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Polystyrene-Poly(ethylene oxide) Diblock Copolymers Form Well-Defined Surface Aggregates at the Air/Water Interface Juliet K. Cox,† Kui Yu, Bruce Constantine, Adi Eisenberg, and R. Bruce Lennox* Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6 Received February 22, 1999. In Final Form: June 22, 1999 A polystyrene-poly(ethylene oxide) containing diblock copolymer, when spread at the air/water interface, spontaneously forms surface aggregates. This surface aggregation is shown to be neither compressioninduced, associated with micellization in the spreading solvent, nor induced by the Langmuir-Blodgett film transfer process. We have previously found that such two-dimensional surface aggregation occurs for diblock copolymers with a polystyrene block and a hydrophilic block of quaternized poly(vinylpyridine), poly(tert-butyl acrylate), poly(n-butyl acrylate), or poly(dimethylsiloxane). The phenomenon has also been observed in films of polystyrene-poly(methyl methacrylate) by Rice and co-workers.1 Indeed, whenever an appropriate imaging technique has been used, phase-separated domains with 30-100 nm length scales have been observed, when amphiphilic diblock copolymers are spread at the air/water interface and transferred to solid substrates at appreciable surface pressures. We therefore believe that the formation of surface aggregates (often well defined) is a general phenomenon for hydrophobic-hydrophilic diblock copolymers. The implications of this phenomenon for the study of diblock copolymers at the air/water interface are discussed, particularly in relation to studies using techniques which report properties averaged over the lateral dimensions of the film in question, such as specular neutron reflectivity, specular X-ray reflectivity, and Brewster angle microscopy.
Introduction In 1991 the first investigation of the lateral structure of polystyrene (PS) containing A-B diblock copolymers at the air-water interface was published.2 Discrete aggregates termed “surface micelles” were observed and assessed using transmission electron microscopy (TEM), atomic force microscopy (AFM), and off-specular X-ray reflectivity. The term “surface micelle” is used in the sense described by Langmuir.3 However this term is often used in a wider sense and seems to refer to a number of different phenomena. We will thus use the more general term “surface aggregate” in this paper. Surface aggregation has been found to be general for a number of hydrophilic B blocks, including quaternized poly(4-vinylpyridine) (PVP+CnI-), poly(tert-butyl acrylate) (PtBA), poly(n-butyl acrylate) (PnBA), and poly(dimethylsiloxane) (PDMS).4,5 Here we expand the repertoire to include PEO and show that the degree of order in the superlattice formed by the surface aggregates is, in many cases, quite remarkable. The use of PEO as the hydrophilic B block is an important test case given that a number of studies6-8 suggest that * To whom correspondence should be addressed. † Present address: LASST, University of Maine, 5764 Sawyer Research Center, Maine, Maine 00469-5764. (1) Rice, S. A.; Lin, B. J. Chem. Phys. 1993, 99, 8308. (2) Zhu, J.; Eisenberg, A.; Lennox, R B. J. Am. Chem. Soc. 1991, 113, 5583. (3) Langmuir, I. J. Chem. Phys. 1933, 1, 756. (4) See for example: Zhu, J.; Eisenberg, A.; Lennox, R. B. Langmuir 1991, 7, 1579. Zhu, J.; Eisenberg, A.; Lennox, R. B. J. Phys. Chem. 1992, 96, 4727. Li, S.; Hanley, S.; Khan, I.; Varshney, S. K.; Eisenberg, A.; Lennox, R. B. Langmuir 1993, 9, 2243. Meszaros, M.; Eisenberg, A.; Lennox R. B. Faraday Discuss. 1994, 98, 283. Li, S.; Clarke, C. J.; Lennox, R. B.; Eisenberg, A. Colloids Surf. A 1998, 133, 191. (5) Li, Z.; Zhao, W.; Quinn, J.; Rafailovich, M. H.; Sokolov, J.; Lennox, R. B.; Eisenberg, A.; Wu, X. Z.; Kim, M. W.; Sinha, S. K.; Tolan, M. Langmuir 1995, 11, 4785. (6) Goncalves da Silva, A. M.; Filipe, E. J. M.; d’Oliveria, J. M. R.; Martinho, J. M. G. Langmuir 1996, 12, 6547.
