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Langmuir 1991, 7, 1067-1071
Phase Behavior of Poly(ethylene oxide)-Poly(propylene oxide) Block Copolymers in Nonaqueous Solution Arianeh A. Samii,t Gunnar Karlstrom,i and Bjorn Lindman'ft Physical Chemistry 1 and Theoretical Chemistry, Chemical Center, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden Received June 18,1990. In Final Form: October 31, 1990 Nonionic block copolymers of the poly(ethy1ene oxide)-poly(propy1ene oxide) type (PEO-PPO)were investigated with respect to phase behavior in several different solvents and were found to display a clouding behavior only in water and in formamide. Also reported is how the addition of different electrolytes affects the cloud point temperature of this nonionic polymer in formamide, and the results are compared with those of aqueous solutions. The phase behavior of this type of polymer is also studied in different ternary systems containing two polar solvents, water and either N-methylformamide, 1,2-propanediol, N-methylacetamide, or dimethylformamide. In some ternary systems, for certain solvent compositions, a deep minimum in the lower consolute temperature is observed. The similarities observed between polymer-formamide systems and polymer-water systems suggest that the mechanism of clouding is the same in the two solvents.
Introduction In recent years, a large amount of work has been carried out in order to understand the phase behavior in aqueous solution of nonionic polymers and surfactants containing the CHzOCHz segment. These systems display a phase separation at elevated temperatures and many different types of investigations have been performed in order to clarify the mechanism behind this and other unusual properties of these The general explanation of the decrease in solubility at higher temperature is obviously a rapid increase in the effective attraction between different solute molecules. The origin of this effect is, however, not fully understood today and three different types of theoretical studies attempt to explain the mechanism behind this behavior, by modeling the systems containing the ethylene oxide group. In the first model? it has been claimed that water forms an ordered structure around the ethylene oxide (EO) chains at low temperatures and, as the temperature is increased, this highly ordered structure breaks down, due to the unfavorable entropy contribution. In the second model: originally based on an idea of Hirschfelder et a1.,I0 it has been proposed that for systems containing EO groups, there is hydrogen bonding between the water molecules and ether oxygen of the EO groups, which is destroyed a t higher temperature. In the third and last model," the decrease in solubility is explained in terms
* To whom correspondence should be addressed.
+ Physical Chemistry 1. Theoretical Chemistrv. (1) Bailey, F. E., Jr.; Koieske, J. V. Poly(ethy1ene oxide); Academic Press: New York, 1976. (2) Saeki, 5.;Kuwahara, N.; Nakata, M.; Kanenko, M. Polymer, 1976, 17,686. (3) Nakagawa, T.; Shinoda, K. In Colloidal Surfactants; Academic Press: New York, 1963; Chapter 2. (4) Nakagawa, T. In Nonionic Surfactants; Schick, M. J., Ed.; Surfactant Science Series;Marcel Dekker: New York, 1967;Vol. 1, Chapter 17. (5) Lindman, B.; Karlstr6m, G. 2.Phys. Chem. 1987,155,199. (6) W-heim, T.;Bohetrbm,J.; Williams, Y. Colloid Polym. Sci. 1988, 266,562. (7) LFg, J. C.; Morgan? R. D. J. Chem. Phys. 1980, 73, 5849. (8) Kjellander, R.;Florin, E. J. Chem. SOC.,Faraday Trans. 1 1981, 77, 2053. (9) Goldstein, R. E. J . Chem. Phys. 1984,80,5340. (10) Hirschfelder, J.; Stevenson, D.; Eyring, H. J . Chem. Phys. 1937, 5,896. (11) Karlatr6m, G. J . Phys. Chem. 1985,89,4962.
