Synthesis and Characterization of Comb Associative Polymers Based

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Langmuir 1997, 13, 6896-6902

Synthesis and Characterization of Comb Associative Polymers Based on Poly(ethylene oxide) Bai Xu, Lin Li, Kewei Zhang, Peter M. Macdonald, and Mitchell A. Winnik* Department of Chemistry and Erindale College, University of Toronto, Toronto, Ontario M5S 1A1, Canada

Richard Jenkins* and David Bassett Union Carbide Corporation, UCAR Emulsion Systems, Research and Development, 410 Gregson Drive, Cary, North Carolina 27511

Dieter Wolf and Oskar Nuyken Institut fu¨ r Technische Chemie, Technische Universita¨ t Mu¨ nchen, Lichtenbergstrasse 4, D-85747 Garching, Germany Received June 21, 1996. In Final Form: July 28, 1997X Model associative polymers based upon poly(ethylene oxide) of Mn ) 8400 with a well-defined comb structure were synthesized by a condensation reaction with isophorone diisocyanate and a long-chain 1,2-diol, purified, and characterized. By 1H NMR we determined that one polymer, Comb-81, contains 47 µmol of C14H29- groups per gram (ca. 2.5 groups per polymer) and Comb-83 contains 55 µmol of C14H29groups per gram (ca. 8.8 groups per polymer). Fluorescence experiments employing pyrene as a probe show that these molecules form micelle-like aggregates in water through self-association. Aggregates of Comb-81 form at very low polymer concentrations (cpol < 50 ppm). Over a range of concentrations, as determined from fluorescence decay experiments, each micelle contains an average of 15 hydrophobic groups. Both dynamic light scattering and pulsed-gradient spin-echo NMR experiments indicate the hydrodynamic radii of the individual micelles formed from Comb-81 are on the order of 20-25 nm. Secondary association occurs at concentrations above ca. 0.2 wt %, and at higher concentrations leads to large increases in solution viscosity. As in other associating polymer solutions, solutions of Comb-81 exhibit a pronounced shear thinning once a critical shear rate is exceeded.

Introduction Concern for the environment is driving a shift in coatings technology from solvent-based to water-borne coatings. One essential aspect of the performance of these coatings, particularly paints, is their rheology. Good performance requires the dispersion to possess a low viscosity at high shear rates to achieve uniform coverage of the substrate and high viscosity at low shear rate to prevent settlement of pigments and increase the shelf life of the formulation. At the same time, at high shear rates the viscosity must be high enough to produce a good film thickness; and at low shear rates, the viscosity must be low enough to exhibit good flow-out and leveling on the surface. These seemingly mutually exclusive demands require the dispersion to exhibit time-dependent rheological properties specific to each particular type of application, in which the viscosity changes in a controlled way with shear rate. One strategy for achieving these rheological properties involves adding to the dispersion a water soluble polymer containing hydrophobic substituents as a rheology modifier. These associative polymers,1-9 when employed in this type of application to provide a more desirable rheology profile, are referred to as “associative thickeners” (ATs).1-5 One major benefit of the use of ATs is that they provide effective thickening of small particle latex dispersions to provide water-borne coatings with high gloss levels. * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, November 15, 1997. (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. Mater. 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, June 1990; Chapter 17.

