Intramolecular Hydrophobic Aggregation of Amphiphilic

Jun 23, 1999 - The surface tension of poly(DMAPAMAIPS) was shown to be larger than that of both the copolymer [poly(DMAPAMAIPS/NaMAI)] with an aromati...
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Articles Intramolecular Hydrophobic Aggregation of Amphiphilic Polysulfobetaine with Various Hydrophobic Groups in Aqueous Solution Der-Jang Liaw,*,† Ching-Cheng Huang,† Hui-Chuan Sang,† and En-Tang Kang‡ Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan 106, Republic of China, and Department of Chemical Engineering, National University of Singapore, Kent Ridge, Singapore 119260 Received June 22, 1998. In Final Form: April 5, 1999

The copolymers derived from zwitterionic DMAPAMAIPS and hydrophobic monomer (AdMAI or NaMAI) were studied in terms of the surface tension, viscometrics, and photophysical measurements. The surface tension of poly(DMAPAMAIPS) was shown to be larger than that of both the copolymer [poly(DMAPAMAIPS/ NaMAI)] with an aromatic hydrophobic residue (naphthalene group) and [poly(DMAPAMAIPS/AdMAI)] with an aliphatic hydrophobic residue (adamantane group). The copolymer [poly(DMAPAMAIPS/NaMAI)] has a larger surface activity than that of [poly(DMAPAMAIPS/AdMAI)]. The reduced viscosity of the copolymers revealed that both zwitterionic and hydrophobic associations were important in polymer aggregation. The aromatic hydrophobic units of the copolymer [poly(DMAPAMAIPS/NaMAI)] were more strongly associated than the units of aliphatic hydrophobic copolymer [poly(DMAPAMAIPS/AdMAI)] due to the stronger hydrophobic aggregation of naphthyl groups. The fluorescence experiment showed that when the hydrophobic adamantyl group was incorporated into the polymer, the environmental polarity of the pyrene residue increases. Coil expansion was due to weaker hydrophobic associations of the adamantane moieties.

Introduction In recent years, there has been a growing interest in the self-organization phenomena of amphiphilic polymers in aqueous solution because of these polymers’ widespread technological applications in water treatment, enhanced oil recovery, paints, coatings, surface modification, drugs, and personal care products.1-8 More recently, much research has also focused on the fluorescently labeled water-soluble polymers to acquire this microscopic, photophysical response, and then comparing this to macroscopic events such as aggregations, phase separation, or latex film formation.9-16 The fluorescent and dynamic * To whom correspondence should be addressed. Tel.: +886 2 27335050 or +886 2 27376638. Fax: +886 2 27376644 or +886 2 23781441. E-mail: [email protected]. † National Taiwan University of Science and Technology. ‡ National University of Singapore. (1) Ezzell, S. A.; Hoyle, C. E.; Creed, D.; McCormick, C. L.; Macromolecules 1992, 25, 1887. (2) Hughes, L. E. U.S. Patent 2,694,688, 1954. (3) Spriestersbach, D. R.; Clarke, R. A.; Couper, M.; Patterson, H. T. U.S. Patent 3,473,998, 1966. (4) Mizuguchi, R.; Ishikura, S.; Takahashi, A.; Uenaka, A. U.S. Patent 4,215,028, 1980. (5) Samour, C. M.; Falxa, M. L. U.S. Patent 3,671,502, 1972. (6) Ishikura, S.; Mizuguchi, R.; Takashashi, A. Jpn Kokai 80,386 and 80,387, 1977. (7) Bahr, U.; Wieden, H.; Rinkler, H. A.; Nischk, G. E. Makromol. Chem. 1972, 161, 1. (8) Kang, E. T.; Neoh, K. G.; Chen, W.; Tan, K. L.; Liaw, D. J.; Huang, C. C. J. Adhesion Sci. Technol. 1996, 10, 725. (9) Itoh, Y.; Abe, K.; Senoh, S. Makromol. Chem. 1986, 187, 1961. (10) Winnik, F. M. Polymer 1990, 31, 2125.

properties of the polymer modulate the photochemical process. The microenvironmental effects of the different side groups modulate the polymer macroscopic and microscopic properties. On an amphiphilic polymer chain, attractive interactions among hydrophobic residues compete with electrostatic interactions among charged segments. There are different electrostatic interactions in various amphiphilic polymer solutions. The amphiphilic polymer with cationic or anionic group shows an electrostatic repulsion. However, the amphiphilic polymer with zwitterionic group shows an electrostatic association. The self-organization of hydrophobic residues form micelle-like microphobic residues, thus forming a micelle-like microphase structure can occur only when the hydrophobic interactions are strong enough to prevail over zwitterionic interactions. When fluorescent hydrophobes are incorporated into an amphiphilic polymer, the photophysical response may effectively probe the self-organization of hydrophobic residues on the microscopic level.17-19 (11) Ringsdorf, H.; Simon, J.; Winnik, F. M. Macromolecules 1992, 25, 5353. (12) Winnik, F. M. Macromolecules 1989, 22, 734. (13) Zhao, C.; Wang, Y.; Hruska, Z.; Winnik, F. M. Macromolecules 1990, 31, 2125. (14) Laschewsky, A. Polysoaps/Stabi1izers Nitrogen-15 NMR; Advances in Polymer Science 124; Springer-Verlag: Berlin, 1995. (15) Salamone, J. Encyclopedia of Polymer Science; Interscience Publishers: New York, 1964; Vol. 11, pp 514-530. (16) McCormick, C.; Hoyle, C.; Clark, M. Macromolecules 1990, 23, 3124. (17) Morishima, Y. ACS Symp. Ser. 1994, 598.

