Langmuir 1994,10,3027-3034
3027
Synthesis, Characterization, and Solution Rheology of Model Hydrophobically-Modified,Water-Soluble Ethoxylated Urethanes? David J. Lundberg,$ Richard G. Brown,§ J. Edward Glass,* and Richard R. Eley” Polymers and Coatings Department, North Dakota State University, Fargo, North Dakota 58105 Received April 14, 1993. In Final Form: May 26, 1994@ The synthesis, characterization, and solution rheology of well-characterized hydrophobically-modified, ethoxylatedurethane (HEUR)water-solublepolymers are described for two types of model HEURs: linear polyloxyethylene) of M n= 26 200 with terminal hydrophobes of different sizes, and terminal hydrophobe groups separated by smaller oxyethylene spacing around a larger internal hydrophobe. In the first series, a terminal isocyanate telechelic prepolymer of poly(oxyethy1ene)is prepared and reacted with amine and alcohol containing hydrophobes of variable size. In the second, terminal hydrophobe HEURs varying in geometry around an internal hydrophobe are synthesized by a one-step addition of an ethoxylated nonylphenol surfactant to a diisocyanate or an isocyanato functional biuret or isocyanurate. Low shear rate viscosities and oscillatory responses of HEUR solutions are examined, alone and in the presence of anionic and nonionic surfactants. HEUR solutions exhibit a maximum inviscositywith increasing surfactant concentrations with both anionic and nonionic surfactants. The viscoelasticity of the solution in the area of the viscosity maximum and factors influencing the magnitude of the viscosity increase are addressed. The surfactant concentration necessary to achieve the viscosity maximum is observed to depend on the concentration and architecture of the HEUR as well as the structure of the surfactant. Differences in the phase separation behavior of aqueous solutions also are observed.
Introduction Hydrophobe-modified, water-soluble polymers have engendered considerable interest over the past decade. Almost all of the materials examined are complex mixtures, but this is rarely acknowledged. For example, in the chain growth polymerization of synthetic polymers (e.g., styrene/maleic acid, methacrylate, acrylate, or acrylamide) a given polymer chain may not contain a hydrophobe-modified (HM) unit or it may contain several HM units. Ifthe synthesis is conducted in aqueous media, micelle formation can lead to blocked sequence^^^^ of the hydrophobe units. In the hydrophobe modification of alkali-swellable emulsions: the cross-linking necessary to maintain insolubility results in a broad variation in molecular weights, and two or more products are obtained with different solubilities and undefined compo~itions.~ In the hydrophobe modification ofhydroxyethyl cellulose (HEC) similar disparities are evident. Depending on the method and extent of derivatization 2-20% of the repeating glucopyranosyl units are not substituted with oxyethylene chains and a significant number of substituted Presented in parts at the American Chemical Society Fall Meeting, Miami, FL, 1989. 3M Center. Building 236-1N-05. St. Paul. M N 55144. 8 Aqualon Co., Research Center, Wilmin&n, DE 19850. It The Glidden Co., Research Center, Strongsville, OH 44136. Abstract published inAdvance ACSAbstracts, August 15,1994. (1) The variety ofchemical structures associated with this technology is presented in Polymers in Aqueous Media, Performance through Association; Glass, J. E., Ed.; Advances in Chemistry 223;American Chemical Society: Washington, DC, 1989;Chapters 8 and 16-28. (2)Peer, W. J . Polymers in Aqueous Media, Performance through Association; Glass, J. E., Ed.; Advances in Chemistry 223;American Chemical Society: Washington, DC, 1989;Chapter 20. (3)Valint, P. L., Jr.; Bock, J.; Schulz, D. N. Polymers in Aqueous Media, Performance through Association; Glass, J. E., Ed.; Advances in Chemistry 223;American Chemical Society: Washington, DC, 1989; Chapter 21. (4)Shay, G. D. Polymers in Aqueous Media, Performance through Association; Glass, J. E., Ed.; Advances in Chemistry 223;American Chemical Society: Washington, DC, 1989;Chapter 25. (5)Fernando, R.H.Ph.D. Thesis, North Dakota State University,
*
@
1986.