S0743-7463(96)00612-9 CCC: $14.00

Urethane-coupled polyethylene glycol polymers containing hydrophobic end groups (HEUR polymers) constitute one class of ATs widely used as rheology modifiers in paints and other coatings applications.1-3 While the structure as written for these telechelic polymers have two hydrophobes on the ends of the polymer backbone, in practice, a significant fraction of these polymers is missing an end group.6 Most HEUR polymers are in fact a mixture of telechelic polymer and nonionic surfactant. (2) (a) Water Soluble Polymers; Glass, J. E., Ed.; Advances in Chemistry Series No. 213; American Chemical Society: Washington, DC, 1986. (b) Polymers in Aqueous Media; Glass, J. E., Ed.; Advances in Chemistry Series No. 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) Franç ois, J. Prog. Org. Coatings 1994, 24, 67. (f) Maechling-Strasser, C.; Clouet, F.; Franç ois, J. Polymer 1993, 33, 1021. (3) (a) Hoy, K. L.; Hoy R. C. US Patent 4,416,485, 1984. (b) Jenkins, R.D.; Bassett, D. R.; Shay, G. D. US Patent 5,292,828, 1994. (c) Emmons, W. D.; Stevens, T. E. US Patent 4,079,028, 1978. (4) Annable, T.; Buscall, R.; Ettelaie, R.; Whittlestone, D. J. Rheol. 1993, 37, 695. (5) (a) Fonnum, G.; Bakke, J.; Hansen, F. K. Colloid Polym. Sci. 1993, 271, 380. (b) Huldeń, M. Colloids Surf., A 1994, 82, 263. (6) Yekta, A.; Nivaggioli, T.; Kanagalingam, S.; Xu, B., Masoumi, Z.; Winnik, M. A. In Hydrophilic Polymers: Performance with Environmental Acceptance; Glass, J. E., Ed.; ACS Symposium Series 248; American Chemical Society: Washington, DC, 1996; Chapter 19. (7) (a) Wang, Y.; Winnik, M. A. Langmuir 1990, 6, 1437. (b) Yekta, A.; Duhamel, J.; Brochard, P.; Adiwidjaja, H.; Winnik, M. A. Macromolecules 1993, 26, 1829. (c) Yekta, A.; Duhamel, J.; Adiwidjaja, H.; Brochard, P.; Winnik, M. A. Langmuir 1993, 9, 881. (d) Yekta, A.; Xu, B.; Duhamel, J.; Adiwidjaja, H.; Winnik, M. A. Macromolecules 1995, 28, 956. (8) Katayama, S.; Shimizu, M.; Akahori, Y. J. Chem. Phys. 1995, 102, 1846. (9) Ou-Yang, H. D.; Gao, Z. J. Phys. II 1991, 1, 1375.

© 1997 American Chemical Society

Comb Associative Polymers

Langmuir, Vol. 13, No. 26, 1997 6897

Table 1. Reactant Weights Used in the Synthesis of Comb Polymers material

a

MWa

grams

Carbowax 8000 (lot UCC-B639) toluene 1,2-hexadecanediol (Aldrich, 90%) Bismuth Hex-Chem 28%d IPDI (Aldrich, 98%) BHT presevative

3RDJY-81 (Comb-81) 500.0 1635 8.49 (7.64)c 7.00 17.88 (17.52) 0.20


3RDJY-82 (Comb-82) 500.0 1665 8.49 (7.64)c 7.00 18.99 (18.61)c 0.20


3RDJY-83 (Comb-83) 270.0 1650 4.58 (4.13)c 7.00 10.46 (10.25)c 0.20

moles

ratiob

0.0591

6

258.5

0.0296

3

222.3

0.0788

8

0.0591

12

258.5

0.0296

6

222.3

0.0837

17

0.0319

18

258.5

0.0160

9

222.3

0.0461

26

8450

8450

8450

Molar mass. b Molar ratio. c Mass of active substance added, based upon vendor’s statement of content.

An alternative strategy for associating polymers involves comblike structures with hydrophobic groups pendant from the backbone. There are in principle many such structures. There is for example a substantial literature on acrylate and acrylamide copolymers containing hydrophobic substituents.2a-d,8 Polymers of this type with bunched hydrophobes often have different properties in water than those in which these groups are more randomly distributed. There are various structural features that one can manipulate in the synthesis of new materials. These include the structure of the hydrophobe, the number of hydrophobes in a bunch, the spacing between hydrophobes in a given bunch, the spacing between bunches along the polymer backbone, the number of bunches on a given polymer chain, the structure of the water soluble polymer segments, and the molecular weight of the polymer. Each of these features affect not only the properties of the polymer in solution but how they interact with other coatings formulation constituents such as latex, pigment, and surfactants.9 Here we are interested in polymers which have structural features in common with the HEUR polymers, but with a comb architecture. We report the synthesis and properties of three polymers, referred to as Comb-81 and Comb-83, whose structures are presented below.

Note that the polymer is prepared by coupling poly(ethylene oxide) (PEO) chains of Mn ) 8400 with isophorone diisocyanate (IPDI). The hydrophobic unit is incorporated in the form of 1,2-hexadecanediol; thus two

d

Mooney Chemical Co., catalyst.

