Effect of Hydrophobic Modification on Phase Behavior and Rheology

15 Nov 1995 - Phase behavior and rheology of polymer mixtures comprising aqueous solutions of oppositely charged polyelectrolytes are investigated...
0 downloads 0 Views 420KB Size
Download PDF
530

Langmuir 1996, 12, 530-537

Effect of Hydrophobic Modification on Phase Behavior and Rheology in Mixtures of Oppositely Charged Polyelectrolytes Krister Thuresson,* Svante Nilsson,† and Bjo¨rn Lindman Physical Chemistry 1, Chemical Center, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden Received December 13, 1994. In Final Form: August 21, 1995X Phase behavior and rheology of polymer mixtures comprising aqueous solutions of oppositely charged polyelectrolytes are investigated. Emphasis is put on the effects of hydrophobic modification of the polymers and addition of salt. The associative phase separation usually observed when mixing oppositely charged polyelectrolytes is effectively prevented over a large miscibility region for the hydrophobically modified polymers. Also, in the extended one-phase region, the viscosity is 3-4 orders of magnitude higher for mixed polyelectrolyte systems compared to that observed for either one of the polymers. Addition of ordinary electrolytes to the mixture decreases the viscosity strongly, and at higher electrolyte contents a phase separation is induced. A mechanism explaining the observations is proposed.

Introduction Graft copolymers comprising both hydrophilic and hydrophobic segments have recently begun to attract large scientific and industrial interest.1 Schematically, these polymers often consist of a large water soluble part, the backbone, to which small amounts of hydrophobic groups (typically less than 5 mol %) have been attached. The appeal for these hydrophobically modified polymers in industrial applications is due to their often superior performance compared to their unmodified relatives. Already, these polymers find important uses as thickeners in water-based paint formulations, stabilizers in emulsions, gelling agents in food, dispatchers of active substance in drug formulations, and in enhanced oil recovery. Apart from their industrial relevance, they also attract considerable scientific interest, due to their sometimes striking physicochemical properties, e.g., their rheological behavior in the presence of surfactants. The viscosity of a solution of a hydrophobically modified polymer can increase by several orders of magnitude on addition of a small amount of surfactant.2-7 This striking behavior is related to the ability of the surfactant molecules to selfassemble at the hydrophobic chains of the polymers.4,8 Studies have so far been performed with both nonionic and ionic hydrophobically modified polymers,4,8,9 but mixtures of one cationic and one anionic hydrophobically modified polyelectrolyte have to our knowledge not yet been examined. This study is focused on such mixtures. However, to understand the observed rheology and phase * To whom correspondence should be addressed. † Present address: Rogaland Research, Prof. Olav Hanssens va ¨g 15, Box 2503, Ullandhaug, 4004 Stavanger, Norway. X Abstract published in Advance ACS Abstracts, November 15, 1995. (1) Polymers in Aqueous Media; Glass, J. E., Ed.; American Chemical Society: Washington, DC, 1989; Vol. 223. (2) Gelman, R. A. Hydrophobically modified hydroxyethylcellulose. International Dissolving Pulps Conference, 1987. (3) Tanaka, R.; Meadows, J.; Phillips, G. O.; Williams, P. A. Carbohydr. Polym. 1990, 12, 443-459. (4) Magny, B.; Iliopoulos, I.; Audebert, R.; Piculell, L.; Lindman, B. Prog. Colloid Polym. Sci. 1992, 89, 118-121. (5) Ka¨stner, U.; Hoffmann, H.; Do¨nges, R.; Ehrler, R. Colloids Surf. 1994, 82, 279-297. (6) Hulde´n, M. Colloids Surf. 1994, 82, 263-277. (7) Nystro¨m, B.; Thuresson, K.; Lindman, B. Langmuir 1995, 11, 1994-2002. (8) Tanaka, R.; Meadows, J.; Williams, P. A.; Phillips, G. O. Macromolecules 1992, 25, 1304-1310. (9) Goddard, E. D.; Leung, P. S. Colloids Surf. 1992, 65, 211-219.

