6240
J. Phys. Chem. B 2009, 113, 6240–6245
Hierarchical Rearrangement of Self-Assembled Molecular Bundle Strands from Poly(oxyethylene)-Segmented Amido Acids Wei-Cheng Tsai‡ and Jiang-Jen Lin†,* Institute of Polymer Science and Engineering, National Taiwan UniVersity, Taipei 10617, Taiwan, and Department of Chemical Engineering, National Chung Hsing UniVersity, Taichung 40227, Taiwan ReceiVed: January 21, 2009
We observed the arrays of molecular bundle strands in ribbon shape and their perpendicular arrangement between the bundle strands from the molecules that consist of symmetrical structure of poly(oxyethylene)segmented bisamido acid (POE-amido acid). The molecules enabled to self-assemble into bundle strands of 5-10 nm width, 1-7 nm height, and 5-120 nm length, which further self-arranged into secondary bundle clusters. By varying the conditions of spin-coating or dip-coating (immersion) on polyethersulfone film surface and drying temperature (26 or 19 °C), the morphologies of the bundle clusters were controllable. Lengthy rattan-like strands with multiple “side-armed” short bundle strands were observed from tapping-mode atomic force microscopy. Different arrays of parallel bundle strands in cluster (by spin-coating method) and rattanlike strands with side arms (by dip-coating method) were observed, with the same bundle units of 5-10 nm in width but varying in height from 0.5 to 7 nm. The bundle height of 0.5 nm obtained by carefully controlled dip-coating into film implies a “self-assembled monolayer (SAM)” formation. The perpendicular bundle side arm arrangement is attributed to the complementary noncovalent bonding forces of POE and -COOH interaction. The presence of a POE crystalline segment (Tm ) 22.6 °C, ∆H ) 85.6 J/g) in the molecules contributed predominately to the formation of bundles and hierarchical parallel clusters or perpendicular “side arms”. Introduction Self-assembling processes are ubiquitous and inspiring for researchers to mimic and learn the “bottom-up” approach for fabricating nanoarrays.1 Synthetic molecules and those that existed in nature such as block copolymers,2-6 surfactants,7 proteins,8 DNA,9,10 peptides,11,12 and peptide amphiphiles13 have been well recognized for their self-assembling behaviors. Various morphologies of the self-assemblies at nanometer to micrometer scale in the forms of rod-coil,14 vesicles,15 fibers,16 bowl-shaped,17 and ringlike18 arrays were observed. With a twodimensional film, the self-assembly monolayer (SAM) is also well-known for generating unique patterns of thin layers with tailored surface properties.19-21 In particular, block copolymers are interesting to serve as templates for controlling the formation of specific patterns. Phase separation of block copolymers into various morphologies have important implications in both of academic interests and practical applications.22 Morphological variation of the self-assemblies from amphiphilic block copolymers is often influenced by their chemical structures and process conditions such as concentration,23 temperature,24 pH,25 and medium.26 The compositions of hydrophobic and hydrophilic domains are thermodynamically driven to generate orderly structures, corresponding to the volume/mass fraction of the distinct blocks.15,27 The block copolymers, consisting of poly(ethylene glycol) or poly(oxyethylene) (POE) segments, are of particular interest because of their potential dipole-dipole noncovalent bonding interaction, crystalline and having practical uses in biomedical applications. * Corresponding author. Tel: +886-2-3366-5312. Fax: +886-2-33665237. E-mail:
[email protected]. † National Taiwan University. ‡ National Chung Hsing University.
