Fabrication and Characterization of a Novel Inclusion Complex of

Jun 12, 2008 - In order to develop a highly ordered polymer dopant to improve the physical properties of polymer materials for microsystems, a novel s...
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Langmuir 2008, 24, 7442-7449

Fabrication and Characterization of a Novel Inclusion Complex of Chiral Monomer Derived from (+)-Camphor with β-Cyclodextrins Jui-Hsiang Liu,*,† Hsien-Jung Hung,† and Akira Harada‡ Department of Chemical Engineering, National Cheng Kung UniVersity, Tainan 70101, Taiwan, Republic of China, and Department of Macromolecular Science, Osaka UniVersity, 560-0043, Japan ReceiVed December 25, 2007. ReVised Manuscript ReceiVed April 13, 2008 In order to develop a highly ordered polymer dopant to improve the physical properties of polymer materials for microsystems, a novel supramolecular inclusion complex (IC) of chiral bornyl 4-(6-acryloyloxyhexyloxy) phenyl4′-benzoate (BAPB) threaded with β-cyclodextrins (β-CDs) was synthesized. The inclusion complex was identified using Fourier transform infrared (FTIR), UV, 13C cross-polarization/magic-angle spinning (CP/MAS) NMR, 1H NMR, and X-ray diffraction (XRD). The construction of the fibrous self-assembled inclusion complex was confirmed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The highly ordered polymerized inclusion complex β-CD-BAPB revealed significant birefringence and was confirmed using polarized optical microscopy (POM). Polymerization of self-assembled nanofibrous monomers with methyl methacrylate was carried out, and the distribution of the nanofibers in the polymer matrix was confirmed using POM. This investigation demonstrates a novel method for the fabrication of polymeric nanofibers with highly ordered, self-assembled functional monomers. The polymeric nanofibers are expected to improve the physical properties of polymer films in the field of microelectric and micromachine systems (MEMS).

Introduction

Scheme 1

Organized molecular systems are well-known in biology and chemistry.1–4 For example, pure molecular compounds form crystals, and surface-active molecular compounds form monolayers at air-water interfaces and vesicles in water. Bilayers of liposomes mimic biological membranes, and biological membranes themselves are good examples of multimolecular organized systems.5–7 Viruses, in particular, are highly organized supramolecular assemblies whose complexity surpasses any man-made assembly.8 Another prime example of a natural organized molecular system is the DNA double helix, which is the result of the highly selective interaction of two complementary singlestrand molecules. Man-made/artificial examples of supramolecular systems include inclusion complexes of macrocyclic receptor molecules and large capping molecules that interrupt two-dimensional hydrogen-bonded networks.9 In these stateof-the-art examples, the structures of all participating molecules are highly specific. Lehn has defined supramolecular chemistry as chemistry beyond individual molecules, that is, the chemistry of intermo* To whom correspondence should be addressed. E-mail: jhliu@ mail.ncku.edu.tw. † National Cheng Kung University. ‡ Osaka University. (1) Sugawara, T.; Matsuda, T. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 137. (2) Sugiyama, K.; Hanamura, R.; Sugiyama, M. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3369. (3) Lecommandoux, S.; Klok, H. A.; Sayar, M.; Stupp, A. I. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 3501. (4) Satoh, T.; Tamaki, M.; Kitajyo, Y.; Maeda, T.; Ishihara, H.; Imai, T.; Kaga, H.; Kakuchi, T. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 406. (5) Nango, M.; Hikita, T.; Nakano, T.; Yamada, T.; Nagata, M.; Kurono, M.; Ohtsuka, T. Langmuir 1998, 14, 407. (6) Yoshida, A.; Hashizaki, K.; Yamauchi, H.; Sakai, H.; Yokoyama, S.; Abe, M. Langmuir 1999, 15, 2333. (7) Park, J. S.; Lim, Y. B.; Kwon, Y. M.; Jeong, B.; Choi, Y. H.; Kim, S. W. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 2305. (8) Huang, Y.; Chiang, C. Y.; Lee, S. K.; Gao, Y.; Hu, E. L.; Yoreo, J. D.; Belcher, A. M. Nano Lett. 2005, 5, 1429. (9) Hawthorne, M. F.; Zheng, Z. Acc. Chem. Res. 1997, 30, 267.

lecular bonds.10 Early work in supramolecular chemistry involved crown ethers and cryptands generating supramolecules based on the interaction of electron pairs and ions and with the possibility of additional ion-ion interaction.11 Cyclodextrins are cyclic polysaccharides containing naturally occurring D-(+)-glucopyranose units in an R-(1,4) linkage. Structurally, the cyclic nature of a cyclodextrin forms a torus, or donutlike shape, having an inner apolar or hydrophobic cavity. The secondary hydroxyl groups are situated on one side of the cyclodextrin torus, and the primary hydroxyl groups are situated on the other side of the torus. The side on which the secondary hydroxyl groups are located has a wider diameter than the side on which the primary hydroxyl groups are located. The (10) Lehn, J. M. Supramolecular Chemistry; Wiley-VCH: Weinheim, 1995. (11) Lehn, J. M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304.

