Amphiphilic Polyelectrolytes and Their Coulombic Complexes with

Chapter 18

Amphiphilic Polyelectrolytes and Their Coulombic Complexes with Surfactants as Novel Photochemical Systems Downloaded via YORK UNIV on December 13, 2018 at 08:41:47 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Y. Morishima, M. Seki, S. Nomura, and M. Kamachi Department of Macromolecular Science, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan

Amphiphilic polyelectrolytes bearing bulky hydrophobic pendant groups form "unimolecular" micelles in dilute aqueous solution due to intramolecular self-organization of the hydrophobic groups. Coulombic complexes of the polyelectrolyte micelles with didodecyldimethylammonium bromide were prepared in dilute aqueous solution. The complexes were soluble in common organic solvents with a range of polarities. Light scattering, GPC, NMR relaxation, 2D-NOESY, ESR spin probe, and fluorescence studies showed that the micellar structure was retained in the complexes in organic solution, particularly so in benzene. Pyrene encapsulated in the core of the complexes exhibited fluorescence and photochemical properties typical of those of a "compartmentalized" chromophore. In recent years, complexes between water-soluble polymers and surfactant molecules have been extensively studied from theoretical and experimental points of view (1-5). These complexes are of interest because they provide self-organized molecular assemblies which may serve as vehicles for chemical reactions relevant to biological phenomena. Because of considerable scientific and technological interests, a number of studies have focused on the complex formation of polyelectrolytes with oppositely charged surfactants below (6) and above (7-11) the critical micelle concentration (cmc). Amphophilic polyelectrolytes bearing bulky hydrophobic substituents adopt micellar structures in aqueous solution due to self-organization of the hydrophobic groups (1217). A remarkable feature for such micelles, as compared with conventional surfactant micelles, is that the clusters of the hydrophobic groups in the amphiphilic polyelectrolyte micelles are rigid and static in the sense that local motions of the hydrophobic groups are highly restricted (13,15,16). When a small molefractionof a hydrophobic photoactive chromophore is covalently incorporated into such amphiphilic polyelectrolytes, it becomes encapsulated in the cluster of the hydrophobic groups. TTius, a photoactive chromophore can be confined to the nonpolar microenvironment in which the chromophore is "protected"fromthe aqueous phase. Furthermore, the molecular motions of the chromophore are restricted due to therigidityof the hydrophobic cluster. Unlike chromophores solubilized in conventional surfactant micelle, the chromophore "compartmentalized" in the cluster of amphiphilic polyelectrolytes shows unique behavior in fluorescence and phosphorescence (18), in energy transfer and migration (16), in photoinduced electron transfer 0097-6156/94/0548-0243$06.00/0 © 1994 American Chemical Society

Schmitz; Macro-ion Characterization ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

MACRO-ION CHARACTERIZATION

244

(19-21), and in photoisomerization (22). Therefore, the compartmentalization of a photoactive group seems to be an interesting approach to the design of photosystems including photon-harvesting, photoinduced charge separation, and photochromic sys tems. Coulombic complexes of such amphiphilic polyelectrolyte micelles with oppositely charged surfactants below the cmc provide new types of molecular assemblies which may have a variety of interesting features worth studying. The complexes can be dis solved in a variety of organic solvents. Therefore, they may act as novel vehicles for photochemical reactions (23,24) in organic solution. However, a question is whether or not the micellar structure of the parent amphiphilic polyelectrolytes remains as such in the Coulombic complexes in organic solution. To answer this question, we prepared Coulombic complexes of amphiphilic polysulfonic acid and cationic double-chain sur factants, and characterized their structures and dynamic properties in organic solution by dynamic light scattering (DLS), GPC, H-NMR relaxation, 2D-NOESY, ESR spin probe, and fluorescence techniques. 1