For example, the POE-containing copolymers can be used to encapsulate large biomolecules for use in medical therapies.28-30 Atomic force microscopy (AFM) is suitable for the observation of nanometer-scale morphologies in a nondestructive manner resulting in high-resolution 3-D topographic images.31,32 The direct observation of the small molecule self-piling phenomenon under different procedural conditions allows us to understand the structure/morphology correlation, and ultimately the principles of molecular crystallization or selfaggregation into a confined nanostructure.33-36 Previously, we have reported for the first time the formation of molecular bundle strands from the water-soluble poly(oxypropylene) (POP)-amido acid sodium salts on an inert polymeric substrate surface.37 The amido acid salts were prepared from the reaction of POPdiamine and trimellitic anhydride at 1:2 molar ratio. The unique combination of symmetrical structure and multiple polar functionalities rendered the molecules self-assembling into primarily molecular bundle strands, bundle strips, or spherical aggregates depending on the pH environment or solubility, concentration, and temperature.37,38 The molecules enabling the formation of molecular bundle strands are actually amphiphilic and behaving like a surfactant which could lower surface tension and toluene/ water interfacial tension.39 The pristine POP-amido acid with the hydrophobic middle POP-block was actually insoluble in water and required the conversion into the corresponding sodium salts before performing self-assembling. Here we synthesize the POE-segmented amido acid by following the same synthetic scheme but involving the hydrophilic POE-diamine at 2000 g/mol molecular weight. Besides the difference in water solubility without the need of converting into sodium salts, the amido acids consisting of POE- or -(CH2CH2O)x- as the middle block instead of POP- or -(CH2CH(CH3)O)x- were found to self-assemble into secondarily bundle clusters and
10.1021/jp902289m CCC: $40.75 2009 American Chemical Society Published on Web 04/08/2009
Poly(oxyethylene)-Segmented Amido Acids
J. Phys. Chem. B, Vol. 113, No. 18, 2009 6241
Figure 1. Synthetic scheme for water-soluble POE- and POP-amido acid sodium salts.39
Figure 2. TM-AFM of the POE-amido acid self-assemblies under different temperatures, (A) topographical, (B) phase images (from spin-coating at 26 °C), and (C) topographical, and (D) phase images (from spin coating at 19 °C).
hierarchical arrays. The presence of the POE crystalline segment in the molecule provides a predominate noncovalent force for the self-assembly. Experimental Section Materials. Hydrophobic poly(oxypropylene) (POP)- and hydrophilic poly(oxyethylene)-diamine (POE-amine) were obtained from Aldrich Chemical Co. or Huntsman Chemical Co. The POE-amine, more specifically poly(oxypropylene-oxyethylene-oxypropylene)-segmented polyether of bis(2-aminopropyl ether) at 2000 g/mol, is water-soluble and crystalline (waxy solid, mp 37-40 °C, amine content 0.95 mequiv/g, 38.7 oxyethylene, and 6 oxypropylene units in the structure). Trimellitic anhydride (TMA) was purchased from Aldrich
Chemical Co. and purified by sublimation. Sliver nitrate (AgNO3, 99.8%) and sodium borohydride (NaBH4, 98%) were purchased from SHOWA Chemical Co. Synthesis of the POE-amido Acid. The POE-amido acid was synthesized from the reaction of POE2000 and TMA at a 1:2 molar ratio according the following experimental procedures. To a 100 mL, three-necked, round-bottomed flask which was equipped with a magnetic stirrer, nitrogen inlet-outlet lines, and a thermometer, POE2000 (10 g, 0.005 mol) in THF (15 mL) was added, followed by a solution of TMA (1.92 g, 0.010 mol) in THF (10 mL) through an addition funnel. During the addition, the mixture was stirred vigorously and the reaction temperature was maintained below 30 °C for 3 h. At the end of reaction, solvent was removed through evaporation under
6242
J. Phys. Chem. B, Vol. 113, No. 18, 2009
Tsai and Lin
Figure 3. TM-AFM of the self-assemblies by dip-coating (immersion) method at 19 °C, (A) topographical and (B) phase images on PES substrate, and at magnification (C) topographical (bundle height of 0.8-3.2 nm) and (D) phase (bundle width of 5.4-6.1 nm and spacing of 4.1-4.2 nm) between two neighboring bundles.
