0 Copyright 1990 American Chemical Society
SEPTEMBER 1990 VOLUME 6, NUMBER 9
Articles Onset of Aggregation for Water-Soluble Polymeric Associative Thickeners: A Fluorescence Study Yongcai Wang and Mitchell A. Winnik’ Department of Chemistry and Erindale College, University of Toronto, 80 St. George St., Toronto, Ontario, M5S 1 A1 Canada Received November 3,1989. In Final Form: March 16, 1990 Poly(ethy1ene oxide) polymers end-capped with n-hexadecyl groups are members of a class of watersoluble polymers which associate at modest concentrations and impart unusual rheological properties to aqueous solutions. We have used pyrene (Py) as a fluorescent probe to study the association process. For each of two samples of these polymers (M,= 40 000,47 000) in water, we find that above a critical polymer concentration the pyrene fluorescence shows a large increase in intensity and mean lifetime. This change corresponds to the formation of micelle-like clusters by the C&% chain ends and partitioningof Py molecules into this hydrocarbon-rich phase. Association commences once the polymer concentration exceeds ca. 0.1 g/L. The association transition is broad, spanning 2 orders of magnitude in polymer concentration.
Introduction Water-soluble polymers containing two or more hydrophobic substituents per chain have unusual properties. The polymers tend to associate in aqueous solution, and these solutions exhibit truly remarkable rheological properties.’ Such materials are often referred to as “associative thickeners”. They are used, for example, as rheology modifiers for paints and for pusher fluids in enhanced oil recovery. Until recently it was feared t h a t these association phenomena might be too complicated to be understood at the molecular level. Nevertheless, there is a need for this level of understanding, and many of the questions one wishes to pose can in fact be answered with modern experimental techniques. The three most basic questions about such systems are t h e following: (i) At what concentrations do the polymers begin to associate? (ii) What is t h e nature of the aggregates formed upon association? (iii) How many polymer molecules associate within each aggregate? While these questions have been asked for many years, answers have not been forthcoming. (1) (a) Schaller, E. In Aduances inEmulsionPolymerizationand Latex Technology;20th Annual Short Course, 1989; Vol. 2, Lecture No. 12. (b) Health, D.; Tadros, T. F. Faraday Discuss. Chem. SOC.1983, 76,203.
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In this paper, we show how the use of pyrene (Py) as a fluorescent probea3 provides rich information about these systems. We examine two samples of n-ClsH33-endcapped poly(ethy1ene oxide), whose chemical structure (1) is shown below. Our experiments measure the critical aggregation concentration (cac) for these polymers in water and show that association leads to the formation of micelle like clusters of the C-16 chains. We are able to identify the location of Py in these clusters. Because we observe Py excimer emission in the system, we can also draw some insights about the average number of C-16 chains per aggregation site.
Experimental Section Two samples of polymeric associative thickener 1 were prepared at Union Carbide under the supervision of Dr. D. Bassett, who provided us with the samples. Molecular weights were determined at Union Carbide by gel permeation chromatography. The synthesis and full characterization of these and other polymers in this series will be published by Bassett and coworkers. In our samples, the poly(ethy1ene oxide) component has molecular mass of 8200 (x F 180) of narrow polydispersity; the polymers themselves are polydisperse with M, (M,/M.) val(2) Gratzel, M.; Thomas, J. K.In Modern Fluorescence Spectroscopy; Wehry, E. L., Ed.; Plenum Press: New York, 1976; p 169. (3) Malliaris, A. Int. Reu. Phys. Chem. 1988, I, 95.
0 1990 American Chemical Society
1438 Langmuir, Vol. 6, No. 9, 1990 C
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Wang and Winnik
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ues of 39 800 (1.6) and 47 500 (1.8) determined by gel permeation chromatography using PEO standards. Note that both samples have very similar M, values (ca. 25 000). These samples are referred t o here as PEO-C16/40 and PEO-C16/47. We purified the polymers by two recrystallizations from methanol. Samples for fluorescence measurements were prepared in the following way: Sufficient pyrene (Py) in acetone was added to M 10.0-mL volumetric flasks t o give final solutions of 6.0 X (or, in some instances, 6.0 X 10-8 M). The acetone was evaporated under a stream of Nz gas. Known quantities of the polymer 1 in aqueous solution were added, then diluted to 10.0 mL with doubly distilled water. The stoppered flasks were heated a t 60 O C for 2 h and then cooled slowly t o room temperature. The final polymer concentration varied from 1mg/L to 10 g/L. Fluorescence spectra were taken on a SPEX Fluorolog 2 spectrometer, using the front-face geometry. Fluorescence decay measurements, Z ( t ) , employed t h e time-correlated singlephoton timing t e ~ h n i q u e . Samples ~ were excited a t 335 nm. Spectra were recorded from 360 t o 550 nm, and Z ( t ) decays were monitored a t 390 nm. Samples were degassed by bubbling argon gas through the solution.
