Langmuir 2006, 22, 8821-8825
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Structural Analysis of PEO-PBO Copolymer Monolayers at the Air-Water Interface Chris S. Hodges,*,† Frances Neville,† Oleg Konovalov,‡ Robert B. Hammond,§ David Gidalevitz,† and Ian W. Hamley|| Institute for Materials Research, Institute of Particle Science and Engineering, and Department of Chemistry, UniVersity of Leeds, Leeds, LS2 9JT, United Kingdom, and European Synchrotron Radiation Facility, B.P. 220, 38043, Grenoble, France ReceiVed March 8, 2006. In Final Form: July 4, 2006 X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD) have been used to examine an oxyethyleneb-oxybutylene (E23B8) copolymer film at the air-water interface. The XR data were fitted using both a one- and a two-layer model that outputted the film thickness, roughness, and electron density. The best fit to the experimental data was obtained using a two-layer model (representing the oxyethylene and oxybutylene blocks, respectively), which showed a rapid thickening of the copolymer film at pressures above 7 mN/m. The large roughness values found indicate a significant degree of intermixing between the blocks and back up the GIXD data, which showed no long range lateral ordering within the layer. It was found from the electron density model results that there is a large film densification at 7 mN/m, possibly suggesting conformational changes within the film, even though no such change occurs on the pressure-area isotherm at the same surface pressure.
Introduction Copolymers of ethylene oxide (represented by E) and butylene oxide (B) have been studied in great detail, particularly by Attwood and Booth and co-workers.1 Most of the research on these copolymers to date covers their bulk behavior, and there exists a dearth of information regarding the surface interactions and conformations taken up by these systems. Interest in these copolymers extends to the pharmaceutical industry, where they have been shown to accept griseofulvin when in their micellar form.2 Ethylene oxide is potentially useful within the body, as it is neutraly charged, water soluble, and biodegradable. To balance this hydrophilic polymer, butylene oxide may be added with various chain lengths to change its hydrophobicity and to alter the overall equilibrium conformations of the copolymer.1 A possible route to prepare novel drug encapsulations, currently being investigated, could involve a drug interacting with a spread copolymer layer at an interface. As such it is important to examine the different conformations taken by the copolymer at interfaces, particularly the air-water interface. Although there are a number of experimental techniques available to study Langmuir monolayers at the air-water interface, very few of them can yield structural information on the molecular level. Two techniques that can are X-ray specular reflectivity (XR) and grazing incidence X-ray diffraction (GIXD), both of which have been used in the present paper. Although small-angle X-ray scattering and smallangle neutron scattering have been used on bulk solutions of these copolymers before,3-11 this is the first time these copolymers have been examined with XR and GIXD at the air-water interface. Compared with the number of surface X-ray studies on homopolymer films, surface studies on copolymer monolayers * To whom correspondence should be addressed. E-mail:
[email protected]. † Institute for Materials Research, University of Leeds. ‡ European Synchrotron Radiation Facility. § Institute of Particle Science and Engineering, University of Leeds. || Department of Chemistry, University of Leeds. (1) Booth, C.; Attwood, A. Macromol. Rapid Commun. 2000, 21, 501. (2) Rekatas, C. J.; Mai, S.-M.; Crothers, M.; Quinn, M.; Collett, J. H.; Attwood, D.; Heatley, F.; Martini, L.; Booth, C. Phys. Chem. Chem. Phys. 2001, 3, 4769.
