Langmuir 1996, 12, 2807-2812
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Relaxation Behavior and Bilayer Formation of Poly(4-vinylpyridinium) Type Polyelectrolytes at the Air/Water Interface Johannes Wagner,† Thomas Michel,* and Walter Nitsch Technische Universita¨ t Mu¨ nchen, Institut fu¨ r Technische Chemie, Lichtenbergstrasse 4, 85748 Garching, Germany Received July 3, 1995. In Final Form: March 4, 1996X Quaternized samples of poly(4-vinylpyridine) with alkyl side chains of various length (C8-C16) and degree of alkylation (ca. 23-97%) were studied by classical film balance. Transfer experiments, ellipsometric characterization and microweighing by quartz crystals show that these polyelectrolyte (PE) systems exhibit a well-defined bilayer formation during compression to areas smaller than the critical area of the monomer unit (so-called collapse), visible as a plateau region in the measured π/A-isotherms. Time dependent measurements of the monolayer area at constant film pressure can be explained and treated by classical nucleation/growth models. Compressed to the beginning of the plateau region and maintained at constant pressure, the monolayers relax exactly to half of the initial area. These relaxation experiments additionally support the postulated concept of elongated, rodlike structures, facilitating the bilayer formation of the presented PE systems.
Introduction In comparison to low molecular weight compounds Langmuir-Blodgett (LB)-systems of polymers are expected to be more stable against temperature and chemical influences. Thus the activities in the field of polymer monolayers and LB-systems have grown rapidly since the first attempts to transfer preformed polymer monolayers in the early 1980s.1 The knowledge of the structure, phase behavior, and transfer capabilities, however, is still not developed to such an extent as for low molecular weight compounds. Among polymers as film-forming materials only minor attention has been paid to the group of polyelectrolytes (PEs), although PEs may be regarded as an especially interesting polymer class caused by the charge distributed along the polymer chain (either by charged side groups or by the charged main chain itself). This charge should lead to additional aspects of structure effects and interaction (especially with oppositely charged small molecules or polymers). Finally due to electrostatic repulsion of the monomer units one may expect that the polymer chain is more likely stretched than a random coil. The amount of literature dealing with charged polymers directly spread at the air/water interface is still very small. For the sake of completeness work on PEs adsorbed from the subphase at spread monolayers of oppositely charged small molecules should be mentioned here. In a previous study on special aspects of polyelectrolyte mono- and multilayers we have shown that in polyelectrolyte systems of the poly(4-vinylpyridinium) type multilayer formation occurs at the air/water interface during compression to small areas.2 Concerning the polyelectrolyte type of the present paper Kawaguchi et al.3 presented examinations * To whom correspondence should be addressed. Phone: + 49 89 3209 3496. Fax:+49 89 3209 3513. E-mail: michel@ tc1.tech.chemie.tu-muenchen.de. † Part of Thesis. X Abstract published in Advance ACS Abstracts, May 1, 1996. (1) For example: Tredgold, R. H. Thin Solid Films 1987, 152, 223. Miyashita, T. Prog. Polym. Sci. 1993, 18, 263. Kawaguchi, M. Prog. Polym. Sci. 1993, 18, 341. (2) Michel, T.; Nitsch, W. Thin Solid Films 1994, 242, 234. (3) Kawaguchi, M.; Itoh, S.; Takahashi, A. Macromolecules 1987, 20, 1052; Macromolecules 1987, 20, 1056.
