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pH-Controlled Polymer Surface Segregation Richard L. Thompson,*,† Sarah J. Hardman,† Lian R. Hutchings,† Amilcar Pillay Narrainen,†,§ and Robert M. Dalgliesh‡ Department of Chemistry, Durham UniVersity, Science Site, Durham DH1 3LE, U.K., and ISIS, Rutherford Appleton Laboratories, Chilton, Didcot, OX11 0QX, U.K. ReceiVed October 28, 2008. ReVised Manuscript ReceiVed December 15, 2008 A new approach to promoting and controlling polymer surface functionalization with acidic or basic polar functional groups is demonstrated and evaluated. Blended polymer films were annealed under pH-buffered conditions, and polar end-functional groups were found to promote surface segregation of the functional polymers. Surface segregation of carboxylic acid (COOH)-functionalized polystyrene increases dramatically with increasing pH from 1.9 to 9.4, whereas the opposite behavior is seen for amine (NH2)-functionalized polystyrene. Neutron reflectometry and nuclear reaction analysis were used to obtain surface excess values for the functional polymers. Subsequent SCFT analysis of the composition versus depth profiles indicates that the affinity of each functional group for the polymer surface changes by about 3kBT over this pH range.
Introduction Control over polymer surface functionality is highly desirable for a diverse range of applications, from wetting and adhesion to cell tissue culture.1,2 Recent advances made by ourselves3-9 and others10,11 have shown that multiple end-functional groups on polymer chains can confer significant changes to the surface properties of polymer blends. It is relatively easy to achieve this using polymers that are end-functionalized with low-surfaceenergy groups such as fluorocarbons3-8,10,12 and siloxanes13,14 because these spontaneously segregate to film surfaces, reducing their surface energy. However, it is less trivial to enrich a surface with polar functional groups. This problem arises because the polar functional groups normally have a significantly higher surface energy than the polymer matrix; therefore, their surface segregation is inhibited. However, by immersing the polymer in a polar liquid, the criteria for surface segregation are reversed, and it is possible to induce the adsorption of polar functional * Corresponding author. E-mail:
[email protected]. Tel: +44 191 3342139. Fax: +44 191 3844737. † Durham University. ‡ ISIS. § Present address: Materials Science Centre, The University of Manchester, Grosvenor Street, Manchester M1 7HS, U.K. (1) Lee, J. H.; Jung, H. W.; Kang, I. K.; Lee, H. B. Biomaterials 1994, 15, 705–711. (2) Goddard, J. M.; Hotchkiss, J. H. Prog. Polym. Sci. 2007, 32, 698–725. (3) Thompson, R. L.; Narrainen, A. P.; Eggleston, S. M.; Ansari, I. A.; Hutchings, L. R.; Clarke, N. J. Appl. Polym. Sci. 2007, 105, 623–628. (4) Narrainen, A. P.; Hutchings, L. R.; Ansari, I.; Thompson, R. L.; Clarke, N. Macromolecules 2007, 40, 1969–1980. (5) Ansari, I. A.; Clarke, N.; Hutchings, L. R.; Pillay-Narrainen, A.; Terry, A. E.; Thompson, R. L.; Webster, J. R. P. Langmuir 2007, 23, 4405–4413. (6) Pillay-Narrainen, A.; Hutchings, L. R.; Feast, W. J.; Thompson, R. L.; Ansari, I. A.; Clarke, N. Macromol. Symp. 2006, 231, 103–109. (7) Narrainen, A. P.; Hutchings, L. R.; Ansari, I. A.; Clarke, N.; Thompson, R. L. Soft Matter 2006, 2, 126–128. (8) Hutchings, L. R.; Narrainen, A. P.; Eggleston, S. M.; Clarke, N.; Thompson, R. L. Polymer 2006, 47, 8116–8122. (9) Hutchings, L. R.; Narrianen, A. P.; Thompson, R. L.; Clarke, N.; Ansari, L. Polym. Int. 2008, 57, 163–170. (10) Li, H.; Zhang, Y. M.; Zhang, H.; Xue, M. Z.; Liu, Y. G. J. Polym. Sci., Part A: Polym. Chem. 2006, 44(12), 3853–3858. (11) Hirao, A.; Sugiyama, K.; Yokoyama, H. Prog. Polym. Sci. 2007, 32, 1393–1438. (12) Narrainen, A. P.; Hutchings, L. R.; Ansari, I. A.; Clarke, N.; Thompson, R. L. Soft Matter 2006, 2, 126–128. (13) Cai, Y. H.; Gardner, D.; Caneba, G. T. J. Adhes. Sci. Technol. 1999, 13, 1017–1027. (14) Lee, H.; Archer, L. A. Macromolecules 2001, 34, 4572–4579.
