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Surface Modification of Poly(tetrafluoroethylene-co-hexafluoropropylene) by Adsorption of Functional Polymers Bryony Coupe, Maria E. Evangelista, Rachel M. Yeung, and Wei Chen* Chemistry Department, Mount Holyoke College, South Hadley, Massachusetts 01075 Received October 23, 2000. In Final Form: January 3, 2001 Surfaces containing controllable densities of hydrophilic functional groups (carboxylic acids and amines) were prepared by the adsorption of functional polymers from aqueous solutions to fluoropolymer film samples. The adsorption behaviors of poly(acrylic acid) (PAA), poly(allylamine hydrochloride) (PAH), and polyethylenimine (PEI) at the poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP)/water interface were studied and compared with the behavior of poly(L-lysine) (PLL), a polymer that has been reported to adsorb at the FEP/water interface. Insight into how polymer structure and adsorption variables (pH, time, ionic strength) contribute to “hydrophobic interactions” (minimization of interfacial free energy) was gained. Wettability of the functionalized FEP film samples was assessed by advancing and receding contact angles (θA/θR), and the adsorbed amounts were determined using X-ray photoelectron spectroscopy. Adsorbed amounts are highest at pH values at which the polymers are not charged; this is predominantly because of lower solubility under these conditions. For the three polyamines studied, the maximum adsorbed amount decreases in the order PLL > PAH > PEI, which is the opposite order of their solubilities. Of the two structurally similar polymers, PAA (-CO2H) promotes greater water wettability than PAH (-NH2). Added salt increases the adsorbed amount dramatically by screening repulsive interactions between ionic groups at pH values when these weak acids and bases are charged. Adsorption of a single functional polymer, PLL, PAA, PAH, or PEI, did not result in significant improvement of wettabilityswater contact angles (θA/θR) changed from 117°/93° (FEP) to 104°/16° (FEP-PLL), 103°/21° (FEP-PAH), 113°/72° (FEPPEI), and 99°/22° (FEP-PAA). Sequential adsorption of two functional polymers, however, resulted in highly wettable surfaces: 39°/15° (FEP-PLL-PAA), 66°/17° (FEP-PAH-PAA), 74°/11° (FEP-PEIPAA), and 61°/17° (FEP-PAA-PAH). Peel tests indicate that the adhesive properties of the fluropolymer are improved after adsorption of functional polymers, especially after adsorption of two functional polymers.
Introduction The most chemically inert and hydrophobic polymers are the Teflons, which include poly(tetrafluoroethylene) (PTFE) and poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP). These materials have been the subject of numerous surface chemistry studies directed at increasing surface energy, primarily to promote adhesion. A review of fluoropolymer surface modification is available.1 Polymers can generally be modified to impart wettability simply by the incorporation of polar functionality (-NH2, -OH, -CO2H) and there are viable chemical approaches for most polymers. This is not the case for fluoropolymers as there are no direct chemical methods for the conversion of CF, CF2, and CF3 groups in perfluoroalkanes to any functionality. The most successful (and only commercially used) chemical modification of fluoropolymers involves single electron reductionsa corrosive reaction that yields a wide range of products.2 Plasma chemistry3-7 and plasma-induced surface grafting8,9 have also been used to (1) Siperko, L. M.; Thomas, R. R. J. Adhes. Sci. Technol. 1989, 3, 157. (2) Costello, C. A.; McCarthy, T. J. Macromolecules 1987, 20, 2819. (3) Vargo, T. G.; Gardella, J. A., Jr.; Calvert, J. M.; Chen, M. S. Science 1993, 262, 1711. (4) Ranieri, J. P.; Bellamkonda, R.; Bekos, E. J.; Vargo, T. G.; Gardella, J. A.; Aebischer, P. J. Biomed. Mater. Res. 1995, 29, 779. (5) Hsieh, M. C.; Farris, R. J.; McCarthy, T. J. Macromolecules 1997, 30, 8453. (6) Nitschke, M.; Menning, A.; Werner, C. J. Biomed. Mater. Res. 2000, 50, 340. (7) Chandy, T.; Das, G. S.; Wilson, R. F.; Rao, G. H. R. Biomaterials 2000, 21, 699. (8) Zhang, J. F.; Cui, C. Q.; Lim, T. B.; Kang, E. T.; Neoh, K. G. J. Adhes. Sci. Technol. 2000, 14, 507. (9) Wang, P.; Tan, K. L.; Kang, E. T. J. Biomater. Sci., Polym. Ed. 2000, 11, 169.
