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Langmuir 2008, 24, 5740-5745
Poly(trifluoroethylene) Adsorption and Heterogeneous Photochlorination Reactions Ilke Anac,*,† Voravee P. Hoven,‡ and Thomas J. McCarthy† Polymer Science and Engineering Department, UniVersity of Massachusetts, Amherst, Massachusetts 01003, and Organic Synthesis Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn UniVersity, Phayathai Road, Pathumwan, Bangkok 10330, Thailand ReceiVed NoVember 14, 2007. ReVised Manuscript ReceiVed January 24, 2008 Heterogeneous (gas-solid) photochlorination reactions of poly(trifluoroethylene) (PF3E) films were studied as a function of reaction time and light intensity. The rate of chlorination was found to be faster in high-intensity light when compared to the reaction in ambient light. PF3E irreversibly adsorbed to oxidized silicon and covalently attached amine monolayers supported on silicon, producing hydrophobic thin films in the thickness range of 8-40 Å. Adsorption conditions such as polymer concentration and solvent composition were investigated. Radical grafting of maleic anhydride to the polymer backbone resulted in increased adsorption on oxidized silicon.
Introduction Adsorption of polymers1 plays a vital role in many scientific and technological applications where interfacial effects are dominant, such as colloid stabilization (e.g., formulation of paints, coatings, and printing inks),2,3 flocculation,4 corrosion,5 adhesion,6–8 and lubrication.9,10 The ultimate goal of research in polymer adsorption is to regulate interfacial properties of materials by controlling the conformation of adsorbed polymer chains. To achieve this goal, it is necessary to understand how the nature of polymer, the substrate, and other prevailing conditions affect the macroscopic properties of the interface. As a result, this field of research has been an area of interest for both experimental11–15 and theoretical groups16–19 for many years. * To whom correspondence should be addressed. E-mail: anacilke@ mail.pse.umass.edu. Tel: +49-6131-379-549. Fax: +49-6131-379-360. Current address: Max Planck Institute for Polymer Research Ackermannweg 10 55128 Mainz, Germany. † University of Massachusetts. ‡ Chulalongkorn University. (1) Netz, R. R.; Andelman, D. Phys. Rep. 2003, 380, 1. (2) Napper, D. H., Polymeric Stabilization of Colloidal Dispersions. Academic Press: London, 1983. (3) Meredith, J. C.; Johnston, K. P. Macromolecules 1998, 31, 5507. (4) Dobias, B., Coagulation and Flocculation: Theory and Applications; M. Dekker: New York, 1993. (5) Nathan, C. C.; Bregman, J. I., Corrosion Inhibitors; National Association of Corrosion Engineers: Houston, 1973. (6) Shull, K. R. Mat. Sci. Eng. R 2002, 36, 1. (7) Bucknall, D. G. Prog. Mater. Sci. 2004, 49, 713. (8) Lee, L.-H. Adhesion and Adsorption of Polymers; Plenum Press: New York, 1980. (9) Dorinson, A.; Ludema, K. C. Mechanics and Chemistry in Lubrication; Elsevier: Amsterdam, 1985. (10) Granick, S.; Kumar, S. K.; Amis, E. J.; Antonietti, M.; Balazs, A. C.; Chakraborty, A. K.; Grest, G. S.; Hawker, C.; Janmey, P.; Kramer, E. J.; Nuzzo, R.; Russell, T. P.; Safinya, C. R. J. Polym. Sci. Pol. Phys. 2003, 41, 2755. (11) Guzonas, D. A.; Boils, D.; Tripp, C. P.; Hair, M. L. Macromolecules 1992, 25, 2434. (12) Konstadinidis, K.; Thakkar, B.; Chakraborty, A.; Potts, L. W.; Tannenbaum, R.; Tirrell, M.; Evans, J. F. Langmuir 1992, 8, 1307. (13) Huang, Y. D.; Santore, M. M. Langmuir 2002, 18, 2158. (14) Costa, A. C.; Composto, R. J.; Vlcek, P.; Geoghegan, M. Eur. Phys. J. E 2005, 18, 159. (15) Coupe, B.; Evangelista, M. E.; Yeung, R. M.; Chen, W. Langmuir 2001, 17, 1956. (16) Muthukumar, M.; Ho, J. S. Macromolecules 1989, 22, 965. (17) Reynolds, B. J.; Ruegg, M. L.; Mates, T. E.; Radke, C. J.; Balsara, N. P. Macromolecules 2005, 38, 3872. (18) Semenov, A. N.; Avalos, B. J.; Johner, A.; Joanny, J. F. Macromolecules 1996, 29, 2179.
