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Langmuir 2008, 24, 13470-13476
Behavior of Thin Films of Poly(oxyethylene)-Poly(oxybutylene) Copolymers Studied by Brewster Angle Microscopy and Atomic Force Microscopy Chris S. Hodges,*,† Robert B. Hammond,‡ and David Gidalevitz†,§ Institute of Materials Research, and Institute of Particle Science and Engineering, UniVersity of Leeds, Leeds LS2 9JT, United Kingdom ReceiVed July 29, 2008. ReVised Manuscript ReceiVed September 25, 2008 Surface films of two copolymers of ethylene oxide (E) and butylene oxide (B), namely E23B8 and E87B18, have been examined by Brewster angle microscopy (BAM) and atomic force microscopy (AFM). Isotherms taken on unsupported films of these copolymers at the air-water interface showed a clear gas to liquid phase transition for E87B18 and a barely discernible phase transition for E23B8. The BAM studies showed a gradual brightening of the films as the surface pressure was increased, which was associated with a film thickening and/or a film densification. Several bright spots were also observed within the films, with the number of spots increasing gradually as the film surface pressure was increased. AFM studies of these films did not show any localized ordering, which fits in with the results from our previous X-ray study of these copolymers [Hodges, C. S.; Neville, F.; Konovalov, O.; Gidalevitz, D.; Hamley, I. W.; Langmuir 2006, 22 (21), 8821-8825], where no long-range ordering was observed. AFM imaging showed two sizes of particulates that were irregularly spaced across the film. The larger particulates were associated with silica contaminants from the copolymer synthesis, whereas the smaller particulates were assumed to be aggregated copolymer. An analysis of the semidilute region of the isotherm showed that while both copolymers had intermixed ethylene oxide and butylene oxide units, the lower molecular weight E23B8 copolymer manifested significantly more intermixing than E87B18.
Introduction When block copolymers are confined to just two dimensions, different kinds of interesting behavior occur that are of significance to a large number of research areas, including adhesion, lubrication, and surfactant studies. The surface properties of the copolymers depend not only on the type of blocks used within the film but also on the ordering of these blocks across the surface. From an analysis point of view, a significant advantage of twodimensional films is that many surface characterization techniques become available, and these can provide detailed descriptions of both long-range ordering (by e.g. X-ray and neutron reflectivity techniques) and much more localized arrangements of the copolymer molecules (e.g., AFM, optical microscopy based techniques). It is the latter category that the present study falls into, with particular emphasis on Brewster angle microscopy (BAM) and atomic force microscopy (AFM). While many studies have been made on films of copolymers with AFM,1-7 fewer studies have been carried out with BAM.8-13 This is, perhaps, * To whom correspondence should be addressed. E-mail: c.s.hodges@ leeds.ac.uk. † Institute of Materials Research, and Institute of Particle Science, University of Leeds. ‡ Institute of Particle Science and Engineering, University of Leeds. § Current Address: Center for Molecular Study of Condensed Soft Matter, Division of Physics, BCPS Department, Illinois Institute of Technology, Chicago, Illinois 60616. (1) Peleshanko, S.; Gunawidjaja, R.; Jeong, J.; Shevchenko, V. V.; Tsukruk, V. V. Langmuir 2004, 20(22), 9223–9227. (2) Zhang, J.; Cao, H.; Wan, X.; Zhou, Q. Langmuir 2006, 22(15), 6587–6592. (3) Peetla, C.; Graf, K.; Kressler, J. Colloid Polym. Sci. 2006, 285, 27–37. (4) Fujioka, M.; Sato, H.; Tsuchia, K.; Ogino, K. Chem. Lett. 2008, 37(3), 272–273. (5) Matmour, R.; Francis, R.; Duran, R. S.; Gnanau, Y. Macromolecules 2005, 38(18), 7754–7767. (6) Gamboa, A. L. S.; Filipe, E. J. M.; Brogueira, P. Nano Lett. 2002, 2(10), 1083–1086. (7) Cao, B. H.; Kim, M. W.; Peiffer, D. G. Langmuir 1995, 11(5), 1645–1652. (8) Faure´, M. C.; Bassereau, P.; Lee, L. T.; Menelle, A.; Lheveder, C. Macromolecules 1999, 32(25), 8538–8550.