10.1021/la7040295 CCC: $40.75  2008 American Chemical Society Published on Web 06/12/2008

Formation of β-CD-BAPB Inclusion Complexes

hydrophobic nature of the cyclodextrin inner cavity allows for the inclusion of a variety of compounds.12,13 Cyclodextrins (CDs) have been used as delivery vehicles for various therapeutic compounds by forming inclusion complexes with various drugs that can fit into the hydrophobic cavity of the cyclodextrin or by forming noncovalent association complexes with other biologically active molecules such as oligonucleotides and derivatives thereof.14,15 Various cyclodextrin-containing polymers and methods of their preparation are also reported in the literature.16–21 In a series of our previous works, we synthesized a polymerizable surfmer and used it as a surfactant to fabricate gradient refractive index plastic (GRIN) rods.22,23 We observed that the existence of a strong molecular interaction could induce the self-assembly of monomers. It is well-known that CDs form inclusion complexes (ICs) with various low molecular weight compounds by including them in cavities. ICs are crystalline adducts in which one component (the host) crystallizes into a matrix, isolating the guest component into cavities of well-defined geometries. Instead of being held together by classical chemical bonds, they are linked by weak forces. In this investigation, in order to develop a highly ordered polymer dopant to improve the physical properties of polymer materials for microelectric and micromachine systems (MEMS), fibrous self-assembled monomers threaded with β-CDs were synthesized. Identification of inclusion complex β-CD-BAPB and investigation of the highly ordered self-assembled polymer nanofibers were carried out. As far as we know, this is a novel method to improve the physical properties of nanosized films using the highly ordered polymer fibers. The results of this investigation represent significant scientific and practical contributions regarding the development of unique supramolecular polymer materials.

Langmuir, Vol. 24, No. 14, 2008 7443 Scheme 2

Experimental Section Instrumentation. Fourier transform infrared (FTIR) spectra were recorded on a Jasco VALOR III (Tokyo, Japan) FTIR spectrophotometer. 1H NMR spectra were obtained on a Bruker AMX-400 (Darmstadt, Germany) high-resolution NMR spectrometer, and chemical shifts were reported in parts per million (ppm) with tetramethylsilane (TMS) as an internal standard. Solid-state carbon nuclear magnetic resonance spectroscopy using magic-angle spinning (13C CP/MAS NMR) was performed on a Bruker Avance 400 spectrometer. Specific rotations were measured at 25 °C in dimethyl sulfoxide (DMSO) using a Jasco DIP-370 polarimeter and the D-line of sodium (λ ) 589 nm) with a precision of (0.001°. Elemental analyses were conducted with a Heraeus CHN-O (Darmstadt, Germany) rapid elemental analyzer. Differential scanning calorimetry (DSC) was conducted with a Perkin-Elmer DSC 7 instrument at a heating and cooling rate of 10 °C/min in a nitrogen atmosphere. Wide-angle X-ray diffraction (WAXD) of the CDs and complexes were recorded on a Rigaku RINT 2500 series instrument with Nifiltered Cu KR (1.5406 Å) radiation. Powder samples were mounted (12) Ni, Y.; Zheng, S. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 1247. (13) Chan, S. C.; Kuo, S. W.; She, H. S.; Lin, H. M.; Lee, H. F.; Chang, F. C. J. Polym. Sci., Part A: Polym. Chem 2007, 45, 125. (14) Chung, I.; Lee, C. K.; Ha, C. S.; Cho, W. J. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 295. (15) Uekama, K.; Hirayama, F.; Irie, T. Chem. ReV. 1998, 98, 2045. (16) Olson, K.; Chen, Y.; Baker, G. L. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 273. (17) Harada, A. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 5113. (18) Araki, J.; Ito, K. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 6312. (19) Harada, A.; Li, J.; Kamachi, M. J. Am. Chem. Soc. 1994, 116, 3192. (20) Miyauchi, M.; Harada, A. J. Am. Chem. Soc. 2004, 126, 11418. (21) Tomatsu, I.; Hashidzume, A.; Harada, A. J. Am. Chem. Soc. 2006, 128, 2226. (22) Liu, J. H.; Yang, P. C.; Chiu, Y. H. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 5933. (23) Liu, J. H.; Wu, D. S.; Tseng, K. Y. Macromol. Chem. Phys. 2004, 205, 2205.

