Dendrimer Monolayers on Solid Substrates - American Chemical Society

Aggregation States and Molecular Motion in Amphiphilic ... The PAMAM molecules sit on the substrate with an oblate shape, in which hydrophilic core...
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Langmuir 2003, 19, 1196-1202

Aggregation States and Molecular Motion in Amphiphilic Poly(amido amine) Dendrimer Monolayers on Solid Substrates Keiji Tanaka, Shiyu Dai, and Tisato Kajiyama* Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Fukuoka 812-8581, Japan

Keigo Aoi Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya 464-8601, Japan

Masahiko Okada College of Bioscience and Biotechnology, Chubu University, Aichi 487-8501, Japan Received June 29, 2002. In Final Form: November 18, 2002 Monolayers of third-generation amphiphilic poly(amido amine) (PAMAM) dendrimer were prepared at the air/water interface and then were successfully transferred onto silicon wafers by a horizontal lifting method. A comprehensive characterization of the PAMAM monolayers was made, structurally and dynamically, by scanning force microscopy in conjunction with X-ray reflectivity and X-ray photoelectron spectroscopy. The PAMAM molecules sit on the substrate with an oblate shape, in which hydrophilic core and hydrophobic alkyl end groups were toward substrate and air sides, respectively. In the PAMAM monolayers, distinct three, Ra-, RH2O- and δ-, relaxation processes were observed on lateral force-temperature curves. They were assigned to segmental motion, plasticized segmental motion by residual water molecules, and a local mode relaxation, respectively.

Introduction Dendrimers including dendrons are a class of synthetic macromolecules composed of a core, repeating units, and end groups and have an unusual architecture of threedimensional hyperbranched structure.1,2 Such macromolecules have been paid a great deal of attention because of the plenty of interdisciplinary applications3-5 since Tomalia et al. first succeeded in synthesizing them in 1985.1 This is because arbitrary functional groups can be easily incorporated into arbitrary portions of dendrimers upon its synthetic process. Besides, interior voids in a dendrimer are intriguing spaces, which can encapsulate functional molecules as well as ions. For applications of dendrimers, electrical, optical, and opto-electrical devices have been extensively explored in connection with the new emerging research field of nanotechnology in the past decade.5 Consequently, it has been widely accepted that highly ordered ultrathin dendrimer films must be prepared as the first benchmark so that highly functionalized devices are promisingly designed and constructed. Langmuir-Blodgett (LB) technique6 enables one to gain direct access to precise thickness * To whom correspondence should be addressed. Fax: +81-92-651-5606. Tel: +81-92-642-3560. E-mail: kajiyama@ cstf.kyushu-u.ac.jp. (1) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Polym. J. 1985, 17, 117. (2) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A., III. Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (3) Matthews, O. A.; Shipway, A. N.; Stoddart, J. F. Prog. Polym. Sci. 1998, 23, 1. (4) Vo¨gtle, F.; Gestermann, S.; Hesse, R.; Schwierz, H.; Windisch, B. Prog. Polym. Sci. 2000, 25, 987. (5) Cagin, T.; Wang, G. F.; Martin, R.; Breen, N.; Goddard, W. A. Nanotechnology 2000, 11, 77.

and orientation controls on the molecular level, and thus, this method seems to be most effective to realize welldefined ultrathin dendrimer films. Hence, in this study, dendrimer monolayers are prepared onto solid substrates by the LB method, and then their aggregation states are studied in detail on the basis of microscopic, scattering, and spectroscopic measurements. The objective here is to elucidate how dendrimer molecules are macro- and microscopically arranged in an ultrathin state and to discuss why such aggregation states are formed. Obtained information would lead to a direction to design for devices. In addition, molecular motion in the films, which is closely related to thermal stability, is important and crucial in reality. Glass transition temperature, Tg, of some dendrimers has been reported.7-13 As a result, it has been revealed that Tg value of dendrimers is generally lower than that of linear polymers with a given molecular weight by virtue of an increased number density of chain ends, although the origin of glass transition for dendrimers seems to be still controversial, for example, usual segmental motion or molecular rotation. Also, rheological properties of dendrimer solutions and melts have been examined.11-16 However, these studies related to molecular (6) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (7) Kim, Y. H.; Webster, O. W. Macromolecules 1992, 25, 5561. (8) Wooley, K. L.; Hawker, C. J.; Pochan, J. M.; Fre´chet, J. M. J. Macromolecules 1993, 26, 1514. (9) Kim, Y. H.; Beckerbauer, R. Macromolecules 1994, 27, 1968. (10) Stutz, H. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 333. (11) Farrington, P. F.; Hawker, C. J.; Fre´chet, J. M. J.; Mackay, M. E. Macromolecules 1998, 31, 5043. (12) Iyer, J.; Fleming, K.; Hammond, P. T. Macromolecules 1998, 31, 8757. (13) Uppuluri, S.; Morison, F. A.; Dvornic, P. R. Macromolecules 2000, 33, 2551.

