be important in relation to the use of this material in the fabrication of devices. The langmuir-Blodgett film deposition technique has recently been used to deposit multilayers onto various substrates for applications in electron lithography: membrane coating,1° and the fabrication of Josephson junction devices." Polymer films must be employed in these applications. The films can either be polymerized in situ at the gaswater interface,12 after deposition of the monomer on a substrate,13J4or in bulk and then spread as a monolayer at the gas-water interface.'O The presence of stearic acid or its salt in monolayers of vinyl stearate could be a major problem. If the film was polymerized at the gas-water interface or after deposition on a substrate, the fatty acid and its salt would be isolated in a polymer matrix. Later applications would subject the films to conditions where unpolymerized material could sublime from the sample.16J6 Stearic acid or its salt would then sublime, thus resulting in the introduction of pinholes in the films. The basis of this study was to determine whether vinyl stearate films were susceptible to hydrolysis at the gaswaster interface under the conditions of typical device fabrication. Poly(viny1 stearate, which had been polymerized in bulk and spread as monolayers, was also examined to determine whether polymer films would hydrolyze at the gas-water interface. The polymer films were exposed to the same conditions as the monomer films in order to determine if the poly(viny1 stearate) sample was less susceptible to hydrolysis than its monomer analogue. 'Present address: Center for Energy Studies, University of Texas at Austin, Austin, TX 78712. (1),Streitwieser, A.; Heathcock, C. "Introduction to Organic Chemistry"; Macmillan: New York, 1976.
2.0. Experimental Section 2.1. Equipment and Procedures. Films were prepared on a Lauda film balance equipped with a computer interface that has been described previously." The solvent used in the study was 99+% heptane (Aldrich, Gold Label). The concentration of the monomer and polymer spreading solutions were 0.5000 mg/mL. The procedure form f i i formation and the preparation of the water substrate has been described previously.'* The Lauda balance was installed in a constant temperature and humidity room. A Brinkman temperature controller was used to maintain the temperature of the trough at 19 "C. This temperature was chosen since it was below the temperature of the room and has been shown previously to reduce the effects of convective currents.19*20The temperature controller was accurate to h0.5 "C. A Beckman Research pH meter equipped with a Fisher Standard Combination electrode (accuracy iO.02) was used to measure the pH of the water substrate. The output from the pH meter was connected to a Brinkman Servogor 220 two-channel recorder in order to facilitate monitoring subphase pH as a function of time. Reagent grade HCl and NaOH was used to adjust the pH of the water subphase. Water subphases were purged with nitrogen prior to film deposition. 2.2. Synthesis and Characterization of Monomer and Polymer. The vinyl stearate was synthesized by a previously described technique." The melting point (35-36 "C)and infrared spectroscopic data ensured that no detectable fatty acids were present in the sample. (2)Davies, J. T.;Rideal, E. K.; 'Interfacial Phenomena"; Academic Press: New York, 1963. (3)Adamson, A. 'Physical Chemistry of Surfaces", 3rd ed.; Wiley: New York, 1976. (4)Gaines, G. L. "Insoluble Monlayers at the Gas-Water Interface";
Interscience Publ., New York, 1966. (5)Alexander, A. E.;Schulman, J. H. Proc. R. SOC.London, Ser. A 1937,161,115. (6)Alexander, A. E.,Rideal, E. K. Proc. R. SOC.London 1937,163,70. (7) Mittelmann, R.; Palmer, R. C. Trans. Faraday SOC.1942,38,506. ( 8 ) Davies, J. t. Trans. Faraday SOC. 1949,45,448. (9) Fariea. G.:Lando. J. B.: Richert. S. E. J. Mater. Sci. 1983.18.3323. (10)uitehak, L.P ~ . DDissertation, . Case Western Reserve university, Cleveland, OH, 1985. (11)Burkhart, C. W.; Larkins, G.;Thompson, E.; Lando, J. B. Thin Solid Films 1983,99,305. (12)OBrien, K.C.;Long, J.; Lando, J. B. Langmuir, in press. (13) Puterman, M.; Fort, T.; Lando, J. B. J. Colloid Interface Sci. 1974,47,705. (14)Cemel, A.; Fort, T.; Lando, J. B. J.Polym. Sci., Polym. Chem. Ed. 1972,10, 2061. (15)Ginnai, 0.Thin Solid Films 1980,68,241. (16)Gregor, H.; Gregor, C. Sci. Am. 1978,238,112. (17)OBrien, K. C. Reu. Sci. Instrum., submitted for publication. (18)O'Brien,K. C.;Rogers, C. E.; Lando, J. B. Thin Solid Films 1983, 102,131.