modify Teflon surfaces. Other methods include excimer laser treatment10 and UV-induced surface modification.11 In general, fluoropolymers are not well-functioning substrates for surface modification; in particular, none of the reported methods introduce discrete functional groups, but rather a mixture of species. While workers in polymer modification research have tried, with limited success, to get things to “stick to Teflon”, it is puzzling that there are numerous reports from other research fields of biopolymers spontaneously adsorbing to fluoropolymers. Proteolytic (R-chymotrypsin) and lipolytic (cutinase) enzymes adsorb to PTFE from aqueous solution,12 β-casein13 and immunoglobulin-G14 adsorb to PTFE from water and change their secondary structures, and bacteria adsorb to PTFE at appropriate pH values and ionic strengths.15 Surfactants such as cetyltrimethylammonium bromide also adsorb to the Teflon/water interface.16 There is one report of a homopolymerspoly(L-lysine)sadsorbing to a fluoropolymer from aqueous solution.17 These workers point out that it is the reduction of interfacial free energy (the displacement of high-energy (10) Revesz, K.; Hopp, B.; Bor, Z. Langmuir 1997, 13, 5593. (11) Noh, I.; Goodman, S. L.; Hubbell, J. A. J. Biomater. Sci., Polym. Ed. 1998, 9, 407. (12) Zoungrana, T.; Findenegg, G. H.; Norde, W. J. Colloid Interface Sci. 1997, 190, 437. (13) Caessens, P. W.; De Jongh, H. H. J.; Norde, W.; Gruppen, H. Biochim. Biophys. Acta, Protein Struct. Mol. Enzymol. 1999, 1430, 73. (14) Vermeer, A. W. P.; Bremer, M. G. E. G.; Norde, W. Biochim. Biophy. Acta, Gen. Subjects 1998, 1425, 1. (15) Rijnaarts, H. H. M.; Norde, W.; Lyklema, J.; Zehnder, A. J. B. Colloid Surf., B 1995, 4, 191. (16) Janczuk, B.; Zdziennicka, A.; Wojcik, W. Eur. Polym. J. 1997, 33, 1093. (17) Shoichet, M. S.; McCarthy, T. J. Macromolecules 1991, 24, 1441.
10.1021/la001488k CCC: $20.00 © 2001 American Chemical Society Published on Web 02/21/2001
Surface Modification by Adsorption of Polymers
water molecules from the fluoropolymer/water interface) that drives polymer or biopolymer adsorption (“hydrophobic interactions”). Wettability, hydrophilicity, and hydrophobicity are currently qualitative terms that cannot be specifically related to molecular or supermolecular structural features. It is certainly well-known that introducing polar functionality to nonpolar surfaces makes them more wettable, but whether introduction of alcohols,18 amines, or carboxylic acids is more favored for a given situation has not been addressed. The concentration (surface density) of functionality required to impart a particular level of wettability to a given surface is unknown as well. In this study, surfaces containing controllable densities of hydrophilic functional groups (carboxylic acids and amines) were prepared by adsorption of poly(acrylic acid), poly(allylamine hydrochloride), and polyethylenimine at the fluoropolymer/water interface under different conditions (varying pH, adsorption time, and solution ionic strength). These studies have led to protocol for a rational control of surface density of discrete and different functional groups and allow for the preparation of a series of samples that are ideally suited for fundamental studies of wettability. Binary mixed fluoropolymer-supported layers were also prepared in an effort to enhance wettability. We have chosen poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) film as a substrate for surface modification for several reasons: (1) Fluoropolymers are popular choices for biomedical devices due to their excellent chemical inertness and mechanical properties as well as their good, compared with most other polymers, biocompatibility. (2) FEP is extremely hydrophobic and we plan to introduce hydrophilic groups; the effects of the introduced groups on wettability will be readily apparent. (3) FEP is chemically dissimilar (containing only CF, CF2, and CF3 groups) from the functionalities that we plan to attach. This makes analysis of surface-chemical changes convenient from changes in the X-ray photoelectron spectra. (4) As a chemically inert substrate, FEP will not interfere with chemistry of the attached functionality. Experimental Section