10.1021/la0261592 CCC: $25.00 © 2003 American Chemical Society Published on Web 01/23/2003

Aggregation States and Molecular Motion

Figure 1. Chemical structure of hemispherical shape thirdgeneration PAMAM dendrimer.

motion are target to bulk systems, meaning that knowledge should not be directly referred to the case of the ultrathin dendrimer films. Besides, from the point of view as property modifier at surface17 and in thin films,18 it is of pivotal importance to elucidate molecular motion in the ultrathin dendrimer films. Therefore, molecular motion in the ultrathin dendrimer films is here investigated as well. To the best of our knowledge, this is a first report to discuss comprehensively about structure, from macroscopically to microscopically, and molecular motion in the ultrathin dendrimer films. Experimental Section Material and Film Preparation. Third-generation poly(amido amine) (G3 PAMAM) dendrimer with a hemispherical shape, in which ethylenediamine and dodecyl chains are the focal point and the end groups, respectively, is used as a material. Figure 1 depicts the chemical structure. The PAMAM dendrimer was synthesized by the reaction of dodecylamine with terminal methyl ester-type PAMAM dendrimer of generation 2.5, which was prepared by stepwise PAMAM dendrimer construction with N-benzyloxycarbonyl-protected ethylenediamine as a halfprotected initiator core, followed by deprotection of the core moiety according to the literature.19,20 The product dendrimer and the methyl ester-type PAMAM dendrimers of generation 0.5, 1.5, and 2.5 were purified by gel filteration in methanol to synthesize uniform compounds.19,20 (14) Hawker, C. J.; Farrington, P. F.; Mackay, M. E.; Wooley, K. L.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1995, 117, 4409. (15) Uppuluri, S.; Keinath, S. E.; Tomalia, D. A.; Dvornic, P. R. Macromolecules 1998, 31, 4498. (16) Emran, S. K.; Newkome, G. R.; Weis, C. D.; Harmon, J. P. J. Polym. Sci., Part B: Polym. Phys. 1999, 37, 2025. (17) Fail, C. A.; Evenson, S. A.; Ward, L. J.; Schofield, W. C. E.; Badyal, J. P. S. Langmuir 2002, 18, 264. (18) Mackay, M. E.; Hong, Y.; Jeong, M.; Hong, S.; Russell, T. P.; Hawker, C. J.; Vestberg, R.; Douglas, J. F. Langmuir 2002, 18, 1877. (19) Aoi, K.; Itoh, K.; Okada, M. Macromolecules 1995, 28, 5391. (20) Aoi, K.; Itoh, K.; Okada, M. Macromolecules 1997, 30, 8072.