Fluoropolymers are applicable for many applications due to their excellent chemical inertness, mechanical properties, and good biocompatibility. Extensive studies on surface modification of fluoropolymers based on chemical means20–25 and physical adsorption have been previously reported by our group. In particular, we have investigated the adsorption of the poly(Llysine) (PLL) to poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) surfaces from aqueous solution in which the adsorption is driven by the unfolding of R-helix structure of PLL26 and adsorption of poly(vinyl alcohol) (PVOH) onto poly(tetrafluoroethylene-co-hexafluoropropylene)(FEP) from aqueous solution in which the adsorption is driven by the crystallization of PVOH and hydrophobic interactions.27 To the best of our knowledge, the adsorption of fluoropolymers to solid substrates has not yet been explored due to their limited solubility. Unlike other fluoropolymers, poly(trifluoroethylene) (PF3E) is soluble in polar solvents such as acetone, methanol, and tetrahydrofuran, due to the highly polar C-H bonds along the backbone. As a result, it can be conveniently cast as a film and serves as a practical precursor to poly(chlorotrifluoroethylene) or polytetrafluoroethylene (Teflon) for surface and coating applications. The presence of C-H bond in the repeat unit also provides a possibility of chemical modification via an introduction of polar groups to the polymer backbone. Even though PF3E is essentially atactic,28 the polymer chains are able to crystallize because of the similarity between C-F and C-H bond lengths and van der Waals radii of fluorine and hydrogen. Therefore, the polymer’s ability to crystallize and to form hydrogen bonds with proton acceptors makes it a suitable candidate for adsorption studies. Gas-phase halogenations have been utilized as a method to improve the surface properties of polymer films, such as adhesion, wettability, gas permeation, and barrier properties. Examples (19) Stepanow, S. J. Chem. Phys. 2001, 115, 1565. (20) Dias, A. J.; McCarthy, T. J. Macromolecules 1987, 20, 2068. (21) Dias, A. J.; McCarthy, T. J. Macromolecules 1984, 17, 2529. (22) Bee, T.; McCarthy, T. J. Macromolecules 1992, 25, 2093. (23) Bening, R. C.; McCarthy, T. J. Macromolecules 1990, 23, 2648. (24) Lee, K.-W.; McCarthy, T. J. Macromolecules 1988, 21, 3353. (25) Shoichet, M. S.; McCarthy, T. J. Macromolecules 1991, 24, 982. (26) Shoichet, M. S.; McCarthy, T. J. Macromolecules 1991, 24, 1441. (27) Kozlov, M.; Quarmyne, M.; Chen, W.; McCarthy, T. J. Macromolecules 2003, 36, 6054. (28) Lovinger, A. J.; Cais, R. E. Macromolecules 1984, 17, 1939.
10.1021/la703562n CCC: $40.75 2008 American Chemical Society Published on Web 05/08/2008
PF3E Adsorption and Heterogeneous Photochlorination Reactions
include halogenations of polyethylene,29 poly(4-methyl-1-pentene),30 polystyrene,31 polysulfone,32 poly(methyl methacrylate),31 and polystyrene/polybutadiene block copolymers.33 Heterogeneous (gas-solid) chlorination and fluorination are commonly used due to their high reaction rates. Gas chlorination studies of polyethylene29 and poly(4-methyl-1-pentene)30 indicate that the thickness of the chlorinated layer (depth of chlorination) and the extent of chlorination (the density of chlorine atoms on the polymer backbone) can be independently controlled by manipulating conditions such as the chlorine gas pressure, light intensity, and exposure time. In this study the chlorination reactions of solution-cast PF3E thin films are discussed. Kinetics of chlorination reactions were determined by attenuated total reflectance infrared spectroscopy (ATR IR) and X-ray photoelectron spectroscopy (XPS). Additionally, the adsorption of PF3E on substrates with different surface chemistry was studied; silanol (oxidized silicon) and amine (3-aminopropyldimethylethoxysilane (APDMES) monolayers on silicon). The amount of adsorbed polymer was controlled by changing the solvent quality and polymer concentration and was determined by XPS, ellipsometry, and contact angle measurements. Tetrahydrofuran (THF) and toluene were chosen as good solvent and nonsolvent for adsorption experiments, respectively. The adsorption of PF3E was further controlled by an introduction of polar functional groups on the backbone of PF3E, through the radical grafting of maleic anhydride (MAH).