pyranosyl units contain more than one oxyethylene chain.6 The oxyethylene adducts have a n average offour segments per graft, with a small percentage achieving ten oxyethylene units. Additional modification of HEC with anionic, cationic, or less reactive oxirane groups add to the oxyethylene adducts and not to the repeating pyranosyl rings of the cellulose chaine7a The process used in the hydrophobe modification of HEC is unknown, but regardless of the position of substitution, the hydrophobes are not significantly removed from the segmentally rigid, glucopyranosyl backbone. Hydrophobically-modified, ethoxylated urethane (HEUR) polymers of commercial importance are prepared by a stepgrowth process and they also exhibit a heterogenous mixture of molecular weights with various degrees of hydrophobe modifi~ation.~ In this paper, the synthesis, characterization, and solution rheology of well-characterized HEUR structures with variable terminal hydrophobe sizes are reported. Two- to four-arm terminal nonylphenol HEURs with a n internal hydrophobe also are prepared. The viscosity behavior of these model HEUR structures is described, alone and in the presence of anionic and nonionic surfactants, and compared with the surfactant behavior of other less well-defined associative thickeners of the hydrophobically-modified, hydroxyethyl cellulose and alkali-swellableemulsion families. The model HEUR materials provide a basis for interpretation of more complex HEURs prepared by a traditional step-growth polymerization. (6)Glass, J. E.;Buettner, A. M.; Lowther, R. W.; Young, C. S.;Cosby, L. A. Carbohyd. Res. 1980,84, 245-163. (7)Glass, J. E.; Shah, S.; Lu, D.-L.; Seneker, S. D. ACS Sympos. Series 240,Polymer Adsorption and Dispersion Stability; Goddard, E. D., Vincent, B., Eds.; ACS Symposium Series 240;American Chemical Society: Washington, DC, 1984;Chapter 7. (8)Seneker, S.D. Ph.D. Thesis, North Dakota State University, 1986. (9)Kaczmarski, J. P.; Glass, J. E. Proc. ACS, Diu. Polym. Mater.: Sci. Eng. 1991,65, 175.
Q743-7463l94l241Q-3Q27$Q4.5QlQ 0 1994 American Chemical Society
3028 Langmuir, Vol. 10,No. 9, 1994
Lundberg et al.
Table 1. Conventional surfactant Molecular Weight Data
theoretical experimental Mn surfactant descriptionQ -OH number -OH number (-OHno.) 14.73f 0.17 3808 (NPO)(EtO)iooH 11.22 4942 11.22 11.35f 0.13 (MeO)(EtO)looH 32.32 0.83 1736 (NPO)(Et0)5oH 28.05 28.97 f 0.33 1936 (MeO)(Et0)5oH 28.05 NP = nonylphenol; Me = methyl; Et0 = oxyethylene units, average values.
*
Experimental Section Reagents. Isophorone diisocyanate (IPDI) was purchased from Huls and was distilled under vacuum before use. The biuret adduct of 1,6-diisocyanatohexane (Mobay), the isocyanurate of isophoronediisocyanate (IPDI-T,Thorson Chemical),in a mixture of cellosolve acetate and xylene, and n-tetramethylxylene diisocyanate (TMXDI, American Cyanamid) were used as received. Poly(ethy1ene glycol) (POE) was provided by Fluka and was designated as having a M, of 35 000. The followingsurfactants ofvaryingoxyethylene content (EtO) and varying hydrophobe groups were used (suppliers): Igepal CO-990(nonylphenol-100EtO) and Igepal CO-890(nonylphenol40 EtO, GAF Chemicals); Methoxy Carbowax5000 (Me-Et0 100) and MethoxyCarbowax2000 (Me-Et040, Union Carbide); Iconol DNP-150 (dinonylphenol-150EtO, BASF); experimentalproduct6017-15 (C1&.3-50 EtO) from Witco. n-Butylamine, di-nbutylamine, and n-octylamine were purchased from Aldrich and nonylphenol was purchased from Fluka. These compoundswere diluted with dry solvent and stirred over molecular sieves. Tetradecanol and docosanol, purchased from Fluka, were recrystallized from benzene/petroleum ether. Toluene, tetrahydrofuran and petroleum ether were stirred for 24 h over 40 mesh calcium hydride and distilled under argon. NJV'-Dimethylformamide was used as received. Water used in viscometric analysis was distilled and passed through a Milli-Q (Millipore)ion exchange filtration system. Dibutyl tin diacetate and dibutyltin dilaurate were obtained from Alfa. Analysis of Starting Materials. Surfactants and POE were analyzed for molecular weight by end group analysis and size exclusion chromatography (SEC). The results are summarized in Table 1. If multiple peaks or shoulders appeared in the chromatographs, the surfactant was discarded. The hydroxyl number titration was performed according to the pyromellitic dianhydride imidazole method for determining hydroxyl numbers.I0 Hydroxyl group titrations ofthe dinonylphenol surfactant indicated that the surfactant contained an average of 110ethylene oxide units. The biuret titrated11 to an average of 3.8 -NCO groups per molecule; thus the reaction yielded on average predominantly four-armed stars with a relative hydrophilic interior. The isocyanate content of the isocyanurate yielded a value of 11.81% isocyanate by weight. A portion of the trimer was reacted with anhydrous methanol for SEC analysis. The chromatograph exhibited one large peak which corresponds to the molecular weight of the trimer and a smaller peak representing a higher molecular weight material. The higher molecular weight peak may be caused by further reaction of the isocyanate with water to produce urea-linked trimeric species. Attempts to further resolve the high molecular weight peak using other size columns were unsuccessful. Synthesis of HEURs with Variable-Size Terminal Hydrophobes. The prepolymer of POE was prepared on a large scale by the stoichiometric reaction of POE, M n (OH number 29 loo), SEC 26 200), with an excess of IPDI (1:200 equivalent ratio) and subsequentlydividing the adduct into smaller portions for reaction with amine and alcohol containing hydrophobes (Scheme 1). The prepolymer was prepared by adding 400 g of POE to a 2000-mL, four-necked, break-away reaction flask, equipped with a Dean Stark water trap and condenser, argon inlet tube, and mechanical stirrer. The POE was dried by azeotropic distillation with toluene. About 600 mL ofthe toluene (10)Kingston, B. H.; Gary, J. J.; Hellwig, W. B. Anal. Chem. 1969, 41 (11, 86. (11)ASTM procedureD2572-87;ASTM: 1916RaceSt.,Philadelphia,
PA 19103.