groups at the polymer-liquid interface. Koberstein et al.15,16 found that by immersing a pure functionalized polymer in water for prolonged periods of time local reorganization could result in a polar functionalized surface. Because of the fact that only local reorganization was possible at ambient temperatures, only functionalities that were already at or adjacent to the film surface could contribute to changes in surface composition. Therefore, the architecture of the functionalized molecules had to be ingeniously designed to ensure that polar groups were initially close to the film surface. More recently, we have shown that surface segregation of polar end-functionalized polymers is possible in blends by annealing blend films above their glasstransition temperature while immersed in a polar nonsolvent.17 This strategy allows the surface to be enriched by any functional group that can diffuse to the surface and so does not require complex polymer architectures or high concentrations of functional groups. However, both approaches are limited in the number of functional groups per unit area that can be brought to a film surface, and neither can discriminate between different kinds of polar functionality that could be simultaneously present in a polymer blend. In an attempt to overcome these limitations, increase the efficiency of surface segregation, and offer control over the nature of polar functionalities that are drawn to a polymer surface, we have explored the influence of pH on the surface segregation of acid (COOH)- and base (NH2)-functionalized polymers. In a series of influential papers, Whitesides et al.18-20 demonstrated that when aqueous solutions were placed on COOH-functionalized surfaces the contact angle of the solution had a strong dependence on pH. Contact angles decreased with increasing pH on COOH-functionalized polyethylene. The increase in wettability was due to the increase in deprotonation (15) Koberstein, J. T. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 2942– 2956. (16) O’Rourke-Muisener, P. A. V.; Jalbert, C. A.; Yuan, C. G.; Baetzold, J.; Mason, R.; Wong, D.; Kim, Y. J.; Koberstein, J. T. Macromolecules 2003, 36, 2956–2966. (17) Narrainen, A. P.; Clarke, N.; Eggleston, S. M.; Hutchings, L. R.; Thompson, R. L. Soft Matter 2006, 2(11), 981–985. (18) Holmes-Farley, S. R.; Reamey, R. H.; McCarthy, T. J.; Deutch, J.; Whitesides, G. M. Langmuir 1985, 1(6), 725–740. (19) Holmesfarley, S. R.; Bain, C. D.; Whitesides, G. M. Langmuir 1988, 4, 921–937. (20) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370–1378.
10.1021/la803583f CCC: $40.75 2009 American Chemical Society Published on Web 02/04/2009
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of COOH as the pH of the contact fluid was increased. Here, we explore the converse situation whereby the pH of a contacting fluid is exploited to control the surface characteristics of a polymer blend. Understanding the response of polymer blend surfaces to pH is not trivial, not the least of which because the elevated temperatures required to overcome the glass transition will also alter the hydrophilicity of polar groups with respect to the contacting solution.21
Experimental Section Blend films comprising 10 wt % end-functional deuterium-labeled polystyrene in hydrogenous polystyrene (Polymer Laboratories, U.K., Mw ) 3065 kg/mol, Mw/Mn ) 1.05) were prepared by codissolving the polymers in toluene and then spin coating them onto silicon blocks. The amine and carboxylic acid end-functional polymers were dPS-NH2 (Polymer Source, Canada; Mw ) 4.9 kg/mol, Mw/Mn ) 1.04) and dPS-2COOH (Mw ) 27.7 kg/mol, Mw/Mn ) 1.31). The dPS-2COOH polymer had two carboxylic acid functional groups at one end of the polymer chain and was prepared in-house as described previously.17 Prior to use, the silicon blocks were cleaned in toluene, followed by permanganic acid, and then thoroughly rinsed with deionized water. The polymer films (1.8 µm thick) were completely immersed in pH-buffered aqueous solutions and then annealed in an autoclave at 122 °C for 2 h. The samples were allowed to cool to 70 °C (i.e., well below the glass-transition temperature, 103 °C) before being removed from the buffered solutions, rinsing in deionized water, and drying at room temperature. By quenching to below the glass-transition temperature while maintaining the pH-controlled environment, the samples were kinetically trapped in their equilibrium conformation at this pH and could not revert back to the preferred conformation for the sample in air. Surface segregation of the deuterium-labeled polymers was investigated by neutron reflectometry using the CRISP reflectometer at the ISIS neutron source, Chilton, U.K. Reflectivity data were collected over a range of scattering vectors from 0.06 < Q/nm-1 < 2.5, which encompassed the full range over which useful data could be obtained from the critical edge to the background. Following normalization and background subtraction, data fitting was carried out using a maximum entropy (model-independent) fitting algorithm22 over 0.15 < Q/nm-1 < 1.5. Fits were found to be very stable with respect to starting parameters. In addition to the maximum entropy fitting method, a simple model for the composition versus depth profile given by eq 1 was used.