on a sample holder and with a scanning speed of 0.5°/min between 2θ ) 8° and 30°. Scanning electron microscopy (SEM) microphotographs were measured with a JEOL HR-FESEM JSM-6700F (Osaka, Japan) instrument. The cross section morphology of the samples was characterized by transmission electron microscopy (TEM) using a JEOL JEM-1200CX-II microscope. Materials. 4-Hydroxybenzoic acid (99%), 6-chloro-1-hexanol (95%), acryloyl chloride (98%), L(-)-borneol (98%), N,N′-dicyclohexyl-carbodiimide (DCC; 99%), p-toluenesulfonic acid monohydrate (p-TSA; 97.5%), 3,4-dihydro-2H-pyran (99%), and β-cyclodextrin hydrate (99%) were purchased from Acros Chemical Co. 4-Dimethylaminopyridine (DMAP; 99%) was purchased from Lancaster Chemical Co. Isopropyl-thioxanthone, 2-benzyl-2-(dimethylamino)-1-[4-(4-morpholinyl)-phenyl]-1-butanone, and trimethylolpropane trimethacrylate were purchased from Waco Chemical Co. All organic solvents were purchased from Aldrich Chemical Co. Dichloromethane (CH2Cl2) was distilled over calcium hydride under argon immediately before use, and other solvents were purified by standard methods. Analytical thin-layer chromatography was conducted on Merck aluminum plates with 0.2 mm of silica gel 60F-254. Anhydrous sodium sulfate was used to dry all organic extracts. Synthesis of Monomers (Schemes 1–3). Synthetic routes for the target monomers are shown in Schemes 1–3. The following monomers were synthesized according to the similar procedures

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Scheme 4

mL) was added dropwise to the solution under vigorous stirring. After completing a 24 h reaction at room temperature, the solution was poured into cold water and the precipitate was filtered. The crude product was washed several times with water and recrystallized twice from EtOH. Yield: 6.3 g (68.5%). Phase transition temperature: crystal 88.0 °C nematic 92.0 °C isotropic. FTIR (cm-1): 2933, 2854 (CH2), 1733 (CdO in Ar-COO-), 1604, 1528 (C-C in Ar), 1291, 1246 (COC), 2672, 2566 (COOH), 1685 (CdC). 1H NMR (CDCl3, δ in ppm): 1.22-2.23 (m, 8H, CH2), 4.00-4.05 (t, 2H, CH2OPh), 4.10-4.20 (t, 2H, COOCH2), 5.44 (dd, 1H, CH2dCH), 5.68 (dd, 1H, CH2dCH), 5.86 (dd, 1H, CH2dCH), 6.91-6.96 (d, 2H, aromatic), 8.03-8.06 (d, 2H, aromatic).

described in the literature24–26 and in our previous reports.27–30 The obtained products were purified and then identified using FTIR and 1H NMR. 4-(6-Hydroxyhexyloxy) Benzoic Acid (1). 4-Hydroxybenzoic acid (16.5 g, 120 mmol) was dissolved in a mixture of EtOH (42 mL) and H2O (18 mL). KOH (17.8 g, 317.8 mmol) and a catalytic amount of KI dissolved in EtOH (50 mL) were added dropwise to the former solution. 6-Chloro-1-hexanol (22.5 g, 165 mmol) was then added, and the solution was heated at reflux for 24 h. The resulting mixture was poured into water and extracted with ethyl ether. The aqueous solution was acidified with HCl diluted with water until it was weakly acidic. The resulting precipitate was filtered and washed several times with water. The crude product was recrystallized from EtOH/ H2O (4/1). Yield: 19.1 g (66.9%), Tm) 139-140 °C. FTIR (cm-1): 3249 (OH), 2932, 2856 (CH2), 1685 (CdO in Ar-COO-), 1600, 1521 (C-C in Ar), 1287, 1251 (COC), 2681, 2561 (COOH). 1H NMR (DMSO-d6, δ in ppm): 1.06-1.46 (m, 8H, CH2), 3.36-3.40 (t, 2H, OCH2CH2), 3.99-4.03 (t, 2H, CH2OPh), 4.33 (s, 1H, OH), 6.97-7.87 (d, 4H, aromatic), 12.56 (s, 1H, COOH). 4-(6-Acryloyloxyhexyloxy) Benzoic Acid (2). Compound 1 (7.15 g, 31.5 mmol), N,N-dimethylaniline (4.0 g, 33.0 mmol), and a catalytic amount of 2,6-di-tert-butyl-p-cresol were dissolved in 1,4-dioxane (50 mL). The solution was cooled with an ice/salt bath, and then acryloyl chloride (9.0 mL, 99.3 mmol) dissolved in 1,4-dioxane (20 (24) Sahlen, F.; Trolls, M.; Hult, A.; Gedde, U. W. Chem. Mater. 1996, 8, 382. (25) Bobrovsky, A. Y.; Boiko, N. I.; Shibaev, V. P. Liq. Cryst. 1998, 24, 489. (26) Hattori, H.; Uryu, T. Liq. Cryst. 1999, 26, 1085. (27) Liu, J. H.; Wang, H. Y. J. Appl. Polym. Sci. 2004, 91, 789. (28) Liu, J. H.; Hung, H. J.; Wu, D. S.; Hong, S. M.; Fu, Y. G. J. Appl. Polym. Sci. 2005, 98, 88. (29) Liu, J. H.; Hsieh, C. D.; Wang, H. Y. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 1075. (30) Yang, P. C.; Chiu, Y. H.; Suda, Y.; Liu, J. H. J. Polym. Sci. Part A: Polym. Chem. 2007, 45, 2026.

