Micellization of and Solute Binding to Amphiphilic Poly(ethylene oxide

Langmuir 1996, 12, 2207-2213

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Micellization of and Solute Binding to Amphiphilic Poly(ethylene oxide) Star Polymers in Aqueous Media Xin Chen and Johannes Smid* Faculty of Chemistry, College of Environmental Science and Forestry, State University of New York, Syracuse, New York 13210-2786 Received July 3, 1995. In Final Form: February 26, 1996X Three-armed poly(ethylene oxide) (PEO) star polymers were synthesized from tris(p-isocyanatophenyl)methane and methoxypoly(ethylene glycol)s of molecular weight 350, 750, and 1900. The polymers exhibit lower critical solution temperature behavior in water, the cloud points decreasing linearly with the molal concentration of added salts. Surface tension and viscometric data reveal the formation of micelles and the possible existence of smaller aggregates prior to micellization. Similar amphiphilic PEO star polymers but with a larger hydrophobic core were found to strongly interact with the optical probes coomassie brilliant blue (CBB) and 1-pyrenebutyrate (PB-), both below and above the critical micelle concentration (cmc). Klotz-type binding plots reveal clear transition points which for PB- are close to the cmc of the star polymers. The binding data confirm that small aggregates may be present below the cmc of the stars.

Introduction Amphiphilic polymers in aqueous media exhibit useful properties stemming from the intra- or intermolecular interaction of the hydrophobic entities or segments in the macromolecules. Many such polymers contain segments of ethylene oxide (EO) units as the hydrophilic entity, and a variety of block, graft, or comb-shaped polymers with sequences of EO units have been reported.1-9 Their commercial interest or potential applications range from emulsifiers and viscosity modifiers to drug release formulations, depending on their particular architecture. We recently reported on the synthesis and properties of amphiphilic star polymers composed of three or four poly(ethylene oxide) (PEO) arms and a large hydrophobic core.10 The latter was derived from either a tri- or a tetrafunctional isocyanate synthesized by hydrosilylation of m-isopropenyl-R,R-dimethylbenzyl isocyanate (m-TMI) with methyltris(dimethylsiloxy)silane or 2,4,6,8-tetramethylcyclotetrasiloxane.11 The two multifunctional isocyanates, abbreviated as T3TMI and D4TMI, are shown in Chart 1. When reacted with methoxypoly(ethylene glycol)s (MPEG’s), the resulting amphiphilic PEO homopolymer stars exhibit lower critical solution temperature (LCST) behavior in water, and their solubility is sensitive to the presence of electrolyte. Surface tension measurements reveal a strong tendency toward micellization. Phase separation into a polymer-rich phase and a polymer-poor phase occurs in aqueous solutions of soX

Abstract published in Advance ACS Abstracts, April 15, 1996.

(1) McCormick, C. L.; Block, J.; Schulz, D. N. Encyclopedia of Polymer Science and Engineering; John Wiley & Sons: New York, 1989; Vol. 17, pp 730-784. (2) Landoll, L. M. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 443. (3) Xu, R.; Winnik, M. A.; Hallet, F. R.; Ries, G.; Croucher, M. D. Macromolecules 1991, 24, 87. (4) Bo, G.; Wesslen, B.; Wesslen, K. B. J. Polym. Sci., Polym. Chem. Ed. 1992, 30, 1799. (5) Tanaka, R.; Meadows, J.; Williams, P. A.; Phillips, G. O. Macromolecules 1992, 25, 1304. (6) Glass, J. E., Ed. Polymers in Aqueous Media; Advances in Chemistry Series 223; American Chemical Society: Washington, DC, 1989. (7) Khan, I. M.; Yuan, Y.; Fish, D.; Wu, E.; Smid, J. Macromolecules 1988, 21, 2684. (8) Nwankwo, I.; Xia, D. W.; Smid, J. J. Polym. Sci., Polym. Phys. Ed. 1988, 26, 58. (9) Strasser, C. M.; Clouet, F.; Francois, J. Polymer 1992, 33, 1021. (10) Zhou, G.; Smid, J. Langmuir 1993, 9, 2907. (11) Zhou, G.; Smid, J. J. Polym. Sci., Polym. Chem. Ed. 1991, 29, 1097.