S0743-7463(95)00962-0 CCC: $12.00
on phase transitions at high molecular areas in monolayers of these polyelectrolytes on salt solutions. Davis et al.4 recently published work on poly(4-vinylpyridines) quaternized with alkyl halides of different chain lengths. They showed that highly charged polyelectrolytes can form stable and transferable monolayers. Especially with respect to the monolayer stability of these polyelectrolyte systems we extended the range of measuring π/A-isotherms toward smaller areas. It is common practice to describe the areas of the isotherm beyond reasonable values of area per monomer unit (or molecule, respectively) as “collapse” regions without specifying what is actually related to the observed collapse, especially if this correlates with distinct discontinuities in film pressure. Consequently these areas are often disregarded or treated as the beginning of monolayer disrupture or disordered three-dimensional particle formation. Recent studies,5-9 however, on monolayers at small molecular areas demonstrate, that this view of disordered layer states does not reflect the actual facts during compression to small areas and should not be generalized. Especially for synthetic polypeptides at the air/water interface, there are several publications describing bilayer formation during compression to small areas,10,11 which was related to the R-helical structure of the polypeptide. Similar to a former study12 (with monolayers of the water-soluble poly(ethylene glycol), the present work demonstrates that the extension of the experiments to smaller areas of the π/A-isotherms may lead to results which can be correlated to structural aspects of polymeric film-forming systems at the air/water interface. (4) Davis, F.; Hodge, P.; Liu, X.-H.; Ali-Adib, Z. Macromolecules 1994, 27, 1957. (5) Albouy, P. A. J. Phys. Chem. 1994, 98, 8543. (6) Rapp, B.; Gruler, H. Phys. Rev. A 1990, 42, 2215. (7) McFate, C.; Ward, D.; Olmsted, J., III. Langmuir 1993, 9, 1036. (8) de Mul, M. N. G.; Mann, J. A., Jr. Langmuir 1994, 10, 2311. (9) Friedenberg, M. C.; Fuller, G. C.; Frank, C. W.; Robertson, C. R. Langmuir 1994, 10, 1251. (10) Takeda, F.; Matsumoto, M.; Takenaka, T.; Fujiyoshi, Y. J. Colloid Interface Sci. 1981, 84, 220. (11) Takeda, F.; Matsumoto, M.; Takenaka, T.; Fujiyoshi, Y.; Uyeda, N. J. Colloid Interface Sci. 1983, 91, 267. (12) Nitsch, W.; Kremnitz, W.; Schweyer, G. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 218.
© 1996 American Chemical Society
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Figure 1. π/A-isotherms of the octyl derivatives of P4VP on pure water.
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Figure 2. π/A-isotherms of the dodecyl derivatives of P4VP on pure water.
Experimental Section NomenclaturesMaterials. The polyelectrolytes are named by using the length of the alkyl bromide as a prefix and the degree of quaternization of the starting polymer as a suffix; e.g., D97 stands for the dodecyl derivative of poly(4-vinylpyridines) ()P4VPs) with 97% of the monomer units being quaternized. The quaternized P4VPs were synthesized by quaternization of a commercially available polymer (Polysciences) of mean molecular mass 300.000 (by viscosity measurements) according to the standard method of Fuoss and Strauss.13 Prior to use the alkyl bromides (C4, C6, C8, C12, C16) were freshly distilled and the degree of quaternization was determined by Br--titration and elemental analysis. Methods. All the polyelectrolytes were soluble in chloroform and thus spread from chloroform solutions of about 0.5-1 mg/ mL. A conventional film balance (Lauda FW 1) at 20 °C with pure millipore water as subphase was used. For transfer, the normal vertical dipping method with a Lauda film lift was used. A standard dipping speed of 10 mm/min was applied. The transfer ratios for both dipping directions were determined by using standard microscope glass slides, which were hydrophobized by trimethylchlorosilane or by immersion into a silicone solution in 2-propanol (Serva), followed by heat treatment at 115 °C for 2 h. No significant differences in transfer behavior on these two substrates were observed. For ellipsometry (Plasmos SD2300) thoroughly cleaned (hot H2SO4/peroxodisulfate) silicon wafers were used which were hydrophobized by etching in 5-10% HF prior to the monolayer transfer. For microweighing, a home-made quartz crystal microbalance (QCM) with an AT crystal at 5 MHz was used. This setup had a sensitivity of 17 ng/(cm2‚Hz), and monolayer deposition can easily be detected even by measuring in the (1 Hz range.