0743-7463/96/2412-0530$12.00/0

behavior better, we also investigated systems in which only one, or none, of the polyelectrolytes was hydrophobically modified. Finally, we also substituted one of the hydrophobically modified polyelectrolytes with a hydrophobically modified nonionic polymer. Normally an aqueous solution of two different polymers phase separates. The phase separation may result in either a separation of the two polymers in different phases or an increased concentration of both polymers in one phase coexisting with a solvent rich phase.10-12 The first phenomenon, referred to as segregative phase separation, is usually observed for uncharged polymers or polyelectrolytes with charges of equal sign and similar charge density. The second phenomenon, dealt with in this paper, is more commonly seen for mixtures of oppositely charged polyelectrolytes. This is generally called complex coarcervation or associative phase separation. Associative phase separation results in a loss of polymer entropy but also in a concomitant release of counterions from the vicinity of the polyelectrolytes leading to a net gain of entropy for the system.13 The associative phase separation often takes place over almost the entire mixing interval. The polyelectrolytes match their charges to form a neutral complex with the excess polymer remaining in the solvent rich phase. A parallel to this phenomenon is seen in the work by Thalberg et al. where a solution of cationic surfactants undergoes an associative phase separation on addition of polyanion.14,15 The phase separation is observed far below the point of charge neutralization, and virtually all polyanion is in the precipitate while the surplus of the surfactant rests in the supernatant. In the present investigation we show that associative phase separation can be prevented in a large mixing region if both the polyelectrolytes are hydrophobically modified. (10) Bungenberg de Jong, H. G. Complex colloid systems. In Colloid Science; Kruyt, H. R., Ed.; Elsevier: Amsterdam, 1949; Vol. II, Chapter 10. (11) Flory, P. J. Principles of Polymer Chemistry, 13 ed.; Cornell University Press: Ithaca, 1953. (12) Smid, J.; Fish, D. Polyelectrolyte complexes. In Encyclopedia of Polymer Science Engineering, 2nd ed.; Mark, H. F., Bikales, N. M., Overberger, C. G., Menges, G., Eds.; Wiley Interscience: New York, 1988; Vol. 11, pp 720-739. (13) Piculell, L.; Lindman, B. Adv. Colloid Interface Sci. 1992, 41, 149-178. (14) Thalberg, K.; Lindman, B.; Karlstro¨m, G. J. Phys. Chem. 1990, 94, 4289-4295. (15) Thalberg, K.; Lindman, B.; Karlstro¨m, G. J. Phys. Chem. 1991, 95, 6004-6011.

© 1996 American Chemical Society

Phase Behavior and Rheology of Polymer Mixtures

Langmuir, Vol. 12, No. 2, 1996 531

Table 1. Data of the Polymers in the Present Investigation molecular weight charge concentration of charges in a 1% (w/w) aqueous solution (mm) mean contour length between chargesa (Å) hydrophobic modification degree (mol %) mean contour length between hydrophobic tailsa (Å) short abbreviation

JR400

LM200

NaPA

NaPA

HM-NaPA

HM-HEC

500 000 positive 10 20

100 000 positive 2 100 5.4 100 HM-P+

20 000 negative 106 2.5

150 000 negative 106 2.5

250 000 nonionic

P20

P150

150 000 negative 99 2.5 3 84 HM-P150

P+

∼2 ∼300 HM-P0

a With 5.45 Å as the length per repeating unit for cellulose polymers (JR400, LM200, and HM-HEC). The length of the repeating unit for the polyacrylates (NaPA and HM-NaPA) equals the distance between consecutive charges.

Figure 2. The chemical structure of the polyacrylates with and without hydrophobic substitution: P20, x ) 0, m ) 2.8; P150 , x ) 0, m ) 20; HM-P150 , x ) 3, n ) 12, m ) 20. Figure 1. The chemical structure of the cationic hydrophobically modified polymer LM200, referred to as HM-P+.

In addition to phase studies, we have also made rheological measurements in the one-phase region, which reveal a large synergistic viscosity increase (by 3-4 orders of magnitude) on addition of one polyelectrolyte to the other. The observations are due to a formation of mixed aggregates consisting of hydrophobic tails from polyelectrolytes bearing charges of opposite sign. Experimental Section Materials. Chemical and physical data, reported in this section, for the polymers are condensed in Table 1. Two cationic polymers, one unmodified (JR400) and one modified with hydrophobic tails (Quatrisoft LM200), are manufactured by Union Carbide Chemicals and Plastics Company, Inc. LM200 is a N,Ndimethyl-N-dodecylammonium derivative of hydroxyethyl cellulose with a molecular weight of approximately 100 000.16 The charges are located at the hydrophobic tails, giving a structure of the hydrophobic side chains resembling that of cationic surfactants, Figure 1.17 The concentration of polymer charges, and tails, was achieved by using a standard Sigma diagnostics titration procedure (procedure 830) to determine the chloride counterion concentration, but it was also determined by nitrogen analysis of the dry polymer.18,19 The hydroxyethyl substitution of LM200 is not known but can possibly vary from MSEO ) 0 to MSEO ) 3.3. Those numbers, corresponding to an unsubstituted sugar unit and the repeating unit of the highly substituted HMHEC, respectively, give the molar mass of the repeating unit to be between 160 and 300 g.3 The true value is probably in between, which is also supported by earlier reports of MSEO ) 2.5 for a typical HEC sample.3,20 This value assumed for further purposes gives the weight of the repeating unit, the hydrophobic substitution degree, and the mean contour length between repeating hydrophobic tails (and charges) of LM200, see Table 1. The unmodified cationic polymer, JR400, is the corresponding N,N,Ntrimethylammonium derivative with a molecular weight of approximately 500 000.16 The charge density is higher than that for LM200 and was determined by titration on the chloride counterions. The data given in Table 1 assume the same weight (16) Dhoot, S.; Goddard, E. D.; Murphy, D. S.; Tirrell, M. Colloids Surf. 1992, 66, 91-96. (17) Brode, G.; Goddard, E. D.; Harris, W. C.; Salensky, G. A. Polym. Sci. Eng. Proc. 1990, 63, 696-698. (18) Guillemet, F.; Piculell, L.; Nilsson, S.; Lindman, B. Phase Behaviour of Mixtures of Associative Polyelectrolyte with Oppositely Charged Surfactant. Cellucon -93., 1993, Lund. (19) Guillemet, F.; Piculell, L. J. Phys. Chem. 1995, 99, 9201-9209. (20) Karlstro¨m, G.; Carlsson, A.; Lindman, B. J. Phys. Chem. 1990, 94, 5005-5015.