S0743-7463(95)00963-2 CCC: $12.00

Chart 1

coined associative PEO star polymers which are endowed with hydrophobic arm ends.12,13 In this paper the synthesis and properties of PEO homopolymer stars possessing a smaller hydrophobic core derived from tris(p-isocyanatophenyl)methane (T3PM) are discussed. Surface tension, viscosity, and cloud point measurements were employed to explore their aqueous solution properties such as phase behavior (LCST) and micellization. The surface tension data reveal peculiar aggregation phenomena prior to micelle formation. The aggregation of the star molecules was also examined by measuring their binding constants with the spectrophotometric probes coomassie blue and 1-pyrenebutyrate. Experimental Section Tris(p-isocyanatophenyl)methane (T3PM) was acquired from Miles, Inc. as an approximately 20 wt % solution in a mixture of monochlorobenzene and ethyl acetate (sold under the name Desmodur RE). The NCO content was determined by adding excess dibutylamine in CH2Cl2 followed by back-titration with HCl in 2-propanol and bromophenol blue as indicator.14 N,N,N′,N′-tetramethylethylenediamine (TMEDA) and the methoxypoly(ethylene glycol)s (MPEG’s) were from Aldrich. The stated molecular weigths (MW’s) of the glycols were 350 (MPEG 350), 750 (MPEG 750), and 1900 (MPEG 1900). 1H NMR spectra yielded average MW’s for these products of 345, 700, and 1940, respectively. Prior to use the glycols were dried azeotropically or freeze-dried from benzene. Solvents were distilled from CaH2, and salts were used without purification. 1-Pyrenebutyric acid (Aldrich) was recrystallized three times from ethanol. Coomassie blue (CBB) was an Eastman Kodak product. Synthesis. The synthesis of the two hydrophobic cores T3TMI and D4TMI (Chart 1) and that of the corresponding star polymers (12) Brown, R. G.; Glass, J. E. Proc. Polym. Mater. Sci. Eng. 1987, 57, 709. (13) Zhou, G.; Smid, J. Polymer 1993, 34, 5128. (14) Furukawa, M.; Yokoyamato, T. J. Polym. Sci., Polym. Lett. Ed. 1979, 17, 175.

© 1996 American Chemical Society

2208 Langmuir, Vol. 12, No. 9, 1996

Figure 1. 1H NMR spectrum of T3PM-MPEG 750 (300 MHz) in CDCl3. T3TMI-MPEG and D4TMI-MPEG using various MPEG’s have been reported elsewhere.10,11 Star polymers with a T3PM core (Chart 1) were made by reacting the isocyanate with the appropriate MPEG. For the higher MW MPEG’s (750 and 1900) excess MPEG was used to reduce reaction times. In a typical experiment, 2.5 g of a Desmodur RE solution (1.6 mmol of T3PM as determined by titration) was mixed with 10 g (13.35 mmol) of MPEG 750 dissolved in 15 mL of toluene. A few drops of TMEDA were added as catalyst. The mixture was kept at 50 °C until no isocyanate (NCO) absorption peak at 2260 cm-1 could be detected in the IR spectrum. Total reaction time was about 3 h. The polymer and excess MPEG were then precipitated in cold hexane to remove toluene and TMEDA. The precipitate was freeze-dried from benzene to remove traces of hexane and toluene and was then dissolved in 300 mL of distilled water. The excess MPEG 750 was removed by ultrafiltration through an Amicon YM2 membrane (cutoff > 1000 MW). About 300 mL of water was then added and the ultrafiltration procedure repeated twice. After rotaevaporating the water, the last traces of water were removed azeotropically with benzene. The waxlike material was obtained in 79% yield. The 1H and 13C NMR spectra of the purified star carbamate T3PM-MPEG 750 are shown in Figures 1 and 2. 1H NMR (CDCl3) δ: 7.3 (d, 6H, H(e)); 7.0 (d, 6H, H(d)); 6.5 (s, 3H, H(C)); 5.4 (s, 1H, H(f)); 3.4-4.3 (m, 186H, H(b)); 3.2 (s, 9H, H(a)). 13C NMR (CDCl3) δ: 153.3 (C(c)); 138.6 (C(g)); 136.2 (C(f)); 129.4 (C(e)); 118.4 (C(d)); 61.3-72.2 (C(b)); 58.6 (C(a)); 54.7 (C(h)). The star polymers T3PM-MPEG 350 (a liquid) and T3PM-MPEG 1900 (a solid, mp 51-52 °C) had nearly identical chemical shifts. Measurements. 1H and 13C NMR spectra were recorded on a Bruker AMX-300 NMR spectrometer with TMS as internal standard. IR spectra were obtained with a Perkin Elmer 1310 instrument. A Waters GPC1A with 500 to 105 Å styragel columns and THF as eluant was used for gel permeation chromatograms (flow rate 0.5 L/min). Concentrations of T3PM-MPEG star polymers were measured spectrophotometrically on a DMS-100, making use of the strong absorption at λm ) 246 nm, the molar absorptivity being 48 000. Intrinsic viscosities were determined in water and THF by means of a Ubbelohde viscometer at 25 ( 0.1 °C. Surface tension (γ) data were obtained with a Du Nouy tension meter calibrated with deionized water and benzene. The critical micelle concentration (cmc) of the stars was taken at the break points of γ versus log concentration plots. Cloud points (Tp) were determined with a thermometer suspended in the polymer solution. The temperature was raised 2 °C per minute and the Tp recorded as the temperature at which the transparent solutions turned

