X-ray Reflectivity Study of Langmuir Films of Amphiphilic Monodendrons

Nov 30, 2000 - We have used X-ray reflectivity and Π-area isotherms to study Langmuir films of a series of second- and third-generation monodendrons ...
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J. Phys. Chem. B 2001, 105, 2170-2176

X-ray Reflectivity Study of Langmuir Films of Amphiphilic Monodendrons Wen-Jung Pao, MacKenzie R. Stetzer,† and Paul A. Heiney*,‡ Department of Physics and Astronomy and Laboratory for Research on the Structure of Matter, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104

Wook-Dong Cho and Virgil Percec* Roy and Diana Vagelos Laboratories, Department of Chemistry and Laboratory for Research on the Structure of Matter, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104 ReceiVed: September 27, 2000; In Final Form: NoVember 30, 2000

We have used X-ray reflectivity and Π-area isotherms to study Langmuir films of a series of second- and third-generation monodendrons with hydrophobic C12H25 alkyl tails at the periphery and hydrophilic COOHCO2CH3 or crown ether groups in the core. Both the limiting molecular areas and the layer thicknesss are consistent with a model in which the chains are directed nearly perpendicularly away from the water surface.

I. Introduction Molecular and supramolecular monodendrons and dendrimers provide powerful building blocks for the construction of giant macromolecular and supramolecular systems with complex architecture and precise shape and functionality.1 We have recently elaborated2 a rational approach to the design, synthesis, and structural analysis of dendritic building blocks. These blocks self-assemble into cylindrical or spherical supramolecular dendrimers, which in turn self-organize into two-dimensional or three-dimensional lattices, respectively. The monodendron shape is determined by the molecular architecture of the repeat unit, the generation number, and the functionality both on the periphery and in the apex. Depending on the width of the aliphatic (peripheral) end and the apex (core), the monodendrons might be described as tapers, half-disks, disks, pyramids, cones, half-spheres, or spheres, resulting in columnar or cubic macroscopic lattices.2 Less is known about the behavior of dendrimers at solid2d,e,3 or liquid4-15 interfaces. The general principles determining molecular conformation at the air-water interface are still unclear. Is the molecular shape near the interface the same as it is in three dimensions? How important are the chemical functionalities of the different parts of the molecule? When monolayers are formed, are they flat, or composed of spherical, ellipsoidal, or cylindrical supramolecular structures? Our knowledge of dendrimer structure in Langmuir films (at the air-water interface) comes largely from pressure-area (Π-A) measurements, which are however open to multiple interpretations.5-15 More detailed information on the molecular dimensions can be obtained from X-ray or neutron14,15 reflectivity measurements, which probe the electron or neutron scattering length density gradient normal to the surface. We have used X-ray reflectivity to study a series of secondand third-generation monodendrons with hydrophobic C12H25 alkyl tails at the periphery and hydrophilic COOHCO2CH3 or † Present address: Department of Physics, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1. ‡ Phone: (215) 898-7918. Fax: (215) 898-2010. E-mail: heiney@ physics.upenn.edu.