Results and Discussion Monolayer Formation and Transfer Behavior and Bilayer Formation. The π/A-isotherms of the QP4VPs are shown in Figures 1-3. Because of complete or partial desorption into the subphase, B93 and H93 are not further discussed here. B93 is completely water soluble, and no film pressure can be measured during compression. Spreading B93 on subphases containing amphiphiles, however, results in stable monolayers which can be transferred quantitatively. Using this system, we were able to prove triple-layer formation during compression.2 A striking and common feature to most of the isotherms is a more or less flat plateau region followed by a steep increase of the film pressure upon further compression. In a good approximation, the molecular area is reduced to half along the plateau, if the steep branch of the second (13) Fuoss, R. M.; Strauss, U. P. J. Polym. Sci. 1948, 3, 747.
Figure 3. π/A-isotherms of the cetyl derivatives of P4VP on pure water.
increase and the kink after the first increase of the film pressure are taken as boundaries. The monolayers are stable (i.e., the change of the area at constant pressure is less than 5% within 1 h) and show negligible hysteresis for film pressures below the kink. For expansion along the plateau, however, there appears a hysteresis. The difference in compression depends on the length of carbon side chain and the degree of alkylation (the longer/greater the side chain/degree of quaternization, the greater the hysteresis). But for all isotherms, the initial isotherms were recovered upon subsequent recompression, indicating that no desorption or irreversible aggregation is related to the hysteresis. This holds for all degrees of quarternization and lengths of carbon side chain except for O39. The latter is obviously partially water soluble and desorbs irreversibly during compression. The measured isotherms agree well with the results of Davis et al.,4 and comparing the different molecular weights of the underivatized polymers (about Mv 300.000 in this work and about Mv 45.000 in ref 4), we can conclude that there is no significant molecular weight dependence on film behavior. Table 1 contains results for measuring the transfer ratios of the monolayers at film pressures smaller than those at the kink. Only during the first one or two down strokes do the transfer ratios change from higher values to those presented in Table 1. After the first transfer cycles, the transfer ratio remains constant up to high numbers of cycles. It can be seen that for smaller degrees
Poly(4-vinylpyridinium) Type Polyelectrolytes
Langmuir, Vol. 12, No. 11, 1996 2809
Table 1. Mean Transfer Ratio of the First Five (C23 and C53 First Four) Dipping Cycles of the Various Quaternized Poly(4-vinylpyridines)a downstroke upstroke
O59
O93
D34
D70
D97
C23
C53
C96
b 0.97
0.73 1.03
0.42 0.97
0.81 0.97
0.93 0.99
0.72 1.02
0.8 1.0
0.9 1.0
a π < plateau - π; glass slides hydrophobized with siliconization solution (SERVA); 10 mm/min. b For down stroke partial respreading for cycle > 2.
Table 2. Comparison of Transfer before and after the Plateau by Mean Frequency Shift in hertz per Transfer Cycle after Transfer to Hydrophobized Quartz Crystala before plateau after plateau
O93
D34
D70
D97
C23
C53
33.4 72.7
28.3 68
37.5 76.3
50.4 101
42.9 102.6
51 93.8
a Data points for O93, D70, and C53 after the plateau were transferred after relaxation, see Figure 7.
Figure 4. Ellipsometric film thickness of transferred quatP4VP for D97 additional data for transfer at π > plateau - π: substrate, HF-etched Si wafer.