are relatively few. X-ray reflectivity has been used before to examine DSPE-PEO,12 PS-PVP,13 MeSt-MAA-co-Et2SiMAA,14 PS-PMS,15 DMAEMA-BMA,16 PS-PBMA,17 and PEE-PEO18 monolayers. In the case of the PS-PVP studies,13 two-dimensional “surface micelles” were found to exist, with a relatively low concentration of water within these surface structures, as confirmed by off-specular scattering studies. Some of the most relevant XR data with respect to the present work is from PEE-PEO, where both neutron reflectivity (NR) and XR were carried out.18 It was found that at medium surface pressures (ca. 10 mN/m) the PEO desorbs to form a brush and the PEE forms a film about 100 Å thick. It was also found that the NR and XR results disagreed with respect to the PEO electron density. When the XR and NR data from the PEE-PEO was attempted to be fit with a simple two-layer box model consisting (3) Kelarakis, A.; Mai, S.-M.; Havredaki, V.; Nace, V. M.; Booth, C. Phys. Chem. Chem. Phys. 2001, 3, 4037. (4) Soni, S. S.; Sastry, N. V.; Patra, A. K.; Joshi, J. V.; Goyal, P. S. J. Phys. Chem. B 2002, 106, 13069. (5) Chaibundit, C.; Mingvanish, W.; Booth, C.; Mai, S.-M.; Turner, S. C.; Fairclough, J. P. A.; Ryan, A. J. Macromolecules 2002, 35, 4838. (6) Xu, J.-T.; Fairclough, J. P. A.; Mai, S.-M.; Chaibundit, C.; Mingvanish, W.; Booth, C.; Ryan, A. J. Polymer 2003, 44, 6843. (7) Xu, J.-T.; Turner, S. C.; Fairclough, J. P. A.; Mai, S.-M.; Ryan, A. J.; Chaibundit, C.; Booth, C. Macromolecules 2002, 35, 3614. (8) Hamley, I. W.; Pedersen, J. S.; Booth C. Langmuir 2001, 17, 6386. (9) Ryan, A. J.; Mai, S.-M.; Fairclough, J. P. A.; Hamley, I. W.; Booth, C. Phys. Chem. Chem. Phys. 2001, 3, 2961. (10) Mingvanish, W.; Kelarakis, A.; Mai, S.-M.; Daniel, C.; Yang, Z.; Havredaki, V.; Hamley, I. W.; Ryan, A. J.; Booth, C. J. Phys. Chem. B 2000, 104, 9788. (11) Kelarakis, A.; Mingvanish, W.; Daniel, C.; Li, H.; Havredaki, V.; Booth, C.; Hamley, I. W.; Ryan, A. J. Phys. Chem. Chem. Phys. 2000, 2, 2755. (12) Boltze, J.; Takahashi, M.; Mizuki, J.; Baumgart, T.; Knoll, W. J. Am. Chem. Soc. 2002, 124, 9412. (13) Li, Z.; Zhao, W.; Quinn, J.; Rafailovich, M. J.; Sokolov, J.; Lennox, R. B.; Eisenberg, A.; Wu, X. Z.; Kim, M. W.; Sinha, S. K.; Tolan, M. Langmuir 1995, 11, 4785. (14) Matsuoka, H.; Mouri, E.; Matsumoto, K.; Rigaku J. 2001, 18 (2), 54. (15) Mu¨ller-Buschbaum, P.; Wolkenhauer, M.; Wunnicke, O.; Stamm, M.; Cubitt, R.; Petry, W. Langmuir 2001, 17, 5567. (16) Su, T. J.; Styrkas, D. A.; Thomas, R. K.; Baines, F. L.; Billingham, N. C.; Armes, S. P. Macromolecules 1996, 29, 6892. (17) Vignaud, G.; Gibaud, A.; Wang, J.; Sinha, S. K.; Daillant, J.; Gru¨bel, G.; Gallot, Y. J. Phys. Cond. Matter. 1997, 9, L125. (18) Wesemann, A.; Ahrens, H.; Steitz, R.; Fo¨rster, S.; Helm, C. A. Langmuir 2003, 19, 709.
10.1021/la060632k CCC: $33.50 © 2006 American Chemical Society Published on Web 09/07/2006
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Hodges et al.
