Polysoaps in Aqueous Solutions: Intermolecular versus Intramolecular

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Articles Polysoaps in Aqueous Solutions: Intermolecular versus Intramolecular Hydrophobic Aggregation Studied by Fluorescence Spectroscopy Didier Cochin,‡,| Frans C. De Schryver,*,† Andre´ Laschewsky,‡ and Jan van Stam*,†,§ Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, BE-3001 Heverlee, Belgium, and Department of Chemistry, Universite´ Catholique de Louvain, Place L. Pasteur 1, BE-1348 Louvain-la-Neuve, Belgium Received August 7, 2000. In Final Form: January 29, 2001 A series of structurally related, poly(methacrylate)-based, cationic polysoaps of various geometries is studied by static and dynamic fluorescence measurements. The aggregation behavior in water is investigated by labeling the end groups of the polysoap backbones with a naphthyl fluorophore. The results strongly favor the model of “regional micelles”, which evolve with increasing concentration from a mainly intramolecular association to an intermolecular one.

1. Introduction A particular subclass of amphiphilic polymers are the so-called polysoaps. They consist of individual low molar mass surfactant fragments attached to a polymer backbone. The interest in polysoaps arises from, for example, their ability to form intramolecular hydrophobic associates, thus lacking an equivalent to a critical micelle concentration, and from the possibility to functionalize the side-chains.1-4 When a polysoap is synthesized, the surfactant fragments can be incorporated in different ways, leading to the so-called head type, mid-tail type, tail-end type, and main chain geometry, as schematically shown in Figure 1. The geometry is decisive for the solution behavior of the polysoap, in addition to other factors, such as the hydrophilic-hydrophobic balance, the chemical structure of the backbone, the density of surfactant side-chains, and the distance between the backbone and the surfactant chain. For example, the geometry of polysoaps strongly influences their surface activity in aqueous solution which can range from negligible to rather substantial.2,4 Still, some generalities are found. Different from hydrophobically associating water-soluble polymers, the viscosity of aqueous polysoap solutions remains low up to high concentrations. This feature originally initiated the study of polysoaps5 and indicates a primary intramolecular * Authors to whom correspondence should be addressed. E-mail: [email protected]; [email protected]. † Katholieke Universiteit Leuven. ‡ Universite ´ Catholique de Louvain. § Present address: Department of Physical Chemistry, Karlstad University, SE-651 88 Karlstad, Sweden. | Present address: SNF Floerger, rue Jean Huss 41, FR-42028 St-Etienne, France. (1) Strauss, U. P. In Polymers in Aqueous Media; Glass, J. E., Ed.; American Chemical Society: Washington, DC, 1989; p 317. (2) Anton, P.; Ko¨berle, P.; Laschewsky, A. Makromol. Chem. 1993, 194, 1. (3) Morishima, Y. Trends Polym. Sci. 1994, 2, 31. (4) Laschewsky, A. Adv. Polym. Sci. 1995, 124, 1.

Figure 1. Schematic presentation of (a) head, (b) mid-tail, (c) tail-end, and (d) main chain type polysoaps.

aggregation, as it demonstrates that no polymer networks are formed, but suggests a primarily intramolecular hydrophobic aggregation. As compared to surfactants of low molar mass which form micelles with a fluidlike interior, the hydrophobic domains of polysoaps seem to be more restricted in their dynamics. Nevertheless, polysoaps exhibit substantial dissolving capacities or may serve as emulsifiers and dispersing agents.4,6-9 A question under debate is the nature of the hydrophobic domains provided by polysoaps. Three models have been proposed in the past, referred to as the “local micelle” model, the “regional micelle” model, and the “molecular micelle” model.4 In the local micelle model, neighboring surfactant side-chains aggregate to form a “string of beads” along the backbone.1,10-12 In the molecular micelle model, the whole polysoap molecule is involved in forming one (5) Strauss, U. P.; Jackson, E. G. J. Polym. Sci. 1951, 5, 649. (6) Palmer, C. P.; Terabe, S. Anal. Chem. 1997, 69, 1852. (7) Palmer, C. P.; Tanaka, N. J. Chromatogr., A 1997, 792, 105. (8) Perrin, P.; Monfreux, N.; Lafuma, F. Colloid Polym. Sci. 1999, 277, 89. (9) Billiot, E.; Warner, I. M. Anal. Chem. 2000, 72, 1740. (10) Cochin, D.; Candau, F.; Zana, R.; Talmon, Y. Macromolecules 1992, 25, 4220. (11) Hu, Y.; Armentrout, R. S.; McCormick, C. L. Macromolecules 1997, 30, 3538. (12) Roualt, Y.; Marques, C. J. Phys. II 1997, 7, 903.

