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Langmuir 1985, I , 514-518
Polymerization of Vinyl Stearate Monolayers: Influence of Pressure Annealing Kevin C. O'Brien,? Jennifer Long, and Jerome B. Lando* Department of Macromolecular Science, Case Western Reserve University, Cleueland, Ohio 44106 Received October 4, 1984. I n Final Form: April 22, 1985 Monolayers of vinyl stearate were irradiated at the gas-water interface while a constant surface pressure on the film was maintained. The polymerization was followed by monitoring the change in the pressure area isotherm for the film and ex situ infrared spectra of the film. The change in area occupied by the film as a function of time could not be used to follow the course of the reaction. The conversion 6f monomer to polymer was increased by increasing the surface pressure maintained on the film and by annealing the film prior to irradiation. A crystallite island model is proposed to describe the morphology of the monolayer before and after pressure annealing,
Introduction A great deal of research has been focused on the polymerization of diacetylenes and butadienes at the gaswater interfa~e.l-~Studies on surface-active compounds containing vinyl groups have focused on the polymerization of vinyl stearate,5p6heptadecenoic acid,I octadecylacrylic acid,8 and octadecyl metha~rylate.~-" Monolayers of vinyl stearate have been polymerized at the gas-water interface through use of UV radiation5 and potassium persulfate.6 These reactions were conducted at a constant area occupied by the film. The extent of the reaction was monitored by following the change in the film's surface pressure with time. The initial surface pressure of the film at the constant area of the polymerization was less than 5 dynlcm. The present study employed reactions at constant surface pressure as opposed to constant area. It has often been suggested that films are more likely to be continuous when a constant surface pressure is maintained on the monolayer throughout the course of the reaction.'*J3 The surface pressures of polymerization for this study were greater than the pressures that have been previously examined. Data on the polymerization of 16-heptadecenoic acid7 suggested that the rate of polymerization could be increased by increasing the surface pressure maintained on the film. Previous studies have shown that monolayer films can be annealed at the gas-water interface.14-19 Pressure annealing occurs when a constant pressure is maintained on the film over a period of time.18J9 This implies that pressure annealing and polymerization occur simultaneously while the film is maintained at constant pressure and irradiated at the gas-water interface. Annealing the film prior to irradiation should also influence the extent of the reaction. Experimental Section Materials and Procedure. The vinyl stearate used for this study was synthesized by a technique which has been previously described.m The details of the computer interface for the Lauda Film Balance, concentration of the spreading solutions, and the method of film preparation have been discussed in previous publications.21s22The Lauda Film Balance was enclosed in a glovebag so that the monolayer polymerization could be conducted under a nitrogen atmosphere. A General Electric Ultraviolet Lamp (25-W Germicidal model without wavelength filters) equipped with Aluminum reflectors was mounted 2 in. above the surface of the water. 'Present address: Center for Energy Studies,University of Texas at Austin, Austin, T X 78712. 0743-7463/85/2401-0514$01.50/0
Table I. Weight Fractions Present in Films of Vinyl Stearate Polymerized at the Gas-Water Interface" fraction present molecular weight 25% 40 % 35%
349 623 1900
'Annealed for 1 h at 16 dyn/cm prior to irradiation. Irradiated for 2 h at 16 dyn/cm.
The water substrate was purged with nitrogen prior to film deposition in order to remove any dissolved oxygen in the subphase. Film preparation prior to irradiation was conducted in the presence of lamps used to handle photosensitive material. The pH of the subphase was maintained between 6.5 and 6.8 to prevent hydrolysis of the ester.22 The glovebag was purged with nitrogen for 1 h prior to reaction to ensure an inert atmosphere above the monolayer. The films were exposed to radiation for 120 min. This time interval was chosen on the basis of data from previous studies of the W polymerization of vinyl teara ate^,^ and heptadecenoic acid.' Studies have shown that less than 100% conversion was obtained when these films were irradiated for 60 min or less at the gas-water interface. Infrared Studies on Monolayer Samples. The preparation (1)Hub, H.; Hupfer, B.; Kock, H.; Ringsdorf, H. J. Macromol. Sci., Chem. 1981, A15, 701. (2) Hupfer, B.; Ringsdorf, H.; Schupp, H. Makromol. Chem. 1981,182,
"_.. 947
(3) Ringsdorf, H.; Schupp, H. J. Mucromol. Sci., Chem. 1981, 1015. (4) Koch, H.; Ringsdorf, H. Makromol. Chem. 1981, 182, 255. (5) Letts, S. A.; Fort, T.; Lando, J. B. J . Colloid Interface Sci. 1976, 56, 64.