because BAM requires copolymers to form structures on a length scale of the order of tens of micrometers, and these features may sometimes be difficult to interpret. However, there is an overlap in scale between AFM and BAM, and by examining the same films with both techniques it is likely that an improved understanding of the film behavior will be obtained. Copolymer films studied by AFM tend to be mounted on either mica or silicon substrates, as the hydrophobicity of these surfaces may be well controlled, plus the substrate itself is exceptionally flat, thus avoiding topographic bulges in the resultant film. However, the chemical nature of the substrate may affect the conformation of the film that is laid down on top of it.12 Indeed, there is a substantial body of work relating to the manner in which a monolayer may be laid down onto a substrate (for example, see ref 14 and references therein). It should be clear that if a monolayer film is at first free to orient itself at an air to water interface as it pleases, this may not result in the same film structure as the deposited Langmuir-Blodgett (LB) film where the underside of the monolayer becomes tethered to the substrate in some way. This is particularly important for amphiphilic molecules such as the copolymers used in the present study. Many investigations have been made regarding the bulk behavior of this EB family of copolymers,20-22 revealing clear (9) Dhanabalan, A.; Ferreira, P. M. S.; Barros, A. M.; van Dongan, J. L. J.; Dynarowicz-La`tka, P.; Janssen, R. A. J. Colloids Surf., A 2002, 198-200, 313– 321. (10) Wu¨stneck, R.; Prescher, D.; Katholy, S.; Knochenhauer, G.; Brehmer, L. Colloids Surf., A 2000, 175, 83–92. (11) Naso, F.; Babudri, F.; Colangiuli, D.; Farinola, G. M.; Quaranta, F.; Rella, R.; Tafuro, R.; Valli, L. J. Am. Chem. Soc. 2003, 125, 9055–9061. (12) Cox, J. K.; Yu, K.; Constantine, B.; Eisenberg, A.; Lennox, R. B. Langmuir 1999, 15(22), 7714–7718. (13) Nieuwkerk, A.; van Kan, E. J. M.; Kimkes, P.; Marcelis, A. T. M.; Sudho¨lter, E. J. R. Langmuir 1998, 14(22), 6448–6456. (14) Lyklema, J. Fundamentals of Interface and Colloid Science; Academic Press: New York, 2005; Vol. II. (15) Hodges, C. S.; Neville, F.; Konovalov, O.; Hammond, R. B.; Gidalevitz, D.; Hamley, I. W. Langmuir 2006, 22(21), 8821–8825.
10.1021/la802445q CCC: $40.75 2008 American Chemical Society Published on Web 11/08/2008
BAM and AFM Studies of E23B8 and E87B18 Thin Films
Figure 1. Structure of the copolymers used in this paper.
micellization points,24 effects of temperature,20 and phase structure.21 However, few researchers have examined the interfacial behavior of these EB copolymers. Previous studies on free-standing copolymer films of this type15,23 indicate that over large areas the films seem to be amorphous. Studies at a finer scale may reveal structure that was too small to be seen previously, or whether these copolymers deposit in a truly random manner.