Figure 1. Structural formula of E23B8.
of one PEE layer and one very rough PEO layer, difficulty was found in fitting the data at large angles. Instead, an extra thin layer of low electron density had to be introduced to account for the roughness of the PEO chains. Indeed, when the thickness of the layers was compared between the XR and NR data, slight differences existed, probably due to the low contrast between the PEO and the water subphase, particularly for the XR data. Materials and Methods The copolymer used in the present work was a poly(oxyethylene)23-poly(oxybutylene)8 (abbreviated as E23B8) diblock (Figure 1) that was part of a set of EB copolymers that were synthesized and characterized in an earlier set of experiments, although only some of these results were published.26 From these experiments it was found that the bulk critical micelle concentration (cmc) was 0.3 g/dm3 and the molecular weight was 1588 g/mol. This copolymer was first dissolved in CH3Cl to 1.5 mg/mL before being further diluted to 0.15 mg/mL, from which the copolymer was spread onto a water surface contained within a Langmuir trough. The X-ray experiments presented here were carried out at the ESRF, Grenoble, France, on the Troika II beamline (ID 10B). The properties of these copolymers have a strong temperature dependence,3,29-32 so all experiments were carried out in an air conditioned room at a temperature of 22 °C. The X-ray source had a beam energy of 8.51 keV and a wavelength of λ ) 1.457 Å. A simple motor drive was used to switch between the two detectors for the XR and the GIXD. In a typical experiment, a home-built shallow Langmuir trough with an area of (44 × 17 ) 748 cm2) and a depth of 3 mm was filled with 18 MΩ cm Millipore water plus a meniscus of approximately 2 mm. This gave a trough volume for the subphase of approximately 370 mL. The trough had first been cleaned with chloroform (three times) and Millipore water (three times) and was used in meniscus mode to enable the X-ray beam to interact with the surface of the water. Careful alignment of the X-ray beam was made before each experiment. In these reflectivity experiments the refractive index of the material, n, is usually expressed as n ) 1 - δ + iβ, where δ ) λ2r0F/2π and β ) µλ/4π for X-ray wavelength λ, r0 is the Thompson electron radius, F is the material electron density, and µ is the material (19) Parratt, L. G. Phys. ReV. 1954, 95, 359. (20) Samoilenko, I. I.; Konovalov, O. V.; Feigin, L. A.; Shchedrin, B. M.; Yanusova, L. G. Crystallogr. Rep. 1999, 44, 310. (21) Konovalov, O.; Myagkov, I.; Struth, B.; Lohner, K. Eur. Biophys. J. 2002, 31, 428. (22) Booth, C.; Yu, G.-E.; Nace, V. M. Block Copolymers of ethylene oxide and 1,2-butylene oxide. In Amphiphilic Block Copolymers: Self-Assembly and Applications; Alexandridis, P., Lindman, B., Eds.; Elsevier: Amsterdam, 1997. (23) Cai, Z.-H.; Huang, K.; Montano, P. A.; Russell, T. P.; Bai, J. M.; Zajac, G. W. J. Chem. Phys. 1993, 98, 2376. (24) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: New York, 1998. (25) Pershan, P. S.; Als-Neilsen, J. Phys. ReV. Lett. 1984, 52, 759. (26) Bedells, A. D.; Arafeh, R. M.; Yang, Z.; Attwood, D.; Heatley, F.; Padget, J. C.; Price, C.; Booth, C. J. Chem. Soc., Faraday Trans. 1993, 89, 1235. (27) Samoilenko, I.; Feigin, L.; Shchedrin, B.; Antolini, R. Physica B 2000, 283, 262. (28) Tanodekaew, S.; Deng, N. J.; Smith, S.; Yang, Y. W.; Attwood, D.; Booth, C. J. Phys. Chem. 1993, 97, 11847. (29) Kelarakis, A.; Havredaki, V.; Yu, G.-E.; Derici, L.; Booth, C. Macromolecules 1998, 31, 944. (30) Castelletto, V.; Caillet, C.; Fundin, J.; Hamley, I. W.; Yang, Z.; Kelarakis, A. J. Chem. Phys. 2002, 116 (24), 10947. (31) Hamley, I. W.; Mai, S.-M.; Ryan, A. J.; Fairclough, J. P. A.; Booth, C. Phys. Chem. Chem. Phys. 2001, 3, 2972. (32) Kelarakis, A.; Havredaki, V.; Booth, C.; Nace, V. M. Macromolecules 2002, 35, 5591.