(6) Fukuda, K.; Shibasaki,Y.; Nakahara, H. Chem. Phys. Chem. Anwendungstech. Grenzflachenakt. Stoffe. Ber. Int. Kongr. 6th, 1972 1973, 11(15). (7) OBrien, K. C., Rogers, C. E.; Lando, J. B. Thin Solid Films 1983, 102, 131. (8) Fariss, G.; Lando, J. B.; Rickert, S. E. J. Mater. Sci. 1983,18,3323. (9) Dubault, A.; Casagrande, C.; Veyssle, M. J . Phys. Chem. 1975, 79,
2254. (10) Beredjick, N.; Burlant, W. J. J . Polym. Sci., Polym. Chem. Ed. 1970, 8, 2807. (11) Elias, H. G. Midl. Macromol. Monogr. 1977, 3. (12) Gaines, G. L. "Insoluble Monolayers at the Gas-Water Interface"; Interscience Publishers: New York, 1966. (13) Adamson, A. "Physical Chemistry of Surfaces", 3rd ed.; Wiley: New York, 1976. (14) Adams, N. K. "The Physics and Chemistry of Surfaces", 3rd ed.; Oxford University Press: London, 1941. (15) Askew, F. A. J. Chem. SOC.1936, 1585. (16) Harkins, W. D.; Carman, E. F.; Ries, H. E. J . Am. Chem. SOC. 1936,58, 1377. (17) Bergeron, J. A.; Gaines, G. L.; Bellamy, W. D. J . Colloid Interface Sci. 1976, 25, 97. (18) O'Brien, K . C., Lando, J. B., Langmuir, in press. (19) Uitenham, L.; OBrien, K. C.; Lando, J. B., Unpublished results. (20) Cemel, A.; Fort, T.; Lando, J. B. J. Polym. Sci., Polym. Chem. Ed. 1972, 10,2061. (21) O'Brien, K. C.; Lando, 3. B, Reu. Sci. Instrum. unpublished results. (22) O'Brien, K. C.; Lando, J. B., Lungmuir, in press.
0 1985 American Chemical Society
Polymerization of Vinyl Stearate Monolayers of KBr pellet samples of the irradiated films has been discussed in previous publications.22The weight percent of acid, monomer ester, and polymer present in the film has been shown to be directly related to the relative area of the 1710-, 1757-, and 1738-cm-' bands.22 The relative error for this technique was determined to be 1 2 % . Molecular Weight Determination. Gel permeation chromatography was used to determine the molecular weight fractions present in the vinyl stearate films polymerized at the gas-water interface. Each gas-water interface polymerization yields less than 1 p of poly(viny1 stearate). Light scattering and solution viscosity could not be employed to determine the molecular weight of the polymer produced at the gas-water interface due to the microgram quantity. The microgram quantities of four different poly(viny1stearate) samples were stored in microliter vials with HPLC grade tetrahydrofuran as the solvent. The samples were injected into the low molecular weight range columns of the instrument under a THF solvent flow rate of 1 cm3/min. The outputs of the LDC Beckman Ultraviolet detector and LDC refract0 monitor were maintained on a strip chart recorder. Poly(ethy1ene glycol) standards were used to calibrate the column. A calibration curve was generated by plotting the molecular weight of the poly(ethy1ene glycol) samples vs. the log of the elution volumne. This calibration curve was then used to determine the molecular weight of the poly(viny1stearate)samples the basis of their elution volumes. Table I lists the typical weight fractions obtained for polymerization of vinyl stearate at the gas-water interface. Samples were maintained at 16 dyn/cm prior to polymerization and then irradiated at the gas-water interface for 2 h while maintaining a constant pressure of 16 dyn/cm on the film. Two weight fractions of oligomer appear to be present, one fraction with a degree of polymerization of approximately two and another with a degree of polymerization of approximately six. The highest weight fraction present in the samples irradiated at the gas-water interface appears to have a molecular weight of 1900.
Results and Discussion Factors That Influence UV Polymerization. Two factors were examined to determine their effect on the polymerization of vinyl stearate at the gas-water interface: (1)surface pressure of polymerization; (2) pressure annealing of sample prior to reaction. The effect of surface pressure on conversion of monomer to polymer has been observed in the UV polymerization of monolayers of heptadecenoic acid,' octadecylacrylic acid: and odadecyl methacrylate?JO Conversion increased with increasing polymerization pressure. Vinyl stearate monolayers were annealed at the gaswater interface by maintaining a constant surface pressure on the film for 60 min prior to polymerization. An annealing pressure of 16 dyn/cm was chosen since this value was less than the collapse pressure of monomer vinyl stearate and was along the linear region in the pressurearea isotherm for the monomer. Films maintained at higher surface pressures were not stable over the time period of the experiment. The extent of the reaction was followed by monitoring changes in the area occupied by the film as a function of time, infrared spectra of the films after irradiation, and pressure-area isotherms for the sample before and after polymerization at the gas-water interface. Change in Area vs. Time. Experiments were conducted under conditions where hydrolysis of the ester was negligible. Changes in the area occupied by the film on irradiation at constant surface pressure could be due to two factors: (1)formation of polymer in the monolayer, (2) annealing of the film at constant surface pressure during irradiation. If a constant surface pressure is maintained on the film during irradiation at the gas-water interface, both of these factors w ill influence the reduction
Langmuir, Vol. 1, No. 4, 1985 515
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Figure 1. Area ratio vs. time for vinyl stearate films polymerized at the gas-water interface at constant surface pressure of 5,12, and 16 dyn/cm. Films were irradiated for 2 h. 1.10
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