Langmuir 1993,9,708-715
708
Behavior of Low Molecular Weight Model Heur Associative Polymers in Concentrated Surfactant Systems A. P. Mast,*$+ R. K. Prud’homme,j and J. E.Glasd DSM Research, Department FA-GF, P.O. Box 18, 6160 MD Geleen, The Netherlands] Department of Chemical Engineering] Princeton University] Princeton, New Jersey 08544, and Polymers and Coatings Department] North Dakota State University, Fargo, North Dakota 58105 Received January 31, 1992. In Final Form: December 7, 1992 The interaction between hydrophobicallymodified water-solublepolymers and surfactants has gained a growing interest in recent years. In this paper we present our results on the behavior of hydrophobically modified ethoxylateurethanes (HEUR)in concentrated surfactant systems, using an ethoxylated sulfate surfactant. The HEURs differ in hydrophobic group, molecular weight, extent of modification, and polydispersity. As will be clear from our results the purification and characterizationof these polymers are very difficult but also essentialfor the understanding of the characteristicbehavior of these materials. Our experimentalresults from rheological experiments,phase-behavior,and flowbirefringenceare presented. Furthermorea mechanism describingthe interaction between the Surfactantsand polymers w i l l be discussed. Introduction The use of water-soluble polymers as thickeners in all kinds of aqueous systems has assumed considerable proportions. Therefore the interaction between polymers and surfactants in aqueous systems has gained a lot of interest the last decennary.l-1° When employingnonionic polymers such as poly(ethy1eneoxide)s(PEO’s)and sulfate surfactants, one can distinguish three different interaction regimes. Below a certain critical surfactant concentration no interaction can be observed. Above this concentration, surfactant molecules form small aggregates along the polymer chain, giving rise to polymer chain uncoiling and extension. This critical concentration is less than the critical micellar concentration (cmc). With increasing surfactant concentration the aggregation increases until a second critical concentration is reached. Above this concentration the polymericchain is saturated and classical surfactant micelles are formed. In order to get significant interaction, the polymer should at least have a molecular weight of 1550. Above 4000 no molecular weight dependence is observed. With increasing ionic strength of the aqueousphase, the onset of aggregation decreasesand the association ratio, i.e. the number of surfactant molecules per monomer unit of the polymer, increases. Ethoxylated sulfate surfactants show considerablyless interaction with nonionic polymers. A number of interaction models have been described to explain the behavior of surfactants and polymers.6Jl-15 + DSM
Research. Princeton University. 5 North Dakota State University. (1) Schwuger, M. J. J. Colloid Interface Sci. 1973,43,491. (2) Murata, M.; Arai, H. J. Colloid Interface Sci. 1973, 44, 475. (3) Cabane, B.; Duplessix, R. J. Phys. (Paris) 1982, 43, 1529. (4) Uhl, J. T.; Prud’homme, R. K. Chem. Eng. Commun. 1982,16,45. (5) Francois, J.; Dayantis, J.; Sabbadin, J. Eur. Polym. J. 1985,21,165. (6)Nagarajan, R. J. Chem. Phys. 1989,90,1980. (7) Witte, F. M.; Engberta, J. B. F. N. Colloids Surf. 1989, 36, 417. (8) Hoffmann, H.; Huber, G. Colloids Surf. 1989, 40, 181. (9)Goddard, E. D.; Phillips, T. S.; Hannan, R. B. J. SOC.Cosmet.
*
Chem. 1975,26,461.
(10) Goddard, E. D. J. SOC.Cosmet. Chem. 1990,41, 23. (11) Smith, M. L.; Muller, N. J.Colloid Interface Sci. 1975,52, 507. (12) Gilanyi, T.; Wolfram, E. Colloid Surf. 1981, 3, 181. (13) Hall, D. G. J. Chem. Soe., Faraday Trans. 1 1985,81, 885. (14) Takisawa, N.; Brown, P.; Bloor, D.; Hall, D. G.; Wyn-Jones, E. J. Chem. SOC.,Faraday Tram. 1 1989,85, 2099. (15) Ruckenstein, E.; Huber, G.; Hoffmann, H. Langmuir 1987,3,382.