crown ether groups in the core (Chart 1). We find they are best described by a model in which the hydrophilic core is at or beneath the water surface, there is a low-density region just above the surface, and the alkyl chains form a high-density sublayer above the surface with the chains directed perpendicular to the interface. II. Synthesis 4′-Methylbenzo-15-crown-5 3,4,5-tris{3′,4′,5′-tris[p-(ndodecan-1-yloxy)benzyloxy]benzyloxy}benzoate [(4-(3,4,5)2)12G2-CO2CH2B15C5] (1),16 4′-methylbenzo-15-crown-5 3,5-bis{3′,4′,5′-tris[p-(n-dodecan-1-yloxy)benzyloxy]benzyloxy}benzoate [(4-3,4,5-3,5)12G2-CO2CH2B15C5] (2),16 4′-methylbenzo-15-crown-5 3,5-bis{3′,4′-bis[p-(n-dodecan-1-yloxy)benzyloxy]benzyloxy}benzoate [(4-3,4-3,5)12G2-CO2CH2B15C5] (3),16 methyl 3,5-bis(3′,5′-bis{3′′,4′′,5′′-tris[p-(n-dodecan-1yloxy)benzyloxy]benzyloxy}benzyloxy)benzoate [(4-3,4,5-(3,5)2)12G3-CO2CH3] (5),16 and 3,4,5-tris[3′,4′,5′-tris(n-dodecan-1yloxy)benzyloxy]benzoic acid [(3,4,5)212G2-COOH] (6)2b were synthesized as was described previously. 2-{2-[2-(2-Hydroxyethoxy)ethoxy]ethoxy}ethyl 3,4,5-tris[3′,4′,5-tris(n-dodecan-1-yloxy)benzyloxy]Benzoate[(3,4,5)212G24EO-OH] (4). From (3,4,5)212G2-COOH (5.0 g, 2.4 mmol), tetra(ethylene glycol) (10 g, 50 mmol), DPTS (0.15 g, 0.48 mmol), and DCC (0.66 g, 3.2 mmol) in 40 mL of CH2Cl2 (reaction time 12 h), a white crystal (4.20 g, 77.5%) was obtained after column chromatography (SiO2; 2:1 hexane/ EtOAc) and recrystallization in a mixture of acetone and CH2Cl2 (5:1). Purity (HPLC): 99+%. TLC (1:1 hexane/EtOAc): Rf ) 0.25. DSC: first heating, k 51 (35.72) Cub 58 (3.53) i; first cooling, i 40 (0.21) Cub 15 (31.70) k; second heating, k 50 (32.69) i. 1H NMR (CDCl3, δ, ppm, TMS): 0.89 (t, 27H, CH3(CH2)11, J ) 6.6 Hz), 1.26 (m, 162H, CH3(CH2)8), 1.75 (m, 18H, CH2CH2OAr), 3.56-3.83 (overlapped m, 18H, (CH2CH2O)3H, CH2OAr, 4-(3′,5′) positions, CO2CH2CH2), 3.89 (t, 8H, CH2CH2OAr, 3,5-(3′,5′) positions, J ) 6.4 Hz), 3.93 (t, 6H, CH2CH2OAr, 3,4,5-(4′) positions, J ) 6.4 Hz), 4.45 (t, 2H, CO2CH2CH2, J ) 5.3 Hz), 5.04 (s, 6H, ArCH2OAr, 3,4,5 positions), 6.60 (s, 2H, ArH ortho to CH2OAr, 4 position), 6.64 (s, 4H, ArH ortho to CH2OAr, 3,5 positions), 7.42 (s, 2H, ArH

10.1021/jp003495n CCC: $20.00 © 2001 American Chemical Society Published on Web 02/24/2001

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CHART 1

ortho to CO2CH2CH2). 13C NMR (CDCl3, δ, ppm, TMS): 14.3 (CH3), 22.9 (CH3CH2), 26.4 (CH2CH2CH2OAr), 29.6 (CH3(CH2)2CH2), 29.9 (CH3(CH2)3(CH2)5), 30.6 (CH2CH2OAr), 32.2 (CH3CH2CH2), 61.9 (CH2OH), 64.3 (CO2CH2), 69.0 (CH2CH2OAr, 3,4,5-(3′,5′) positions), 69.2-70.8, 72.7 (CH2(OCH2CH2)2OCH2), 71.9 (ArCH2OAr, 3,5 positions), 73.5 (CH2CH2OAr, 3,4,5-(4′) positions), 75.3 (ArCH2OAr, 4 position), 105.9 (ArC ortho to CH2OAr, 3,5 positions), 106.3 (ArC ortho to CH2OAr, 4 position), 109.9 (ArC ortho to CO2CH2CH2), 125.4 (ArC ipso to CO2CH2CH2), 131.9 (ArC para to CH2OAr, 3,5 positions), 132.6 (ArC para to CH2OAr, 4 position), 137.9 (ArC ipso to CH2OAr, 3,4,5 positions), 142.8 (ArC para to CO2CH2CH2), 153.2 (ArC meta to CH2OAr, 4 position), 153.5 (ArC meta to CH2OAr, 3,5 positions), 166.2 (CO2CH2CH2). Anal. Calcd for C144H256O18: C, 76.01; H, 11.34. Found: C, 76.13; H, 11.25.

