0 Copyright 1993 American Chemical Society
APRIL 1993 VOLUME 9, NUMBER 4
Letters Association Structure of Telechelic Associative Thickeners in Water A. Yekta, J. Duhamel, H. Adiwidjaja, P. Brochard,l and M. A. Winnik* Department of Chemistry and Erindale College, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 1Al Received November 9,1992. In Final Form: February 1 , 1993
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We have examined aqueous solutions of an associative thickener [AT] polymer (poly(ethy1eneoxide),
M,, = 8200, totalM,, 34 QQQ, with C16H330- end groups) by a variety of techniques, including fluorescence and fluorescencedecay measurementsin conjunction with pyrene derivatives as fluorescentprobes. These establish that ca. 20 AT end groups associate to form a hydrophobic cluster, and that these clusters have a very high microviscosity (more than an order of magnitude greater than that of classical surfactant micelles). We also establish that the clusters persist upon dilution of the sample and under rheological stress. We propose a new model to account for this behavior.
Associative thickeners [AT'S] are water-soluble polymers containing hydrophobic substituents which exhibit remarkable rheological properties. At modest concentrations (1-2 wt % 1, they impart high viscosity to aqueous solutions, and these solutions undergo shear thinning with little elasti~ity.~!~ Many experiments have been carried out on linear (telechelic), urethane-coupled poly(ethy1ene oxide)s [PEOsl with hydrophobic end groups. These are often referred to as HEUR-type AT'S. An example of one such structure, AT 22-2, is shown below. From these studies, a model has evolved in which the HEUR end groups associate to form micelle-like clusters [MLC'sl Polymer chains bridge these clusters, causing a large increase in solution viscosity. In this model, shear thinning is accompanied by the breakup of the clusters and the generation of free chain ends. .293
(1) Current address: Thomson TRT Defense, Rue Guynemer, 78283 Guyancourt, France. (2) (a) WaterSoluble Polymers;Glass, J . E.,Ed.;Advancesin Chemistry Series 213; American Chemical Society: Washington, DC, 1986. (b) Polymers in Aqueous Media; Glass, J. E., Ed.;Advances in Chemistry Series 213; American Chemical Society: Washington, DC, 1989. (c) Polymers a8 Rheology Modifiers; Schulz, D. N., Glaes, J. E., E&.; ACS Symposium Series 462; American Chemical Society: Washington, DC, 1991. (3) Jenkins, R. D. Ph.D. Thesis, Lehigh University, Bethlehem, PA, 1990.
This is a molecular-level model, and it needs to be tested with appropriate experiments. It is only very recently that experiments have been carried out which examine molecular properties of these Our contribution4was to introduce pyrene [Pyl as a fluorescent probe to study this sample in aqueous solution. In this way we established that the C16H330- groups in these polymers do in fact associate to form hydrophobic clusters, and that polymer association occurs at polymer concentrations greater than a few parts per million. The central issues which one needs to understand concern the number of chain end groups which associate to form a cluster (the mean aggregation number Nag&, the origin of the shear-thinning behavior, and the effect of shear on the structure of the MLC's. In this paper we summarize a number of new results, which will be published later in full detail, that indicate that the above model is incorrect. We propose here a new model for the mechanism of AT action in solution. Finally (4) Wang, Y.; Winnik, M. A. Langmuir 1990,6, 1437. (5) Richey, B.; Kirk, A. B., Eisenhart, E. K.; Fitzwater, S.;Hook, J. J. Coat. Technol. 1991,63,31. (6) (a) Maechling-Strasser, C.; Franpoise, J.; Clouet, F.; Tripette, C. Polymer 1992,33,627. (b)Persson, K.; Abrahmsh, S.;Stilba,P.; Hansen, F. K.; Walderhaug, H. Colloid Polym. Sci. 1992,270,465. (c) Ou-Yang, H. D.; Gao, 2.J. Phys. 11 1992, 1 , 1375.
0143-1463/9312409-0881$04.00/00 1993 American Chemical Society
Letters
882 Langmuir, Vol. 9,No. 4, 1993
9.0
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Figure 1. Fluorescence spectra of 1 X lo4 M dipyme in (a) cyclohexanol and (b) an aqueous solution (10 g/L) of AT 22-2. The spectra are normalized to their peak value at 395 nm.