Chen and Smid cloudy. Similar measurements were carried out for saltcontaining polymer solutions. Reproducibility was better than 0.5 °C. Binding measurements with CBB and pyrene butyrate (PB-) were conducted spectrophotometrically. CBB solutions were prepared by adding 0.0923 g of CBB powder to a mixture of 10 mL of 95% ethanol and 5 mL of 85% phosphoric acid, followed by dilution with water to 100 mL. After overnight stirring, the solution was filtered and stored in the dark. It was stable for several weeks. The CBB concentration was calculated from the absorptivity at 535 nm ( ) 7500), the isosbestic point in the spectrum of a mixture of bound and free CBB. Binding experiments were performed by varying the star polymer concentration at constant [CBB]. The fractions of free and bound CBB (see also Figure 8) were calculated from the absorptivities of free CBB at λm ) 470 nm (m ) 34 000) and λ ) 621 nm ( ) 5100), and that of bound CBB was calculated from those at λm ) 621 nm (m ) 34 000) and λ ) 470 nm ( ) 2000). The four absorptivities were calculated using the reported  ) 7500 at 535 nm.15 Solutions of PB- were obtained by dissolving 1-pyrenebutyric acid in water with a slight excess of LiOH. The [PB-] was measured from the λm at 341 nm ( ) 41 000).16 Fractions of free and bound PB- (see also Figure 9) were obtained from the known absorptivities of free PB- (m ) 41 000 at λm ) 341 nm and  ) 24 000 at λ ) 344 nm) and that of polymer bound PB- (m ) 39 000 at λm ) 344 nm and  ) 25 000 at λ ) 341 nm).