of the repeating unit for JR400 as for LM200. Both cationic polymers were used in their chloride salt form. Polyacrylates with two different average molecular weights, 20 000 and 150 000 (in the acid form), as given by the manufacturer, were chosen as the negatively charged polyelectrolytes. The high molecular weight sample has either no hydrophobic substitution or a substitution degree of 3 mol % dodecyl chains grafted to the polymer backbone, Figure 2.21 The sodium polyacrylates without hydrophobic modification were supplied in the acid form by Polyscience, Inc. Prior to use they were neutralized with NaOH and freeze-dried. The hydrophobically modified polyacrylate was received as a kind gift from Dr. Ilias Iliopoulos, Paris, and was used as received. In addition to the charged polymers, an uncharged hydrophobically modified hydroxyethyl cellulose (HM-HEC) was used. The structure of this polymer resembles the structure of LM200, but the hydrophobic tails of HM-HEC consist of C12-C24 alkyl chains. The polymer which was supplied by Aqualon (UK) Ltd. has a hydroxyethyl substitution degree of MSEO ) 3.3 and a hydrophobic substitution degree of ca. 2 mol %. The average molecular weight has been reported to be ca. 250 000.3 Prior to use, HM-HEC was rinsed from unreacted hydrophobic tails by extensive extraction with acetone several times. After extraction, the polymer sample was dried from acetone and dissolved in water to a concentration of approximately 1 g polymer per 100 g of water (1% (w/w)). High molecular weight impurities not soluble in water were centrifuged off, leaving a clear supernatant phase. Low molecular weight impurities, such as salt, in the supernatant were removed by dialysis against Millipore water in a Filtron Ultrasette devise. The conductivity of the expelled water was measured, and when no further decrease in the conductivity was obtained, the dialysis was stopped. This occurred after ca. 70 h and at a conductivity of ca. 2 µS/cm. The remaining polymer sample was freeze-dried. The dialysis and freeze-drying procedure was also used for the LM200 and JR400 samples. Throughout the text the following notation will be used for the polymers: P+ for a cationic polymer, P- for an anionic polymer, and finally P0 for an uncharged polymer. HM as in HM-P+ indicates that the polymer is hydrophobically modified. A , refers to the molecular weight subscript, for example 150 in P150 of the polymer. Methods. After preparation of the samples, they were mixed with care at room temperature before any measurements were performed. The low-viscosity samples were mixed by turning end over end for at least 24 h, while the samples with higher viscosity were mixed with a magnetic stirrer. Phase boundaries were obtained by a titration procedure. A 1% (w/w) solution of one of the polymers was titrated with a 1% (w/w) solution of another polymer. On titration, the solution (21) Wang, K. T.; Iliopoulos, I.; Audebert, R. Polym. Bul. 1988, 20, 577-582.

532

Langmuir, Vol. 12, No. 2, 1996

Thuresson et al.

turned from one phase (1Φ) to two phase (2Φ). The mixtures were taken to be 2Φ when a ruled scale visually observed through the stirred solution appeared blurred. The 2Φ solution was then titrated back into the clear 1Φ region by using a 1% (w/w) solution of the first polymer. The difference between titration and back titration values was less than (0.5 in percent units. The uncertainty is mainly caused by the high viscosity of the solutions. The absence of hysteresis means that the values are on the binodal line. Most rheology measurements were carried out on a Bohlin VOR rheometer in the oscillatory mode. The experiments were performed as frequency sweeps (0.01-20 Hz) and the complex viscosity η* is presented at 0.1 Hz while the storage modulus G′ and phase angle δ are presented at 5 Hz. η* is defined as

η* )

(G′2 + G′′2)1/2 2Πf

G′′ represents the loss modulus and f the frequency. For measurements on low-viscosity samples, a double gap concentric cylinder was used, while for the samples with higher viscosity an ordinary concentric measuring system (cup and bob) was chosen. In addition to the Bohlin rheometer a Carri-Med CSL 100 rheometer equipped with automatic gap setting was used for the measurements in which both oscillatory and flow measurements were performed. Depending on the viscosity of the sample one out of two different cone and plate measuring geometries, 4 and 6 cm in diameter, was used. Before all rheology measurements, precautions were taken to prevent bubble formation in the sample that could distort the result. Phase boundaries were obtained at room temperature and all rheological measurements at 25 °C.

