5360
Langmuir 2002, 18, 5360-5365
Dissolution and Aggregation of a Poly(NIPA-block-sulfobetaine) Copolymer in Water and Saline Aqueous Solutions Janne Virtanen,† Michel Arotc¸ are´na,‡ Bettina Heise,‡ Sultana Ishaya,‡ Andre´ Laschewsky,§,| and Heikki Tenhu*,† Laboratory of Polymer Chemistry, University of Helsinki, PB 55, FIN-00014 HY, Finland, Universite´ Catholique de Louvain, De´ partment de Chimie, Place L.Pasteur 1, B-1348 Louvain-la-Neuve, Belgium, Fraunhofer Institut fu¨ r Angewandte Polymerforschung, Geiselbergstr. 69, D-14476 Golm, Germany, and Universita¨ t Potsdam, P.O. Box 60 15 53, D-14415 Potsdam, Germany Received December 17, 2001. In Final Form: May 13, 2002 Thermal properties of the novel, double thermosensitive block copolymer, poly(N-isopropyl acrylamide)block-poly(3-[N-(3-methacrylamido-propyl)-N,N-dimethyl]-ammonio propane sulfonate) (PNIPA-b-PSPP) have been studied in pure and saline (NaCl) aqueous solutions by dynamic laser light scattering. The block copolymer [Mn(PNIPA) ) 10 800 g/mol and Mn(PSPP) ) 9700 g/mol] exhibits both upper (UCST about 9 °C) and lower (LCST about 32 °C) critical solution temperatures in pure water. The addition of NaCl enhances the solubility of the zwitterionic block, PSPP, leading to the disappearance of the UCST. On the other hand, the solubility of PNIPA in water decreases as NaCl is added. At 20 °C, the copolymer shows a bimodal size distribution through the NaCl concentration range of 0-0.93 M above a certain limiting polymer concentration. The slow and fast components of the diffusion coefficients of the polymer have been calculated. A gradual addition of salt turns the mutual interactions from zwitterionic attractions between PSPP blocks to hydrophobic attractions between PNIPA blocks. The formation of the aggregates and the aggregate sizes at T < UCST and T > LCST are influenced by polymer and salt concentrations. Below the UCST, the aggregates in saline polymer solutions are somewhat larger than those in pure polymer solutions. Above LCST, the aggregate size is determined by the salt concentration.
Introduction Water-soluble amphiphilic diblock copolymers are capable of forming micelles in water.1-3 Micelle formation in water occurs spontaneously when the one of the blocks is hydrophobic. In this case, the hydrophilic block forms a shell preventing the aggregates of hydrophobic chains from precipitating. When a block copolymer consists of two hydrophilic blocks, micelle formation can be induced by different chemical and/or physical stimuli.4-10 Depending on the solvent quality, micelles with either a hydrophilic or a hydrophobic core may build up. Because of the interplay of various interaction forces between the blocks and the solvent, it is possible to switch from a macromolecular self-assembly to another. * To whom correspondence should be addressed. Tel: +358 9 191 50334. Fax: +358 9 191 50330. E-mail: Heikki.Tenhu@ Helsinki.fi. † University of Helsinki. ‡ Universite ´ Catholique de Louvain. § Fraunhofer Institut fu ¨ r Angewandte Polymerforschung. | Universita ¨ t Potsdam. (1) Tuzar, Z.; Kratochvil, P. In Surface and Colloid Chemistry; Matijevic, E., Ed.; Plenum Press: New York, 1993; Vol. 15, p 1. (2) Gao, Z.; Varshney, S. K.; Wong, S.; Eisenberg, A. Macromolecules 1994, 27, 7923. (3) Bo, G.; Wessle´n, B.; Wessle´n, K. B. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 1799. (4) Virtanen, J.; Baron, C.; Tenhu, H. Macromolecules 2000, 33, 336. (5) Virtanen, J.; Tenhu, H. Macromolecules 2000, 33, 5970. (6) Virtanen, J.; Lemmetyinen, H.; Tenhu, H. Polymer 2001, 42, 9487. (7) Thu¨nemann, A. F.; Beyermann, J.; Kukula, H. Macromolecules 2000, 33, 5906. (8) Gohy, J. F.; Varshney, S. K.; Je´roˆme, R. Macromolecules 2001, 34, 3361. (9) Sedlak, M.; Antonietti, M.; Co¨lfen, H. Macromol. Chem. Phys. 1998, 199, 247. (10) Qi, L. M.; Co¨lfen, H.; Antonietti, M. Angew. Chem., Int. Ed. 2000, 39, 604.