Langmuir, Vol. 19, No. 4, 2003 1197 Chloroform of spectroscopic grade was used as a spreading solvent. A chloroform solution of the PAMAM with the concentration of 7.66 × 10-6 M was prepared for spreading. Water (pH ) 5.8) used here was house-deionized, then further purified with a Milli-QII system (Millipore Co. Ltd.) with the initial resistivity greater than 18 MΩ. Surface pressure-area, π-A, isotherm measurement, and monolayer preparation were carried out with a microprocessor controlled film balance system (USI, Co., FSD20). The surface pressure was determined by the Wilhelmy balance technique with a filter paper plate. The PAMAM monolayers formed were tried to transfer onto solid substrates at a given surface pressure by either a vertical dipping6 or a horizontal lifting method.21 The transferred PAMAM monolayers were dried under vacuum at an ambient temperature for, at least, a day, and then, various characterizations were made. A silicon wafer was immersed into a mixed solution of concentrated H2SO4 and 30% H2O2 (70/30 vol) at 363 K for 1 h to obtain hydrophilic surface, and it was used as a substrate for the vertical dipping method. In the horizontal lifting method, a silicon wafer was soaked into a water solution of ammonium bifluoride with the concentration of 13.4 vol % for 90 s, resulting in hydrophobic surface, and then was used. Aggregation States in Transferred Monolayers. AFM observations for the PAMAM monolayers transferred onto solid substrates were made in air using an Explorer (Topometrix), which was equipped with a 2 µm dry scanner. A commercially available cantilever with the bending spring constant of 0.02 N‚m-1 (Olympus Optical Co., Ltd.) was used. Observations were carried out in contact mode, using the smallest possible setpoint value to minimize normal force during scanning. The monolayer thickness was evaluated by specular XR measurements. A monochromator was set next to the exit of the X-ray source (RU-300, Rigaku, Ltd., Co.) operated at 50 kV and 250 mA. The angular dependence of reflectivity was gathered by a series of incident angle, θ-2θ scans with the θ step width of 0.01 deg.‚step-1. The accumulation time for a step was 100 s with aluminum attenuators until θ ) 0.35 deg., and after that angle, it was changed to be 150 s removing the attenuators. The reflected beam was detected by a scintillation counter after the beam passed through the slit. Reflectivities were calculated from model electron density profiles along the direction perpendicular to the film surface by using an algorithm of Parratt22 on the basis of a recursive calculation method and fitted to the experimentally measured reflectivities. Aggregation states in the PAMAM monolayers were further studied by X-ray photoelectron spectroscopy (XPS) in addition to AFM and XR. XPS (PHI 5800 ESCA system, Physical Electronics, Co., Ltd.) measurements were carried out with a monochromatized Al KR source at 14 kV and 24 mA. Molecular Motion in Transferred Monolayers. Molecular motion in the transferred PAMAM monolayers was examined by lateral force microscopy (LFM, SPA 300 HV, Seiko Instruments Industry Co., Ltd.) with an SPI 3800 controller. Actually, the usage of “surface molecular motion” should be preferred rather than “molecular motion” because a probe tip contacts with the sample surface for the measurements. However, the thickness of the PAMAM monolayers is a few times an indentation depth of the tip, and thus, molecular motion in the whole film normal to the surface would be detectable by the measurements. Therefore, both terms will be used as the same meaning in this paper. LFM measurements were carried out at various temperatures in a vacuum. How to calibrate surface temperature was already stated elsewhere.23 A cantilever (Olympus Optical Co., Ltd.), of which both sides were coated by gold, was used. The bending spring constant was obtained to be 0.13 ( 0.01 N‚m-1 on the basis of the resonance frequency change after vapor deposition of gold thin layer onto the back of the cantilever. Also, the tip radius estimated using monodisperse colloidal gold was 42.8 ( 2.6 nm. The applied force to the cantilever was set to be 10 nN in a repulsive force region. (21) Fereshtehkhou, S.; Neuman, R. D.; Ovalle, R. J. Colloid Interface Sci. 1986, 109, 385. (22) Parratt, L. G. Phys. Rev. 1954, 95, 359. (23) Satomi, N.; Tanaka, K.; Takahara, A.; Kajiyama, T.; Ishizone, T.; Nakahama, S. Macromolecules 2001, 34, 8761.

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Figure 2. Surface-pressure isotherms of PAMAM monolayer at 293 K. Thick arrows denote surface pressures at which the monolayers were transferred onto solid substrates.