Results Phase Behavior. First we focus on mixtures of two oppositely charged polyelectrolytes. As was mentioned in the Introduction a phase separation usually takes place when two different polymers are mixed. Figure 3a shows in a broad a phase separation between HM-P+ and P150 mixing regime. As expected with oppositely charged polymers the observed phase separation is associative as is found by analyzing the composition of the phases (data not shown). At a total polymer concentration of 1% (w/w) the 2Φ region extends from about 1.7% to 99% by weight . A dramatic change was seen when of polymer P150 P150 was replaced by its hydrophobically modified analogue, HM-P150 . The 2Φ area decreases strongly to extend only from about 2.0% to 21% by weight HM-P150 . Figure 3b shows a set of relative volumes of the top and bottom phases in the 2Φ region for the mixture of HM-P+ and HM-P150 . The bottom phase swells on addition of HM-P150 from the small relative volume close to the charge neutralization point of the mixture. Close to the other phase border (at higher weight fractions of HMP150 ) the bottom phase occupies the major part of the volume. Addition of 16 mm NaCl induces a salting out of a 1 wt % HM-P+ solution.19 The low salt concentration indicates that without its charges the polymer would have been insoluble in water. Thus, HM-P+ is an intrinsically insoluble polymer that owes its solubility to the entropy arising from the presence of counterions. The resistance to phase separation on addition of salt is much higher in the mixture of the two hydrophobically modified polyelectrolytes. In fact ca. 200 mm NaCl can be added before a phase separation occurs in an aqueous solution of 0.5% (see the Rheology (w/w) HM-P+ and 0.5% (w/w) HM-P150 section below). Finally, when the nonionic polymer HM-P0 is used, instead of HM-P+, the solution displays a 1Φ behavior over the entire mixing regime (from 1% (w/w) HM-P0 to

Figure 3. (a) One- (1Φ) and two-phase (2Φ) regions in the polymer mixtures on substituting HM-P+ with P150 or 0 HM-P150 and on substituting HM-P with HM-P150, bottom to top. The total polymer concentration is always 1% (w/w). The charge neutralization in the polyanion-polycation mixtures is indicated. (b) Relative volumes of coexisting polymer rich bottom and solvent rich upper phases vs the weight fraction of HM-P150. 1% (w/w) HM-P150 ). No associative, nor segregative, phase separation is observed for this system. Rheology. We start this section by presenting the viscosity of a P+ and a HM-P+ solution as a function of added electrolytes, ranging from ordinary salts, with different valence of the anions, to polyelectrolytes. The amount of added salt, NaCl, Na2SO4, or Na3PO4, is presented in terms of ionic strength. Figure 4 shows the viscosity of each of the polymer solutions as a function of the ionic strength. It is clear that the changes are almost independent of the identity of the added salt. When salt was added, the viscosity of the P+ solution decreased, whereas the opposite trend was observed for the HM-P+ solution. However, addition of polyanions to modified and unmodified polycation solutions results in viscosity increases for both systems. The recorded relative increase of the viscosity ranges from about 70 to 400 times and from approximately 25 to 100 times that of the pure polycation/water systems for HM-P+ and P+, respectively. The magnitude of the increase depends on the added

Phase Behavior and Rheology of Polymer Mixtures

Figure 4. The complex viscosity η* at 0.1 Hz for a 1% (w/w) aqueous solution of P+ (filled symbols), or HM-P+ (open symbols), as a function of cosolute addition. The concentrations of simple electrolytes are presented as ionic strengths, while the polyanion concentrations are presented as concentration of charges. is represented by O or b, HM-P150 by 0 or 9, Addition of P150 P20 by ] or [, NaCl by 4 or 2, Na2SO4 by 3 or 1, and Na3PO4 by + or ×, respectively. The broken lines are only drawn to facilitate the interpretation.

Langmuir, Vol. 12, No. 2, 1996 533

Figure 6. The storage modulus G′ (O) and the loss modulus G′′ (]) as a function of frequency in solutions of 1% (w/w) HMP+ with 0.01% (w/w) P20 (full line) or P150 (dashed line).

Figure 7. The complex viscosity η* as a function of frequency in 1% (w/w) HM-P+ plus 0.01% (w/w) P20 (O) or P150 (]), 2% + (w/w) HM-P plus 0.02% (w/w) P20 (×) or P150 (+), and 3% (w/w) HM-P+ plus 0.03% (w/w) P20 (b) or P150 ([).

Figure 5. The viscosity η as a function of shear rate in the solution of 1% (w/w) w/w% HM-P+ with 0.01 % (w/w) P20 (line) or P150 (]). The upper of the two mutually connected curves indicates a sweep toward higher shear rates. polyanion, with P20 resulting in the smallest and P150 in the largest increase at 0.1 Hz. According to the Phase Behavior section above, it was only possible to add a low concentration of simple salts or polyanions to HM-P+. Above this, the polymer is salted out or an associative phase separation occurs. Both flow and oscillatory measurements were performed in a series of polymer solutions where the ratio between polyanion and polycation was kept constant but where the total polymer concentration was increased. The series consists of six different solutions containing 1%, 2%, or 3% (w/w) HM-P+ with 0.01%, 0.02%, or 0.03% (w/w) P20 or P150 , respectively. The flow measurements were performed as cycles in which the shear rate first was increased and later decreased. Each sweep, up or down, lasted for 30 min. In the oscillatory measurements, the frequency was only swept toward higher values. Figure 5 shows the viscosities η of the samples with the lowest polymer content (1% (w/w) HM-P+ with 0.01% (w/w)