φ(x) ) φbulk +
( ( ))
φs - φbulk xs - x 1 + erf 2 ws
(1)
Here, φ is volume fraction dPS as a function of depth x below the film surface (x ) 0). The surface and bulk volume fractions of dPS are given by φs and φbulk, respectively, and xs defines the spatial extent of the adsorbed layer whose interface with the subphase is characterized by the width ws. A slightly more complex model in which the surface excess layer was followed by a layer that was depleted with respect to φbulk was also considered
φ(x) ) φdep +
( ( )) ( (
φs - φdep xs - x 1 + erf + 2 ws x - xdep φbulk - φdep 1 + erf 2 wdep
))
(2)
where φdep, xdep, and wdep are the volume fraction, maximum depth, and lower interface width of the depleted region, respectively. Finally, nuclear reaction analysis (NRA) experiments were carried out on representative samples of the blended films following neutron reflectometry. For these experiments, the samples were irradiated (21) Carey, D. H.; Ferguson, G. S. J. Am. Chem. Soc. 1996, 118, 9780–9781. (22) Sivia, D. S.; Hamilton, W. A.; Smith, G. S. Physica B 1991, 173, 121– 138.
Figure 1. Neutron reflectivity data and fits for (a) dPS-NH2 and (b) dPS-2COOH end-functionalized polymers in an hPS matrix. The insets show the same data in the R(Q) vs Q format.
with a beam of 0.7 MeV 3He+ ions at 83° to the sample normal. The energy of backscattered protons resulting from the reaction between 3 He and 2H within the polymer was analyzed to determine the composition versus depth profile of the deuterated functional polymer. NRA has an inherently poorer depth resolution than neutron reflectometry and is therefore less well suited than neutron reflectometry to determine the surface excess of low-molecularweight, weakly adsorbing polymers. However, the analysis of NRA data is unambiguous and can be used to validate the neutron reflectometry results. This technique is described in greater detail elsewhere.23-25
Results Typical neutron reflectivity data and fits are shown as RQ4 versus Q in Figure 1. The broad peak in the reflectivity found in all data sets in the range of 0.1 < Q/nm-1 < 1.0 arises from the excess perdeuterated polymer at the film surface and would be absent if the functional polymer had not adsorbed to the film surfaces. It is evident from the increase in the size of this peak in the raw data that the surface excess increases with increasing pH for dPS-2COOH and decreasing pH for dPS-NH2. Derived concentration profiles are shown in Figure 2 and demonstrate that pH exerts a strong influence on the near-surface concentration of the adsorbed functional polymers. The surface excess, z* () ∫0∞ [φ(x) - φbulk] dx), was calculated by numerically integrating the derived volume fraction profiles. The uncertainty in the z* (23) Payne, R. S.; Clough, A. S.; Murphy, P.; Mills, P. J. Nucl. Instrum. Methods Phys. Res., Sect. B 1989, 42, 130–134. (24) Geoghegan, M. MeV Ion Beam Profiling of Polymer Surfaces and Interfaces. In Polymer Surfaces and Interfaces; Richards, R. W., Peace, S. K., Eds.; John Wiley & Sons Ltd.: Chichester, U.K., 1999; Vol. III, pp 43-73. (25) Chaturvedi, U. K.; Steiner, U.; Zak, O.; Krausch, G.; Schatz, G.; Klein, J. Appl. Phys. Lett. 1990, 56, 1228–1230.