Materials. Poly(acrylic acid) (Mw ) 90 000) was obtained from Polysciences. Poly(L-lysine hydrobromide) (Mw ) 331 000) was obtained from Sigma. Poly(allylamine hydrochloride) (Mw ) 70 000), polyethylenimine (Mw ) 750 000), potassium chlorate, and inhibitor-free THF were obtained from Aldrich. Other chemicals were purchased from Fisher. All reagents were used as received. Water was purified using a Millipore Milli-Q system that involves reverse osmosis followed by ion-exchange and filtration steps. Methods. Either HCl or NaOH was used for pH adjustments. An Accumet AR15 pH meter was used for pH measurements. Contact angle measurements were made with a Rame´-Hart telescopic goniometer and a Gilmont syringe with a 24-gauge flat-tipped needle. The probe fluid was water, purified as described above. Dynamic contact angles, advancing (θA) and receding (θR), were recorded while the probe fluid was added to and withdrawn from the drop, respectively. Each reported angle represents an average of at least six measurements. X-ray photoelectron spectra (XPS) were recorded on a Perkin-Elmer Physical Electronics 5100 spectrometer with Mg KR excitation (15 kV, 400 W). Spectra were recorded at two different take off angles, 15° and 75°, between the plane of the sample surface and the entrance lens of the detector optics. Atomic concentration data were determined using sensitivity factors obtained from samples of known composition: C1s, 0.250; O1s, 0.660; N1s, 0.420; F1s, 1.000. Contact angle measurements and XPS analyses were (18) Adsorption behavior of poly(vinyl alcohol) will be reported in a separate paper.
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Figure 1. Adsorption of functional polymers to the FEP/water interface. done immediately after sample preparation. Peel tests were performed manually with an angle of 180° between the film surface and tape (3M no. 810). Substrate. FEP films (Berghof, 5 mil) were cleaned in 25 mL of concentrated H2SO4 solution containing 0.5 g of KClO3 for 2 h. The film samples were rinsed with Milli-Q water (5×) and THF and then dried (room temperature, 24 h). Preparation of FEP-PLL.17 Clean FEP films were immersed in 0.5 mM (based on repeat units) PLL aqueous solution (pH 11) either with or without 1 M NaCl at room temperature for 72 h. Film samples were then rinsed with Milli-Q water (3×) and dried (room temperature, 24 h). Preparation of FEP-PAA, FEP-PAH, FEP-PEI. Clean FEP films were immersed in 0.01 M (based on repeat units) PAA, PAH, or PEI aqueous solution, at a prescribed pH either with or without 1 M NaCl at room temperature for a prescribed amount of time. Film samples were then rinsed with Milli-Q water (3×) and dried (room temperature, 24 h). Preparation of FEP-PLL-PAA, FEP-PAH-PAA, and FEP-PEI-PAA. FEP-PLL samples were prepared by immersing in 0.5 mM (based on repeat units) PLL aqueous solution (pH 11) at room temperature for 72 h. FEP-PAH and FEP-PEI samples were prepared by introducing clean FEP films to 0.01 M (based on repeat units) PAH and PEI aqueous solutions (pH 11.5) at room temperature for 72 h, respectively. The film samples were then rinsed with Milli-Q water (3×) and dried (room temperature, 24 h). The samples were then introduced to 0.01 M PAA aqueous solution at a prescribed pH at room temperature for a prescribed amount of time. The films were then rinsed with Milli-Q water (3×) and dried (room temperature, 24 h). Preparation of FEP-PAA-PAH. Clean FEP films were immersed in 0.01 M PAA aqueous solution (pH 2) at room temperature for 24 h. The film samples were then rinsed with Milli-Q water (3×) and dried (room temperature, 24 h). The samples were then introduced to 0.01 M PAH aqueous solution at a prescribed pH at room temperature for a prescribed amount of time. The films were then rinsed with Milli-Q water (3×) and dried (room temperature, 24 h). Desorption of FEP-PLL, FEP-PAA, FEP-PAH, and FEP-PEI. After FEP-PLL, FEP-PAA, FEP-PAH, and FEPPEI samples were prepared according to the procedures described above, they were then introduced to Milli-Q water at a prescribed pH at room temperature for a prescribed amount of time. The films were then rinsed with Milli-Q water (3×) and dried (room temperature, 24 h).