Figure 1. (a) FTIR spectra of β-CD (dashed line) and β-CD-BAPB (solid line) inclusion complex and (b) comparison of FTIR spectra of (β-CD + BAPB), IC, and BAPB.

Formation of β-CD-BAPB Inclusion Complexes Ethyl 4-(Tetrahydro-2-pyranyloxy) Benzoate (3). Ethyl 4-hydroxybenzoate (25.0 g, 150.6 mmol) was dissolved in CH2Cl2 (200 mL) with a catalytic amount of p-toluenesulfonic acid monohydrate (pTSA) at 30 °C. 3,4-Dihydro-2H-pyran (37.9 g, 451.8 mmol) was added dropwise, and the solution was then stirred for 24 h. The reaction mixture was washed with 5% aqueous NaHCO3 and water, dried over MgSO4, and evaporated. The white powder was recrystallized twice from EtOH. Yield: 23.7 g (62.9%). FTIR (cm-1): 2941, 2867 (CH2), 1684 (CdO in Ar-COO-), 1601, 1498 (C-C in Ar), 1290, 1244 (COC). 1H NMR (CDCl3, δ in ppm): 1.31-1.35 (t, 3H, CH3), 1.40-1.66 (m, 6H, CH2), 3.64 (t, 2H, OCH2CH2), 4.40 (m, 2H, OCH2CH3), 5.48 (t, 1H, OCHCH2), 7.05-7.97 (d, 4H, aromatic). 4-(Tetrahydro-2-pyranyloxy) Benzoic Acid (4). Compound 3 (23.0 g, 92.0 mmol) was dissolved in CH2Cl2 (60 mL). KOH (15.4 g, 276.0 mmol) dissolved in EtOH/water (5/1, 100 mL) was then added and heated at reflux for 24 h. After reaction, the solvent was removed. The residue mixture was poured into water, and the solution was neutralized with diluted HCl to pH ) 5. The resulting precipitate was filtered and washed with water for several times. It was then recrystallized twice from EtOH/H2O (6/1). Yield: 15.1 g (73.9%), Tm) 199-200 °C. FTIR (cm-1): 2945, 2873 (CH2), 1674 (CdO in Ar-COO-), 1608, 1505 (C-C in Ar), 1278, 1244 (COC), 2663, 2540 (COOH). 1H NMR (acetone-d6, δ in ppm): 1.65-1.83 (m, 6H, CH2), 3.59 (t, 2H, OCH2CH2), 5.56 (t, 1H, OCHCH2), 7.11-7.97 (d, 4H, aromatic). Bornyl 4-(Tetrahydro-2-pyranyloxy) Benzoate (5). Compound 4 (14.5 g, 65.3 mmol) and (-)-borneol (10.2 g, 65.3 mmol) were dissolved in CH2Cl2 (100 mL) at 30 °C. N,N′-Dicyclohexylcarbodiimide (DCC) (40.7 g, 195.9 mmol) and 4-dimethylaminopyridine (DMAP) (0.7 g, 6.5 mmol) dissolved in CH2Cl2 (50 mL) were then added to the former solution and stirred for 2 days at 30 °C. The resulting solution was filtered, dried over MgSO4, and evaporated. The crude product was purified by column chromatography (silica gel, ethyl acetate/hexane ) 1/6) and recrystallized twice from EtOH. Yield: 10.9 g (46.6%). FTIR (cm-1): 2950, 2873 (CH2), 1710 (CdO in Ar-COO-), 1608, 1510 (C-C in Ar), 1275, 1244 (COC). 1H NMR (CDCl3, δ in ppm): 0.81-0.86 (m, 9H, CH3), 1.12-2.45 (m, 12H, CH2), 3.61 (t, 2H, OCH2CH2), 4.09 (m, 1H, CHCH2), 5.13 (t, 2H, OCHCH2), 7.07-8.00 (d, 4H, aromatic). Bornyl 4-Hydroxybenzoate (6). Compound 5 (10.2 g, 28.4 mmol) was dissolved in a mixture of CH2Cl2/MeOH (1/1, 60 mL) with a catalytic amount of p-toluenesulfonic acid monohydrate (p-TSA). The mixture was stirred for 12 h at 50 °C, and the solvent was then evaporated. The crude product was purified by column chromatography (silica gel, CH2Cl2) and recrystallized twice from EtOH. Yield: 5.9 g (75.8%), Tm) 134-135 °C. FTIR (cm-1): 3324 (OH), 2960, 2878 (CH2), 1679 (CdO in Ar-COO-), 1613, 1510 (C-C in Ar), 1229, 1280 (COC). 1H NMR (acetone-d6, δ in ppm): 0.81-0.86 (m, 9H, CH3), 1.31-2.45 (m, 7H, CH + CH2), 5.12 (t, 1H, OCHCH2), 6.90-7.90 (d, 4H, aromatic), 10.5 (s, 1H, OH). Bornyl 4-(6-Acryloyloxyhexyloxy) Phenyl-4′-benzoate (BAPB) (7). Compound 2 (8.5 g, 29.1 mmol) and compound 6 (7.9 g, 29.0 mmol) were dissolved in CH2Cl2 (100 mL) at 30 °C. N,N′Dicyclohexyl-carbodiimide (DCC) (18.0 g, 87.0 mmol) and 4-dimethyl-aminopyridine (DMAP) (0.34 g, 2.9 mmol) were dissolved in CH2Cl2 (50 mL) and then added to the former solution. The reaction mixture was stirred at 30 °C for 48 h. The resulting solution was filtered and washed with water, dried over MgSO4 and then evaporated. The crude product was purified by column chromatography (silica gel, ethyl acetate/hexane)1/8) and recrystallized twice from EtOH. Yield: 6.1 g (38.4%), Tm) 55-56 °C. [R]D) -18.8. FTIR (cm-1): 2945, 2873 (CH2), 1741 (CdO in Ar-COO-), 1608, 1505 (C-C in Ar), 1198, 1244 (COC), 1633 (CdC). 1H NMR (CDCl3, δ in ppm): 0.82-0.87 (m, 9H, CH3), 1.26-2.48 (m, 15H, CH + CH2), 4.05-4.09 (t, 2H, OCH2), 4.17-4.21 (t, 2H, OCH2), 5.11-5.19 (t, 1H, OCHCH2), 5.82 (dd, 1H, CH2dCH), 6.14 (dd, 1H, CH2dCH), 6.43 (dd, 1H, CH2dCH), 6.97-7.01 (d, 2H, aromatic), 7.27-7.30 (d, 2H, aromatic), 8.12-8.17 (d, 4H, aromatic). Anal. Calcd for C33H40O7 (548): C, 72.26; H, 7.30. Found: C, 72.31; H, 7.28.