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aggregate by folding the backbone and so enabling virtually all surfactant side-chains to aggregate.13-15 The regional micelle model is intermediate to these two models in that it assumes the association of a small number of neighboring chains into local segments, which cluster together, for example, because of a folding of the backbone.2,3,16,17 As in the model of the local micelle, the hydrophobic chains may only partially be involved in the aggregation. In this article, a series of structurally related cationic poly(methacrylate) soaps of the head type and of the tailend type are studied by time-resolved fluorescence measurements. By labeling the end groups of the polysoap backbones with a fluorophore, here a naphthalene derivative, the aggregation behavior is investigated. Particular conclusions can be drawn when comparing the fluorescence decay at several polysoap concentrations with similar measurements on an unlabeled polysoap with the analogous unbound fluorophore added. In addition, possible aging of such polysoap solutions is investigated. On one hand, the poly(methacrylate) soaps may be potentially degraded because of the hydrolytically labile ester bonds of the surfactant fragments as well as the ester bond of the labels. On the other hand, slow rearrangements of polysoap self-organization in aqueous solution have been reported in the past,1,18-20 a behavior investigated for the specific polysoap systems used in this study. By steady-state fluorescence measurements of labeled polysoaps and by comparison with a degradable model compound, the aging was followed.

Cochin et al. Chart 1. Chemical Structures of the Compounds Used

2. Experimental Section 2.1. Synthesis and Labeling of the Polysoaps. The compounds used are shown in Chart 1. The synthesis of 1-3 and 5-7 was reported before.21,22 Homopolymer 5 was obtained by free radical polymerization of N-(11-methacryloyloxyundecyl)N-2-hydroxyethyl-N,N-dimethylammonium bromide in ethanol using the naphthalene-labeled initiator 6. Copolymers 1-3 were made analogously from N-2-methacryloyloxyethyl-N,N,N-trimethylammonium bromide (“choline methacrylate”) and N-2methacryloyloxyethyl-N-decyl-N,N-dimethylammonium bromide. Copolymer 4 is newly synthesized under identical conditions from choline methacrylate and N-2-methacryloyloxyethyl-Ndecyloxycarbonylmethyl-N,N-dimethylammonium bromide.23 According to the analysis of poly(methyl methacrylate) prepared under identical conditions, the polymers should bear on average 1.2 naphthalene end groups, resulting from mixed chain termination by combination and by disproportionation. The strong absorbance of these end groups enabled us to estimate molar mass data via end-group determination, as this method is independent of the physical state of the macromolecules. Otherwise, for polymers of the kind used here, that is, for associative polyelectrolytes,24 molar mass data must be taken with much caution, as it is almost impossible to exclude (at least (11) Kammer, U.; Elias, H.-G. Kolloid Z. Z. Polym. 1972, 250, 344. (12) Finkelmann, H.; Jahns, E. In Polymer Association Structures; El-Nokaly, M. A., Ed.; ACS Symposium Series 384; American Chemical Society: Washington, DC, 1989; p 1. (13) Hamad, E.; Qutubuddin, S. J. Chem. Phys. 1992, 96, 6222. (14) Borisov, O. V.; Halperin, A. Langmuir 1995, 11, 2911. (15) Borisov, O. V.; Halperin, A. Macromolecules 1997, 30, 4432. (16) McCormick, C. L.; Chang, Y. Macromolecules 1994, 27, 2151. (17) Yeoh, K. W.; Chew, C. H.; Gan, L. M.; Koh, L. L.; Ng, S. C. J. Macromol. Sci., Chem. 1990, A27, 711. (18) Kramer, M. C.; Welch, C. G.; Steger, J. R.; McCormick, C. L. Macromolecules 1995, 28, 5248. (19) Anton, P.; Laschewsky, A. Makromol. Chem., Rapid Commun. 1991, 12, 189. (20) Cochin, D.; Laschewsky, A.; Nallet, F. Macromolecules 1997, 30, 2278. (21) Yegorov, V. V.; Batrakova, Y. V.; Zubov, V. P. Polym. Sci. U.S.S.R. 1990, 32, 861. (22) Cochin, D.; Hendlinger, P.; Laschewsky, A. Colloid Polym. Sci. 1995, 273, 1138.