Results and Discussion Synthesis and Characterization. The reaction between the trifunctional isocyanate T3PM and MPEG with TMEDA as catalyst proceeds smoothly to completion at 50 °C in about 3 h. However, small quantities of higher molecular weight polymers are found when equivalent amounts of isocyanate and hydroxyl groups are used in the reaction with MPEG 750 and MPEG 1900. This most likely results from the well-known side reaction between an unreacted NCO group and the carbamate function of the star polymer. To minimize this, excess MPEG is used. Removal of the excess glycol by ultrafiltration is facilitated by the tendency of T3PM-MPEG star polymers to form aggregates in water at high concentration (vide infra) while MPEG does not. The same was found for the T3TMIMPEG star polymers.10 Some product is lost as a result of the ultrafiltration step, but the yields are still about 80%. Gel permeation chromatography (GPC) tracings of the purified star polymer show only one peak, while two peaks are found before ultrafiltration.13 The 1H and 13C NMR spectra, shown for T3PM-MPEG 750 in Figures 1 and 2, are in agreement with the expected structure. The number average MW (Mn) can be calculated from the 1H NMR spectrum assuming that T3PM is a trifunctional isocyanate. For T3PM-MPEG 750 an average number of 2.95 arms was calculated, giving an Mn ) 2430. While T3TMI is a pure trifunctional isocyanate,11 the commercial T3PM is known to contain small amounts of isocyanates with different functionalities. This should also be reflected in the star polymer product. However, the chief component certainly is the tristar. Aggregation of T3PM-MPEG Star Polymers. Plots of the surface tension (in dynes/cm) versus the log of the star polymer concentration for the three T3PM-MPEG star polymers are depicted in Figure 3. Their shapes are more complex than those found for the corresponding D4TMI-MPEG stars. The latter exhibit only one transition coinciding with the formation of micelles. The T3PMMPEG plots have additional discontinuities in the low concentration range. The critical micelle concentrations (cmc’s) are 3 × 10-3 M at γ ) 48.2 for T3PM-MPEG 350, (15) Read, S. M.; Northcote, D. H. Anal. Biochem. 1981, 116, 53. (16) Roland, B.; Smid, J. Polymer 1984, 25, 1166.

Amphiphilic Poly(ethylene oxide) Star Polymers

Figure 2.

13C

Langmuir, Vol. 12, No. 9, 1996 2209

NMR spectrum of T3PM-MPEG 750 (75.6 MHz) in CDCl3.

Figure 3. Surface tension dependence on log [star] for T3PMMPEG star polymers in water: (b) T3PM-MPEG 350; (9) T3PM-MPEG 750; (2) T3PM-MPEG 1900.

5.9 × 10-3 M at γ ) 46.4 for T3PM-MPEG 750, and a value larger than 2 × 10-2 M for T3PM-MPEG 1900. For the same MPEG arm length the cmc’s for the corresponding D4TMI-MPEG star polymers are 0.9 × 10-4, 1.6 × 10-4, and 3.0 × 10-4 M, respectively.10 This is more than an order of magnitude lower than those for the T3PM stars and is the result of the much smaller size of the hydrophobic T3PM core relative to that of D4TMI. The cause of the discontinuities in the surface tension plots at concentrations around 10-4 and 10-5 M (Figure 3) is not immediately obvious. Initially we did not find them for the D4TMI-MPEG stars,10 but a repeat of the γ measurements for the D4TMI-MPEG 1900 star polymer revealed a second transition close to 10-5 M, besides that at 3 × 10-4 M. Other authors have recently reported peculiar observations in aqueous solutions of amphiphilic macromolecules. For example, two transition points in surface tension plots were reported for comblike polymers with short PEO branches.17 The phenomenon was attributed to the formation of different polymer aggregates, each with their own characteristic surface absorption (17) Sivadasan, K.; Somasundaran, P. Colloids Surf. 1990, 49, 229.

Figure 4. Viscosity plots for the T3PM-MPEG 750 star polymer at 25 °C in (9) THF and (2) water.

behavior. Surface pressure/area isotherms for monolayers of amphiphilic diblock ionomers18,19 and starlike octopus molecules20 have also revealed more than one transition, probably caused by changes in the polymer conformation of the macromolecular species or their aggregates at the air/water interface.18-20 Such changes are likely to affect the surface tension of these macromolecules. Our binding studies (vide infra) suggest that below the cmc of the star polymers aggregates of a smaller size exist. Viscosity Measurements. Plots of reduced viscosity, nsp/c, versus concentration of the star polymer T3PMMPEG 750 in THF and H2O are shown in Figure 4. While the Huggins plot in THF is linear, a clear transition point at 1.4 g/dL or 5.7 × 10-3 M star polymer is found in water. This is very close to the cmc value of 6.0 × 10-3 M found for this star from surface tension measurements. Note the downward inflection in the plot, as was also observed for D4TMI-MPEG stars.10 Aggregation in aqueous (18) Zhu, J.; Eisenberg, A.; Lennox, R. B. J. Am. Chem. Soc. 1991, 113, 5583. (19) Zhu, J.; Eisenberg, A.; Lennox, R. B. Macromolecules 1992, 25, 6556. (20) Conner, M.; Kudelka, I.; Regen, S. L. Langmuir 1991, 7, 982.