Materials which respond to external stimuli have recently become of major interest. Poly(N-isopropyl acrylamide) (PNIPA) is a water-soluble polymer, which undergoes an abrupt phase transition upon heating at its lower critical solution temperature (LCST) around 32 °C.11,12 The transition is reversible, meaning that when temperature is decreased, PNIPA redissolves in water. The LCST of PNIPA can be affected in various ways. By incorporation of comonomers into the polymer structure, the LCST can be noticeably changed.13,14 The addition of organic molecules and/or electrolytes also has an influence on the phase transition.15,16 Polyampholytes are zwitterionic polymers which possess both positively and negatively charged moieties. Poly(sulfobetaine)s are zwitterionic polymers which are insoluble or sparingly soluble in water, and they exhibit hydrogel characteristics. Their insolubility in water emerges from polyelectrolyte complexes which build up owing to ionic cross-links between two opposing charges in the polyions.17 However, the water solubility of poly(sulfobetaine) is achieved by the addition of a simple salt; this phenomenon is not observed with usual polyelectrolytes. The solvating power of the electrolyte is due to the extent of the site-binding ability of the cation and the anion.18 As a salt (e.g., KCl) is added to an aqueous solution (11) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163. (12) Vakkalanka, S. K.; Peppas, N. A. Polym. Bull. 1995, 36, 221. (13) Lu, T.; Vesterinen, E.; Tenhu, H. Polymer 1998, 39, 641. (14) Lowe, T. L.; Benhaddou, M.; Tenhu, H. Macromol. Chem. Phys. 1999, 200, 51. (15) Lowe, T. L.; Tenhu, H. Macromolecules 1998, 31, 1590. (16) Lowe, T. L.; Virtanen, J.; Tenhu, H. Polymer 1999, 40, 2595. (17) Chang, H. J. Pet. Technol. 1978, 1113. (18) Liaw, D.; Lee, W.; Whung, Y.; Lin, M. J. Appl. Polym. Sci. 1987, 34, 999.
10.1021/la0118208 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/19/2002
Poly(NIPA-block-sulfobetaine) Copolymer Chart 1
of the polybetaine, Cl- binds to the quaternary ammonium group to a greater degree than K+ to the sulfonate group. The overall result is that the poly(sulfobetaine) begins to behave increasingly as a polyanion while extending the chain conformation. Poly(3-[N-(3-methacrylamido-propyl)N,N-dimethyl]-ammonio propane sulfonate), PSPP, exhibits, like other zwitterionic polymers, an upper critical solution temperature (UCST) in water that increases with the molar mass. This is attributed to the strong mutual intermolecular attraction of the zwitterionic betaine groups.19-21 In contrast to the abrupt collapse of PNIPA, the phase separation of PSPP occurs gradually. Recently, Laschewsky et al.21 have synthesized PNIPAb-PSPP block copolymers employing the RAFT polymerization technique.22 They studied the thermal behavior of these polymers in pure water by 1H NMR spectroscopy, viscometry, UV spectroscopy, and fluorescence spectroscopy. The block copolymers retained their water solubility in a temperature range from 0 to 100 °C. Above the LCST of PNIPA, the block copolymer formed colloidal aggregates of collapsed PNIPA solubilized by a PSPP shell. On the other hand, when cooled below the UCST of PSPP, the block copolymer formed reverse aggregates solubilized by a PNIPA shell. The authors showed, by fluorescence measurements, that microdomains in the aggregates were rather polar below UCST, whereas above the LCST microdomains were nonpolar. In this paper, we have studied the behavior of the PNIPA-b-PSPP block copolymer in aqueous solutions by dynamic laser light