0743-1463/85/2401-0533$01.50/00 1985 American Chemical Society
O'Brien and Lando
534 Langmuir, Vol. 1, No. 5, 1985
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Figure 1. Infrared spectra of bulk vinyl stearate. The 1757-cm-* band is due to the C=O stretch and the 1645-cm-' band is due to the C=C stretch. Radiation polymerization of vinyl stearate was performed as described by Restaino et Monomer vinyl stearate was sealed in a glass tube that had been evacuated to lo4 atm. This sample was then placed in a Cobalt-60 y source and exposed to approximately 10 Mrads at room temperature. Residual monomer was removed from the polymer by dissolving the sample in benzene and precipitating the polymer in acetone which had been cooled to -5 "C. The polymer was then dried under vacuum overnight after three precipitations. The details of this purification are described in the literature." Figures 1and 2 are infrared spectra of monomer vinyl stearate and purified poly(viny1stearate) in KBr. The absence of the c=C stretch at 1645 cm-' verifies that no monomer remains in the sample. The C=O stretch has shifted from 1757 cm-' in the monomer to 1738 cm-* in the polymer. This is due to the loss of the slight conjugation between the vinyl group and the carboxyl group which exists in the monomer. Gel permeation chromatography (GPC) studies also indicated the absence of monomer in the bulk polymer. Infrared and GPC studies on unpurified poly(viny1 stearate) indicated that the percent conversion to polymer after "in source'' polymerization was approximately 100%. The intrinsic viscosities of poly(viny1stearate) in benzene at 25 "C were measure using a Ubbelohde suspended level dilution viscometer. The viscosity average molecular weight of poly(viny1 stearate) was calculated by using the Mark-Houwink equation. The Mark-Houwink coefficients for poly(viny1 stearate) in benzene have been previously c a l ~ u l a t e d .The ~ ~ M,and melting point of the polymecwere determined to be 6000 and 49-52 "C, respectively. The M , for poly(viny1 stearate) prepared in this fashion was comparable to values obtained by Restaino et aLZ1 2.3. Infrared Studies on Monolayer Samples. Polymer and monomer monolayers were collapsed at the gas-water interface and removed with a spatula. These samples (approximately 100 pg of material) were then dried under nitrogen in a glovebag (Instruments for Research and Industry, Cheltenham, PA) overnight and then stored in a vacuum desiccator under a vacuum of lo4 a h to remove residual water from the solid sample. Pellet fabrication was conducted in a glovebag to reduce the absorption of atmospheric water vapor by the KBr powder. Infrared spectra were obtained from a DIGILAB FTS-14 infrared spectrometer. The spectra were analyzed on a VAX 11/780 computer operated under VMS version 3.2 through use of spectral manipulation Software SPECMANIP.24 The 1710-, 1738-, and 1757-cm-' bands were examined to determine whether the area underneath the bands could be directly related to the amount of compound that was present in the sample. (19) Velarde, M. G.; Normad, C. Sci. Am. 1980, 243,92. (20) Pearson, J. R. A. J.Fluid Mech. 1958, 4, 489. (21) Restaino, A. J.; Merobian,R. B.; Morawetz,H.; Ballantine, D. S.; Dienes, G. J.; Mertz, D. J. J.Am. Chem. SOC. 1956, 78, 2939. (22) Port, W.S.;Hansen, J. E.; Jordan, E. F.; Dietz, T. J.; Swern, D. J . Polym. Sci. 1951, 7, 207.
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Figure 2. Infrared spectra of purified bulk poly(viny1stearate) prepared by "in sourcen polymerization. The 1738-cm-' band is due to the C=O stretch in the polymer. lQ3.
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Figure 3. Peak area of 1710-cm-l band vs. weight percent stearic acid from mixed monolayers studies. The infrared spectra of mixed monolayers of known composition were examined to determine if Beer's law was followed for the acid, monomer ester, and polymer ester bands. Figures 3-5 are plots of the percent area vs. weight percent of component for stearic acid, monomer vinyl stearate, and poly(viny1 stearate). These plots show that the weight percent of acid, monomer ester, and polymer are directly related to the relative area of the 1710-, 1757-,and 1738-cm-' bands. The relative error for this technique was determined be +2%.