Results and Discussion I. Monolayer Behavior at the Air/Water Interface. At first, monolayer behavior of the PAMAM at the air/ water interface is examined so that the ultrathin PAMAM films are prepared onto solid substrates by the LB technique. Figure 2 shows π vs A isotherm for the PAMAM at the air/water interface. The surface pressure gradually increased with decreasing surface area and reached the constant value of approximately 45 mN‚m-1. Thus, it is clear that the PAMAM molecules form a stable monolayer on the water surface and that the monolayer is in a liquid expanded state24 at a lower surface pressure. The lift-off surface area, at which the π value started to increase, was 13.2 nm2‚molecule-1. Frank, Hawker, and co-workers discussed the molecular size dependence of the lift-off point for PAMAM dendrimer monolayer using the published data by Tomalia25 and proposed that the relation can be given by (2.30 × 10-3) ( (8 × 10-5) nm2‚(molecular weight).26 Since the theoretical molecular weight of the G3 PAMAM used here was 5489,27 the lift-off value was calculated to be 12.6 ( 0.4 nm2‚molecule-1. Our experimental result is in good accordance with this value. In general, the value of limiting area, which is specified by extrapolation to π ) 0 mN‚m-1 of the straight portion of the π-A curve, has been widely used to estimate a molecular occupied area being in a closely packed state for solid, or crystalline, monolayers.28 Invoking that this notion can be simply extended to monolayers in a liquid expanded state, the size of a PAMAM molecule at the air/water interface was deduced to be 7.8 nm2‚molecule-1, as shown in Figure 2. According to molecular dynamics (MD) calculation, the PAMAM molecule was in a flattened conformation on the water surface and its projected area was in the range of 7.1-9.6 nm2‚molecule-1.29 Hence, it is plausible that a PAMAM molecule sits on the water surface with not spherical but oblate shape. The core of the PAMAM molecule including focal point is hydrophilic, and thus, attracts to the water surface. In contrast, the (24) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; John Wiley & Sons: New York, 1997. (25) Sayed-Sweet, Y.; Hedstrand, D. M.; Spinder, R.; Tomalia, D. A. J. Mater. Chem. 1997, 7, 1995. (26) Kampf, J. P.; Frank, C. W.; Malmstro¨m, E. E.; Hawker, C. J. Langmuir 1999, 15, 227. (27) In general, branching of PAMAM dendrimer synthesized from the core portion was not perfect upon successive reaction step to grow the generation. After each step, the products were purified as mentioned in the Experimental Section, and the defects evaluated by 13C NMR were approximately 2%. Thus, the exact molecular weight of the employed G3 PAMAM dendrimer would be smaller than its theoretical value of 5489. Also, the number of end groups is possible to be smaller than 16. However, the essential discussion will be not altered at all because the extent of such defects is trivial, if any. (28) Gains, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; John Wiley & Sons: New York, 1966. (29) Yanagida, M. Ph. D. Dissertation, Kyushu University, 2000.

Figure 3. Surface morphology of PAMAM monolayers transferred onto silicon wafers (a) at π ) 15 and (c) 30 mN‚m-1 and (b) and (d) corresponding line profiles.

end groups of the molecule are hydrophobic dodecyl chains and prefer to stay away from water molecules. Such a situation leads to the oblate conformation of the PAMAM molecules at the air/water interface. The employed molecule is the 16 end type of the G3 PAMAM dendrimer,27 as shown in Figure 1. The cross-sectional areas of an alkyl chain in a fatty acid monolayer being in a hexagonal crystalline state and an amorphous one have been reported to be 0.21 and 0.30 nm2‚molecule-1, respectively.30 Hence, the molecular occupied area of the PAMAM should be 3.4 ()16 × 0.21) nm2‚molecule-1 or, at most, 4.8 nm2‚molecule-1 so that alkyl chains extend perpendicularly to the water surface. However, these values are much smaller than the experimental occupied area of a PAMAM molecule estimated by the limiting area on the basis of the π-A isotherm, indicating that the hydrophobic alkyl chains are present with highly disordered conformation on the hydrophilic core. To strengthen this discussion, electron diffraction (ED) measurement of a transferred PAMAM monolayer was made. The result did not exhibit any signatures of a crystalline state. This result strongly advocates the discussion for the conformation of alkyl chain end groups. II. Aggregation States of Transferred Monolayers. The usual vertical dipping method6 using hydrophilic silicon wafers treated by piranha solution was applied to transfer the PAMAM monolayers. Although the transfer ratio was almost unity, AFM observation revealed that most area at the substrate surface was not covered with the monolayer. Instead, protruded aggregates were clearly observed at the periphery of the substrate. This might be arisen from a situation that the PAMAM molecules slipped from the substrate during the upward stroke or the drying process. Since such could not be overcome by changing stroke speed, a horizontal lifting method21 was applied in lieu of the vertical dipping technique. In this case, a substrate is gently attached from the air side onto the monolayer surface, which mainly consists of hydrophobic alkyl tails. Thus, a silicon wafer was treated by water solution of ammonium bifluoride with the concentration of 13.4 vol %, resulting in a hydrophobic surface. The monolayer transfer ratio for this method was again almost unity. Figure 3 shows AFM images of the PAMAM dendrimer monolayers so transferred at π ) 15 and 30 mN‚m-1 and (30) Kajiyama, T.; Oishi, Y.; Uchida, M.; Morotomi, N.; Ishikawa, J.-I.; Tanimoto, Y. Bull. Chem. Soc. Jpn. 1992, 65, 864.