III. Methods of Physical Characterization X-ray reflectivity (XR) measurements were conducted at beam line X22B of the National Synchrotron Light Source, Brookhaven National Laboratory, using a liquid surface spectrometer as described previously.17,18 A monochromatic X-ray beam (λ ) 1.533 Å) was deflected toward the water surface via Bragg reflection from a Ge(111) monochromator. The instrumental resolution was determined primarily by the slits between the sample and the detector. These slits were always set to optimize the resolution in the scattering plane while increasing the signal by lowering the resolution out of the scattering plane. The resolution in the direction along Qz was 0.008 Å-1. The H2O used for all subphases was purified using a Millipore filtration system with a resulting resistivity F > 18.2 MΩ cm.

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TABLE 1: Limiting Molecular Areas Extracted from Π-A Isotherms of the Monodendrons Studied monodendron

molecular area (Å2)

area per end group (Å2)

1 2 3 4

250 150 117 235

28 36 30 26

Figure 1. Characteristic X-ray reflection intensity profile, from monodendron 4 at a spreading pressure Π ) 8 mN m-1. Intensities are normalized to that obtained from total (specular) reflection.

Small quantities of solute (∼10-3 g) were weighed on a Denver Instruments A-200DS balance to an accuracy of 10-5 g. Including the effect of subsequent solvent evaporation, the typical mass uncertainty was 3-5%. The solvent employed was HPLC grade chloroform (99.9% pure). All monolayers were spread from 10-3-10-4 M chloroform solutions. The Langmuir trough was cleaned between experiments using a hexane rinse followed by an ethanol rinse and three rinses with purified H2O. X-ray reflectivity measurements employed a movable barrier/ Langmuir float trough as previously described.17 All measurements were made at T ) 24-25 °C. Using previously determined isotherms as a guide, the film was compressed from the gas phase to the desired surface pressure immediately before each measurement. The barrier speed was typically 10 cm min-1, corresponding to a compression rate of 10-25 Å2 molecule-1 min-1. All isotherms were both reversible (showing the same surface pressure upon compression and expansion) and reproducible (showing the same surface pressure after multiple compressions) to better than 5% in surface pressure and area. The molecular areas were also consistent to within 10% between measurements made several weeks apart on different troughs with the same spreading solution. Between two successive reflectivity measurements, either the barrier was retracted and recompressed (with the reversibility of the isotherm carefully monitored) or a new film was deposited. When the initial spreading pressure was less than Π ) 20 mN m-1, we found that the surface pressure decreased by less than 10% during the 3 h time span of an XR measurement. When the initial surface pressure was greater than 25 mN m-1, it typically decreased by 6 mN m-1 over the same time period. The combined uncertainty in molecular area (due to measurement and deposition error and surface pressure variation during the measurement) was on the order of 10%. The XR profiles were analyzed as described previously.17,19 A typical raw reflectivity scan is shown in Figure 1. The raw data were divided by the Fresnel reflectivity, and the resultant reflectivity profile was compared via least-squares fits to an empirical model for the electron density profile F(z). To achieve

monodendron

molecular area (Å2)

area per end group (Å2)

5 (bottom of rise) 5 (top of rise) 6

520 375 210

43 31 23

Figure 2. Pressure-area isotherms of the monodendrons studied. Measurements were made under compression, at a typical barrier speed of 10 cm min-1, corresponding to a compression rate of 10-25 Å2 molecule-1 min-1.