M n
M
(SEC) 3755 4434 1817 1734
W
(SEC) 4668 5166 2199 1826
MdMn
(SEC) 1.3 1.2 1.2 1.0
Scheme 1. Telechelic Synthesis of Linear HEURa with Variable Terminal Hydrophobe Sizes
X
H
H
was removed through the Dean Stark trap, and 800 g of dry, distilled tetrahydrofuran and 467 g of IPDI were added. A small portion of the solution was withdrawn from the mixture after 36 h at 45 "C and placed in methanol to provide samples for size exclusion chromatographic analysis. The remainder of the solution was precipitated into petroleum ether, filtered and dried under vacuum. The isocyanate content was determined by din-butylamine titration and the prepolymer was stored under argon at dry ice temperature until reacted with amines or alcohols with variable hydrophobe sizes. The following description of the hydrophobic modification of the telechelic prepolymer with tetradecanol is representative of all end-capping reactions with hydrophobic, active-hydrogen containing compounds. In a 500-mL three-necked, round-bottom flask equipped with a mechanical stirrer, argon inlet tube, and thermometer, 100 g of the telechelic prepolymer (0.287% -N=C=O, 0.00683mol)was dissolved in 300 mL of dry, distilled THF. Recrystallized tetradecanol(36.50 g, 0.171 mol -OH; 25 mol excess of -OH to -N=C=O) was dissolvedin a small amount of THF and added to the flask and the contents were heated to 40 "C. A drop of dibutyltin diacetate was added to catalyze the addition reaction and the mixture stirred for 12 h. No catalyst was added when hydrophobic amines were used. Synthesis of Simplistic J3EURs with an Internal Hydrophobe. The synthesis of terminal hydrophobe HEURs varying in geometry around an internal hydrophobe involved the one-step addition of an ethoxylated (50-100 mol averages) nonylphenol with an isocyanato functional biuret (Scheme 2a) or isocyanurate (Scheme 2b), or a diisocyanate (Scheme 2c). The following description is typical of such a polymer synthesis. An ethoxylated nonylphenol surfactant (267.8 g, -OH number = 14.73 k 0.17, 0.071 mol -OH) was added to a reaction flask followingthe procedure described in the preceding section. About 400 mL of dry, distilled THF was added to the flask and the isocyanurate solution (25.8g, 0.072 mol -N=C=O), diluted with small portions of THF, was rinsed into the flask. The contents were heated to 40 "C, with stirringfor 18h. After 12h dibutyltin diacetate (0.06g, 2.0 wt % based on isocyanate weight) was added. The reaction was allowed to proceed at 45 "C for 6 h at which time FT-IR analysis indicated that the isocyanate was completely reacted. The reaction solvents were removed on a rotary evaporator and the solid polymer was redissolved in THF. The polymer was precipitated twice into dry petroleum ether and the powder was vacuum filtered and dried. A small shoulder was observed in the chromatographs due to a small amount of unreacted ethoxylated surfactant. Efforts to remove residual surfactant by fractionation from solventhonsolvent blends were unsuccessful. Dialysis on the branched polymers in membrane
Langmuir, Vol. 10,No.9,1994 3029
Synthesis of HEUR Polymers Scheme 2. Synthesis of HEURa with Internal Hydrophobes by the Addition of Ethoxylated Nonylphenol a. To Biuret of Hexamethylene Diisocyanate b. To Isocyanurate of Isophorone Diisocyanate c. To m-Tetramethylxylene Diisocyante
MICRONS 4.0 4,Z 4.4 4.6
.tzt
5.0
55
6,O
6,s 7,O I1
a 9
0
OCN-(CH2)-N-C-N-(CH2)-N-C-N-(CH2)-NC0 66.0 H 6 C=O H N -(CH2) -NCO
+
RO-ICH2CH20);H
OCN-(CH2)-N
H
6 H
e
P 9 9 R RO-(CH2CH2O)-C-Y -(CH2) -N-C-N-(CH2)-N-C-N-(CH~)gN-C-(OCH2CH2);OR "0 H 6 C = 0H RO-(CH~CHZO)-C-N-(CH,)-N H 6 H
"
6C=O H 0 H N-(CH2)-N-C-(OCH2CH2);OR H 'H
ob
WAVE NUMBERS
'
Figure 1. Fourier transform infrared spectra of(a)IP(Et0)sgsIP and (b) NPIP(Et0)595NPIP.