In more recent years a growing interest to understand the behavior of a fairly new class of thickeners, hydrophobically modified water-soluble polymers, has developed. These substances are also known as polymeric surfactants or associative polymers. Most of the studies describe the self-associating properties of these materials in aqueous systems.1c22 These associative polymers enhance the low shear viscosity of water. With increasing shear rate, shear thickening or thinning is observed depending on the concentration, the molecular weight, and the kind of hydrophobic group. The viscosity enhancement is best when end-substituted instead of random copolymersare used. With increasing chain length of the hydrophobic group, the viscosity enhancement increases. The viscosity enhancement is just as with classical surfactants very salt sensitive.’a The effect of added surfactant on the self-association properties of these polymeric surfactant has gained some attention, only at low surfactant concentration.23-26We are more interested in systems having a higher surfactant concentration. Because of application in the paint industry the interaction of these associative polymers with latsx particles has been studied e~tensively.~~-~l Different models have been developed to explain the observed behavior. Gen(16) Wang, Y.; Winnik, M. A. Langmuir 1990, 6, 1437. (17) Siano, D. B.; Bock, J.; Myer, P.; Valint, P. L., Jr. Polym. Mater. Sci. E M . 1987.57.609. (18) bock, J ; Vbint, P. L., Jr.; Pace, S. J.; Siano,D. B.; Schulz, D. N.; Turner, S. R. Proc. Natl. Meet. ACS 1988, 147. (19) McCormick, C. L.; Nonaka, T.; Johnson, C. B. Polymer 1988,29,
.--.
7.11
(20) Wang, K. T.; Iliopoulos, I.; Audebert, R. Polym. Bull. 1988,20, 577. (21) Jenkins, R. D.; Silebi, C. A.; El-Aasser, M. S. PMSE R o c . ACS 1989, 61, 629. (22) Jenkins,R. D. Thesis, Lehigh University, Department of Chemical Engineering, Bethlehem PA, 1990. (23) Lundberg, D. J.; Glass, J. E.; Eley, R. R. PMSE Proc. ACS 1989, 61, 533. (24) Lundberg, D. J.; Fossum, E.; Glass, J. E. PMSEProc. ACS 1990, 62, 663. (25) Lundberg, D. J. Thesis, North Dakota State university, Department of Polymers and Coatings, Fargo, ND, 1990. (26) Ma, Z.; Kaczmaraki, J. P.; Glaaa,J. E. PMSEProc. ACS 1991,65, 61.5.
(27) Char, K.; Frank, C. W.; Gast, A. P. Langmuir 1989,5, 1335. (28) Murakami, T.; Fernado, R. H.; Glaaa, J. E. JOCCA 1988,71,315. (29) Karunasena, A,; Glass, J. E. Polym. Mater. Sci. Eng. 1989, 61, 544. (30) Santore, M. M. In Thesis, Princeton University, Department of Chemical Engineering, Princeton, NJ, 1990.
0743-746319312409-0708$04.00/0 0 1993 American Chemical Society
Polymers in Concentrated Surfactant Systems
Langmuir, Vol. 9, No. 3, 1993 709
erally these models can be divided into two major mechanisms. According to the first mechanism a network is formed in which the latex particles are the cross-linking entities. The hydrophobic groups of the polymers adsorb onto the particle surface. These polymers are capable of bridging between particles and in this way a network is formed. The latex particles are more or less fixed depending on the strength of the adsorption and the elasticity of the polymeric network. On the other hand a network can be formed in which the cross-linkingentities are the aggregates formed by the hydrophobic groups on the polymers. Bieleman et al.31 proposed a concept of bridged micelles forming a network. The latex particles are trapped in this network and are therefore restricted in their motion. Our interest focuseson the behavior of hydrophobically modified ethoxylate urethanes (HEUR) in concentrated aqueous surfactant systems. We want to study the specific interaction between the surfactant and the polymers. The supramolecular structures formed could give us information about the thickening mechanism. In the study reported here, we use an ethoxylated sulfate surfactant. Some results from nonpolymeric rheologicalmodifiers are included as well. The importance of the polymeric chain can thus be evaluated. The synthesis and characterization of the HEUR polymers will only be described briefly since these have been published elsewhere.32 Results from rheological experiments, phase behavior, and flow birefringence are presented. Furthermore a mechanism describing the interaction between the surfactant micelles and the HEUR polymers is discussed.
Methods, Materials, and Samples Methods. The rheological experiments are performed on a Rheometrics fluid spectrometer, RFS2, with fluid transducers of 10 and 100 g cm. A Couette-geometry is thermostated at 25 “C. The air above the couette is saturated with water to prevent possible evaporation of the sample. This apparatus can be employed for both steady-state experiments and dynamic mechanical analysis. During steady shear flow experiments a constant stress is applied to the fluid. The steady shear viscosity is defined as t
-712Ii.
