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Preparation and Modification of Poly(methacrylic acid) and Poly(acrylic acid) Multilayers C. Mengel,† A. R. Esker,‡ W. H. Meyer, and G. Wegner* Max-Planck Institut fu¨ r Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany, Giesecke & Devrient GmbH, Prinzregentenstrasse 159, 10, D-81607 Mu¨ nchen, Germany, and Virginia Polytechnic Institute and State University, Department of Chemistry, Blacksburg, Virginia 24061 Received August 17, 2001. In Final Form: April 25, 2002 By employment of a strategy of post-transfer modification, precursor Langmuir-Blodgett (LB) films of poly(tert-butyl methacrylate) (PtBMA) and poly(tert-butylacrylate) (PtBA) can be converted to poly(methacrylic acid) (PMAA) and poly(acrylic acid) (PAA) through acid-catalyzed hydrolysis in the gas phase. X-ray reflectivity studies show that these films possess surface roughnesses and controllable thicknesses, which are consistent with the retention of the “two-dimensional” configuration of the precursor polymers at the air/water interface. On this basis, the PMAA and PAA films with presumable layered architecture can be obtained, even though PMAA and PAA are too hydrophilic to undergo direct LB-multilayer formation. A combination of infrared spectroscopy, contact angle measurements, and sorption experiments confirms the chemical transformation and increased hydrophilicity of the films. Using the same approach, gasphase reactions with organic amines convert the reactive carboxylic acid groups to their corresponding ammonium salts, thereby leading to the formation of polyelectrolyte LB films. When these films are heated to elevated temperatures, amide bonds are formed. The use of difunctional amines opens up the possibility to cross-link the PMAA or PAA films efficiently. Fourier transform infrared measurements and X-ray reflectivity studies clearly indicate the conversion from PtBMA or PtBA LB films to ultrathin, highly swellable network films.
Introduction Several approaches leading to the formation of polyelectrolyte films exist. A frequently utilized pathway is the irradiation-induced graft polymerization of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) onto suitable substrates.1,2 Another method is the preparation of hyperbranched PAA films by grafting poly(tert-butyl methacrylate) (PtBMA) to a monolayer of self-assembled mercaptoundecanoic acid and subsequently hydrolyzing the tert-butyl groups.3-5 Besides these approaches for the covalent attachment of polymers to a substrate, methods of physisorption such as cast techniques have also been applied.6 An improvement on these approaches in terms of controlled architecture is hybrid films,7 which combine the Langmuir-Blodgett (LB) film technique with strategies for polyelectrolyte film formation through polyion selfassembly.8 More recently, Rulkens et al. have produced a specially tailored 1:1 poly(p-phenylene) copolymer derivative, where one of the aromatic rings of the repeating unit possesses a C12 alkyl chain and the other carries a sulfonate group as a tetrabutylammonium salt, which forms multilayer LB films on its own.9 * Corresponding author. † Giesecke & Devrient GmbH. ‡ Virginia Polytechnic Institute and State University. (1) Kaji, K. J. Appl. Polym. Sci. 1986, 32, 4405. (2) Okada, A.; Inchinose, N.; Kawanishi, S. Polymer 1996, 37, 2281. (3) Zhou, Y.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M.; Wells, M. J. Am. Chem. Soc. 1996, 118, 3773. (4) Bruening, M. L.; Zhou, Y.; Aguilar, G.; Agee, R.; Bergbreiter, D. E.; Crooks, R. M. Langmuir 1997, 13, 770. (5) Zhou, Y.; Bruening, M. L.; Liu, Y.; Crooks, R. M.; Bergbreiter, D. E. Langmuir 1996, 12, 5519. (6) Dong, J.; Ozaki, Y.; Nakahima, K. J. Polym. Sci., Part B: Polym. Phys. 1997, 35, 507. (7) Lvov, Y.; Essler, F.; Decher, G. J. Phys. Chem. 1993, 97, 13773. (8) Decher, G.; Hong, J. D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. Decher, G. Science 1997, 277, 1232