of alkylation and length of alkyl side chain the monolayers tend to transfer not perfectly during the down stroke. Nevertheless, for all derivatives the transfer ratio during the up stroke is almost 1. For the octyl derivative we find a slightly different transfer behavior than Davis et al.4 reported. They found no transfer during the down stroke, compared with a transfer ratio of about 0.7 of the octyl derivative used by us. This may be related to the effect of different substrates used and/or to a molecular weight effect (our starting P4VP had a Mv ) 300.000 compared to Mv ) 45.000 in ref 4). Nevertheless, our few experiments made with samples of Mv ) 50.000 in order to check molecular weight effects showed no significant differences in film behavior (also see comparison of isotherms above). Consequently, this difference in transfer behavior may be mainly related to substrate effects. Even the states of the film after the plateau region can be transferred. Table 2 summarizes the results of quartz crystal micoweighing experiments. Listed are the averages of the frequency shifts of up to seven transfer cycles from the states after the plateau compared to those before the plateau. Interestingly it turns out that, at least for the derivatives with a high degree of quarternization and hence fairly good transfer ratios in both dipping directions, the deposited mass after the plateau is indeed twice as high as that before the plateau. The same situation is illustrated in Figure 4, where ellipsometric film thickness measurements at film pressures below and above the plateau pressure for D97 are shown, and in Table 3, where
the direct comparison in ellipsometric film thickness at both sides of the plateau is presented. The QCM results clearly show that during the compression along the plateau region no desorption takes place. All the material remains at the interface and can be transferred to the substrate. Furthermore, the ellipsometric measurements reveal that the mean thickness per transfer cycle at both sides of the plateau agrees very well with the factor of 2. This must be seen as an additional proof for the postulated bilayer formation during compression. Already Davis et al.4 succeeded in transfer of poly(4vinylpyridinium) type PE with carbon side chains of C g 12. They reported a discrepancy in the measured film thickness (by X-ray) and those calculated by using spacefilling molecular models. They considered the measured thicknesses (and d-spacing, respectively) to be too small and related these findings to the interdigitation of the carbon chains during transfer. In our measurements the ellipsometric thickness of the transferred monolayer state is also found to be too small, compared to the calculated length of a monomer unit with a completely stretched carbon side chain as described by Figure 5, if one assumes that the polymer main chain lies flat on the water surface and the carbon side chain is oriented perpendicular to the main chain and that this orientation reflects the PE structure in its monolayer state. This discrepancy increases with increasing length of the carbon side chain, as summarized in Table 4. That means that, on the basis of ellipsometric data of overall film thickness, we may derive partial interdigitation of monolayers in the transferred multilayers. For the case of the C12-derivative our data (32.8 Å for two layers, see Table 4) agree well with those reported by Davis (bilayer spacing of 34.3 Å). For the C16-derivative, however, our data (32.8 Å for two layers, see Table 4) tend to show a stronger interdigitation of the transferred multilayers than Davis’ results (bilayer spacing of 39.1 Å); possible differences in transfer behavior cannot explain this difference. With respect to interdigitation, Davis et al. mainly based their discussion on derivatives with a chain length of C ) 22. A more detailed discussion and explanation for this difference cannot be given on the basis of the present data. For the C12-derivative ()D97) we have a few additional X-ray data (kindly measured by Dr. Hans Riegler, now at MPI fu¨r Kolloid-und Grenzfla¨chenforschung, Berlin-Adlershof, Germany). On the basis of the Bragg peak the bilayer spacing is 31.5 Å, and the total thickness by analyzing the Kiessig fringes is 468.5 Å (for a 30 monolayer system, that means 31.2 Å per bilayer); thus, our ellipsometric and X-ray data are comparable. Comparing the ellipsometric film thickness after transfer at both sides of the plateau, however, for the transferred bilayer state of D97 gives a factor of about 2.5 compared to the ideal factor of 2. This discrepancy may be explained if one postulates that the transferred bilayer state is not interdigitated and leads to an increased total film thickness. Due to the lack of structural data (e.g. X-ray or neutron reflectivity), however, a more detailed discussion of this structural effect is not yet possible. Relaxation Processes. As mentioned above, for a first approximation all monolayers are stable for film pressures below the plateau region. As discussed in this section, this statement is strictly not correct. According to Figures 1-3, except for O93, the plateau regions are not perfectly flat and the transition of the film pressure at the beginning of the plateau is not abrupt and covers a range in area. It is not surprising that the shape and the film pressure of the plateau depends on the compression speed of the monolayer, which was proven by our own experiments.
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Table 3. Comparison of the Ellipsometric Film Thickness of quat-P4VP after Transfer at π < and π > plateau - π on a HF-Etched Si Wafer before plateau
O93 D70 C53 D97
after plateau
p (mN/m)
dipping cycles
total thickness (nm)
thickness per cycle (nm)
30 25 28 28
20 16 20 20
55.8 39.1 65.2 66.8
2.79 2.44 3.26 3.34
p (mN/m)
dipping cycles
total thickness (nm)
thickness per cycle (nm)
38 32 35 38
10 8 10 19
56.6 41.5 69.1 150.4
5.66 5.18 6.91 7.91
Figure 6. Relative area of D97 versus time at constant pressure: subphase, pure water; for isotherm compare Figure 2.