(1)
in which r12 is the shear stress and i. is the shear rate. During oscillatory flow a sinusoidal shear strain is applied to the fluid, resulting in a sinusoidal stress in the fluid. The storage modulus, G’, is the stress component in phase with the strain. The loss modulus, G”, is the stress component out of phase with the strain. These moduli are defined as
G’ -rmaxljlmax COS
in which rmaris the maximum shear stress, is the maximum shear rate, and 4 is the phase shift between stress and strain. The storage modulus can be related to the elasticity of the material, while the loss modulus is related to the purely viscous properties. A lot of studies have already shown that both surfactant dispersions and polymeric solutions often exhibit viscoelastic properties.2633 The Couette geometry is cleaned before use and just the amount of sample needed to fill the cup is inserted using a syringe. Then the bob is lowered very slowly while the normal stress on the bob is monitored and is allowed torelax. Then the sample was allowed to rest for a t least 30 min. The same measurement cycle was (31) Bieleman, J. H.; Riesthuis, F. J. J.; van der Velden, P. M. Polym. Paint Colour J. 1986, 176, 450. (32) Glass, J. E.; Kaczmarski, J. P. PMSE Proc. ACS 1991, 65, 175. (33) Rehage, H.; Hoffmann, H. J. Phys. Chem. 1988, 92, 4712.
performed for each sample. This procedure guarantees a shear history which is the same for all samples. The phase behavior of the different samples is tested visually a t room temperature. Flow birefringence experiments are performed on the prototype Rheometrics optical analyzer, with a closed Couette geometry a t room temperature. In this optical analyzer the polarized light travels along the z-axis with respect to the steady shear flow field imposed on the system. As a polarization state generator a rotating polarizer is used; a circular polarizer is used as the polarization state analyzer.34The optical anisotropy of materiels subject to shear can be evaluated measuring the transmitted light during flow. The birefringence, An’, is a measure for the anisotropy of the system, while its orientation angle, x, relates to the degree of orientation. Results from flow birefringence thus give an indication of the orientation and extension in the system during flow. The birefringence and ita orientation angle can be related to the rheological stress tensor through the stress optical rule. A number of molecular theories, including flexible chain models and rigid-rod polymers in dilute and semidilute solutions,predict such a stress optical rule.w6 This stress optical rule is given by An’sin 2% = 2Cr12 An’ cos 2x = C(rll - rzz)
(3)
in which C is the stress optical factor and (711 - 722) is the first normal stress difference. The stress optical factor is characteristicfor the kind of polymer used but is independent of molecular weight.36 Materials. In this study we used Akyposal BA56, an ethoxylated sulfate surfactant, R(OCH&H2),0S03-Na+ (R = c13+5 and n = 3), which is supplied as a 56 wt % active matter dispersion in water (CHEM-Y). The nonpolymeric rheological modifiers are Aminol A15 and Aminol N (CHEM-Y), both basically nonionic surfactants. Aminol A15, R(OCH&Hz),OCH2CONHCH2CH20H (R = c13-c15 and n = 1.5), is supplied as a liquid containing 97 wt % active matter. Aminol N, RCONH(CH2CH20),H (R = CS-C~Sand n = 2.5), is supplied as a liquid containing 90-95 wt % active matter. All these materials are used without further purification. The HEUR polymers are synthesized starting from commercially available PEO’s of molecular weights 8000 (Carbowax 8000, Union Carbide), 12000, 20000, and 35000 (Fluka). End modification is accomplished using octadecylisocyanate (Aldrich, technical grade). Internally modified polymers were synthesized using dicyclohexylmethane diisocyanate (HlpMDI) (Mobay) and tetramethylxylidene diisocyanate (TMXDI) (American Cyanamid). A nonylphenol surfactant (IgepalCO-990, GAF Chemicals) having 100 EO units as described by the supplier is also modified using the above-mentioned diisocyanate substances. One of the materials used in our study is a commercially available product, known as PEG 6000 distearate (AKZO). A comparable PEO 8000 distearate is synthesized by end modifying a PEO (8000) with stearic acid (Aldrich, 99.5+%). All synthesized materials were characterized using a slight variation of the PMDA/IMDA pr0cedure,3~infrared spectroscopy and size exclusion chromatography (SEC). In order to get an indication about the extent of modification (EOM) H NMR is used. Having a EOM of 100% or more suggests full substitution, while an EOM under 90% suggests that still nonmodified EO tails are present. The HEUR polymers are all purified very carefully before use. Samples. The surfactant concentration used in the samples is 9.4 wt % active matter Akyposal BA56. By adding NaCl (Aldrich, 99+%) the ionic strength is adjusted to 0.43 M. A number of samples have been studied a t low surfactant concentration, which contains 0.7 wt % active matter, at the same ionic strength. Every time a surfactant dispersion is prepared fresh to which the proper amount of salt and polymer solution is added. The samples are placed on a rollerbench overnight and (34) Fuller, G. G. Annu. Rev. Fluid Mech. 1990,22,387. (35) Fuller, G. G. Private communication. (36) Rehage,H.; Hoffmann, H.; Wunderlich, I. Ber. Bunsen-Ces. Phys. Chem. 1986,90, 1071. (37) Kingston, B. H. M.; Garey, J. J.;Hellwig, W. B. Anal. Chem. 1969, 41, 86, meth.1.