b
Table 2. SEC Molecular Weight data for Linear Telechelie Hydrophobe Polymers
R n
G-NH-C-(OCHZCH~)-OR n
tubing with molecular weight cutoffs below the molecular weight of the isocyanurate adduct, but above the molecular weight of the surfactant, resulted in no change in the molecular weights as measured by SEC. The IPDI-Tisocyanurate provides a large, internal hydrophobe unit, with an average of three hydrophobes per polymer. An approximate four-arm star polymer was prepared using the biuret adduct of 1,6-diisocyanatohexane (Scheme 2a, NCO % = 21.98, equiv) wt 191.08 g/equiv) and monofunctional ethoxylated surfactants. The biuret contained approximately 3.8 -NCO groups per molecule so the reaction yielded an average four-arm star with a more hydrophilicinterior. This general reaction sequencealso was employedinthe synthesis of a linear HEUR with terminalhydrophobes with a small internal hydrophobe, via the reaction of the ethoxylated surfactant with the diisocyanate, TMXDI. Characterizationof Polymers. The isocyanate content was determined using the di-n-butylamine method. Fourier transform infrared (FT-IR)spectra,recorded using a Mattson Cygnus 25 FT-IR spectrophotometer, of the telechelic prepolymer and the starting POE are presented in Figure 1. The spectrum of the prepolymer contains a discrete absorbance band between 2275 and 2250 cm-l (cumulated double bond stretching, -N=C=O) and bands between 1675 and 1525 cm-1 (urethane C-0 stretch and urethane N-H stretch)which are not present in the starting POE. The reactions of the ethoxylated nonylphenol surfactant with isocyanates were monitored by the disappearance of the isocyanate peak in the spectra. The molecular weights of branched thickeners were measured by vapor pressure osmometry using a Wescan Instruments VPO,toluene as the solvent, and sucrose octaacetate as a standard. Calibration curves were constructed at 30 "C. SEC analysis was performed using three Ultrastyragel columns (Waters, 500 A and (2x) 100 A). Calibration curves were constructed from poly(styrene) standards and had effective range of 50-10000 Da and 2000-160000 Da, respectively. Molecular weight distributions were measured by size exclusion chromatography using a Waters M730 Data
MIl
MW
MwfMn
26 200 22 500 18 600 25 400 21 000 24 000 19 800 19 700
33 800 29 300
1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.4
23 800
33 100 27 000 31 800 26 700 28 200
Module, a Waters R401 differential refractometer, tetrahydrofuran as the mobile phase, a flow rate of 1.0 mumin, and a column temperature of 30 "C. Procedure. Stock solutions were made by rolling a welldispersed polymer in water slowly, until uniform solutions were attained. Fresh stock solutions of the surfactant were prepared for each study.12 The surfactants examined for their influence on HEUR solution viscosities in the latter part of this study were Tergitol15-S-9 (Union Carbide), with an average of 13 carbons as a branched hydroprobe and an average of nine ethoxylate units (b-C13H2,0(EtO)gH), and sodium dodecyl sulfate, SDS (Aldrich,98%pure). The surfactants were used as received. Cone and plate viscometers were used in all solution viscosity determinations. The G and G values were obtained using a Carri-Med controlled stress rheometer in the frequency range of 0.1-10 Hz. The cone diameter was 4 cm with a cone angle of 2 deg. The data were taken in the linear viscoelastic region. Intrinsic viscosities were determined with Ubbelohde capillary viscometers.
Results and Discussion The synthetic schemes described above allowed a number of HEURs with different hydrophobes to be studied by varying the size of the hydrophobic alcohol or amine in the second step of Scheme 1. The telechelic prepolymer was reacted with methanol for SEC analysis. The absence of a higher molecular weight peak indicated that chain extension did not occur t o a n extent detectable by SEC. The IPDI hydrophobically-modified poly(oxyethylene) polymers (Scheme 1) were observed to have slightly lower molecular weights than the starting materials (Table 2). The molecular weights by SEC and vapor pressure osmometry of the ethoxylated nonylphenol thickeners with the isocyanurate internal hydrophobe (the most hydrophobic of the internal hydrophobe thickeners studied) are listed in Table 3. The experimental molecular weights are in agreement with the theoretical molecular weights. (12) Batina, N.; Cosovic, B.; Filipovic-Vincekovic, N. J. Colloid Interface Sci. 1988,125, 69.
Lundberg et al.
3030 Langmuir, Vol. 10, No. 9, 1994 Table 3. Molecular Weight Data for HEUR with Small Oxyethylene Spacings and InternalHydrophobea calculated mol wt
polymer descriptiona
(NPO(EtO)loo)3(IPDI-T)
12 458 15 860 (NPO(EtO)50)3(IPDI-T) 6242 (CH~O(E~O)~O)~(IPDI-T) 6 842
(CH3O(EtO)loo)3(IPDI-T)
a
M, M, M, (VPO) (SEC) (SEC) 9 732 10 547 14 830 10 064 11 036 17 349 6432 6351 10970 6 416 6 209 10 750
Scheme 2b adducts.