Materials and Methods The copolymers used in the present work, poly(oxyethylene)23co-poly(oxybutylene)8 and poly(oxyethylene)87-co-poly(oxybutylene)18, have been synthesized and characterized previously,20,21 and are labeled in this paper as E23B8 and E87B18, respectively. The chemical structure of the copolymers used in this study is shown in Figure 1 where m is either 23 or 87, and n is either 8 or 18. The shorter copolymer, E23B8, is quoted20 as being similar to E27B7, which was stated as having an uncertainty in chain length as (2 E units and (1 B units and a molecular weight ratio, Mw/Mn ) 1.05 based on GPC measurements. The molecular weight distribution of E87B18 is Mw/Mn ) 1.02. The solubility of the oxybutylene is much lower than that of the oxyethylene in water, so it would seem likely that the copolymer would orientate itself preferentially with the oxyethylene units within the water subphase, and the oxybutylene units in the air at the air-water interface, although the behavior of one of these copolymers as examined by X-ray reflectivity15 seems to be more complex than this. The molecular weight of E23B8 was 1588 g/mol, and that of E87B18 was 5124 g/mol. Brewster Angle Microscopy Experiments. The Brewster angle microscope (BAM) used was a Nanofilm Technologie GmbH BAM2plus and was mounted directly above a Langmuir trough. The BAM was fitted with a 10× long working distance objective lens, and both the objective and the detector were mounted at the Brewster angle for water at 532 nm of 53.15°. The field of view of the lens was approximately 440 µm by 330 µm, and its numerical aperture was 0.21, with a resolution of approximately 2 µm. Due to the nature of the laser used (40 mW continuous), great care was required when aligning the instrument. The fact that the lens is at an angle means that only a thin portion of the total field of view will be in focus at any one time. This means that to obtain a complete image the software has to adjust the focus of the lens sequentially as it scans across the surface of the sample and then couple these individual scans together. This limits the rate at which the data may be obtained to approximately one image every 5 s. The distortions to the resultant images due to the angle of incidence of the laser are corrected by software supplied with the BAM. A piece of angled black glass was mounted at the bottom of the trough to ensure that no double reflection from the laser could occur. The Langmuir trough used was custom built,16 rectangular and made from PTFE and mounted directly under the BAM on a specially designed stage. The trough was usually filled to approximately twothirds the maximum depth due to the particular arrangement of the barriers and Wilhelmy plate in this instrument. The barriers consist of two pairs of coupled 15 cm pieces of PTFE hinged together in (16) Murray, B. S.; Nelson, P. V. Langmuir 1996, 12(25), 5973–5976. (17) Hutter, J. L.; Bechhoefer, J. ReV. Sci. Instrum. 1993, 64(7), 1868–1873. (18) Booth, C.; Attwood, D. Macromol. Rapid Commun. 2000, 21(9), 501– 527. (19) des Cloizeaux, J. J. Phys. (Paris) 1975, 36, 281. (20) Tanodekaew, S.; Deng, N.; Smith, S.; Yang, Y.; Attwood, D.; Booth, C. J. Phys. Chem. 1993, 97, 11847–11852.
Langmuir, Vol. 24, No. 23, 2008 13471 a rhombohedral arrangement, so that the maximum surface area for a spread film is 225 cm2. A single drive pushes or pulls on one pair of hinges to close or open the barriers uniformly at rates as slow as 0.3 mm/min. The compression ratio of the trough as used in these experiments was 7. A typical measurement first involves spreading the appropriate volume of copolymer, dissolved in chloroform, onto the surface of the subphase (MilliQ 18.2 MΩcm water) using a microliter syringe. The copolymer film was then allowed to settle for 20 min before any experiment was carried out. This allowed sufficient time for both evaporation of the chloroform and relaxation of the copolymer film. The surface pressure measurements were made using a mica Wilhelmy plate, and this was calibrated each time before any experiment was started. The entire equipment was encased in a steel cabinet both to minimize dust settling onto the spread monolayer film and to meet the safety requirements for using the class 4 green laser of the BAM. Since the barriers close symmetrically in this trough, the Wilhelmy plate was positioned in the center of the trough to allow maximum barrier movement. At the start of the experiment, the BAM was set so that the polarizers in front of the laser source and CCD camera detector let the minimum amount of light through to the detector. There will always be some roughness of the air-water interface that prevents all light from the laser from being blocked by the polarizers. A calibration image was then collected with no monolayer deposited on the subphase surface, so that any imperfections in the CCD camera lens collecting the laser light might be background corrected. When a film is deposited onto the surface of the water, the Brewster angle of the resultant monolayer-air interface will be slightly altered so that some laser light will be allowed through to the CCD camera. By scanning the CCD camera laterally, an image of the deposited film is formed. Atomic Force Microscopy Experiments. Langmuir-Blodgett (LB) films of the copolymers were deposited onto freshly cleaved muscovite mica over a variety of surface pressures used in the BAM experiments. The films formed were then examined by both contact mode and tapping mode imaging in a Digital Instruments Nanoscope IV instrument (Santa Barbara, CA). The largest scan size possible with the EV-type scanner mounted was 14 µm × 14 µm. This is just large enough to be able to compare some surface features with features visible in the BAM. For larger area scans, a Topometrix Explorer atomic force microscope (Veeco, Santa Barbara, CA) with a maximum scan size of 100 µm × 100 µm was used. The cantilevers used were standard NPS silicon triangular levers (Veeco, Santa Barbara, CA) with typical spring constants of 0.12 N/m as determined by the thermal resonance method.17