of a change in the conformational structure of the EO chains as a function of temperature. At low temperature, the water-soluble conformation with the highest dipole moment, which is the gauche-trans isomer of the ethylene oxide segment (gauche around the C-C bond and trans around the C-0 bond), is preferred, whereas a t higher temperatures other conformers with smaller or no dipole moment and low aqueous solubility will become energetically favored. There has been little attention paid to exploring phase behavior of this type for polymers or surfactants in nonaqueous solvents. Such studies could aid in understanding the underlying mechanism of clouding in aqueous solution. There exist, however, very few studies. Saeki et aL2 observed clouding of poly(ethy1ene oxide) (PEO) in tertbutyl acetate, which is a solvent without hydrogen bonds, and Warnheim et have recently found a lower consolute temperature (LCT) for a system containing a nonionic surfactant of the EO type in formamide; the phase behavior is qualitatively the same as the one observed in water. There also exist reports' on several other solvents that can dissolve PEO. Moreover Lattes et al.12 points out the importance of the balance between hydrophilic and hydrophobic parts of some nonionic block copolymers in nonaqueous microemulsions. In a preliminary report,13 we discussed the phase behavior of nonionic block copolymers of poly(ethy1ene oxide)-poly(propy1ene oxide) (PEO-PPO) in different polar solvents. In this paper, we have extended our investigations by studying the effect of electrolytes NaCl and NaI, on this type of polymer, both in water and in formamide solutions. The cloud point variation of this nonionic polymer with solvent composition in different ternary systems containing PEO-PPO, a polar solvent (with and without hydrogen bonds), and water will also be reported.
Materials and Methods The block copolymers of poly(ethy1ene oxide-propylene oxide) with a molecular weight of 2917 and 3438 used in this study were obtained from Polyscience Inc. The polymers have one poly(12) Lattes, A.; Rico, I.; De Savignac, A.; Samii, Ahmadzadeh A. Tetrahedron 1987,43, 1725. (13) Samii, A. A.; Lindman, B.; Karlstrijm, G. h o g . Colloid Polym. Sci. 1990,82, 280.
0743-7463/91/2407-1067$02.50/0 0 1991 American Chemical Society
1068 Langmuir, Vol. 7,No. 6,1991
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Table I. Some Relevant ProDerties for the Solvents Used molecular dipole dielectric moment molecular volume permittivity IL,D
molecule
water 1.85 formamide 3.73 acetaldehyde 2.69 N-methylformamide 3.83 acetamide 3.76 dimethyl sulfoxide 3.96 N-methylacetamide 3.73 3.82 NJ-dimethylformamide a The value of the transition energy from ref 14.
v, A3
ILIV
r21v
29.89 65.98 57.24 97.01 98.21 117.78 126.81 127.91
0.0619 0.0560 0.0470 0.0390 0.0383 0.0340 0.0294 0.0292
0.1145 0.2109 0.1263 0.1512 0.1440 0.1331 0.1097 0.1141
ethylene oxide block and one polypropylene oxide block, with a molar ratio of EO/PO of 0.33 for the one with MW 3438 and 0.8 for the one with MW 2917. In this study we have used the following notation: PEO-PPO with MW 3438 is "polymer A" and PEO-PPO with MW 2917 is "polymer B". Both polymers were used as received. All other chemicals were of analytical grade and were used as supplied. All components were weighed and compositions are expressed in percent by weight. The cloud point measurements were done in an electrically thermostated bath with, a t least three determinations for each sample. The cloud point is taken as the temperature where the last visible sign of clouds disappears on cooling. The clouding observed in formamidesystems is a rather indistinct haziness and we estimate the accuracy of our measurements to be 0.5 OC.