Experimental Section Materials. Poly(chlorotrifluoroethylene) (PCTFE) powder (Kel-F 81) was obtained from 3M. Tributyltin hydride (Bu3SnH), azobisisobutyronitrile (AIBN), APDMES, chlorine gas (99.5%), benzoyl peroxide (BPO), anhydrous THF, and anhydrous toluene were obtained from Aldrich. MAH, hexane (HPLC grade), heptane (HPLC grade), ethanol, and 2-propanol were purchased from Fisher. Carbon dioxide (coleman grade) was purchased from Merrian Graves. MAH was recrystallized from chloroform. BPO was recrystallized from chloroform/methanol. Surface chlorinations were run on both sides of a free-standing PCTFE film. Ambient light and a high-intensity UV light (UVP Inc., Black-Ray B-100A, 6000 µW/cm2 at 365 nm) were used in the chlorination reactions. Silicon wafers were obtained from International Wafer Service (100 orientation, P/B doped resistivity, 20-40 Ω cm, thickness 450-575 µm) and cleaned by a Harrick Scientific expanded O2 plasma cleaner at high power settings for 5 min prior to use. The oxide layer on the wafers after plasma treatment was determined to be ∼25 Å by ellipsometry. Water was purified using a Milipore Mili-Q water system that involves reverse osmosis, ion exchange, and filtration steps; we will refer to water purified by this method as Milli-Q water. Instrumentation. Contact angle measurements were performed using a Rame´-Hart telescopic goniometer and a Gilmont syringe with a 24 gauge flat-tipped needle. Dynamic advancing (θA) and receding (θR) angles were recorded while the probe liquid was added to and withdrawn from the surface, respectively. The probe liquid was Milli-Q water. Each reported angle represents an average of at least four measurements. XPS spectra were obtained on a Physical Electronics Quantum 2000 Scanning (29) Cross, E. M.; McCarthy, T. J. Macromolecules 1992, 25, 2603. (30) Levasalmi, J. M.; McCarthy, T. J. Macromolecules 1995, 28, 1733. (31) Shimada, J.; Hoshino, M. J. Appl. Polym. Sci. 1975, 19, 1439. (32) Mohr, J. M.; Paul, D. R.; Pinnau, I.; Koros, W. J. J. Membr. Sci. 1991, 56, 77. (33) Csernica, J.; Rein, D. H.; Baddour, R. F.; Cohen, R. E. Macromolecules 1991, 24, 3612.
