Langmuir 2005, 21, 3229-3231
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Reentrant Morphological Transitions in Copolymer Micelles with pH-Sensitive Corona O. V. Borisov* and E. B. Zhulina Institute of Macromolecular Compounds of the Russian Academy of Sciences, 199004 St. Petersburg, Russia, and DRFMC/SI3M, CEA-Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France Received December 15, 2004. In Final Form: February 26, 2005 In contrast to self-assembled aggregates of conventional ionic (including polymeric) surfactants the equilibrium micelles of diblock copolymer with a pH-sensitive polyelectrolyte block can exhibit two inverse sequences of morphological transitions triggered by an increase in solution salinity. The direct sequence of the sphere-cylinder-lamella transitions is similar to that for the copolymer with a strongly dissociating ionic block and occurs at a high salt concentration in solution. The abnormal reversed sequence of the lamella-cylinder-sphere transitions is predicted to occur at relatively low ionic strength in solution. The origin of the reentrant transitions is coupling between aggregation and ionization in copolymer micelles.
Amphiphilic block copolymers self-assemble into micelles and mesophases of different morphologies in selective solvents.1,2 This capacity of polymeric amphiphiles finds diverse industrial applications in the controlled encapsulation, delivery, and release of drugs, agrochemicals, food additives, and so forth. Design of the systems that can undergo the controlled structural transformations (change in shape, size, aggregation state, etc.) in response to external stimuli is, therefore, not only a fundamental but also an important technological problem. The structure of the equilibrium aggregates formed by amphiphilic copolymers in selective solvents is determined by the balance of the attractive forces driving the aggregation (e.g., the attraction between hydrophobic blocks) and the repulsive forces (e.g., the Coulomb repulsion between ionic blocks) limiting the aggregation. By tuning the strength of these interactions one can affect the structure and even the morphology of aggregates. Aqueous systems offer the best opportunity to tune the strength of the Coulomb repulsion by variations in the solution salinity. For polymeric amphiphiles with strongly dissociating ionic blocks an increase in the solution salinity leads to the same sequence of sphere-cylinder-lamella morphological transitions as for conventional nonpolymeric ionic surfactants.2 As we discuss in this letter, the response of the aggregates formed by copolymers with weakly dissociating (pH-sensitive) ionic blocks to variations in the concentration of added salt is dramatically different. We focus on micellar aggregates formed by flexible diblock copolymer with a hydrophobic block B and a pHsensitive ionic block A. In dilute aqueous solution of concentrations exceeding the critical micelle concentration such copolymer gives rise to micelles with the dense core of radius R comprised by hydrophobic B blocks and the hydrophilic charged corona of radius D comprised by A blocks. Below we express R and D in units of monomer size a which is assumed to be the same for both blocks. For pH-sensitive (weakly dissociating) polyelectrolyte (1) Halperin, A. In Polymeric vs. Monomeric Amphiphiles: Design Parameters; Ciferri, A., Ed.; Supramolecular Polymers; Marcel Dekker: New York, 2000. (2) Fo¨rster, S.; Abetz, V.; Mu¨ller, A. H. E. Adv. Polym. Sci. 2004, 166, 173-210
(such as, e.g., poly(acrylic acid)), the degree of corona ionization can be tuned by variations in both the pH and the solution salinity. Importantly, the ionization of the pH-sensitive block is coupled to the polymer density in the corona and, therefore, to the number of copolymer molecules associated in the micelle.3 This ionization/ aggregation coupling is a direct manifestation of the annealing polyelectrolyte nature of a pH-sensitive corona block. Strongly dissociating ionic polymer (e.g., sulfonated polystyrene) does not exhibit this feature.2,4-7 Equilibrium characteristics of a micelle with morphology i (where i ) 1, 2, and 3 corresponds to lamella or cylindrical or spherical micelle, respectively) are determined by the (i) , excess free balance of the corona free energy F corona energy of the core/corona interface Fsurface ) γs(R) (γ is the interfacial free energy per unit area), and free energy of 2 2 2 2 the core9 F (i) core ) biR /NB (b1 ) π /8, b2 ) π /16, b3 ) 3π / 80), where NB is the degree of polymerization of the core block. We describe the corona of a micelle in the framework of mean-field theory of annealing polyelectrolyte brushes8 and assume additivity of the electrostatic and the excluded volume monomer-monomer interactions. We also use the local electroneutrality approximation (LEA) which implies that the electrostatic screening length inside the corona is much smaller than D and that excess local charge in the corona is almost 0. The LEA together with the Donnan equilibrium for small ions determines the local pH in the corona that is always smaller than the pH in the bulk solution. As a result, the degree of ionization of an acidic monomer R(r) in the corona (at distance r e D from the center of micelle) is less than its bulk value Rb (specified by the pH and the pK as Rb/(1 - Rb) ) 10pH-pK). The degree of ionization R(r) depends also on polymer density cp(r) in the corona and on the total concentration of monovalent ions Φion in the solution. (3) Zhulina, E. B.; Borisov, O. V. Macromolecules 2002, 35, 9191. (4) Marko, J. F.; Rabin, Y. Macromolecules 1992, 25, 1503. (5) Wittmer, J.; Joanny, J.-F. Macromolecules 1993, 26, 2691. (6) Shusharina, N. P.; Nyrkova, I. A.; Khokhlov, A. R. Macromolecules 1996, 29, 3167. (7) Borisov, O. V.; Zhulina, E. B. Macromolecules 2002, 35, 4472. (8) Zhulina, E. B.; Birshtein, T. M.; Borisov, O. V. Macromolecules 1995, 28, 1491. (9) Semenov, A. N. Sov. Phys. JETP 1985, 61, 733.