Results and Discussion Phase Diagrams for PEO-PPO in Water and Formamide Solutions. In our recent study on the phase behavior of nonionic block copolymers, PEO-PPO, in different polar solvent^,'^ we showed that these polymers display a clouding phenomenon in water and in formamide but not in a large number of other polar solvents. As reported by Warnheim et al.,6J4some nonionic surfactants of the ethylene oxide type give similar results. These different observations raise the question of why clouding appears only in water and in formamide solutions. Obviously, both water and formamide are polar and hydrogen bonding solvents, but we have investigated several other hydrogen bonding solvents, with both higher and lower dielectric permittivity, in which even a t elevated temperature no clouding phenomenon appears.I3 An interesting question is if we can understand why the clouding phenomenon occurs for the studied system in some polar solvents but not in the other. The main idea in our work is that this is related to the difference in the solvation energy of the polar and nonpolar conformers of the studied polymer. If this difference is large, then the polymer solution will cloud, whereas if this difference is small, one would not expect clouding. In a first approximation one would expect that this difference in solvation energy would depend on the dielectric constant of the medium. One may recall that simple dielectric theory relates the dielectric permittivity to p2/1.3kT for a hard sphere dipolar fluid15and that the solvation energy for a dipole in such a medium can be obtained from 2
Gsolv
e - 1 Pwlute
- -2~ + 1 r,3
In a first approximation r, will be independent of the particle size and thus the solvation energy will depend on t or p2so~v/ V. However r,, in eq 1 to some degree depends (14)Wiirnheim, T.; SjBberg, M. J. Colloid Interface Sci. 1989, 131,
402.
(15) Woodward, C. E.;
Nordholm, S. Mol. Phys.
1986,59,1177-1200.
t
hydrogen bonding
ET,^
kcal/mol at 25 O C
78.5 109 182.4 49 175.7 37
on the size of the solvent particles and this can be understood from the interaction energy between two dipoles PIP2
W(r) = -(angular factor) (2) r123 From eq 2 it is obvious that larger solvent molecules cannot come as close to the solute molecule and one is thus tempted to assign a dependance of r,, on the size of the solvent molecules. One may thus conclude that GsOlv should depend on the solvent properties according to f ( p 2 / V)(l/Va) where the first factor comes from the dielectric constant dependence and the second factor comes from the dependence of r,, on particle size. It is thus tempting to test models where the dependence on Vis larger or the p dependence smaller that the p2/ Vsuggested by dielectric theory. In Table I we show values for one such model ( p / V)which is in agreement with experimental observations. One can see that the value of p / V is larger for formamide and water than for the other polar solvents. This can be interpreted as if the solvation energy obtained for a solute molecule in different hard sphere solvents, with the same dielectric permittivity but with different radii and dipole moments, is larger in the solvent with the smaller molecules. Thus the solvent molecules that are small and polar would favor phase separation at higher temperature. Figures 1and 2 present the phase diagrams for the two polymers studied in water and in formamide. From these figures one can observe that the lower consolute temperature in formamide systems (Figures l b and 2b) is higher than that in water (see Figures l a and 2a). From the point of view cited above, we would assume that formamide is less polar than water. This is also confirmed by different empirical parameters, like the one established by Kosower,lGwhich gives an order of polarity for different solvents. Kosower has derived a solvent parameter called "Z", which corresponds to the transition energy (ET)for the longest wavelength absorption band for 1-ethyl-4-carbomethoxypyridinium iodide. The value of the transition energy defined by relation 3 is very sensitive to the nature of the solvent and increases with its polarity. In Table I, the value of ET listed for some nonaqueous solvents classifies formamide to be less polar than water Z = ET = hNc/X (kcal/mol)
(3)
where h is Planck's constant, N is Avogadro's number, and c / X is the frequency of the absorption. The higher value of the cloud point occurring in formamide solution compared to water could be explained by a smaller (16) Kosower, E. M. J.Am. Chem. SOC.1968, BO, 3253. We may notice here that the charge transfer between ground and excited states is associated with the transitionenergy of the substituted pyridinium iodide. A polar environment creates a relative stabilization and supports greater charge seperation in a transition state and larger dipole moments.