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Figure 2. UV-vis spectra of BAPB with β-CD in DMSO at [BAPB] ) 1 mM, [β-CD]/[BAPB] ) 0-13.0.

Figure 3. 1H NMR spectrum of the β-CD-BAPB inclusion complex. Scheme 5

Preparation of β-CD-BAPB Inclusion Complex. β-CD (0.925 g, 0.8 mmol) was dissolved in 40 mL of H2O at 60 °C, and BAPB (0.50 g, 0.9 mmol) was then added into the aqueous solution with stirring at 60 °C for 24 h. The turbid solution was ultrasonically agitated at room temperature for 15 min, and then the clear solution was allowed to stand at room temperature for 2 days. The precipitate was collected, washed with 5 mL of THF three times and then with 10 mL of water three times to remove the residual BAPB and CD, and dried at 60 °C under vacuum. The inclusion complex of β-CDBAPB was obtained as a white powder with a yield of around 70%. The pure β-CD-BAPB inclusion complex without free β-CD and BAPB monomers was conformed using DSC, WAXD, and FTIR analytical methods. Polymerization of β-CD-BAPB Inclusion Complex. Prepared β-CD-BAPB inclusion complex (0.5 g) in 5 mL of methyl ethyl ketone (MEK) was dropped on a substrate and then dried. An equal molar mixture of isopropylthioxanthone and 2-benzyl-2-(dimethylamino)-1-[4-(4-morpho-linyl)-phenyl]-1-butanone was used as a photoinitiator. A mixture of trimethylolpropane trimethacrylate (0.1 g) and photoinitiator (0.1 g) in 5 mL of MEK was dropped on the inclusion complex and then dried under vacuum. The dried substrate