Results and Discussion Adsorption of One Functional Polymer to the FEP/ Water Interface. Our approach is to take advantage of the high FEP/water interfacial free energy and adsorb polymers with discrete functional groups as indicated in Figure 1. Poly(acrylic acid) (PAA), poly(allylamine hydrochloride) (PAH), and polyethylenimine (PEI) were our choices.18 We also used poly(L-lysine) (PLL) as a control to compare our results with literature data. Adsorption behavior of the chosen functional polymers was studied as a function of pH, adsorption time, and ionic strength of the solutions. Figure 2 shows advancing and receding water contact angle data of FEP-PAA and FEP-PAH prepared at different solution pH values; other
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Coupe et al. Table 1. XPS % O (15° take off angle) of FEP-PAA as a Function of Ionic Strength at Different pH Values (0.01 M PAA, 24 h, room temperature) pH
without salt
with 1 M NaCl
2 5 9
7.97 1.81 0.7
8.31 6.82 3.67
Table 2. XPS Atomic Composition Data (15° take off angle) and Advancing and Receding Water Contact Angles (deg) for FEP Derivatives
Figure 2. Adsorption of PAA (0.01 M PAA, room temperature, 72 h: θA (b); θR (O)) or PAH (0.01 M PAH, room temperature, 120 h: θA (2); θR (4)) to FEP as a function of pH.
Figure 3. Adsorption kinetics of PAH to FEP (0.01 M PAH, room temperature, pH 11.5): θA (b); θR (O).
adsorption parameters remained constant. At pH 2 for PAA and pH 11.5 for PAH, minimum water contact angle values were observed due to highest adsorbed amounts. The lower solubility of PAA and PAH in water at these pH values at which they are not charged (pKa values for -CO2H and -NH3+ are ∼ 4.5 and ∼ 10.5, respectively.19) is an additional driving force. The dependence of pH on adsorption of PLL and PEI follows the same trend as that of PAH. Adsorption kinetics of these polymers at optimized (to yield greatest adsorbed amounts) pH values were also determined. As indicated in Figure 3, for FEP-PAH, receding water contact angles dropped rapidly within the first few hours and reached a plateau after 24 h; advancing water contact angles did not change significantly as a function of time and reached a minimum at 72 h adsorption time. Seventy two hours of adsorption for PAH, 24 h of adsorption for PAA (not shown), and 72 h of adsorption for PEI (not shown) were chosen as the adsorption time for further experiments. The effect of ionic strength was also investigated. Table 1 shows the oxygen content determined using XPS at a 15° take off angle for the FEPPAA sample that had been prepared with and without 1 M NaCl. When PAA is partially charged (pH 5) and PAA is fully charged (pH 9), the added salt increases the adsorbed amount. This is rationalized because the salt both increases the surface tension of water, increasing the FEP/water interfacial free energy, and screens repulsive interactions between ionic groups on PAA. Salt (19) Brown, H. C.; McDaniel, D. H.; Ha¨flinger, O. Determination of Organic Structures by Physical Methods; Braude, E. A., Nachod, F. C., Eds.; Academic Press: New York, 1955; Vol. 1, pp 567-662.