Results and Discussion Onset of Aggregation. In Figure 1 we present both the intensity of pyrene emission (I)and its mean decay time (7)as a function of polymer concentration. At low polymer (or end-group) concentrations, or in water itself, Py decays exponentially, with 7 = 200 ns. At polymer concentrations above 0.1 g/L, the I ( t ) profile becomes noticeably nonexponential, and ( 7 ) increases with increasing polymer concentration. The parallel increase in7 and ( 7 )indicates that P y is partitioning into a second phase (i.e., a micellar phase) where its nonradiative decay rates are suppressed. The 300-ns lifetime found at high polymer concentration is typical of that found for Py in, for example, sodium dodecyl sulfate micelle^.^ Upon addition of 1 to an aqueous solution of Py, we observe a small but important shift in the excitation s p e c t r u m . I n p u r e w a t e r a n d a t low polymer concentrations, the (0,O) excitation band appears at 333 nm. I t is s h i f t e d t o 335.5 nm a t high polymer concentrations, consistent with transfer of Py from an aqueous to a less polar environment.6 More useful information about the Py environment is obtained from changes in the vibrational fine structure in i t s emission spectrum, the intensity ratio 1 1 / 1 3 of the (0,O) band to the (0,2) band.7 These values range from 1.87 in water to 0.6 in aliphatic hydrocarbon solvents.7b Values of 1.1-1.2 are typical of simple aqueous micelles,'* implying that Py is located in the surface region of the micelle hydrocarbon core. In Figure 2 we observe that this ratio drops sharply above a certain polymer concentration in (4) OConnor, D. V.; Phillips, D. Time Correlated Single Photon Counting; Academic Press: London, 1984. (5) (a) Lianos, P.; Zana, R. J. Colloid Interface Sci. 1981,84, 10. (b) Turro, N. J.; Kuo, P. L. Langmuir 1986,2,438. (6) Thomas, J. K. Chem. Reu. 1980,80, 283. (7) (a) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. SOC.1977, 99,2039. (b) Dong, D. C.; Winnik, M. A. Can. J. Chem. 1984,62,2560.
Figure 1. Plot of fluorescence intensity ( 0 , O ) (lower two curves) a) (upper two curves) as and mean fluorescence decay time (0, a function of Dolvmer concentration in moles Der liter based ipon
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the vicinity of 10 mg/L and reaches limiting values of 1.23 for PEO-C16/47 and 1.21 for PEO-C16/40. The data in Figures 1 and 2 indicate that polymer association begins a t concentrations of 1 above approximately 0.1 g/L and is lower for PEO-C16/40 than for PEO-C16/47. The association transition appears to be very broad, spanning almost a 100-fold range in polymer concentration. Part of this broadness is due to the partition equilibrium of pyrene into the hydrophobic clusters. We estimate the cac in the following way: we draw a tangent to the rising portion of each curve in Figure 1 passing through the inflection point. The intersection of this line with the horizontal line passing through the lowconcentration I values is taken to be the cac.lob From this analysis we find cac values of 0.1 g/L for PEO-C16/40 and 0.2 g/L for PEO-C16/47. Nature of the Aggregates. The increase in ( 7 ) and Z shown in Figure 1and the decrease in 11/13in Figure 2 imply the formation of micelle-like aggregates of the C 1 a ~ end groups. Py partitions into the micelle-like phase, where t h e phase experiences a more hydrocarbon-like environment. Some aspects of the behavior here are strikingly different from that of systems which form simple nonionic micelles.* For C1,J-IB-(EO)x surfactants, the cmc values increase exponentially with the weight fraction of ethylene oxide (EO) units? increasing, for example, from 5 X lo+ M for the surfactant carrying 17 EO units to 2 (8) Tanford, C.; N o d i , Y.; Rohde, M. F. J.Phys. Chem.1977,81,1565. (9) Barry, B. W.; Eini, D. I. D. J . Colloid Interface Sci. 1976,54,339, 348.