reduced pressure. The product was obtained as a yellowish waxy solid and characterized by FT-IR, with the absorption at 1651 cm-1 (carbonyl), 1546 cm-1 (NH in amide), and 1716 cm-1 (COOH). Doping Silver Nanoparticles (AgNP) onto the POE-amido Acid. To a glassware reactor, equipped with a stirrer and nitrogen inlet-outline lines, was placed with POE- amido acid (0.60 g in 32 mL of deionized water) and added with the AgNO3 solution (0.63 g in 30 mL of water). The mixture was stirred for 1 h and then NaBH4 (0.17 g in 70 mL of water) was added slowly at room temperature. Under the nitrogen flow, the solution changed its color into black and showed the characteristic UV-vis absorption of 410 nm, indicating the appearance of silver nanoparticles. Sample Preparation. Two film coating methods were used for preparing the POE-amido acid molecular bundle samples. By the spin-coating method, POE-amido acid (0.1 wt %) in deionized water was spin-coated on polyethersulfone (PES) film at a spinning rate of 1500 rpm for 15 s and then 2000 rpm for
10 s under controlled temperature (at 26 or 19 °C). By the dipcoating method, the samples were prepared by immersing a PES film in the sample solution (0.1 wt % in water) for 3 s and drying the film at 19 °C. Analytical Methods. The analysis was carried out by using a tapping mode atomic force microscopy, SPA-400HV with an SPI3800N controller (Seiko Instruments Industry Co., Ltd.) for a constant temperature. The cantilever was fabricated from Si with a spring constant of 16 N/m and a resonance frequency of 139 kHz. The scan speed was in 1.0 Hz with a scanning density of 512 lines/frame, and approximately 3 min for each scan using the tapping mode TM-AFM. Fourier transform infrared spectroscopy (FT-IR) was recorded on a Perkin-Elmer Spectrum One FT-IR Spectrometer in the range of 4000-400 cm-1. Samples were prepared by dissolving in THF and evaporating into a thin film on a KBr plate. Thermal properties of melting and crystallizing temperatures were characterized on a differential scanning calorimeter (DSC, Perkin-Elmer Pyris 6). Samples in the amount of 3-8 mg on a sealed aluminum pan were generally
Poly(oxyethylene)-Segmented Amido Acids
J. Phys. Chem. B, Vol. 113, No. 18, 2009 6243
Figure 4. Conceptual diagram of the bundle strands (A) from the spin-coating method: formation of single ribbon-like bundle strands and their parallel strand clusters (corresponding to Figure 2), through molecular stretching and intermolecular alignment to form bundle strands, and (B) from the dip-coating (immersion) method: the formation of “rattan-like” long strand and perpendicularly connected “side arm” (corresponding to Figure 3), through the periphery carboxylic acids and POE noncovalent bonding interaction.
Figure 5. TM-AFM images showing the coexistence of POE-amido acid bundles and aggregated AgNP at 15-30 nm in diameter, (A) topographical, (B) phase, and (C) representative bundle width of 6.3 nm.
used. The temperature range of -50 to +50 °C at a heating rate of 10 °C min-1 with a nitrogen flow of 20 mL min-1 was used. The melting point (Tm) was determined from the thermo-
gram while the enthalpy of crystallization (∆H) was measured by integrating the peak area. UV-vis spectrum was recorded on Shimadzu UV-1240.
6244
J. Phys. Chem. B, Vol. 113, No. 18, 2009
Results and Discussion Synthesis of the POE-amido Acids. Previously, the amphiphilic copolymers with a hydrophobic POP middle block and two amido acid termini were prepared from the reaction of POPdiamine and trimellitic anhydride, and investigated for their selfassembling property.37,38 The POP block in the middle of the symmetrical POP-amido acid is actually amorphous but contributed to the hydrophobic interaction along with the carboxylate salts to form intermolecular bundle strands. The hydrophobic POP-diamine at 2000 g/mol Mw is insoluble in water, but becomes soluble after converting into sodium salt. The presence of carboxylate salts in the molecule was required for the water solubility as well as the self-assembling into bundles. In the present study, the hydrophilic and crystalline POE2000 (2000 g/mol) segment or -(CH2CH2O)x- is constructed in the amido acid and we found it is highly water-soluble in the form of carboxylic acid. The structure is mainly characterized by the FT-IR absorption for the amide formation. Compared to the amorphous POP, the POE counterpart is hydrophilic and crystalline, implying its strong noncovalent bonding force. According to the DSC analysis, the POE segment contributes to the molecular crystalline at Tm ) 34 °C (∆H ) 145.9 J/g) for the pure POE2000 starting material and Tm ) 22.6 °C (∆H ) 85.6 J/g) for the POE-amido acid. The chemical structures of the POE- and POP-amido acids are shown in Figure 1. Self-assembly by Spin-Coating Method. The self-assemblies of POE-amido acid were prepared by spin-coating the samples of 0.1 wt % concentration in water on the PES substrate at 26 °C. As observed in Figure 2, A and B, the POE-amido acid tends to form molecular bundle strands having a similar height and width, in a high degree of homogeneity. Average dimension for these bundles is 5-10 nm in width, 1-4 nm in height, and 5-30 nm in length. The theoretical calculation for a fully stretched molecular length of the POE2000 backbone amounted to 11.1 nm,40 similar to the observed rod width. In control experiments, the POE2000-diamine failed to form bundle morphology but showed spherical particle shapes observed on the substrates of mica, glass, and PES (images shown in Figure S1, Supporting Information). With similar spin-coating procedures but at 19 °C, the molecular bundle strands are individually lengthy and form a more orderly secondary morphology such as ribbon-like patterns. As shown in Figure 2, C and D, the bundle strands further aligned into clusters as the secondary arrays in a large area, in which the lengthy bundle strands are pointed more or less in parallel direction. The ribbon-like cluster patterns consist of individual bundle strands with an average size of 5-10 nm in width, 2-7 nm in height, and 10-120 nm in length. The distance between two neighboring bundle strands in parallel is in a common range of 4-6 nm space, perhaps owing to the presence of a repulsion interaction between the bundle side groups of carboxylic acids.41 This repulsion force apparently participated the formation of secondary array morphologies (Figure 2C,D). In contrast, the POP analogues could only selfpile into a maximum length of ca. 30 nm with a similar width. Apparently, the length is much longer than the formation in Figure 2B by the self-assembly at a higher temperature, but with the same width that is consistent with the stretched POE length. With the same bundle width, their height increased during the growth of the lengths. The lower temperature, 19 °C instead of 26 °C, favored the formation of lengthy bundle strands, perhaps due to a thermodynamic equilibrium for the POE crystallization (Tm ) 22.6 °C).