To maximize the surface density of ionizable groups while retaining a well-defined architecture and small surface roughness, an ideal polyelectrolyte system would be LB films based on salts of PAA or PMAA. Cross-linked systems based on these polymers and their salts would be ideal models for superabsorbent polymers (SAP)10 or cushioning layers for supported membranes.11,12 Unfortunately, these materials are too hydrophilic to directly form LB multilayers. Some attempts to overcome this problem include copolymers containing PAA13 and LB multilayers of PAA complexed with docosylamine.14 However, these attempts suffer from the same limitations as the systems described above. Recently, the formation of ultrathin PAA and PMAA films with layered architecture, formed by post-transfer modification of LB films of precursor polymers poly(tert-butylacrylate) (PtBA) and PtBMA, respectively, has been reported.15 For this approach, isobutene is liberated from the precursor polymers under gas-phase hydrolysis conditions based on an analogous strategy for preparing regenerated cellulose from trimethylsilylcellulose LB films.16-18 Building on these successful strategies, this study details the preparation of PMAA and PAA salts in the gas phase from LB(9) Rulkens, R.; Wegner, G.; Enkelmann, V.; Schulze, M. Ber. BunsenGes. Phys. Chem. 1996, 100, 707. (10) Modern Superabsorbent Polymer Technology; Buchholz, F. L., Graham, A. T., Eds.; Wiley & Sons: New York, 1998. (11) Sackmann, E. Science 1996, 271, 43. (12) Sigl, H.; Brink, G.; Seufert, M.; Schulze, M.; Wegner, G.; Sackmann, E. Eur. Biophys. J. 1997, 25, 249. (13) Tamura, M.; Sekiya, A. Chem. Lett. 1991, 399. (14) Bardosova, M.; Tredgold, R. H.; Ali-Adib, Z. Langmuir 1995, 11, 1273. (15) Esker, A. R.; Mengel, C.; Wegner, G. Science 1998, 280, 892. (16) Schaub, M.; Wenz, G.; Wegner, G.; Stein, A.; Klemm, D. Adv. Mater. 1993, 5, 919. (17) Buchholz, V.; Wegner, G.; Stemme, S.; O ¨ dberg, L. Adv. Mater. 1996, 8, 399. (18) Buchholz, V.; Adler, P.; Ba¨cker, M.; Ho¨lle, W.; Simon, A.; Wegner, G. Langmuir 1997, 13, 3206.
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film precursors and an assessment of the changes in hydrophilicity through contact angle and sorption experiments. Additionally, the cross-linking of these films with simple amide chemistry to achieve network structures is presented. Experimental Section Polymers. Three different polymer samples were prepared via anionic polymerization. All of these were end-functionalized since the introduction of cross-linkable groups offers the opportunity to construct well-defined networks in solution as well as in films, as long as the molecular weight is small.19 The first, linear R,ω-styryl-functionalized PtBMA was obtained by using naphthylsodium as the bifunctional initiator and p-vinylbenzylbromide as the terminating agent.20 A 5-fold excess of LiCl was added to the starter solution as it is known that living polymerization of acrylates can be performed only by reducing the reactivity of the carbanions via the addition of a suitable σor π-ligand.21 Size exclusion chromatography using poly(methyl methacrylate) (PMMA) standards showed that the polymer had a number-average molecular weight of Mn ) 25.8 kg mol-1 and a polydispersity of Mw/Mn ) 1.18. The second polymer, linear R,ω-allyl-functionalized PtBA, was prepared by modifying a method described by Anderson et al.22 1,1,4,4-Tetraphenyldilithiobutane (TPDLB) served as the bifunctional initiator, while allylbromide was used as the terminating agent. The molecular weight data for this polymer were Mn ) 28.8 kg mol-1 and Mw/Mn ) 1.50. The degrees of functionalization for PtBMA and PtBA were estimated as 95 ( 5% and 85 ( 5% by 1H NMR spectroscopy (300 MHz, Bruker). The final polymer prepared by anionic polymerization was a three-armed star PtBMA polymer (Mn ) 36 000 g/mol and Mw/ Mn ) 1.07). This polymer was prepared using a novel initiator, 1-(2-anthryl)-1-phenylhexyl-lithium (APH-Li), and 1,3,5-trisbromomethylbenzene (TBMB) as the terminating agent. A detailed description of the synthesis is available elsewhere.23 The different polymers are abbreviated as Stn-Polymerm for star polymers or R,ω-functionalized-Polymerm for linear bifunctional polymers, where n stands for the degree of functionalization of the branch points and m represents the degree of polymerization determined by GPC of the individual arms or chains, respectively. Substrate Preparation. Silicon wafers and glass and quartz slides were used as substrates for LB-transferred and spin-coated films. All of the samples were cleaned by ultrasonicating for 15 min in dichloromethane, boiling for 30-60 min in a mixture of 30% H2O2/concentrated NH4OH/H2O (1:1:5 by volume), thoroughly rinsing with water, and blow-drying with nitrogen. The samples were then hydrophobized by one of two procedures. The glass