10.1021/la980728h CCC: $18.00 © 1999 American Chemical Society Published on Web 06/23/1999

Intramolecular Hydrophobic Aggregation of Amphiphilic Polysulfobetaine

Previous works have examined a series of poly(betaine)s, and their corresponding cationic polyelectrolytes with different electron-withdrawing groups and varied methylene units between the charged groups.20 Previous reports have also thoroughly examined the difference in dilute aqueous solution properties such as viscosity, cloud point, and degree of binding.21-27 These properties provide a reasonable assessment of the macroscopic behavior; however, a detailed analysis requires using a more sensitive characterization. When naphthalene labels were incorporated into the acrylamide-containing water-soluble polymer, photophysical properties and the behavior of polymers in aqueous solution could be clearly defined.26,27 Moreover, in recent years attention has focused on aqueous solution properties, photophysics, photochemistry, and the applications of the water-soluble copolymers containing hydrophobic residues and charged units.17-19 Zwitterionic group incorporation would affect solution properties of copolymer. The solution properties can become dependent on pH and/or electrolyte concentration. Moreover, if the zwitterionic group is farther from the copolymer backbone, this appears to affect the nature of the hydrophobic associations.26-29 If a hydrophobic group and a zwitterionic group are incorporated into the same vinyl monomer unit, water solubility and surface activity can result.28,29 In this studies, we prepared copolymers of new watersoluble monomer methacryloyl isocyanate containing N,Ndimethylpropanediamine quaterinized by propanesultone (DMAPAMAIPS). The present paper describes the hydrophobic aggregation behavior of amphiphilic polymer in aqueous solution with an emphasis on the qualitative comparison between the various types of hydrophobic groups in the copolymers. The synthesis, viscometrics, surface tension, and fluorescence measurement of the amphiphilic polymer with various hydrophobic groups are reported. Experimental Section Materials. Methacryloyl isocyanate (MAI) (Nippon Paint Co.) was prepared from methacrylamide and oxalyl chloride as reported in previous literature30 and purified by distillation at reduced pressure (52-53 °C/39 mmHg). The N,N-dimethylaminopropylamine, 1-adamantol, and 1-naphthanol materials were purchased from Acros. The 1-pyrenemethanol was used directly as received from Aldrich. The 1,3-propanesultone was purchased from TCI and distilled before using. The solvents were purified according to standard methods. Preparation of Monomers. Methacryloyl Isocyanate Containing N,N-Dimethylaminopropylamine (DMAPAMAI). In a 100 mL flask equipped with a stirrer and thermometer, (18) Morishima, Y.; Itoh, Y.; Nozakura, S. Makromol. Chem. 1981, 182, 3135. (19) Morishima, Y. Trends Polym. Sci. 1994, 2, 31. (20) Liaw, D. J.; Huang, C. C.; Lee, W. F.; Borb’ely, J.; Kang, E. T. J. Polym. Sci. Chem. Ed. 1997, 35, 3527. (21) Liaw, D. J.; Huang, C. C.; Chou, Y. P. Eur. Polym. J. 1997, 33, 829. (22) Liaw, D. J.; Huang, C. C. Polym. Int. 1996, 41, 267. (23) Liaw, D. J.; Huang, C. C. Polymer 1997, 38, 6401. (24) Liaw, D. J.; Huang, C. C.; Kang, E. T. J. Polym. Sci. Phys. Ed. 1998, 36, 11. (25) Liaw, D. J.; Huang, C. C.; Kang, E. T. Colloid Polym. Sci. 1997, 275, 922. (26) Liaw, D. J.; Huang, C. C.; Sang, H. C.; Kang, E. T. Langmuir 1998, 14, 3195. (27) Liaw, D. J.; Huang, C. C.; Sang, H. C.; Kang, E. T. Submitted for publication in Langmuir. (28) Kramer, M. C.; Welch, C. G.; Steger, J. R.; McCormick, C. L. Macromolecules 1995, 28, 5248. (29) Koberle, P.; Laschewsky, A.; van den Boogaard, D. Polymer 1992, 33, 4029. (30) Urano, S.; Mizuguchi, R. Eur. Pat. Appl. 0,143,613; Chem. Abstr. 1984, 102, 124076u.