Results and Discussion Isotherms for both copolymers are shown in Figure 2. The isotherm for the lower molecular weight copolymer (E23B8) is quite indistinct, with only a single very small phase transition at around 600 Å2 per molecule (7.7 mN/m), and this copolymer never reached a collapse pressure even after several hundred milliliters of solution had been spread at several different concentrations. The higher molecular weight copolymer, E87B18, has two clear phase transitions corresponding to the so-called gas-to-liquid transition at 3000 Å2 per molecule (2.4 mN/m) and then another “gel-phase” transition at 750 Å2 per molecule (12.8 mN/m). The names given here to the different regions of the isotherm are only intended to be descriptive and are common in Langmuir film work. This copolymer could only be compressed up to a surface pressure of 42 mN/m, where the film collapsed. Presumably, the longer ethylene oxide chains prevent the E87B18 molecules from sliding over each other, forcing the copolymer to remain at the air-water interface until a maximum packing density is achieved. The much shorter ethylene oxide chain of (21) Castelletto, V.; Caillet, C.; Fundin, J.; Hamley, I. W.; Yang, Z.; Kelarakis, A. J. Chem. Phys. 2002, 116, 10947–10958.
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Figure 2. Isotherms for free-standing films of E23B8 (left) and E87B18 (right) at the air-water interface. The isotherms were carried out at 25 °C and compressed at 0.29 cm2/s.
Figure 3. BAM images at the air-water interface on E23B8 at surface pressures of (a) 6.8 mN/m, (b) 9.6 mN/m, (c) 13.4 mN/m, and (d) 16.9 mN/m.
Figure 4. BAM images at the air-water interface on E87B18 at surface pressures of (a) 6.9 mN/m, (b) 9.5 mN/m, (c) 20.5 mN/m, and (d) 23.5 mN/m.
E23B8 seems to allow much more rearrangement of the molecules to occur at the interface, perhaps even forcing multilayers to form at higher surface pressures. If true, this could lead to more topography being present in films of E23B8 than in similar films of E87B18 assuming that these multilayers transfer during the LB deposition. It is possible that E23B8 becomes partially soluble at higher surface pressures, in which case no extra topography would be found on the LB films. The equivalent bulk concentration of each copolymer was much lower than the critical micellization concentration (cmc) (about 10 g/dm3 for E23B8 at 300 K and approximately 0.01 g/dm3 for E87B1822) even for the largest volumes of copolymer added for the higher surface pressure parts of the isotherm. Thus, while we cannot completely rule out the possibility of surface micelles being formed at lower concentrations than the bulk cmc, we feel that the isotherms in Figure 2 show the majority of the spread copolymer remains at the air-water interface. Rippner et al.23 describe liquid surface films of EB copolymers and show a similar shaped surface tension isotherm to Figure 2. They mention the effect of film age on the position of the isotherm, an effect that was found to be very minor with the copolymers used in our experiments. The results of the BAM experiments on E23B8 are shown below (Figure 3). All experiments were carried out at room temperature (25 °C). The low surface pressure images are generally featureless, except for a few brighter spots widely
separated from each other. As the surface pressure is increased, more of the bright spots appear, and the mean separation between each bright spot decreases. At the same time, there is a slightly larger amount of laser light reflected, visible as a small increase in the background brightness in each image as the surface pressure is increased. This may be interpreted as either a possible thickening of the surface film, as each molecule is forced to alter its conformation due to the vicinity of its nearest neighbors with the increased compression from the barriers of the Langmuir trough, or a densification of the packing of the copolymer molecules. Both film thickening and densification of the free-standing films were observed in our previously published X-ray reflectivity data.15 The BAM images obtained using E87B18 copolymer films are shown in Figure 4. The BAM images obtained on films of E87B18 show a small number of large bright spots at low surface pressures. The number of these large bright spots does not appear to change much as the surface pressure is increased. However, above about 20 mN/m, smaller bright spots begin to appear in the film, and the number of these smaller spots increases steadily with surface pressure. The brightening of the images as the surface pressure increases is also more obvious with E87B18 than with E23B8, and again this brightening is quite sudden when the pressure is increased to near 20 mN/m, as can be seen in Figure 4c and d. This would suggest a phase transition of some sort at this pressure,
BAM and AFM Studies of E23B8 and E87B18 Thin Films
Figure 5. Zoomed BAM images of (a) E23B8 at 16.9 mN/m and (b) E87B8 at 23.5 mN/m.