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was exposed to UV light under nitrogen atmosphere. Absorption of vinyl groups around 1640 cm-1 decreased obviously after UV exposure. After exposure, the sample was analyzed via polarized optical microscopy (POM). Fabrication of Self-Assembled β-CD-BAPB Inclusion Complex Reinforced Films. Syhthesized β-CD-BAPB ICs were mixed with 75 wt % methyl methacrylate (MMA), 20 wt % trimethylolpropane trimethacrylate, and 5 wt % photoinitiator of an equal molar mixture of isopropylthioxanthone and 2-benzyl-2-(dimethylamino)-1-[4-(4morpholinyl)phenyl]-1-butanone, and a proper amount of MEK was added to adjust the viscosity. The mixture was coated on a substrate, and after removing the MEK solvent the substrate was exposed to UV light under a nitrogen atmosphere. After exposure, a film sample was prepared and was analyzed via polarized optical microscopy (POM). Preparation of β-CD-BAPB Samples for TEM, SEM, and POM Analyses. In order to study the cross section of the selfassembled β-CD-BAPB and to prepare TEM samples, a few drops of β-CD-BAPB ICs in MEK were dropped on a cured epoxy substrate. After drying, another part of epoxy resin was coated on the IC sample and then was cured. The fixed β-CD-BAPB IC sample was then cut into thin films with a microtome for TEM analysis. Furthermore, for SEM and POM analysis, a few drops of β-CDBAPB ICs in MEK were dropped on a substrate. After drying, the samples were analyzed using SEM and POM instruments. For regular TEM analysis, a Cu-grid was used as a substrate.

Results and Discussion Preparation of β-CD-BAPB Inclusion Complex (Scheme 4). Schemes 1–3 show the synthesis of chiral monomer BAPB containing a bornyl group. The synthesized products were identified by 1H NMR, 13C NMR, UV, X-ray diffraction (XRD), and FTIR spectroscopy. The molecular length of BAPB is approximately 4 nm, and the depth of the β-CD molecule was 0.8 nm. Theoretically, as seen in Scheme 4, five β-CD molecules could be threaded with one BAPB group. The β-CD-BAPB inclusion complex was prepared by an ultrasonically agitated chiral monomer BAPB in β-CD aqueous solution at room temperature. After ultrasonic agitation and 2 days of stagnation, a precipitate of the β-CD-BAPB inclusion complex was obtained. The results suggest that the molecular polarity and interaction between the host and guest molecules are important factors for the formation of inclusion complexes. For BAPB, the sterically hindered bornyl group may block one of the terminals of the BAPB molecule leading to the formation of the inclusion complex. Figure 5. Wide-angle X-ray diffraction patterns of (a) β-CD, (b) β-CDBAPB inclusion complex, and (c) BAPB.

Figure 6. DSC curves of β-CD-BAPB IC, BAPB guest compound, and (β-CD + BAPB) physical mixture (10 °C/min). Figure 4. 13C CP/MAS NMR spectra of β-CD and β-CD-BAPB inclusion complex.

Identification of Inclusion Complex β-CD-BAPB. FTIR is a simple way to confirm the presence of both the host and guest

Formation of β-CD-BAPB Inclusion Complexes

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Figure 8. POM textures of a (a) polymerized self-assembled inclusion complex and (b) composite PMMA film blended with self-assembled inclusion complexes. Figure 7. (a) SEM image of β-CD-BAPB inclusion complex and (b) TEM image of a cross section of the columnar β-CD-BAPB inclusion complex.

components in the inclusion complex β-CD-BAPB and may give some information about the binding mode of the inclusion complexes.31–33 In Figure 1a, the dashed line and solid line show the spectra of β-CD and β-CD-BAPB, respectively. Absorption peaks due to functional groups were identified, and they are noted in Figure 1a. As seen in Figure 1a, the spectrum of β-CD shows a broad band at 3215-3385 cm-1 due to the symmetric and antisymmetric O-H stretching mode, and three other intense bands at 1635 cm-1 (C-O-C glycosidic bridge) coupled with peaks at 1040 (C-C) and 1019 (C-O) cm-1. Positions and relative intensities of a few bands due to both the host and the guests are affected by the formation of the inclusion complex. The most distinctive bands in the β-CD-BAPB inclusion complex spectra appear in the 1700-1800 cm-1 region, and they are assigned to the CdO stretching bands for BAPB. A new band appearing at approximately 1750 cm-1 in the β-CD-BAPB spectrum is absent in the pure CD spectrum. Furthermore, the multipeaked band around 1750 cm-1 is due to the carbonyl groups