In
73
i
t
t
501
9
30
s
PI
0' 0.0
0.5
1.5
1.0
2.5
2.0
3.0
3.5
4.0
4.5
I
5.0
CONCENTRATION (wt.%)
P i "
8
16
24
32
CONCENTRATION Iwt.%l
Figure 2. Low shear rate (2 s-l) viscosity (mPa*s)as a function of the aqueous solution concentration of modified isocyanurate 0, [MeOtrimer (Scheme 2b): A, [NPO(E~O)~OO]~(IPDT)T; *, [NPO(E~O)~O~~(IPDT)T; 0 , [MeO(Et0)5013(Et0)lools(IPDT)~; (IPDT)T.
Low Shear Rate Viscosities. Phase separation is observed in the isocyanurate HEURs when the arms of this class (Scheme 2b) contain less than 50 oxyethylene units, with terminal nonylphenol groups. This also is true for the adducts of methoxy initiated monohydroxy functional (no terminal hydrophobes) oligomers with less than 30 oxyethylene units. To avoid phase separation with these and with other HEURs exhibiting phase separation, a small amount ( 500) of oxyethylene units between terminal hydrophobes, exhibit viscosity increases in aqueous solutions in proportion to their terminal hydrophobe sizes (Figure 3) due to intermolecular hydrophobic associations.14 This behavior is consistent with the behavior of HMHEC15 and of conventional surfactants in aqueous media. The interconnecting isophorone groups do not promote hydrophobic domain formation (fluores(13)Char, K.; Frank, C. W.; Gast, G. P.; Wing, W. T.Macromolecules 1987, 20 (8), 1833. (14)Karunasena, A.; Brown, R. G.; Glass, J. E. Polymers in Aqueous Media, Performance through Association; Glass, J. E., Ed.; Advances in Chemistry 223; American Chemical Society: Washington, DC, 1989; Chapter 26. (15) Gelman, R. A.; Barth, H. G. Water-Soluble Polymers: Beauty with Performame; Glass, J. E., Ed.; Advances in Chemistry Series 213; American Chemical Society: Washington, DC, 1986, Chapter 6.
/ 1
0 0
6
a/'! I
1
2
3
4
5
CONCENTRATION (wt.%)
Firmre 3. Low shear rate (2 9-l) viscositv as a function of the aqueous solution concentration of linear HEUR polymer (Scheme 1): (a, top) (W) HO(Et0)595H,( 0 )CH3IP(Et)595IPCH3, (A) C4HJP(EtO)595IPC4Hg, (*) ~ - C ~ H I ~ P ( E ~ ) ~ ~ ~ I (b, P-~-OC~HI
cence measurements9 and the CH30IP(Et0)595IPOCH3
HEUR exhibits the same viscosity profile as the unmodified POE (Figure 3a). Viscosity increases can be realized with the addition of conventional anionic17 and nonionic18 surfactants. A review of prior surfactant studies is helpful in understanding the nature of the H E W s u r f a c t a n t interactions. SurfactantInteractions: Prior Art. Mixed solutions of homologous surfactants exhibit ideal mixing behavior in micelles and monolayers; the homologue with the longer hydrophobe is the most interfacially active. Mixed micelles and monolayers of anionic and nonionic surfactants, however, exhibit a strong deviation from ideal behavior in enhanced association and surface activity.lg At low concentrations, micelles of the nonionic component are dominant. With increasing concentration, a n enhanced incorporation of the anionic surfactant occurs below the critical micelle concentration of the pure anionic surfactant. The difference arises from a decrease in electrostatic repulsions among the ionic groups, facilitated by the oxyethylene chains of the nonionic surfactant.20 The (16) Discussed in the following paper in this issue. (17)Sau, A. C.; Landoll, L. M. Polymers in Aqueous Media, Performance through Association; Glass, J. E., Ed.; Advances in Chemistry 223; American Chemical Society: Washington, DC, 1989; Chapter 18. (18)Bergh, J. S.; Lundberg, D. J.; Glass, J. E. Prog. Org. Coat. 1989, 17, 155. (19) Kurzendorfer, C. P.; Schwuger, M. J.; Lange, H. Ber. BunsenGes. Phys. Chem. 1978, 82, 962. (20) Meguro, K.; Akasu, H.; Ueno, M. J . Am. Oil Chem. SOC.1976, 53, 145.