3.0. Results and Discussion
The surface pressures maintained on the films during these studies were chosen on t h e basis of typical pressures employed in the Langmuir-Blodgett deposition of vinyl stearate and poly(viny1 stearate) on various substrates. Values for typical deposition pressures were greater t h a n 10 d y n / c m and were contained along the so-called condensed regime of t h e pressure-area isotherm for vinyl stearate and its polymer. The constant pressures maintained on films in this study were greater than those employed in previous studies on the hydrolysis of methyl and ethyl stear'ate monolayer^.^*^ 3.1. Hydrolysis of Monomer Vinyl Stearate. Acid (23) Burlant, W.; Adicoff, A. J. Polym. Sci. 1958, 27,269. (24) Gillette, P. "Computer Analysis of Infrared Spectra"; Case Western Reserve University: Cleveland, 1981.
Langmuir, Vol. 1, No. 5, 1985 535
Hydrolysis of Vinyl Stearate and Poly(viny1 stearate)
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or base attack on the ester group in vinyl stearate would result in a change in the area occupied by the film if the monolayer is maintained at a constant surface pressure. This area change would result from the formation of stearic acid or ita ionized analogue. Carbonyl bands at 1710 and 1563 cm-’ in the ex situ infrared spectra of the films would indicate the presence of either stearic acid or its salt.26 Figure 6 is a plot of the area ratio, A / A o ,vs. time, where A ( t ) is the area occupied by the film at time t and A . is the initial area occupied by the film. The pH of the substrate was varied from 6.5 to 11.3 while a constant surface pressure of 15 dyn/cm was maintained on all the films for approximately 6 h. The reduction in the area ratio appears to increase rapidly with increasing pH. The data in Figure 6 indicate that approximately 50% of the original area of the f i ihas been lost after 200 min for the sample exposed to a pH of 11.3. Infrared spectra of these samples were also examined in order to determine whether stearic acid or ita salt were present in the film. Carbonyl bands due to stearic acid or ita salt were not detected in spectra for films spread on subphases with a pH of 6.5. Infrared samples could not be obtained form films spread on subphases with a pH of 10.5 and 11.3 due to the gellike nature of the samples. The drastic reduction in area occupied by the film correlates with the previous observations of Adam and Miller for fatty acid fims spread on basic subphases (pH range &12).26 They found a large reduction in the area (25) No. 18324 ‘Sadtler Standard Spectra”;Samule P. Sadtler and Sons Inc.: Philadelphia, PA, 1961. (26) Adam, N. K.;Miller, J. G. F. R o c . R. SOC.London, Ser. A 1933, 142, 401.
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y*ww”s (CW I Figure 7. Infrared spectra of vinyl stearate monolayer on water substrate, pH 2, maintained at 12 dyn/cm for 6 h. occupied by the films spread on basic subphases when the monolayers were maintained at a constant surface pressure. The films also appeared to become very gellike in nature. They attributed the loss in the integrity of the fatty acid films to a reduction in the lateral adhesion of the molecules in the film due to the formation of ionized fatty acid. Vinyl stearate films were also exposed to acidic subphases in order to determine the susceptibility of the ester group to acid attack. Films were spread on subphases of pH 2 and maintained at 12 dyn/cm for 6 h. Plots of A / & vs. time for these samples indicated that the area ratio remained constant within experiment error. The spectrum of this sample was examined to determine the presence of stearic acid or ita salt in the f i . The absence of a band in the 1563-cm-’ region indicated that ionized acid was not present in the monolayer. The 1800-1700-~m~~ region of the infrared spectrum for a sample exposed to these conditions is shown in Figure 7. The carbonyl band for the monomer ester appears a t 1757 cm-l, while a shoulder is apparent a t 1738 cm-I and a small peak occurs at 1710 cm-’. These peaks are carbonyl bands due to the presence of poly(viny1 stearate) and stearic acid. The relative weight precent of monomer, acid, and polymer were determined from peak areas and the extinction coefficients for the bands. Table I lists the bands and the weight percent of each component as calculated from Figure 7. Films of vinyl stearate were also spread on acidic subphases (pH 2) and maintained at 6 dyn/cm to determine
O'Brien and Lando
536 Langmuir, Vol. 1, No. 5, 1985 Table I. Infrared Data for Vinyl Stearate Monolayers on Substrate of pH 2 for 6 h band, cm-' 1757 1738 1710
origin C=O monomer C=O polymer C=O acid
wt%
87.8 10.2 2.0