good agreement between the model and data with the minimum number of parameters, we found it convenient to model the electron density using a combination of uniform slabs, or boxes, each with a different electron density Fi and thickness Li, together with Gaussian profiles20 with areal density γj, profile centers zj, and widths δj. Specifically, the density profiles for compounds reported in this paper were all well described by a single slab with thickness L supplemented by a single Gaussian with negative density close to the water surface, such that the overall profile had a minimum in the electron density just above the water surface. There were then seven independent parameters for each least-squares fit: an overall amplitude (constrained however to lie within 5% of the measured incident intensity), the roughness of the water surface, the density, thickness, and roughness of the density slab, and the density, position, and width of the Gaussian profile. The details of the fitting procedure, including the calculation of the reflectivity due to a combination of flat slabs and Gaussian peaks, have been presented elsewhere.17,20 IV. Results Typical Π-A isotherms are shown in Figure 2, and limiting molecular areas obtained by extracting the vertical rise down to zero surface pressure are given in Table 1. The isotherms for compounds 1-4 and 6 are all characteristic of good amphiphiles, displaying a gas phase with nearly zero spreading pressure followed by a sharp rise in surface pressure at lower area. Measurements made on another trough with a greater expansion ratio showed either plateau formation or complete collapse at smaller areas, almost certainly indicating multilayer formation. As discussed above, all these isotherms were highly reproducible and showed almost no hysteresis. The isotherm for compound 5 was also reproducible with little hysteresis, but with a much larger molecular area and a more gradual rise. For this compound, Table 1 provides both the limiting molecular area at low surface pressure and the area at the beginning of the plateau (Π ≈ 17 mN m-1).

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TABLE 2: Parameters Extracted from Fits to Reflectivity Profilesa monodendron

Π (mN m-1)

σw(Å)

F1(e-/Å3)

σ1(Å)

L1(Å)

γ2(e-/Å2)

σ1(Å)

d2(Å)

Ltot(Å)

1 1 2 2 3 3 4 4 5 5 6 6

5 20 5 26 6 26 8 27 4 12 3 8

4.95 3.69 2.08 2.22 1.69 1.91 2.40 3.11 2.56 2.65 2.83 2.72

0.264 0.233 0.277 0.275 0.217 0.257 0.258 0.276 0.273 0.278 0.263 0.240

3.48 2.53 3.45 3.66 5.00 3.54 4.72 5.53 3.75 4.10 4.22 4.04

17.5 20.4 21.1 23.1 18.8 24.9 21.3 23.8 19.1 19.8 18.1 17.5

-1.23 -1.97 -1.44 -1.65 -2.18 -1.79 -1.44 -1.46 -1.17 -1.21 -1.11 -1.31

5.33 5.84 5.57 6.34 6.16 6.19 4.41 4.33 3.75 3.69 3.59 3.43

2.75 4.12 4.30 4.81 8.38 5.19 7.42 7.91 6.31 5.96 5.47 4.29

21.7 23.6 24.5 27.6 22.2 28.5 26.3 29.9 23.5 24.6 23.1 26.9

a σ is the roughness of the water surface. ρ , σ , and L are the electron density, roughness, and thickness of the slab. γ , σ , and d are the w 1 1 1 2 2 2 electron density, width, and position of the Gaussian peak. The layer thickness Ltot was defined to be equal to the distance from the water surface at which the electron density was equal to 10% of the water density.

Figure 3. X-ray reflectivity normalized to the Fresnel reflectivity RF(q) for compounds 1-3 at indicated spreading pressures Π (mN m-1). Solid dots are data; for clarity, only every fifth data point is shown. Solid curves are model fits as discussed in the text.

Figure 4. X-ray reflectivity normalized to the Fresnel reflectivity RF(q) for compounds 4-6 at indicated spreading pressures Π (mN m-1). Solid dots are data; for clarity, only every fifth data point is shown. Solid curves are model fits as discussed in the text.

Although there is substantial spread in the measured molecular areas, the third column in Table 1 shows a strong correlation in the areas per end group. For compounds 1-4, the areas per end group are all in the range 26-36 Å2. This is only slightly larger than the typical 20 Å2 cross-sectional area for an alkyl chain (although close to the expected area for a chain containing phenyl rings). This indicates that the molecular area must be largely, but not entirely, determined by the sum of loosely packed individual alkyl chains, with the chains extending radially away from the surface. Compound 6 has a somewhat smaller area, but not unphysically so. The limiting molecular area for 5 is much larger, but the area after compression to the plateau is again in the same range. A provisional explanation for this effect could be that the molecule lies essentially “flat” on the surface in the gas phase, but lifts off and extends under compression.