P20 or P150 ). The solution with the low molecular weight polyanion is in the Newtonian regime at low shear rates, and the measured viscosity follows the same curve independent of whether the shear rate is swept toward higher or lower values. The sample with the high molecular weight polyanion, on the other hand, behaves differently. The Newtonian plateau is not reached at the lowest shear rates used, and the measured viscosity is higher in the scan where the shear rate is increased indicating that the viscosity is dependent on the history of the sample. In Figure 6, the storage, G′, and the loss, G′′, moduli are plotted as a function of the frequency. At high frequencies both samples show elastic behavior and the responses of the two are quite similar. However, at displays low frequencies the sample containing P150 elastic properties, while the sample containing P20 is more viscous. Figure 7 shows the complex viscosity η* as a function of the frequency for the six samples. The response becomes more similar for the two different polyanion molecular weights at higher frequencies and at increasing polymer content. For the HM-P+ solution with P20 added, the Newtonian plateau (now in η*) is approached at the lowest total polymer content, while it is added (compare Figure 5). At higher not with P150 polymer concentrations, however, η* becomes strongly frequency dependent both for the sample containing P150 and for that with P20 . This shear thinning behavior

534

Langmuir, Vol. 12, No. 2, 1996

Figure 8. The viscosity η as a function of shear rate in the mixture of 3% (w/w) HM-P+ and 0.03% (w/w) P20 (line) or P150 (]). The upper of two mutually connected curves indicates a sweep toward higher shear rates.

Figure 9. The complex viscosity η* as a function of the frequency in the mixture of 1% (w/w) HM-P+ and 0.015% (w/w) P20 (O) or P150 (]).

and lack of Newtonian plateaus for both samples at the highest concentration is reproduced in the flow curves, Figure 8. Furthermore, at this concentration the history seems important for both the samples as the measured viscosity is higher when the shear rate is increased. In Figure 9 the concentration of the polyanion has been increased, and η* as a function of the frequency for samples or containing 1% (w/w) HM-P+ and 0.015% (w/w) P20 P150 is shown. The difference in η* between the samples has increased compared to that in Figure 7. However, at increasing frequency, the response again becomes less dependent on the molecular weight of the added polyanion. In the samples above it was only possible to add a small amount of the polyanion to the polycation before phase separation took place, whereas in the case of HM-P+ and HM-P150 it is possible to make 1:1 mixtures of the two polyelectrolytes at a total polymer concentration of 1% (w/w) (vide supra). Such a mixture shows a viscosity increase by 3-4 orders of magnitude compared to the 1% (w/w) solutions of either one of the polymers, Figure 10. On addition of NaCl to this mixture, the storage modulus G′ decreases strongly and the phase angle δ increases. G′

Thuresson et al.

Figure 10. The complex viscosity η* at 0.1 Hz of mixtures of HM-P+ and HM-P150 as a function of the weight fraction of HM-P. The total polymer concentration is kept at 1% (w/w). 150

Figure 11. The effect of added NaCl on the storage modulus G′ (.) and the phase angle δ (0) at 5 Hz in the high-viscosity region. The aqueous solution contains 0.5% (w/w) HM-P+ and 0.5% (w/w) HM-P150 . The solution phase separates at ca. 200 mm NaCl.

and δ show a regular change in the entire range of the 1Φ region which reaches up to about 200 mm NaCl, Figure 11. Finally, on mixing HM-P0 with a polyanion, η* changes in a monotonic manner from the viscosity of an aqueous solution of HM-P0 to the viscosity of a solution of HMP150 alone, Figure 12. A total polymer content of 2% (w/w) was used. It is also clear that η* in the 1:1 mixture decreases only moderately on addition of 100 mm NaCl. Discussion Phase Behavior. The 2Φ region in the mixture conis replaced with HMtracts dramatically when P150 P150, despite the fact that covalently bonded hydrophobic tails obviously increase the attraction between the polymers. Moreover, hydrophobic modification increases the hydrophobicity of the polymer complex, which should also make the polymer complex less soluble and increase the size of the 2Φ region. Hence, the increased attraction