partial) aggregation of the macromolecules in solution. On the basis of the assumption of 1.2 naphthalene end groups per macromolecule, end-group analysis via UV spectroscopy gives number average molar masses of polymers 1-5 of 85 000, 110 000, 60 000, 45 000, and 115 000, respectively. In the case of homopolymer 5, an analogous, unlabeled sample 5* was prepared by polymerization with the initiator AIBN under otherwise identical conditions. All polymers are freely watersoluble. The reference fluorophore 8 was made by standard procedures from isobutyric acid chloride with 2-naphthylethanol. The use of label molecules can give rise to problems, leading to difficulties in interpretation or even resulting in misinterpretation of the experimental data. For example, labels may disturb the structure to be studied; for example, they may modify the aggregation of the polysoaps, labels may be found in an atypical environment, or they may be distributed over various, different environments, causing difficulties in deconvolution. To minimize these possible drawbacks, an end-tagged naphthalene label was chosen for two main reasons. First, naphthalene is a small, rather hydrophobic molecule, that hardly shows any solvatochromism but is still sensitive to the polarity of its environment. There is only a minor possibility that the naphthalene moiety would interfere with the association process of the hydrophobic chains of the polysoap. The hydrophobicity and the lack of solvatochromism make naphthalene an almost ideal

Polysoaps in Aqueous Solutions fluorescent probe to study changes in the local polarity of the hydrophobic domains without changes in spectral shape due to differences in the polysoap charge densities. Second, to reduce the number of labels as much as possible and, hence, minimize the possible disturbances, the label was end-tagged. The end-labeling strategy is superior to the statistical copolymerization of a small amount of polymerizable label with the other monomers or to the statistical chemical modification of the preformed polymers, as it ensures a minimum of disturbance of the polymer chain and allows only a defined number of labels per chain. Moreover, having only one or two labels per macromolecule, their position at the end(s) of the polymer chain probably makes them more sensitive to changes in aggregation mode, for example, going from intramolecular aggregation to intermolecular. 2.2. Methods. Time-resolved fluorescence measurements were performed with previously described equipment.25 The excitation wavelength, as obtained by frequency-doubled pyrromethene 556 (Exciton) emission, was 290 nm, and the emission was monitored at 325 nm. All fluorescence decays were observed at the magic angle. They contained about 10 000 peak counts in 512 channels of the multichannel analyzer, of which about 450 were used in the fittings, starting from the rising edge. The time increment was 48 ps/channel, and the reference compound for deconvolution was POPOP (p-bis[2-(5-phenyloxazolyl)]benzene, solvent ) methanol, decay time ) 1.1 ns). The steady-state spectra were recorded in the right-angle mode on a SPEX Fluorolog 1680 combined with a SPEX Spectroscopy Laboratory Coordinator DM1B. To avoid intensity changes due to changes in the equipment, all long-time study intensities were normalized to the intensity of the background level. The samples (polysoaps 1, 3, 4, and 5) for stability measurements were kept in sealed fluorescence cells in the dark and measured at several times during 1 year (except for 4 that was investigated for about 4 months). Fresh solutions of the same polysoaps were hydrolyzed by NaOH in order to determine the effect of hydrolysis. NaOH was also used to hydrolyze the aged stability samples at the end of the long-time studies to check whether they were degraded or not. All measurements were performed at 20 °C. Deionized water of Milli-Q quality was used for all solutions. All polymer concentrations are given in % w/w. 2.3. Polysoap Aging. The fluorescence probe, 8, is a naphthalene derivative. Even though naphthalene does not show notable differences in its steady-state emission spectrum when going from a polar environment to a nonpolar one, its quantum yield increases by a factor of 1.6 when transferred from water to the nonpolar interior of a micelle.26-28 This can be used to probe the long-time stability of the polysoaps, as a gradual degradation of the polymer will lead to more fluorescent probe molecules in the aqueous bulk and, consequently, to a lower fluorescence intensity. The magnitude of this effect was estimated by control experiments performed on 1 and 4. To fresh solutions of these polymers, NaOH was added to yield a final concentration of 1 M. The strong base induces polysoap degradation via enforced ester hydrolysis. The presence of the strong base did not induce a change in shape or position of the steady-state fluorescence spectra. The fluorescence intensity of 1 and 4 at the emission maximum as a function of time is given in Figure 2a,b. During the first 4 h, the intensity decreases by 50-60% of the original value, whereupon it remains constant. This decrease corresponds very well to the difference in quantum yield between naphthalene in a nonpolar and in a polar environment, respectively. It is indicative of a polymer degradation and subsequent removal of the naphthalene moiety from the hydrophobic micellar interior into the aqueous bulk. (23) Khalil, M. M. H.; Boens, N.; Van der Auweraer, M.; Ameloot, M.; Andriessen, R.; Hofkens, J.; De Schryver, F. C. J. Phys. Chem. 1991, 95, 9375. (24) Reekmans, S. Ph.D. Thesis, Katholieke Universiteit Leuven, Heverlee, Belgium, 1993. (25) Nowakowska, M.; Loukine, N.; Gravett, D. M.; Burke, N. A. D.; Guillet, J. E. J. Am. Chem. Soc. 1997, 119, 4364. (26) Evans, C. H.; Partyka, M.; van Stam, J. J. Inclusion Phenom. Macrocyclic Chem. 2000, 38, 381.