film samples
%C
%F
%O
%N
θA/θR
FEP FEP-PLL FEP-PAH FEP-PEI FEP-PAA FEP-PLL-PAA FEP-PAH-PAA FEP-PEI-PAA FEP-PAA-PAH
34.04 54.75 47.59 43.98 42.68 54.11 53.03 47.32 48.92
65.32 38.02 44.68 50.41 49.36 27.27 27.15 36.73 38.61
0.64 3.53 3.43 4.48 7.97 16.88 18.51 15.07 9.41
3.70 4.31 1.13 1.73 1.31 0.88 3.07
117/93 104/16 103/21 113/72 99/22 39/15 66/17 74/11 61/17
concentration does not significantly affect the adsorbed amount when PAA chains are not charged (pH 2). In all systems studied, the addition of salt had very little affect on the maximum adsorption amount at pH values at which the polymers are uncharged. Table 2 shows XPS atomic composition data (15° take off angle) and water contact angle data for FEP, FEPPLL, FEP-PAH, FEP-PEI, and FEP-PAA adsorption experiments carried out under conditions that maximize adsorbed amounts. Both 15° and 75° take off angle XPS data were acquired for all samples prepared in this study. These analyses assess the composition of the outermost ∼10 and ∼40 Å of the samples, respectively.20 In all cases, take off angle dependent fluorine content was observed indicating a thin film of adsorbed polymer. This indicates that the thickness of the adsorbed functional polymers is very thin, PAH > PEI, which is the opposite order of the solubilities of the three polyamines. Of the two structurally similar polymers, PAA and PAH, less PAA is adsorbed to FEP. The water contact angles of FEPPAA are similar to those of FEP-PAH; this indicates that PAA (-CO2H) contributes more to wettability than PAH (-NH2). The adsorbed functional polymers affect receding contact angles significantly (except for FEP-PEI due to low adsorbed amount) but not advancing contact angles indicating hydration of these polymers during analysis and that the adsorbed amounts are very low and that perhaps the adsorbed layers are discontinuous. The extent of modification of FEP with PLL, PAH, PEI, and PAA is minimal based on XPS and contact angle data (Table 2). To increase wettability on modified surfaces, sequential adsorption experiments using two functional polymers were carried out. Desorption studies of the first adsorbed functional polymer were carried out to assess the stabilities of the initial adsorbed layer during the adsorption of the second polymer. (20) The values are calculated using 14 Å as the mean free path of C1s electrons ejected using Mg KR irradiation. This value was measured in poly(p-xylylene): Clark, D. T.; Thomas, H. R. J. Polym. Sci.: Polym. Chem. Ed. 1977, 15, 2843.
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Figure 4. Sequential adsorption of functional polymers to the FEP/water interface. Table 3. Advancing and Receding Water Contact Angles (deg) and XPS % O (15° take off angle) for FEP-PAA as a Function of Desorption Time and pH after 1 h of desorption
after 24 h of desorption
pH
θA/θR
%O
θA/θR
%O
2 5 9
65/19 99/28 105/30
11.32
97/18 83/16 99/15
7.83
Desorption of the Adsorbed Functional Polymers. For each system, desorption studies for 1 and 24 h in water at different pH values were carried out. For FEPPAA, water contact angle data and oxygen content (XPS 15° take off angle) after desorption are presented in Table 3. At pH 2, desorption data after 1 h indicate that the advancing contact angle is lower and the oxygen content is higher than before desorption (Table 2), likely due to a reorientation of -CO2H groups in pH 2 water (destruction of the acid dimer held together by hydrogen bonding) and hydration of the PAA layer. After 24 h of desorption at the same pH, an increase of the advancing contact angle and a decrease of the oxygen content indicate the occurrence of desorption at longer times. It is difficult to determine the importance of desorption at different pH valuessa combination of reorientation of -CO2H groups, hydration, and desorption is at play. For FEP-PAH, the advancing and receding contact angles are identical to those measured before desorption (Table 2) at all pH values and at both desorption times; this indicates that adsorption is irreversible and that desorption does not occur under these conditions. FEP-PLL behaves similarly to FEPPAH