Mast et al.
710 Langmuir,Vol. 9, No.3, 1993
Table I. Composition and Results from the Characterization for All Associative Polymers Used in This Study SEC results description of the" Mb (theo) MnC(titr) M"d MW' PDIf EOMB % associative polymer 9573 10512 1.10 PEO(6K) C17h 6562 PEO(8K) C17 8562 8652 9497 10686 1.12 89.9 1.10 78.9 8592 9438 10360 11442 PEO(8K) C18 PEO(8K) C18 8592 10238 11238 12081 1.07 110 16600 18823 1.13 PEO(12K) C18 12592 18303 23562 1.29 59.1 PEO(2OK) C18 20592 20549 PEO(2OK) C18 20592 21677 19308 21705 1.12 104 PEO(35K) C18 35592 28111 32141 38051 1.18 123 PEO(8K) HlzMDI 16264 17492 24444 1.40 PEO(8K) HlzMDI-2 24528 20164 26786 1.32 PEO(8K) HlzMDI-3 32792 22650 32350 1.42 PEO(8K) TMXDI 16246 16970 21139 1.25 PEO(8K) TMXDI-2 24492 19591 25736 1.31 PEO(8K) TMXDI-3 32738 27042 36872 1.36 PEO(8K) TMXDI/C18 16838 18533 24594 1.33 PEO(8K) TMXDI-2/C18 25084 23744 35557 1.50 PEO(8K) TMXDI-3/C18 33330 21419 33234 1.55 PEO(4K) TMXDI/NPi 9452 8151 9532 1.17 PEO(4K) HizMDI/NP' 9470 7393 9456 1.28 ~
PEO(8K) means a PEO block of molecular weight 8OOO; C18 means octadecyl end modified; TMXD1-x means diisocyanate modified in which 1c represents the average number of TMXDI blocks per molecule ( x = 1is omitted); TMXDIs/C18 means octadecyl and diisocyanate modified. * Theoreticalcalculated molecular weight. Number average molecular weight aa determined by titrati0n.g Number average molecular weight using size exclusion chromatography. e Weight average molecular weight using size exclusion chromatography. f PDI = polydispersity index defined aa MJM,. 8 EOM = extend of modification. h The commercial product PEG 6OOO distearate. i Diisocyanate modified Igepal co-990. afterward allowed to stand for approximately 24 h a t room temperature to let the introduced air bubbles diffuse out of the sample.
Results and Discussion In Table I the results of the characterization are summarized for all associative polymers used in our study. As one can see the stearate end modified PEO, having a molecular weight of 6000 according to the supplier, resembles extremely well the stearate end modified PEO(8K),based on the SEC results. Furthermore we note that the diisocyanate-modifiedpolymers are polydisperse. This is probably due to the residual PEO present and the fact that there is no 100%conversion. These diisocyanatemodified PEOs are used as a starting material for the octadecyl/diisocyanate modified PEOs. This means that these materials are not defined very well and are very polydisperse. Diisocyanate-Modified PEO's. The diisocyanate modified PEO's do not exhibit viscoelastic behavior and no significant rise in low shear viscosity is observed. The shear viscosity decreases continuously with increasing shear rate and the polymers give slightly turbid dispersions both at low (0.7 wt % active matter) and high (9.4 wt % ' active matter) surfactant concentration. These results were compared with results from nonmodified PEO's with molecular weights of 20 OOO (Fluka)and 6 OOO OOO (Aldrich) at the same concentrations. The dispersions containing the nonmodified PEO's give comparable rheological behavior as was observed with the diisocyanate-modified PEO's, while at low surfactant concentration the nonmodified PEO's give clear dispersions. At high surfactant concentration these PEOs give slightly turbid (20 000) to phase-separated (5 OOO 000)systems. These observations coincide with the behavior normally observed for nonadsorbing polymers in latex dispersions. At high surfactant concentration this leads to depletion flocculation and turbid dispersions. The turbidity generated by the diisocyanate modified PEOs at low surfactant concentration could either be due to impurities present or from the aggregation of the small hydrophobic groups. The extensive purification process which is performed on the
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to an ionic strength of 0.43 M. Key: 0, no polymer added, 0 , PEO(6K) C17,l.l mM; A,Amino1A15,50mM; 0,PEO(8K) C18 (EOM = 79%), 1.0 mM; . , PEO(2OK) C18 (EOM = 59%),0.9 mM; A, PEO(8K) C18 (EOM = 110%)' 1.0 mM.