PS-PEO diblock copolymers form a homogeneous film at the air/water interface. These polymers show a variety of morphologies at the air/water interface including circular aggregates, rods, ribbons, and large lamellae. We consistently find that if highly regular “superlattices” of circular aggregates arise, then the surface pressure/area isotherms exhibit a plateau or inflection which is not present in the isotherms of the other morphologies. The form of surface aggregation is dependent on the relative length of the two blocks in a fashion similar to that observed for block copolymers in solvents and in the bulk. The films are prepared by carefully spreading polymer, from a stock solution, onto a clean water surface. Transfer via standard Langmuir-Blodgett (LB) techniques leads to the polymer film being deposited on a variety of solid substrates. Various processes could be responsible for the aggregation phenomenon: polymer self-assembly at the water surface, compression-induced surface aggregation during the film balance experiment, or the simple deposition of aggregates formed in the spreading solution. We establish that neither of the latter routes is viable and that, by inference, an in situ self-assembly process is operative. Rice and co-workers1 have used static and dynamic evanescent wave light scattering to conclude that spread films of PS-PMMA form “thin disklike aggregates containing about 240 molecules”. No region of their surface pressure/area isotherm is found to correspond to isolated copolymer chains. There is some evidence that a compression-induced aggregation occurs with diblock copolymers of poly(methyl methacrylate)-(poly(dimethylamino)(7) Goncalves da Silva, A. M.; Simoes Gamboa, A. L.; Martinho, J. M. G. Langmuir 1998, 14, 5327. (8) Bijsterbosch, H. D.; de Haan, V. O.; de Graaf, A. W.; Mellema, M.; Leermakers, F. A. M.; Cohen Stuart, M. A.; van Well, A. A. Langmuir 1995, 11, 4467.
10.1021/la9901940 CCC: $18.00 © 1999 American Chemical Society Published on Web 08/31/1999
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Table 1. Characteristics of the Polymers Used in This Study polymer
Mw total/103
Mw/Mn of the PSa
mol % PS
PS(125)-b-PEO(94) PS(180)-b-PEO (120) PS(140)-PEO(80) PS(215)-b-PEO(113) PS(125)-b-PEO(30) PS(215)-b-PEO(37)
17 24 18 27 14 24
1.07 1.09 1.09 1.09 1.07 1.09
57 60 64 66 81 85
a Obtained by the relative ratio of the NMR signals. b From GPC measurements with PS standards.
ethyl methacrylate)9 and a diblock copolymer of poly(2ethyl-2-oxazoline)-poly(2-nonyl-2-oxazoline).10 It appears that this compression-induced aggregation is governed by different interactions than the surface aggregation of PS containing diblock copolymers. It is significant that when these films are imaged they show much less order than the PS-PEO copolymers described below. Hashimoto and Kumaki11 recently published AFM images of phase separation in a film of a diblock copolymer of poly(methyl methacrylate)-poly (octadecyl methacrylate), where both blocks are hydrophilic. These observations suggest that domain formation is not restricted to diblock copolymers with one hydrophobic block and one hydrophilic block. Despite our extensive work which shows that there are well-defined lateral inhomogeneities in diblock copolymer monolayers, a number of recent studies of PS-PEO diblock copolymers at the air-water interface6-8 do not establish that aggregation occurs once the material is deposited on the water surface. Instead the polymer is assumed to exist in the form of discrete molecules on the water surface. The PEO block is then usually believed to be forced into the water to form a brush on compression. Given our experience of the surface aggregation phenomenon to date and its apparent generality, it seemed to be more likely that the PS-PEO system also forms surface aggregates rather than remaining as isolated molecules at the air/ water interface. Thus, we have studied the lateral structure of a number of PS-PEO diblock copolymers cast onto the water surface. The results confirm our suspicion that, at least for these PS-containing diblock copolymers, surface aggregates are always formed at the air/water interface. This paper is arranged in the following manner. First the range of morphologies observed and their dependence on relative block length are presented, together with a note on the treatment of PEO-containing samples. An investigation into the source of the aggregation is then discussed. Finally this work is considered in terms of the previous papers in this field, and various implications for experiments on diblock copolymers at the air/water interface are examined. Experimental Section The diblock copolymers used in this study were synthesized by sequential anionic polymerization12 and have a polystyrene block with a low polydispersity index (less than 1.1). The characteristics of the polymers studied are summarized in Table 1. Spectroscopic grade chloroform (BDH, Toronto) was used as the spreading solvent without further purification, after establishing that freshly distilled chloroform did not alter the results. (9) An, S. W.; Su, T. J.; Thomas, R. K.; Baines, F. L.; Billingham, N. C.; Armes, S. P.; Penfold, J. J. Phys. Chem. B 1998, 102, 387. (10) Baermark, T. R.; Sprenger, I.; Ruile, M.; Nuyken, O.; Merkel, R. Langmuir 1988, 4, 4222. (11) Kumaki, J.; Hashimoto, T. J. Am. Chem. Soc. 1998, 120, 423. (12) O’Malley, J. J.; Marchessault, R. H. Macromol. Synth. 1972, 4, 35.