Tsai and Lin Self-assembly by Dip-Coating (Immersion) Method. The samples were prepared by immersing a PES film into the 0.1 wt-% of POE-amido acid in deionized water for 3 s and then conditioned at 19 °C. It was found that the lengthy bundle strands were more pronounced over a large area of micrometers. As shown in Figure 3A and 3B, the rattan-like patterns extended into 1-4 µm in length with a constant 5-10 nm in width and 0.5-4 nm in height. A bundle width of 5-10 nm is still the same, but some bundle strands congregated into a broader 25 nm rattan-like lines perhaps caused by a secondary bundle alignment of molecular length. Another example of bundle with a height of 0.8-3.2 nm and width of 5.4-6.2 nm was observed (Figure 3, C and D, respectively). Particularly interesting, the border areas between the rattan-like arrays were tethered with shorter bundle strands in parallel directions. The perpendicular arrangement between the rattan peripheries and the “side-arm” bundle is a unique arrangement (Figure 3, A and B), implying a new type of bundle arrays. It was measured that the “rattanlike” main chain bundle strands had an average height of 0.5 nm or 5 Å, indicating a thickness of nearly molecular level. The SAM in angstrom-scale structure seems to occur in this morphology. It was noticed that the formation of such a “sidearm” bundle could be obtained only under the conditions of low-temperature and the dip-coating method, implying a kinetic control for forming such a morphological arrangement. Since the POE2000-amido acid was constructed from the reaction of POE-diamine and TMA, the structure consists of symmetric arylamido acids at both ends of the POE block. With potential noncovalent bonding forces including hydrogen bonding42 and π-π aromatic stacking43 that are integrated with the hydrophilic POE segment3,44 in the structure, the molecules are able to self-pile by intermolecular interactions into molecular bundles. These bundles had a common width of approximately 10 nm, representing the stretched POE length. In contrast to the previously reported amorphous POP as the backbone, the POE block in the structure may be crystallized at a temperature below 22.6 °C. The self-assemblies were found to be temperature dependent. Nevertheless, the formation of molecular bundles and their secondary arrays via POE backbone stretching and intermolecular aligning is conceptually described in Figure 4. Based on these observations, a self-assembly model was proposed to explain the formation of lengthy rattan-lines and short “side-arm” bundle strands. Both types of the noncovalent bonding forces involving the POE backbone crystalline and the POE/COOH interaction45,46 between two bundle strands are attributed to the formation of these side-armed bundles. Since the self-assemblies were largely controlled by the intermolecular noncovalent bonding forces, the self-assembled morphologies may be affected by the presence of metal ions or metal nanoparticles. It is well-known that amide,47 carboxylic acid,48 and POE49 functionalities are capable of chelating with silver ions during the formation of silver nanoparticle (AgNP). By mixing POE-amido acids and AgNO3 in water, a homogeneous solution was obtained. Under the reduction with sodium borohydride, AgNP were formed and evidenced by the characteristic UV-vis absorption at 410 nm. The homogeneity and stability of the AgNP slurry indicated the intensive dispersing ability of the POE-amido acid. By using the same conditions of preparing the POE self-assemblies, the AgNP dispersion was evaporated into film and examined by AFM. As shown in Figure 5, the AgNP were uniformly produced at the average diameters of 15-30 nm size distribution, but aggregated into particle strings as the main morphology. In the same micrograph, there is also observed a small portion of the molecular bundle strands