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N,N-dimethylaminopropylamine (DMAPA) (3.06 g, 0.03 mol) with dried THF (20 mL) were charged under argon atmosphere and the contents were stirred below 5 °C. A mixture of methacryloyl isocyanate (MAI) (3.33 g, 0.03 mol) and dry THF was added dropwise for 1 h. Following the addition, the mixture was stirred for 8 h at the same temperature. The solution was then concentrated under reduced pressure to remove THF and unreacted DMAPA. Diethyl ether was added to precipitate the undesired product. After filtration, the solution was condensed to remove diethyl ether. The structure of DMAPAMAI was identified by IR, 1H NMR, and 13C NMR. IR (KBr) ν/cm-1: νN-H (3200), νCdO (1677, 1709), νCdC (1623). 1H NMR (CDCl3): δ (ppm) 1.95 (3H, CH3), 5.5, 5.9 (2H, CH2dC), 8.6, 8.9 (1H, NH), 2.2 (6H, N(CH3)2), 3.3 (2H, -NH-CH2-CH2-CH2-N(CH3)2), 1.68 (2H, -NH-CH2-CH2-CH2-N(CH3)2), 2.3 (2H, -NH-CH2-CH2CH2-N(CH3)2) (Figure 1A). 13C NMR (CDCl3): δ (ppm) 18.5 (CH3), 119.3 (CH2dC), 140.2 (CH2dC), 45.4 (-N(CH3)2), 39.8 (-NHCH2-CH2-CH2-N(CH3)2), 25.8 (-NH-CH2-CH2-CH2N(CH3)2), 59.0 (-NH-CH2-CH2-CH2-N(CH3)2). Elem. Anal. Calcd: C, 56.34; H, 8.92; N, 19.72. Found: C, 56.46; H, 8.79; N, 19.87. UV: max ) 9763 cm-1 L mol-1, λmax ) 235.7 nm.

Methacryloyl Isocyanate Containing N,N-Dimethylamino Propylamine Quaternized by Propanesultone (DMAPAMAIPS). In a 100 mL flask, DMAPAMAI (6.39 g, 0.03 mol) was added with anhydrous acetone under argon and stirred at 0 °C. The 1,3-propanesultone (4.03 g, 0.033 mol) was added for 2 h. Following the reaction, the monomer (DMAPAMAIPS) was precipitated and accumulated from acetone. DMAPAMAIPS was washed by acetone and dried under vacuum at 30 °C. Yield: 86% (9 g). IR (KBr) ν/cm-1: νN-H (3200), νCdO (1677, 1709), νCdC (1623). 1H NMR (D2O): δ (ppm) 1.9 (3H, CH3), 5.3, 5.8 (2H, CH2d C), 8.1, (1H, NH) (Figure 1B). Elem. Anal. (C13H25N3O5S). Calcd: C, 42.98; H, 7.46; N, 12.54. Found: C, 42.97; H, 7.47; N, 11.56.

Methacryloyl Isocyanate Containing Adamantanol (AdMAI). The AdMAI monomer was prepared as described in the procedure. First, 1-adamantanol (15.2 g, 0.1 mol) and ethyl acetate (50 mL) were charged into a flask (150 mL). The mixture was stirred at room temperature under argon, while MAI (12.1 g, 0.11 mol) was added by dropping funnel over a period of 30 min. After completion of the addition, the mixture was stirred at room temperature for 4 h. The product was purified by recrystallization from ethyl acetate and dried under vacuum at room temperature. The structure was confirmed by IR, NMR spectra, and elemental analysis. The 1H NMR spectra of AdMAI monomer is shown in Figure 2A. The monomer was soluble in methanol, ethyl ether, chloroform, benzene, toluene, pyridine, 1,4-dioxane, tetrahydrofuran, N,N-dimethylformamide, dimethyl sulfoxide but insoluble

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Figure 1. 1H NMR spectra of monomers (A) DMAPAMAI in CDCl3 solvent and (B) DMAPAMAIPS in D2O solvent. in n-hexane. IR (KBr) ν/cm-1: νN-H (3256), νCdO (1739, 1675), νCdC (1629). 1H NMR (DMSO-d6): δ (ppm) 1.83 (3H, CH3), 5.53, 5.77 (2H, CH2dC), 10.2 (1H, NH), 1.62 (6H, CH2), 2.14 (3H,CH), 2.08 (6H, CH2). 13C NMR (DMSO-d6): δ (ppm) 167.66, 150.24, 139.57,122.75,80.79,41.37,36.10,30.74,18.85.Elem.Anal.(C15H21O3N). Calcd: C, 68.42; H, 8.04, N, 5.32. Found: C, 68.32; H, 8.03; N, 5.37.

Methacryloyl Isocyanate Containing Naphthanol (NaMAI). The synthetic method of NaMAI was the same as AdMAI.

The 1H NMR spectra of NaMAI monomer is shown in Figure 2B. IR (KBr) ν/cm-1: νN-H (3210), νCdO (1705), νCdC (1622). 1H NMR:

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Figure 2. 1H NMR spectra of monomers (A) AdMAI (DMSOd6) and (B) NaMAI (DMSO-d6). δ (ppm) 1.8 (3H, CH3), 5.3, 5.7 (2H, CH2dC), 10.1 (1H, NH), 6.8-8.2 (7H, aromatic) (Figure 2B). Elem. Anal. (C15H13O3N). Calcd: C, 70.59; H, 5.10; N, 5.49. Found: C, 70.44; H, 5.22; N, 5.53. Synthesis of Polymers. Poly(DMAPAMAIPS). The polymer was prepared by free radical polymerization initiated by 4,4′-azobis-4-cyanovaleric acid (ACVA), with the following procedure. A glass ampule containing DMAPAMAIPS (3.35 g, 0.01 mol) and ACVA (0.028 g, 10-4 mol) in 50 mL of H2O was degassed by three freeze-pump-thaw cycles on a vacuum line. The sealed ampule was maintained at 60 °C for 24 h. After polymerization, the mixture was poured into a large excess of acetone to precipitate the resulting polymers. The polymers were purified by repreciptation from H2O into acetone and dried under vacuum at 60 °C with 65% yield. The intrinsic viscosity was calculated to be 0.95 dL/g in 1.5 M NaCl solution with an Ubbelohde viscometer at 30 °C. The molecular weight was determined by light-scattering measurement (Otsuka DLS-7000), Mw ) 6.50 × 105. Poly(DMAPAMAIPS/AdMAI) and Poly(DMAPAMAIPS/ NaMAI). The polymers were prepared by free radical polymerization initiated by 2,2′-azobisisobutyronitrile (AIBN), with the procedure for the copolymer as follows. A glass ampule containing DMAPAMAIPS (0.01mol) (50 mol % on the basis of the monomers), one of the hydrophobic monomers (AdMAI) (0.01mol) (50 mol % on the basis of the monomers)31 and AIBN (10-4 mol) in N,N-dimethylformamide (30 mL) and water (10 mL) (3/1) cosolvent was degassed by three freeze-pump-thaw cycles on a vacuum line. The sealed ampule was maintained at 60 °C in a water bath. After polymerization, the mixture was poured into a large excess of acetone to precipitate the resulting polymers. The polymers were purified by three repreciptations from DMF/H2O cosolvent into acetone and dried under vacuum at 70 °C. Poly(DMAPAMAIPS/AdMAI) was obtained. In a similar manner, poly(DMAPAMAIPS/NaMAI) was obtained. The compositions of polymers were determined for both elemental analysis (Perkin-Elmer 2400) and NMR spectra (JEOL EX400). Poly(31) Morishima, Y.; Tominaga, Y.; Kamachi, M.; Okada, T.; Hirata, Y.; Mataga, N. J. Phys. Chem. 1991, 95, 6027.

Intramolecular Hydrophobic Aggregation of Amphiphilic Polysulfobetaine (DMAPAMAIPS/AdMAI) consists of 49.4 mol % DMAPAMAIPS and 50.6 mol % AdMAI. Mw[Poly(DMAPAMAIPS/AdMAI)] ) 5.77 × 105. Poly(DMAPAMAIPS/AdMAI) consists of 50.1 mol % DMAPAMAIPS and 49.9 mol % AdMAI. Mw[Poly(DMAPAMAIPS/AdMAI)] ) 5.82 × 105. These polymers [poly(DMAPAMAIPS/AdMAI), poly(DMAPAMAIPS), and poly(DMAPAMAIPS/NaMAI)] were all soluble in H2O but insoluble in the most of the other common organic solvents (e.g. acetone, methanol, ethanol, THF, benzene, toluene, DMF, DMSO, and n-hexane). Preparation of Pyrene-Labeled Polymers. In a 100 mL flask equipped with a stirrer and a thermometer, 1-pyrene methanol (0.232 g, 0.001 mol) and dried THF (20 mL) were added under an argon atmosphere and the contents were stirred below 5 °C. A mixture of methacryloyl isocyanate (MAI) (0.229 g, 0.002 mol) and dry THF was added dropwise for 1 h. Following the addition, the mixture was stirred for 10 h at the same temperature. The solution was then condensed under reduced pressure to remove THF and unreacted 1-pyrene methanol. Methanol was added to precipitate the undesired product. After filtrated, the solution was condensed to remove methanol. The contents were poured into a large excess of water to precipitate the resulting compound. The precipitated yellow crystals were collected by filtration and dried under reduced pressure for 24 h. Methacryloyl isocyanate containing 1-pyrene methanol (PyMMAI) was obtained. Elem. Anal. (C22H17O3N). Calcd: C, 76.97; H, 4.96; N, 4.08. Found: C, 76.88; H, 5.03; N, 3.98. IR (KBr) ν/cm-1: νN-H (3215), νCdO (1704), νCdC (1623). 13C NMR (CDCl3): δ (ppm) 18.63 (CH3), 120.65 (H2CdC), 139.8 (H2CdC), 122.59-128.37 (pyrene group carbon). The synthesis reaction is given below.