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Chen and Smid

Figure 5. Phase diagram for T3PM-MPEG 750 in water.

solutions of linear amphiphilic polymers with a random distribution of hydrophilic and hydrophobic groups usually results in an increase in the slope of the reduced viscosity plot. In our system core/core interaction between the individual star molecules may well lead to their partial alignment in the aggregate. To a degree it would resemble the situation when rodlike macromolecules aggregate. In that case the deviation in the Huggins plot is in the same direction as that found in our system. Extrapolation of the Huggins plot in water below the cmc yields an intrinsic viscosity [η] ) 0.038, considerably less than the value of 0.09 obtained from the plot in THF. This may suggest a more extended conformation of the star polymer in THF or the presence of small aggregates in water below the cmc, as argued earlier from the peculiar surface tension plots. The Huggins plot above the cmc extrapolates to a value of [η] ) 0.058. Aggregates of T3PM-MPEG 750 may be compared to a multiarm star polymer. It has been argued21 that in a θ solvent the intrinsic viscosity of a multiarm star polymer is below that of a linear polymer whose MW is twice that of the star arm. From the value [η] ) 0.17 dL/g for MPEG 5000 in H2O22 and the relationship [η] ) 0.0125 M0.78 for low MW PEO in water23 an [η] ) 0.066 dL/g can be calculated for a PEO of MW ) 1500 (twice that of the MPEG arm of our T3PM-MPEG 750 star). Hence the [η] ) 0.058 dL/g for our star polymer micelle is slightly below that calculated for MPEG 1500, consistent with observations of Bauer et al.21 for regular star polymers. Lower Critical Solution Temperature and Cloud Points. The incipient phase separation diagram for an aqueous solution of T3PM-MPEG 750 is depicted in Figure 5. The experimental points are cloud points measured for solutions containing different weight percentages of the star polymer. The minimum in the Tp curve as a function of wt % star is found at 2 ( 1 wt % T3PM-MPEG 750, the LCST being 72 °C. Cloud points for the three T3PM-MPEG star polymers were determined for 3 wt % solutions. In the region Tp is essentially identical to the LCST of the polymer. Figure 6 shows a plot of Tp values versus the HLB number of the polymers. This number is defined as the ratio 20MH/(MH + ML), where MH and ML denote the respective formula weights of the hydrophilic and lipophilic parts of the molecules. The plot for D4TMI-MPEG stars obtained by Zhou10 is also reproduced in Figure 6. It nearly overlaps (21) Bauer, B. J.; Fetters, L. J.; Graessley, W. W.; Hadijichristides, N.; Quack, G. F. Macromolecules 1989, 22, 2337. (22) Zhou, G. Ph.D. Thesis, State University of New York, Syracuse, NY, 1992. (23) Brandrup, J.; Immergut, E. H. Polymer Handbook, 2nd ed.; John Wiley & Sons: New York, 1975; p IV-23.

Figure 6. Plot of cloud point versus the hydrophile-lipophile balance (HLB) number for T3MP- and D4TMI-MPEG star polymers. [Star] ) 3 wt %. Table 1. Tp° and Ks Values for T3PM-MPEG Star Polymers Ks polymer

MW

Tp°, °C

salt

°C/m

-71.3 43.1 Na2SO4 NaH2PO4 -44.1 T3PM-MPEG 750 2460 75.2 Na2SO4 -96.8 NaH2PO4 -50.1 T3PM-MPEG 1900 6190 104.0 Na2SO4 -140.1 NaH2PO4 -76.0