Aggregation States and Molecular Motion

their height profiles along the lines in each image. At π ) 15 mN‚m-1, the surface was quite smooth except the intentionally scratched area to estimate the monolayer thickness, as shown in part a. The root-mean-square roughness, Rrms, of an unscratched area of (1 × 1) µm2 was 0.21 ( 0.05 nm. Hence, it seems reasonable to claim that the PAMAM dendrimer monolayer was successfully transferred from the water surface onto the solid substrate. The step height of the scratched area by an AFM tip was approximately 1.4 nm, as shown in the panel of b, which was smaller than a hypothetical radius of a PAMAM molecule with a circular conformation deduced by MD simulation.29 This means that the PAMAM molecules transferred onto the solid substrate are in an oblate shape.31 On the other hand, in the PAMAM monolayer transferred at π ) 30 mN‚m-1, a mottled surface morphology was observed, as shown in part c. White and dark regions correspond to higher and lower regions in height, respectively. The height and width of the protruded regions at the surface were roughly evaluated to be a few nanometers and 50-100 nm, respectively, as shown in part d. The Rrms values of an unscratched area of (1 × 1) µm2 and an apparently smooth area of (50 × 50) nm2 were 1.28 ( 0.06 and 0.26 ( 0.02 nm, respectively. To estimate the film thickness, the monolayer was scratched by an AFM tip. The mean step height of the hole, corresponding to the thickness of the monolayer, was about 1.0 nm. For the PAMAM monolayer, the collapse pressure was 45 mN‚m-1, as shown in Figure 2. Nevertheless, the surface in the monolayer transferred at π ) 30 mN‚m-1 was heterogeneous in terms of height. Although it might be possible that aggregation states of the monolayer were changed upon its transfer process, such could be hardly thought. This is because for the monolayer transferred at π ) 15 mN‚m-1, protruded regions were not observed at all. Thus, it is conceivable that the monolayer started to collapse even at the surface pressure of 30 mN‚m-1, which was well below the collapse pressure based on the π-A isotherm. To further examine thickness and aggregation states of the transferred PAMAM monolayers, XR measurements were made.32-34 Figure 4a shows the scattering vector, kz [)(2π/λ)sin θ], dependence of X-ray reflectivity for the PAMAM dendrimer monolayers. The data for the monolayer transferred at π ) 30 mN‚m-1 was offset by one decade for the sake of clarity. The solid lines denote the best-fit calculated reflectivity to the experimental data on the basis of the model electron density profiles along the perpendicular direction. Figure 4b illustrates schematic representations of the PAMAM monolayers on silicon wafers, which were drawn on the basis of the model electron density profiles. Since the calculated curves are in good agreement with the experimental reflectivity, it can be claimed that each model shown in part b corresponds well to the aggregation states of the PAMAM dendrimer monolayers transferred at π ) 15 and 30 mN‚m-1 on silicon wafers. The thickness and the surface roughness of the monolayer transferred at π ) 15 mN‚m-1 were 1.5 and 0.28 nm, respectively. These values are in good accordance with the AFM results mentioned above. Also, the density of the dendrimer monolayer was 1.3 g‚cm-3. Although this (31) Bliznyuk, V. N.; Rinderspacher, F.; Tsukruk, V. V. Polymer 1998, 39, 5249. (32) Russell, T. P. Mater. Sci. Rept. 1990, 5, 171. (33) Johnson, M. A.; Santini, C. M. B.; Iyer, J.; Satija, S.; Ivkov, R.; Hammond, P. T. Macromolecules 2002, 35, 231. (34) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171.

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Figure 4. (a) X-ray reflectivity for PAMAM monolayers transferred onto silicon wafers at π ) 15 and 30 mN‚m-1 and (b) models to fit experimental reflectivity.