and quartz slides, as well as silicon wafers used for infrared spectroscopy (polished on both sides), were hydrophobized by heating them for 2 h at 80 °C in the presence of 1,1,1,3,3,3hexamethyldisilazane (Aldrich, 98%). Silicon wafers used for X-ray reflectivity experiments (polished on one side) were hydrophobized by a 5 min treatment in an ammonium fluoride etchant solution (Selectipur, Merck). Isotherms and Film Formation. Spreading solutions of polymer, 0.5 g/L, were prepared in chloroform (Uvasol, Merck) and spread onto a water subphase (Milli-Q, Millipore) housed in a Lauda FW1 trough. The isotherms were recorded at constant compression rates of 3 mm min-1. LB films for all of the polymers were prepared at a constant surface pressure of Π ) 15 mN m-1 except for the PtBMA star polymer which was transferred at 12 mN m-1. Transfer onto hydrophobized substrates was conducted (19) Mengel, C. Dissertation, University of Mainz, Mainz (Germany), 1998. (20) Lutz, P.; Masson, P.; Beinert, G.; Rempp, P. Polym. Bull. 1984, 12, 79. (21) Anderson, B. C.; Andrews, G. D.; Arthur, P., Jr.; Jacobson, H. W.; Melby, L. R.; Playtis, A. J.; Sharkey, W. H. Macromolecules 1981, 14, 1601. (22) Fayt, R.; Forte, R.; Jacobs, C.; Jerome, R.; Ouhadi, T.; Teyssie, P.; Varshney, S. K. Macromolecules 1987, 20 (6), 1442. Teyssie, P.; Fayt, R.; Hautekeer, J. P.; Jacobs, C.; Jerome, R.; Leemanns, S.; Varshney, S. K. Makromol. Chem., Macromol. Symp. 1990, 32, 61. (23) Mengel, C.; Meyer, W. H.; Wegner, G. Macromol. Chem. Phys. 2001, 202, 1138.
Mengel et al. at a dipping speed of 1.0 cm min-1 resulting in quantitative transfer (transfer ratios ) 1.00 ( 0.05). Chemical Derivatization of the Films. Acid-catalyzed hydrolysis of PtBMA and PtBA films to PMAA and PAA was accomplished by heating the films above approximately 2 mL of concentrated hydrochloric acid for 6 h at a temperature of T ) 60 °C. For conversion to the respective ammonium salts, the hydrolyzed films were exposed to the atmosphere of the amine for 2 h at room temperature. For removal of residual amine, the films were then subsequently dried in the vacuum oven for at least 30 min at room temperature. For conversion to the respective amids, the films were heated under vacuum for at least 2 h at T ) 150 °C. Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectra (200 scans, 4 cm-1 resolution) were recorded with a Nicolet 850 FTIR spectrometer in the transmission mode. Due to the extremely hydrophilic nature of the acids and their salts, the films were dried under vacuum at 50 °C for at least 2 h and were immediately placed into the sample cell upon removal from the oven. The samples were then cooled to room temperature under nitrogen for 1 h before the spectra were recorded. LB films were transferred onto only one-half of the substrate so that the other half could be used for the background correction. X-ray Reflectivity. Measurements were performed on a Philips PW 1820 powder diffractometer using unfiltered Cu KR radiation (λ ) 1.542 Å). The analysis of the data was performed according to the previously established procedure.24 In accord with recent work,25 sorption measurements were performed by placing a small vessel of solvent inside the X-ray chamber. After allowing 15 min for the sample chamber to saturate with vapor and the film to swell, reflectivity profiles were obtained to quantify the increase in film thickness. To observe changes in thickness accompanying chemical transformations of the films beyond hydrolysis, X-ray reflectivity measurements were carried out on a modified diffractometer (Scintag, Santa Clara, CA) using Cu KR radiation with a wavelength of 1.542 Å at the NIST Center for Neutron Research, Gaithersburg, MD. With the NIST instrument, it was possible to observe the critical angle. Hence, a standard multilayer fitting routine was used to model the reflectivity data.26 Contact Angle Measurements. Static contact angles between water and the LB films were measured on a device from Spindler & Hoyer. Water was distilled through a Milli-Qapparatus (Millipore). Reported values reflect the average of five measurements on each sample. Ultraviolet-Visible Spectroscopy (UV-Vis). UV-vis spectra were recorded on a Perkin-Elmer Lamda 9 UV spectrometer. As with the FTIR substrates, LB films were transferred onto only one-half of the substrate so that the other half could be used for the background correction.