Typical X-ray reflectivity profiles are shown in Figures 3 and 4, together with fits to an empirical profile derived from a Gaussian + slab model as described in section III. For each compound, we present one measurement at relatively low surface pressure, close to the limiting molecular area, and one measurement at higher surface pressure. Good agreement between the model and data was obtained for all of these fits. The corresponding electron density profiles are shown in Figures 5 and 6. All samples showed the same qualitative features: a “dip” in the electron density (produced mathematically by a Gaussian peak subtracted from a slab) followed by a relatively sharp maximum. It was necessary to incorporate this dip to obtain reasonable agreement with the reflectivity data; its physical significance will be discussed below. Saville et al.14 noted a similar effect in their neutron reflectivity study of polyether dendrimers, and modeled it using a three-slab profile.

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Figure 5. Electron density profiles calculated for compounds 1-3 at indicated spreading pressures Π (mN m-1). z is the distance from the water interface.

Figure 6. Electron density profiles calculated for compounds 4-6 at indicated spreading pressures Π (mN m-1). z is the distance from the water interface.

Note also that the peak in the density is always farther from the water surface (z ) 0) at higher spreading pressure than it is at low spreading pressure. The parameters obtained from fits to the data are given in Table 2. σw is the roughness of the “water surface”, which in this case has no physical significance except that it contributes to the shape of the intensity dip. F1, σ1, and L1 are the electron density, roughness, and thickness of the first (and only) slab. γ2, σ2, and d2 are the electron density, width, and position of the Gaussian peak. The most significant parameter is the total thickness of the layer. We took the layer thickness Ltot to be equal to the distance from the water surface at which the monolayer electron density was equal to 10% of the water density. The tabulated layer thickness provides quantitative confirmation of the trend noted above, that the layer thickness in every case increases upon compression. All layer thicknesses are in the 22-30 Å range, roughly consistent with that expected for fully extended chains. Figure 7 shows a possible conformation of compound 1, in both side and top views. Although the energy was minimized using Chem3D from a hypothesized starting configuration, this figure should be thought of as an aid to discussion rather than the result of a true molecular energy minimization calculation. The chains are taken to extend directly away from the surface, and the crown ether group to extend into the subphase. This model embodies many features seen in the data. We assume that the crown ether moiety is fully embedded in the water surface. The distance from the “bottom” of the molecule (excluding the crown ether) to the top of the chains is approximately 23 Å, consistent with that obtained by XR measurements. The upper part of the molecule, consisting of alkyl chains, is relatively dense, while the lower part, consisting

primarily of the phenyl rings in the “inner” part of the molecule, is relatively dilute, thus accounting for the dip in electron density. The conformation shown would have a molecular area on the order of 400 Å2, substantially larger than the 250 Å2 limiting molecular area observed in Π-A isotherms. The most likely explanation is that as lateral pressure is applied the chains can move closer together, at the expense of some strain energy in the inner core of the dendrimer. A similar effect was recently observed in an amphiphilic dendrimer formed by attaching 10,12-pentacosadiynoic acid to a third-generation poly(amidoamine) dendrimer core.13 One can also envision the formation of thicker aggregates, but in this case the X-ray reflectivity should measure a thickness larger than that of one extended molecule. We were unsuccessful in attempts to measure the X-ray reflectivity of films at very low spreading pressure, in the gas phase. The reflectivity intensity was found to be anomalously noisy, with sudden positive or negative jumps in intensity. We speculate that these jumps resulted from floating “islands” of film drifting in and out of the X-ray beam. The reason for the anomalous behavior of compound 5 remains unclear. It seems likely, however, that the amphiphilic character of the molecules is reduced due to the relatively low hydrophilicity of the central CO2CH3 group. This may result in films that are relatively unstable to multilayer collapse compared with the other compounds studied. V. Comparison with Previous Results We now compare our observations to previously reported measurements on Langmuir films of dendrimeric amphiphiles. One is often faced with the task of deducing as much as possible about the molecular topography given only the molecular area