Langmuir, Vol. 15, No. 16, 1999 5207 Scheme 1

Scheme 2

The pyrene-labeled polymers were prepared by free radical polymerization initiated by 2,2′-azobisisobutyronitrile (AIBN). The procedure for the polymer was as follows. A glass ampule containing DMAPAMAIPS (0.01 mol) (99 mol % on the basis of the monomers) and pyrene containing monomer (PyMMAI) (10-4 mol) (1 mol % on the basis of the monomers)31 and AIBN (10-4 mol) in N,N-dimethylformamide (30 mL) and water (10 mL) (3/ 1) cosolvent was degassed by three freeze-pump-thaw cycles on a vacuum line. The sealed ampule was maintained at 60 °C in a water bath. After polymerization, the mixture was poured into a large excess of acetone to precipitate the resulting polymers. The polymers were purified by three repreciptations from DMF/ H2O cosolvent into acetone and dried under vacuum at 70 °C. (Scheme 2). In a similar manner, a glass ampule containing DMAPAMAIPS (0.0049 mol) (49 mol % on the basis of the monomers), one of the hydrophobic monomers (AdMAI or NaMAI) (0.005 mol) (50 mol % on the basis of the monomers), pyrene containimg monomer (PyMMAI) (10-4 mol) (1 mol % on the basis of the monomers),31 AIBN(10-4 mol) in N,N-dimethylformamide (30 mL), and water (10 mL) (3/1) cosolvent was degassed by three freeze-pump-thaw cycles on a vacuum line. The sealed ampule was maintained at 60 °C in a water bath. After polymerization, the mixture was poured into a large excess of acetone to precipitate the resulting polymers. The polymers were purified by three reprecipitations from DMF/H2O cosolvent into acetone and dried under vacuum at 70 °C (Scheme 3). The corresponding pyrene-labeled polymers [poly(DMAPAMAIPS/PyMMAI), poly(DMAPAMAIPS/AdMAI/PyMMAI), and poly(DMAPAMAIPS/ NaMAI/PyMMAI)] were obtained. Poly(DMAPAMAIPS/

PyMMAI) consists of 99.1 mol % DMAPAMAIPS and 0.9 mol % PyMMAI. Mw[Poly(DMAPAMAIPS/PyMMAI)] ) 6.60 × 105. Poly(DMAPAMAIPS/AdMAI/PyMMAI) consists of 48.9 mol % DMAPAMAIPS, 50.2 mol % AdMAI, and 0.9 mol % PyMMAI. Mw[Poly(DMAPAMAIPS/AdMAI/PyMMAI)] ) 5.80 × 105. Poly(DMAPAMAIPS/NaMAI/PyMMAI) consists of 49.4 mol % DMAPAMAIPS, 49.7 mol % AdMAI, and 0.9 mol % PyMMAI. Mw[Poly(DMAPAMAIPS/NaMAI/PyMMAI)] ) 5.73 × 105. These pyrene-labeled polymers were also soluble in H2O, but insoluble in most of other common organic solvent (such as acetone, methanol, ethanol, THF, benzene, toluene, DMF, DMSO, and n-hexane). The solubilities of the pyrene-labeled polymers are the same as that of their corresponding homopolymer and copolymers. The polymers were also hygroscopicity. Analytical Methods. Viscometric measurements were carried out with a Ubbelohde viscometer (flow time of 79.19 s with pure

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Scheme 3

Figure 3. Relationships between surface tension of polymers and various polymer concentration in aqueous solution. (b) Poly(DMAPAMAIPS), (9) poly (DMAPAMAIPS/AdMAI), and (2) poly(DMAPAMAIPS /NaMAI) in aqueous solution.

water) at 30.00 ( 0.01 °C. The fluorescence spectra were recorded by a Shimadzu RF-5031 spectrophotometer. The polymer solutions were mixed at predetermined ratios. All the fluorescence measurements were performed at room temperature by excitation at 330 nm wavelength. IR spectra were recorded in the range 4000-400 cm-1 for the synthesized monomers and polymers using KBr disks (JASCO IR-700 spectrometer). Elemental analyses were recorded using a Perkin-Elmer 2400 instrument). NMR spectra were recorded using a JEOL EX400. The weight-average molecular weights (Mw) of the polymers were determined on dilute samples in NaCl aqueous solutions with an Otsuka DLS-7000 spectrophotometer.

Results Surface Tensions. When a hydrophobic group and an zwitterionic group are incorporated into the polymer, surface activity can be observed. Figure 3 shows the relationship between surface tension and the concentration of amphiphilic copolymers with various hydrophobic groups in aqueous solution at 30 °C. The surface tensions of the aqueous solutions of the amphiphilic copolymers decreased with an increase in the polymer concentration. The surface tension of the aqueous solution of the copolymer with aromatic hydrophobic units (naphthyl units) [poly(DMAPAMAIPS/NaMAI)] is lower than that of copolymers with aliphatic hydrophobic groups (adamantyl groups) [poly(DMAPAMAIPS/AdMAI)]. The inflection point of the surface tension curve denotes hydrophobic aggregation, as also illustrated in Figure 3. This corresponds to the critical concentration at which intermolecular cooperative associations through hydrophobic interactions begin to occur among the amphiphilic copolymers in various hydrophobic groups. There is no inflection point of surface tension curve of poly(DMAPAMAIPS). Reduced Viscosity. Reduced viscosity is generally accepted to be an effective measure of the macroscopic solution property of water-soluble polymer. Therefore, determining the reduced viscosity of the amphiphilic copolymers with various hydrophobic groups should reflect the influence of these different hydrophobic associations on the hydrodynamic volume of the polymer chain for poly-

Figure 4. Relationships of reduced viscosity of polymers and various polymer concentration in aqueous solution (9) poly(DMAPAMAIPS), (b) poly(DMAPAMAIPS/AdMAI), and (2) poly(DMAPAMAIPS/ NaMAI).