Langmuir, Vol. 24, No. 11, 2008 5741 Scheme 1. Reductive dechlorination of PCTFE
Scheme 2. Maleation of PF3E in ScCO2
ESCA Microprobe. Spectra were recorded at 15° and 75° takeoff angles between the plane of sample and the entrance lens of the detector. Ellipsometric measurements were done on a Rudolph Research Model Auto SL-II automatic ellipsometer with a He-Ne laser source (λ ) 632.8 nm). The angle of incidence (from the normal to the plane) was 70°. Each reported data represents an average of at least four measurements on different locations on each sample. Film thickness was calculated from the ellipsometric parameters (∆ and Ψ) using DafIBM software, assuming a transparent double layer model (silicon substrate/silicon oxide + APDMES layer/PF3E/air and silicon substrate/silicon oxide + PF3E/air). The following parameters were used for the calculation: Silicon substrate ns ) 3.858, ks ) 0.018 (imaginary part of the refractive index); air, no ) 1; PF3E, n1 ) 1.42; silicon oxide + APDMES layer, n2 ) 1.462. ATR IR spectra were recorded using an IBM 38 FTIR at 4 cm-1 resolution with a germanium internal reflection element (45°). Gel permeation chromatography (GPC) was performed with a Polymer Laboratory LC1120 high performance liquid chromatography (HPLC) pump equipped with a Waters differential refractometer detector. The mobile phase was THF with a flow rate of 1 mL/min. Synthesis of PF3E (Scheme 1).34 A solution of Bu3SnH (22.7 g, 78 mmol) in THF (112 mL) was added to a solution of AIBN (1.2 g, 7.3 mmol) in THF (30 mL) in a nitrogen-purged roundbottom flask containing PCTFE (6 g, 52 mmol, Mn ) 700 000-800 000 g/mol). The mixture was refluxed at 66 °C for 24 h. An additional 1.2 g of AIBN in 30 mL of THF was added to the mixture after the reaction had proceeded for 12 h. The product was precipitated in 600 mL of cold hexane (in dry ice) and purified by Soxhlet extraction in heptane for 24 h. The product was dried under vacuum at 80 °C for 48 h. The product had a Mw ) 573 640 g/mol and a polydispersity of 1.3. Maleation of PF3E (Scheme 2). A PF3E sample (0.49 g, 5.9 mmol) was placed in a stainless steel, high-pressure vessel that was machined from a hexagonal stock (4 in. long × 7/16 in. i.d.) and fitted with a 5000 psi Omega pressure gauge and a needle valve at one end and a NPT plug at the other end. MAH (0.2 g, 2.0 mmol) and BPO (0.08 g, 0.3 mmol) were also added to the vessel which was then put in an ethylene glycol/water bath (60 °C) before it was filled with ∼5.8 g of carbon dioxide (2500 psi). The reactants were heated to 60 °C for 4 h before transferring the vessel to a silicone oil bath (125 °C) for 4 h. The vessel was quenched in cold, running water immediately after removal from the oil bath. (Scheme 2). The purification procedure involved precipitating the product four times in THF/water. The polymer film of the maleated PF3E (PF3E-g-MAH) was then cast and dried in a vacuum oven at 75 °C for 48 h. Preparation of PF3E Films. Polymer films were cast from a 5% w/v solution of PF3E in THF onto the surface of a Petri (34) Cais, R. E.; Kometani; J. M., Macromolecules 1984, 17, 1932. (35) Kwok, D. Y.; Neumann, A. W. Colloids Surface A 2000, 161, 49.
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Figure 1. Gas/vacuum manifold used for heterogeneous chlorination reactions.
Figure 2. Vapor-phase chlorination of PF3E.
dish. The dish was covered with a perforated sheet of aluminum foil to allow slow evaporation of the solvent at room temperature overnight. The resulting polymer film was detached from the dish by introducing water at the edges of the film and allowing it to float-off. Films were dried in a vacuum oven at 75 °C for 24 h. Chlorination of PF3E Films (Figure 2). Heterogeneous chlorination reactions of PF3E films were carried out in a secondary manifold (Figure 1) that could be evacuated and filled with nitrogen and chlorine. Reactions were carried out under atmospheric chlorine pressure in the dark and using either ambient light or a high-intensity UV light source which was placed at the base of an aluminum foil funnel below the sample container. For reactions that were run in the dark, the manifold was wrapped with a thick, black cloth. The sample vessel (containing the tared sample), the chlorine reservoir and the secondary manifold were evacuated and refilled with nitrogen several times before each addition. Chlorine gas (5 psi) was introduced after the last evacuation. The overpressure of chlorine gas in the manifold was depressurized through a bubbler to allow the pressure inside to equilibrate with the outside pressure. The reaction was stopped by evacuating the system and replacing the unreacted chlorine and product HCl with nitrogen. Samples were dried/degassed under vacuum at room temperature for 24 h. Preparation of Silicon-Supported Amine Surfaces. Clean silicon wafers (1.5 × 1.5 cm2) were supported on a specially built glass wafer holder which was introduced in a Schlenk tube. The wafer holder was designed to ensure no contact between the smooth side of the wafers and the glass. The Schlenk tube was capped, and APDMES (0.5 mL) was added via syringe to the bottom of the tube such that there was no contact between the silane and the clean wafers. Silanization was performed in the vapor phase at 70 °C for 24 h. After silanization, the wafers were rinsed with toluene, 2-propanol, ethanol, and water (in that order) and dried under vacuum at room temperature for 30 min. Adsorption Studies (Figure 3). PF3E solutions of different concentration in THF and THF/toluene mixtures (2.5:7.5, 5:5, 3:7 v/v) were prepared. PF3E solutions in THF/toluene mixtures were made by addition of the desired amount of toluene to a homogeneous solution of PF3E in THF. PF3E dissolves in THF very slowly upon heating so the solutions were prepared a day prior to adsorption studies and toluene was added immediately before adsorption. Substrates were submerged in the PF3E solutions at room temperature for 24 h. The samples were then
Figure 3. Adsorption of PF3E (a) or PF3E-g-MAH (b) onto oxidized silicon (Si/SiO2) and silicon-supported amine surfaces (Si/SiO2APDMES).