Langmuir, Vol. 7, No. 6, 1991 1069
Phase Behavior of PEO-PPO in Nonaqueous Solution a
3
0
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E
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6o 50
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Figure 1. (a) Partial phase diagram for polymer A in water. (b) Partial phase diagram for polymer A in formamide. a
60 two phase region
o^
e
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Figure 2. (a) Partial phase diagram for polymer B in water. (b) Partial phase diagram for polymer B in formamide. difference in polarity between the polymer and formamide than between polymer and water. Effect of Electrolytes on the Cloud Point of PEOPPO. Studies of the influence of electrolytes on the phase separation in systems containing nonionic polymers will give us some further information on the different interactions in these systems. Concerning this point, we have examined the effect of the phase behavior of one nonionic block copolymer of adding two different salts, NaCl and NaI, in both formamide and water solutions. In Figures 3 and 4, the cloud point variations for the polymer A are presented. I t can be seen that both electrolytes affect the cloud point temperature in water as well as in formamide. NaCl lowers the cloud point temperature in both solvents,
0.2
0.4
0.6
0.8
weight % (NaCI)
Figure 3. (a) Partial phase diagram for 1% polymer A in water as a function of weight percent of NaCl added (14,one-phase region; 24, two-phase region). (b) Partial phase diagram for 1% polymer A in formamide as a function of weight percent of NaCl added. whereas the addition of NaI raises the lower consolute temperature. Qualitatively the same electrolyte effect has been observed for other nonionic polymers and surfactants in water1’+’ and it has been commonly referred to by the concept of “salting in” and “salting out” cosolutes.20 From Figure 3 it should be noted that a t the same low NaCl concentration (about 0.3%), the depression of the lower consolute temperature is more pronounced in formamide (15 OC, Figure 3b) than in water (1“C, Figure 34. The difference is qualitatively the same when we compare the effect of NaI in water and formamide solutions (Figure 4). Moreover, in formamide solutions, there is essentially no further variation in cloud point temperature above 0.3 % of NaCl concentration (Figure 3b), whereas, as can be seen in Figure 4a, the slope of the curve is rather constant up to high NaI concentrations. These observations can be related to the solubility of salt in formamide. In fact, a large anion like iodide is found to be quite soluble in formamide.21 A possible explanation may be that, close to the solubility limit of salt in formamide, the salt is not dissociated into ions but exists as ion pairs. The qualitative similarities of the effects of electrolytes on cloud point (CP) in water and in formamide suggest that the underlying mechanism is the same. As has already been reported on for aqueous systems, this effect may be discussed in terms of the distribution of the electrolyte in the system and the relative polarity of the solvent, polymer, and the cosolute.l* From this point of view, NaCl added will probably interact with the solvent, either water or formamide, and make the solvent effectively more polar, (17) Florin, E.; Kjellander, R.; Ericsson, J. C. J. Chem. SOC., Faraday Trans. 1 1984,80, 2889. (18) KarlstrBm, G.;Carlsson,A.; Lindman,B.J. Phys. Chem. 1990,94, 5005.
(19) Marezall, L. Colloid Surf. 1987,25, 279. (20) Schott, M.; Royce, A. E.; Han,S. K. J. Colloid Interface Sci. 1984, 98,196. (21) Gautier, M.; Rico, I.; Samii, Ahmadzadeh A.; De Snvignac, A.; Lettee, A. J. Colloid Interface Sci. 1986, 112, 484.
1070 Langmuir, Vol. 7, No. 6, 1991 a
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4
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58
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Figure 5. Partial phase diagrams for (a) polymer A, water, and formamide (polymer concentration is constant a t 1%and the ratio between water and formamide is varied); (b) polymer A, water, and N-methylformamide (polymer concentration is constant at 1% and the ratio between water and N-methylformamide is varied); (c) polymer B, and 1,2-propanediol (polymer concentration is constant at 1% and the ratio between water and 1,2-propanediol is varied). NB! Polymer B is chosen in this system instead of polymer A because of the insolubility of the latter in 1,Zpropanediol. For each of the curves the one-phase region is below the curve and the two-phase region is above the curve.
weight% (Nal) Figure 4. (a) Partial phase diagram for 5% polymer A in water as a function of weight percent of NaI added. (b) Partial phase diagram for 5% polymer A in formamide as a function of weight percent of NaI added.