as assessed by the desorption studies. No desorption occurs for FEP-PEI after 1 h at all pH values (data not shown). Different systems behave differently in desorption studies; however, no obvious desorption occurs after 1 h at all pH values studied. Sequential Adsorption of Two Functional Polymers: Binary Mixed Polymer Layers. On the basis of XPS and contact angle data (Table 2), the extent of modification of FEP with PLL, PAH, PEI, and PAA is minimal. Even though low water contact angle and high oxygen content data were observed after desorption of FEP-PAA at pH 2 for 1 h (Table 3), we suspect that the stability of this surface is poor and not suitable for any application. In an effort to increase wettability, sequential adsorption studies using two functional polymers were undertaken. After initial adsorption, PAA, PAA, PAA, and PAH were adsorbed to FEP-PLL, FEP-PAH, FEP-PEI, and FEP-PAA surfaces, respectively, under different conditions (pH and time). Figure 4 is a schematic representation of the sequential adsorption of PAH and PAA to FEP. The type of interaction between PAH and PAA depends on pH: hydrogen bonding occurs at low pH (shown in Figure 4) and at high pH when PAA is charged and PAH is neutral; electrostatic interaction occurs at intermediate pH values when both polymers are charged. Figure 5 shows advancing and receding contact angles of
Figure 5. Adsorption of PAA (0.01 M PAA, room temperature) for 1 h to FEP-PAH as a function of pH: θA (b); θR (O).
FEP-PAH-PAA as a function of pH values of PAA solutions after 1 h adsorption. Receding contact angles are essentially independent of pH. The advancing contact angle is the lowest at pH 2 (66°), intermediate at pH 5 (86°), and highest at pH 9 and 11.5 (∼100°). At intermediate pH values (e.g., pH 5) where both PAA and PAH are charged, strong electrostatic attraction between -CO2and -NH3+ results in flat polymer conformations, low adsorbed amount, and high advancing contact angle. At low pH values (e.g., pH 2), weaker but favorable hydrogen bonding between -NH3+ (a good donor) and -CO2H (a good acceptor) results in more loops and tails, a higher adsorbed amount and lower advancing contact angle. At high pH values (e.g., pH 11.5), unfavorable hydrogen bonding between -CO2- (a good acceptor) and -NH2 (a relatively poor donor) results in a low adsorbed amount and high advancing contact angle. For the same reason, the highest adsorbed amounts and lowest contact angles were obtained in systems of FEP-PAA-PAH, FEPPLL-PAA, and FEP-PEI-PAA at pH 2. Rubner et al. report pH-dependent multilayer adsorption of PAA and PAH onto glass and Si wafers.21 The pH-dependent adsorption behavior observed here differs from that reported by Rubner. This is likely due to differences in substrates and polyelectrolyte interactions. Sequential adsorptions were carried out for 24 h at pH 2. For FEP-PLL and FEP-PAH, where desorption of the first functional polymer did not occur, the contact angles of the surfaces after adsorbing PAA did not change as a function of time. For FEP-PAA and FEP-PEI where desorption occurred after 1 h, the advancing contact angles of the surfaces after adsorbing the second functional polymer are higher after 24 h of adsorption than after 1 h of adsorption. XPS atomic composition (15° take off angle) and contact angle data of the surfaces after sequential adsorptions are in Table 2. Again, fluorine is present in large quantities for all samples indicating that the binary mixed polymer layers are very thin. The nitrogen content is very low indicating low functional group densities on these surfaces. The high oxygen content is most likely due to hydrogen-bonded water in these nanolaminates. It is apparent that the enhanced wettability of sequentially modified surfaces is due not to high functional group densities but to large quantities of trapped water molecules. (Binary mixed layers are more effective than singular layer at hydrogen bonding of water even at XPS operating pressure, 10-8 Torr.) As mentioned earlier, the adsorbed amount of the three polyamines decreases in (21) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213.