Figure 5. Schematic drawing of the molecular model of D97 for calculating the maximal length (simply as the length of the vector between the two atoms indicated by arrows) of the completely stretched monomer unit. The same calculation for O93 and C96 leads to 1.42 and 2.42 nm, respectively (The drawing is produced by the program Hyperchem for Win, version 2.0, HyperCube Inc., and the distance between the marked atoms is also calculated by the program). Table 4. Comparison of Measured (HF-Etched Si Wafer) and Calculated (See Figure 5) Monolayer Thickness O93 D97 C96
dellips (nm)
dmodel (nm)
1.33 1.66 1.64
1.69 2.19 2.69
The presented π/A-isotherms in Figures 1-3, however, are recorded with the lowest possible compression speed of the used film balance (2 mm/min, which means about 1-2 Å2/(repeat unit‚min)). In order to get a more detailed picture of the process occurring during the compression along the plateau, we measured the decrease of molecular area with time at constant film pressure. The results are best illustrated by D97 in Figure 6. Each plot represents an individual and independent measurement after new spreading. As can be seen, the monolayer tends to be unstable even at film pressures significantly below the plateau region shown in Figure 2. For 33 mN/m, this instability leads to a spontaneous and rapid decrease in the molecular area to 0.4-0.5A0 (A0 ) molecular area at the start of the relaxation measurement). If the driving force for this relaxation becomes smaller, there exists an induction period for this process, as can be seen in Figure 6 by the plot for 28 mN/m. The starting point for the relaxation lies between 26 and 28 mN/m, and this is significantly lower than the critical film pressure at the
kink of the isotherm for D97 in Figure 2. This indicates the above-mentioned (at the beginning of this paragraph) influence of compression speed on the shape of the π/Aisotherm. The compression speed of the used Langmuir film balance cannot be lowered any further, as noted in the Experimental Section, and this introduces a significant difference between the actually measured isotherm and the ideal equilibrium (infinitely slow compression) isotherm for these particular polymer systems. These experimental findings qualitatively represent the two-dimensional equivalent of characteristic phenomena in classical nucleation and subsequent particle growth in bulk phases. That means for low “supersaturation”, e.g. 28 mN/m for D97 in Figure 6, slow nucleation processes cause an induction period followed by fast growth. For higher film pressure (33 mN/m), we have a high “supersaturation” and the initial concentration of nuclei is high enough, so that within the accuracy of the experimental setup and procedure the induction period vanishes. The final state of this “particle growth” is that all of the growing nuclei are in contact with each other and the polymeric material is transferred completely to the bilayer state. Perhaps the most striking result of these relaxation experiments is that the final state of this spontaneous relaxation coincides with half of the initial molecular area, as illustrated by Figure 7, showing the results of relaxation experiments for monolayers of different degrees of alkylation and lengths of the side chain. This is strong evidence for the postulated bilayer formation. The chosen film pressures are always around the kink at the beginning of the plateau. Furthermore, the layers of the QCM measurements represented in Table 2 were deposited just after the relaxation plotted in Figure 7. These QCM data and the ellipsometric measurements prove that there is no desorption during the relaxation. Consequently, the most plausible interpretation of the monolayer behavior and the experimental findings indeed is a bilayer formation. An attempt was made to interpret our own relaxation results in more detail with respect to particle nucleation/
Poly(4-vinylpyridinium) Type Polyelectrolytes
Langmuir, Vol. 12, No. 11, 1996 2811 Table 5. Summary of Basic Geometry and Nucleation/ Growth Model for Monolayer Relaxation with Resulting Characteristic Exponent x in Eq 1a
Figure 7. Relaxation of quat-P4VP (different degree of alkylation and length of side chain) monolayer area versus time at constant film pressure, π, at the beginning of or slightly below the plateau: π ) 30 mN/m.
growth models. Vollhardt et al.14 presented a series of studies describing the relaxation of stearic acid monolayers in the collapse region. They related the measurable loss of normalized area to the overall growth rate of 3D particles, considering different rate laws of the initial nucleation (instantaneous or progressive), the geometry of the growing particle, and the overlap of the grown particles. Their theory led to a generalized equation for any nucleation model of the form
A0 - A ) 1 - exp(-Kxtx) A0 - A∞
a l represents the length of the postulated main axis of the rodlike polymer structure built up along the polymer main chain lying flat on the surface.
