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Fluorescence Studies of an Alkaline Swellable Associative Polymer in Aqueous Solution Eugenia Kumacheva,† Yahya Rharbi,† Mitchell A. Winnik,*,† Liang Guo,‡ Kam C. Tam,*,‡ and Richard D. Jenkins§ Department of Chemistry and Erindale College, University of Toronto, Toronto, Ontario, Canada M5S 1A1, School of Mechanical & Production Engineering, Nanyang Technological University, Singapore 639798, Republic of Singapore, and Union Carbide Corporation, UCAR Emulsion Systems, 410 Gregson Drive, Cary, North Carolina 27511 Received June 21, 1996. In Final Form: October 16, 1996X Rheology measurements and fluorescent probe experiments were used to study the association process in aqueous solutions of an alkali swellable copolymer of methacrylic acid, ethyl acrylate, and an ethoxylated hydrophobic monomer. The pyrene-probe experiments monitor hydrophobic cluster formation in the polymer solution and indicate a rearrangement of the system as a function of pH. The polymer, present as a latex at pH < 4, dissolves at pH 5.7. In this solution, many small hydrophobic domains are present. As the pH is raised and the polymer further neutralized, the viscosity of the solution increases, accompanied by formation of a smaller number of larger hydrocarbon domains, detected as a large increase in the extent of pyrene excimer emission.
Introduction Polymers that associate in solution via physical interactions are often efficient rheology modifiers.1-4 When associating water-soluble polymers are used in this way, they are referred to as associative thickeners (ATs). In water, AT-polymers experience association due to intraor intermolecular interaction of their hydrophobic substituents. An understanding of the association mechanism is important, because this should lead to improved control of the rheology of aqueous fluids. A useful technique for studies of the association process and structure evaluation in aqueous AT solutions involves fluorescent probes.5-12 Analysis of the selective dye partitioning between the †
University of Toronto. Nanyang Technological University. Union Carbide Corporation. X Abstract published in Advance ACS Abstracts, December 15, 1996. ‡ §
(1) (a) Jenkins, R. D. Ph.D. Thesis, Lehigh University: Bethlehem, PA, 1990. (b) Jenkins, R. D.; Silebi, C. A.; El-Aasser, M. S. Polym. Mat. Sci. Eng. 1989, 61, 629. (c) Jenkins, R. D.; Silebi, C. A.; El-Aasser, M. S. In Advances in Emulsion Polymerization and Latex Technology: 21st Annual Short Course; El-Aasser, M. S. Ed.; Lehigh University: Bethlehem, PA, 1990; Chapter 17. (2) (a) Water Soluble Polymers; Glass, J. E., Ed.; ACS Advances in Chemistry Series 213; American Chemical Sociey: Washington, DC, 1986. (b) Polymers in Aqueous Media; Glass, J. E., Ed.; ACS Advances in Chemistry Series 213; American Chemical Society: Washington, DC, 1989. (c) Polymers as Rheology Modifiers; Schulz, D. N., Glass, J. E., Eds.; ACS Symposium Series 462; American Chemical Society: Washington, DC, 1991. (d) Hydrophilic Polymers: Performance with Environmental Acceptance; Glass, J. E., Ed.; ACS Advances in Chemistry Series 248; American Chemical Society: Washington, DC, 1996. (e) Franc¸ ois, J. Prog. Org. Coat. 1994, 24, 67. (f) Maechling-Strasser, C.; Clouet, F.; Franc¸ ois, J. Polymer 1993, 33, 1021. (3) Jenkins, R. D., DeLong, L. M., Bassett, D. R. In Hydrophilic Polymers; Glass, J. E., Ed.; ACS Advances in Chemistry Series 248, American Chemical Society: Washington, DC, 1996; p 425. (4) (a) Tam, K. C., Tan, C. B., Farmer, M. L. Proc. Am. Soc. Rheol. Meet., 67th 1995. (b) Tirtaatmadja, V.; Tam, K. C.; Jenkins, R. D. Macromolecules, 1996, submitted for publication. (5) (a) Zana, R. In Surfactant Solutions: New Methods of Investigation; Zana, R., Ed.; Marcel Dekker: New York, 1986. (b) Chu, D. Y.; Tthomas, J. K. In Photochemistry and Photophysics; Rabek, J. F., Ed.; CRC Press: Boca Raton, FL, 1991; Vol. 3, pp 49-102. (6) (a) Yekta, A.; Duhamel J.; Brochard, P; Adiwidjaja H., Winnik, M. A. Macromolecules 1993, 26, 1829. (b) Yekta, A.; Xu, B.; Duhamel J.; Adiwidjaja H.; Winnik, M. A. Macromolecules 1995, 28, 956. (7) Infelta, P. P. Chem. Phys. Lett. 1979, 61, 88. (8) (a) Yekta, A., Aikawa, M., Turro J. Chem. Phys. Lett. 1979, 63, 543. (b) Warr, G. G., Grieser, F. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1813.
