Langmuir 1991,7,834-839
a34
The Langmuir Lectures Monolayers of Polymers George L. Gaines, Jr. Chemistry Department, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 Received November 26, 1990
A selection of results on spread monolayers of synthetic macromolecules on water surfaces is presented, including recent results from our laboratory and some related work by others. Emphasis is on the variety of surface pressure-area characteristics that have been observed for both homopolymers and copolymers, how these should be interpreted, and selected newer optical techniques that provide information complementary to that obtained from E A curves. Introduction Agnes Pockels was almost certainly the first to spread and study molecular films of polymeric materials on water surfaces. She, of course, was the young German lady who originated many of the standard techniques for the study of what are now recognized as monomolecular films spread at air-water interfaces. In the early 1890s,she described measurements on films of “colophony”and “mastic”among other substances.’ These observations actually predate the concepts of either polymer or monolayer. Since then, there have been a vast number of reports on the properties of spread films of macromolecules. Devaux observed the spreading of albumin in 1903.2 The first synthetic polymer monolayers were described by Katz and Samwel in 1928; the vinyl polymers that they spread included poly(viny1 acetate) and poly(methy1 a ~ r y l a t e ) .Synthetic ~ condensation polymers of w-hydroxydecanoic acid were studied by Harkins, Carman, and Ries in 193L4 In 1946,Crisp reported a comprehensive survey of surface pressure and potential measurements (as well as qualitative viscosity observations) on a variety of synthetic polymer films and attempted to establish criteria for the significance and interpretation of these measurements.5 Nevertheless, some question remains as to whether the expression “polymer monolayer” is in fact an oxymoron. In bulk solution, polymer molecules generally exist as random three-dimensional coils whose bounding dimensions may be some tens or hundreds of angstrom units. Most authors, following the early workers already mentioned, have assumed that in the spread monolayers the polymer molecules are extended at the interface, with every monomer segment in the surface layer. (The main evidence for this assumption is the fair agreement between the area requirement in the spread and compressed film and measurements on the projected area of molecular models of the monomer segment.) How likely is it that the spreading process can produce such a profound change in average polymer molecule conformation (Figure l)? Recently, the possibility of building up spread monolayers to form deposited multilayer films, using the Langmuir-Blodgett (L-B) technique, has attracted renewed (1) Pockels, A. Nature 1892,46,418; 1893,48, 152. (2) Devaux, H. C. R. Hebd. Seances Acad. Sci. 1935,201,109. (3) Katz, J. R.; Samwell, P. J. P. Naturwissenschaften 1928,16,592. (4) Harkins, W. D.; Carman, E. F.; Riee, H. E. J. Chem. Phys. 1935, 3, 692. (5) Crisp, D. J. J . Colloid Sci. 1946, 1, 49, 161.
Figure’l. Is it possible that during the spreadingprocess, every polymer segment (part of a three-dimensional random coil in solution in a spreading solvent) can take up a position exactly at the air-water interface? attention? While much of this work has focused on monolayers of smaller molecules, the special properties of films of macromolecules, including their thermal and mechanical stability, suggest that L-B films of polymers may have future technological importance. In addition to the spreading of preformed polymers, work has been reported on the polymerization of spread monomeric species either in the water-supported monolayer or after transfer to a solid substrates7 Clearly, there is a far larger body of reported information on spread films of macromolecules than can be considered in this report.* In addressing the question of the true nature of spread polymer f i i s posed above, I shall consider in some detail surface pressure-area curves for a variety of systems and their interpretation. I shall also discuss the application of some newer techniques that have been applied in our laboratory and elsewhere. In all of this, I shall consider only films of preformed synthetic polymers, omitting any consideration of proteins or of polymerization reactions in films. (6) Roberta, G. G. Langmuir-Blodgett Films;Plenum Press: New York, 1990. (7) See, for example, numerous papers in the proceedings of the first four International Conferenceson L-B Films. Thin Solid Films 1983,99, 1985, 132-134; 1988, 159-160; and 1989,178-180 (8) In addition to references already cited, earlier work on monolayers
of polymers may be traced through the following sources: (a) Crisp, D. J. In Surface Phenomena in Chemistry and Biology; Danielli, J. F., Pankhurst, K. G. A., Riddiford, A. C., Eds.;Pergamon Press: New York, 1958, p 23. (b) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley: New York, 1966. (c) Gaines, G. L., Jr. In MTP International Reoiew of Science- Volume 7, Surface Chemistry and Colloids; Kerker, M., Ed.; Butterworths: London, 1972; p 1.