scattering (DLS). The chemical structure of the polymer is shown in Chart 1. The lengths of the chemically different blocks are Mn(PNIPA) ) 10 800 g/mol and Mn(PSPP) ) 9700 g/mol. In pure water, the UCST is about 9 °C and the LCST is about 32 °C. Such polymeric structures consisting of PNIPA have been suggested for applications in drug delivery systems owing to their capability of collapsing and redissolving in aqueous solutions as a function of temperature.23-26 The purpose of this study was to find out the details and the mechanism of aggregation of the recently synthesized new zwitterionic block copolymers.21 Experimental Section Copolymer Synthesis. The copolymer PNIPA-b-PSPP was prepared by the RAFT polymerization technique. The details of the synthesis are published separately.21 The average content of SPP in the copolymer was determined by comparing the integrals (19) Huglin, M. B.; Radwan, M. A. Polym. Int. 1991, 26, 97. (20) Ko¨berle, P.; Laschewsky, A.; Lomax, T. D. Makromol. Chem., Rapid Commun. 1991, 12, 427. (21) Arotc¸ arena, M.; Heise, B.; Ishaya, S.; Laschewsky, A. J. Am. Chem. Soc. 2002, 124, 3787. (22) Le, T. P.; Moad, G.; Rizzardo, E.; Thang, S. H. PCT Int. Appl. WO 98/01478, 1998. (23) Hoffman, A. S. Macromol. Symp. 1995, 98, 645. (24) Uhrich, K. E.; Cannizzaro, S. M.; Langer, R. S.; Shakesheff, K. M. Chem. Rev. 1999, 99, 3181. (25) Chung, J. E.; Yokoyama, M.; Yamato, M.; Aoyagi, T.; Sakurai, Y.; Okano, T. J. Controlled Release 1999, 62, 115. (26) Yuk, S. H.; Bae, Y. H. Crit. Rev. Ther. Drug Carrier Syst. 1999, 16, 385.
Langmuir, Vol. 18, No. 14, 2002 5361 of the signals in the 1H NMR spectra that are characteristic for PNIPA with those which are characteristic for PSPP. Size exclusion chromatography (SEC) results confirm the conclusions by Laschewsky et al.: Mn was 19 000 g/mol, and polydispersity was 1.35. No sign of homopolymers was observed. SEC measurement was conducted with Waters equipment using a Waters 2410 refractive index detector. The eluent was 0.1 M aqueous NaNO3. Poly(ethylene oxide) (PEO) standards (Polymer Laboratories) were used for the calibration. Light Scattering. DLS measurements were conducted with a Brookhaven Instruments BI-200SM goniometer and a BI9000AT digital correlator. The light source was a Lexel 85 Argon laser (514.5 nm, power range of 15-150 mW). Time correlation functions were analyzed with a Laplace inversion program (CONTIN), with which it is possible to calculate the fast and slow diffusion coefficients from a bimodal sample. The range of polymer concentrations was 0.01-2.0 g/L, and the concentration of NaCl varied from 5 × 10-4 to 0.5 M. Experiments were carried out in a temperature range from 4 to 45 °C, at the scattering angle of 90°. The samples were both cooled and heated slowly and rapidly from ambient temperature to 4-6 °C and 29-45 °C. The slow heating/cooling rate was on average 0.1 °C/min. At each temperature, the solutions were stabilized for 20-90 min. In the fast cooling/heating, the samples were placed into the goniometer where the temperature was 4 or 45 °C. The samples were equilibrated for 20-25 min at 4 °C and for at least 1-2 h at 45 °C. Preparation of Pure and Saline Polymer Solutions. All the solutions were filtered through Millipore 0.45 µm membranes before mixing. A known amount of aqueous NaCl solution was added to a pure aqueous polymer solution in order to obtain a desired salt concentration. While adjusting the salt concentration, the polymer concentration was kept constant by adding an appropriate amount of concentrated polymer solution.