aqueous medium and hydrophobic environments in the polymer provides important information about the clusters arising through association and often permits quantitative characterization of the resulting structure. In the present paper we extend the fluorescent probe approach to studies of hydrophobic alkali swellable associative emulsion (HASE) polymers which represent an important group of water-soluble ATs. HASE polymers are usually synthesized by emulsion copolymerization of an associative monomer, a carboxylic acid monomer, and a nonassociative flexible monomer to yield a polymer in the form of a latex dispersion. At low pH, the carboxylic groups are not charged and polymer is water-insoluble. An increase in pH results in ionization of the acid groups leading to polymer solubility in water. The thickening efficiency in HASE polymers is achieved both by the associative mechanism and by the expansion of the macromolecule backbone. The chain expansion has two components, the first is due to the compact-particle-tocoil expansion upon dissolution and the second is due to electrostatic repulsion between the neutralized carboxylic groups. While HASE polymers have been used commercially for many years, they have not received much attention from a mechanistic point of view, particularly when compared to nonionic ATs with hydrophobic end groups, for which large amounts of information are now available.1,2 Recently, scientists at Union Carbide developed a new variety of HASE ATs in which the hydrophobic group is separated from the polymer backbone by an oligomeric poly(ethylene oxide) (PEO) chain. Model polymers were prepared and characterized,3 and publications describing their rheology are starting to appear.3,4 The long term goal of these experiments is to provide a molecular level understanding of the association mechanism in these polymers and its connection to bulk solution rheological properties. (9) (a) Nakajima, A. J. Lumin. 1976, 11, 4229. (b) Dong, D. C.; Winnik, M. A. Can. J. Chem. 1984, 62, 2560. (10) (a) Kalyanasundaram, K; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (b) Kalyanasundaram, K. Langmuir 1988, 4, 842. (c) Kalyanasundaram, K; Thomas, J. K. J. Phys. Chem. 1977, 81, 2176. (11) (a) Turro, N. J.; Kuo, P.-L. Langmuir 1986, 2, 438. (b) Turro, N. J.; Kuo, P.-L. J. Phys. Chem. 1986, 90, 837. (c) Lianos, P.; Lang, J.; Zana, R. J. Colloid Interface Sci. 1983, 91, 276. (12) Wilhelm, M.; Zhao, C.-L.; Wang, Y.; Xu, R.; Winnik, M. A. Macromolecules 1991, 24, 1033.
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Here we combine the fluorescent probe technique and rheology experiments to study the association in aqueous solutions of one such polymer, prepared from methacrylic acid, ethyl acrylate, and a C20H41O-EO35 macromonomer.3,4 We presume that the hydrophobic groups, responsible for association, are randomly distributed along the polymer chain. We study the association process as a function of polymer concentration in the solution and as a function of the extent of carboxy-group neutralization. For convenience we refer to the hydrophobic clusters formed upon association as micelles or micelle-like structures. We show that an increase in pH results in a rearrangement of the associative structure in the polymer solution. Using pyrene as a fluorescence probe, we find that, at pH < 6, excimer emission is low because the pyrenes are distributed among a large number of small micelles. At higher pH, the concomitant increase in the excimer emission intensity and the solution viscosity indicates a rearrangement of the system: further ionization of the carboxy groups shifts the system toward intermolecular association. Experimental Section Materials. The polymer examined in the present work has the following formula, where R is C20H41, p ) 35, and the mole ratios x:y:z are 49.05:50.04:0.9.