in monomeric BAPB. The FTIR spectra in Figure 1 show that β-CD-BAPB inclusion complexes have been prepared successfully through β-CD threading onto the BAPB chain as shown in Scheme 4. Furthermore, the existence and the purity of inclusion complexes were confirmed using a spectrophotometer and the dependence of the UV-vis spectra on the molar ratio of β-CD and BAPB. These results are summarized in Figure 2, where a hyperchromic effect can be observed. Increasing the molar ratio of β-CD and BAPB increases the intensity of absorbance around 270 nm and shows a blue shift. The results suggest that β-CDBAPB inclusion complexes were likely formed.34 The IC samples were dissolved in DMSO, and an equilibrium between free CD and included CD is expected. Consequently, it is hard to find spectral differences between free CD and ICs when using UV-vis spectrometry. (31) Dai, X. H.; Dong, C. M.; Fa, H. B.; Yan, D.; Wei, Y. Biomacromolecules 2006, 7, 3527. (32) Rusa, C. C.; Luca, C.; Tonelli, A. E. Macromolecules 2001, 34, 1318. (33) Huang, L.; Allen, E.; Tonelli, A. E. Polymer 1999, 40, 3211. (34) Goto, H.; Furusho, Y.; Yashima, E. J. Am. Chem. Soc. 2007, 129, 109.

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Figure 9. Schematic of the formation of the β-CD-BAPB inclusion complex: (a) β-CD-BAPB inclusion complex, (b) model of the inclusion complexes, and (c) formation of the self-assembled fibrous inclusion complex.

Precipitates of the inclusion complex of β-CD-BAPB were found to dissolve in DMSO. Figure 3 shows the 1H NMR spectrum of the inclusion complex of BAPB with β-CD(s) in DMSO-d6. In the 1H NMR spectrum, the peaks were identified and were found to belong to both β-CD and BAPB molecules. The results suggest that the precipitate obtained from ultrasonically treated β-CD-BAPB may be the inclusion complex. Furthermore, the integrated areas at 4.81 ppm (C1H protons of β-CD) and 5.03 ppm (Hi proton of BAPB) are 9.04 and 0.44, respectively. Given that a molecule of β-CD theoretically contains seven C1-H protons and a molecule of BAPB contains one Hi proton, the number of threaded CD onto BAPB is estimated to be 2.9.35 These results suggest that BAPB is threaded within three β-CDs, which formed inclusion complexes. Unlike in Scheme 4, the actual image of the inclusion complex of β-CD-BAPB might also be expressed as the model shown in Scheme 5. The terminal acryloyloxy and chiral bornyl groups may exist without the outer β-CD cages.

Liu et al.

The 13C CP/MAS NMR spectra of β-CD and β-CD-BAPB are shown in Figure 4. The bands observed in the 10-120 ppm region for β-CD and β-CD-BAPB are different. The host β-CD within β-CD-BAPB had fewer splitting resonance peaks compared with the multiple resonance peaks of free β-CD. β-CD within β-CD-BAPB adopted a more symmetrical cyclic conformation, while β-CD in its pure crystal had a less symmetrical conformation. This suggests that β-CD-BAPB formed through β-CD threading onto BAPB.36,37 WAXD is a useful method to elucidate the structure of these inclusion complexes in the solid state. The wide-angle X-ray diffraction patterns of β-CD, β-CD-BAPB, and BAPB are shown in Figure 5. As compared with peaks in Figure 5a, new strong diffraction peaks at 11.5°, 17.2°, and 17.8° were observed in Figure 5b. The diffractogram of β-CD-BAPB showed a diffraction pattern different from those of β-CD and BAPB. This constitutes primary evidence that a different crystal type was formed. The results also reveal that the β-CD-BAPB inclusion complex contained negligible free BAPB guest monomers and β-CD molecules. According to the powder X-ray diffraction patterns of β-CD-BAPB, the formation of a channel-type structure was suggested because the reflection peaks are similar to those of reported channel-type inclusion complexes.31,33,38–40 The melting and crystallization behavior of β-CD, free BAPB monomer, and the β-CD-BAPB inclusion complex were investigated using DSC, as presented in Figure 6. The maximal melting peaks were observed for free BAPB at 68.0 °C and for a (β-CD + BAPB) physical mixture in the heating run at 62.0 °C. No significant exothermic peak was obtained for BAPB monomers in the cooling cycle. In the case of a (β-CD + BAPB) mixture, only a small endothermic peak could be seen. This result may be due to a structural disturbance between β-CD and BAPB. Moreover, no melting peak was observed for β-CDBAPB in the heating run. For the cooling cycle, no significant peak was obtained. These observations indicate that the original crystallization of BAPB within the IC was completely suppressed in the β-CD cavities and that the β-CD-BAPB inclusion complex contained negligible free BAPB guest monomers.33 Parts a and b of Figure 7 show the SEM image and the TEM cross-sectional image, respectively, of the β-CD-BAPB inclusion complex. The appearance of the aggregated inclusion complex was estimated as a self-assembled columnar construction. Figure 7a shows the developed columnar construction, in which the outer surface appears relatively smooth and the column diameter is 451 nm. To further examine its structure, the self-assembled columnar construction was fixed with an epoxy matrix and then cut with a microtome. The TEM image of the cross section of the column is shown in Figure 7. A bundle of nanosized fibers constructed the column. The deformation of the circular structure is due to the stress from curing the epoxy matrix. Polymerization of Self-Assembled ICs. The acryl groups are known to undergo rapid polymerization upon irradiation even with low-intensity UV light in the presence of a photoinitiator.41 (35) Liu, Y.; Yang, Y. W.; Chen, Y.; Zou, H. X. Macromolecules 2005, 38, 5838. (36) Okumura, H.; Okada, M.; Kawaguvhi, Y.; Harada, A. Macromolecules 2000, 33, 4297. (37) Huh, K. M.; Ooya, T.; Sasaki, S.; Yui, N. Macromolecules 2001, 34, 2402. (38) Liu, Y.; Chen, G. S.; Chen, Y.; Zhang, N.; Chen, J.; Zhao, Y. L. Nano Lett. 2006, 6, 2196. (39) Okumura, H.; Kawaguvhi, Y.; Harada, A. Macromolecules 2001, 34, 6338. (40) Li, J.; Yan, D. Macromolecules 2001, 34, 1542. (41) Kang, S. H.; Jang, K. S.; Theato, P.; Zentel, R.; Chang, J. Y. Macromolecules 2007, 40, 8349.