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Figure 2. Stability studies of the polysoaps. (a and b) Normalized fluorescence intensity as a function of time for the control measurements. To freshly prepared solutions of polysoap 1 (a) and 4 (b), NaOH was added to yield a final concentration of 1 M, whereupon the emission spectra were measured at several times during 5 h. The decrease in intensity is due to hydrolysis of the polymers, leading to an increased amount of the probe fragment 8 or of 2-naphthylethanol in the aqueous bulk. (c) Long-term study of the stability of polysoap 4. Note that the time axis is in the logarithmic scale. (d) Hydrolysis by NaOH of the sample of polysoap 4 used for the long-time study, as described in the text.

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The aging behavior of polysoaps 1, 3, 4, and 5 in aqueous solution was investigated. By this choice of polysoaps, the possible effect of hydrophobic content, potentially labile moieties, and the geometry could be studied. Note that the additional ammonioacetate moiety in copolymer 4 is particularly sensitive to hydrolysis; aqueous solutions of the monomer show notable hydrolysis in the 1H NMR spectrum after storage for 1 week under ambient conditions. Figure 2c shows the normalized fluorescence intensity of 4 as a function of time as an illustrative example. The analysis of the fluorescence intensity changes by time gave further insight about the stability of the investigated polymers. For copolymer 1, the intensity remained constant during the whole study. Homopolymer 5 shows a constant intensity as a function of time too, indicating that no physical aging or substantial degradation occurs. For the copolymers 3 and 4, however, the average intensity goes slowly down with time, after a period of constant intensity (approximately 100-150 days). In parallel, polymer solutions in D2O were checked regularly by thin-layer chromatography and by 1H NMR spectroscopy. No indication for degradation could be detected after 5 months. Most probably, these methods are less sensitive to small changes than fluorescence spectroscopy. Concerning potential hydrolysis, the discussion is complicated by the fact that three potential weak sites are found in the polymers: (i) the link of the surfactant fragment to the poly(methacrylate) backbone, (ii) the link of the end-group label, and in the case of polysoap 4 (iii) the link of the hydrophobic chain to the cationic moiety. For times longer than 6 months, there may be some subtle differences in hydrolytic stability, but for shorter periods the polysoaps are fully stable. Thus, such polymers, and presumably all related poly(methacrylate)-based soaps which are widespread in the literature,4 are indeed useful systems for fundamental studies. Concerning applications, long time stability is given under ambient conditions at neutral pH, whereas the sensitivity of the systems should be accounted for at high pH values. This conclusion is supported by studies on the induced hydrolysis of 1 year old solutions of 1, 4, and 5. To these solutions, NaOH was added to yield a final concentration of 1 M. The fluorescence intensity was measured as a function of time, and the result of 4 is shown in Figure 2d. All samples show a decrease in fluorescence intensity with time. Polysoap 4 shows a pronounced decrease, explained by the presence of particularly labile ester moieties. Polysoaps 1 and 5 show almost identical decrease rates, strengthening the conclusion that they are similar in stability.

3. Results and Discussion Fluorescence Decay Measurements. The fluorescence decay of a probe molecule is sensitive to changes in its local environment. For a polysoap, this means that changes in the hydrophobic association, for example, going from purely intramolecular aggregation to an additional intermolecular one, are likely to induce changes in the decay profile. For naphthalene, it is known that the decay time in water is shorter than in a less polar environment, for example, the interior of a hydrophobic microdomain.26 This property is used to probe the aggregation behavior of the polysoaps. For polysoaps 1-3, of the head type, dilution experiments were performed, and the fluorescence decays at 0.5%, 0.05%, and 0.005% polymer concentration were measured (Figure 3). Because of the multiexponential character of the decays, at least a triexponential decay function is needed to describe the decays; only qualitative conclusions will be drawn from these measurements. In the case of polysoaps 1 and 2, there is not much of a difference between the two highest concentrations, whereas at the lowest concentration the decay is differing significantly. The decays of polysoap 3, however, are diverging also at the two highest concentrations. Keeping in mind that there is a significant difference in hydrophobicity between the latter and the first two (see Chart 1), this