0 1991 American Chemical Society
The Langmuir Lectures
Figure 2. Typical films (a) of the expanded type [poly(ethyl acrylate)]and (b)condensed type [poly(ethylmethacrylate)].ll is the surface pressure, A V is the surface potential, and j t is the apparent moment, plotted against area. AB, nonhomogeneous region (ll negligible; A V variable); BC, expanded CD, linear region; and DE, collapse region. Reprinted with permission from ref 8a. Copyright 1958 Pergamon Press. Surface Pressure-Area (II-A) Curves CrispSnoted that not only completely nonpolar polymers (poly(ethylene), poly(propy1ene))but also some polymers of very polar monomer units (nylon, poly(acrylonitri1e)) could not be spread on a water surface. Presumably the attractive forces between the chain segments in the latter class of materials are much larger than the hydration forces, and hence the spread state is less stable than a threedimensional polymer aggregate. He also distinguished films of the expanded and condensed types, based in part on the character of their ll-A curves (Figure 2). Expanded films were more compressible and exhibited reversible collapse. Fowkesg offered a molecular interpretation for their distinction, suggesting that in expanded films the polymer segments are miscible with water molecules in the surface layer, while in the condensed films the polymer chains are in contact and water is substantially excluded. Measurements during compression-expansion cycles suggest another categorization of spread polymer films. These are illustrated in Figure 3. Reversible behavior (Figure 3a) is observed when the cycle can be stopped with no decay in surface pressure, and the expansion (and subsequent repeated compression) curves reproduce the initial compression. Some films (Figure 3b) exhibit reversible behavior below a collapse surface pressure which may not be well-defined during continuous compression but becomes obvious when the compression is stopped-the surface pressure at constant area falls more or less precipitously. In cases of reversible collapse, the film respreads when the available area is increased, and below the collapse pressure the compression and expansion curves are again identical. In cases of irreversible collapse (Figure 3c), once the film has collapsed the material does not respread, although subsequent (or initial) compressionexpansions limited to surface pressures below collapse are reversible. Finally, several kinds of rearrangements may (9) Fowkes, F. M. J. Phys. Chem. 1964,68, 3515.
Langmuir, Vol. 7, No. 5,1991 835 occur, where the initial compression curve bears little relation to (generally appearing much more compressible than) subsequent compressions or expansions of the precompressed film. (Similar behavior has been noted with some nonpolymer monolayers; in one case it has been shown to be due to the formation of difficulty deformable patches of film during initial spreadingthese then distort during the initial compaction to cover the available surface more uniformly.'") An extremely interesting study of films of hydroxypropylcellulose and hydroxyethylcellulose has been reported very recently by McNally and ZografLll The surface pressure isotherms for spread films are reproduced in Figure 4. Adsorbed layers on aqueous solutions of these water-soluble polymers exhibit the same plateau surface pressures, essentially independent of concentration, and the viscoelastic properties of spread and adsorbed films (estimated by surface light scattering) are virtually identical. Analysis of the surface pressure isotherms led to the conclusion that in both adsorbed and spread and compressed films, approximately 49 % of the hydroxyethylcellulose monomer units and 64 % of hydroxypropylcellulose monomers occupy positions in the surface layer, while the remainder are "desorbed" into the aqueous subphase. Copolymers and films of mixtures of different polymers provide additional informative examples of different n-A curve behavior. Figure 5 illustrates the behavior of monolayers of random copolymers of vinyl acetate (whose homopolymer forms an expanded, highly compressible film) and vinyl stearate (whose long saturated chains produce a homopolymer film which is condensed and incompressible). The introduction of only 10% of the stearate monomer leads to a substantial contraction of the poly(vinylacetate) film, and when 59% vinyl stearate is present, the film is actually slightly less compressible than the stearate homopolymer monolayer. (This behavior may result from the flexibility of the vinyl acetate units between stearate monomers, which permits better packing of the long chains.) Poly(methy1methacrylate) and poly(dimethylsi1oxane) are both spreadable to yield relatively expanded and compressible monolayers. Both block copolymers and mechanical mixtures of these two species spread to yield films whose II-A characteristics are quite different than would be expected from the behavior of the homopolymers (Figure 6). No explanation for these observations is available.12 As noted above, poly(acrylonitri1e) although polar does not spread on water. Poly(4-vinylpyridine) on the other hand is so water soluble that attempts to spread and compress it on a water surface lead to no detectable surface pressure. Yet a random copolymer containing 48 mol % vinylpyridine yields a stable monolayer (Figure 7). At pressures below 5 dyn/cm, the film is stable, and the area requirement is such that every vinylpyridine monomer might be immersed in the subphase. As the area is reduced further, the monolayer begins to rearrange and collapse. Early attempts at theoretical interpretation of the E A curves of polymer monolayers were mostly based on twodimensional analogues of bulk solution lattice theories, such as that due to Singer.13 As already noted, Fowkese had developed an extension of this model to allow for miscibility with water in the surface layer. More recently, (10) Bergeron, J. A.; Gaines, G. L., Jr.; Bellamy, W. D. J. Colloid Interface Sci. 1967,25, 97. (11) McNally, E. J.; Zografi, G. J . Colloid Interface Sci. 1990,198,61. (12) Gaines, G. L., Jr. Adu. Chem. Ser. 1975, No. 144,338. (13) Singer, S. J. J. Chem. Phys. 1948, 16, 872.