in agreement with Figure 2 where a gradient change at a surface pressure of about 20 mN/m is observed. Figure 5 shows a comparison between two zoomed BAM images from each surface film. It is apparent that both films contain roughly similar sized bright spots of about 5 µm in diameter, but no further information regarding the film structure may be obtained, since it is below the resolution of the Brewster angle microscope. The similar size and appearance of the bright spots observed in Figure 5 suggests that the same feature is present in both copolymer films. We may discount dust from the air as being a possible candidate for these bright spots, since considerable care was taken to prevent dust by covering the entire BAM with a set of shutters. When the shutters were opened and the film surface examined over time, several very large and much brighter angled features appeared (not shown), which we believe were settled dust particles. More likely is that the bright spots seen in Figure 5 are either from lumps formed in the copolymer film from aggregation of the copolymer on compression, or that impurities from the copolymer synthesis have been incorporated into the surface film. Such globules have been observed previously by BAM studies on copolymers of polystyrene-polyethylene oxide (PS-PEO) by Faure´ et al.8 In fact, the BAM images obtained on the PS-PEO copolymer films have a very similar appearance to the films studied here. It was claimed8 that for the PS-PEO films the existence of these bright domains in the BAM images showed clear evidence for a transition from “adsorbed layer to brush” within the plateau region of the isotherm. It would seem unlikely that such a transition exists within our uncharged EB copolymer films, since a brush layer would lead to long-range ordering that would have been picked out in the X-ray studies,15 and a brush layer is more likely to lead to a clear collapse pressure in the copolymer isotherm (not seen in Figure 2 for E23B8). Further information on the detail of the film structure may be obtained from LB films on mica, imaged by AFM (Figure 6). At 3, mN/m the E23B8 film is in the gaseous phase based on the isotherm shown in Figure 2. The 500 nm image in Figure 6a illustrates a sparsely populated area, with occasional groups of molecules dispersed randomly over the image. It should be noted that our published X-ray work15 on this system suggested a highly coiled ethylene oxide block at all surface pressures leading to a very small free-standing film thickness (3.5 Å). The dried and collapsed films that were imaged on these LB films are likely to measure much smaller heights than those of the free-standing films, particularly if the film is thinly distributed, as seems to be the case in Figure 6a. The rms roughness measured from Figure 6a was 0.7 ( 0.3 Å, which means that only a small fraction of the area is covered in copolymer and most of the area is just the mica substrate surface. It should be noted that mica has a high surface charge density that may cause the adsorbed copolymer to lie more closely to the interface than would be found on say a silica substrate. Occasionally when imaging, a larger and harder (based on the phase contrast seen in tapping mode imaging)
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structure was found, examples of which are shown in Figure 6c. It is likely that these harder structures are silica particles remaining in the polymer solution from the synthesis of these copolymers.18 The largest structures shown in Figure 6c are less than 1 µm in size, that is, below the resolution limit for the BAM and much smaller than the bright spots shown in Figure 5. The AFM images presented in Figure 7 show the appearance of the E23B8 film in the central region of the isotherm (Figure 2). Over the smallest scan size (Figure 7a), the film appears to be still randomly oriented but more raised than was observed at 3 mN/m (Figure 6). Measured film heights were indeed larger (6 Å) than those observed at 3 mN/m, although as mentioned before these heights represent a much compressed film thickness from the collapse of the film under its own weight in air. Film thickening (to 13 Å) at this intermediate pressure was also observed in the published X-ray reflectivity studies of this copolymer15 and corresponded to a conformational rearrangement of the ethylene oxide parts of