Scheme 1

of the carbons become part of the polymer backbone, with the remaining C14H29 forming the pendant group. In this structure IPDU stands for the diurethane formed from the reaction with IPDI. In the general structure shown above the subscript w denotes the number of “teeth” in a “bunch” of hydrophobes, y indicates the spacing (i.e., the number of EO units) between hydrophobe bunches, and the z refers to the degree of polymerization of the polymer. Here w ) 1; the polymers are simple combs with randomly distributed hydrophobe chains, with w ) 1, y ) 2, and z ) 3 for Comb-81, w ) 1, y ) 2, and z ) 6 for Comb-82, and w ) 1, y ) 2, and z ) 9 for Comb-83. The reaction used to prepare the polymers is summarized in Scheme 1. Experimental Section Materials and Synthesis. The associative thickeners Comb81, Comb-82, and Comb-83 were prepared at Union Carbide as described below, and the recipes are given in Table 1. A four-neck, 1 L, round bottom reaction flask equipped with a heating mantle, Dean-Stark trap, condenser, thermometer, nitrogen bubbler, nitrogen purge line, and stirrer was charged with 1635 g of toluene, 500 g of Carbowax 8000 poly(oxyethylene) (Union Carbide Corporation) of number average molecular weight of 8456, as determined by hydroxyl number end-group analysis, and 8.49 g of 1,2-hexadecanediol (90% active, used as supplied from Aldrich). With nitrogen purge, the solution was heated to reflux at about 113 °C and azeotroped to completely remove trace water from the reaction mixture. A 150 g quantity of water/ toluene azeotrope mixture was drawn off from the Dean-Stark trap. The reaction solution was subsequently cooled to 90 °C, and 7 g of Bismuth Hex Chem 28% Bismuth Octoate catalyst (Mooney Chemical Co.) was charged and allowed to mix well in the toluene solution. Then 17.88 g of isophorone diisocyanate (98% active, used as supplied from Aldrich) was charged to the reaction vessel.

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Table 2. Intrinsic Viscosities of the Unpurified Comb Polymers in 40:60 Butyl Carbitol-Water sample

[η] (dL/g)

Mv

K′ a

K′′ b

Comb-81 Comb-82 Comb-83

0.815 1.24 1.51

97 000 184 000 248 000

0.45 0.43 0.33

-0.11 -0.12 -0.16

a

Huggins coefficient. b Kraemer parameter.

After a 2 °C exotherm, the reaction mixture became viscous, and the reaction was allowed to proceed 1 h and 30 min at 90 °C and then cooled to 70 °C before adding 0.2 g of 2,6-di-tert-4methylphenol (BHT) preservative. The reaction mixture was poured into a stainless steel pan with large surface area to facilitate drying. The final product was a waxy material, designated as thickener 3RDJY81. Purification. To purify the polymers, we take advantage of the temperature-dependent solubility of PEO in ethyl acetate. A polymer solution (ca. 5 wt %) was prepared by dissolving the polymer (2.03 g) in warm ethyl acetate (35.0 g, 50 °C). The solution was cooled in ice and then centrifuged (Sorvall Superspeed RC-2) at 10 000 rpm at 5 °C for 30 min. The supernatant was discarded and the precipitant was redissolved in a similar amount of warm ethyl acetate. This procedure was repeated three times for each sample. The yield of purification is around 85%. Residual solvent was removed from the sample by freeze drying (at 10 wt %) from benzene. Last traces of solvent were removed by placing the sample under vacuum (1 Torr, 40 °C) for 15 h. Structural Characterization. 1H NMR spectra were obtained on a Varian VXR 400S spectrometer using solutions containing 1-3% of polymer in CDCl3. The peaks in the spectra were assigned and integrated areas were evaluated and compared to the peak integral of p-difluorobenzene (triplet at 6.99 ppm) added as an internal standard, as previously described.6 In this way, three independent linear equations relating peak integrals to the amount of substituents were derived (see below). The 1H NMR characterization shows that Comb-81 contains 47 µmol of hydrophobic groups per gram of polymer and Comb-83 contains 55 µmol of hydrophobic groups per gram of polymer. Intrinsic viscosity measurements were carried out on all three comb polymers prior to purification by recrystallization. In a 40:60 mixture of diethylene glycol monobutyl ether (butyl carbitol) and water at 25 °C, plots of the reduced and inherent viscosities of Comb-81, -82, and -83 samples (before recrystallization from ethyl acetate) depend linearly on concentration and yield identical values of the intrinsic viscosity within experimental error. The values of the Huggins coefficient K′ for all three parameters were between 0.3 and 0.45, and the values of the Kraemer parameter (K′′) were between -0.1 and -0.15, so that the differences between K′ and K′′ were within 10% of 0.5. These results, presented in Table 2, are typical of noninteracting polymers in a good solvent and agree with the notion that the solvent system selected has minimized intermolecular associative interactions. Association is characterized by a value of the Huggins parameter greater than 1 and differences between K′ and K′′ larger than 0.5 as is found for telechelic associative polymers, and for Comb-81, in aqueous solution. Polyethylene glycol standards in the 40:60 solvent mixture gave intrisic viscosites satisfying the expression [η] (dL/g) ) (4.31 × 10-4)Mv0.657, from which Mv values of the three comb polymers could be calculated. Polymer molecular weights were estimated from gel permeation chromatography (GPC) measurements on recrystallized samples of Comb-81 and Comb-83 in dimethylformamide (Aldrich, HPLC grade) by comparison with poly(ethylene oxide) (PEO) standards. Measurements in tetrahydrofuran are problematic because of the tendency of high molecular weight PEO to adsorb to the column. The GPC system employed a Knauer Model 64 HPLC pump, a Knauer RI detector, a Waters HR 5E type WAT044229 column (range 2000-4000000), and a solvent flow rate of 0.5 mL/min at 25 °C. Eight PEO (and polyethylene glycol) standards, ranging from M ) 1470 to 760 000 were used in the column calibration. The Mn value determined in this way for Comb-81 is 38 400 (Mw/Mn ) 2.3) and for Comb-83 is 97 600 (Mw/Mn ) 2.1). In combination with the 1H NMR results, we calculate from the Mn values that there are 1.8 hydrophobic groups per chain for Comb-81 and 5.4 hydrophobic groups per