Results and Discussion Influence of NaCl at 20 °C. The solubility of the zwitterionic polymer PSPP in water increases with increasing temperature. Also, the solubility of zwitterionic poly(sulfobetaine)s is promoted by the addition of salt in a wide range of salt concentrations.27,28 The improved solubility is reflected in an increase of the hydrodynamic volume of PSPP when the conformation of PSPP changes from a more or less compact coil to an extended one,18 a phenomenon commonly termed the anti-polyelectrolyte effect. In contrast, the solubility of PNIPA increases when the temperature is lowered and decreases when salts are added.29,30 According to the study of Hoffman and Park,29 the LCST of linear and cross-linked PNIPA is lowered from 32 to 22 °C when the concentration of NaCl increases from 0.1 to 1.0 M, respectively. The critical NaCl concentration (Cs,crit) needed to induce phase separation of PNIPA in aqueous solutions increases linearly with decreasing temperature. A temperature of T ) 20 °C corresponds to Cs,crit ∼ 1.2 M, and T ) 4 °C corresponds to Cs,crit ) 2.5 M. Dhara and Chatterji30 found that the critical chloride concentration for the phase separation of PNIPA at 24-25 °C was about 0.8 M. Following the discussion above, it is clear that PSPP and PNIPA behave oppositely, and therefore the block copolymer PNIPA-b-PSPP senses opposite effects when NaCl is added. With increasing salt concentration, the quality of the aqueous solvent becomes better for PSPP but poorer for PNIPA. The effect of salt on the solubility of PSPP is best observed at low salt concentrations, and that on PNIPA at high salt concentrations. (27) Kathmann, E. E. L.; White, L. A.; McCormick, C. L. Macromolecules 1997, 30, 5297. (28) Xue, W.; Champ, M.; Huglin, M. B. Eur. Polym. J. 2001, 37, 869. (29) Park, T. G.; Hoffman, A. S. Macromolecules 1993, 26, 5045. (30) Dhara, D.; Chatterji, P. R. Polymer 2000, 41, 6133.
5362
Langmuir, Vol. 18, No. 14, 2002
Figure 1. Intensity of scattered light from saline polymer solutions with varying NaCl concentrations, Cs, at 20 °C. The polymer concentration is 2.0 g/L.
Figure 1 shows the variation in the scattered light intensity of the polymer solution with Cp ) 2.0 g/L as a function of NaCl concentration at room temperature. A pure aqueous polymer solution was slightly opaque, but with increasing NaCl concentration the solution became completely clear to the eye. No change in the intensity of the scattered light was observed at this stage, probably because while the polymer dissolves the size of the scattering entities grows and these two processes have opposing effects on the scattering. At around Cs ) 0.73 M, the intensity of the scattered light started to increase gradually, and at Cs ∼ 0.93 M, the intensity increased rapidly indicating the phase separation and aggregate formation of PNIPA blocks. The colloidal aggregates were electrostatically stabilized by PSPP having polyanion character. At 20 °C, the size distribution of the salt-free polymer solution with Cp ) 2.0 g/L was bimodal. The distribution remained unchanged throughout the whole studied NaCl concentration range. However, at Cs ) 0.96 M, the two maxima in the size distributions became narrower. From the bimodal distributions, slow and fast components of the diffusion coefficients have been calculated, and they are shown as functions of salt concentration in Figure 2. The slow diffusion is a result of clustering of polymers. The mechanism of the cluster formation may be expected to change upon the addition of salt. In a salt-free solution, large temporary clusters build up due to ionic interactions, whereas in the high salt concentration regime the clustering results from the salting out of PNIPA chains, that is, it is of a hydrophobic nature. Because the chains are more or less bound to each other, the fast diffusion is assumed to be that of loops or mobile chain ends. As is seen in Figure 2, the function describing the slow diffusion coefficient, Ds, against salt concentration is far from being continuous. Two continuous regions may be distinguished. At low salt concentrations (Cs < 0.3 M), Ds decreases linearly with increasing Cs. With 0.5 < Cs < 0.9 M, Ds has a constant value. The salt concentration region where the sudden change in Ds takes place (between 0.3 and 0.5 M) is expected to be the region where the principal mechanism of clustering changes from ionic to hydrophobic. The coefficient of the fast diffusion changes more gradually, although the suggested change in the mechanism of clustering has an effect on this as well: with low salt concentration the PNIPA blocks diffuse freely, whereas with increasing salt concentration their motional freedom
Virtanen et al.