Langmuir, Vol. 6, No. 9, 1990 1439
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PEO-C16/40; ( 0 )PEO-C16/47. concentration: (0)
X 10-5 M for that with 60 EO units. Here the cac values are much lower. Curious differences appear in the polymer concentration dependence of (T),and of Il/I3, for the two polymers (Figures 1and 2). The change in 11/13 is more pronounced at lower concentrations of the PEO-C16/40 polymer. Over this concentration range, Py partitions between the aqueous and micelle-like phases. Both the fluorescence decay profiles and the steady-state fluorescence spectra contain a superposition of signals due to Py in the aqueous phase and that in the micelles. Were it not for excimer formation (see below), the I(t) profile could be analyzed in terms of Py in two environments, characterized by two independent decay timesaloNevertheless, it appears from Figure 2 that the two polymers form aggregates of different cluster size (see below), and this affects the partition coefficient of Py between the aqueous and micellar phases. Excimer Formation. Figure 3 indicates that the onset of aggregation is accompanied by excimer formation, which increases with increasing polymer concentration, passes (10) (a) Zhao, C.-L.; Winnik, M. A.; Riess, G.; Croucher, M. D. Langmuir 1990,6,514. (b) The assumptionsin this analysis, and a more rigorous approach to obtaining cac values, will be described in a forthcoming paper: Wilhelm, M.; Zhao, C.-L.; Wang, Y.;Xu, R.; Winnik, M. A.; Mura, J.-L.; Riess, G.; Croucher, M. D. Macromolecules, submitted for publication. (11) (a) A similar phenomenon has been observed above the cmc of micelles formed from a CloHzl-substituted crown ether. Here the Py associates specifically with the crown either both above and below the cmc. T w o , N. J.; Kuo, P.-L. J. Phys. Chem. 1987,91, 3321. (b) The aggregation number for these CloHzl crown ether micelles is known (N = 88),and from the published data11. it appears that the maximum in the &/Im va surfactant concentrationplot occurs when the Py concentration is twice the micelle concentration.
through a maximum, and falls off eventually to zero.lla At low polymer concentration, there are few micelles and, as a consequence, little excimer emission. At high polymer concentration, the number of micelles exceeds that of pyrene molecules. Few micelles contain two pyrenes, and there is also little excimer emission. Only when the concentration of micelles is comparable to that of Py does one expect I, to be significant. Indeed, repeating these experiments with a Py concentration of 6 X lo4 M largely reproduces the data in Figures 1 and 2 (the data are noisier), but the spectra show essentially no detectable excimer emission. We can estimate the mean aggregation number (&'I of the micelle-like cluters by making two assumptions: first, the cluster size is independent of polymer concentration; second, the maxima in Figure 3 correspond to the situation where the Py concentration is exastly twice the micelle concentration.11bJ2 We calculate N = 13 for PEO-C16/ 40 and N = 23 for PEO-C16/47, values which are of comparable magnitude with known micellar aggregation numbers for the series of C16H33-(EO)x nonionic surfactants with long EO chains.8pe The N values are numberaveraged aggregation numbers, since the calculations use M,,to determine the end group concentration per weight equivalent of polymer. . These results lead to the following general picture of polymeric associative thickener behavior in aqueous solution: in very dilute solution, the molecules are molecularly dispersed. Association begins once a critical concentration (the cac) is reached. The association transition is broad, and the broadness may only reflect the molecular weight polydispersity of the polymers we examined. Association leads to the formation of micellelike aggregates of the C16H33 chain ends. These clusters are similar in many ways to micelles formed from C16H33(EO), surfactants with long EO chains. One of the important differences between the surfactants and 1 is that in 1 the two chain ends can act cooperatively. At high polymer concentrations (1wt %), bridging interactions lead to a gel-like network of micelles, which has a profound impact on the solution rheology.
Acknowledgment. We thank NSERC Canada and the Ontario Centre for Materials Research for support of this research and Dr. David Bassett of Union Carbide for stimulating our interest in this problem and for providing samples of the polymers examined. Registry No. Py, 129-00-0; 1 (copolymer), 127818-89-7. (12)A more quantitative treatment of the data is possible when all Py molecules are bound to the micelles. Selinger,B. K.; Watkins, A. R. Chem. Phys. Lett. 1978,56,99.