the copolymer. Again, at the larger scan sizes, two particle sizes are visible, with the smaller particle size being much more common than the larger particle. Only a few small particles were visible in Figure 6c at 3 mN/m, whereas more are visible in Figure 7c. This would suggest that these smaller particles may be formed from the copolymer as the film is being compressed. The smaller particles are approximately the same size (130 nm across) in both figures. The images shown in Figure 8 were taken with the copolymer film compressed into the gel phase of the isotherm, where in the BAM images a sudden brightening of the reflected laser signal occurred, and many bright spots were visible (Figure 3). The 500 nm image, Figure 8a, shows a quite different structure from that of the films studied at either 7 or 3 mN/m. The small thin strands that were previously visible (Figures 6a and 7a) have now formed into small rounded lumps about 24 nm across and with an average height of 6 Å, as if the copolymer molecules themselves have been squashed together to form a highly globular film. The fact that the film height remains the same when the surface pressure is increased from 7 to 27 mN/m suggests a densification of the film. However, the fits to the X-ray data15 did not show a densification of the film, suggesting that some of the copolymer was being lost from the surface film into the bulk liquid at 27 mN/m. This would explain why there was no collapse pressure found on the isotherm for E23B8 (Figure 2). The fast Fourier transform of image (a) in Figure 8 showed no identifiable pattern, indicating that even at this high surface pressure the globules remain disordered. Rippner et al.23 found adsorbed amounts of 2-3 mg/m2 for E106B16 on hydrophobic silica by ellipsometry and listed the copolymer layer thickness as up to 10 nm at its maximum, that is, above the micellization point. For our dried films, there does not appear to be any sign of micellization, both from the AFM images and from the shape of the isotherm. In fact, it is possible that the EB molecules in our surface films deposited on mica lie nearly flat as a result of the higher surface charge of mica. The hydrated micelle size for E23B8 is 5-7 nm,21 which would be expected to collapse to maybe 2-4 nm on drying. None of the measured heights in the AFM data shown here are close to these values, but instead the heights are nearer to the thickness of a single E23B8 molecule. Hence, we conclude that our data show no evidence for the formation of surface micelles. The larger scale AFM images, Figure 8b and c, show two types of particle. The smaller particles appear very similar both in size (130 nm) and number to those seen in Figure 7, as if once the copolymer has formed these aggregates, only a certain density (22) Booth, C.; Attwood, D.; Price, C. Phys. Chem. Chem. Phys. 2006, 8, 3612–3622.
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Figure 6. AFM images on Langmuir-Blodgett films of E23B8 at a surface pressure of 3 mN/m on a mica substrate: (a) a 500 nm scan size, (b) a 4 µm scan size, and (c) a 14 µm scan size.
Figure 7. AFM images on Langmuir-Blodgett films of E23B8 at a surface pressure of 7 mN/m on a mica substrate: (a) a 500 nm scan size, (b) a 4 µm scan size, and (c) a 14 µm scan size.
Figure 8. AFM images on Langmuir-Blodgett films of E23B8 at a surface pressure of 27 mN/m on a mica substrate: (a) a 500 nm scan size, (b) a 4 µm scan size,and (c) a 14 µm scan size.
Figure 9. Topometrix Explorer AFM images on a 27 mN/m Langmuir-Blodgett film of E23B8 on a mica substrate: (a) an 80 µm scan size, (b) a 40 µm scan size, and (c) a 20 µm scan size.
of them can exist at the interface The larger particle (300 nm across) is probably mechanically harder than the smaller particle, since a significant phase contrast was observed on these particles when they were imaged in tapping mode. We suggest that these larger, harder particles are remaining silica particles from the synthesis of the copolymers. The particle sizes measured from the AFM images above are much smaller than the bright spots seen in the BAM images. At the highest surface pressures, the mean spacing between bright spots on the BAM images is at least 20 µm. To address this issue, we took our samples onto a Topometrix Explorer atomic force microscope with a tripod scanner capable of imaging a 100 µm scan size.