Hydrophobic Self-Organization in Amphiphilic Polyelectrolytes in Aqueous Solution Amphiphilic polyelectrolytes with bulky hydrophobic groups were prepared by freeradical terpolymerization of sodium 2-(acrylamido)-2-methylpropanesulfonate (AMPS), methacrylamides bearing bulky hydrophobic substituents, and small mole fractions of methacrylamides bearing spectroscopic probes (16,18,21,22,25). The lauryl (La), cyclododecyl (Cd), 1-adamantyl (Ad), and 1-naphthylmethyl (Np) groups were employed as the bulky hydrophobic substituents. The 1-pyrenyl (Py) and 2,2,6,6-tetramethylpiperidine-N-oxide (NO) groups were employed as fluorescence and spin labels, respectively. The terpolymenzations were performed in N,Ndimethylformamide (DMF) at 60 °C in the presence of 2,2'-azobis(isobutyronitrile). The compositions of the terpolymers were found to be virtually the same as the monomer feed compositions. In the text, the terpolymers are abbreviated as poly(A/R/P), where A, R, and Ρ represent the AMPS, hydrophobic, and probe units, respectively.

poly(A/Np/Py) The amphiphilic terpolymers are soluble in water to give transparent solutions in which "unimolecular" micelles are formed owing to intramolecular self-organization of the hydrophobic groups (13-16). H-NMR spectroscopy provides information about the hydrophobic self-organization. Considerable line broadening of the resonance peaks due to protons of the hydrophobic groups occurs when they form a hydrophobic cluster in water (13,15,24,25). In 2D-N0ESY spectra of the terpolymer poly(A/Np/Py) in EfcO at room temperature, cross peaks were observed due to dipolar J


18. MORISHIMA ET AL.

245

Amphiphilic Polyelectrolytes

ÇH CH Ç— Ç:0 NH 3

—J-CHrÇH-J C=0 I

CH-rÇ-

r

100-x-y

NH I

CH3~~C"~ CH3

poly(A/La/2-Np) x=32, y=4 mol% poly(A/Cd/2-Np) x=27, y=3moi%

poly(A/Ad/2-Np) x=32, y=4mol%

ÇH

CHj-Ç—

f-CHjrÇH J

CxO

100-x-y

a

CHjrÇC:0

NH

NH CH3~"C~ CH3 CH

CH

3

2

SOg-Na*

è

/

poly(A/La/Py) x=49, y=1 mol% poly(A/Cd/Py) x=50, y=1 mol%

poly(A/Ad/Py) x=49, y=1 mol%

Scheme I


246


interactions between the naphthyl and methyl and/or methylene protons (see Figure 3a). NMR relaxation techniques are useful tools to study local segment motions in poly mers. Spin-spin relaxation time (Γ2) for the naphthyl resonance inpoly(A/Np/Py) in D2O was found to be very short, being one order of magnitude shorter than that ob served in DMF-J7 (see Table I). This is in agreement with the fact that the naphthyl protons in poly(A/Np/Py) show much broader resonance lines in D2O than in OMF-dj. Thesefindingsare indicative of strongly restricted motions of the naphthyl groups due to their cluster formation in aqueous solution. The spin-lattice relaxation occurs most efficiently through molecular motion whose frequency is comparable to the NMR fre quency. Therefore, spin-lattice relaxation time (Γι) decreases with a decrease in molecular motion, reaches a minimum, and then increases with a further decrease in the molecular motion. The long Γι observed for the naphthyl resonance in poly(A/Np/Py) in EfeO, in spite of the very short Γ2, is attributable to a longitudinal relaxation in a highly restricted motion. Evidence for Unimolecular Micelle A question may be asked as to whether the hydrophobic self-organization is an in tramolecular or intermolecular event. A study of intermolecular energy transferfroma naphthalene-labeled polymer to a pyrene-labeled polymer may give an answer to this question. Amphiphilic polyelectrolytes labeled with 2-Np [poly(A/R/2-Np)] and with Py [poly(A/R/Py)], where R is La, Cd, or Ad, were employed for this experiment (Scheme I). In these polymers in aqueous solution, 2-Np and Py are encapsulated in the hydrophobic clusters of La, Cd, and Ad. If the hydrophobic self-organization oc curs intramolecularly, 2-Np and Py should be isolatedfromeach other in each separate polymer and no energy transferfromphotoexcited 2-Np to Py should be observed. On the other hand, if an intermolecular association takes place, there should be a chance for Py to come close to 2-Np within the Fdrster radius (/?o=2.86 nm for transfer from 1methylnaphthalene to pyrene (26)). A mixture of poiy(A/R/2-Np) and poly(A/R/Py) in aqueous solution was irradiated at 290 nm, at which virtually 2-Np can be excited selectively. Fluorescence emissions from 2-Np and Py were monitored at 340 and 395 nm, respectively. Figure 1 shows the ratios of the intensities of Py fluorescence and 2-Np fluorescence as a function of the total polymer concentration. The ratios were normalized by dividing by the ratios at the lowest polymer concentration (0.03 wt %) for ease of comparison. In the case of Ad as the hydrophobic group, practically no Py fluorescence was observed at polymer concentrations below ca. 4 wt%. This implies that a "unimolecular micelle is formed as a result of intramolecular self-organization of Ad, when the polymer concentration is lower than ca. 4 wt%. For the amphiphilic terpolymers with Cd, the formation of the unimolecular micelle was also implied, when the polymer concentration was below ca. 2 wt%. On the other hand, amphiphilic polyelectrolytes with La showed a much stronger tendency for intermolecular association. A unimolecular micelle was sug gested to be formed at polymer concentrations lower than ca. 0.5 wt%, being one order of magnitude lower than the Ad and Cd cases. 11

