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Association Behavior of Poly(methyl methacrylate-block-methacrylic acid) in Aqueous Medium Jia Yao,†,§ Palaniswamy Ravi,† Kam C. Tam,*,† and Leong H. Gan‡ Singapore-MIT Alliance, School of Mechanical and Production Engineering, and Natural Science, National Institute of Education, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 Received August 19, 2003. In Final Form: December 15, 2003 The atom transfer radical polymerization technique was used to synthesize the poly(methyl methacrylateblock-methacrylic acid) (P(MAA-b-MMA)) copolymer in order to study the aggregation behavior in aqueous solution over the course of neutralization. Combinations of static and dynamic light scattering (SLS, DLS) and potentiometric titration techniques were used to investigate the size and shape of the micelle at various degrees of neutralization (R). By comparing the effect of different polymer chain length with similar MMA/MAA ratio on the aggregation behavior during neutralization, we found relatively strong entanglement of long MMA polymer chains. The comparison between the different MMA/MAA ratios showed that longer MMA chains produced more entanglements. Conductometric titration was used to determine the counterion condensation phenomenon during the course of neutralization. At a critical micellar charge density observed at R ∼ 0.4, Na+ ions are condensed on the polymer chains. The amount of condensed Na+ was evaluated by the conductivity change, yielding the condensation ratio when the polymer was completely neutralized.
Introduction Association of block copolymers in selective solvents can produce structures such as micelles and physical networks, which have potential applications ranging from controlled release to rheological modifications.1 With the advances made in the synthesis of block copolymers, specific macromolecular architecture can be created for specific end-use applications.2 By varying the block lengths or adjusting the pH and ionic strength of the solution, one can control the size and the aggregation number of the micelles.3 An understanding of the structure-property relationship of block copolymer micelles is necessary for the development of such systems for different applications, such as in enhanced drug delivery. Recently, much interest is being focused on the theoretical models describing the structure of charged polyelectrolyte micelles.4 A spherical micelle can be thought of as a polymer brush at a curved interface. Theories for both micelles and polymer brushes have been developed for “quenched polyelectrolyte”, where the charges are fixed along the polymer chain, and for “annealed” systems, where the charge distribution is * To whom correspondence may be addressed. E-mail:
[email protected]. † Singapore-MIT Alliance, School of Mechanical and Production Engineering. ‡ Natural Science, National Institute of Education. § Present address: Department of Chemistry, Zhejiang University, P.R. China 310027. E-mail:
[email protected]. (1) (a) Kwon, G. S.; Naito, M.; Yokoyama, M.; Okano, T., Sakurai, Y.; Kataoka, K. Pham. Res. 1995, 12, 92. (b) Kataoka, K.; Kwon, G. S.; Yokoyama, M.; Okano, T.; Sakurai, Y. J. Controlled Release 1993, 24, 119. (c) Harada, A.; Kataoka, K. Macromolecules 1995, 28, 5294. (d) Scholz, C.; Iijima, M.; Nagasaki, Y.; Kataoka, K. Macromolecules 1995, 28, 7295. (2) (a) Sukhorukov, G. B.; Antipov, A. A.; Voigt, A.; Donath, E.; Mohwald, H. Macromol. Rapid Commun. 2001, 22, 44. (b) Sauer, M.; Meier, W. Chem. Commun. 2001, 1, 55. (3) Zhang, X.; Matyjaszewski, K. Macromolecules 1999, 32, 1763. (4) (a) Misra, S.; Varanasi, S.; Varanasi, P. P. Macromolecules 1989, 22, 4173. (b) Pincus, P. Macromolecules 1991, 24, 2912. (c) Dan, N.; Tirrell, M. Macromolecules 1993, 26, 4310. (d) Shusharina, N. P.; Nyrkova, I. A.; Khokhlov, A. R. Macromolecules 1996, 29, 3167. (e) Hariharan, R.; Biver, C.; Mays, J.; Russel, W. B. Macromolecules 1998, 31, 7506.