T3PM-MPEG 350

1400

°C/unit charge -17.8 -22.0 -24.2 -25.0 -35.0 -38.0

with that of the T3TMI-MPEG stars (not shown in Figure 6). The results demonstrate that, for the same HLB number, star polymers with a T3PM core have a much lower cloud point than those possessing the larger T3TMI or D4TMI core.10 This is especially true for stars with short MPEG arms. For example, D4TMI-MPEG 750 has an HLB ) 14.5 and a Tp ) 84 °C,10 while T3PM-MPEG 350 has nearly the same HLB number (14.6) but its Tp is only 43.1 °C (Table 1). To explain this observation, it should be pointed out that the Tp measurements were carried out at a concentration exceeding the cmc of the star polymers. The more planar T3PM structure may well result in a higher aggregation number for its star polymer micelles than those for micelles containing the more bulky T3TMI or D4TMI core. For large aggregates the EO units close to the hydrophobic cores will be less exposed to water molecules due to the high density of MPEG chains around the hydrophobic region. This in turn adversely affects the water solubility of the stars, especially when the MPEG arms are short. For the same HLB number a T3PM star has a much shorter MPEG arm than a D4TMI star (vide supra). Therefore, the lower Tp for the former stars may not be surprising. The salt effect on Tp was checked with two “salting out” electrolytes, NaH2PO4 and Na2SO4. Plots of Tp versus molal salt content (Cs) for three T3PM-MPEG star polymers in NaH2PO4-containing solutions are shown in Figure 7. Similar plots were obtained with Na2SO4. Consistent with results reported for D4TMI-MPEG star polymers10 a linear correlation exists between Tp and Cs

Tp ) Tp° + KsCs

(1)

where Tp° denotes the cloud point in the salt-free solution and Ks is a constant which reflects the effectiveness of the salt to modify the stability of aqueous star polymer solutions.

Amphiphilic Poly(ethylene oxide) Star Polymers

Langmuir, Vol. 12, No. 9, 1996 2211

Figure 7. Plots of cloud point versus [NaH2PO4] for T3PMMPEG star polymers. [Star] ) 3 wt %.

Ks values for the T3PM-MPEG star polymers are listed in Table 1. They have been expressed in units of °C/molal and °C/unit charge, the latter being preferred when comparing salts with different valences. For example, on a molal basis Na2SO4 is much more effective in lowering Tp than the monovalent NaH2PO4, but on a unit charge scale the difference is minimal, the biphosphate salt being slightly more effective. Garvey and Robb24 have reviewed the salt effect for polymers with segments of EO units. It is believed to originate from a disturbance of the hydration layer around PEO segments. Salt-deficient zones are created around these segments causing a nonuniform distribution of ions, a phenomenon that is chiefly controlled by the nature of the salt anion. The salt effect is more pronounced for stars with long MPEG arms (Table 1). For T3TMI-MPEG and D4TMIMPEG polymers a linear correlation exists between Ks and the inverse of the MW of the MPEG arm.10 This is not the case for the three T3PM-MPEG star polymers, the effect of salt being unusually large for T3PM-MPEG 1900. This can be rationalized by noting that a 3 wt % star polymer solution (the concentration used for Tp measurements) falls above the cmc’s of T3PM-MPEG 350 and T3PM-MPEG 750 but below that of T3PM-MPEG 1900. Hence, for the latter polymer more EO units may be exposed to the salt solution than if it were aggregated. This would make it more sensitive to the effects of salt. Other complications may arise from changes in the cmc due to the presence of salt and from the complex aggregation pattern of T3PM-MPEG stars (vide supra). Binding of Hydrophobic Molecules. It may be anticipated that in an aqueous solution the rather large hydrophobic cores of the star polymers will attract hydrophobic organics. This could provide additional information on the micellization in the polymer systems, since the interactions are expected to be influenced by a change in the aggregation state of the star polymers. Coomassie brilliant blue (CBB), a dye frequently used to assay proteins,15,25 and 1-pyrenebutyrate (PB-) were the probes of choice. Figure 8 depicts changes in the optical spectrum of 10-5 M CBB on adding the star polymer T3TMI-MPEG 550. The reddish-brown cationic form of this dye (λm ) 470 nm) is replaced by a polymer-bound species with λm ) 621 nm and an absorptivity about thrice that of the free dye. The isosbestic point (λm ) 535 nm) implies an equilibrium between the two species. The polymer-bound species is probably the neutral form of (24) Garvey, M. J.; Robb, I. D. J. Chem. Soc., Faraday Trans. 1979, 75, 993. (25) Compton, S. J.; Jones, C. G. Anal. Biochem. 1985, 151, 369.