Results and Discussion Isotherms and LB Films of PtBMA and PtBA. The first step in producing ultrathin films with layered architecture of polyelectrolytes is finding suitable precursor polymers. Methacrylate and acrylate polymers such as PtBMA and PtBA form monomolecular films at the air/water interface (A/W) when spread from an organic solvent like chloroform.27-31 In addition to the isotherms, these studies have also probed the rheological and dynamic properties of PtBMA at A/W.29-31 In the case of (24) Schaub, M.; Fakirov, C.; Schmidt, A.; Lieser, G.; Wenz, G.; Wegner, G.; Albouy, P.-A.; Wu, H.; Foster, M. D.; Majrkzak, S.; Satija, S. Macromolecules 1995, 28, 1221. (25) Meier, F.; Schiewe, B.; Esker, A.; Wegner, G. Macromol. Symp. 1999, 145, 161. (26) Anker, J. F.; Majkrzak, C. F. Proc. SPIE-Int. Soc. Opt. Eng. 1992, 1738, 260. (27) Sutherland, J. E.; Miller, M. L. Polym. Lett. 1969, 7, 871. (28) Gabrielli, G.; Guarini, G. G. T. J. Colloid Interface Sci. 1978, 64, 185. (29) Kawacuch, M.; Sauer, B. B.; Yu, H. Macromolecules 1989, 22, 1735. (30) Saccetti, M.; Yu, H.; Zografi, G. Langmuir 1993, 9, 2168. (31) Kim, S.; Yu, H. Prog. Colloid Polym. Sci. 1992, 202, 2.
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Figure 1. Π∠A isotherms for linear (R,ω-styryl-PtBMA180) and three-arm star (St3-PtBMA90) polymers at T ) 8 °C and linear R,ω-allyl-PtBA225 on water at T ) 8 °C and 20 °C.
PtBMA15,32-34 and other methacrylates,35,36 the possibility of LB-multilayer formation is also established. Figure 1 shows comparisons of the surface pressure-area per monomer isotherms (Π∠A) for linear PtBMA (R,ω-styrylPtBMA180) and three-arm PtBMA star polymers (St3PtBMA90) as well as linear PtBA (R,ω-allyl-PtBA225) at different temperatures. The isotherms are consistent with previously published results.27,28 These isotherms show a condensed, liquid-crystalline-like surface phase for the monolayer state. The only effect of the star topology on the isotherm is a decrease in the collapse pressure (Πc ) 15.0 vs 19.5 mN m-1 for the linear polymer). For this reason, the star was transferred at a pressure of Πt ) 12 mN m-1 compared to Πt ) 15 mN m-1 for the linear polymer. This behavior can be contrasted with a more expanded monolayer structure of the linear polymers at low pressure (Π < 4 mN m-1). At higher pressures, the isotherms are consistent with a more condensed, liquidcrystalline-like monolayer phase, which has a higher collapse pressure (Πc ) 26.8 mN m-1 at 8 °C). Thus, LB transfer at Πt ) 15 mN m-1 is facilitated. The highly condensed monolayers are consistent with strong lateral chain interactions between the hydrophobic tert-butyl groups. In the case of PtBA, the more expanded isotherm is consistent with a monolayer conformation in which the absence of the methyl group along the polymer backbone leads to greater flexibility allowing all of the tert-butyl groups to be oriented into the air out of the aqueous subphase. This is supported by the large difference in the glass transition temperature (Tg ) 112 °C for PtBMA) associated with the absence of the methyl group in PtBA (Tg ) 40 °C).37 However, the formation of liquid-crystallinelike monolayer phases allowing LB-multilayer formation indicates that water must plasticize these films leading to significantly lower effective Tg values. Using the transfer conditions mentioned above, LB multilayers were fabricated onto silicon substrates. X-ray reflectivity measurements were performed on these LB (32) Clays, K.; Armstrong, N. J.; Penner, T. L. J. Opt. Soc. Am. B 1993, 10, 886. (33) Clays, K.; Armstrong, N. J.; Ezenyilimba, M. C.; Penner, T. L. Chem. Mater. 