chain for Comb-83. In Comb-81 in particular, there appears to be a low molecular weight tail in the GPC which lowers the Mn value. If the hydrophobe content is calculated on the basis of the “peak” molecular weights (67 300 for Comb-81; 238 000 for Comb83), we obtain 3.0 hydrophobic groups per chain for Comb-81 and 13 hydrophobic groups per chain for Comb-83. Fluorescence quenching measurements were carried out as described previously for analogous telechelic polymers.7 Dynamic Light Scattering Measurements. The size of aggregates in solution was monitored with a Brookhaven Instruments BI-90 particle sizer. This instrument measures the diffusion coefficient of particles by dynamic light scattering and calculates the particle size from the Stokes-Einstein equation (see below). A built-in cumulant analysis program was used to obtain the effective diameters and polydispersity. An inverse Laplace transform method was used as well as a control to monitor the size distribution and the validity of the log-normal distribution hypothesis used in the cumulant analysis. Pulsed-Gradient Spin-Echo NMR Diffusion Measurements. Samples for pulsed-gradient spin-echo (PGSE) NMR diffusion measurements10-12 were prepared by serial dilution of a stock solution consisting of 1.0 wt % polymer in 2H2O. Proton self-diffusion studies were performed using an MRI (magnetic resonance imaging) probe with actively shielded gradient coils (Doty Scientific, Columbia, SC) installed in a Chemagnetics CMX 300 NMR spectrometer operating at 300 MHz for protons. A standard Stejskal-Tanner PGSE sequence10,11 [(90°x)-τ-(180°y)τ], with gradient pulse during t, was employed. Two levels of gradient strength, 0.796 and 1.04 T/m, were used. This gradient strength was calibrated with a sample of 10 wt % PEO in 2H2O for which the diffusion coefficient is known. The experimental error of the diffusion coefficients measured here is less than (5%. All measurements were performed at 25 °C, and the temperature was controlled by an air flow regulator, yielding a temperature stability of (0.5 °C. According to Stejskal and Tanner,10 the amplitude I of the spin-echo signal induced by a (90°-τ-180°-τ) rf pulse sequence in the presence of a pair of magnetic field gradient pulses of amplitude G and duration δ, separated by a time ∆, is given by

I ) I0 exp[-(γG δ)2 Ds (∆ - δ/3)]