Phase Behavior and Rheology of Polymer Mixtures

Langmuir, Vol. 12, No. 2, 1996 535

could easily be thought to increase the tendency for an associative phase separation. With polymers that carry opposite charges and not are hydrophobically modified, association occurs through ion pair formation; a precipitation forms by association of polymers to form complexes that are electrically neutral. If excess polymers of either type are present, they remain in solution in the aqueous phase. With polymers that are hydrophobically modified, association also occurs through hydrophobic bonds; the complexes that are formed between different macromolecules have almost the same composition as the mixture as a whole. The electrostatic force results in a selection rule where hydrophobic tails of the cationic polymer preferentially combine with hydrophobic tails of the anionic polymer. Consequently the overall charge need not be zero unless the mixture has a particular composition. Thus, precipitation is prevented over most of the composition range. Given that the charge density is low enough to allow phase separation, the volume fraction of the polymer rich phase will increase with increasing charge density of the complex due to the counterion entropy, Figure 3b. In a recent investigation the interaction of HM-P+ polymers with an anionic surfactant was discussed in similar terms.19 The stoichiometry of the complex also explains the observation that more than 10 times the salt concentration that can be added to the HM-P+ solution can be added to the mixture of 0.5% (w/w) HM-P+ and 0.5% (w/w) HM-P150 before a phase separation occurs. This salt concentration is, furthermore, substantially less than what , which is can be added to a 1% (w/w) solution of P150 soluble in an aqueous solution saturated with NaCl (ca. naturally de6 m).22 Hydrophobic modification of P150 creases the solubility of the polymer, but Magny added up without observing to 15 wt % NaCl (ca. 3 m) to HM-P150 23 any phase separation. The enhanced salt tolerance of HM-P+ in the presence of HM-P150 can be explained by the formed polymer complex having a net charge higher

than that of HM-P+ but lower than that of HM-P150 . The limiting NaCl concentration that can be added to the mixture before phase separation should therefore be intermediate to what can be added to the two polymers alone. Overall charge neutrality of the polymer complexes is reached on substituting approximately 2 wt % of HM-P+ by HM-P150 . This ratio corresponds well to the onset of the 2Φ region observed in the HM-P+ rich regime, see Figure 3a. Near charge neutralization polymer complexes with zero net charge form, and these phase separate from the solution. Note that a mixture of P+ and P- is expected to phase separate prior to charge neutralization if no extra interactions, e.g., a hydrophobic, exist between the polymers. Related to the present system Magny et al. have recently shown that no phase separation exists until close to charge neutralization in a mixture of HM-P150 and a cationic surfactant. The phase separation found on adding a small amount of cationic surfactant to a solution is hence effectively prevented by hydrophobic of P150 modification of the polymer. It was suggested that the presence of hydrophobic attractive forces, in addition to the electrostatic ones, cause the surfactant molecules to distribute more evenly over the hydrophobically modified polymer.24 Finally, no phase separation was observed when the uncharged polymer HM-P0 was mixed with HM-P150 . This observation, true for all polymer ratios, is an expected result, since this polymer system has nothing to gain by separating into two phases. Both an associative and a segregative phase separation would result in an increased , followed by a concentration (into one phase) of HM-P150 loss of counterion and polymer entropy. Nevertheless, a may still form, but complex between HM-P0 and HM-P150 as no driving force for a phase separation exists, no 2Φ region is observed. Rheology. The decrease of the viscosity in the P+ solution on addition of simple electrolytes is an effect of a decreased persistence length of the polymer chain. The opposite behavior was seen for HM-P+, where a small viscosity increase was observed as salt was added, Figure 4. This is explained by an enhanced aggregation of the hydrophobic tails due to screening of the repulsive electrostatic interactions between the polymer charges. The combination of the hydrophobic tails results in an aggregate, or a three-dimensional network, which tends to increase the viscosity of the system.21 The increased affinity for network formation on addition of electrolyte has the same physical basis as the lowering of the cmc of ionic surfactants in the presence of salt.25 Adding a negatively charged polyelectrolyte, as P150 , + P20, or HM-P150, to a solution of HM-P results in a viscosity increase which is much more pronounced than that observed on addition of simple electrolytes. Note that the viscosities of 1% (w/w) aqueous solutions of pure HM-P150 , P20 , and P150 are comparable and lower than the viscosities of 1% (w/w) aqueous solutions of HM-P+ or P+. The viscosity increase could be explained by the increased tendency toward clustering of the hydrophobic tails when the charges on HM-P+ are screened by those of the anionic polymer in analogy with addition of ordinary electrolytes. or P20 to a P+ On the other hand, on the addition of P150 solution, a large viscosity increase is also observed even though none of the polymers bears hydrophobic tails.

(22) Buscall, R.; Corner, T. Colloids Surf. 1986, 17, 25-38. (23) Magny, B. Polyelectrolytes associatifs : me´thodes de synthe`se, comportement en milieux dilue´ et semi-dilue´. Thesis, University Pierre et Marie Curie Paris, 1992.

(24) Magny, B.; Iliopoulos, I.; Zana, R.; Audebert, R. Langmuir 1994, 10, 3180-3187. (25) Lindman, B.; Wennerstro¨m, H. Micelles. amphiphile aggregation in aqueous solution. Top. Curr. Chem. 1980, 87, 1-83.

Figure 12. The complex viscosity η* at 0.1 Hz for the mixture of HM-P0 and HM-P150 with a total polymer concentration of 2% (w/w) as a function of the weight fraction of HM-P150 (O). The viscosity of the mixture containing 1% (w/w) HM-P0 and 1% (w/w) HM-P150 decreases on addition of 100 mm NaCl (]).