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Figure 3. Comparison of three neutron reflectometry fitting methods ((-) maximum entropy, (0) eq 1, and (9) eq 2) for the dPS-2COOH blended film annealed at pH 9.4. The fits to the experimental data shown in the inset obtained using eqs 1 and 2 have been offset by successive factors of 10%.
Figure 2. Concentration profiles for (a) dPS-NH2 and (b) dPS-2COOH blends adjacent to solutions of pH 1.9 (9), pH 3.4 (0), pH 7.3 (b), and pH 9.4 (O). The SCFT concentration profile corresponding to the surface excess for the dPS-2COOH pH 9.4 system is shown as a dotted curve.
values is dominated by the uncertainty in φbulk adjacent to the surface, which was estimated from the standard deviation in φ(x) over the range of 20 < x/nm < 30. Some small oscillations are apparent in the derived concentration profiles of the functional polymer adjacent to the surface. This is almost certainly due to “ringing”, an artifact of the maximum entropy fitting procedure.22 In some cases, a broader dip in φ(x) is apparent over a range of 20-40 nm. To explore the significance of this broader depletion and to test the uniqueness of the maximum entropy fits to the neutron data, the analysis was repeated using the model concentration profiles defined by eqs 1 and 2. Previously, the simple error function model (eq 1) has been used successfully to describe equilibrium concentration profiles of adsorbed endfunctional polymers at film surfaces.5,26,27 In fitting our reflectometry data, φs, xs, and ws were the adjustable parameters. Although the quality of the fits to the reflectometry data are slightly poorer than those obtained by the more flexible maximum entropy model, the fits are still very good, and the derived surface excess values are also in excellent agreement. Derived concentration profiles and fits for the three fitting methods are shown in Figure 3 and clearly show that all methods yield consistent results. Interestingly, using the more complex model (eq 2) in which φdep was also an adjustable parameter did not provide any improvement over the best fit that could be obtained with the (26) Clarke, C. J.; Jones, R. A. L.; Edwards, J. L.; Shull, K. R.; Penfold, J. Macromolecules 1995, 28, 2042–2049. (27) Hopkinson, I.; Kiff, F. T.; Richards, R. W.; Bucknall, D. G.; Clough, A. S. Polymer 1997, 38, 87–98.
simpler model. The calculated reflectivities using these models (inset, Figure 3) are shown offset from each other and the experimental data because they were indistinguishable to the eye. We have also repeated the surface analysis on some of the more strongly adsorbing samples by nuclear reaction analysis17,24 and have found surface excess values to agree within the uncertainty of the measurements. We are therefore confident that the surface excess values derived from the neutron reflectometry data are robust. Typical results for composition versus depth obtained by NRA and fits to these data are shown in Figure 4. Fits for the near-surface regions were obtained by approximating the variation in composition versus depth by a series of thin, homogeneous layers. For the dPS-NH2 blend, a simple two-layer model in which one layer described the surface excess and a second layer described the bulk composition of the film was sufficient. For the films containing dPS-2COOH, a third interstitial layer was required in which the volume fraction of dPS-2COOH was depleted relative to the bulk composition. Surface excess values can readily be converted to an equivalent number of adsorbed functional groups per unit area using a conversion factor of n/V, where n is the number of functional groups per adsorbed polymer chain and V is the volume of the chain. By increasing the pH of the annealing environment from 1.9 to 9.4, the areal density of functional groups increases from 0.05 to 0.34/nm2 for COOH and decreases from 0.39 to 0.11/nm2 for NH2. Although these values appear to cover similar ranges, it should be stressed that the two functional polymers differ significantly in both number of functional groups per chain and molecular weight. In this case, the enhanced adsorption that might be expected for the difunctional polymer relative to the monofunctional polymer is largely negated by its comparatively high molecular weight.4
Discussion Before considering the influence of functional groups and the external environment on the surface segregation of polymers, it is important first to eliminate the other possible sources of surface segregation. It is well known that isotopic substitution can cause surface segregation in polymer blends, particularly when the molecular weight of both components is high.28 For (28) Jones, R. A. L.; Norton, L. J.; Shull, K. R.; Kramer, E. J.; Felcher, G. P.; Karim, A.; Fetters, L. J. Macromolecules 1992, 25, 2359–2368.