(1)
or in a linearized form
{ln[(1/(A0 - A))/(A0 - A∞)]}1/x ) K1/x x t
(2)
where A is the total monolayer area at time t, A0 is the initial monolayer area, A∞ is the area at t ≈ ∞, and K is a constant specific for the applied geometry and nucleation models represented by the characteristic exponents x and 1/x, respectively. As summarized in Table 5, for a disklike geometry of constant height and growing at the circular edge of the disk, the exponent x in eq 1 for instantaneous nucleation should be 2 and for progressive nucleation it was expected to be 3, respectively. As a basis for the following discussion we start from the assumption of a relatively elongated, cylinder-like shape lying flat (more or less) on the water surface, built up by the polymer backbone of the PE, without introducing any further assumption of the structure of the polymer rod (e.g. helical or not). One might expect that such a shape should greatly facilitate the monolayer to bilayer transfer. Such phenomena were, in fact, reported. For tetrasubstituted phthalocyanine derivatives, which aggregate to highly ordered columnar states (cylinders), an extremely sharp phase transition to a bilayer state (and even triple and quadruple layer states) was observed.5 Within this context the bilayer formation of R-helical (and therefore elongated) polypeptides of derivatized polyglutamate type can also be referred to.10,11 Similar to these structures, we take as a reasonable hypothesis that our PE also is of (14) Vollhardt, D.; Retter, U. J. Phys. Chem. 1991, 95, 3723; Vollhardt, D.; Retter, U.; Siegel, S. Thin Solid Films 1991, 199, 189; Vollhardt, D.; Retter, U. Langmuir 1992, 8, 309.
Figure 8. Plot of area relaxation for D97 versus time for different characteristic exponents x according to eq 2 at 20 °C assuming a one-dimensional growth geometry: π ) 28 mN/m; subphase, water.
a “rodlike” structure, facilitating the phase transition of the PE to bilayer states. We slightly extended the above summarized model of Vollhardt et al. of nucleation and particle growth by introducing a one-dimensional (“rodlike”) growing geometry with a nucleus of constant length l, as indicated in Table 5. One dimensional here means that the nucleus grows only in directions perpendicular to its initial length axis and that the initial length l of the particel does not increase. Using this geometry and applying exactly the same formalism as described for the other geometries in ref 14, it turns out that for the instantaneous type the exponent should be x ) 1 and that for the progressive type x ) 2, respectively. In Figures 8 and 9, eq 2 is plotted versus time for the system D97 at 28 mN/m for 20 °C (for the plot A/A0 ) f(t) at 20 °C compare Figure 6) and 40 °C, respectively, assuming a one-dimensional particle. For both temperatures a fairly good linear plot results with x ) 2 at 20 °C but x ) 1 at 40 °C.
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Figure 9. Relative area versus time for D97 according to eq 2 at 40 °C (other conditions same as for Figure 8).