(DMAPAMAIPS), poly(DMAPAMAIPS/NaMAI), and poly(DMAPAMAIPS/AdMAI). Figure 4 shows the plots of the reduced viscosity of various amphiphilic copolymers with varying concentrations of the copolymer in salt-free aqueous solution. Copolymers with various hydrophobic units showed negatively sloping plots in dilute regime, which is typical behavior of polybetaines.23 This reveals a decrease in the reduced viscosities in the order poly(DMAPAMAIPS/ NaMAI) < poly(DMAPAMAIPS/AdMAI) < poly(DMAPAMAIPS) at low polymer concentration (0.01-0.1 g/dL). That is, the reduced viscosities for the copolymer with aliphatic hydrophobic groups [poly(DMAPAMAIPS/ AdMAI)] in aqueous solution are significantly larger than those for the copolymers with aromatic hydrophobic groups [poly(DMAPAMAIPS/NaMAI)]. Furthermore, poly(DMAPAMAIPS) without hydrophobic group has the largest reduced viscosity in dilute polymer solution. When hydrophobic groups (naphthyl or adamantyl group) are incorporated into the polymer, the hydrodynamic volume of dilute polymer solutions decreased with an increase in the hydrophobic associations. Photophysical Property. Figure 5A compares fluorescence spectra in aqueous solution of the pyrene labels in the hydrophobic terpolymers, poly(DMAPAMAIPS/ AdMAI/PyMMAI) and the reference copolymer without hydrophobes, poly(DMAPAMAIPS/PyMMAI). The peak

Intramolecular Hydrophobic Aggregation of Amphiphilic Polysulfobetaine

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Figure 6. I3/I1 ratio of poly(DMAPAMAIPS/AdMAI/PyMMAI) in aqueous solution with different KCl concentrations. [poly(DMAPAMAIPS/AdMAI/PyMMAI)]: 0.2 g/dL in H2O.

Discussion

Figure 5. Fluorescence spectra (A) poly(DMAPAMAIPS/ PyMMAI) (s) and poly(DMAPAMAIPS/AdMAI/PyMMAI) (- - -) in aqueous solution (B) poly(DMAPAMAIPS/AdMAI/ PyMMAI) in aqueous solution with various KCl concentrations.

maxima at 372, 380, 385, 392, and 415 nm arise from the fluorescence emission of isolated pyrenes (monomer emission). The broad, structureless band centered 535 nm resulted from emission of excited dimeric pyrene (excimer). Poly(DMAPAMAIPS/PyMMAI) exhibits a remarkable excimer emission. Figure 5B shows the fluorescence spectra of poly(DMAPAMAIPS/AdMAI/PyMMAI) in aqueous solution with various KCl concentrations. The KCl addition could not enhance the excimer emission of poly(DMAPAMAIPS/AdMAI/PyMMAI). The ratio of the third to the first vibronic bands (I3/I1) in the fluorescence spectra of pyrene is known to depend on the polarity in the media where pyrene exists, the I3/I1 ratio being larger in less polar media.32-34 The I3/I1 ratio of poly(DMAPAMAIPS/ PyMMAI) of 0.53 and the poly(DMAPAMAIPS/AdMAI/ PyMMAI) ratio of 0.46 are both lower than those reported for pyrene in aqueous solution,17,19 implying that the pyrene residues in polymer are exposed to the more polar aqueous phase which resulted from the interaction of R4N+ group and SO3- group. Figure 6 shows a plot of the I3/I1 ratio as a function of the KCl concentration in polymer solution. As salt (KCl) is added, the I3/I1 ratio increases, perhaps due to the interactions between zwitterionic groups and KCl. (32) Dubin, P., Bock, J., Davies, J. M., Schulz, D. N., Thies, C., Eds. Macromolecular Complexes in Chemistry and Biochemistry; SpringerVerlag: New York, 1994. (33) Schild, H. G.; Tirrell, D. A. Langmuir 1991, 7, 1319. (34) Kwon, G. S.; Naito, M.; Kataoka, K.; Yokoyama, M.; Sakurai, Y.; Okano, T. Colloids Surf B: Biointerfaces 1994, 2, 429.

Zwitterionic group incorporation was shown to affect the water solubility of polymer. Aqueous solution properties can become dependent on pH and/or electrolyte concentration. When a hydrophobic group and a zwitterionic group are incorporated into the polymer, surface activities due to the formation of hydrophobic microdomains exhibit different surface tensions of the amphiphilic copolymers in solution.26-29 The surface activity of amphiphilic copolymers with various hydrophobic groups in aqueous solution is shown in Figure 3. The surface tensions of the aqueous solutions of the amphiphilic copolymers decreased with an increase in the polymer concentration. The surface tension of the aqueous solution of the copolymer with aromatic hydrophobic units (naphthyl units) is lower than that of copolymers with aliphatic hydrophobic groups (adamantyl groups). That is, the aromatic naphthyl-containing copolymers [poly(DMAPAMAIPS/NaMAI)] showed a larger surface activity than did the aliphatic adamantane-containing copolymers [poly(DMAPAMAIPS/AdMAI)]. Furthermore, hydrophobes containing polymers [poly(DMAPAMAIPS/NaMAI) and poly(DMAPAMAIPS/AdMAI)] exhibited relatively higher surface tensions than their corresponding polymer without hydrophobes [poly(DMAPAMAIPS)]. The phenomenon due to surface-active effects of polymers increased with the contents of the hydrophobic unit increased. The inflection point of the surface tension curve is caused by hydrophobic aggregation. This inflection point corresponds to the critical concentration at which intermolecular cooperative associations through hydrophobic interactions begin to occur among the amphiphilic copolymers with various hydrophobic groups. The critical concentration of naphthalene-containing copolymer [poly(DMAPAMAIPS/ NaMAI)] is similar to that of adamantane-containing copolymer [poly(DMAPAMAIPS/AdMAI)]. However, there is a relatively remarkable inflection point of surface tension curve of poly(DMAPAMAIPS/NaMAI). This is because aromatic hydrophobic association is more dominant than the aliphatic hydrophobic association for amphiphilic copolymers, due to the better packing ability of naphthalene groups. Polymer architecture would affect the inter- and intramolecular association. Reduced viscosity is generally accepted to be an effective measure of the hydrodynamic volume of the polymer containing the similar molecular weight. Therefore, determining the reduced viscosity of the amphiphilic copolymers with various hydrophobic groups should reflect the influence of these different hydrophobic associations on the hydrodynamic volume of the polymer chain for poly-