density value is a little larger than those of usual synthetic polymers, it is the same as the density obtained for PAMAM dendrimer monolayer with a higher generation by Tsukruk and co-workers.34 There are two unique layers on the models: adsorbed water and SiO2 layers. Assuming these two layers, the fitting to the experimental data became better. In general, it is not so easy to remove water molecules from a monolayer, which are introduced upon monolayer transferring process, without heating it up. Such residual water molecules in the monolayer might accelerate surface oxidation of the substrate. If that is the case, it might be possible that PAMAM molecules turn upside down on the substrate during the drying period to minimize the interfacial free energy. Internal aggregation states in the monolayer, such as alkyl chain distribution, can be somehow seen by XR measurement as a change in electron density profile. However, the discrepancy of electron density between the hydrophilic tails and the hydrophobic core is not so remarkable. In a sense, an increase in parameters for the fitting procedure may lead to ambiguity for the analysis in this case. Hence, we have restricted ourselves to claim the thickness of the monolayer and the interfacial roughness, and the internal aggregation structure is deferred to XPS, which is more direct method. In the PAMAM monolayer transferred at π ) 30 mN‚m-1, the thicknesses of flat and protruded regions were 1.1 and 2.4 ()1.1 + 1.3) nm, respectively, as shown in the right cartoon of part b. This result seems to imply how the monolayer was compressed at the air/water interface. At a lower surface pressure, the thickness of the monolayer increased with decreasing occupied area on account of applied lateral pressure. However, the monolayer was in part collapsed at a higher surface pressure around 30 mN‚m-1 because the monolayer could not stand under such an increased lateral pressure. Once the lateral stress in the monolayer was released by forming protrusions, the thickness of the monolayer probably decreased. Also, the mean height of the protruded regions was 2.4 nm from the substrate surface, which was approximately twice the thickness of the flatten region.

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Figure 6. Schematic representation of monolayer transferred onto silicon wafer. Figure 5. XPS integral intensity ratio for PAMAM monolayers transferred onto silicon wafers at π ) 15 and 30 mN‚m-1. The left and right ordinates stand for IC1s/ ISi2p and IC1s/IN1s values, respectively. Solid curves are drawn on the basis of model calculation. See text in detail.

Hence, it can be envisaged that the protruded regions are mainly composed of bilayer structure of the PAMAM molecules. The protruded regions covered the surface of 64%. Figure 5 shows the angular-dependent XPS results. The analytical depth of the measurement is given by 3λ‚sin θ, where λ and θ are inelastic mean-free path and emission angle of photoelectrons, respectively.35 Hence, the abscissa of Figure 5 was expressed by sin θ. The ordinate stands for integrated intensity ratio of photoelectrons of carbon 1s to silicon 2p or to nitrogen 1s. Since the monolayer thickness was much smaller than the current maximum analytical depth of 10 nm, photoelectrons arisen from Si2p were detectable even though the substrate surface was covered with the monolayer, resulting in a finite value of IC1s/ISi2p. The IC1s/ISi2p value for the monolayer transferred at π ) 30 mN‚m-1 was larger than that at π ) 15 mN‚m-1 at a given sin θ because of a discrepancy of the thickness and increased with decreasing analytical depth for both. As the X-ray attenuation in the surface layer is negligible, the intensity of photoelectrons emitted from atoms at the depth x is proportional to exp(-x/λsin θ).35 The solid curves in Figure 5 trace the IC1s/ISi2p calculated by exp(-x/λsin θ) using an overlayer and a patchy overlayer models.35 These models were essentially the same as those shown in panel b of Figure 4 except for the following two points. The first is that water adsorbed layer was not counted for the model calculation because the XPS measurement was made under ultrahigh vacuum. The second is that the surface coverage of the PAMAM monolayers was not perfect but 96% to obtain the best-fit curves for both monolayers transferred at π ) 15 and 30 mN‚m-1. Taking into account a usual experimental accuracy of XPS measurements, approximately 10%, it seems reasonable to consider that the XPS results are in agreement with the AFM and XR results. The atomic ratio of carbon to nitrogen in a PAMAM molecule based on the chemical structure is 6.78. To compare this number with the XPS results, some corrections must be made, resulting in 4.21. Since the intensity of photoelectrons exponentially decays with the distance from the outermost surface, the IC1s/IN1s values shown by filled and open triangles in Figure 5 seem to be quite reasonable. The IC1s/IN1s values for both monolayers increased as the analytical depth became shallower. Since nitrogen atoms are present only in the core portion of a PAMAM molecule, the data clearly indicates that alkyl chain end portions are located at the side of air. Such (35) Andrade, J. D. Surface and Interfacial Aspects of Biomedical Polymers; Plenum Press: New York, 1985.