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Figure 7. Space-filling model showing a possible conformation of compound 1. The left-hand image shows a “side view,” while the right-hand image shows a “top view,” looking down on the water surface from above. The crown ether group is assumed to extend into the water, and the chains to extend vertically away from the water surface.

and thickness. In the solid state many dendrimers, including those under consideration here, self-assemble into rods or spheres, with several molecules making up one subunit. The sphere radius, for example, is determined largely by steric effects. The two-dimensional versions of such structures are also attractive models for the structure of the Langmuir films. However, a plot of the molecular area versus number of chains per molecule is not in itself sufficient to establish the topography. If the area is linearly proportional to the number of chains, this could be consistent with a planar model (in which molecules lie essentially flat on the surface with all end groups pointing away from the surface), or with multiple molecules selfassembled into half-spheres or half-cylinders of constant radius. Conversely, a nonlinear relationship between molecular area and chains per molecule implies that the molecules assume the form of spheres, ellipsoids, or other three-dimensional objects whose dimensions depend on the size of the molecules. Most, although not all, reported measurements of molecular area fall into the first category. Π-A isotherms from a wide variety of dendrimers display an approximately linear dependence of the molecular area on the number of end groups,5-10,14 although the area per chain can also depend on the functionality of the core and the length of the chains.8,10 By contrast, in some compounds the molecular area increased more slowly than the number of end groups,10,11 an effect which may be associated with multilayer formation.10 Although we did not achieve as great a dynamic range in the number of end groups as some of the studies discussed above, we too found that the area per end group did not change significantly when the number of end groups was varied from 4 to 12. Furthermore, our XR measurements have established the layer thickness in all compounds studied is primarily determined by the chain length, features consistent with a “planar” model with the chains extending away from the surface. We anticipate that future grazing-incidence X-ray diffraction studies may prove extremely useful in establishing the intermolecular distances along the surface, and further distinguish between planar, cylindrical, spherical, or other models. Acknowledgment. We thank E. DiMasi for her assistance with the reflectivity measurements. This work was supported by the MRSEC Program of the National Science Foundation