I u.0
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Figure 2. Z ~ Z Mvs concentration (right axis, an indicator of microscopicviscosity) for dipyme in aqueous solutions of AT-22 (A) and relative macroscopic Viscosity ( ~ ~ 1as) a function of concentration for AT 22-2 ( 0 )and the control polymer (0).
we emphasize that the conclusions of many otherwise quantitative experiments are limited by our knowledge of the detailed structure and compositional heterogeneity of moleculeslike AT 22-2,whose properties we examinehere. To proceed further in this field, one will have to establish anew standard for the characterizationof these materials.’
cyclohexanol, implying that it is an order of magnitude more viscous than a typical surfactant micelle. The second set of observations (Figure 2) concerns the absence of any effect of dilution on the fluorescence properties of dipyme. I ~ I remains M constant, as does 11/13,a measure of the polarity of the dipyme microenC I~H~~~-DI-[(OCH~CH~)~-DI]~-(OCH~CH~)~-DI-OC~~H~~ vironment.8 Even at 0.1 g/L polymer concentration,where AT 22-2 (Mn = 34,000, y = 3, x 180) the solution is almost as fluid as water itself, the hydrophobic clusters remain unchanged. HO(CH2CH20)x-DI-[(OCH2CH2)x-DI]y. I-(OCH~CH~)~OH Using pyrene as a probe, it is normally possible to obtain Control (y 3) aggregation numbers for micellar cluster^.^ Because of the exceptionally high local viscosity, excimer formation within the cluster is slow.1o Once we realized the feature 0 retarding excimer formation, we could use fluorescence CH3, ,CH2NH& decay measurementsto determinethat there are on average 20 chain ends (c1&33& groups) per micelle-like cluster. This mean aggregation number is independent of polymer concentration from 2.5 to over 10 g/L, covering more than 2 orders of magnitude in bulk solution viscosity. dipyme DI The third type of information we need to understand the system is provided by viscosity measurements in water. The first set of relevant experiments involve the At modest to high concentrations in water (Figure 2), the fluorescent probe dipyme [bis(l-pyrenylmethyl) ether] .8 solution viscosity increases almost exponentially with This probe is soluble in organic solvents and is essentially increasing concentration. This behavior is taken to be an insoluble in water. It can be solubilized in water by indication of the effect of end group association which surfactant micelles. The control polymer is unable to leads to large aggregates. At concentrations below 1 g/L, solubilizedipyme, whereas in the presence of A T in water, the relative viscosities of the AT samples have a much dipyme behaves as though it were dissolved in a very smaller dependenceon concentration,and yield an intrinsic viscous medium. Dipyme forms an intramolecularexcimer viscosity [J value of 1.1dL/g for AT 22-2. by a molecular folding mechanism. The rate of this process A reasonable explanation for the low concentration behavior is that the large-scaleaggregates have dissociated into individual micelles, which in the case of the AT 22-2 is a spherical “rosette” comprised of 10 cyclized chains. For the micelles to be made up of only cyclized chains, all is resisted by the local friction of the environment. In the polymer molecules must contain two CleH&- end Figure 1one notes less excimer (smaller I ~ I Min)the AT groups. From the Einstein expression for hard spheres solution than in cyclohexanol(9= 65 cP), asolvent 2 orders of magnitude more viscous than a fluid solvent such as [TIM= (10x/3)NAR,3 (1) cyclohexane ( r ) = 0.66 cP). Thus, the hydrophobic cluster Using M = lOM, for the molecular weight of the micelle, which solubilizes dipyme is locally more “viscous” than we calculate an effective hydrodynamic radius R h of 18 nm for the micelle. By GPC in THF solution, the AT (7) BothAT22-2and thecontrolpolymer 23-2areprepared byreacting PEO of M,, = 8200 with isophorone diisocyanate. In the former case sample has a relatively broad molecular weight distribuhexadecanol is added to the reaction mixture, and the structures and tion, M,IM,, = 1.7. Using M = lOM,, we obtain R h = 21 compositions indicated are those inferred from reaction stoichiometry. 5
Some supporting data from light scattering, ‘HNMR, and membrane osmometry are available. ( 8 ) (a) Georgescauld, D.; DesmasBz, R.; Lapouyade, R.; Babeau, A.; Richard, H.; Winnik, M. A. Photochem. Photobiol. 1980, 31, 539. (b) Winnik,F. M.; Winnik, M. A.;Ringsdorf, H.; Venzmer, J. J. Phys. Chem. 1991,95, 2583.