536

Langmuir, Vol. 12, No. 2, 1996

Apparently, in addition to promoting aggregation of hydrophobic tails on HM-P+, added P- polymers also promote the formation of an electrostatic network. To get a further understanding of the network formation we first focus on the addition of polyanions to polycation solutions. As the concentration of positive charges is 2 mm in a 1% (w/w) HM-P+ solution, the mean distance between these can be estimated to be approximately 94 Å (assuming a cubic lattice). The distance between consecutive repeating charges on HM-P+ is ≈100 Å, Table 1, which is valid if the tails are distributed at equal distances from each other (a blocky structure, or aggregation of tails, results in larger distances between the positive charged centers). Since the distance between repeating (positive) charges in a HM-P+ chain is on the order of the average distance between (positive) charges in the cubic lattice, there is a significant probability that the added polyanion can associate with more than one polycation. It should be noted that the added polyanion can associate with the charges of the polycation without geometrical constraints, since the distance between the repeating (negative) charges is short (2.5 Å). The contour length of P20 is about 700 Å, implying that this polymer only can cross-link a small number of different polycations, while P150 , having a contour length of ≈5300 Å, can be much more entangled and, hence, cross-link a larger number of polycations. Corresponding calculations for the P+ solution give a mean distance in a cubic lattice and a distance between consecutive charges of 55 and 20 Å, respectively. Figures 5 and 6 show that the sample containing the ) has a broader high molecular weight polyanion (P150 distribution of relaxation times than the sample containing P20 , since neither the Newtonian low shear plateau in η nor the high-frequency plateau in G′ are observed. The displays the rheological response sample containing P150 of a gel, while the sample with P20 behaves more like a liquid. Furthermore, the shear experiment indicates that the first sample relaxes slowly on the time scale set by the experiment (0.5 h) as η is smaller in the first sweep (where the shear rate is increased). This is not observed in the . Together these results indicate a sample containing P20 larger extent of entanglement in the P150 system. Figure 7 shows that the rheological response for the two molecular weight polyanions systems becomes more similar as the frequency or the polymer concentration is increased. The increase in total polymer concentration is accompanied by a decrease of the mean distance between to positive charges (from 94 to 65 Å), which allows P20 associate to a larger number of polyanions. The difference in rheological properties on addition of polyanions with different molecular weight is thus decreased, and hence, the Newtonian plateau is not reached. Now the viscosity shows a dependence on the sample history for both polyanions, Figure 8. Increasing the P- concentration (keeping the HM-P+ concentration constant) shows that the relative increase in η*, at low frequencies, is larger with the long chained polyanion (Figures 7 and 9). These results are in agreement with complex formation and the entanglement idea, i.e., the degree of entanglement increases with the length of the polyanion. HM-P150 polymers have a long persistence length in the salt free system since the charge density is high (the presence of 0.5% (w/w) HM-P+ will not influence the persistence length of HM-P150 , since it only corresponds to 1 mm charges). Addition of salt to the mixture of 0.5% and 0.5% (w/w) HM-P+ induces a decrease (w/w) HM-P150 of G′ and a concomitant increase of δ and finally a phase separation in analogy with simple polyelectrolyte solu-

Thuresson et al. Table 2. Calculated Persistence Length, Pel, of HM-P150 at Various NaCl Concentrations, Using the Debye-Hu 1 ckel Approximationa

NaCl (mm)

persistence length (Å)

0 10.1 14.8 23.1 35.4

259 177 113 74

a In the calculations the polymer is set to be fully dissociated, R ) 0.5. The charge and length of the repeating units are q ) -1, and a ) 1.25 Å (carbon-carbon distance), respectively. The dielectric constant of the solution is set to D ) 0r, where r ) 78.5 is the dielectric constant of water. The temperature is T ) 298 K.

tions, Figure 11. These observations can be explained by two mechanisms. First, increasing ionic strength leads to decreasing attractive electrostatic interactions, resulting in fewer cross-links which may also have shorter lifetimes, and thus decreasing G′ and increasing δ on a given time scale. Second, increasing ionic strength reduces by the persistence length of, in particular, HM-P150 electrostatic screening. The formed network thus becomes less rigid. This is supported by calculations of the as a electrostatic persistence length, Pel, of HM-P150 function of added salt by means of the Debye-Hu¨ckel approximation26

Pel )

R2Γ2 16Πκ2kbTD

Here R is the degree of ionization of the monomers, Γ ) q/a where q is the charge of the monomers and a is the length of the repeating units, kb is the Boltzmann constant, T is the temperature, and D is the dielectric constant of the solution. Finally, κ-1 is the Debye screening length. This simple calculation shows that the persistence length of the polyanion is on the order of the mean distance between positive charges in a solution of 0.5% (w/w) HMP+ (≈120 Å), Table 2. Already at low concentrations of added salt, Pel decreases below this value. The decreased persistence length of the polyanions is reflected in G′. The two mechanisms could be regarded as two extremes with the real system being in between. does not result in a large Mixing HM-P0 with HM-P150 synergistic increase of the viscosity, as was observed for , Figure 12. The the mixtures of HM-P+ and HM-P150 strong electrostatic attraction between different polymer system, types is not present in the HM-P0 - HM-P150 leading to a smaller tendency to form mixed aggregates. Thus, the viscosity displays a monotonic change as a function of the mixing ratio. Still it is plausible that mixed aggregates form, which parallel the formation of mixed micelles in a solution containing one charged surfactant, either cationic or anionic, and one nonionic, micelle or vesicle forming, surfactant.27-29 Recently it has been shown that HM-P150 interacts with nonionic micelles and vesicles. These mixtures are under certain conditions (26) Skolnick, J.; Fixman, M. Macromolecules 1977, 10, 944-948. (27) Shirahama, K.; Nishiyama, Y.; Takisawa, N. J. Phys. Chem. 1987, 91, 5928-5930. (28) Kresheck, G. C.; Kale, K.; Erman, J. Thermometric and surfactant selective potentiometric titration studies of surfactant binding to phospholipid vesicles. In Solution Behaviour of Surfactants; Mittal, K. L., Fendler, E. J., Eds.; Plenum Press: New York, 1982; Vol. 1, pp 677-692. (29) Takasaki, M.; Takisawa, N.; Shirahama, K. Bull. Chem. Soc. Jpn. 1987, 60, 3849-3853.