Figure 8. Changes in the optical spectrum of coomassie blue in water on adding T3TMI-MPEG 550. [CBB] ) 10-5 M; [star polymer] ) 2 × 10-5 to 6.4 × 10-3 M.

Figure 9. Changes in the optical spectrum of 1-pyrenebutyrate (PB-) on adding D4TMI-MPEG 550 ([PB-] ) 2 × 10-5 M).

CBB known to absorb at 650 nm.15,25 The hypsochromic shift to 621 nm appears reasonable considering that a similar shift occurs for the dye methyl orange on binding to the hydrophobic domain of an amphiphilic polymer.26 Pyrene and its derivatives have frequently been employed as optical and fluorescent probes in the study of micelles27 and amphiphilic macromolecules.28 Their optical spectra exhibit a bathochromic shift when solubilized in the hydrophobic domain of amphiphilic macromolecules in water.16 Figure 9 shows changes in the spectrum of an aqueous PB- solution when adding the star polymer (26) Roland, B.; Kimura, K.; Smid, J. J. Colloid Interface Sci. 1984, 97, 392. (27) Rosen, M. J.; Cohen, A. W.; Dahanayake, M.; Hua, X. Y. J. Phys. Chem. 1982, 86, 54. (28) Thomas, J. K. Acc. Chem. Res. 1977, 10, 133.

2212 Langmuir, Vol. 12, No. 9, 1996

Chen and Smid Table 2. Binding Parameters for CBB and PB- in Aqueous Solutions of T3TMI-MPEG 550 and D4TMI-MPEG 550

Figure 10. Klotz plots for the binding of CBB to (b) T3TMIMPEG 550 ([CBB] ) 5.85 × 10-5 M; [star] ) 4.0 × 10-5 to 1.5 × 10-3 M) and (9) D4TMI-MPEG 550 ([CBB] ) 3.56 × 10-5 M; [star] ) 1.5 × 10-5 to 1.1 × 10-3 M).

Figure 11. Klotz plots for the binding of PB- to (b) T3TMIMPEG 550 ([PB-] ) 1.77 × 10-5 M; [star] ) 4.0 × 10-5 to 5.5 × 10-4 M) and (9) D4TMI-MPEG 550 ([PB-] ) 1.9 × 10-5 M; [star] ) 2.6 × 10-5 to 4.3 × 10-4 M).

D4TMI-MPEG 550. The sharp absorption maxima of the free PB- at 326 and 341 nm are replaced by maxima at 328.5 and 344 nm, respectively. Although the shifts are even smaller than found earlier for the system PB-/ poly(vinylbenzo-18-crown-6),16 the maxima of free and bound PB- are clearly distinguishable, as are the isosbestic points. The fractions of free and bound solute can be computed from the respective spectra (see Experimental Section). The data were plotted in the form of the Klotz expression29

1/R ) 1/n + 1/nKA

(2)

where 1/R denotes the ratio of the total star polymer concentration to bound CBB or PB-, A is the molar concentration of the free probe, and 1/n represents the minimum number of star molecules bound per CBB or PB- molecule. Klotz plots for the binding of CBB to T3TMI-MPEG 550 and to D4TMI-MPEG 550 are shown in Figure 10. Figure 11 has similar plots for PB-. All four plots exhibit two linear sections with a clear transition point. The discontinuities most likely are related to changes in the aggregation state of the respective star polymers as their concentration is increased. Intercepts and slopes of the respective plots were determined in the concentration range below and above the transition point. (29) Klotz, I. M.; Walker, F. M.; Pivan, R. B. J. Am. Chem. Soc. 1946, 68, 1486.