(9) (a) Yekta, A.; Aikawa, M.; Turro, N. J. Chem. Phye. Lett. 1979,63, 543. (b) Infelta, P. P.;Gritzel, M. J. Chem.Phye. 1979,70,179. (c) Atik, S. S.;Nam, M.; Singer, L. A. Chem. Phys. Lett. 1979,67,75. (10)Yekta, A.; Duhamel, J.; Brochard, P.;Adiwidjaja,H.; Winnik, M. A. Macromolecules, in press.
Langmuir, Vol. 9, No. 4, 1993 883
Letters Tetramer Unimer
Micelle
Microgel
Figure 3. A cartoon depicting the microgel model for the associative thickener structure in aqueous solution. At higher concentrations, the network spans the entire solution, and the system gels.
nm. From dynamic light scattering [DLSI measurements, we find Rh = 25 nm. Because of the synthetic method used to prepare AT 22-2,7 there is a significant probability that some of the chains contain only one hydrophobic end group. These molecules would bind to a cluster at their hydrophobic end, with the other end free in solution. In the low concentration regime where single micelles exist, these chains would make a greater contribution to Rh than that calculated from eq 1, explaining the larger Rh value obtained by DLS. Extrapolating from the recent work of Devanand and Selser,ll we can estimate the radius of gyration RG for a length of PEO in water half the size of our AT polymer. They find
RG = 0.0215M2583nm (2) from which we calculate RG = 8.6 nm. Thus, the packing of polymers into a single micelle leads to substantial stretching of the PEO chains in the corona. Two important fluorescence experiments provide evidence on the influence of shear on the AT structure in aqueous solution. A group at Rohm and Haas prepared (11) Devanand, K.; Selser, J. C. Macromolecules 1991, 24, 5943.
an HEUR AT with pyrene groups incorporated into the hydrophobe.5 The clusters formed from these end groups exhibit intense excimer emission. Using a cone-and-plate device, fluorescence was monitored as a function of shear rate, and no change inIdlM was observed. In experiments described in a recent thesis,12Py and dipyme were used as probes to monitor fluorescence as a function of the extensional strain rate for solutions of a polymer similar to AT-22 in water. Under conditions in which large deviations from Newtonian flow behavior was found to prevail, again no change in fluorescence was observed. To explain these results, we need a model in which the structure of the MLC’s is conserved as the system responds to stress. The model presented in Figure 3 accounts for many of the known properties of AT’S in aqueous solution. Over the whole concentration range for c > cmc (the critical micelle concentration), each hydrophobic cluster is made up of 20 chain ends. In the concentration range 1-4 g/L, the micelles associate to form doublets, triplets, quartets, or higher aggregates of finite size, accompanied by rearrangement so that many chains span adjacent clusters. We refer to these aggregates as “microgels”. At sufficiently high concentration, here in the region of 5 wt 9% ,the system may form a gel. The microgel model allows us to explain the influence of shear on the AT structure. In our view, when the AT solution is subjected to sufficient shear, the microgel responds by breaking up into a larger number of smaller microgel units, accompanied by rearrangement of the end groups. In this way, the mean number of hydrophobic end groups per MLC remains constant. Thus, diluting the sample and imposing shear have similar effects on the structure of the solution. As a final comment, we note that the deduction of 20 chain ends per MLC depends upon our knowing the molecular mass (M,) of the AT and ita end group content. As our skills at characterizing these materials improve, the exact value of NW reported here may have to be revised. Acknowledgment. The AT and control samples were provided by Dr. D. R. Bassett at Union Carbide. Financial support was provided by the Ontario Centre for Materials Research, by NSERC, by Union Carbide, and by Aqualon. We thank D. R. Bassett and R. D. Jenkins (Union Carbide) and D. Vlahiotis and D. F. James (University of Toronto) for helpful discussions. (12) Vlahiotis, D. Ph.D. Thesis, University of Toronto, Toronto, Canada, 1992.