Without introducing any inconsistency, these findings can be explained by the above-mentioned assumption of cylinder-like growing geometry. Furthermore, a geometry other than a one-dimensional one (“rod”) does not lead to the exponent x ) 1 for instantaneous kinetics. Consequently, the experimental findings are interpreted as reasonable support for our model of elongated, cylindrical or rodlike, i.e. “one-dimensional”, PE structure. The found change in nucleation kinetics for the temperature change from 20 °C to 40 °C is plausible; it is not surprising that for increasing temperature the initial “concentration” of the nuclei gets closer or is even at its maximum value for subsequent particle growth. We interpret this more detailed analysis of the relaxation experiments with respect to the hypothesis of elongated polymer structure facilitating such a welldefined bilayer formation. Within this context the results reported by Brinkhuis and Schouten15 have to be mentioned. They reported a few relaxation measurements with monolayers of isotactic poly(methyl methacrylate) and considered nucleation processes with distinct growth geometry (based on Avrami’s theory of crystallization, which was also the basis in the work of Vollhardt et al.) resulting in double-helix formation during compression. In summary, the experimental findings clearly demonstrate a spontaneous bilayer formation of the polyelectrolyte monolayers. Supported by relaxation measurements of the PE-layers, we postulate an elongated and rodlike structure of the polymers and we assume that such a structure greatly facilitates the spontaneous bilayer formation. It is much more unlikely that a completely disordered and coiled structure of the polymer could lead to the reported monolayer and relaxation behavior. On the basis of our own experiments no further discussion of the PE mono- or bilayer structure is possible yet. It may be speculated that during compression along the plateau region the PE rods roll on top of each other forming a bilayer similar to that reported for columnar and liquid crystalline structures in e.g. ref 5. On the other hand it may be possible that during the compression of the PE monolayer helical (rodlike) structures are formed, similar to results reported for isotactic poly(methyl methacrylate)15 (they reported double-helix formation). A thorough examination done by X-ray and/or neutron reflectivity, however, is necessary in order to get more detailed (15) Brinkhuis, R. H. G.; Schouten, A. J. Macromolecules 1991, 24, 1487.
Wagner et al.
information about the polymer structure in mono- and bilayer states. Without such structural information a discussion of whether the PE bilayer may be a head to head or tail to tail geometry (in a classical amphiphilic sense) or simply the result of rods rolling on top of each other is highly speculative and preliminary. The interpretation of preformed “rods” is favored by the authors as a simple and plausible model and may be supported by considering the generally low areas per repeat unit found in the isotherms. This may indicate that there are possibly preformed helical or at least folded structures of the polymer main chain within (or even building) these rods. Otherwise one must assume that all polymer main chains lie flat on the surface with each of their monomer units, resulting in highly ordered, parallel oriented domains with any gaps in order to reach these small areassthis is considered to be very unlikely. The reported behavior is obviously not restricted to a particular degree of quaternization or length of carbon side chain. There is, however, an influence of the polymer structure with respect to steric hindrance by chain cohesion or other energy barriers on the process of bilayer formation. There seems to be the tendency that there is an optimum for C8 in chain length at a high degree of alkylation or lower degree of alkylation for higher chain lengths. Conclusion The main intention of this work was to present a detailed analysis of the stability of polyelectrolyte monolayers at low molecular areas. As a striking result it turns out that the polyelectrolytes used in this work show a striking tendency for self-organization to bilayer structures in a well-defined manner. Depending on the degree of quaternization and length of carbon side chains, the π/Aisotherms show a distinct plateau region, which can be clearly related to bilayer transitions. Transfer experiments of the stable layer at both sides of the plateau and characterization of the multilayers by ellipsometry and microweighing clearly prove the formation of a stable bilayer state. The strongest evidence for this bilayer formation could be derived from time dependent measurements of molecular area at constant film pressure. The relaxation of the monolayer reaches a final value of exactly half of the initial area. By a preliminary analysis this relaxation could be explained by classical rate laws of nucleation and particle growth. This well-defined behavior of the reported polyelectrolyte systems led to the hypothesis of stretched and thus rodlike polymer structure at the air/water interface, facilitating the bilayer formation. We interpret the results of the more quantitative analysis of our relaxation experiments as an additional support of compact and rodlike polymer structure. For further investigations the question arises whether this structural behavior is a general phenomenon, caused by electrostatic repulsion/ interaction of highly charged polymers at interfaces. It was demonstrated that characterization of polymer monolayers at small molecular areas, often simplified as collapse regions, can offer a very interesting path for describing aspects of e.g. structure, interaction, and transfer behavior of polymer monolayers. Acknowledgment. This work was partly supported by the Deutsche Forschungsgemeinschaft within Sonderforschungsbereich 266. LA950962X