star polymer

1/n

T3TMI-MPEG 550 D4TMI-MPEG 550

1.0 2.5

T3TMI-MPEG 550 D4TMI-MPEG 550

4.9 4.3

1/n1

10-4K1, M-1

104, M

CBB 0.81 0.85

9.4 9.0

1.19 1.69

7.2 4.2

PB1.05 2.69

12.5 7.6

1.96 4.04

2.1 1.8

10-4K1, M-1

Those below this point are denoted by 1/n and K1, and those above by 1/n1 and K11, where K1 ) nK and K11 ) n1K1 are the first binding constants for the particular system. The data are collected in Table 2, together with the star polymer concentration at which the transition point in the respective plots is observed and which we have equated with the cmc of the star polymer. For the two star polymers only the cmc of D4TMIMPEG 550 was measured by surface tension, its value being 1.3 × 10-4 M.10 This is three times lower than the value 4.2 × 10-4 M found for the transition point in the star polymer solution with CBB (Table 2). The reason for this difference may be the CBB solution itself, which contains 10% ethanol and 5% phosphoric acid. The presence of ethanol is likely to raise the cmc of the star polymer solution. The PB- solution contains no ethanol, and the transition point of 1.8 × 10-4 M is much closer to the surface tension cmc value. That it is not exactly the same is not surprising, since cmc values from dye binding studies often yield values different from those of surface tension measurements. The dye binding disturbs the aggregate, especially when the dye is charged, and a slightly higher cmc value is, therefore, not unreasonable. The 1/R values of Figures 10 and 11 were calculated on the assumption that the star polymer in solution is present in the form of single molecules or unimers. If that were the case, 1/n should be unity or less, depending on whether one or more than one probe molecule is associated with the star polymer molecule. For three of the four systems 1/n is greater than unity (Table 2). This suggests that aggregates may exist even below the cmc of the star polymer, albeit that the degree of aggregation is less than that above the cmc, as indicated by the much higher value for 1/n′. The bound probe itself may, of course, alter the aggregate structure. Also, the accuracy of the 1/n values is affected by the uncertainty in the extrapolation of the respective plots. Nevertheless, the conclusion that small aggregates may persist below the cmc is not unreasonable considering the surface tension results for star polymers with a T3PM or a D4TMI core. Repulsion effects are likely to limit the number of PBanions bound to a star polymer aggregate. This is suggested by earlier data on the binding of PB- anions to the hydrophobic domain of poly(vinylbenzo-18-crown-6) in water. A polymer chain of one hundred vinylbenzyl18-crown-6 units can adsorb a maximum of only four PBanions.16 This number increases to forty when the neutral amphiphilic polymer is converted into a polycation by adding crown ether-complexable cations such as K+ or Cs+ ions. This neutralizes the bound PB- ions and reduces the electrostatic repulsion. The values of 1/n and 1/n′ only yield the minimum number of star polymer unimers per bound CBB or PBmolecule, not the actual size of a star aggregate. Information on their size could possibly be obtained from lightscattering measurements, but an accurate instrument was not at our disposal. In all four systems the 1/n′ values are in the neighborhood of ten. This suggests that above the

Amphiphilic Poly(ethylene oxide) Star Polymers

cmc the star aggregates may contain roughly ten unimers, or a multiple of ten if more than one probe molecule is bound to the aggregate. Finally, judging from the values of the first binding constants, the interaction between probe molecules and star polymers is quite strong, both above and below the cmc of the stars. For example, at 25 °C K1 values for PBbinding to the two stars are two to six times the value found with the poly(crown ether).16 In our recent studies on the phase separation of associative PEO star polymers in water, it was found that hydrophobic organics accumulate in the polymer-rich phase because of its high

Langmuir, Vol. 12, No. 9, 1996 2213

hydrophobic content.12,13 The results with the homopolymer stars demonstrate that binding of such organics to polymer species present in the polymer-poor phase cannot be ignored and hinders their transfer to the polymer-rich phase. Acknowledgment. The authors gratefully acknowledge the financial support of the Petroleum Research Fund (Grant No. 25836AC7P) administered by the American Chemical Society. LA950963P