Toluene (BDH, Toronto) was also used to test the effect of the spreading solvent. Muscovite mica (JB Scientific), silicon wafers (Applied semiconductors, Virgina), and carbon-coated TEM grids were used for the film transfer. The mica was cleaved just before submersion, and the silicon was cleaned by soaking in boiling chloroform. Copper TEM grids (JBS Supplies, Canada) were first coated with a Formvar film (JBS Supplies, Canada) and then a thin layer of carbon in a procedure described previously.2 The polymers once synthesized and dried were stored, sealed, in a freezer and in the dark. Likewise, once the solutions were made, they were stored, sealed, in the refrigerator and used within 2 weeks. These precautions were taken because both PEO solutions and the dry polymer are known to be highly sensitive to degradation via oxidation.13 The surface pressure/area isotherms and LB deposition were performed using a film balance (KSV 3000) with a platinum Wilhelmy plate. The subphase was prepared from house-purified water which was subsequently passed through a Milli-Q water purification system equipped with an organic removal cartridge. The subphase water temperature was maintained at 28 ((0.5) °C. In a typical experiment, 75-100 µL of the polymer solution (∼1 mg/mL) was spread evenly over the (thermally stabilized) water surface in small (ca. 5 µL) drops. After a further 30 min delay to allow for the evaporation of the solvent, compression at a constant rate of 5 mm/min (25 mm2/s) began. For a LB experiment, the substrates were immersed in the subphase before the polymer solution was spread. The layer was then compressed until a surface pressure of 2 mN/m was reached. The substrates were slowly (1 mm/min) removed from the subphase, vertically through the interface, transferring the polymer layer to the solid surface, while a constant surface pressure was maintained. The TEM grids were subsequently shadowed with a Pt/C mixture, at an angle of 15 or 18° from the horizontal. The TEM images were obtained using a Philips EM400 transmission electron microscope operating at an acceleration voltage of 80 kV. The AFM used was an Autoprobe CP (Park Scientific Instruments, CA) in intermittent contact mode with sharp, silicon, cone tips (2.0 and 0.6 µm thick Ultralevers, Park Scientific Instruments, CA).
Results The range of morphologies that have been obtained in this study fall into two broad categories, namely, ordered circular aggregates (panels b and d of Figure 1) and mixed morphologies of rods and pancakes (panels a and c of Figure 1). The morphology adopted is dependent on the relative block size, where 57-66 mol % PS yields circular aggregates. Above this PS content a mixture of morphologies is apparent with neither rod-only or pancake-only regimes present. We have not yet investigated the effect of lowering the PS content still further, but previous work suggests that lateral segregation still occurs as the PS block becomes smaller.4 The circular core radii range from 16 to 25 nm, and they are separated by 33-42 nm, dependent on the size of the diblock copolymer. Unfortunately, the difficulties inherent in making this measurement and the distribution in the core sizes make the error rather large (∼18%). The determination of the core height is dependent on the method used, with TEM giving larger numbers. The measured range for a number of polymers is between 2 and 7 nm. The surface pressure/area isotherms obtained from some of the diblock copolymers are shown in Figure 2. One might suppose that the surface aggregate morphology can be inferred from the shape of the isotherm. However, we do not believe that one can interpret the film structure simply from the shape of the surface pressure/area isotherm, as discussed later. We assume, as with the previous experiments that the hydrophobic PS block dominates in the higher cores and the PEO is either underneath or radially (13) Bailey, F. E.; Koleske, J. V. Poly(ethylene oxide); Academic Press: NewYork, 1976.
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Figure 3. Schematic depiction of the proposed mechanisms for the formation of surface aggregates. Here black represents the PS block and gray the PEO block in the diblock copolymer.
Figure 1. TEM and AFM images of films from two pairs of polymers with the same PS block and different PEO block lengths: (a) PS(125)-b-PEO(30); (b) PS(125)-b-PEO(94); (c) PS(215)-b-PEO(37); (d) PS(215)-b-PEO(113). The TEM grids were shadowed at an angle of 18 or 15° with a mixture of platinum and carbon. The scale bars on these images correspond to 500 nm. The AFM images are 5 µm2 scans.