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Figure 4. Volume fraction versus depth data obtained for (a) dPS-NH2, pH 1.9 and (b) dPS-2COOH, pH 3.4 (4) and pH 9.4 (2). The dPS2COOH pH 9.4 data and fit have been offset by 0.3.
polystyrene systems, this becomes significant when the molecular weight of both polymers approaches 103 kg/mol. Because our additives are more than an order of magnitude below this threshold, their blends are too far from the coexistence point for the overall molecular weight to induce surface segregation of one component or the other. The large difference in the molecular weight of the blend components has itself been shown to cause significant surface segregation of the lower-molecular-weight component.29 However, this effect appears to be very small when measured for blends that are of comparable molecular weight and composition to those that we have studied here.30 Above all, it should be noted that whereas these influences may make small contributions to the measured surface excesses their contributions will be systematic in nature and will not have any pH dependence. Our neutron reflectometry data and derived concentration profiles shown in Figures 1 and 2, respectively, clearly show that there is a strong pH dependence to the composition profiles and that their variation with pH is systematic in nature. The increasing surface excess with increasing pH for dPS-2COOH is consistent with the contact angle data reported by Whitesides et al.18-20 Both results clearly show that the affinity for the acidic COOH functional groups toward the contacting solution increases with increasing pH. The reversal of this trend when the dPS is functionalized with a basic (NH2) group is also clearly apparent from our results and is consistent with the anticipated behavior. Whereas this result could be regarded as somewhat obvious, it is significant because it excludes the possibility that our results (29) Hariharan, A.; Kumar, S. K.; Russell, T. P. J. Chem. Phys. 1993, 98, 4163–4173. (30) Kiff, F. T.; Richards, R. W.; Thompson, H. L.; Bucknall, D. G.; Webster, J. R. P. J. Phys. II 1997, 7, 1871–1891.
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could arise from any other systematic change that could conceivably have some pH dependence. Moreover, the data shown in Figure 3 for the most strongly adsorbing dPS-2COOH (pH 9.4) blended films further demonstrate that our results are consistent, even when two very different approaches to fitting the neutron reflectometry data are used. Despite the lower depth resolution of the NRA measurement compared to that of neutron reflectometry, our results shown in Figure 4 indicate that this technique is relatively sensitive to variations in composition that persist over tens of nanometers. (This is because neutron reflectometry relies on gradients in scattering length density, which for small, gradual changes in composition are negligible.) We would therefore argue that while NR provides the most accurate measure of the surface excess, where the composition of the film varies rapidly with depth, NRA is the better technique for measuring the depletion in the polymer adjacent to the surface, over which φ(x) varies slowly. The presence of the depleted layer adjacent to the surface excess indicates that these films were not fully equilibrated by the annealing process.5,31 This depleted region is likely to exist given the comparatively short annealing times used in these experiments, especially when the affinity of the functional groups for the polymer solution interface is large enough to inhibit the desorption of the end functional polymers from the film surface. After 2 h of annealing at 120 °C, the expected displacement of a linear polystyrene of 27.7K (equal to that of dPS-2COOH) is approximately 40 nm, which corresponds closely to the mean distance of the depleted region from the film surface. This observation confirms that the annealing regime used was not quite sufficient to achieve equilibrium adsorption. Although it is more difficult to predict the equivalent diffusion behavior for dPS-NH2, whose Mw is below the entanglement molecular weight for polystyrene, it will undoubtedly have a much larger diffusion coefficient. The expected displacement of dPS-NH2 is well over 100 nm; therefore, the region that could contribute to the measured surface excess extends to a greater depth than the NRA measurement. This explains why no depletion region was found in the NRA data for annealed blends of this polymer. Using a self-consistent field theory (SCFT) model,32,33 it is possible to relate the surface excess concentration of a polymer to the thermodynamic affinity of the end-functional group to the surface. One of the most appealing aspects of this model is that the influence of the end-functional groups on polymers of different molecular weights, such as those that we have studied, may be compared directly. The affinity of the end-functional groups for their respective surfaces, β/kBT, commonly referred to as the sticking energy, is shown in Figure 5 and increases from approximately 1.2 to 4.5 over the measured pH range. The error bars in beta were estimated from the uncertainty in the surface excess values. Previously, we found that β for the same dPS2COOH material that we discuss here was 3.3kBT when annealed under glycerol.17 Therefore, by controlling the pH we have established that it is possible to increase or decrease the affinity of dPS-2COOH to film surfaces relative to our earlier work. The sticking energy of dPS-NH2 decreases similarly with increasing pH. Because this trend is in the opposite direction to that observed for dPS-2COOH, it is clear that the functional groups, which are the only distinguishing feature of the polymers, must be responsible for the observed trends. It appears that the range of variation in sticking energy is marginally greater for the difunctional end-functional polymer than for the monofunctional (31) Clarke, C. J.; Jones, R. A. L.; Clough, A. S. Polymer 1996, 37, 3813– 3817. (32) Shull, K. R. J. Chem. Phys. 1991, 94, 5723–5737. (33) Shull, K. R. Macromolecules 1996, 29, 2659–2666.