Full details of the synthesis and characterization of this and similar HASE polymers are reported by Jenkins.3 Tirtaatmadja et al. 4 refer to this sample as RDJY31-5. In short, a macromonomer was prepared first by ethoxylation to p ) 35 of the C20 alcohol, followed by reaction with R,R-dimethyl-m-isopropenylbenzyl isocyanate. This macromonomer was introduced into a conventional seeded semicontinuous emulsion polymerization under monomer-starved conditions with appropriate amounts of methacrylic acid and ethyl acrylate. The resulting latex, at pH ) 3.4, was purified by dialysis through a cellulose membrane. In this work we designate this polymer 1-C20. According to the structure and composition of the polymer, it contains 2.30 × 10-2 g of C20H41 groups/g of polymer (8.16 × 10-5 mol/g of polymer). Pyrene (Aldrich) was recrystallized from ethanol and dissolved in acetone to obtain a stock solution of 0.1 mM. NaOH and KCl (spectrograde) were used as received, and water was purified through a Millipore Milli Q system. Sample Preparation. To prepare HASE polymer solutions, the latex was diluted to 9.3 g/L and then neutralized with NaOH solution (0.5 M) to pH ) 9.5. At pH ) 5.7, the polymer dissolves and the solution becomes transparent. The ionic strength of the stock solution was adjusted with KCl to 10-4 M. This solution with C ) 9.0 g/L and pH ) 9.5 was used as a stock system. Polymer solutions of varying concentrations were prepared by the dilution of the stock solution with a (NaOH + KCl) solution of the same pH and ionic strength. All polymer solutions were kept in dark and stored for not more then one week.
Langmuir, Vol. 13, No. 2, 1997 183 The pyrene saturation experiments followed that described by Yekta et al.6 An excess of pyrene was coated onto the bottom of several centrifuge tubes by adding 2.0 mL aliquots of the pyrene solution in acetone (c ) 10-4 M) to each tube and by allowing the solvent to evaporate at 40 °C for 24 h. A polymer solution of known concentration was added to each tube and stirred at ambient temperature (22 °C) in the dark for 72 h to allow the system to come to equilibrium. Then the solutions were centrifuged (15 000 rpm, 1 h) to separate pyrene microcrystals from the solution. The presence of these microcrystals in the solution can be recognized from the fluorescence and excitation spectra of the solutions.6 The effect of acid group neutralization was studied in a different series of experiments at constant polymer concentration (Cpol ) 2.0 g/L) and pH values varying from pH ) 5.7 to pH ) 12. Solutions saturated with pyrene were prepared at pH ) 5.7. Centrifugation was carried out at only 4000 rpm for 1 h to prevent possible sedimentation of the polymer. After each fluorescent intensity measurement, the pH was incremented by a successive addition of microliter quantities of 0.1 M NaOH to the polymer solution, so that the change in polymer and pyrene concentration at the highest pH (pH ) 12) did not exceed 10%. Upon addition of NaOH to the sample, the pH reached a steady value within 15 min and did not change further with time. All of these pH and fluorescence intensity measurements were carried out on the same solution in the same cuvette. Methods. UV absorption measurements were carried out with a Hewlett-Packard 8452A diode-array spectrophotometer with 2 nm resolution. For pyrene solubilization experiments, an equivalent reference with the same polymer concentration but without pyrene was used in all measurements. The absorbance at each wavelength was taken relative to the absorption at 402 nm. The extinction coefficient of micellized pyrene at the major absorption peak 338 nm (3.58 × 104 M-1 cm-1) was taken as that reported6 for pyrene solubilized in a linear associative thickener polymer solution. Fluorescence measurements were made with a SPEX Fluorolog 2 spectrometer with double-grating monochromators, a redsensitive photomultiplier, and a photon-counting detector. Slit widths were set at 1.0 mm. The spectra were recorded in sample and reference (S&R) mode