Figure 2. A typical isotherm of E23B8 showing a very small phase transition at 600 Å2. absorption coefficient. This means that values of δ are directly proportional to changes in the material electron density F, a fact that is invaluable for monolayer films. Since the refractive index, n, usually deviates by only one part in 106, we take the usual kinematic approximation and assume a Fresnel-type reflection from an infinitely sharp interface disturbed only by interference from the spread surface layer.16 The critical angle of the interface is given by Rc ) x2δ, and for angles significantly larger than this, the reflection intensity will be given by R 1 )| RF Fsub
∫F′(z) e
iq z
z dz|2
where Fsub is the subphase electron density, qz is the scattering vector perpendicular to the interface, and F′(z) is the gradient of the scattering length density along the surface normal (and is therefore implicitly related to the electron density over this interface). The scattering length density profiles obtained are used as inputs for the calculation of the molecular parameters.19 For the modeling of the results, the surface layer is split into slabs each with a homogeneous electron density, and an iterative program was then used to best fit the data.20,27 The GIXD experiments were carried out at an angle of 0.1°, which was 70% of the critical angle, and the XR experiments were carried out between 0.15° and 3°. Typically, 56 µL of the 0.154 mg/mL E23B8 was spread onto the water subphase to form the monolayer, which on the trough used gave a start surface pressure of 3mN/m, measured using a small piece (3 mm wide) of filter paper as the Wilhelmy plate. The compression speed was set to 0.2 mm/s. Before the start of each X-ray experiment, the trough was completely sealed by a clear PTFE cover before the air was then replaced by water-saturated helium to stop parasitic scattering of the X-rays before they reached the sample surface and to reduce subphase evaporation. The diffracted beam was detected by a linear positionsensitive detector for GIXD that records an intensity profile as a function of the vertical scattering angle. For details of the GIXD technique, the reader is referred to ref 19.
Results and Discussion A typical isotherm carried out on the E23B8 monolayer at 25 °C at the air-water interface is shown in Figure 2. Since there are no sharp gradient changes in the isotherm of E23B8, it may be expected that the resultant monolayer is not particularly structured. Experiments were carried out in a custommade one-barrier Langmuir trough with a compression ratio of approximately 7 (for more details, see earlier text). It is apparent that at areas > 600 Å2 per molecule the film is in a loose and expanded state before undergoing a slow transition to a more condensed phase. It should be noted here that the calculation of
Structural Analysis of PEO-PBO Monolayers
Langmuir, Vol. 22, No. 21, 2006 8823
Figure 3. X-ray reflectivity data for E23B8 at 3 mN/m (squares), 7 mN/m (circles), and 27 mN/m (triangles). The data have been offset for clarity.
the area per molecule has been made by assuming that the entire area of the trough is occupied by the monolayer. As will be demonstrated later, this is not necessarily a valid assumption with these types of monolayer films. No collapse point on the isotherm was found, although further addition of copolymer caused the pressure to rise slightly for a few seconds before relaxing back to the maximum pressure shown in Figure 2. This maximum pressure showed no sign of decay with time over the course of the experiment (a couple of hours), thus ruling out any significant diffusion of E23B8 into the bulk water phase for this volume of copolymer added (56 µL), although it is possible that very slow diffusion rates could take place over a longer time period. The raw reflectivity data for E23B8 have been divided by the reflectivity at the critical angle to produce the normalized plot shown in Figure 3. The reflectivity plot shows a smooth profile over the entire angular range with no visible Kiessig fringes. These fringes are expected for a uniform homogeneous layer, as may be expected for E23B8 given the isotherm shown in Figure 2. There could be two possible explanations for the lack of Kiessig fringes: (a) the copolymer layer is not homogeneous across the area examined by the X-ray beam (a few cm2) or (b) the surface roughness of the copolymer film is too large to allow the fringes to be observed. The other interesting point about the data in Figure 3 is that the width of the peak in the data at the highest pressure (27 mN/m) is different than the other two pressures. A different peak width usually indicates a change in thickness of the layer, with a narrower peak translating as a thicker layer. Grazing incidence X-ray diffraction was carried out on the E23B8 film