10.1021/la0469203 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/15/2005
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The free energy f{cp(r)} per unit volume of the corona accounts for the nonuniform local stretching of blocks A, the nonelectrostatic monomer-monomer interactions with the second virial coefficient vA > 0, the difference in the osmotic pressure of ions inside and outside of the corona, and the mixing entropy of charged and neutral monomers along the chain.3 As long as dilute solutions are considered, the effects arising from interactions between the aggregates can be neglected. The polymer density profile cp(r) is determined by (i) ) minimization of the corona free energy, F corona ∫Vf{cp(r)} dV, where V is the volume of the corona. As a (i) as a function result we find the corona free energy F corona of area per chain at the core/corona interface s(R), degree of polymerization of the corona block NA, second virial coefficient vA, degree of ionization Rb, and ion concentration Φion in the bulk solution. Depending on degree of ionization of the polyelectrolyte block and extent of screening of Coulomb interactions in the corona we specify the following states of the corona: In the annealing osmotic state (osm), when Rbcp(r)/Φion . 1 the degree of ionization R(r) is strongly coupled to cp(r). Here, the corona is swollen by the excess osmotic pressure of the counterions. As a result of the smaller local pH in the corona the ionization is suppressed,8 and R(r) ≈ {[Rb/(1 - Rb)][Φion/2cp(r)]}1/2 , Rb. Here, an increase in salt concentration Φion at fixed pH leads to redistribution of ions in the corona and promotes ionization of the coronal chains. As a result, the aggregation number decreases whereas the corona thickness D increases. In the salt dominance state (salt), Rbcp(r)/Φion , 1, the concentrations of ions inside the corona are almost the same as in the bulk solution. Here, R(r) and cp(r) are decoupled, and the degree of ionization approaches its maximal value, R ≈ Rb. An increase in Φion leads to progressive screening of the electrostatic repulsion between coronal blocks, and the aggregation number increases whereas the corona thickness D weakly decreases. In the quasi-neutral state (qn), the coronal block is almost nonionized (at low salt) or strongly ionized, but the Coulomb repulsion is strongly screened (at high salt). In both limits the micelle behaves as a neutral copolymer aggregate, that is, interaction between the coronal chains is dominated by the excluded volume repulsion. Morphological transitions can be triggered by the external stimuli (e.g., by variations in salinity of the solution) when micelles have the so-called crew-cut shape10 with corona thickness H ) D - R , R. Under these (i) can be expanded with respect to core conditions, F corona curvature with the accuracy of linear in-curvature terms. In a crew-cut micelle the corona is almost planar, and the (i) area per chain s(R) is determined by the balance of F corona (1) (1) ≈ F corona and Fsurface. Here, F corona is the free energy of the planar brush with area s(R) per chain. Therefore, equilibrium area per chain s(R) and the leading contribution (1) to the free energy per chain, F (i) ≈ F corona + Fsurface, are almost the same for all morphologies i. The core contribu(1) tion F (i) core and the corrections to F corona due to the core (1) = Fsurface. curvature are small with respect to F corona However, these small contributions determine the transition lines (binodals) F (i) ) F (i(1) that separate regions of thermodynamic stability of the crew-cut micelles with morphology i and i ( 1. The transitions between morphologies occur as the first-order phase transitions, with the possibility of existence of metastable states. (10) Borisov, O. V.; Zhulina, E. B. Macromolecules 2003, 36, 10029.
Letters
Figure 1. Schematic presentation of the “reverse” and “direct” morphological lamella-cylinder-sphere transitions induced by an increase in salinity in the solution of aggregates of diblock copolymers with pH-sensitive polyelectrolyte blocks.