The smaller scale image of Figure 9c clearly shows that the larger lumps are themselves composed of smaller parts that have aggregated together to form features a few micrometers across. Since these aggregates are the same size as the bright spots observed in the BAM image (Figure 5), it is likely that they are the same entity. It would therefore seem possible that the E23B8 copolymer films contain individual globules of copolymer approximately 130 nm across (Figure 8) that are able to aggregate together into micrometer-sized lumps visible as bright spots in the BAM image. The large cone shaped feature visible in Figure 9a is approximately 15 µm across and very smooth in appearance. These features should be easily visible in the BAM images, but
BAM and AFM Studies of E23B8 and E87B18 Thin Films
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fully hydrated copolymer layer at the air-water interface. For the lighter of the two copolymers (E23B8), Figure 10 shows a gradient of 2.11, which means that the adsorbed layer at the air-water interface is almost completely three-dimensional in nature. Thus, it is not surprising that little ordering was observed in the AFM images shown here (Figures 6-8).
Conclusions
Figure 10. Double logarithmic plot of the surface pressure versus the inverse surface area (σ ) 1/A) for E23B8 (squares) and E87B18 (circles). The corresponding linear fits are drawn over each data set, with the gradient of the fitted line indicated alongside. E23B8 has a gradient of 2.11, and E87B18 has a gradient of 2.69.
they were not observed, indicating that it most probably results from the LB film deposition process. A zoomed portion of Figure 9a is shown in Figure 9b. As well as the large cone shaped feature, just visible on the surface of this cone are small lumps, each of which are about 300 nm across. This corresponds to the size of the particles described above in Figure 8 that were assumed to be residual silica from the copolymer synthesis. The AFM data for E87B18 (not presented) demonstrated very similar results to those shown for E23B8 in Figures 6-8. The observed features were of approximately the same size as those found for E23B8, and no long-range order was observed. Silica particles on the E87B18 films were just as prevalent and of a similar size, lending weight to our hypothesis that the 300 nm particles observed in Figures 6-8 are residual contaminant silica from the copolymer synthesis. We do not believe that the presence of the silica particles significantly alters the ability of the copolymer film to structure, since the copolymers are overall uncharged and therefore unlikely to be attracted to or repelled from the negatively charged silica. It is worth noting that Faure´ et al.8 also had an unstructured film despite the presence of the polystyrene in their copolymer. This suggests that the oxyethylene plays an important role in maintaining good mixing within the surface film, probably from its very high hydrophilicity. The behavior of the copolymer films examined in this paper may be examined with regard to the degree of mixing within the adsorbed layer. Following the work of des Cloizeaux,19 at intermediate pressures (in the semidilute region) just below the plateau of the isotherm for each copolymer, π/T ∝ A-y where y ) Vd/(Vd - 1), V is the critical exponent of the excluded volume, d is the spatial dimensionality, π is the surface pressure, A is the area occupied by each molecule, and T is the absolute temperature. If the copolymer forms a two-dimensional array in this concentration regime, then y ) 3, and if the array is fully threedimensional then y ) 2.25. The data in Figure 10 show that, for the larger of the two copolymers (E87B18), the layer is intermediate between forming a two-dimensional and a three-dimensional array (gradient ) 2.69) in this semidilute region. The fact that no significant structure was observed agrees with this, as did our earlier published model from the X-ray reflectivity data,15 where we concluded that there should be a significant degree of intermixing between the ethylene oxide and the butylene oxide parts of the copolymer within the (23) Rippner, B.; Boschkova, K.; Claesson, P. M.; Arnebrant, T. Langmuir 2002, 18, 5213–5221. (24) (s) Ribeiro, E. M. N. P.; Oliviera, S. A.; Ricardo, N. M. P. S.; Mai, S.; Attwood, D.; Yeates, S. G.; Booth, C. Int. J. Pharm. 2008, 362, 193–196.