Poly(oxyethylene)-Segmented Amido Acids in ca. 6.3 nm width, as similarly observed for the original 5-10 nm dimension without the AgNP presence. Since the control experiments of AgNO3 reduction into silver mirror with the presence of POE2000, the dispersion ability or the POE/AgNP interaction may have largely distracted the formation of bundle strands and clusters. Presumably, the AgNP in string arrays are directed by the presence of POE-amido acids during the generation and growth of AgNP. Conclusion The self-assembled morphologies derived from the secondary arrangement of molecular bundle strands were observed from the symmetrical POE2000-backboned bisamido acids. The primary molecular bundle strands (5-10 nm in width) may rearrange into hierarchical arrays including rattan-like and “sidearm” morphologies. The crystalline POE segment is attributed to the formation of these orderly arrays. The self-assembling is sensitive to the environmental conditions particularly the temperature since the POE is crystallized at 22.6 °C (measured by DSC). Intermolecular interactions through hydrogen bonding with the adsorbed moisture on the POE portion and amidocarboxylic acids may dictate the variation of array shapes. The perpendicular “side-arm” shapes were derived from the arrangement of two bundle strands connected by the interaction of the bundle peripheries (-COOH) and the tethered POE bundle strands. Unique formation of the arrays, in particular, the perpendicular arrangement of the bundle morphologies with a constant range of 5-10 nm in width and different heights in the ranges of 1-7 nm or 0.5-4 nm, is influenced by the POE crystalline force which could extend the bundle length up to 4 µm. The bundle arrays may be used as the templates for forming AgNP in string-like aggregate during the water evaporation. The manipulation of molecular bundles as the template for forming metal nanowires will be pursued. Acknowledgment. We acknowledge the financial support from the Ministry of Economic Affairs and National Science Council (NSC) of Taiwan. Supporting Information Available: AFM images of POE2000-diamine starting material. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418–2421. (2) Li, Z.; Kesselman, E.; Talmon, Y.; Hillmyer, M. A.; Lodge, T. P. Science 2004, 306, 98–101. (3) Jain, S.; Bates, F. S. Science 2003, 300, 460–464. (4) Pochan, D. J.; Chen, Z.; Cui, H.; Hales, K.; Qi, K.; Wooley, K. L. Science 2004, 306, 94–97. (5) Raez, J.; Manners, I.; Winnik, M. A. J. Am. Chem. Soc. 2002, 124, 10381–10395. (6) Zheng, R.; Liu, G.; Yan, X. J. Am. Chem. Soc. 2005, 127, 15358– 15359. (7) Zana, R.; Talmon, Y. Nature (London) 1993, 362, 228–230. (8) Ringler, P.; Schulz, G. E. Science 2003, 302, 106–109. (9) Rothemund, P. W. K. Nature (London) 2006, 440, 297–302. (10) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature (London) 1996, 382, 607–609.