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Table 4. XPS Atomic Composition Data (15° take off angle) of the Tape after Peel Tests film samples
%C
%O
%F
tape, control tape (FEP) tape (FEP-PAH) tape (FEP-PAH-PAA)
87.23 86.42 83.17 74.48
12.77 12.80 10.91 12.96
0.77 5.92 12.57
the order of PLL > PAH > PEI. Comparing surfaces of FEP-PLL-PAA, FEP-PAH-PAA, and FEP-PEI-PAA, advancing contact angles increase in the same order and the total adsorbed amount is least (fluorine content was the highest) for FEP-PEI-PAA and comparable for FEPPLL-PAA and FEP-PAH-PAA. This suggests that wettability of surfaces modified by sequential adsorption are determined to a large extent by the adsorbed amount of the first functional polymer. Overall, sequential adsorption of two functional polymers is much more effective in improving the wettability of FEP than adsorption of one functional polymer. Mechanical Integrity of the Nanolaminates. In order for the method of adsorbing functional polymers to fluoropolymers to be practical, the adsorbed nanolaminates must have mechanical integrity and must adhere to the substrate. We have carried out simple peel tests using pressure-sensitive adhesive tape to determine the locus of failure in substrate/layer(s)/adhesive tape composites. The tape was applied and peeled from the samples and XPS spectra of both surfaces were compared with spectra obtained before the adhesive joint was formed. For all of the modified FEP surfaces studied as well as the virgin FEP control, the film surfaces were indistinguishable to virgin FEP (Table 2) after peel tests, indicating that all the adsorbed functional polymers were transferred to the tape. Some surface composition data of the tape after peel tests from FEP samples are given in Table 4. The tape composition changed only slightly after peeling from virgin FEP indicating adhesive failure at the FEP/tape interface. That almost 6% fluorine (15° take off angle) is present on the tape peeled from FEP-PAH suggests that cohesive failure occurred within FEP substrate. Nitrogen from the functional polymer was less than 1% of the atoms detected on the tape; the PAH chains are covered by FEP chains resulting from the cohesive failure. Similar results were obtained with other FEP samples adsorbed with one functional polymer. These indicate that the mechanical strength of the adsorbed
nanolaminates and the adhesive strength between FEP and the nanolaminates are stronger than the cohesive strength of the substrate. Peel tests were also performed on FEP with two sequentially adsorbed functional polymers. Even more fluorine (12.57%) is present on the tape peeled from FEP-PAH-PAA. Peel tests with pressuresensitive adhesive tape indicate that the adhesive properties of FEP film samples are improved after adsorption of functional polymers, especially after adsorption of two functional polymers. Conclusions The adsorption behaviors of poly(acrylic acid), poly(allylamine), and polyethylenimine at the poly(tetrafluoroethylene-co-hexafluoropropylene)/water interface were studied as a function of solution pH, adsorption time, and solution ionic strength. Wettability of the functionalized FEP film samples was assessed by advancing and receding water contact angles; the adsorbed amount was determined using X-ray photoelectron spectroscopy. The adsorbed amount is the highest at pH values when these polymers are not charged due to lower solubility under these conditions. Of the three polyamines, the adsorbed amount decreases in the order of PLL > PAH > PEI, which is the opposite order of their solubilities. Of the two structurally similar polymers, PAA (-CO2H) contributes more to wettability than PAH (-NH2). Added salt increases the adsorbed amount dramatically by screening repulsive interactions between ionic groups at pH values at which these weak acids and bases are charged. Adsorption of one functional polymer, PLL, PAA, PAH, or PEI, does not result in significant improvements of wettability. Sequential adsorption of two functional polymers results in very wettable surfaces. Peel tests indicate that adsorption of functional polymers, especially adsorption of two functional polymers, improves the adhesive properties of the fluoropolymer. Acknowledgment. We thank Mount Holyoke College, the Camille & Henry Dreyfus Foundation, the Balfour Foundation, and the Reese Foundation for financial support and the NSF-sponsored Materials Research Science and Engineering Center (MRSEC) for use of the central facilities at University of Massachusetts at Amherst. LA001488K