Preparation of Coulombic Complexes of Amphiphilic Polyelectrolytes and Double-Chain Surfactants Figure 2a shows a histogram for the size distribution of the unimolecular micelle of poly(A/Ad/Py) in aqueous solution measured by DLS. At the polymer concentration of 0.32 wt% employed for the DLS measurement, the possibility of intermolecular asso ciation can be excluded as discussed in the previous section. The particle size distribu tion of the poly(A/Ad/Py) micelle in water was found to be rather narrow, ranging from ca. 6 to 13 nm in diameter.


18.

MORISHIMA E T

AI*

247

Amphiphilic Polyelectrolytes

Table I. T\ and 7*2 for naphthyl protons and for methylene protons in the DDAB residues sample solvent 72 (ms) Γ ι ί ms) naphthyl DDAB naphthyl DDAB poly(A/Np/Py) P20 726 4 poly(A/Np/Py) 854 45 DMF-i/7 poly(A/Np/Py)/DDAB benzcne-ifc 598 655 11 137 poly(A/Np/Py)/DDAB DMF«/ 712 792 26 737 7

r

r

j h C H - ÇH — OO

ι" ! 3

Τ*Λ

r

h CH - C

2

2

NH

CH - Ç 2

\

NH

CH3~""C~ CH3 CH I

2

I

S01Ma+ 3

x=49, y=4 mol% poly(A/Np/NO)

4

Ο

CH -ÇH2

O O -nOO-x-y

L

NH I CH3—C— CH3 ÇH2 I

S0

-?J(CH2)iiCH

5^

3 7 C

H

3

(CHaJnCHa

x=49, y=4 mol% poly(A/Np/NO)/DDAB

0.1 1 Polymer Concentration (wt%) Figure 1. Ratio of the fluorescence intensities for the Py and 2-Np probes as a function of the total concentration of the mixture of poly(A/R/Py) and poly(A/R/2-Np) in aqueous solution, where R is La, Cd, or Ad.

American Chemical Schmitz; Macro-ion Characterization Society library

ACS Symposium Series; American Chemical Society: Washington, DC, 1993. m Λ me* tf»4L Of

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When an aqueous solution of didodecyldimethylammonium bromide (DDAB) was added to an aqueous solution of poly(A/Ad/Py), the solution became turbid because of phase separation of the resulting Coulombic complexes. The complexes, which were collected by centrifiigation, were purified by dialysis in a water suspension and recovered by freeze-drying. The complexes are abbreviated in the text as poly(A/R/Py)/DDAB, where R represents the hydrophobic groups.

—|-CH -

Amphiphilic Polyelectrolytes and Their Coulombic Complexes with

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