J. Phys. Chem. B, Vol. 113, No. 18, 2009 6245 (11) Tovar, J. D.; Claussen, R. C.; Stupp, S. I. J. Am. Chem. Soc. 2005, 127, 7337–7345. (12) Cornelissen, J. J. L. M.; Donners, J. J. J. M.; Gelder, R.; Graswinckel, W. S.; Metselaar, G. A.; Rowan, A. E.; Sommerdijk, N. A. J. M.; Nolte, R. J. M. Science 2001, 293, 676–680. (13) Silva, G. A.; Czeisler, C.; Niece, K. L.; Beniash, E.; Harrington, D. A.; Kessler, J. A.; Stupp, S. I. Science 2004, 303, 1352–1355. (14) Jenekhe, S. A.; Chen, X. L. Science 1999, 283, 372–375. (15) Discher, D. E.; Eisenberg, A. Science 2002, 297, 967–973. (16) Ma, M.; Krikorian, V.; Yu, J. H.; Thomas, E. L.; Rutledge, G. C. Nano Lett. 2006, 6, 2969–2972. (17) Liu, X.; Kim, J. S.; Wu, J.; Eisenberg, A. Macromolecules 2005, 38, 6749–6751. (18) Chen, Z.; Cui, H.; Hales, K.; Li, Z.; Qi, K.; Pochan, D. J.; Wooley, K. L. J. Am. Chem. Soc. 2005, 127, 8592–8593. (19) Chowdhury, D.; Maoz, R.; Sagiv, J. Nano Lett. 2007, 7, 1770– 1778. ´ ; Colonna, B.; Echegoyen, L. Proc. Natl. Acad. Sci. (20) Herranz, M. A U.S.A. 2002, 99, 5040–4047. (21) Kim, J. H.; Rahman, M. S.; Lee, J. S.; Park, J. W. J. Am. Chem. Soc. 2007, 129, 7756–7757. (22) Dimitrova, I.; Trzebicka, B.; Mu¨ller, A. H. E.; Dworakb, A.; Tsvetanov, C. B. Prog. Polym. Sci. 2007, 32, 1275–1343. (23) Bhargava, P.; Zheng, J. X.; Li, P.; Quirk, R. P.; Harris, F. W.; Cheng, S. Z. D. Macromolecules 2006, 39, 4880–4888. (24) Agut, W.; Bruˆlet, A.; Taton, D.; Lecommandoux, S. Langmuir 2007, 23, 11526–11533. (25) Mountrichas, G.; Pispas, S. Macromolecules 2006, 39, 4767–4774. (26) Alexandridis, P.; Yang, L. Macromolecules 2000, 33, 5574–5587. (27) Antonietti, M.; Fo¨rster, S. AdV. Mater. 2003, 15, 1323–1333. (28) Groll, J.; Ademovic, Z.; Ameringer, T.; Klee, D.; Moeller, M. Biomacromolecules 2005, 6, 956–962. (29) Liu, Z.; Sun, X.; Nakayama-Ratchford, N.; Dai, H. ACS Nano 2007, 1, 50–56. (30) Jo, S.; Park, K. Biomaterials 2000, 21, 605–616. (31) Ho¨rber, J. K. H.; Miles, M. J. Science 2003, 302, 1002–1005. (32) Adler-Abramovich, L.; Reches, M.; Sedman, V. L.; Allen, S.; Tendler, S. J. B.; Gazit, E. Langmuir 2006, 22, 1313–1320. (33) Zhu, L.; Cheng, S. Z. D.; Calhoun, B. H.; Ge, Q.; Quirk, R. P.; Thomas, E. L.; Hsiao, B. S.; Yeh, F.; Lotz, B. J. Am. Chem. Soc. 2000, 122, 5957–5967. (34) Hansma, P. K.; Elings, V. B.; Marti, O.; Bracker, C. E. Science 1988, 242, 209–216. (35) Jeong, S. M.; Park, J. W. J. Am. Chem. Soc. 2008, 130, 3497– 3501. (36) Liu, X.; Zhang, Y.; Goswami, D. K.; Okasinski, J. S.; Salaita, K.; Sun, P.; Bedzyk, M. J.; Mirkin, C. A. Science 2005, 307, 1763–1766. (37) Wang, C. H.; Tsai, W. C.; Wei, K. L.; Lin, J. J. J. Phys. Chem. B 2005, 109, 13510–13514. (38) Lin, J. J.; Tsai, W. C.; Wang, C. H. Langmuir 2007, 23, 4108– 4111. (39) Wei, K. L.; Hung, F. Y.; Lin, J. J. J. Polym. Sci., Polym. Chem. 2006, 44, 646–652. (40) Lin, J. J.; Cheng, I. J.; Wang, R.; Lee, R. J. Macromolecules 2001, 34, 8832–8834. (41) Zanuy, D.; Casanovas, J.; Alema`n, C. J. Am. Chem. Soc. 2004, 126, 704–705. (42) Aucagne, V.; Leigh, D. A.; Lock, J. S.; Thomson, A. R. J. Am. Chem. Soc. 2006, 128, 1784–1785. (43) Liu, L.; Robinson, J. T.; Sun, X.; Dai, H. J. Am. Chem. Soc. 2008, 130, 10876–10877. (44) Wang, H.; Keum, J. K.; Hiltner, A.; Baer, E.; Freeman, B.; Rozanski, A.; Galeski, A. Science 2009, 323, 757–760. (45) Bhattacharjee, R. R.; Mandal, T. K. J. Colloid Interface Sci. 2007, 307, 288–295. (46) Lu, X.; Weiss, R. A. Macromolecules 1995, 28, 3022–3029. (47) Kim, J. H.; Min, B. R.; Won, J.; Joo, S. H.; Kim, S. H.; Kang, Y. S. Macromolecules 2003, 36, 6183–6188. (48) Wu, B.; Zhang, J.; Wei, Z.; Cai, S.; Liu, Z. Macromolecules 2001, 105, 5075–5078. (49) Kim, C. K.; Kim, C. K.; Lee, B. S.; Won, J.; Kim, H. S.; Kang, Y. S. J. Phys. Chem. A 2001, 105, 9024–9028.
JP902289M