(1)

where I0 is a constant, γ is the magnetogyric ratio for the nuclei studied, Ds is the self-diffusion coefficient, and the effects of the transverse relaxation time T2 are included in the term I0. In our experiments, ∆ is kept constant while δ is varied. The selfdiffusion coefficient is calculated from eq 1 using at least 10 different values of δ and the calibrated value of the field strength. Details about the technique may be found in reviews by Stilbs11a and by Soderman and Stilbs.11b Viscosity Measurements. Stock solutions consisting of 3.0 wt % polymer in deionized water were prepared. Each sample was prepared by dilution of the stock solution. Viscosities of dilute polymer solutions (cpol < 1 wt %) were measured with a capillary viscometer at 25 °C. Viscosities at higher concentrations were measured on a Rheometrics RAA analyzer in a cone and plate geometry (φ ) 50 mm, 0.04 rad cone angle). The instrument is controlled by a 486 personal computer for on-line data acquisition. The AT solution is placed in the gap between the cone and the plate. The cone is connected to a transducer, and the plate is either rotated at a constant speed or operated in the oscillatory mode. The linear viscoelastic region was determined to be below 20% strain by strain sweep experiments at frequencies 1, 10, and 100 Hz.

Results and Discussion Hydrophobic Group Analysis. In Figure 1 part of the 1H NMR spectrum of the polymer Comb-81 in CDCl3 is presented. In addition, there is a triplet at 6.99 ppm from 1,4-difluorobenzene added as an internal standard (10) Stejskal, E. O.; Tanner, J. E. J. Chem. Phys. 1965, 42, 288. (11) (a) Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 1. (b) So¨derman, O.; Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1994, 26, 445. (12) Persson, K.: Abrahmsen, S.; Stilbs, P.; Hansen, F. K.; Walderhaug, H. Colloid Polym. Sci. 1992, 270, 465.


Langmuir, Vol. 13, No. 26, 1997 6899

Figure 1. 1 H NMR spectrum of Comb-81 (230 mg in 1.809 g of CDCl3 containing p-difluorobenzene (5.75 × 10 -3 M) as an internal standard).

and the CH2O peak at 3.6 ppm. Peaks from IPDU appear in the 0.70-2.0 ppm region (Figure 1), which overlap peaks due to the C14H29- group. Our objective is to separate the contributions from the IPDU and alkyl (R) resonances in the region 0.7-2.0 ppm. Our analysis is based upon assignment of the resonances of the IPDU group, which is present as a cis-trans mixture in the sample.13,14 Three independent linear equations can be written which make it possible to relate the integrated intensities of different regions of the 1H NMR spectrum to the number of moles of R and IPDU per gram of sample. This information is obtained independent of the knowledge of the polymer molecular weight.6,15 The three equations for the C14H29- substituted comb polymers are

I(0.70-1.10) ) 12(IPDU) + 3R

(2)

I(1.10-1.90) ) 12(IPDU) + 26R

(3)

I(1.10-1.40) ) 12(IPDU) + 24R

(4)

Here I is the integrated NMR intensity in the region indicated, IPDU is the linking group, and R is the C14H29 alkyl chain. From these equations, we find that Comb-81 contains 47 µmol of hydrophobic groups per gram of polymer and Comb-83 contains 55 µmol of hydrophobic groups per gram of polymer, corresponding, for example, to 3.0 hydrophobic groups per chain for Comb-81 (of Mpeak ) 67.000). An illustration of the structures of Comb-81 and Comb-83, showing a representative number of pendant groups, is presented in Figure 2. Self-Association. In HEUR polymers, the hydrophobic end groups associate into micelle-like structures, and a similar phenomenon occurs in Comb-81 and Comb-83. There are two classic methods to demonstrate that polymer substituents in solution associate to form hydrophobic domains. First, one can examine the ability of these polymers, at various concentrations, to solubilize a poorly soluble dye like pyrene in aqueous solution.7 The solubility of pyrene in water is relatively small (ca. 7 × 10-7 M) and pyrene will partition strongly into any hydrophobic regions. An example is shown in Figure 3, where we use UV absorption measurements to determine the quantity of pyrene solubilized in aqueous solutions of Comb-81 at different polymer concentrations c81. The amount of (13) Wendlich, D.; Reiff, H.; Dietrich, D. Angew. Makromol. Chem. 1986, 141, 173. (14) (a) Auf der Heyde, W.; Hubel, W.; Boese, R. Angew. Makromol. Chem. 1987, 153, 1. (b) Hatada, K.; Ute, K. J. Polym. Sci., Part C: Polym. Lett. 1987, 25, 477. (c) Born, L.; Wendlich, D.; Reiff, H.; Dietrich, D. Angew. Makromol. Chem. 1989, 171, 213. (d) Hatada, K.; Ute, K.; Oka, K.-I. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 3019. (e) Bialas, N.; Hocker, H. Makromol. Chem. 1990, 191, 1843. (15) Xu, B.; Yekta, A.; Li, L.; Masoumi, Z.; Winnik, M. A. Surf. Colloids, A 1996, 112, 239.