at each pressure (3, 7, and 27 mN/m) looking for any lateral structuring within the monolayer (compare this with XR, where vertical structure through a monolayer is sensed). The measurements were made at 70% of the critical angle (Rc ) 0.144°) with a beam energy of 8.51 keV and a beam wavelength of 1.457 Å, with the horizontal scattering vector qxy ranging between 1.5 and 3.4 Å-1. No peaks were observed in the resulting data at any pressure, indicating that there was no long-range 2-D lateral ordering of the copolymer molecules at the air-water interface. This indicates that the E23B8 film is quite amorphous over the length scales investigated by GIXD and is therefore not straightforward to model. To test out these ideas about the data, we decided to use a “box” model20 as shown in Figure 4. In this model, each interface is divided into a number of slabs, or “boxes”, of uniform electron density. The experimental data are then least-squares fitted to
Figure 4. An example of a “box” model used to fit the XR data, alongside a sketch of E23B8 showing which part of the copolymer each layer of the model corresponds to. Each layer has electron density, F; real and imaginary components of the refractive index, δ and β; thickness, d; and roughness, σ.
the model, where the number of slabs, slab thickness, electron density, and roughness at the interface are variables. There are two reasonable box models that may be tried to fit the XR data: a one-layer model or a two-layer model. In the one-layer model, the copolymer could be visualized as forming surface micelles or a mixed conformation such that the X-ray beam cannot distinguish between either of the blocks of the copolymer. In the two-layer model the copolymer could be supposed to align in the plane vertically out of the water so that, on average, most of the oxyethylene blocks lie toward the bulk water and most of the oxybutylene blocks face the air. The model outputs values for the interfacial roughness (σ, defined as the deviation away from a mean planar interface) for each of the layers, the layer thickness (d), and values of δ and β from which the electron density of the layer may be calculated (see above). In the two-layer version of the model, layer 1 represents the upper layer (oxybutylene) and layer 2 the lower layer (oxyethylene), producing three interfaces: the air-layer 1 interface, the layer 1-layer 2 interface, and the layer 2-subphase interface. These three interfaces may each then be given a roughness, σ1, σ2, σ3. The results of both models (one and two-layers) compared with the reflectivity data are shown in Figure 5. The results shown in Figure 5 demonstrate that both models seem to fit the data well. However, on further examination the fits were found to be subtly different over the three pressures used, as is shown in Figure 6. From Figure 6 it may be seen that while both fits agree with the data at 3 mN/m, small deviations away from the data occur for the one-layer model at 7 and 27 mN/m. Values for χ2 were typically below 10-6 in this fitting routine, so it is reasonable to interpret the deviations away from the data by the one-layer model as real. It is also easier to see the change in peak width at 27 mN/m when the data is plotted as in Figure 6, so the model output should reflect this as a sudden thickness increase at 27 mN/m. The output from both models is shown in Tables 1 and 2. The model outputs show quite clearly from Tables 1 and 2 that the one-layer model (Table 1) indicates only a slight thickening of the copolymer at low surface pressures, and no further change in thickness occurs at high surface pressure. This is contrary to what is expected on the basis of the width of the peak in the XR data, as mentioned earlier. The two-layer model, however,
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Hodges et al. Table 1. One-Layer Model Resultsa d1 σ1 σ2 δ1 β1 F1
3 mN/m
7 mN/m
27 mN/m
9.9 1.6 11.3 4.5 × 10-6 4.5 × 10-7 0.47
12.7 1.5 15.8 5.5 × 10-6 4.8 × 10-7 0.58
12.8 2.4 12.7 4.7 × 10-6 3.7 × 10-7 0.49
a
The outputs from the one-layer and two-layer models used to fit the XR data for E23B8: layer thickness, d (Å); layer roughness, σ (Å); and layer electron density, F (e- per Å3). The monolayer film pressure used is listed at the top of each column. Table 2. Two-layer Model Resultsa d1 d2 σ1 σ2 σ3 δ1 δ2 β1 β2 F1 F2
3 mN/m
7 mN/m
27 mN/m
3.5 5.2 1.4 2.2 12.2 4.1 × 10-6 4.6 × 10-6 1.0 × 10-7 4.4 × 10-7 0.43 0.48
3.4 5.6 1.5 15.0 18.8 5.5 × 10-6 5.8 × 10-6 8.3 × 10-7 1.8 × 10-7 0.58 0.61
3.9 9.8 1.9 1.5 12.6 3.9 × 10-6 4.7 × 10-6 6.6 × 10-7 4.0 × 10-7 0.49 0.49
a The outputs from the one-layer and two-layer models used to fit the XR data for E23B8: layer thickness, d (Å); layer roughness, σ (Å); and layer electron density, F (e- per Å3). The monolayer film pressure used is listed at the top of each column.