Figure 2. Surface pressure/area isotherms from some of the polymers studied here: (a) (PS(125)-b-PEO(30). (b) PS(140)b-PEO(80); (c) (PS(215)-PEO(113). Note that the isotherm from a polymer which gives mixed morphologies (PS(125)-b-PEO(30)), in the LB film, shows less structure than that from a polymer which gives circular aggregates (PS(215)-PEO(113) and PS(140)-b-PEO(80)).
extended around the core. As yet we have been unable to find a method of chemically distinguishing the relative position of the two blocks in the film. It is worth noting that if the spreading solutions are not fresh or the dry polymer has not been stored carefully, the surface aggregation is different and more disordered, probably due to degradation of the PEO block. There are contrary opinions as to the origin of this type of diblock copolymer surface aggregate. Some authors believe it to be a transfer of micelles in the spreading solution to the surface,7 while others think it is driven by compression of the films, in the form of a surface concentration dependent critical micelle concentration
(cmc).9,14 Others (especially us) believe it to be a spontaneous surface aggregation process which is neither compression nor spreading solvent dependent. These different aggregation mechanisms are shown schematically in Figure 3. We set out to test both of the former two possibilities. In panels a and b of Figure 4 the image obtained from a nonselective spreading solvent (chloroform) is compared to that from toluene, a solvent which is highly selective for the PS. Although the diblock copolymer solution in chloroform would be expected to be predominately in the form of single chains, micelles with a PEO core would be anticipated in toluene. This has been tested by dynamic light scattering measurements on the polymer films, where neither spreading solution shows any large aggregates at the concentrations used, though the toluene sample does show large aggregates at a significantly higher concentration (13 mg/mL). Obviously, as the spreading solvent evaporates, the polymer concentration will increase. If the observed surface aggregation is simply a manifestation of micellization in the spreading solvent, toluene would be expected to yield PEO islands, or at very least a different morphology from the chloroform-spread film. However, as can be seen from Figure 4, there is no difference in the morphology. The polymer must therefore be rearranging from its solution configuration when it comes into contact with the water surface to yield the observed morphologies. A TEM image of the spread film transferred at “zero” surface pressure (Figure 4c) establishes that aggregation occurs prior to compression. This sample was obtained by spreading the solution, allowing the solvent to evaporate, and then vertically removing the substrate, through the surface. It is not possible to maintain the surface pressure constant during this LB transfer. Comparison with the deposition at 2 mN/m (Figure 4a) establishes that the circular aggregates spontaneously form on the water surface with no movement of the barriers and are therefore not compression-induced. Discussion We have shown that PS-PEO surface aggregates are not dependent on either the spreading solvent or compression. The question remaining is: what is the driving force behind the surface aggregation? It must be the (14) Israelachvili, J. Langmuir 1994, 10, 3774.
Surface Aggregates of Diblock Copolymers
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Figure 5. Schematic representation of the reorganization of the polymer molecules on compression, proposed by Goncalves and co-workers for PS-PEO diblock copolymers. At low surface pressures, the PS forms an “insoluble globule” with the PEO arrange in a “flattened, pancake-like structure” which does not penetrate the subphase. After compression the PS anchors the PEO to the water surface and the PEO block stretches into the subphase forming an extended brush; thus the structure becomes three-dimensional. The PS block is represented by black and the PEO by gray.
Figure 4. Comparison of the films obtained from solutions of PS(140)-b-PEO(80) in (a) a nonselective spreading solvent (chloroform) and (b) a selective spreading solvent (toluene), both films transferred at a surface pressure of 2 mN/m. In (c) a LB film for the same polymer transferred at “zero” surface pressure is shown. These TEM grids have been shadowed with a mixture of platinum and carbon at an angle of 15°, the scale bars correspond to 200 nm.