Figure 2. An illustration of the structure of Comb-81 and Comb-83 chains in their non-associated form. (a) Comb-81 is shown with three hydrophobic groups reflecting the result from 1 H NMR analysis suggesting that each chain (based upon Mpeak in the GPC) contains on average 3.0 hydrophobic groups. (b) A molecule of Comb-83 is shown with nine hydrophobic groups.

Figure 3. Saturation concentration of pyrene in aqueous solutions in the presence of various concentrations of Comb-81 ([AT]).

pyrene solubilized increases linearly with increasing c81, and the limit of zero polymer concentration, we recover the normal water solubility of pyrene. The second approach to establishing the presence of hydrophobic domains involves measuring the intensity ratio (I1/I3) of first and third emission peaks in the fluorescence spectrum of pyrene.16 This ratio provides spectroscopic evidence about the polarity of the dye microenvironment in the system. This ratio varies from 1.71 in water to 0.62 in cyclohexane. Figure 4 shows a fluorescence spectrum of pyrene solubilized in an aqueous solution of Comb-81. Here I1/I3 is equal to 1.20 ( 0.03 and almost invariant with polymer concentration. This value is identical to that found for the corresponding linear HEUR polymer with C16H33O- end groups, suggesting that the hydrophobic domains formed from these two polymers have features in common and are about as hydrophobic as a sodium dodecyl sulfate micelles.7,16 Hydrophobe Aggregation Number. A key structural feature characterizing associating polymers in water is the number of hydrocarbon groups NR that associate to form a micelle-like cluster. Values of NR can, in many instances, be determined by fluorescence quenching experiments. For the linear C16 end-capped HEUR (16) (a) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (b) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: Orlando, FL, 1987; Chapter 2.

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Figure 4. Fluorescence spectrum of a pyrene-saturated aqueous solution of Comb-81 (cpol ) 5 g/L) in which peaks I1 and I3 are identified.

polymers, we employed fluorescence decay measurements of pyrene excimer formation kinetics to determine values of NR ≈ 20.7d These experiments work well if the system satisfies the following features required by the data analysis: (i) The pyrene decay in the limit of low pyrene concentration (cPY) can be described by a simple exponential function. This result implies that the pyrenes occupy similar sites in all micelle-like clusters and that a negligible fraction of pyrene molecules are in the water phase. (ii) The pyrene fluorescence decays at higher values of cPY fit well to the micelle-quenching (Poisson distribution of quenchers) model. This implies that the two key fitting parameters of eq 5 behave as expected with variation in cPY. To satisfy these criteria, one must find that the parameter n*, the mean number of pyrenes per micelle, increases linearly with cPY, and that kt, which describes the rate at which a pair of pyrenes react within a micelle, should be independent of cPY. Here we apply this technique to Comb-81. The fluorescence decay data were fitted to the equation

{

I(t) ) I(0) exp -

[Q] t (1 - e-kt) 0 [mic] τ

}

(5)

In this expression, I(t) is the fluorophore decay rate, [mic] is the molar concentration of micelles in the solution, and [Q] is the quencher (i.e., pyrene) concentration. Note that n* ) [Q]/[mic]. The t/τ0 term describes the exponential decay of fluorophore in micelles containing no quencher. The second term describes the contribution to the total observable fluorescence intensity from micelles containing i quenchers (i ) 1, 2, 3, ...) and kt is the corresponding pseudo-first-order rate coefficient for the intramicellar reaction with one quencher. Our data for Comb-81 are shown in Figure 5. In the lower portion of the figure we plot n* vs cPY, and in the upper portion of the figure we plot the corresponding values of kt obtained from the data analysis. Note that values of kt remain constant when polymer concentration changes. From these data, we calculate NR ) 15 for Comb81.17 Size of the Micelles. Dynamic Light Scattering. One way to measure the size of aggregates in solution is by dynamic light scattering (DLS).9 In the BI-90 particle sizer, the diffusion coefficient of the mobile species is determined by a built-in cumulant analysis of the autocorrelation function decay assuming a log-normal fit to (17) Rao, B. H.; Uemura, Y.; Dyke, L.; Macdonald, P. M. Macromolecules 1995, 28, 531.