structure is totally opposite to how the monolayers were transferred onto the substrates. Besides, O1s photoelectrons were clearly observed after drying the monolayers, meaning that a native oxide layer was again formed at the surface of the substrate, as deduced after the XR measurements. Hence, it seems most likely that the PAMAM molecules turned upside down on the substrate probably with the aid of residual water molecules to minimize the interfacial free energy upon drying process of the monolayer. In addition, active molecular mobility of the PAMAM molecules at room temperature, which will be later discussed, might help such a structural change. Figure 6 illustrates a possible model of structural change in the PAMAM monolayer transferred at π ) 15 mN‚m-1 upon the drying process. Also, it is of interest to compare the aggregation states observed here, in which chain ends are toward the air phase, with those by a computational study.36 Lescanec and Muthukmar anticipated that chain ends of dendritic macromolecules might be migrated into the interior side.36 This simulational prediction was not consistent with what was observed here. The IC1s/IN1s values were coincident for both monolayers within the error bars, even though there were many protruded regions at the surface of the monolayer transferred at π ) 30 mN‚m-1. This means that the alkyl end groups of the outermost PAMAM molecules in the protruded regions contacts the air phase. Hence, it is plausible that the protruded regions of the monolayer transferred at π ) 30 mN‚m-1 mainly consists of z-type bilayer, in which hydrophobic portions of each layer are toward the air phase.6 III. Molecular Motion in Transferred Monolayers. We now come to molecular motion in the PAMAM monolayers transferred onto the solid substrates. Since the manifestation of frictional force of polymeric materials is closely related to their viscoelastic properties, it is possible to examine molecular motion in the PAMAM monolayers by using LFM.37-40 When the energy dissipation increases in the monolayer because of molecular motion, lateral force increases. Hence, it can be postulated that lateral force alteration with measuring temperature is essentially similar to the temperature dependence of dynamic loss modulus or loss tangent.37-40 Since the monolayer transferred at π ) 30 mN‚m-1 possessed many protruded regions on its surface, scanning could be hardly made without facing such regions for the (36) Lescanec, R. L.; Muthukumar, M. Macromolecules 1990, 23, 2280. (37) Kajiyama, T.; Tanaka, K.; Takahara, A. Macromolecules 1997, 30, 280. (38) Kajiyama, T.; Tanaka, K.; Satomi, N.; Takahara, A. Macromolecules 1998, 31, 5150. (39) Hammerschmidt, J. A.; Gladfelter, W. L.; Haugstad, G. Macromolecules 1999, 32, 3360. (40) Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 2000, 33, 7588.

Aggregation States and Molecular Motion

Figure 7. Temperature dependence of lateral force for PAMAM monolayer transferred at π ) 15 mN‚m-1 at scanning rates of 1 and 8 µm‚s-1.

distance of a few micrometers. Hence, only the monolayer transferred at π ) 15 mN‚m-1 was focused here. Figure 7 shows lateral force variation with temperature at two different scanning rates. The LFM measurements were truncated after 340 K because scanning was unstable probably because of thermal partial decomposition of the PAMAM molecules. Figure 7 makes it clear that three relaxation peaks appeared for the PAMAM monolayer in the temperature range employed. The lateral force peak marked by Ra was observed at 260-270 K, which was the highest temperature in comparison with other peaks and not so sensitive to the scanning rate as shown in Figure 7. On the basis of measuring frequency and reciprocal number of peak temperature, the apparent activation energy of this relaxation process was roughly estimated to be 180 kJ‚mol-1, which was a reasonable value for surface micro-Brownian motion.40 Hence, it seems reasonable to consider that the relaxation that appeared at 260-270 K is assigned to segmental motion in the PAMAM monolayer. So far, literature value of bulk glass transition temperature, Tgb, for the PAMAM has not been available. Thus, a direct comparison of the Ra-temperature in the PAMAM monolayer with the corresponding Tgb could not be made. Tgb of the PAMAM should be much smaller than Tgb of usual polyamides, ranging from 320 to 380 K,41 because of a large number density of chain end groups. Hammond et al., using differential scanning calorimetry, have systematically studied thermal properties of linear poly(ethylene oxide) (PEO)-PAMAM diblock copolymers with various generations and chain end groups. 12 Consequently, Tgb of the PAMAM block with 16 end groups was in the range of 250-284 K, depending on chain end chemistry. Of course, this value cannot be simply referred as Tgb of the PAMAM used because of discrepancies between theirs and ours such as primary sequence and chain end structure, although Tgb of the PAMAM used might not be so different. A more conclusive study is necessary to claim to what extent molecular motion in the PAMAM monolayer is different from that in the bulk in this hyperbranched system. The lateral force peaks were also observed at approximately 210 K for 1 µm‚s-1 and 230 K for 8 µm‚s-1. In contrast to the Ra-relaxation process, its temperature position was strongly dependent on the scanning rate. This relaxation and Ra-relaxation peaks seem to be merged into a broad peak at a hypothetical higher scanning rate, which could not be experimentally attained. In general, it has been widely accepted that absorbed water molecules give rise to an additional relaxation process to the segmental motion at a lower temperature side.42 The PAMAM monolayers were dried under vacuum at an (41) Brandrup, J.; Immergut, E. H.; Grulke, E. A. Polymer Handbook, 4th ed.; John Wiley & Sons: New York, 1999.