Synthesis of HEUR Polymers
Langmuir, Vol. 10, No. 9, 1994 3031
interactions are attributed to a partial positive charge loo transfer to the ether oxygen of the oxyethylene groups and, consequently, a n increased attraction of the hydro80 phobic group of the nonionic surfactant to the anionic surfactant. 60 Poly(oxyethy1ene) (POEI-sodium dodecyl sulfate (SDS) mixtures have been the most extensively investigated among polymer/surfactant interactions and are the most 40 pertinent to this study; HEUR thickeners are generally ’95% oxyethylene units by chemical composition. In one 20 of the first studies of POE and SDS solutions using conductance, surface tension, and viscosity measurements, 0 two transition points were observed and interpreted in 0 0.01 0.02 0.03 0.04 terms of a polymer-surfactant complex or micelle.21 In SDS CONCENTRATION (Molar) dilute POE SDS solutions, a critical SDS concentration is noted for the formation of the aggregates. The SDS Figure 4. Effect of SDS surfactant concentration (M) on low micelles boundz2 to POE are smaller than the micelles shear rate viscosity (Paas, 2 s-*) of a (0) 4 wt % CaH170IP(Et0)995IPOCaH17 and (m) 3 wt % NPIP(Et0)595IPNP; 1 CMC formed in the absence of POE. NMR data indicate that SDS = 8 x M. the surfactantlpolymerlwater interface tends to retain a certain stoichiometric composition. The POE-SDS aggregate is a mixed micelle in which some of the polymer 100 A units are wrapped around a given micelle,23with a free energy of 10-20 kT for the initially bound micelles; other 0 oxyethylene units are randomly spaced above the micelle interface.24 The gain in free energy, dG, per additional bound micelle decreases as the repulsions between micelles 200 within a POE/SDS aggregate become more important. As long as the free energy is greater than kT,all the available micelles bound to POE are equally distributed. Beyond a stoichiometric number of micelles per aggregate, the micelles remain free or are bound weakly to the aggregate~.~~ SurfactantMEUR Interactions. For a given HEUR structure below its critical aggregation concentration (CAC), the viscosity will increase with increasing surfactant c ~ n c e n t r a t i o n , ~ achieve ~ J ~ a maximum, and then decrease. The concentration of surfactant required to achieve the maximum, the viscoelasticity of the solution 200 in the area of the viscosity maximum, and factors influencing the magnitude of the viscosity increase in model hydrophobically-modified, water-soluble polymers 100 are addressed below. The viscosity of the linear HEURs without an internal 0 hydrophobe and a large (Le., high molecular weight poly100 (oxyethylene)) spacer unit (Scheme 1)was examined in both anionic (SDS) and nonionic (b-C13H27(OEt)90H) surfactant solutions. The anionidnonionic mixed sur0 factant interactions noted in prior art are reflected in the 0 1 2 3 4 5 6 7 8 9 10 viscosity maxima near the CMC (0.8 f0.2,0.008 M Figure FREQUENCY (Hz) 4)of SDS. The magnitude of the viscosity increase of a Figure 5. Storage modulus ( G , Pa, 0 )and loss modulus ( G , 3 wt % NPIP adduct is greater than that of a 4 wt % Pa, A) dependence on frequency of 3 wt % NPIP(Et0)695IPNP C8H171P(EtO)5951PC~H1, adduct. Oscillatory data of the solutions with varying SDS concentration: (A) 0 M SDS;(B) SDS solutions reveal a n increasing elastic (storage 4x M SDS; (C) 8 x M SDS;(D)2 x M SDS. modulus, Figures 5 and 6) response a t the SDS concentrations effecting the viscosity increase. When viewed in the classical perception of shear stress A viscosity increase is observed a t high nonionic as a function of shear rate, the NPIP(Et0)595IPNP surfactant (Figure 7) concentrations (-300 CMCs) in solutions (Figures 10 and 11)exhibit a n “apparent yield NPIP(Et0595IPNP solutions, and the presence of an elastic stress” a t the optimized concentrations with both surnetwork in this nonionic combination is reflected in the factant types, supporting the presence of a transient high storage modulus (Figure 8) of the solutions in the network with the viscosity increase a t low deformation area of the viscosity maximum. A viscosity maximum is rates. The observation of the maximum in the b-Ci3not observed over a broad range of nonionic surfactant H27(OEt)90H solutions is somewhat surprising in that concentrations with the 4 wt % C B H ~ ~ I P ( E ~ O ) ~ ~ ~ I Psuch C ~ Hmaxima I~ with nonionic or ethoxylated anionic surand there is no evidence of a n elastic network (Figure 9). factants are not significant in hydrophobically-modified, hydroxyethyl cellulose solutions (Figure 12) or in hydro(21)Jones, M.J. J . Colloid Interface Sci. 19f37,23,36. phobically-modified, alkali-swellable emulsion disper(22)Zana, R.;Lang, J.; Lianos, P. Microdomains Polymer Solutions; sions.26 Dubin, P., Ed.; Plenum Press: New York, 1985;Chapter 20. (23)Cabane, B.J . Phys. Chem. 1977,81,1639. (26)Tamg, M.-R. Ph.D. Thesis, North Dakota State University, in (24)Cabane, B.;Duplessix, R.J.Phys. 1982,43,1529. progress. (25)Cabane, B.;Duplessix, R. Colloids Surf. 13 (l),19-33, 66-2.