1993, 5, 1032. (34) Wijekoon, W. M. K. P.; Wijaya, S. K.; Bhawalkar, J. D.; Prasad, P. N.; Penner, T. L.; Armstrong, N. J.; Ezenyilimba, M. C.; Williams, D. J. J. Am. Chem. Soc. 1996, 118, 4480. (35) Duda, G.; Schouten, A. J.; Arndt, T.; Lieser, G.; Schmidt, G. F.; Bubeck, C.; Wegner, G. Thin Solid Films 1988, 159, 221. (36) Mumby, S. J.; Rabolt, J. F.; Swalen, J. D. Thin Solid Films 1985, 133, 161. (37) van Krevelen, D. W.; Hoftyzer, P. J. Properties of Polymers: Their Estimation and Correlation with Chemical Structure; Elsevier Science: Amsterdam, 1976; p 57.
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Figure 2. Representative X-ray reflectivity profiles from 30 layer LB films of PtBA and PtBMA.
films of PtBMA and PtBA. In both cases, the transfer proceeds via Y-type deposition, in which one layer is transferred on each up and down stroke of the substrate. A sample reflectivity profile for PtBMA and PtBA can be found in Figure 2. In both cases, well-pronounced Kiessig fringes up to fairly large wave vectors of q can be seen, indicating homogeneous surfaces with low surface roughnesses. The most significant difference between the two curves is the presence of a Bragg peak, which is sometimes observed for PtBA but is never seen for PtBMA. The Bragg peak at a wave vector of q ≈ 0.35 Å-1 corresponds to a double-layer thickness of 18 Å. However, Bragg peaks are observed only if the number of deposited layers is sufficiently large (>30) and the surface roughness of the substrate is sufficiently low (σs < 7 Å). As Bragg peaks are never observed for transferred films of PtBMA, the methyl groups along the backbone seem to prevent a conformation at A/W in which all of the methyl groups can be oriented away from the interface. This interpretation is also consistent with calculated film densities of 1.00 g cm-3 for PtBMA and 0.91 g cm-3 for PtBA. For PtBMA, this density is essentially the same as the bulk value of 1.02 g cm-3, while PtBA films have a significantly larger bulk density of 1.00 g cm-3 indicating different packing for PtBA at A/W compared to the bulk.38 By fitting the X-ray reflectivity profiles according to established methods,24 film thicknesses and roughnesses can be obtained. Figure 3 shows the film thickness plotted against the number of transferred layers for PtBA and its derivatives, while Table 1 and Table 2 show the relevant film parameters extracted from the experimental curves. From the slope, a thickness per layer value of d ) 9.3 ( 0.4 Å for PtBA can be extracted. High-quality LB multilayers require materials to possess a suitable balance between flexible and rigid (shapepersistent) moieties. In the case of PtBMA (and PtBA by analogy), this rigidity must come from the backbone as these materials lack long, rigid side chains. This is reasonable considering PtBMA has a larger persistence length, 19 Å, than similar simple vinyl polymers.38,40 Still, this is significantly smaller than the 60 Å value for cellulose ethers or ≈500 Å for polyglutamates which represent hairy-rod type polymers.41 Hairy rods show clear (38) Bicerano, J. Prediction of Polymer Properties; Marcel Dekker: New York, 1993. (39) Wegner, G. Mol. Cryst. Liq. Cryst. 1993, 235, 1. (40) Koyhokaryu, M.; Skazka, S.; Berdnikova, K. G. Vysokomol. Soedin. 1967, 8, 1063. (41) Schwiegk, S.; Vahlenkamp, T.; Xu, Y.; Wegner, G. Macromolecules 1992, 25, 2513.