The Langmuir Lectures
836 Langmuir, Vol. 7, No. 5, 1991
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less modelistic analysis has been attempted, and in particular a surface solution model with scaling law analysis proposed by Vilanove and Rondelez" has shown promise. This treatment leads to the equation
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(2v - 1) with Y the critical exponent for the radius of gyration of the polymer chain. In two dimensions, v is expected to have the value 0.50 for a theta solvent and 0.77 for a good solvent. A log-log plot of pressure-surface concentration data permits the evaluation of v, and Figure 8 shows such a plot for the poly(4-vinylpyridine-co-acrylonitrile)data of Figure 7 (stable monoIayer region). The slope leads to v = 0.75, indicating that water is a good solvent in the surface layer. Kim and co-workers16 have also applied this analysis to some different polymer systems. Some Other New Tools To Study Polymer Monolayers I turn now to a brief consideration of several additional (mostly optical) techniquesthat have provided information on polymer monolayers complementary to that obtained from classical n-A measurements. (It should be emphasized that these are only a few of many new techniques that are being applied to both monolayers on water and L-B films; for reviews discussing additional methods of characterization, see refs 6, 7, 16, and 17.) R.; Rondelez, F. Phys. Rev. Lett. 1980, 45, 1502. (15) (a) Kim, M. W.; Chung, T. C. J. Colloid Interface Sci. 1988,124, 365. (b)Kim, M. W.; Liu, 5.-N.;Chung, T. C. Phys. Rev. Lett. 1988,60, 2745. (16) Swalen, J. D.; et al. Langmuir 1987,3,932. (17) Stroeve, P.,Fransee, E., Eds. Thin Solid Films 1987, f52, Nos. 1-2. (14) Vilanove,
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Figure 4. Surface pressure isotherms (25 "C) for spread monolayers of several molecular weight grades of hydroxypropylcellulose and hydroxyethylcellulose. H and L are high (-W) and low (-1Oa), while G is intermediate (-3 X W) and E is 'extralow" (-6 X 104). Reprinted with permission from ref 11. Copyright 1990 Academic Press.
Ellipsometry, while it is not a new method, has only recently been applied to study polymer films in situ at air-water interfaces. Kawaguchi et al.'* provided several examples. Sauer et al.l9 have reexamined some of these systems and added others. The former authors concluded that poly(ethy1eneoxide),poly(tetramethy1eneglycol),and (18) (a) Kawaguchi,M.;Yohyama,M.;Mutoh,Y.;Takshaehi,A.Longmuir 1988,4,407. (b)Kawaguchi, M.; Yohyama, M.; Takahashi, A. Longmuir 1988, 4 , 411. (19) Sauer, B. B.; Yu, H.; Yazdanian, M.; Zograf, G.; Kim, M. W. Macromolecules lSS9,22, 2332.
Langmuir, Vol. 7, No. 5, 1991 837
The Langmuir Lectures
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Figure 5. n-A curves for vinyl acetatevinyl stearate copolymers: (1) poly(viny1 acetate) (PVA); (2) 90% PVA-10% PVS copolymer; (3) 41% PVA-59% PVS copolymer; (4) 17% PVA83%PVS copolymer; (5) poly(viny1stearate) (PVS). Reprinted with permission from ref 8b. Copyright 1966 Wiley.
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Figure 6. Surface pressure-areacurvesfor (a)MMA-DMS block copolymer, 44.6% DMS,and (b) mixture of dimethylsiloxane and methyl methacrylate homopolymers,42.2%siloxane. Dashed curve: calculated for additivity of single component monolay-
ers. poly(vin 1acetate) spread films could have thicknesses of 50-400 i.e., have many segments immersed in the subphase. Sauer et al. used a different method of analyzing similar experimental data and concluded that the thicknesses were only 4-6 A. The latter workers also concluded that the thickness of a s read hydroxypropylcellulose monolayer was about 8.0 , compared with a thickness estimated from molecular models of a flat extended chain
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Figure 8. Scaling law plot for vinylpyridine-acrylonitrile copolymer monolayer data of Figure 7. The slope of the line is 3.03, yielding u = 0.75 according to eq lb.