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ambient temperature for, at least, a day. However, actually, water molecules would be still present in the monolayer, as observed in XR measurements. If that is the case, it is not surprising that a plasticized effect of residual water molecules on segmental motion, namely, the RH2O-relaxation, was observed. The apparent activation energy of this process was 1/5∼1/8 of the surface Rarelaxation process, being a reasonable reduction by the plasticized effect.42 The most intense peaks were observed near the lower temperature limit of our measurements. The overall shape of these peaks could not be seen, and especially, the peak top did not even show up in the case of 8 µm‚s-1. Thus, it is quite difficult to discuss what the origin of this relaxation is. However, taking into account a fact that the peaks were observed at extremely low temperatures, it is plausible that they correspond to a relaxation process with a quite small size. Hence, for the moment, the most probable assignment of this δ-relaxation process is a rotational relaxation of end methyl groups or a local mode relaxation of methylene groups in the end alkyl tails. Finally, it should be considered why the lateral force peaks corresponding to the δ-relaxation were intense in comparison with Ra- and RH2O-relaxation peaks. When a probe tip contacts with the surface and travels for a given distance, lateral force is manifested. In this experiment, the outermost surface of the PAMAM monolayer mainly consisted of disordered alkyl end groups, and amido amine repeating units existed beneath the layer of alkyl groups, as shown in the cartoon of Figure 6. This means that the displacement imposed by the tip mostly dissipates, or absorbs, in the outermost alkyl layers. Since the displacement also propagates into the region of amido amine repeating units through the alkyl layers,43 the Ra- and RH2O-relaxation peaks are supposed to be detected. If that is the case, it seems quite reasonable to consider that the δ-relaxation peak in the outermost surface layer is much larger than the Ra- and RH2O-ones in the underneath layer. In a sense, this notion indirectly supports the assignment of the δ-relaxation process. Conclusions Third-generation poly(amido amine) (PAMAM) dendrimer monolayers were prepared at the air/water interface and then were successfully transferred from the air side onto hydrophobic silicon wafers, which were treated using water solution of ammonium bifluoride, by the horizontal lifting method. Aggregation states in the PAMAM monolayers were studied by atomic force microscopic observation, X-ray reflectivity, and X-ray photoelectron spectroscopic measurements. The surface of the monolayer transferred at π ) 15 mN‚m-1 was molecularly smooth, whereas the monolayer was in part collapsed even at π ) 30 mN‚m-1 being well below its collapse pressure of 45 mN‚m-1 on the basis of the π-A isotherm. Also, the hydrophobic substrate was oxidized by an ambient atmosphere, resulting in hydrophilic surface, and then the PAMAM molecules were eventually turned upside down on the substrate during the drying process. The PAMAM molecules sit on the substrate with an oblate shape, in which hydrophilic core and hydrophobic alkyl end groups were toward substrate and air sides, respectively. Lateral force microscopy revealed three distinct, Ra-, RH2O-, and δ-, relaxation processes in the PAMAM (42) McCrum, N. G.; Read, B. E.; Williams, G. Anelastic and Dielectric Effects in Polymeric Solids; Dover: New York, 1967. (43) Satomi, N.; Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 2001, 34, 6420.

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monolayer. The Ra- and RH2O-relaxations were assigned to segmental motion and plasticized segmental motion by residual water molecules, respectively. Also, the δ-relaxation observed at 120-130 K was assigned to a rotational relaxation of end methyl groups or a local mode relaxation of methylene groups in the end alkyl tails. Acknowledgment. We are most grateful for helpful discussions with Prof. Atsushi Takahara, Kyushu Uni-

Tanaka et al.

versity, and thank Hirohiko Yakabe for his help with XR measurement. This was in part supported by a Grantin-Aid for Scientific Research (A) (#13355034) and by aGrant-in-Aid for Scientific Research in the Priority Area of “Molecular Synchronization for Design of New Materials System” (# 404/13022253) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. LA0261592