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Table 1. Important Fitting Parameters for X-ray Reflectivity Profiles from LB Films of PtBMA and Its Derivativesa PtBMA
PMAA
PMAA-EDA
PMAA-x-EDA
PMAA-x-EDA-NaOH
layers transferred
d [Å]
σp [Å]
σs [Å]
d [Å]
σp [Å]
σs [Å]
d [Å]
σp [Å]
σs [Å]
d [Å]
σp [Å]
σs [Å]
d [Å]
σp [Å]
σs [Å]
20 30 40 50
185 273 365 453
12 17 17 28
10.5 9.5 10 13
128 201 246 296
16 13 12 20
13 12 12 15
163 257 328 363
21 19 20 24
15 12 20 14
119 162 229 293
17 18 13 23
13.5 10 16 12
121 178 238 266
16 24 17 19
15 12 16 13
a
σp, roughness of the air/polymer interface; σs, roughness of the polymer/substrate interface. Table 2. Important Fitting Parameters for X-ray Reflectivity Profiles from LB Films of PtBA and Its Derivativesa PtBA
PAA
PAA-EDA
PAA-x-EDA
PAA-x-EDA-NaOH
layers transferred
d [Å]
σp [Å]
σs [Å]
d [Å]
σp [Å]
σs [Å]
d [Å]
σp [Å]
σs [Å]
d [Å]
σp [Å]
σs [Å]
d [Å]
σp [Å]
σs [Å]
20 30 40 50
185 263 361 463
9 28 9 10
9 14 9 9
92 124 171 213
13 18 14 14
9 12 9 9
141 189 367 324
14 24 19 18
8 13 11 10
100 135 176 244
12 24 18 16
8 11 9 8
111 147 209 264
13 24 17 22
9 11.5 11 11
a
σp, roughness of the air/polymer interface; σs, roughness of the polymer/substrate interface.
Figure 3. Thickness vs the number of LB layers transferred for PtBA and its derivatives.
polarization effects in both IR and UV spectra with orientation of the rigid backbone parallel to the transfer direction.39,41 In contrast, FTIR spectra of PtBMA and PtBA, such as those shown in Figure 4, show no polarization. One can deduce from these observations that PtBA LB films are made up of monolayer-thick layers in which the polymers exist in nearly two-dimensional random-coillike conformations. In the following section, simple chemical modifications will be presented, under which the layered structure as well as the low surface roughness are both believed to be retained. Scheme 1 presents schematically the chemical pathway chosen. In the first step, LB films of PtBMA or PtBA are fabricated. In the second step, gas-phase hydrolysis gives access to the corresponding PMAA or PAA films. Gasphase reaction with suitable amines transforms the polyacids into the corresponding ammonium salts (step three). As outlined in step four, heating these films can generate amide bonds. If difunctional or polyfunctional amines are used, networks are formed. These networks are insoluble in water or aqueous, dilute NaOH. Thus, by placing these films in NaOH, cross-linked polyelectrolyte films of PMAA or PAA can be obtained. In the following discussion, the different steps will be elucidated in more detail. Ultrathin Films of PMAA and PAA. Acid-catalyzed hydrolysis of tert-butyl esters yields carboxylic acids through the elimination of gaseous isobutene. Posttransfer modification of the LB films in the previous section is accomplished by subjecting them to hydrochloric acid
Figure 4. Representative FTIR spectra from LB films of PtBMA (a), PMAA (b), PMAA-EDA (PMAA-ethylenediammonium salt) (c), PMAA-x-EDA (cross-linked PMAA network via amide bond formation) (d), and the corresponding Na salt PMAA-x-EDA-Na (e).