thus increasing the difference in polarity between the solvent and the polymer, which favors phase separation. The partitioning of NaI, with a large negative ion I-, will be slightly in favor of the polymer, which means that the difference in polarity between solvent and solute diminishes which leads to an increase in CP. Finally, it could be mentioned that some additional experiments were performed in which different concentrations of NaCl were added to solutions of the same nonionic polymers in other polar solvents, such as dimethyl sulfoxide, N-methylacetamide, and N-methylformamide. None of these systems shows a clouding behavior. Phase Behavior of PEO-PPO in Ternary Systems. As part of the characterization of the nonaqueous systems, we have investigated the phase behavior of the following systems at a constant polymer concentration: 1, 1% polymer A/water/formamide; 2, 1% polymer A/water/ N-methylformamide; 3, 1% polymer B/water/ 1,2-propanediol; 4, 5 % polymer A/water/N-methylacetide; 5 , 5 % polymer A/water/N,N-dimethylformamide. In Figures 5-7, we present the partial phase diagrams of these different systems as a function of weight percentage of water. From Figure 5 it can be seen that for the first three systems (1,2, and 3), the addition of water decreases the cloud point temperature of the polymers. These results are as expected from the discussion given above in terms of the relative polarity of the different components present in the solution. With similar argumenta, one may state that water, which is the most polar solvent, will preferably interact with these nonaqueous solvents and thereby make the media more polar. This will increase the difference in polarity between the solvent and the polymer, which leads to a depression of the cloud point temperature. By a more careful examination of Figure 5, one can remark that the curves a and b reach a plateau above a certain water concentration, whereas in curve c, the change in cloud point is roughly linear. For the first two systems, it seems as if, at higher water contents, the polymer molecules are in their near proximity mainly surrounded
100
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40
60
80
100
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Figure 6. Partial phase diagram for polymer A, water, and NJVdimethylformamide. Polymer concentration is constant a t 5 % and the ratio between water and NJV-dimethylformamide is varied.
0
1 phase
0
8 L
1
2 phase region h
40
-1
-40 0
20
40 60 weight % (water)
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100
Figure 7. Partial phase diagram for polymer A, water, andN-methylacetamide. Polymer concentration is constant a t 5% and the ratio between solvents is varied.
by water molecules. One possible explanation might be that the polymer molecules are preferably solvated by a hydrogen bond donor solvent like l,Zpropanediol, as compared to less hydrogen bond donor solvents like formamide and N - m e t h y l f ~ r m a m i d e . ~ ~ ~ ~ ~ (22) Nelander, B.; Nord, L. J. Phys. Chem. 1982,86,4375. (23)Engdahl,A.; Nelander, B. J. Phys. Chem. 1989,91,8604.
Phase Behavior of PEO-PPO in Nonaqueous Solution The phase diagrams for the last two systems, with N-methylacetamide and N&-dimethylformamide (see Figures 6 and 71, are similar, but very different from the previous systems. Thus the one phase region has been found to be situated at very low temperatures for certain (intermediate) solvent compositions. In order to get insight into this unusual phase behavior, we have started the determination of the complete ternary phase diagram for the N-methylacetamidelpolymerA/ water system, work which is now in progress. In conclusion, we can see that the phase behavior of nonionic block copolymers in formamide is qualitatively
Langmuir, Vol. 7,No. 6, 1991 1071 similar to the behavior in water. That means that the driving force toward phase separation in the two solvents is probably the same. Our results are in agreement with both the hydrogen bond9J0 and the EO conformational models." The water structure model! on the other hand has to be ruled out, since that would imply that water and formamide have the same structure.
Acknowledgment. The Swedish Natural Sciences Research Council is kindly thanked for financial support. Registry No. (PEO)(PPO)(block copolymer), 106392-12-5; NaCl, 7647-14-5; NaI, 7681-82-5.