Films of two copolymers of ethylene oxide and butylene oxide (E23B8 and E87B18) were investigated with both BAM and AFM. The isotherm obtained from free-standing films of E87B18 spread at the air-water interface showed two phase transitions: a gaseous to liquid-expanded transition at 2.4 mN/m and a liquid condensed to solid transition at 12.8 mN/m, followed by a film collapse pressure at 42 mN/m. The lower molecular weight copolymer (E23B8) only had a very small transition at 7.7 mN/m and did not show any sign of film collapse. Instead, an upper surface pressure limit was found at 28 mN/m that could not be increased by adding more copolymer to the surface or by compressing an existing film still further. Above 28 mN/m, it is possible that some E23B8 moves away from the air-water interface into the bulk to relieve the surface stresses. BAM images obtained on each copolymer demonstrated a gradual brightening in the image as the surface pressure was increased. This was correlated with a gradual thickening and/or densification of the surface layer, in agreement with our earlier X-ray reflectivity study of these copolymers.15 The BAM data for E87B18 brightened significantly more than the E23B8 films and was linked with the ability of the E87B18 to sustain high surface pressures, allowing a much higher surface density to occur than was possible with E23B8. At the same time as the brightening of the surface film, bright spots of approximately 5 µm across also appeared within the BAM images. The number of bright spots increased as the surface pressure was increased, and each of these spots was associated with aggregates of small (130 nm) copolymer lumps that were identified from AFM images of the same copolymer films. Otherwise, the BAM images did not show any long-range ordering for either of the copolymers. The AFM images obtained from LB films of these copolymers on mica showed unstructured, thinly spread strands of copolymer a few nanometers in length. These strands became more globular at 27 mN/m, although the measured film thickness was very small (6 Å) compared with the values of film thickness derived from X-ray data15 of free-standing films of the same copolymer. These small film thickness measurements by AFM were associated with film collapse and drying before imaging in air. However, the trend in the copolymer thickness measured by AFM correlated quite well with what had been found from the X-ray modeling on E23B8. Thus, there is significant film thickening between 3 and 7 mN/m for E23B8, but almost no further film thickening occurs at surface pressures higher than this. When a larger area of the film was examined by AFM, all surface pressures demonstrated two types of particulate that existed within the copolymer film. One type consisted of soft lumps approximately 130 nm in diameter that could aggregate into much larger micrometer-sized lumps that were visible in the BAM images. We believe that these lumps were local features of the copolymer, since their number gradually increased as the surface pressure increased. The other particulate type appeared to be much harder (based on larger phase contrast in tapping mode imaging) and also larger at 300 nm in diameter. We suggest that these larger particulates are residual silica from the synthesis of the copolymer. A brief analysis of the intermixing within the copolymer layer at the air-water interface at the semidilute region of each isotherm
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was made, and we conclude that in this region of the isotherm the two copolymers behave differently. E87B18 appears to be a much more two-dimensional surface film; that is, the butylene oxide blocks and ethylene oxide blocks seem to lie the same way up across the film. This makes sense in terms of the isotherm for E87B18 (Figure 2), where the surface pressure increases dramatically up to the collapse pressure, probably indicating that each molecule cannot move past its neighbors and is trapped within the surface film. This is not true for E23B8, however, as this film was found to be almost completely three-dimensional and no collapse pressure was found in the isotherm. We suggest that, at high surface pressures, some of the E23B8 molecules are forced away from the air-water interface into the bulk liquid, implying that the ethylene oxide chains in E23B8 are not as tangled as may be the case with the much longer ethylene oxide chains of E87B18.
Hodges et al.
Overall, the findings in this paper consolidate the X-ray work already published15 and suggest that these copolymers, although simple in form, may not always behave in a simple manner. In fact, because no long-range structure or short-range structure has been found with these copolymers at the air-water interface, the analysis is more complex and suggestive. However, the possibility of being able to tune the relative hydrophobicity of these simple copolymers may yet lead to interesting applications at an air-liquid interface. Acknowledgment. We are grateful to Dr. Brent Murray from the Proctor department of Food Science here at Leeds University for the extended time allowed to us for the brewster angle microscope used in this paper. LA802445Q