+
Lundberg et al.
3032 Langmuir, Vol. 10,No. 9,1994 100
1
[
1
100
I
A
I9
A
0 100
0 300 100
-2-
n
-
200 60 40
n Q 100
B
4.
20
8ol /1
60
40t
+
tp
2o
/
0 0
1
2
I
,
,
7
8
9
F ~ R E ~ U ~ N C(HZ) ~Y
300
10
Figure 6. Storage modulus ( G , Pa, 0)and loss modulus ( G , Pa, A) dependence on frequency of 4 wt % C8H17IP(Et0)595IPCsH17 solutions with varying SDS concentration: (A) 0 M SDS; (B) 4 x low3M SDS; (C) 8 x M SDS (0.02 M SDS solutions gave no oscillatory response).
1
2
3
4
5
6
7
8
9
10
FREQUENCY (Ht) Figure 8. Storage modulus ( G , Pa, 0)and loss modulus ( G , Pa, A ) dependence on frequency of 3 wt % NPIP(Et0)595IPNP solutions with varying b-C13H270(EtO)gHconcentration: (A) 0 wt % b-C13H270(EtO)gH;(B) 0.003 wt % b-C13H270(EtO)gH;(C) 0.03wt %; (D)0.30 wt %.
100 u)
sm
0
8 0 1
I
>
100
A 80
60 40 20
0 10'~
10'
IO'
b-C, 3H27(Et0)90HConcentration (wt%) Figure 7. Effect of b-C13H270(EtO)gH nonionic surfactant aqueous solution concentration(wt %) on low shear rate viscosity (Pa-s,2 s-l) of a 3 wt % NPIP(EtO)6gsIPNP,1CMC b-C13H270(Et0)gH 0.003 wt %.
HEUR GeometricalInfluences. A. Viscosity M a ima. The surfactant concentration necessary to produce the viscosity maximum is dependent on the HEUR concentration and its structure. The general influence of associative thickener concentration on the amount of anionic surfactant (SDS) required to achieve that maximum is illustrated in Figure 13 with the linear NP(OEt)looTMXDI(EtO)looNPHEUR of Scheme 2c. There is a statistical ratio involved in the achievement of higher viscosity maxima that will be addressed in future studies. The structure of the model HEUR also influences the amount of surfactant required to achieve the viscosity increase (Figure 14). As the number of hydrophobes is increased in HEURs with an internal hydrophobe geometry, the amount of SDS required increases. The 3.8average arm HDI-biuret HEUR (Scheme 2a) maximum requires a SDS concentration near the level where SDS solutions, alone, increase in viscosity. This also is true
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Figure 9. Storage modulus ((7,Pa, 0) and loss modulus ( G , Pa A) dependence on frequency of 4 wt % C S H ~ ~ I P ( E ~ O ) ~ ~ ~ IPCsH17 solutionswith varying b-C13H270(EtO)gHconcentration (wt %): (A) 0; (b) 0.003; (C) 0.03; (D) 0.45.
for [NP(OEt)looTMXDI(EtO)looNP HEUR maximum a t high b-ClsHzs(OEt)sOH concentrations (Figure 15)l. B . Phase Separation. Solutions with a sharp distinction between a liquid (aqueous) and a gel-like (thickener) phase are described as phase separated. All of the internal
Synthesis of HEUR Polymers
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Langmuir, Vol. 10,No.9,1994 3033
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SHEAR RATE (5' ') Figure 10. Shear stress (Pa) dependence on shear rate (s-l) of 3 wt % NPIP(Et0)595IPNPsolution: (-) no SDS; (- - -) 4 x M SDS;(- - -1 8 x M SDS; 2 x M SDS. (a*.)
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SDS Concentration (mmoi/L) Figure 13. Effect of SDS concentration on viscosity (mPa*s, 40 8-l) of solutions of [C~H~~O(E~O~OO)I~TMXDI: 0 , 4 w t %; A, 7 wt %; 0,10 wt %. 0.5
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SHEAR RATE (s-') Figure 11. Shear stress (Pa) dependence on shear rate (8-l) of 3 w t % NPIP(EtO)595IPN'Psolution: (-)no b-C13Hz70(EtO)gH; (- - -) 0.0015wt % b-C13H270(EtO)gH;(- - -1 0.003 wt % b-C13H270(EtO)gH;(- - -) 0.3 wt % b-C13H270(EtO)gH;(. .) 3.0 wt % b-C13H270(Et0)9Hs
100
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SDS Concentration (mmoilL) Figure 14. Effect of SDS concentration on viscosity (mPa*s, 40 8-l) of HEUR thickeners at 0, 4 wt % [CgHl~O(Et0)1001~~]~(HDI)B TMXDI solution; B, 4 wt % [ C ~ H ~ ~ O ( E ~ O ) ~ ~(Scheme 2al solution; 0, SDS only. 0.3
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b-C, 3H2,(EtO)90H Conc. (wt%)
Concentration of Surfactant (Wt%) Figure 12. Effect of ~ - C I ~ H ~ ~ O ( E ~ O )(0) ~ Sand O ~SDS N H(A) ~ aqueous solution concentration (wt %) on low shear rate viscosity (2 s-l) of a 1.0 wt % hydrophobically-modified,hydroxyethyl cellulose.