(NSF) under Award Number DMR96-32598 and by NSF DMR99-96288. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Research was carried out in part at the National Synchrotron Light Source, Brookhaven National Laboratory, supported under US DOE Contract No. DE-AC02-98CH10886. References and Notes (1) For recent reviews on dendritic building blocks see: (a) Fre´chet, J. M. J.; Science 1994, 263, 1710. (b) Newkome, G. R.; Moorehead, C. N.; Vo¨gtle, F. Dendritic Molecules: Concepts, Synthesis, PerspectiVes; VCH: Weinheim, 1996. (c) Moore, J. S. Acc. Chem. Res. 1997, 30, 402. (d) Tomalia, D. A.; Esfand, R. Chem Ind. 1997, 416. (e) Smith, D. K. Diederich, F. Chem. Eur. J. 1998, 4, 1353. (f) Frey, H. Angew Chem., Int. Ed. 1998, 37, 2193. (g) Vo¨gtle, F., Ed. Dendrimers. Topics in Current Chemistry; Springer: Berlin, 1998; Vol. 197. (h) Mathews, O. A.; Shipway, A. N.; Stoddart, F. J. Prog. Polym. Sci. 1998, 23, 1. (i) Schlu¨ter, A. D. Top. Curr. Chem. 1998, 197, 165. (j) Fischer, M.; Vo¨gtle, F. Angew Chem., Int. Ed. 1999, 38, 884. (k) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. ReV. 1999, 99, 1665. (l) Berresheim, A. J.; Mu¨ller, M.; Mu¨llen, K. Chem. ReV. 1999, 99, 1747. (m) Moore, J. S. Curr. Opin. Colloid Interface Sci. 1999, 4, 108. (n) Roovers, J. Comanita, B. AdV. Polym. Sci. 1999, 142, 179. (o) Schlu¨ter, A. D.; Rabe, J. P. Angew. Chem., Int. Ed. 1998, 197, 165. (2) (a) Percec, V.; Johansson, G.; Ungar, G.; Zhou, J. J. Am. Chem. Soc. 1996, 118, 9855. (b) Balasurusamy, V. S. K.; Ungar, G.; Percec, V.; Johanson, G. J. Am. Chem. Soc. 1997, 119, 1539. (c) Hudson S. D.; Jung, H. T.; Percec, V.; Cho, W. D.; Johansson, G.; Ungar, G.; Balagurusamy, V. S. K. Science 1997, 278, 449. (d) Percec, V.; Ahn, C.-H.; Ungar, G.; Yeardley, D. J. P.; Mo¨ller, M.; Sheiko, S. S. Nature 1998, 391, 161. (e) Percec, V.; Ahn, C. H.; Cho, W. D.; Jamieson, A. M.; Kim, J.; Leman, T.; Schmidt, M.; Gerle. M.; Moller, M.; Prokhorova, S. A.; Sheiko, S. S.; Cheng, S. Z. D.; Zhang, A.; Ungar, G.; Yeardley, D. J. P. J. Am. Chem. Soc. 1998, 120, 8619. (f) Percec, V.; Cho, W.-D.; Mosier, P. E.; Ungar, G.; Yeardley, D. J. P. J. Am. Chem. Soc. 1998, 120, 11061. (g) Percec, V.; Cho, W.-D.; Ungar, G.; Yeardley, D. J. P. Angew. Chem., Int. Ed. 2000, 39, 1598. (h) Ungar, G.; Percec, V.; Holerca, M. N.; Johanson, G.; Heck, J. A. Chem. Eur. J. 2000, 6, 125. (i) Yeardley, D. J. P.; Ungar, G.; Percec, V.; Holerca, M. N.; Johansson, G. J. Am. Chem. Soc. 2000, 122, 1684. (j) Percec, V.; Cho. W. D.; Mo¨ller, M.; Prokhorova, S. A.; Ungar, G.; Yeardley, D. J. P. J. Am. Chem. Soc. 2000, 122, 4249. (3) (a) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171. (b) Sheiko, S. S.; Eckert, G.; Ignateva, G.; Muzafarov, A. M.; Spickermann, J.; Rader, H. J.; Mo¨ller, M. Makromol. Rapid Commun. 1996, 17, 283. (c) Sheiko, S. S.; Gauthier, M.; Mo¨ller, M. Macromolecules 1997, 30, 2343. (d) Sheiko, S. S.; Muzafarov, A. M.; Winkler, R. G.; Getmanova, E. V.; Eckert, G.; Reineker, P. Langmuir 1997, 13, 4172. (e) Zhou, Y. F.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M.; Wells, M. J. Am. Chem. Soc. 1996, 118, 3773. (f) Stocker, W.; Schu¨rmann, B. L.; Rabe, J. P.; Fo¨rster, S.; Lindner, P.; Neubert, I.; Schlu¨ter, A. D. AdV. Mater.