vapor at 60 °C for 6 h.15 A sample reflectivity profile for PAA is shown in Figure 5. The films retain their smooth surfaces as seen in Tables 1 and 2, as well as a linear dependence on the number of layers originally transferred as shown in Figure 3. From the slopes in Figure 3, a monolayer thickness of 4.1 ( 0.2 Å for PAA is obtained. This value along with the transfer area can be used to estimate a density value of 1.14 g cm-3 for PAA. On the basis of the thickness changes, it appears that the hydrolysis is quantitative. Remarkable in this context is the high density of carboxylic acid groups which can be generated by this approach. Calculation of the packing density gives a value of σ ) 3.71 carboxylic acid groups per nm2. For this calculation, the surface area of a nonhydrolyzed monomer has been used. Confirmation of the chemical transformation can be seen in IR spectra such as those shown in Figure 4 and the analysis summarized in Table 3. The predominant changes are the growth of an O-H stretch which is extremely broad
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Scheme 1. Schematic Pathway to Ultrathin Polyelectrolyte PMAA and PAA Networks
Table 3. FTIR Peak Frequencies (in Wavenumbers) for Important Molecular Motions in LB Films of PtBMA, PtBA, and Their Derivatives ν [cm-1] 3400-2500 3000-2800 1724 1704 1540-1560 1523 1450 1365 1270 1250 1170 1140
PtB(M)A CH2, CH3 COOR
P(M)AA OH CH2, CH3 COOH (dim)
CH2, CH3 C(CH3)3 C-O-C
P(M)AA-EDA OH, NH2 CH2, CH3 COO-
P(M)AA-x-EDA OH, NH2 CH2, CH3 COOH (dim)
CH2, CH3
CH2, CH3
amide II CH2, CH3
C-O C-O
C-O C-O
C-O C-O
C-O-C
due to strong hydrogen bonding in carboxylic acids from 3500 to 2500 cm-1, broadening of the carbonyl stretch
P(M)AA-x-EDA-Na OH, NH2 CH2, CH3 COOamide II CH2, CH3 C-O C-O
type valence-stretch valence-stretch valence-stretch valence-stretch valence-stretch bending-stretch deformation deformation valence-stretch valence-stretch valence-stretch valence-stretch
because of overlapping monomeric 1704 cm-1 and dimeric 1724 cm-1 acid bands, and attenuation of the peaks below
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Figure 6. Experimental schematic for the X-ray sorption experiments.
Figure 5. Representative X-ray reflectivity profiles from 50 layer LB films of PtBA (a), PAA (b), PAA-EDA (c), PAA-x-EDA (d), PAA-x-EDA after washing in water (e), PAA-x-EDA-Na (f), and PAA-x-EDA-Na after washing in water (g).
1500 cm-1 associated with the loss of the deformation modes of the tert-butyl ester moiety. The linear relationship between film thickness and the number of layers transferred suggests that PMAA and PAA may also exist in two-dimensional random coil conformations. If this is true, it means that the elimination of gaseous isobutene does not destroy the layered structure even if the double-layered structure is lost (no Bragg peak).15 Moreover, their potential existence as layered films mean that they possess all of the properties of an LB film even though PMAA and PAA are too hydrophilic to form LB films directly. The possible stability of the chain conformations would be consistent with the relatively high glass transition temperatures of the respective polyacids (Tg(PMAA) ) 228 °C, Tg(PAA) ) 106 °C)42 and the low hydrolysis temperatures relative to the surface Tg of PtBMA.43 Contact Angle Measurements for the Precursor LB and Hydrolyzed Films. Two methods were used to follow the changes in hydrophilicity upon hydrolysis. The first of these methods was contact angle measurements. Table 4 shows contact angles of water against LB films of the precursor polymers and polyacids. For the PtBMA series, the presence of the methyl group along the backbone means that the surface is rather hydrophobic resulting in a large contact angle with water. Even after hydrolysis, no significant change in the water (42) Polymer Handbook, 3rd ed.; Brandrup, J., Immergut, E. H., Eds.; Wiley & Sons: New York, 1989. (43) Y.-K.; Cha, J.; Chang, T.; Ree, M. Langmuir 2000, 16, 2351.
Figure 7. Swelling ratios (D/D0) for LB films of PtBMA (a), PMAA (b), PtBA (c), and PAA (d) plotted against the total Hildebrand solubility parameter, δt, for a variety of solvents. Table 4. Static Contact Angles with Water for LB Films of PtBMA, PtBA, PMAA, and PAA polymer
θ [deg]
polymer
PtBMA PtBA
85 ( 2 75 ( 2
PMAA PAA
θ [deg] 73 ( 2