whether cationic polymerization would occur at lower surface pressures. No change was observed in the areas of the films over a 6-h time frame. Infrared spectra indicated that only monomer ester was present in the samples. Bands due to poly(viny1stearate), stearic acid, or its salt were not detected. The presence of polymer was not due to sample contamination or photopolymerization, since films were prepared under yellow lamps used to handle photosensitive materials. It appears that the vinyl stearate underwent cationic polymerization as well as hydrolysis at the gaswater interface. These reactions appeared to be favored by an increase in surface pressure maintained on the film. The infrared data in Table I demonstrates that cationic polymerization is favored over hydrolysis. 3.2. Hydrolysis of Bulk Poly(viny1 stearate). The hydrolysis of monolayers of poly(viny1 stearate) at the gas-water interface were also examined. The polymer was polymerized in the bulk and was of low molecular weight (A& = 6000). Poly(viny1stearate) was exposed to the same conditions as vinyl stearate to determine whether it would be less susceptible than its monomer to acid or base attack. The pH of the substrate could also influence the rate of hydrolysis of poly(viny1 stearate) monolayers. The poly(viny1 stearate) was prepared by "in source" polymerization. Previous work has suggested that under these conditions, the major means of chain termination should be hydrogenation.n Hydrolysis of the polymer monolayer could result in the formation of poly(viny1alcohol) as well as stearic acid and its salt. Analysis of the cange in the area occupied by the film with time is complicated by the fact that one of the products of the hydrolysis, poly(viny1 alcohol), is surface active, therefore it will either remain in the monolayer or dissolve into the subphase depending on the pH of the subphase. Infrared spectroscopy will not be useful in detecting the presence of poly(viny1 alcohol) because the major band used to detect the presence of alcohols, the hydrogen bonded OH stretch at -3300 cm-1,2salways appears in the spectra of collapsed films due to the occlusion of water from the subphase. The presence of poly(viny1 alcohol) may be detected from the change in area occupied by the film and the isotherm of the exposed monolayer. Isotherms of poly(vinyl alcohol) and similar polymers indicate that this compound collapses at 2-3 d y n / ~ m and ~ ~should lie horizontally flat on the water's surface.30 These facts imply that the formation of poly(viny1 alcohol) during the hydrolysis of poly(viny1 stearate) would result in large changes in the area occupied by the film and changes in the shape of the pressure area isotherm of the exposed sample. Films of poly(viny1 stearate) were exposed to similar conditions as monomer films. Polymer films were also maintained at lower surface pressures (6 dyn/cm) on subphases of various pH in order to examine the effect of surface pressure on hydrolysis of the polymer monolayer. (27) Morawetz, H. J. Polym. Sci., Part C 1963, 1, 65. (28) Colthup, N.;Daly, L.; Wiberley, S."Introductionto Infrared and Raman Spectroscopy"; Academic Press: New York, 1964. (29) Crisp, D. J. "Surface Chemistry Supplementary Research";Buttersworth: London, 1949. (30) Ries, H.; Walker, D. J. Colloid Sci. 1961, 16, 361.
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Figure 8 is a representative plot of the area ratio vs. time for a poly(viny1 stearate) film maintained at 15 dyn/cm on a substrate of pH 6.5 and 12. The decrease of approximately 2% in the area ratio is exactly the same value observed for other samples that were exposed to acidic subphases (pH 2) and others that were maintained at low surface pressures (6 dyn/cm.). This phenomena has been observed in various monolayer systems and is referred to as pressure annealing of the film.10J2*31-34 Figure 9 depicts the infrared spectrum for the poly(viny1 stearate) film described in Figure 8 which was exposed to a subphase of pH 12. This spectrum is identical with spectra obtained for polymer samples exposed to acidic subphases (pH 2) and those maintained at a lower surface pressures (6 dyn/cm). The only band present in the 1800-1700-cm-' region is the carbonyl stretch of poly(viny1 stearate) at 1738 cm-'. Bands due to stearic acid (1710 cm-l) or its salt (1563 cm-') were not present in any of the spectra. ~~~~~~
~~~
(31) Adam, N. K."The Physics and Chemistry of Surfacen",3rd ed.; Oxford University Press: London, 1941. (32) Askew, F. A. J . Chem. SOC. 1936, 1585. (33) Harkins, W. D.;Carman, E. F.; Ries, H. E. J . Am. Chem. Soc. 1936.58. ..__1377. -I - - 7
(34) Bergeron,J. A.;Gaines, G. L.; Bellamy, W. D. J.Colloid Interface Sci. 1967, 25, 97.