Phase Behavior and Rheology of Polymer Mixtures

highly viscous.30 A weak aggregation between HM-P0 and HM-P150 is supported by the results presented in Figure 12, showing that the viscosity of the mixture decreases on addition of NaCl. Note that the added electrolyte, 100 mm NaCl, does not significantly affect the viscosity of a solution containing only HM-P0 and that the viscosity of a HM-P150 solution increases (see also Figure 5).21,31 The observed viscosity decrease is explained by assuming that the two polymers form a network and that the persistence decreases with increasing salt conlength of HM-P150 centration. Concluding Remarks A stoichiometric mixture of two oppositely charged homopolymers will have a strong tendency for associative phase separation, the driving force being the entropy of the counterion distribution. There is no driving force for an excess of one of the polyelectrolytes to enter the polymer rich phase, and so it will appear in the water rich phase. The same result will be obtained if one of the polymers is hydrophobically modified. However, if both polyelectrolytes are hydrophobically modified, there is an additional mechanism, the tendency to form mixed aggregates; this is similar to the strong tendency toward mixed aggregate formation in solutions of two surfactants or of one surfactant and one hydrophobically modified polymer. This would give a net charge of the macromolecular part of the concentrated phase and an entropy loss in the counterion distribution. To eliminate the increased free energy the concentrated phase swells and phase separation is reduced or completely eliminated, depending on the charge stoichiometry. This explains the much reduced two-phase region in mixtures of two HM polyelectrolytes. An analogous result is obtained for mixtures of one HM polyelectrolyte and an oppositely charged surfactant, where the mechanism is basically the same.19,24 The mixture of one charged and one uncharged HM polymer is expected to behave differently. These mixtures will also have a tendency to form mixed aggregates. However, now associative phase separation would be accompanied by an important loss of entropy and is disfavored by electrostatic effects, as also observed. We (30) Sarrazin-Cartalas, A.; Iliopoulos, I.; Audebert, R.; Olsson, U. Langmuir 1994, 10, 1421-1426. (31) Thuresson, K. Unpublished results.

Langmuir, Vol. 12, No. 2, 1996 537

note the analogy with mixtures of a clouding polymer and an ionic surfactant. Here addition of surfactant reduces the tendency toward phase separation of the polymer (increased cloud point) since with associated surfactant this would result in an entropy loss. On addition of small amounts of salt, this effect is eliminated and a major decrease in cloudpoint is obtained.20,32,33 In the present case the cloud point is high (above 100 °C), and within the limited salt concentration used we see no phase separation. The rheology results can in many ways be understood on the basis of the interactions inferred from the phase behavior. That the viscosity increases on salt addition for HM polyelectrolytes but not for the corresponding homopolymers is a direct consequence of increased hydrophobic association. For mixtures of HM-P+ and HMP- at low electrolyte contents association between oppositely charged chains is favored whereas at higher ionic strength the hydrophobic association becomes electrostatically unselective, i.e., association between similarly and oppositely charged chains becomes equally likely. Hence, increasing the ionic strength results in fewer crosslinks between oppositely charged polyelectrolytes (which may also have shorter lifetimes). This in combination with the decreased persistence length of the polyanions is the explanation for the reduction of the storage modulus on a given time scale. The rheological properties of the network show the normal dependence on polymer molecular weight and concentration. Acknowledgment. We are grateful to Dr. I. Iliopoulos for kindly supplying us with the hydrophobically modified polyacrylate, Drs. L. Piculell and G. Karlstro¨m for helpful discussions, and Dr. F. Tiberg for valuable comments on the manuscript. The Division of Food Technology is thanked for allowing us to use the Bohlin VOR rheometer. This work was supported by the National Bord for Industrial, and Technical Development (NUTEK) and Bo Rydin foundation. Nils and Dorthi Troe¨dsson research foundation is acknowledged for a grant for the Carri-Med rheometer. LA941003Q (32) Carlsson, A.; Karlstro¨m, G.; Lindman, B. Langmuir 1986, 2, 536-537. (33) Carlsson, A.; Karlstro¨m, G.; Lindman, B.; Stenberg, O. Colloid Polym. Sci. 1988, 266, 1031-1036.