Figure 2. Slow diffusion coefficients, Ds (open squares), and fast diffusion coefficients, Df (filled squares), at different NaCl concentrations at 20 °C. The polymer concentration is 2.0 g/L. Lines are drawn to guide the eye. Vertical dotted lines show the values of n(NaCl)/n(SPP).
decreases substantially. In the figure, the salt concentrations have been indicated where the molar ratio of NaCl to the repeating units of SPP, n(NaCl)/n(SPP), gets the values 31 and 155. In Figure 2, both the Df and Ds values are seen to decrease abruptly in a high salt concentration limit, Cs ∼ 0.93 M, where the polymer starts to precipitate. At this Cs, the LCST of PNIPA is 20 °C. At 20 °C, the size distributions of the more concentrated polymer samples were bimodal regardless of the salt concentration, as mentioned above. However, for a more dilute solution with Cp ) 0.2 g/L, the size distributions were monomodal at 20 °C and broadened when salt was added. Due to the effect of salt on the clustering of the copolymer at room temperature, the block copolymer is not in a molecularly dissolved, unimeric state at high polymer concentration. This may be the case also at low polymer concentration, and because the polymers at room temperature are not unimers the aggregation starts above and below the critical temperatures by clustering of small aggregates. Slow Heating and Cooling of Polymer Solutions. A pure aqueous polymer solution and two different saline polymer solutions were slowly heated and cooled. Polymer concentrations were 0.2 and 2.0 g/L. In both saline polymer solutions, the molar ratio of NaCl to the repeating units of SPP was kept constant; n(NaCl)/n(SPP) ) 31 corresponds to Cs ) 0.1 and 0.01 M and n(NaCl)/n(SPP) ) 155 corresponds to Cs ) 0.5 and 0.05 M when Cp ) 2.0 and 0.2 g/L, respectively. Figure 3 shows the intensity of the scattered light as a function of temperature from a pure polymer solution with Cp ) 0.2 g/L. Upon slow cooling, the UCST behavior was observed only with the pure aqueous solution. This is explained as follows. In the course of the addition of NaCl to the solution, the solubility of PSPP increases due to increased solvent-polymer interactions and polyanion characteristics. Therefore, the intermolecular attraction between zwitterionic groups gradually disappears, and no phase separation is observed. However, the phase transition occurred at 16 and 17 °C for aqueous copolymer solutions with polymer concentrations of 0.2 and 2.0 g/L, respectively. Figure 4 shows the size distributions of the copolymer with Cp ) 0.2 g/L in saltless aqueous solution at 6, 20, and 35 °C. At 4-6 °C, the size distributions of the pure polymer
Poly(NIPA-block-sulfobetaine) Copolymer
Figure 3. Intensity of scattered light from aqueous polymer solutions with a polymer concentration of 0.2 g/L during slow heating and cooling.
Figure 4. Size distribution of the copolymer in a saltless aqueous solution at 6, 20, and 35 °C. The polymer concentration is 0.2 g/L.
solutions were monomodal with Rh ) 916 nm (Cp ) 2.0 g/L) and 275 nm (Cp ) 0.2 g/L). When the solutions were heated back to room temperature, the aggregates broke down relatively fast, but still somewhat slower than they built up (see Figure 3). The LCST of the polymer changed with both polymer and NaCl concentrations. The collapse of the polymer coil was very distinct in all solutions, however, slightly more gradual as the polymer concentration decreased. The cloud points of aqueous solutions with a copolymer concentration of Cp ) 2.0 g/L were 33, 31, and 26 °C for a salt-free solution, for Cs ) 0.1 M and for Cs ) 0.5 M, respectively. For more dilute solutions with Cp ) 0.2 g/L, the cloud points were 34, 34, and 33 °C for a salt-free sample, for Cs ) 0.01 M and for Cs ) 0.05 M, respectively (see Figure 5). The cloud point of the copolymer decreases as the NaCl concentration increases (see Figure 6). This is in good agreement with the results reported by Hoffman and Park.29 Ten-fold dilution of the polymer solution increased the LCST only 1 °C, and therefore it can be concluded that the LCST depends critically not on the polymer concentration but on the salt concentration. As the temperature increases, the polymer coil collapses and forms aggregates with a narrow size distribution, both in pure aqueous and in saline solutions. Figure 7 shows the average hydrodynamic radii of the polymer for the concentration of Cp ) 0.2 g/L at 35 and 40 °C in dependence on the amount of added salt. The graph illustrates that
Langmuir, Vol. 18, No. 14, 2002 5363
Figure 5. Intensity of scattered light from aqueous polymer solutions during slow heating. Cp ) 2.0 g/L (a, b, c) and 0.2 g/L (a′, b′, c′). n(NaCl)/n(SPP) ) 0 (a and a′), 31 (b and b′), and 155 (c and c′).