of 5.7 A; this is consistent with the extent of immersion of segments estimated as described aboveell I t would appear that ellipsometry has the potential to elucidate features of polymer monolayer structure, but only if an unambiguous interpretation is possible. Second-order nonlinear optical processes, including second harmonic generation (SHG) and sum frequency generation (SFG), are highly surface selective, because these processes are forbidden in centrosymmetric media.20 Berkovic and Shen21utilized SHG, together with the ratio of s-polarized to p-polarized SHG output, to characterize spread monolayers of poly(viny1 stearate) and poly (octadecyl methacrylate). Their results indicated that the carbonyl (>C=O) bond is perpendicular to the water surface in both of these long-chain ester films. We have also measured second harmonic output from monolayers of poly(4-vinylpyridine-co-acrylonitrile)during compression-expansion cycles. Figure 9 shows typical data. While we do not yet have phase measurements required to permit definitive interpretation, these results appear to be consistent with the hypothesis that the vinylpyridine (20) For a review, 888 Shen, Y. R. Nature 1989,337,519. (21) Berkovic, G.; Shen, Y. R. In Nonlinear Optical andElectroactiue Polymers; Prasad, P. N., Ulrich, D. R., Ede.; Plenum Press: New York, 1988; p 157.
838 Langmuir, Vol. 7, No. 5, 1991
The Langmuir Lectures 4-
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Figure 11. Ellipsometric thickness measurements, based on refractive index of solid polymer, for L-B films of syndiotactic poly(methy1methacrylate) deposited on hydrophobic silicon at 10 dyn/cm. (‘0.5 dip“ point is for single monolayer transferred to a hydrophilic silicon substrate.) ~VMONOMER
Figure 10. E A curve for syndiotactic poly(methy1 methacrylate) on water, 25 OC: initialcompression cycle (solid line)limited to stable region, second compression cycle to region of reversible collapse. Thickness at top is calculated on basis of density of solid polymer. moiety is oriented perpendicular to the water surface, and its orientation does not change during compression. Langmuir-Blodgett Films of Polymers Finally, I give two examples of studies of built-up multilayer films of polymers deposited on solid substrates by the Langmuir-Blodgett (L-B) technique. Several years ago, we examined several samples of poly(methy1 methacrylate) and concluded that a syndiotactic preparation gave the most satisfactory film deposition.22 Figure 10 shows the II-A behavior of this monolayer, which exhibits reversible collapse above about 15dyn/cm, but stable and reversible behavior to a t least 10 dyn/cm. L-B deposition of this material was carried out a t 10 dyn/cm, and Figure 11illustrates the results of ellipsometric measurements of the thickness of these L-B films. There is good linearity of total thickness with number of layers, and the ellipsometric thickness per layer (9.1 A) is close to the thickness (22) Karas, P. L.; Gaines, G. L., Jr. Unpublished resulta.
estimated from the II-A curve (10.9A). (More work would be required to decide whether this difference is real.) Mumby, Swalen, and Rabolt23reported one of the first detailed characterizations of L-B films of preformed polymers. They applied polarized infrared spectroscopy, both in transmission and grazing incidence reflection, to layers of poly(octadecy1 acrylate) (PODA) and poly(octadecyl methacrylate) (PODMA) deposited on evaporated aluminum films. In these L-B films, the carbonyl group was oriented perpendicular to the substrate surface, independent of polymer type and deposition surface pressure. As noted previously, SHG measurements on PODMA monolayers on water indicated the same orientation of the carbonyl group, so it appears that no rearrangement of this functionality occurs during the deposition process. Conclusions In view of the potential technological interest in L-B films of polymeric materials already alluded to, and the availability of new, powerful tools for the study of the precursor monolayers, progress in our understanding of spread films of macromolecules should be rapid. At the (23)Mumby, S.J.; Swalen, J. D.; Rebolt, J. F. Macromoleculee 1986,
19, 1054.
The Langmuir Lectures present time, however, we cannot give a general, definitive answer to the question posed at the beginning of this paper. For Some films, such as those of P o b e r s composed of highly insoluble monomer units (e& PODA Or PODMA), have it is quite certain that well-spread segment lying in the surface. In a few cases, such as hydroxyethyl- and hydroxypropylcellulose, it is also most likely that a substantial fraction of the monomers are immersed. More work is required to decide many inbetween cases.
Langmuir, Vol. 7, No. 5, 1991 839
Acknowledgment. I am grateful to the General Electric Research and Development Center, where most of my own work reported here was carried out. The Naval Air Development Center provided financial (under Contract Number N2269-83-M-3281)for our study on L-B Mrs*pe L*Karas assisted in that work. Nonlinear optical measurements were performed at RPI, with help and advice from G. S. Frysinger and G . M. Korenowski.