Figure 4. XPS survey and C1s high-resolution spectra of PCTFE (A,C) and PF3E (B,D) films analyzed using a 75° takeoff angle.
rinsed with the same solvent used for adsorption and dried under vacuum overnight at room temperature. Desorption studies were done by immersing the samples in THF for 24 h. The PF3E was found to be irreversibly attached to the silicon surface and no desorption was observed after exposure to THF for 24 h.
Results and Discussion Synthesis of PF3E. PF3E was synthesized by the reductive dechlorination of PCTFE (Scheme 1). This radical reaction was conducted in THF at 66 °C where AIBN and Bu3SnH were used as the initiator and reducing agent, respectively. The mechanism is illustrated in the Supporting Information. An excess molar ratio of Bu3SnH was used to ensure the complete dechlorination. PF3E films were prepared by casting from a 5% w/v THF solution. The films were transparent, colorless, and flexible. The PF3E films exhibited contact angles of 94° ( 2°/68° ( 2°. Our dynamic contact angle data were in close agreement with the static angle (92°) predicted by Neumann et al., 35 whose analysis evaluated solid surface tension using experimental contact angle patterns. Figure 4 shows the XPS survey and C1s spectra of PF3E and PCTFE films acquired at a 75° takeoff angle. The stoichiometry of PF3E product from quantitative XPS (75° takeoff angle) was C ) 29.1%, F ) 70.8%, O ) 0.2%, Sn ≈ 0%, and Cl ≈ 0%. This stoichiometry and the disappearance of the Cl2p peak indicates both the complete removal of residual tin compounds (Bu3SnCl and unreacted Bu3SnH) and complete dechlorination. The C1s peak of PCTFE appeared as a broad spectrum, which is a combination of carbons from CF2 and CFCl. Upon
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Figure 6. Kinetics of chlorination of PF3E using ambient light (0) and high-intensity UV light (9), determined by XPS at a 15° takeoff angle.
Figure 5. ATR-IR spectra of PF3E films before and after chlorination by high-intensity UV irradiation in comparison with PCTFE.