Figure 6. Phase transition temperatures of two copolymer solutions with different polymer concentrations as a function of NaCl concentration: Cp ) 0.2 g/L (a and a′) and 2.0 g/L (b and b′). Slopes of the UCSTs drawn in the figure are estimates.
the addition of a small amount of NaCl (n(NaCl)/n(SPP) ) 31) results in smaller aggregates than those observed in salt-free aqueous solutions. However, with increasing concentration of NaCl (n(NaCl)/n(SPP) ) 155), the aggregate sizes increase again. Furthermore, at 40 °C in a pure polymer solution the aggregates are larger than those in a solution with n(NaCl)/n(SPP) ) 155. This is not the case at 35 °C. These observations demonstrate the competition between different interactions in the solutions and may well be understood in terms of the suggested change in the mechanism of aggregation with changing salt concentration. The aggregates dissolved only slowly when the temperature was brought back to room temperature. Similar hysteresis in the heating and cooling cycles was observed in earlier studies on the present polymer as well as on PNIPA in pure water.21,31 The effect was explained by the formation of associative intrachain structures.31 Fast Cooling and Heating of Polymer Solutions. Aqueous polymer solutions with polymer concentrations varying from 0.01 to 0.2 g/L were cooled rapidly to 4 °C. These studies were performed on pure aqueous solutions as well as on saline ones. In the latter case, the ratio (31) Wu, C.; Wang, X. Phys. Rev. Lett. 1998, 80, 4092.
5364
Langmuir, Vol. 18, No. 14, 2002
Figure 7. Hydrodynamic radii, Rh, of aggregates formed during slow heating at 35 and 40 °C. The polymer concentration is 0.2 g/L.
Figure 8. Size distributions of aggregates in saltless aqueous polymer solutions after fast cooling to 4 °C. Cp ) 0.01 g/L (a), 0.05 g/L (b), 0.1 g/L (c), and 0.2 g/L (d).
n(NaCl)/n(SPP) was kept constant (31 or 155) within a given series of experiments. The hydrodynamic radii Rh were measured after the period of time (20-25 min) which was needed to reach a constant intensity of the scattered light. For pure aqueous polymer solutions as well as for the solutions with higher salt concentrations (n(NaCl)/n(SPP) ) 155), the behavior of the polymer did not depend on the cooling rate. However, at intermediate salt concentrations (n(NaCl)/n(SPP) ) 31), fast cooling led to the formation of aggregates, although aggregates were not formed when cooling these solutions slowly. This peculiar behavior implies that the occurrence of the phase separation is kinetically controlled in solutions with low salt concentration where zwitterions as well as anionic groups coexist. This suggests that the aggregates observed are metastable, though this has not yet been experimentally proven. Again, the mechanism of aggregation in the samples with different salt contents is suggested to be different. As illustrated in Figures 8 and 9, the size distributions of the aggregates were rather broad. In all cases, the size of the aggregates decreases with decreasing polymer concentration, though the effect is more distinct for saline polymer solutions than for pure aqueous ones.
Virtanen et al.
Figure 9. Size distributions of aggregates in saline aqueous polymer solutions after fast cooling to 4 °C. Cp ) 0.05 g/L (a), 0.1 g/L (b), and 0.2 g/L (c). In all solutions, n(NaCl)/n(SPP) ) 31.
Figure 10. Hydrodynamic radii, Rh, of the aggregates formed during fast heating at 45 °C. n(NaCl)/n(SPP) equals 0 (a), 31 (b), and 155 (c).