in 50 nm steps, integrating counts for 1 s. For linearity of response, the sample (S) counts were always kept to fewer than 3 × 105 counts/s, and the reference (R) to less than 0.06 mA. In analysis of emission spectra, the S&R data were converted to the S/R ratio. To determine the intensities of pyrene excimer and monomer emission, we calculated the integrals of two domains: 365-392.5 nm (ascribed to monomer, IM) and 450-550 nm, excimer range, IE. Some changes in the emission spectra of the polymer solutions were noted over prolonged sample storage. For example, IE/IM values showed a ca. 10% increase for samples that had been stored in the dark at room temperature for more than a week. Excitation spectra were run at λem ) 372 and 481 nm to test for the presence of pyrene dimers and microcrystals in the polymer solutions. Fluorescent decays were measured by the time-correlated single-photon-counting method as described previously.6 Solutions of pyrene in the aqueous polymer were excited with a flash lamp (0.5 atm D2) at 338 nm, and emission was observed at 376 (monomer) and 520 nm (excimer). The monomer decays were fitted to the expression
I(t) ) I(0) exp{-t/τ0 - n[1 - exp(-k1(t)]}
(1)
the classic micelle Poisson quenching model7,8 using four fitting parameters: the initial intensity I(0), the mean number of pyrene molecules per micelle n, the unquenched pyrene lifetime τ0, and the first-order rate constant k1 for self-quenching of an excited pyrene by an unexcited one inside a micelle. The unquenched pyrene lifetime τ0 was also determined independently by a similar experiment at low pyrene concentration, and a value similar to that found above was obtained. We used χ2 and the weighted residuals as criteria for the goodness of fit. Steady-shear viscosity measurements were performed using a Contraves Low Shear 40 Rheometer at (25 ( 1) °C over six decades of shear rate. The rheometer was fitted with a Couette cell: the radii of the bob and cup were 5.5 and 6 mm, respectively.
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Figure 1. Amount of pyrene solubilized in a 1-C20 solution at pH ) 9.5, as measured by UV spectrometry, as a function of polymer concentration.
Figure 2. Fraction of the micellized pyrene in polymer solution calculated from eq 2 using the data from Figure 1. The length of the bob was 8 mm. Here we present data obtained for shear rates 0.011 and 1.01 s-1.
Results and Discussion Pyrene Partitioning into Hydrophobic Domains. In water, pyrene has a limiting solubility at room temperature of [Py]wsat ) 7 × 10-7 M.5,6 The presence of the HASE polymer increases the solubility of pyrene in aqueous medium, and the amount solubilized increases in proportion to the amount of polymer in solution. The example shown in Figure 1 is for a series of polymer solutions at pH ) 9.5. The distribution of pyrene between the hydrophobic phases within the polymer and the aqueous phase is described by the expression
[Py]mic/[Py]tot ) aCpol/(1 + aCpol)
(2)
Here a is expressed in moles of pyrene/g of polymer. A plot of the fraction of micellized pyrene [Py]mic/[Py]tot vs polymer concentration is shown in Figure 2. The sharp increase in the total solubility of pyrene in the system in the presence of the polymer indicates the formation of hydrophobic domains in the polymer solution where pyrene can reside. From the data in Figure 1, we can determine the equilibrium partition coefficient K of pyrene in the polymer solution. We assume the pseudophase micelle model, which treats the hydrophobe as a separate phase with the density of the corresponding alkane, here C20H42. According to this model, pyrene partitions into the hydrophobic domains in proportion to their volume fraction, and one ignores any possible complication due to domain size. According to Figure 1, the saturation concentration
Figure 3. I1/I3 ratio (s) and viscosity at pH ) 9.5 vs polymer concentration. The I1/I3 data refer to solutions saturated with pyrene. The viscosity data refer to shear rates of (4) 0.011, and (3) 1.01 s-1.