In the annealing osmotic limit, the binodals are given by
)
(
RbΦion 1 - Rb
=
ifi+1
NB2/9γ4/3 1/9 βi NA14/9
i ) 1, 2
(1)
where βi ) {i(i + 1)[(i + 1)2bi+1 - i2bi]}, whereas in the salt dominance limit the binodals are specified as
( ) Rb2 2Φion
i+1fi
=
NB10/9 2/3 5/9 γ βi NA16/9
i ) 1, 2
(2)
The latter equation is similar to that found for strongly dissociating block polyelectrolytes10 at high ionic strengths in the solution. It indicates that transitions from morphology i + 1 to the morphology i, i ) 1, 2 occur upon an increase in the solution salinity, Φion. On the contrary, as follows from eq 1, a sequence of salt-induced morphological transitions in the low salt regime is the opposite, that is, an increase in Φion leads to transition from morphology i to morphology i + 1, i ) 1, 2. Only copolymer with relatively short polyelectrolyte block, NA , NB5/8γ3/8vA-9/16, can form thermodynamically stable spherical (S), cylindrical (C), and lamellar (L) crewcut micelles with corona thickness H , R. As a resul tof the van der Waals attraction forces, lamellar sheets associate together, and copolymer solution segregates into a lamellar mesophase and water.11 Solutions with cylindrical and spherical aggregates are expected to remain homogeneous. Upon an increase in the solution salinity, micelles with pH-sensitive corona first undergo the “reversed” L-C-S transitions at low salinity (when micellar coronae are found in the osmotic annealing regime) and then “direct” S-C-L transitions at high salinity in the solution (under the conditions of salt dominance in the coronae). The two sequences of salt-induced morphological transitions are schematically depicted in Figure 1. This behavior is dramatically different from that of copolymer with strongly dissociating polyelectrolyte block. The latter can undergo only the “direct” S-C-L transitions at high salinity in the solution. Variations in the ionic strength in the bulk solution lead to the nonmonotonic evolution of all micelle characteristics accompanied by abrupt jumps at the transition lines. Figure 2 demonstrates schematically the (11) Bendejacq, D.; Ponsinet, V.; Joanicot, M. Eur. Phys. J. E 2004, 13, 3.
Letters
Figure 2. Reduced core radius R/NB versus salt concentration Φion in double logarithmic coordinates. The slopes are indicated.
dependence of reduced size R/NB of hydrophobic domain B on the salt concentration Φion. Nonmonotonic evolution of micelle with pH-sensitive corona is governed by the competition of two effects: promotion of corona ionization and screening of the electrostatic interactions due to added salt. The former is dominant at low solution salinity, whereas the latter becomes important at high solution salinity. As a result, the strength of repulsive interaction between the coronal chains varies nonmonotonically as a function of salt concentration Φion and reaches its maximum in the intermediate salinity range. Here, the micelle characteristics exhibit extrema: the core radius R and aggregation number reach their minimal values, whereas the corona thickness H reaches its maximum. Conclusions. To summarize, we predict that equilibrium micelles formed by diblock copolymer with weakly dissociating, pH-sensitive polyelectrolyte block can exhibit the unusual sequence of L-C-S morphological transitions upon an increase in the ionic strength in solution. This sequence is “reversed” with respect to what was theoretically predicted10,12 and experimentally observed13 for copolymer with strongly dissociating polyelectrolyte block. The reversed succession of morphological transitions is (12) Netz, R. R. Europhys. Lett. 1999, 47, 391.
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expected at low solution salinity, whereas at high salt concentration the aggregation behaviors of diblock copolymer with weakly and strongly dissociating polyelectrolyte blocks are similar. Here, “direct” S-C-L morphological transitions are expected upon an increase in solution salinity. The origin of the predicted effect is competition between screening of the electrostatic repulsion and enhancement of the corona ionization upon an increase in ionic strength in the bulk solution. To the best of our knowledge, the experimental observations on reversed transitions were not reported in the literature. We emphasize that earlier theoretical predictions14,8 on the nonmonotonic swelling of the weakly dissociating polyelectrolyte brush preceded and stimulated the experiments15,16 that confirmed the theoretical findings. We believe that the results reported in this letter would also stimulate new experimental activity in the field of selfassembling polyamphiphiles. Acknowledgment. The authors greatly acknowledge the hospitality of Dr. A. Halperin in CEA-Grenoble. This work has been partially supported by the Russian Foundation for Basic Research (Grant 02-03-33127), by the Dutch National Science Foundation (NWO) program “Computational approaches for multi-scale modelling in self-organizing polymer and lipid systems” No. 047.016.004, and by the EUROCORES program within the project “Higher levels of Self-Assembly of Ionic Amphiphilic Copolymers” (SONS-AMPHI). LA0469203 (13) Kra¨mer, E.; Fo¨rster, S.; Go¨ltner, C.; Antonietti, M. Langmuir 1998, 14, 2027; Fo¨rster, S.; Hermsdorf, N.; Leube, W.; Schnablegger, H.; Regenbrecht, M.; Akari, S.; Lindner, P.; Bo¨ttcher, C. J. Phys. Chem. B 1999, 103, 6652. Regenbrecht, M.; Akari, S.; Fo¨rster, S.; Mo¨hwald, H. J. Phys. Chem. B 1999, 103, 6668. (14) Israels, R.; Leermakers, F. A. M.; Fleer, G. J. Macromolecules 1994, 27, 3087. (15) Guo, X.; Ballauff, M. Phys. Rev. E 2001, 64, 05146. (16) Currie, E. P. K.; Sieval, A. B.; Fleer, G. J.; Cohen Stuart, M. A. Langmuir 2000, 16, 8324.