Figure 5. (upper) Plot of the fitting parpameter kt, representing the pseudo-first-order rate coefficient for pyrene excimer formation in a micelle containing two pyrenes, vs the global concentration of Comb-81 ([AT]) in each experiment. (lower) Plot of the number of pyrenes per micelle ([Py]/[mic]) multiplied times the Comb-81 concentration ([AT]) vs pyrene concentration ([Py]) in the system. Values of [Py]/[mic] are obtained as the fitting parameter (n*) in the Poisson-quenching micelle model of fluorescence decay analysis.

the distribution of diffusing species. Hydrodynamic radii RH are calculated through the Stokes-Einstein expression, eq 6, where η is the viscosity of the solvent, Dm is the mutual diffusion coefficient, kB is the Boltzmann constant, and here T is the temperature.

RH ) kBT/6πηDm

(6)

From the curve in Figure 6, we observe a smooth decrease in the effective diameter (2RH) of the aggregates with decreasing polymer concentration cpol. The effective diameter approaches a constant value of ca. 40 nm as the polymer concentration decreases to the detection limit of our instrument. Both the cumulant analysis and the inverse Laplace transform analysis of the autocorrelation decay data indicate that the polydispersity of sizes becomes smaller as the concentration decreases. The aggregates appear to be uniform in size in the very dilute regime. Pulsed-Gradient Spin-Echo NMR Measurements. PGSE measurements monitor the random motion of individual molecules or aggregates during a time interval (∆ - δ/3).10 The relationship between the mean-squared displacement in one dimension 〈X2〉 and the self-diffusion coefficent Ds is given by

〈X2〉 ) 2Ds(∆ - δ/3)

(4)


Figure 6. Plot of the effective diameter Dm, as obtained by dynamic light scattering measurements, for Comb-81, as a function of polymer concentration in water.

Langmuir, Vol. 13, No. 26, 1997 6901

Figure 8. Steady shear viscosity (η) of Comb-81 solutions as a function of shear rate at different polymer concentrations.

studied previously,15,17 this radius pertains to the size of the micelle-like aggregates, with an onset of association that takes place at concentrations well below the detection limit of either the DLS or the PGSE NMR measurements. From the size of the aggregated species one can estimate the number of polymer molecules it contains. For a flowerlike micelle,18 a hard-sphere model provides a good description of its contribution to the solution viscosity. Then the mass of the aggregated species, M, is related to its hydrodynamic volume, (4π/3)RH3, through the intrinsic viscosity [η], via the Einstein equation15,17

[η]M ) 2.5(4π/3)RH3NA

Figure 7. Plot of the self-diffusion coefficient Ds, as obtained by 1H PGSE NMR measurements, for Comb-81, as a function of polymer concentration in water.

For diffusion coefficients typical of these polymer systems (Ds ) 10-12 m2 s-1), and using our value of ∆ ) 350 ms, the root-mean-squared displacements are many times larger than the gyration radius of either individual polymer molecules or their micellar aggregates. Consequently, the motion monitored by the PGSE NMR experiment corresponds to pure center-of-mass diffusion. Figure 7 shows diffusion coefficients measured via PGSE NMR for Comb-81 as a function of polymer concentration. Over the range 0.1 < cpol < 0.72 wt % Ds values decrease with increasing concentration. An important feature of our PGSE NMR measurements at the polymer concentrations shown in Figure 7 is that, within experimental accuracy, we always observed monoexponential echo attenuations. Furthermore, there was never any dependence of the apparent diffusion coefficent on diffusion time (∆ - δ/3). This indicates that either there is a narrow distribution of diffusion coefficents about the mean value or there is fast exchange on the time scale of the measurements (∆ - δ/3) between different sites having different diffusion characteristics. As in the case of dynamic light scattering data, the effective hydrodynamic radius of the diffusing units can be estimated from the Stokes-Einstein expression, eq 6, in which Ds replaces Dm. In the limit of cpol f 0, these two diffusion coefficients become equal. Extrapolating Ds to cpol ) 0 yields Do ) 7.9 × 10-12 m2 s-1, corresponding to a hydrodynamic radius of about 25 nm. This hydrodynamic radius is very similar to that obtained by DLS. For Comb-81, like the telechelic associative polymers we have