of pyrene is 3.0 × 10-6 mol/(g of polymer). From the composition of the polymer, we calculate that each gram of polymer contains 2.3 × 10-2 g of C20H42 groups. Assuming a hydrophobe cluster density close to the density of n-eicosane C20H42 (ca. 0.8 g/mL), we calculate a partition coefficient K ) 1.5 × 105 (L of water/L of cluster). It is known that the spectroscopic properties of pyrene are very sensitive to the polarity of its environment, and that this sensitivity can be exploited to study the partitioning of pyrene into hydrophobic domains in aqueous solution.9-11 We observed that two features of the pyrene spectrum change as polymer is added to a solution of pyrene in water. First, there is a shift in the low energy (0,0) band of the (S2 r S0) absorption of pyrene, from λmax ) 334 nm in water to 338 nm in the polymer solution.12 In addition, the ratio of the first-to-third band in the pyrene fluorescence spectrum (I1/I3) decreases as the polymer concentration is increased from Cpol ) 0.25 to 8.3 g/L, as shown in Figure 3, leveling off at a value of I1/I3 ) 1.175 ( 0.025. This value is typical for pyrene solubilized in the hydrophobic core of surfactant micelles and in the micelle-like domains of other associative polymers in aqueous solution. The sharp decrease in I1/I3 with increasing Cpol (Figure 3) is also consistent with the data on pyrene partitioning into the polymer hydrophobic domains as shown in Figure 2. We also compare in Figure 3 the results of the I1/I3 measurements with the concentration dependence of the solution viscosity, obtained at two different shear rates, 0.011 and 1.01 s-1. Concentrations of polymer at pH 9.5 which lead to significant partitioning of pyrene into hydrophobic domains also make a significant contribution to the solution viscosity. Note that when the concentration of 1-C20 is less than 2 g/L, increasing the shear rate by 2 orders of magnitude has little effect on the solution viscosity. At higher concentrations, the polymer undergoes significant shear thinning. pH Effects on Domain Structure. The experiments described above were all carried out at pH ) 9.5. Figure 4 shows fluorescent spectra of pyrene taken at different pH values for polymer solutions with Cpol ) 2 g/L. At pH 5.7, curve a, where the polymer has just dissolved, the spectrum is characterized by an intense monomer peak and a weak excimer. At pH ) 10, the monomer band has decreased significantly, accompanied by an increase in excimer emission. The presence of an isoemissive point at 446 nm throughout the titration suggests the presence of two “states” of the pyrene, corresponding to two types of hydrophobic domains, whose relative importance shifts as the degree of neutralization of the polymer is increased.
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Figure 6. Effect of pH on relative excimer to monomer intensity Ie/Im (2) and on the viscosity (4) of the polymer solution. Figure 4. Fluorescence emission spectra of pyrene solubilized in 1-C20 polymer solution (Cpol ) 2 g/L) at various pH: a, pH ) 5.7; b, pH ) 7.0; c, pH ) 10.
Figure 5. Effect of pH on change in excimer (9) and monomer (0) emission intensity, Cpol ) 2 g/L.
One explanation consistent with these observations is that at low pH there are a larger number of small micelles in the system. A distribution of pyrenes among these micelles would have a lower probability of finding two pyrenes in the same micelle than at high pH, where we envision a smaller number of larger micelle-like domains. More information about this issue is provided in Figure 5, where IE and IM are plotted as a function of pH. One sees that the decrease in monomer intensity is accompanied by an increase in the excimer intensity over the same pH range. This is the type of behavior expected if excimer formation were dynamic in origin and provides further evidence against the presence of pyrene ground state dimers or pyrene aggregates in the system. The most significant variation in IE and IM occurs in the pH range 6.0-7.0, while at pH > 9 these values have leveled off. IE/IM and the Solution Viscosity η. Further information is provided in Figure 6, where both the IE/IM ratio and the solution viscosity η are plotted as a function of pH. Both begin to change over the same narrow range of pH values. The growth in η is sharper, indicating that dissolution of the latex polymer leads to an immediate large increase in solution viscosity. Under these circumstances, the methacrylic acid groups in the polymer are only partially neutralized. The change in IE/IM is somewhat broader, occurring over the range of pH from 6 to 8. This indicates that there are changes which occur in the polymer morphology between pH 7 and 8 which are not reflected in the solution viscosity. However, it is also possible that the sharper viscosity response to increase in pH may be due to the difference in the solution concentrations used in rheological and fluorescence measurements.