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efficiency of surface functionalization with this approach. Although the limited film stability when annealing under aqueous solutions meant that it was not possible to increase the total surface concentrations with higher annealing temperatures or longer annealing times, we anticipate that this could be achieved by using thicker polymer films or hydrophobic substrates. Moreover, surfaces modified in this way could be defunctionalized or regenerated with a different level of functionalization simply by repeating the annealing process at a different pH, and we are currently exploring these possibilities with further experiments.
Conclusions
Figure 5. Influence of pH on the thermodynamic sticking energy per functional polymer chain end with annealing environment. Data are shown as (O) for dicarboxylic acid (2COOH) groups and (9) amine (NH2) groups, respectively.
end-functional polymer. This suggests that further increasing the number of functional groups per polymer chain could be exploited to achieve greater ranges of surface coverage or control of segregation over a narrower pH range. A typical composition versus depth profile derived via SCFT is included for the dPS-2COOH blend at pH 9.4 in Figure 2. The SCFT model profiles predicts less adsorbed chain stretching than was implied by the fits to the neutron data. Whereas agreement between SCFT model profiles and NR results is frequently imperfect, this result indicates that the adsorbed functional polymers had not relaxed to their equilibrium distribution. Neither the SCFT profile nor the simple error function profile (eq 1) can include the region of depleted functional polymer concentration adjacent to the film surfaces that is apparent in the maximum entropy profile for dPS-2COOH annealed at pH 9.4 or either of the dPS-2COOH composition profiles determined by NRA. The limited equilibration period used suggests that for dPS-2COOH the surface excess values and sticking energies that were derived from these profiles should be regarded as lower limits of the true values. However, because there is no basis for the pH external to the film surface to influence the kinetics of diffusion within the film it is safe to conclude that the qualitative trends in z* and β are accurate and that there is further scope for improving the
The pH of a nonsolvent may be used to control the surface segregation of acidic or basic end-functionalized polymers. We have demonstrated this unambiguously for a model polystyrene blend in which deuterium-labeled COOH- or NH2-functionalized polymers were blended with unfunctionalized hydrogenous polystyrene. The end-functionalized polymers were attracted to film surfaces when immersed in pH-buffered solutions and annealed above the glass-transition temperature. Neutron reflectometry and nuclear reaction analysis were used to quantify the excess concentration of each functional polymer at the film surface after annealing at controlled pH between 1.9 and 9.4. With increasing pH, there was a systematic increase in the surface excess for the COOH-functionalized polymer whereas the surface excess of the NH2-functionalized polymer decreased. In these experiments, the areal density of functional groups could be varied from approximately 0.1 to 0.4/nm2, although increased values would be likely with lower polymer molecular weights, more functional groups per chain end, and prolonged annealing at higher temperatures. Acknowledgment. We thank Professor Ken Shull (Northwestern University) for provision of the SCFT modeling code and Dr. Devinder Sivia (ISIS, U.K.) for making available the maximum entropy data analysis software. We gratefully acknowledge STFC for provision of the neutron scattering facilities at ISIS. Supporting Information Available: Surface excess values derived from NR experiments. This material is available free of charge via the Internet at http://pubs.acs.org. LA803583F