Figure 5. One-layer (top) and two-layer (bottom) box model fits to the XR data for E23B8.
indicates a significant thickening of the copolymer at 27 mN/m, and most of this thickening occurs in the lower part of the film (corresponding to the oxyethylene block of the copolymer). This would suggest that the oxyethylene is becoming more extended and possibly less hydrated as the surface pressure increases. Both models (one and two layers) indicate a significantly higher value of electron density at 7 mN/m than at 3 or 27 mN/m. The reason for this extremely high electron density is not known, but it possibly indicates some rearrangement taking place within the copolymer film that does not significantly affect the surface pressure, since there is no sign of a sharp phase transition in the isotherm (Figure 2). If water were expelled from the copolymer
film during this rearrangement, then the electron density would be expected to increase, although whether it could increase to the values stated in Table 2 is not clear. Further work would be required to verify if this densification of the copolymer film is real or just an artifact of the model. It should be pointed out that many attempts were made to fit these data from different starting values, so we confidently believe that the results quoted in Tables 1 and 2 are at the global minimum for the fitting routine and not just a local minimum. These box models usually fit best when there are strong features in the reflectivity data, as these features strongly restrict the number of local minima that the model can find. The roughness values found from both models were extremely large for the oxyethylene-water interface. This probably indicates that the model is not well suited to the copolymer film, most likely because the film is amorphous (as suggested from the GIXD data) with small localized variations that are not well matched by a box model approximation. Even so, it is interesting to note that at 7 mN/m the two-layer model outputs excessively large roughness values for the two lower interfaces (σ2 and σ3), whereas at both 3 and 27 mN/m only one large roughness (σ3) is output. This backs up the high electron density result obtained at 7 mN/m in the two-
Figure 6. The one-layer (dotted line) and two-layer (solid line) model fits to the E23B8 XR data at 3, 7, and 27 mN/m when plotted versus qz(R/RF).4
Structural Analysis of PEO-PBO Monolayers
Figure 7. A cartoon of possible conformations for E23B8 at 3, 7, and 27 mN/m as established from two-layer fits to the data from the X-ray reflectivity analysis. The layers as used in the two-layer model of the system are shown, and the differently colored regions represent the roughness values for each interface.