2176 J. Phys. Chem. B, Vol. 105, No. 11, 2001 1998, 10, 793. (g) Stocker, W.; Karakaya, B.; Schu¨rmann, B. L.; Rabe, J. P.; Schlu¨ter, A. D. J. Am. Chem. Soc. 1998, 120, 7691. (h) Prokhorova, S. A.; Sheiko, S. S.; Mo¨ller, M.; Ahn, C. H.; Percec, V. Macromol. Rapid Commun. 1998, 19, 359. (4) (a) Karthaus, O.; Ijiro, K.; Shimomura, M.; Hellmann, J.; Irie, M. Langmuir 1996, 12, 6714. (b) Tsukruk, V. V. AdV. Mater. 1998, 10, 253 and references therein. (5) Cui, G.; Xu, Y.; Liu, M.; Fang; F.; Chen, Y.; Li, Y. Macromol. Rapid Commun. 1999, 20, 71. (6) Felder, D.; Gallani, J.-L.; Guillon, D.; Heinrich, B.; Nicoud, J.-F.; Nierengarten, J.-F. Angew. Chem., Int. Ed. 2000, 39, 201. (7) Kampf, J. P.; Frank, C. W.; Malstro¨m, E. E.; Hawker, C. J. Langmuir 1999, 15, 227. (8) Sayed-Sweet, Y.; Hedstrand, D. M.; Spinder, R.; Tomalia, D. A. J. Mater. Chem. 1997, 7, 1199. (9) Mulders, S. J. E.; Brouwer, A. J.; Kimkes, P.; Sudho¨lter, E. J. R.; Liskamp, R. M. J. J. Chem. Soc., Perkins Trans. 2 1998, 1535. (10) Schenning, A. P. H. J.; Elissen-Roma´n, C. E.; Weener, J.-W.; Baars, M. W. P. L.; van der Gaast, S. J.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120, 8199. (11) Iyer, J.; Hammond, P. T. Langmuir 1999, 15, 1299. (12) Sheiko, S. S.; Buzin, A. I.; Muzafarov, A. M.; Rebrov, E. A.; Getmanova, E. V. Langmuir 1998, 14, 7468. (13) Sui, G.; Micic, M.; Huo, Q.; Leblanc, R. M. Colloids Surf., A 2000, 171, 185.

Pao et al. (14) (a) Saville, P. M.; White, J. W.; Hawker, C. J.; Wooley, K. L., Fre´chet, J. M. J J. Phys. Chem. 1993, 97, 293. (b) Saville, P. M.; Reynold, P. A.; White, J. W.; Fre´chet, J. M. J.; Wooley, K. L.; Penfold, J.; Webster, J. R. P. J. Phys. Chem. 1995, 99, 8283. (15) Kirton, G. F.; Brown, A. S.; Hawker, C. J.; Reynolds, P. A.; White, J. W. Physica B 1998, 248, 184. (16) Percec, V.; Cho, W.-D.; Ungar, G. Unpublished results. (17) (a) Gidalevitz, D.; Mindyuk, O. Y.; Heiney, P. A.; Ocko, B. M.; Henderson, P.; Ringsdorf, H.; Boden, N.; Bushby, R. J.; Martin, P. S.; Strzalka, J.; McCauley, J. P., Jr.; Smith, A. B., III. J. Phys. Chem. B 1997, 101, 10870. (b) Gidalevitz, D.; Mindyuk, O. Y.; Stetzer, M. R.; Heiney, P. A.; Kurnaz, M. L.; Schwartz, D. K.; Ocko, B. M.; McCauley, J. P., Jr.; Smith, A. B., III. J. Phys. Chem. B 1998, 102, 6688. (c) Gidalevitz, D.; Mindyuk, O. Y.; Heiney, P. A.; Ocko, B. M.; Kurnaz M. L.; Schwartz, D. K. Langmuir 1998, 14, 2910. (18) Ocko, B. M.; Wu, X. Z.; Sirota, E. B.; Sinha, S. K.; Gang, O.; Deutsch, M. Phys. ReV. E 1997, 55, 3164. (19) (a) Kjaer, K.; Als-Nielson, J.; Helm, C. A.; Tippman-Krayer, P.; Mo¨hwald, H. J. Phys. Chem. 1989, 93, 3200. (b) Als-Nielsen, J.; Jacquemain, D.; Kjaer, K.; Leveiller, F.; Lahav, M.; Leiserowitz, L. Phys. Rep. 1994, 246, 251. (20) Magnussen, O. M.; Ocko, B. M.; Regan, M. J.; Penanen, K.; Pershan, P. S.; Deutsch, M. Phys. ReV. Lett. 1995, 74, 4444. (b) Heiney, P. A.; Gru¨neberg, K.; Fang, J. Y.; Dulcey, C.; Shashidhar, R. Langmuir 2000, 16, 2651.