diisocyanate modified PEO's and the following characterisation suggest that no byproducts are present. But on the other hand the analysis of these materials is certainly very difficult and should be developed further.32 From the results presented here we can conclude that the small diisocyanate groups do not contribute to any associative properties at high surfactant concentration. The interaction between surfactant and polymer is negligible. Ethoxylated sulfate surfactants are known to have less interaction with PEO's than simple sulfate surfactants.1-16 Lundberg et have also observed that ethoxylated nonionic surfactants show considerable less interaction with associative polymers than sulfate surfactants. Octadecyl or Stearate End Modified PEO's. Considerable viscosity enhancement and strong viscoelastic behavior are observed for end modified POE's (Figures 1 and 2 and Table 11). A surfactant dispersion containing only surfactant (9.4 wt % active matter) has a low shear viscosity of 0.5 Pa s and a dispersion containing 0.7 wt %
Polymers in Concentrated Surfactant Systems
Langmuir, Vol. 9, No. 3, 1993 711
LL W
2l
a
0.
I
200
0.0 0.00
0
Figure 2. Storage (G’) and loss modulus (G”) as a function of frequency (w). All dispersions contain 9.4 w t % active matter surfactant, adjusted to an ionic strength of 0.43 M. Key: 0 , G’(w) PEO(8K) C17,l.l mM; m, G”(o) PEO(8K) C17,l.l mM; 0,G’(w) PEO(20K) C18 (EOM = 59%), 0.9 mM; 0,G”(w) PEO(20K) C18 (EOM = 59%), 0.9 mM.
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=‘
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23 9.7 3.0
2
3
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0
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Figure 3. Flow birefringence results for a dispersion containing 9.4 wt 7% active matter surfactant, an ionic strength of 0.43 M, and associative polymer PEO(6K) C17 a t a steady shear rate of 1.0 s-l: 0,birefringence An’; 0,orientation angle x. 8ooT=os-1
Table 11. Low Shear Viscosities of Dispersions Containing 9.4 wt ?& Active Matter Akyposal BA56 and Associative Polymers or Nonpolymeric Rheological Modifiers at an Ionic Strength of 0.43 M concentration wt% mMa 0.0 0.0 Aminol A15 2.0 50 Aminol N 2.0 49 PEO(6K) C17 0.74 1.1 PEO(6K) C17 0.83 1.3 PEO(6K) C17 2.0 3.1 PEO(8K) C17 (EOM = 90%) 0.91 1.1 PEO(8K) C18 (EOM = 79%) 0.82 1.0 PEO(8K) C18 (EOM = 79%) 2.0 2.4 PEO(8K) C18 (EOM = 110%) 0.86 1.0 PEO(12K) C18 1.3 1.0 PEO(20K) 2.0 1.0 PEO(20K) C18 (EOM = 59%) 1.8 0.9 2.0 1.0 PEO(2OK) C18 (EOM = 59%) PEO(20K) C18 (EOM = 104%) 1.4 0.7 PEO(8K) TMXDI-3 2.0 0.6 PEO(8K) TMXDI-3/C18 0.50 0.2 PEO(8K) TMXDI-3/C18 0.94 0.3 PEO(8K) TMXDI-YC18 0.97 0.4 PEO(8K) TMXDI/C18 0.95 0.6 PEO(4K) H12MDI/NP 0.95 1.0 PEO(4K) H12MDI/NP 2.0 2.1 PEO(4K) TMXDI/NP 0.96 1.0 PEO(4K) TMXDI/NP 2.0 2.1
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