layer model, since a large degree of intermixing between the oxybutylene and oxyethylene blocks could both densify and roughen the film as far as the model is concerned. The stretched linear chain length of the oxybutylene block is approximately 26 Å, which is clearly much larger than the layer 1 thickness (for the two-layer model) at any pressure. In fact, the model values for layer 1 thickness are nearer to the maximum width of an oxybutylene monomer (5.2 Å). This would suggest that the oxybutylene block must lie very flat at the air-water interface. The very low value of roughness quoted for this oxybutylene block within the two-layer model suggests either a highly ordered arrangement or that only part of the surface is covered, perhaps because some polymer has diffused away from the interface, leaving small vacant spaces. However, the electron density obtained for layer 1 in the two-layer model is always well above that of water (0.33 e- per Å3), ruling out the vacant spaces idea. Our ideas as to how these results may be interpreted are shown in a cartoon of possible conformations in Figure 7. It should be noted that the cartoon is two-dimensional and cannot therefore show any three-dimensional structure. For example, at 27 mN/m it is likely, based on the earlier comment regarding the oxybutylene blocks lying flat at the air-water interface, that several of these chains could occupy the same surface area allowed by the assumption of close packing, so as to give an overall average area per molecule that would be significantly smaller than would be calculated from the oxybutylene block length alone. This would then resolve the issue that, at 27 mN/m, the area per molecule as calculated from the pressure-area isotherm is only 54 Å2, which if a circular domain per molecule was assumed would lead to a radius of only 4.1 Å per molecule. The cartoon shows that at the lowest pressure (3 mN/m) the E23B8 molecules probably have an open structure and may significantly overlap each other, particularly in the top (oxybutylene) layer, where the density difference between the air and the PBO is such that the PBO simply flops over to one side. Note that if overlapping does occur within the film, this would not be sensed by GIXD unless a significant portion of the film had the same type of overlap, i.e., with a preferred orientation. Sometimes more than one PBO chain may overlap to form a relatively dense, but still quite smooth, top layer. The lower (oxyethylene) layer at 3 mN/m is represented as taking up more than one conformation, effectively creating a third low-density layer that extends into the water subphase. This would partially account for the large values of σ3 in the two-layer fits to the data, although more likely
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it represents the patchiness of the copolymer film and hence the unsuitability of the box model within this system. At 7 mN/m the overlapping remains, but the mean intermolecular spacing decreases as illustrated (i.e. densification of the copolymer film occurs). At the highest pressure (27 mN/m) any trapped (or overlapping) parts of the copolymer slip over each other to allow a smaller overall area per molecule. There may still exist several different conformations of the copolymer molecule at this pressure. It is important to note that the Wilhelmy plate used to record the changes in surface pressure to form the isotherm (Figure 2) measures only an average surface energy change in a localized region near to the plate itself. If the copolymer film is significantly heterogeneous in its structure, several phases may exist locally and not necessarily show up in the isotherm. This indicates that if several different conformations of the copolymer are present at any particular pressure, a significant roughness will be determined from the XR modeling that may not appear to correspond to any measured change on the isotherm. In summary, this copolymer, E23B8, appears to be highly amorphous over the length scales that the X-ray techniques employed here examine (several cm2) and shows no sign of sharp phase transitions. It is possible that more localized examination of the copolymer film by Brewster angle microscopy and by atomic force microscopy may reveal additional detail and explain the anomalously high electron densities found by the models employed here. Using these techniques it may be possible to establish a more complete understanding of the behavior of these block copolymers.
Conclusions An oxyethylene-b-oxybutylene (E23B8) block copolymer has been investigated by X-ray specular reflectivity and grazing incidence X-ray diffraction when spread as a monolayer film on the surface of a pure water subphase. The X-ray reflectivity results were then modeled with both a one- and a two-layer box system, which output values for the layer thickness, roughness, and electron density. Only the two-layer model showed that, as the copolymer film pressure increases, the overall film thickness increases, as expected. The hypothesis of intermixing within the copolymer film agrees with the large roughness values found and with the GIXD data, where no peaks were observed. These results suggest that the E23B8 copolymer film is highly amorphous and quite uniform on the length scale investigated by these X-ray techniques. On the basis of the two-layer model results, the oxybutylene blocks probably lie very nearly flat at all surface pressures, whereas the oxyethylene blocks significantly change conformation, possibly partially dehydrating during these changes, leading to densification of the copolymer film. Acknowledgment. We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities and for allowing us to use beamline ID10B. We thank the EPSRC for providing funding for this work under the auspices of “A Collaboration into Research on Nanoparticles” (ACORN). We are grateful to Prof. Steve Evans for allowing us to use his Langmuir trough. We acknowledge Dr. Zhuo Yang for synthesizing the copolymers used in this paper within the group of Dr. Colin Booth at the University of Manchester. LA060632K