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Figure 7. Proposed model of the interaction between hydrophobic units and zwitterionic units in polymer chains. (A) Poly(DMAPAMAIPS/PyMMAI) and (B) poly(DMAPAMAIPS/AdMAI/PyMMAI) in aqueous solution.

(DMAPAMAIPS), poly(DMAPAMAIPS/NaMAI), and poly(DMAPAMAIPS/AdMAI). Copolymers with various hydrophobic units show negatively sloping plots (Figure 4). The steepness of the negative slopes reflects the extent of chain expansion upon dilution. If intramolecular hydrophobic interactions are strong enough to prevent the expansion of a polymer coil, the reduced viscosity would not increase upon dilution. In the case of amphiphilic copolymers with various hydrophobic units and sulfobetaine units, which have both positive and negative charges, these polymers exhibit both intermolecular and intramolecular associations.23,37,38 The incorporated hydrophobicgroups would interrupt the associations of R4N+ groups and SO3- groups, but at the same time these groups also provide the hydrophobic associations, which are due to the aggregation of the polymer chains. The hydrophobic groups interrupt both the long- and/or short-range interactions of the zwitterions wherein longer-range interactions would not be affected as much. The negative slope tends to become flatter as the surface activity of the hydrophobic units in the copolymers increases (0.01-0.1 g/dL). The amphiphilic copolymers with aromatic hydrophobic groups (naphthyl group) have a marked tendency to allow a compact conformation in dilute aqueous solution as compared to the copolymers with aliphatic hydrophobic groups (adamantyl group) and the surface activity is dominant for the copolymers with aromatic hydrophobic units as compared to the copolymers with aliphatic hydrophobic units. Viscosity is not only related to molecular weight but also reflects the extent of hydrophobic association of hydrophobic residues on which the hydrodynamic volume of the polymer coils would strongly depend. Figure 4 shows a decrease in reduced viscosity in the order poly(DMAPAMAIPS/NaMAI) < poly(DMAPAMAIPS/AdMAI) < poly(DMAPAMAIPS) in the dilute aqueous solution (0.01(35) Chang, Y.; McCormick, C. L. Macromolecules 1993, 26, 6121. (36) Weers, J. Langmuir 1991, 7, 855. (37) Schulz, D. N.; Peiffer, D. G.; Agarwal, P. K.; Larabee, J.; Kaladas, J. J.; Soni, L.; Handwerker, B.; Garner, R. T. Polymer 1986, 27, 1734. (38) Salazar, L. C.; McCormick, C. L. Polym. Preprints 1989, 30(2), 344.

0.1 g/dL). That is, the reduced viscosities for copolymers with aliphatic hydrophobic units [poly(DMAPAMAIPS/ AdMAI)] in salt-free aqueous solutions are significantly large compared to those for copolymers with aromatic hydrophobic units [poly(DMAPAMAIPS/NaMAI)]. This result suggests that the aromatic copolymers easily shrink, as opposed to aliphatic copolymers due to the stronger hydrophobic aggregation of the aromatic units, even under salt-free conditions. The chain expansion in aqueous solutions is much more difficult for poly(DMAPAMAIPS/ NaMAI) with aromatic hydrophobic units, thus this polymer shows a relatively smaller hydrodynamic volume. Coil expansion was due to weaker hydrophobic associations of the adamantane moieties. Furthermore, poly(DMAPAMAIPS), without a hydrophobic group, has the largest reduced viscosity in dilute polymer solution. When hydrophobic groups (naphthyl or adamantyl group) are incorporated into the polymer, the hydrodynamic volume of polymer solutions decreased with an increase in the hydrophobic associations (Figure 4). When fluorescent hydrophobe (pyrene group) is incorporated into the copolymer, the photophysical response may effectively indicate solution behavior on the microscopic level. The intensity of excimer emission relative to that of the monomer emission (IE/IM) reflects intra- and intermolecular interactions and microscopic conformation of polymer in solution. The ratio of the third to the first vibronic bands (I3/I1) in fluorescence spectra of pyrene is known to depend on the polarity in media with pyrene, the I3/I1 ratio being larger in less polar media. The I3/I1 ratio of 0.53 for poly(DMAPAMAIPS/PyMMAI) and the ratio of 0.46 for poly(DMAPAMAIPS/AdMAI/PyMMAI) are lower than those reported for pyrene in aqueous solution [for example, the I3/I1 ratio for poly(2-acrylamido2-methylpropanesulfonic acid/laurylmethacrylamide/1pyrenylmethylmethacrylamide) is 0.80].17,19 This implies that the pyrene residues in polymer are exposed to the aqueous phase and those residues are confined within the interaction of R4N+ group and/or SO3- group, as indicated in Figure 5.32-35 In H2O, pyrene-labeled zwitterionic sulfobetaine copolymer [poly(DMAPAMAIPS/