Figure 16. Effect of surfactant on the viscosity (40 8-l) of a 4 w t % [ C ~ H I ~ ( E ~ O ) ~ ~ ~ ]solution: ~ T M X D. I b-C13H270(EtOhH; ,
hydrophobe HEURs containing nonylphenol terminal hydrophobes exhibit phase separation. The isocyanurate (Scheme 2b) HEUR phase separates (lower layer ca. 50% by volume) due to strong intramolecular hydrophobic bonding and the limited solubility of the isocyanurate center. The cloudy dispersion of a 4 wt % solution becomes clear when SDS is added (0.115wt % SDS,0.5CMC), but
phase separation was observed in solutions containing the b-C13H2,(OEt)90H(0.0015-0.6wt %) surfactant. There is a critical concentration of the nonionic surfactant required to achieve complete solubility with the different internal hydrophobe HEURs. The linear HEUR thickeners with a significant amount of oxyethylene units (595 average) between the hydrophobes do not exhibit phase
0, b-C13H270(EtO)gHonly.
3034 Langmuir, Vol. 10, No. 9, 1994
Lundberg et al.
The synthesis of the linear-model HEURs by a telechelic route has allowed investigation of the properties of well
characterized water-soluble polymers containing variable size terminal hydrophobes. With increasing HEUR concentration, the size of the terminal hydrohobe influences the magnitude of the viscosity achieved. The anioniclnonionic mixed surfactant interactions noted in prior art are reflected in HEUR solution viscosity maxima with increasing anionic surfactant concentrations. A viscosity maximum also is observed with the terminal nonylphenol linear HEUR with a nonionic surfactant, b-C13HdOEt)90H. This is surprising since the neutralization of electrostatic repulsions is absent, and conventional nonionic or ethoxylated anionic surfactants do not promote significant viscosity maxima in other hydrophobically-modified, water-soluble polymers (e.g., HMHEC and HASE). With both surfactants, elastic networks are formed with HEUR thickeners, and a t low deformation rates, the HEUWsurfactant solutions exhibit yield stress behavior. When small spacer distances are used in the synthesis of HEURs with internal hydrophobes, phase separation in the aqueous solutions is observed. This is related to a dominance of intramolecular hydrophobic associations. The phenomenon can be eliminated with the addition of SDS but not with branched-chain nonionic hydrophobes a t moderate concentrations. The associations of HEURs of the linear telechelic type synthesized in this study and of direct monoisocyanate additions to oxyethylene of various oligomeric sizes were evaluated in various association models and found to fit a face centered cubic model a t SDS concentrations promoting viscosity maxima. Upon detailed analysis, however, the direct addition linear HEURs were found to be impure components, consisting of blends of unreacted, mono-, and disubstituted oxyethylene components. Modeling the behavior of hydrophobically-modified, water-soluble polymers can be a misleading endeavor.
(27) Kaczmarski,J.Philip; Glass, J. Edward Proc. ACS Diu. Polym. Mater.: Sci. & Eng. 1992, 67, 284. (28) Lundberg, D. J. Ph.D.Thesis, North Dakota State University, 1990.
Acknowledgment. The financial support of these studies from the Hercules and E.1. DuPont Companies is gratefully acknowledged.
separation behavior, but form a loose gel, with an applesauce consistency. C . Modeling. The contribution of the oxyethylene spacing to the total viscosity was addressed in these early studies through the direct addition of octadecyl isocyanate to POEs of varying molecular weights. The spacing difference appears to be a primary factor in the greater surfactant concentration required to achieve a viscosity maximum and in the magnitude ofthe viscosity achievedeZ7 A three-dimensional network was envisaged in which a cubic close packed structure provides the highest density of cross-link points per unit volume. Ideally, a t the observed viscosity maximum with surfactant addition, the highest possible density of cross-link points would be present. The descriptive ability of a face centered cubic modelz8was assessed by comparing the experimentally determined surfactant concentration a t the viscosity maxima to the concentration predicted by the model. The approximations necessary to apply the model (the size of the micelle cross-link points, uniform length of the oxyethylene spacers, etc.) were approximate to conventional anioniclnonionic systems and the predicted SDS concentrations were approximate to the experimental SDS concentrations a t the viscosity maxima, without modification of the model for the variety of interactions cited in the prior art section. Upon detailed analysis, however, the octadecyl linear HEURs of this earlier study were found to be impure components, consisting of blends of unreacted, mono-, and disubstituted oxyethylene components. Modeling the behavior of hydrophobicallymodified, water-soluble polymers can be a misleading endeavor.
Conclusions