For any given polymer solution, the size distributions of the aggregates formed in moderately saline polymer solutions (n(NaCl)/n(SPP) ) 31) at 4 °C were broader than in pure water. This may be explained as follows. At the cloud point, the copolymers form fairly large core-shelllike aggregates. The core consists mainly of PSPP and perhaps of some PNIPA, and the shell consists of pure PNIPA. The added salt enhances the solubility of PSPP and thus leads to a looser packing of the aggregate core, broadening the distribution of aggregate sizes. This result is well in accordance with the recent fluorescence studies on PNIPA-b-PSPP.21 Below the UCST, the core of the aggregate mainly consisting of PSPP blocks is polar and contains a relatively high amount of water. This prevents the compression of the core resulting in the widening of the size distribution. This hypothesis is supported by the observation that the intensity of light scattered from the aggregates is much lower from saline polymer solutions than from the pure aqueous solutions. The normalized intensity ratio at equilibrium (It - I0)/I0 was 3-12 for saline and 8-53 for a pure aqueous solution. Here, It is the scattering intensity from a sample equilibrated at 4 °C and I0 is the intensity before cooling. A similar
Poly(NIPA-block-sulfobetaine) Copolymer
Langmuir, Vol. 18, No. 14, 2002 5365
Figure 11. Size distributions of aggregates in saline aqueous polymer solutions after fast heating to 45 °C. Cp ) 0.01 g/L (a), 0.05 g/L (b), 0.1 g/L (c), and 0.2 g/L (d). In all solutions, n(NaCl)/ n(SPP) ) 31.
solubilizing effect has also been observed with PNIPAgraft-PEO copolymers with an increasing number of PEO grafts in the copolymer.5 The solutions were also quickly heated to 45 °C, where they were annealed at least for 2 h. A common feature of the pure and saline polymer solutions was the decrease of the hydrodynamic radius with decreasing polymer concentration (see Figures 10 and 11). This result, as well as the decrease of the aggregate size with decreasing polymer concentration during the fast cooling, can be explained by the competition between intramolecular and intermolecular interactions of the polymer chains: as the number of polymer chains in the solution decreases, the intramolecular interactions dominate, thus resulting in smaller aggregates.5,32 The difference between the sizes of aggregates in different solutions is very similar to that observed during slow heating. The polymer in pure water forms the largest aggregates at every concentration. The addition of only a small amount of NaCl, namely, n(NaCl)/ n(SPP) ) 31, leads to the smallest aggregates. The aggregate size increases as the concentration of NaCl further increases. In the experiments illustrated in Figures 10 and 11 and discussed above, the ratio salt/polymer was kept constant. Figure 12 illustrates the situation when the polymer concentration was constant and the salt concentration was varied. Polymer solutions (Cp ) 0.1 g/L) with varying NaCl contents (0-0.889 M) were heated fast to 45 °C. The aggregate size first increased swiftly with increasing NaCl concentration but reached a virtually constant value above 0.15 M of NaCl (see Figure 12). In this sample, 0.15 M (32) Qiu, X.; Wu, C. Macromolecules 1997, 30, 7921.
Figure 12. Hydrodynamic radii, Rh, of the aggregates in aqueous polymer solutions with varying NaCl concentrations after fast heating to 45 °C. The polymer concentration is 0.1 g/L.
NaCl corresponds to n(NaCl)/n(SPP) ) 932. This indicates that the measurements described in this report have been conducted in a relevant salt concentration region, where the aggregation process is strongly dependent on the added salt. Conclusions The upper and lower critical temperatures of the aqueous solutions of the poly(NIPA-b-SPP) block copolymer are observed as changes in the size distributions of the samples. The polymer is soluble in pure water in the whole studied temperature range but forms aggregates at T < UCST and T > LCST. The aggregation processes are strongly dependent not only on temperature but also on both the salt and polymer concentrations. The important observation was that at 20 °C the coefficient of the slow diffusion, Ds, of the polymer changes discontinuously with increasing salt concentration, whereas the coefficient of the fast diffusion changes more progressively. The salt concentration regime where Ds shows the discontinuity is assumed to be the regime where the mechanism of temporary clustering of the macromolecules changes from an ionic into a hydrophobic one. Acknowledgment. The work was supported by the Finnish Technology Agency (TEKES) and by the Belgian State (Services du Premier Ministre - Services Fe´de´raux des Affaires Scientifiques, Program PAI4/11). LA0118208