Intramolecular Hydrophobic Aggregation of Amphiphilic Polysulfobetaine

PyMMAI)] has extensive intermolecular and intramolecular association. The intramolecular association results from interactions between the quaternary amino group (R4N+) and the sulfonate group (SO3-) within the poly(DMAPAMAIPS/PyMMAI) chain.23,25,36-38 That is, the structure of betaine exhibits a dipole, typically an intramolecular interaction is characterized by a dipoledipole interaction along the same chain. Meanwhile, the intramolecular and intermolecular association results from interactions between the quaternary amino group (R4N+) and the sulfonate group (SO3-) at the different poly(DMAPAMAIPS/PyMMAI) chains. The association of the quaternary amino group (R4N+) and the sulfonate group (SO3-) would neutralize the zwitterionic condition of poly(DMAPAMAIPS/PyMMAI) (Figure 7A). The degree of association increases, interaction between isolated, covalently bound fluorescent hydrophobes (pyrenes) allows the formation of dimeric, sandwich-like conforms that subsequently lead to excimer formation.39 Hence, poly(DMAPAMAIPS/PyMMAI) shows a relatively remarkable excimer emission. When adamantyl groups are incorporated into the amphiphilic polymer, the pyrene labels would be in higher polar media. The adamantyl groups of poly(DMAPAMAIPS/AdMAI/PyMMAI) may not be able to sterically fit well with the pyrene moiety because these groups are rigid and have a three-dimensionally bulky structure. The incorporated adamantyl groups would interrupt the associations of R4N+ and SO3- and the excited dimer (pyrene) formation. The zwitterionic interaction also interrupts the clusters of hydrophobes formation (Figure 7B). The microenvironmental polarity of pyrene labels would increase. A relatively low I3/I1 ratio and intensity of excimer emission of poly(DMAPAMAIPS/AdMAI/ PyMMAI) were observed (Figure 5A). In general, polybetaine has the intramolecular and intermolecular association of the zwitterionic group on the polymer.37 If the zwitterionic groups are predominantly participating in these types of associations, the hydrophobic characteristics are increased and the polar character of the moiety was reduced consequently. In this study, the incorporated hydrophobes interrupt these types of associations and the zwitterionic groups interrupt the hydrophobic associations, thereby causing the polar character of the moiety to reduce. The similar behavior had also been observed (39) Kramer, M. C.; Steger, J. R.; Hu, Y.; McCormick, C. L. Macromolecules 1996, 29, 1992.

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in naphthalene-labeled styrene-containing polybetaine.26 As salt (KCl) is added, the I3/I1 ratio increases with decreasing microenvironmental polarity of pyrene labels (Figure 6). The addition of KCl, can shield the R4N+ group and/or SO3- group of poly(DMAPAMAIPS/AdMAI/PyMMAI) chains and can reduce the interruptions of R4N+ and/or SO3- groups and adamantyl groups for each other. The pyrene labels of polymer side chain would be in less polar media, therefore, the I3/I1 ratio increases with increasing KCl concentration in polymer solution (Figure 6). While more salt was added, the positive charge of R4N+ group and the negative charge of SO3- group within the polymer conformation were effectively screened out. The I3/I1 ratio would keep constant in higher salt concentration. Although the R4N+ groups and SO3- groups of poly(DMAPAMAIPS/AdMAI/PyMMAI) were screened out in KCl aqueous solutions, the three-dimensionally bulky structure of adamantyl groups interrupts the formation of excimer. Hence, there is no remarkable excimer emission of poly(DMAPAMAIPS/AdMAI/PyMMAI) in KCl aqueous solutions (Figure 5B). Conclusions The synthesis, solution, and fluorescence properties of the zwitterionic water-soluble copolymers, poly(DMAPAMAIPS/ NaMAI) and poly(DMAPAMAIPS/AdMAI) having hydrophobic groups (naphthalene or adamantane) were studied. The surface tension of aqueous solution of the copolymer with aromatic (naphthalene) groups is shown to be lower than that of copolymer with aliphatic (adamantane) groups. The reduced viscosity indicates that the incorporated hydrophobic groups would interrupt associations of R4N+ groups and SO3- groups, which provide the hydrophobic associations due to the aggregation of the polymer chains. The fluorescence spectrum showed that rigid and bulky groups (adamantane) incorporated into the polymer side chain would interrupt the interactions of R4N+ groups and SO3- groups, resulting in an increase in the microenvironment polarity of pyrene groups. Acknowledgment. The authors would like to thank the National Science Council of the Republic of China for their support on this work under Grant NSC 87-2216E011-014. LA980728H