Figure 3. Fluorescence decays. (a-c) Different concentrations of polysoaps 1 (a), 2 (b), and 3 (c), labeled by the fluorophore 8. The polysoap concentrations were 0.5% (2), 0.05% (9), and 0.005% (b). (d) Comparison of the decays of 0.5% 1 (b), 2 (9), and 3 (2).

behavior would be indicative of a mainly intramolecular aggregation at the lowest concentration, whereas at higher ones, an intermolecular aggregation is superposed. As copolymer 3 has a much higher hydrophobic content than its analogues 1 and 2, this polysoap will form aggregates more easily, leading to a fluorescence decay with a higher

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Figure 4. Fluorescence decays at different ratios of labeled 5 to unlabeled 5*. The overall polysoap concentration was 0.8%, and the ratios were 1:0 (b), 1:10 (9), 1:25 ([), 1:50 (2), and 1:100 (1). Table 1. Results of the Fittings of a Biexponential Decay Function to (i) the Decays of 5 at Different Ratios of Labeled to Unlabeled 5/5* and (ii) the Decays of 8 Dissolved in Micelles of 7a composition 1:0 1:10 1:25 1:50 1:100 22 10 5 1

R1

τ1 [ns]

R2

(i) 0.8% polysoap, 5 to 5* ratio 0.33 57.2 0.67 0.23 56.0 0.77 0.21 53.2 0.79 0.14 60.4 0.86 0.14 58.0 0.86 0.08 0.17 0.16 0.16

(ii) 0.3% 7 55.9 0.92 56.0 0.83 60.7 0.84 58.7 0.84

τ2 [ns]

χν2

19.9 18.3 15.7 15.0 17.2

1.03 1.20 1.15 1.07 1.18

31.5 27.2 31.5 33.9

1.05 1.05 1.17 1.11

a In (i), the overall polysoap concentration was 0.8%, and in (ii) it was 0.3%. The loading of 8 in the micelles in (ii) is given as relative to the amount in labelled 5; that is, 22 means that the amount of 8 to 7 is 22 times higher than in the case of labelled 5.

contribution of the more long-lived decay component. This is also observed when comparing the decays of the three polysoaps at the same concentration (Figure 3d). In terms of the models for polysoap aggregation presented in the Introduction, this means that the local micelle model can be discarded, as it is incompatible with changes in the decay pattern upon dilution. Whether the intermolecular aggregates are composed of regional micelles or molecular micelles cannot be decided from these data only. Homopolymer 5, which is a tail-end type polysoap, was investigated in a slightly different manner. Instead of diluting the solution as such, the concentration of labeled 5 was diluted by mixing labeled and unlabeled 5* at a constant total concentration of polysoap. The decays measured in this way are shown in Figure 4. As for polysoaps 1-3, the decays are obviously not monoexponential at any composition. This means that the fluorophore probe must be distributed over different environments, for example, hydrophobic domains, mixed domains, and the aqueous bulk. The decays were analyzed by fitting a biexponential decay function to the decay data, the results of this analysis being compiled in Table 1. It was found that the contribution of the short-lived component increases when more unlabeled polysoap is added. This obervation shows that naphthalene is replaced from a hydrophobic to a more polar environment upon addition of 5*. In other words, the added unlabeled polymer interacts with the aggregates formed by 5 to form new

Figure 5. Fluorescence decays at different contents of the probe 8, dissolved in the monomeric reference surfactant 7. The content of 8 is given as relative to the amount in labeled 5; that is, 1 means that the amount of 8 to 7 is equal to what is found in the case of labeled 5. The fluorescence decay of 5 is inserted for comparison. The overall concentration corresponds to a polymer concentration of 0.3%: (b) labeled 5, (9) 8 to 7 is 22 times more than labeled 5, ([) 8 to 7 is 10 times more than labeled 5, (2) 8 to 7 is 5 times more than labeled 5, and (1) 8 to 7 is equal to labeled 5.