At pH ) 5.7-5.9, the polymer dissolves to the point that the solutions become clear. The low values of the viscosity in this range of pH suggest that hydrophobic interactions between the pendant C20H41, groups is internal to each polymer. The subsequent increase in viscosity would then correspond to the onset of interpolymeric association. In other words, hydrophobic groups randomly distributed along the same polymer chain form small hydrophobic clusters. Our sample contains polymers with molecular weights in the range 100 000200 000. For z ) 0.9 and p ) 35, we estimate the number of hydrophobic monomers per chain to be on the order of 5-10. Further neutralization of the carboxylic groups leads to a structural rearrangement of the polymer. We imagine that at pH > 6.0 there is an increase in swelling of individual chains, accompanied by interpolymeric association of the hydrophobic substituents. As a result, the viscosity of the solution dramatically increases. Strictly speaking, the ionization of the methacrylic acid groups, and changes in the polymer coil dimensions, can also contribute to the viscosity growth. This effect, however, is expected to be much smaller than bridging and “crosslinking” provided by interchain association. The pyrene excimer experiments tell us that the hydrophobic domains formed in this way have a larger size, and the occupation probability for more than one pyrene molecule per micelle increases. As a result, the amount of excimer in the system increases and IE/IM grows with the extent of polymer neutralization. At pH > 9 the number of hydrophobic domains in the solution and their size do not show any significant change: IE/IM and η values stabilize. However, at sufficiently high pH (>12-12.5) some reduction in IE/IM and viscosity occurs, probably due to an increase in ionic strength. One further comment is in order concerning the weak pH effect on the ability of the polymer to solubilize pyrene in aqueous solution. In the system examined here, raising the pH appears to change the domain size but does not affect the amount of hydrocarbon phase in the system. Fluorescence Decay Experiments. In Figure 7 we plot a typical fluorescent decay curve of pyrene monomer for pyrene at saturation, solubilized in an aqueous solution of the 1-C20 at pH ) 9.5 and Cpol ) 4.1 g/L. Except for the first channel, the data fit reasonably well to the Poisson quenching model, eq 1. This type of fit is found for all polymer solutions so far examined, with χ2 in the range 1.1-1.35. In this system, if we treat τ0 as a fitting parameter, we find an unquenched pyrene lifetime τ0 ) 220 ns, which is in a good agreement with the value obtained independently in experiments with linear associative thickeners at low pyrene concentration. One prediction of the Poisson quenching model is that the parameter n, the number of pyrenes per micelle, should
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polymer and pyrene concentrations. Thus, determining meaningful aggregation numbers requires more careful experiments than we have currently carried out. We anticipate describing these results in a future publication. Summary We have examined the rheology and the association properties of an alkali swellable associative polymer in water as a function of polymer concentration and pH. A large increase in thickening efficiency occurs in the region of pH > 6 and is associated with the formation of interpolymer hydrophobic bridges. Pyrene-probe experiments suggest that as the pH of the solution is raised, small intrapolymeric hydrophobic domains are replaced with larger interpolymeric domains. Figure 7. Fluorescence decay curve for pyrene solubilized in aqueous solution of 1-C20 polymer, Cpol ) 4.1 g/L, pH ) 9.5.
increase in proportion to the pyrene concentration. This type of behavior is observed over only a limited range of
Acknowledgment. The Toronto authors thank NSERC Canada for support of this research. LA960614A