balance of interactions between air, water, and the two polymer blocks, analogous to surface-induced phase separation in thin films of diblock copolymers.15 The attraction between the water and the PEO is sufficiently strong that the PEO will spread at the surface. Likewise the repulsion of the PS block for both the water and the PEO drives it to aggregate. The result is domains which are PS rich and which minimize the contact with both the PEO and the water. The spreading solvent acts as a medium for the polymer to find a quasi-equilibrium conformation. We do not believe that it is simply the interaction of the hydrophobic PS block with the water that drives the surface aggregation but rather the relative interaction of the two blocks with the water and the air. Hence copolymers with two hydrophilic blocks can also phase separate,11 due to both the repulsive interactions between the blocks and the difference in the interactions of each block with the water subphase. These results are obviously very different from the recent interpretation of the surface pressure/area isotherm for PS-PEO diblock copolymers, given by Goncalves et al.6 In their work they apply the scaling theories of Alexander16 and Ligoure17 for polymeric brushes at a solid/ liquid interface to the surface pressure/area isotherm and thereby conclude that the isotherm relates to a transition (evolution) of the PEO from a pancake-like to brush-like conformation as depicted schematically in Figure 5. Bijsterbosch and co-workers8 investigated the same polymers by neutron reflection and conclude, from limited data, that at high surface pressures the PEO does indeed extend in a brush-like conformation, with a parabolic volume fraction profile. This is a different perspective to our work where the compression is seen to cause reor(15) Spatz, J. P.; Moller, M.; Noeske, M.; Behm, R. J.; Pietralla, M. Macromolecules 1997, 30, 3874. (16) Alexander, S. J. Phys. (Paris) 1977, 38, 963. (17) Ligoure, C. J. Phys. II 1993, 3, 1607.
ganization of the surface aggregates,18 not individual polymer molecules. Our polymer samples have slightly different molecular weights, higher PS contents, and somewhat differently shaped surface pressure/area isotherms (with much less pronounced plateaus). Molecular weight is unlikely to be a defining issue however. In addition, isotherms of PS-PVP+ show both well-defined plateau and surface aggregation,5 so the isotherm shape is not directly indicative of surface aggregation. The application of solid/liquid theories to the water/air interface is difficult as both the high surface tension of the water and the possibility of the polymer extending into the air present a very different environment for the polymer than adsorption at an impenetrable surface. While the formation of a polymeric brush with varying grafting density on a liquid surface would have tremendous potential, the possibility of such a conformation using a water subphase seems unlikely, as discussed by Richards et al.19 They point out that the low surface energy of the PEO means that (particularly for this polymer) the graft density required to reduce the surface tension of the water below that of the PEO and hence produce a driving force for brush formation, is far above the point of film collapse. This group notes that the information available from previous work8 is limited by the use of only one solvent contrast and the small range of the reflectivity data. Indeed the only clear evidence for brush formation on a liquid has been presented by Kent and co-workers20 for PSPDMS on an ethyl benzoate subphase, which has a relatively low surface tension (γ ≈ 34 mN/m compared to 72 mN/m for water).20 Recently, Goncalves et al.7 have expanded on their previous work and mention the possibility of aggregates in their spreading solutions. They do not however, include aggregation in their interpretation of the surface pressure/ area isotherm and do not actually observe surface aggregates in the polymer film. An et al.,9 on the other hand, propose that a compression-induced aggregation may have occurred. Again no confirmatory evidence, such as LB visualization of the film, is available. Given that the LB transfer technique is a relatively easy extension of the Langmuir film balance experiment, this lack of corroboration greatly limits both the discussion and (18) Cox, J. K.; Yu, K.; Eisenberg, A.; Lennox, R. B. Phys. Chem. Chem. Phys., in press. (19) Richards, R. W.; Rochford, B. R.; Webster, J. R. P. Polymer 1997, 38, 1169. (20) Kent, M. S.; Lee, L. T.; Farnoux, B.; Rondelez, F. Macromolecules 1992, 25, 6240.
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analysis. This is especially true when one compares the variety of interpretations given to very similar shaped surface pressure/area isotherms. Conclusions We have demonstrated that PS-PEO diblock copolymers spontaneously form well-defined circular aggregates at the air/water interface. This work highlights the need for complementary experimental evidence such as AFM or TEM visualization of LB films in order to interpret the surface-pressure/area isotherms of diblock copolymers. Though the surface pressure/area isotherm shapes may be qualitatively similar to those for small amphiphilic molecules, the source of the change in surface pressure is very different and conclusions cannot simply be drawn
Cox et al.
from the isotherm. Indeed many other techniques used at the air/water interface will not detect phase separation on the scale that we describe here. For instance neither Brewster angle microscopy, specular X-ray reflectivity, nor neutron reflectivity would be capable of differentiating a laterally homogeneous film from surface aggregation with domain length scales in the nanometer range. Offspecular reflectivity techniques will show little unless the surface aggregates self-organize into a highly periodic superlattice. One is thus faced with the reality that complementary information (of which ex situ LB visualization is the most definitive) is required to assess the structure of diblock copolymer films at gas/liquid interfaces. LA9901940