dechlorination, the C1s spectrum was characterized by two peaks at 289.25 (47.65%) and 291.47 eV (52.35%) that corresponded to (-FCHCF2-)n and (-FCHCF2-)n carbons, respectively. Spectra A and E of Figure 5 show the ATR-IR spectra of PF3E and PCTFE films, respectively. Complete dechlorination was also confirmed by the disappearance of the signal attributed to C-Cl vibration, which appears at 950 cm-1. Chlorination of PF3E. Two series of photochlorination reactions were carried out at atmospheric chlorine pressure using both ambient light and a high-intensity UV light source. The mechanism should be based on a radical chain reaction, commonly known for chlorination of saturated hydrocarbons in the presence of light. ATR-IR spectra for PF3E samples photochlorinated by this method are shown in Figure 5. The appearance of a C-Cl stretching band (950 cm-1) in the chlorinated samples proved that the chlorination experiments were successful. The intensity of the C-Cl stretching band of chlorinated PF3E film samples increased with reaction time, until a plateau was reached at ∼100 min. Comparison of transmission IR (data not shown) with ATR IR spectra indicated that the chlorination of PF3E is surface selective; only small peak changes were observed by transmission IR after reaction. Kinetics of chlorination was studied by XPS, as shown in Figure 6. The slow increase of the Cl/C ratio with exposure time using ambient light indicated that, under these conditions, the chlorination proceeded very slowly and with low yield. The chlorination using a high-intensity UV light occurred faster and resulted in a more densely chlorinated layer. Under UV irradiation, the Cl/C ratio reached a limiting value of ∼0.45 after 10 min of chlorination, close to the value of a fully chlorinated PF3E sample (Cl/C of PCTFE is 0.5). The data recorded from 15° and 75° (see Supporting Information) takeoff angles were similar, indicating that there was little or no surface selectivity in the XPS sampling region (within the outer 40 Å) and that the chlorination proceeded throughout the XPS sampling depth. Chlorination reactions were also conducted in the dark, but no chlorine was detected even
Figure 7. Transmission IR spectra of (a) PF3E (b) PF3E-g-MAH after precipitation; (c) PF3E-g-MAH after drying.
after 20 min of exposure time. This demonstrates a significant effect of photointensity on the extent of chlorination. The C/F ratio remains almost unchanged (0.67 ( 0.02 or close to 2/3) after chlorination regardless of the condition used for chlorination (time or light intensity). This suggests that only hydrogen atom in each repeat unit of PF3E is replaced by a chlorine atom meaning that only C-H bond is homolytically cleaved during the process of chlorination via radical mechanism and C-F bond is unaffected. Maleation of PF3E. The maleation of PF3E via radical grafting of MAH was attempted by using two different initiators. In the first case, AIBN was used as the radical source and the reaction was conducted in homogeneous solutions of THF at 65 °C. These reactions proved to be unsuccessful. The other initiator used was BPO, which is commonly used as a radical source for grafting reactions.36 The longer half-life of BPO (60 h at 60 °C)37 compared to AIBN (20 h at 60 °C)38 prevented the use of either THF or methanol as solvents for the reaction. Therefore, maleations were conducted in supercritical carbon dioxide (ScCO2). Figure 7 shows the transmission spectra of PF3E and its maleated derivatives after precipitation and drying. The carbonyl peak, which appears at 1787 cm-1 and attributed to grafted MAH, was a clear indication of successful reaction. The ratio of the intensities of the carbonyl peak at 1787 cm-1 (υCdO) to that at 1718 cm-1 (assigned to unreacted MAH) increased when the sample was dried and (36) Sathe, S. N.; Rao, G. S. S.; Devi, S. J. Appl. Polym. Sci. 1994, 53, 239. (37) Huang, N. J.; Sundberg, D. C. J. Polym. Sci. Pol. Chem. 1995, 33, 2533. (38) Grignard, B.; Jerome, C.; Calberg, C.; Detrembleur, C.; Jerome, R. J. Polym. Sci. Pol. Chem. 2007, 45, 1499.
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Figure 8. F/Si ratio (calculated from XPS composition data at 75° takeoff angle) of PF3E film adsorbed on Si/SiO2 from THF/toluene (3:7; v/v) (0) and THF (9) as a function of PF3E solution concentration at room temperature.
stabilized after 2 days of drying. An O-H stretching band was not observed above 3000 cm-1, implying that hydrolyzed monomer did not exist in the product. Adsorption of PF3E and Maleated PF3E. Adsorption kinetics of PF3E and its maleated derivatives on silicon surfaces was measured by following the change in surface chemical composition with time by XPS. The results showed no clear trend with adsorption time, so we arbitrarily chose a 24 h period to determine the effect of other variables on adsorption. The effect of PF3E concentration on adsorbed layer composition is presented in Figure 8. PF3E isotherms on Si/SiO2 (a water contact angle ≈ 0°) with silanol groups (Si-OH) are shown for two different solvent compositions: THF and THF/toluene (3:7; v/v). For both compositions studied, the F/Si ratio remained constant until a polymer concentration of 2.5 mg/mL. At this point, the amount of adsorbed material increased considerably. The data suggest that at low polymer concentrations (