intermolecular aggregates with a lower content of (labeled) end groups. This is incompatible with the molecular micelle model, as it assumes large aggregates involving the whole polymer. Simply, no influence of added unlabeled 5* is expected within this model. As long as only intramolecular aggregation is assumed, the local micelle model should also be independent of added 5*. However, this model is, as well, incompatible with the experimental observations, even for intermolecular aggregate formation. Such a process would increase the probability of finding the naphthalene end-label in an aggregate instead of decreasing it. Instead, to obtain a consistent picture of this system we have to assume that adding unlabeled 5* leads to a displacement of the naphthalene moieties from the regional micelle aggregates to the aqueous phase. The estimated decay times (Table 1) are consistent with this picture, as they resemble the decay times found for naphthalene exchanging between micelles and the aqueous bulk: approximately 50-60 and 20-30 ns, respectively.26 To verify this, a third set of decay measurements was performed. Instead of using labeled and unlabeled polysoap 5, solutions of the analogous “monomer” 7, being a micelle-forming surfactant, and the unbound probe 8 were studied. To resemble intramolecular aggregation, micelles can be used. Dissolving the probe 8 in a micellar solution of 7 will yield a situation close to a system of local micelles. The emission spectra of such solutions, not shown, show that the steady-state properties of the bound and unbound probe are identical, within experimental error. The timeresolved measurements, however, show a different pattern. Starting with a solution containing 22 times more probe 8 per monomer unit of 7 than for 5 labeled with 1.2 naphthalene moieties on average, the stepwise dilution of probe 8 yields the decays shown in Figure 5. The result of the numerical analysis of the decays is given in Table 1. This analysis shows that approximately the same decay times are found in these systems as were found for the naphthalene probe bound to the end groups of 5, consistent with a distribution of the unbound probe 8 between the micelles and the aqueous bulk. The contribution of the short-lived component, however, is more pronounced in the micellar systems. This leads to an overall impression of a shorter decay and also proves that the fluorophore 8

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has a significant solubility in water, as expected. The important finding from these measurements is that the decays are identical, regardless of the concentration of the probe 8, except for the very highest one. The somewhat faster decay at the highest probe concentration is indicative of excimer formation, leading to a quenched emission. This is consistent with a micellar solution and with a distribution of fluorescence probes between the micelles and the bulk. Accordingly, these systems, which serve as a model for an intramolecular aggregation, yield a decay pattern clearly different from what was found in the polysoap 5 systems. Hence, these measurements, together with the previous ones, support the conclusion that the investigated polysoaps form purely intramolecular hydrophobic domains only at very low concentrations. Further, the character of the domains does not fit to the local micelle or molecular micelle models but more likely fits to the regional micelle model. Anionic copolymers with a structure analogous to 1-4 have been studied by others with light scattering and fluorescence methods, using a naphthyl label as the fluorescent probe.29,30 The results reported suggest that an intramolecular association is accompanied by an intermolecular one, in accordance with the present results. 4. Conclusions The studied polysoaps form at low concentrations basically intramolecular hydrophobic domains, but at higher concentrations the formation of intermolecular domains is superposed. Dilution studies support the model (27) Noda, T.; Morishima, Y. Macromolecules 1999, 32, 4631. (28) Yamamoto, H.; Morishima, Y. Macromolecules 1999, 32, 7469.

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of regional micelles for the polysoaps under investigation, whereas the model of local micelles is not compatible. In view of the previous results on polysoap 5*, concerning the concentration dependence of its dissolving capacity as well as of its reduced viscosity,2 the model of molecular micelles is most improbable, too. From time-resolved measurements, a correlation is found between the hydrophilic/hydrophobic balance in the composition of the copolymers and the aggregation behavior: For polysoaps with a significant difference in hydrophobicity, the change in fluorescence decay pattern is different upon dilution. The most hydrophobic polysoap forms more microdomains, which also are more stable to dilution effects. For all investigated polysoaps, it is concluded that the aggregation is mainly intramolecular at the lowest concentration, whereas at higher concentrations intermolecular aggregation is superposed. Steady-state fluorescence measurements show that the studied polysoaps exhibit no detectable physical or chemical aging in aqueous solutions under ambient conditions, for a period of at least 6 months. Acknowledgment. The authors thank Moheddin ElGuweri and Dr. Erik Wischerhoff for help in synthesizing polymer 4. Dr. Wouter Verbouwe and Els Rosseau are thanked for maintenance of the single photon counting equipment. J.v.S. thanks the K.U. Leuven for financial support. Financial support from DWTC, Belgium, through Grant IUAP/PAI-4/11 and FWO/FNRS, Belgium, is gratefully acknowledged. LA0011329