(7)

where NA is Avogadro’s number. We measured the intrinsic viscosity Comb-81 polymer in aqueous solution, and find a value of 290 cm3/g. Applying eq 7, with RH ) 25 nm, yields a value of M ) 3.4 × 105. Comb-81 has Mn ) 38 400. With this value, we calculate that each micelle comprises ca. nine polymers. These polymers of Mn ) 38 400 contain on average 1.8 hydrophobes per chain. Thus each micelle would contain 9 × 1.8 ) 16 hydrophobes. Note that based upon the Mpeak value of 67 000 with three hydrophobes per chain, we calculate that each micelle contains 5 polymers and 15 hydrophobes. From fluorescence quenching (pyrene excimer) experiments, we have determined that each micelle contains NR ) 15 C14H29 chains. These numbers are remarkably consistent. Steady Shear and Shear-Thinning. In Figure 8 we plot steady-state viscosity vs shear rate for Comb-81 at various concentrations in water. Unlike the C16-endcapped HEUR polymers, the higher concentration solutions are not Newtonian at these low shear rates. The viscosity shows a gradual decrease as the shear rate is increased from 0.01 to 10 s-1. Once a critical shear rate is reached, a strong shear thinning effect is observed for all polymer concentrations. A shear thickening effect is observed prior to the onset of shear thinning in the case of polymer concentrations of 12 and 15 g/L. In the highshear-rate region, the viscosity vs shear rate dependence can be described by a power law similar to the behavior found for ordinary concentrated polymer solutions.19 The exponent of the power law is close to -1. This implies that the stress is constant across the sample in the shear thinning region. The value of the critical shear rate for the onset of shearthinning shifts smoothly to lower shear rate with increas(18) Semenov, A. N.; Joanny, J.-F.; Khokhlov, A. R. Macromolecules 1995, 28, 1066.

6902 Langmuir, Vol. 13, No. 26, 1997

ing polymer concentration. If the onset of shear thinning is associated with a relaxation time of the system, this result implies that this relaxation time becomes longer as the polymer concentration is increased. A detailed analysis of the viscous and viscoelastic properties of these solutions is presented in an accompanying paper.20 Summary We report the synthesis and characterization of a comb associative polymer based upon poly(ethylene oxide) (PEO), with pendant C14H29 groups separated by ca. 185 EO units. Fluorescence experiments with pyrene as a probe indicate a very low onset of association, at polymer concentrations below 50 ppm. Fluorescence decay measurements of the polymer solution indicate that on average 15 hydrophobes associate to form individual micelle-like objects. Both dynamic light scattering and pulsed(19) Grassley, W. W. In Physical Properties of Polymers; American Chemical Society: Washington, DC, 1984; p 97. (20) Xu, B.; Yekta, A.; Winnik, M. A.; Sadeghy-Dalivand, K.; James, D. F.; Jenkins, R. D.; Bassett, D. Langmuir 1997, 13, 0000.

Xu et al.

gradient spin-echo NMR measurement indicate a narrow distribution of sizes for species present at low concentrations (ca. 0.1 wt %) with a hydrodynamic radius of 20-25 nm. By combining this value with the measured intrinsic viscosity, we infer that about 5 to 9 polymer molecules, containing a total of 15 hydrophobic substituents, combine to form individual flower-like micelles in this concentration range. Thus the diffusion measurements and the fluorescence quenching measurements give very similar descriptions of the polymeric species. At higher concentrations, the system exhibits very large increases in solution viscosity, indicating that the micelles undergo secondary association to form larger aggregates. These solutions exhibit a pronounced shear thinning once a critical shear rate is exceeded. This critical shear rate, which is on the order of 100 s-1, shifts to lower values with increasing polymer concentration. Acknowledgment. The authors thank NSERC Canada for their support of this research. In addition, we would like to thank Mr. Nick Plavac and Mrs. Zarah Masoumi for their assistance in the NMR measurements. LA960612Q

Synthesis and Characterization of Comb Associative Polymers Based

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