Formation of β-CD-BAPB Inclusion Complexes

ICs of self-assembled monomers threaded with CDs were polymerized under UV irradiation. In order to promote the efficiency of the polymerization of ICs, a small amount of trimethylolpropane trimethacrylate was added as a cross-linking agent. Figure 8a shows the birefringent properties of polymeric nanofibers observed via POM. The bright fibrous pattern shows the highly ordered ICs. The birefringent character might be due to the highly ordered ICs. The polymerization of the selfassembled IC monomers was identified using FTIR. Unsaturated CdC absorption around 1640 cm-1 decreased with an increase in UV irradiation. After polymerization, the highly ordered arrangement of the self-assembled ICs was kept. As shown in Figure 8a, a bright birefrigent texture under crossed POM could be seen. Moreover, the synthesized β-CD-BAPB ICs were further copolymerized with comonomers. Figure 8b shows the POM texture of the synthesized composite film with self-assembled ICs. The polymerization of the monomers was tracked using FTIR. The absorption of vinyl groups around 1640 cm-1 decreased obviously during UV exposure. The formation of polymer films gives further evidence of the occurrence of polymerization. After polymerization, the self-assembled β-CD-BAPB ICs are fixed in the PMMA matrix. Theoretically, the synthesized film is a kind of nanofiber reinforced film. The synthesized nanofibers are expected to improve the physical properties of polymer films in the field of microelectric and micromachine systems (MEMS), although the physical properties of the films were not investigated. The degree of polymerization and the molecular weight of the polymerized self-assembled β-CD-BAPB ICs were not studied. According to the above results, a reasonable schematic representation of the formation of the β-CD-BAPB inclusion complex was proposed. Figure 9 shows that, due to the selfassembly interaction between the terminal acryloyloxy, bornyl groups, and hydroxyl groups on β-CD, the β-CD-BAPB inclusion complex aggregated and formed a columnar structure. Polar-polar interactions between the carbonyl groups, π-π interactions

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between the vinyl groups, and hydrogen bonding between the OH groups on β-CD were considered to be the main factors drawing the molecules together and leading to the formation of the columnar architecture. As shown in Figure 9, the fibrous construction consists of highly ordered functional monomers. Significantly, these structures could potentially be used as dopants to synthesize polymer films and other plastic devices. This investigation demonstrates an easy method to fabricate selfassembled fibrous constructions with high-ordered functional monomers, which is expected to promote the physical properties of the polymer films. The results of this investigation present significant scientific and practical contributions regarding the development of unique supramolecular polymer materials.

Conclusion A novel method for the preparation of polymerizable selfassembled fibrous constructions was achieved by the addition of β-CD into BAPB monomers. Based on SEM, TEM, XRD, and POM analyses of the fibrous architecture, a reasonable schematic representation for the formation of self-assembled β-CD-BAPB inclusion complexes was proposed. Highly ordered fillers in polymer matrices usually increase the physical properties of films. The polymerizable self-assembled fibrous monomer clusters fabricated in this investigation are expected to be used as fillers to promote the physical properties of polymer films and devices. Acknowledgment. The authors would like to thank the National Science Council (NSC) of the Republic of China (Taiwan) for financially supporting this research under Contract No. 96-2221-E006-009. Supporting Information Available: Figures showing FTIR spectra of β-CD, β-CD-BAPB inclusion complex, and β-CD+BAPB physical mixture; FTIR spectra of IC and β-CD+BAPBphysical mixture; and FTIR spectra of compounds used in this investigation. This material is available free of charge via the Internet at http://pubs.acs.org. LA7040295