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Langmuir 1985, 1, 537-541
537
The carboxyl group of the ester is the site of attack of the OH- and H+ during ester hydro1ysis.l The submerged polymer backbone could sterically hinder attack on the carbonyl. Polymers that contain vinyl groups in the hydrophobic portion of the molecule would not be as resistive to chemical attack from beneath the water's surface. This was evidence by the esterification of poly(octadecylacry1ic acid) at the gas-water i n t e r f a ~ e . ~ 4.0. Conclusions
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Figure 10. Isotherm for poly(viny1 stearate) prepared in bulk and spread on a subphase pH 12 and maintained at 15 dyn/cm for 6 h.
The presence of another possible product of the hydrolysis of poly(viny1stearate), poly(viny1alcohol), can be detected by examining pressure-area isotherms for the polymer film. The isotherm for the sample described in Figures 8 and 9, spread on a subphase of pH 12 and maintained at a constant surface pressure of 15 dyn/cm, is pictured in Figure 10. This isotherm is identical with those for other samples exposed to acidic subphases and those maintained at lower surface pressures. The isotherm in Figure 10 is identical with pressure-area isotherms for vinyl stearate polymerized in bulk and spread on subphases of approximately neutral pH (pH -6.5). The molecular weight of the polymer was not large = 6000), but the resistivity of the polymer film to chemical attack was much greater than that of the monomer. This resistivity could be due to the location of the polymer backbone relative to the ester group. The vinyl group in vinyl stearate is located in the hydrophilic portion of the molecule which is submerged in the water.35 Polymerization occurs through the reaction of the vinyl group, therefore the polymer backbone should be submerged in the water subphase.
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(35) Fort, T.; Alexander, A. E. J. Colloid Sci. 1959, 14,190.
Ester groups in vinyl stearate monolayers appear to be susceptible to hydrolysis at the gas-water interface under typical conditions employed in Langmuir-Blodgett deposition. A drastic reduction in the area occupied by monomer films occurred when the samples were exposed to basic subphases (pH -12) for a 6-h time frame. This was attributed to the formation of stearic acid and its salt which would contribute to the loss in lateral adhesion between molecules in the film, which would explain the dramatic loss in film area. Infrared spectra for vinyl stearate films spread on acidic subphases (pH 2) and maintained at surface pressures of 12 dyn/cm indicated that cationic polymerization as well as ester hydrolysis was occurring in the film. These reactions could cause major problems if vinyl stearate films were spread on basic or acidic subphases during device fabrication. Monolayers of low molecular weight poly(viny1stearate) = 6000) were maintained at pressures of 15 and 6 dyn/cm on subphases of pH 2 and 12 in order to determine the susceptibility of the polymer to acid or base hydrolysis. Area ratio vs. time data, infrared spectra, and pressurearea isotherms suggest that, unlike the monomer monolayer, the poly(viny1 stearate) films were resistant to chemical attack for time periods up to 6 h. This was attributed to the steric hinderance which the polymer backbone poses to ions in the water subphase. The chemical resistivity of the bulk polymer monolayer is a major advantage of using bulk poly(viny1 stearate) as opposed to monomer vinyl stearate in the fabrication of membrane and electronic devices.
Acknowledgment. We acknowledge the financial support of NSF Grant DMR-81-1441 for this work.
Computer-Controlled Apparatus for Interfacial Electrochemical Studies Larry Cunningham and Henry Freiser* Strategic Metals Recovery Research Facility, University of Arizona, Tuscon, Arizona 85721 Received October 11, 1984. I n Final Form: M a y 23, 1985 This paper described a computer-controlled apparatus for acquisition of electrochemical data at the interface of two immiscible electrolyte solutions (ITIES). The system is based on a previously described four-electrode current scan polarographic apparatus and is equipped with an improved solvent delivery system which allows precise control of flow rates. The apparatus allows rapid, simultaneous acquisition of both current-voltage and electrocapillary c w e s and represents a significant improvement over currently available alternatives with respect to the speed, precision, and accuracy by which such data can be obtained.
Introduction Electrochemistry at the interface of two immiscible electrolyte solutions (ITIES) has been reviewed by Korytal (1) Koryta, J. Ion-Sel. Electrode Reu. 1983,5, 131-164.
0743-7463/85/2401-0537$01.50/0
and more recently by Vanysek et ale2 Apparent from these papers is a resurgent interest in the elucidation of the structure of this interface by methods such as double-layer (2) Vanysek, P.;Buck, R. P. J. Electroanal. Chem. 1984, 163, 1-9.
0 1985 American Chemical Society