allowed to vary along the polymer chain, as in a weak polyacid or polybase.5 In annealed polyelectrolyte micelles and brushes, the pH controls the degree of charge on the micelle corona or polymer brush, which can induce swelling due to electrostatic repulsions.6 Systematic studies on polyelectrolyte amphiphilic block copolymers of poly(styrene-block-methacrylic acid) (PS-b-MAA)7 and poly(styrene-block-acrylic acid) (PS-bPAA)8 with respect to micellar size and structure have been reported. Hybrid polymeric micelles with compact polystyrene core and poly(methacrylic acid)/poly(ethylene oxide) shells were produced in a 1,4-dioxane (80 vol %)/ water mixture.9 Because of the hydrophobic character of polystyrene, sample solutions were prepared either by heating for a significantly long time or by stepwise dialysis from organic solvent to water, which limits the potential application of these systems. By adoption of a less hydrophobic segment such as poly(methyl methacrylate) (PMMA), the solubility can be increased. In addition, the R-methyl groups on the MAA chain make it quite different from the PAA system. For example, the configuration of MAA at low pH is much more compact than that of PAA.10 (5) (a) Zhulina, E. B.; Borisov, O. V. Macromolecules 1996, 29, 2618. (b) Israels, R.; Leermakers, F. A. M.; Fleer, G. J. Macromolecules 1994, 27, 3087. (6) (a) Baines, F. L.; Billinggham, N. C.; Armes, S. P. Macromolecules 1996, 29, 3416. (b) Biesalski, M.; Rifihe, J.; Johannsmann, D. J. Chem. Phys. 1999, 111, 7029. (c) Groenewegen, W.; Lapp, A., Egelhaaf, S. U.; van der Maarel, J. R. C. Macromolecules 2000, 33, 4080. (7) (a) Prochazka, K.; Kiserow, D.; Ramireddy, C.; Tuzar, Z.; Munk, P.; Webber, S. E. Macromolecules 1992, 25, 454. (b) Ramireddy, C.; Tuzar, Z.; Prochazka, K.; Webber, S. E.; Munk, P. Macromolecules 1992, 25, 2541 (c) Prochazka, K.; Martin, T. J.; Munk, P.; Webber, S. E. Macromolecules 1996, 29, 6518. (d) Karymov, M. A.; Prochazka, K.; Mendenhall, J. M.; Martin, T. J.; Munk, P.; Webber, S. E. Langmuir 1996, 12, 4749. (e) Stepanec, M.; Podhajecka, K.; Prochazka, K.; Teng, Y.; Webber, S. E. Langmuir 1999, 15, 4185. (f) Stepanec, M.; Prochazka, K. Langmuir 1999, 15, 8800. (g) Kriz, T.; Masar, B.; Pospisil, H.; Plestil, J.; Tuzar, Z.; Kiselev, M. A. Macromolecules 1996, 29, 7853. (8) (a) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728. (b) Zhang, L.; Eisenberg, A. Macromolecules 1996, 29, 8805. (c) Zhang, L.; Shen, H.; Eisenberg, A. Macromolecules 1997, 30, 1001. (d) Yu, K.; Zhang, L.; Eisenberg, A. Langmuir 1996, 12, 5980. (9) Stepanec, M.; Podhajecka, K.; Tesarova, E.; Prochazka, K.; Tuzar, Z.; Brown, W. Langmuir 2001, 17, 4240.
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Table 1. Molecular Characteristics of P(MMA-b-MAA), Concentrations of Solutions, and Counterion Condensation Ratios
sample
Mw/Mn
concn (wt %)a
counterion condensation ratio for neutralized system
MMA194-MAA114 MMA22-MAA175 MMA10-MAA102 MMA6-MAA54
1.24 1.24 1.20 1.28
0.010 0.014 0.082 0.040
0.21 0.29 0.62 0.40
a
The polymer concentrations were calculated from titration.
We have examined the micellization behaviors of P(MMA-b-MAA) in aqueous solutions, where the hydrophobic segment of MMA is short.11 Although the coreshell like micelle could be produced in solution, the micellization behavior may be varied by changing the hydrophilic-lipophilic balance values via the length of hydrophilic and hydrophobic segments. In this study, P(MMA-b-MAA) with different molecular weights and lengths of MMA block were synthesized, which is a natural extension of our previous work. The effects of MMA and MAA segment length on the micellization behavior were examined. Of particular interest is the evaluation of counterion condensation phenomenon using conductometric titration techniques. Experimental Section Polymer Synthesis. The P(MMA-b-MAA) copolymer was synthesized using the atom transfer radical polymerization (ATRP) technique as described in our previous paper.11 Four different polymers were prepared, and their compositions and molecular weights are summarized in Table 1. The molecular weight and molecular weight distribution were examined by GPC traces with THF as the mobile phase. The compositions were determined by 1H NMR in CDCl3 solution. Preparation of P(MMA-b-MAA) Polymer Solution. To remove traces of impurity, the block copolymers were first dissolved in methanol and dialyzed against water for several days. Because methanol is not a good solvent for MMA, the sample MMA194-MAA114 dissolved very slowly, and during dialysis, precipitation occurred as water content increased. But for the polymers with short MMA chains, it readily dissolved in methanol, and during dialysis, precipitation also occurred because of high polymer concentrations. Other researchers prepared PSMAA micelle solutions using dialysis from a 1,4-dioxane-water mixture,7,9 and they obtained good core-shell structures. In the present work, our objective is to study the association behavior of a MMA-MAA block copolymer in aqueous medium. Copolymers with short MMA chains readily dissolved to form micelles in alkaline aqueous environment. As a comparison, MMA194MAA114 with long MMA chain was also prepared using this protocol, and large aggregates may be produced because of chain entanglement of MMA and MAA chains. The protocol chosen to prepare the test solutions is by first titrating the polymer solution with 1 M NaOH solution to a pH of ∼11. The polymer solution was continuously stirred for 1-2 h, until the solution became homogeneous, and the pH was readjusted to ∼3 using 1 M HCl solution. The polymer solution remained transparent, confirming the homogeneity of the solution, which was subsequently used for potentiometric and conductometric studies. Potentiometric and Conductometric Titrations. The pH and conductometric titrations were performed using an ABU93 Triburet Titration System. The system was equipped with a Radiometer pHG201 pH glass electrode, Radiometer REF201 reference electrode, and conductivity electrode. All the titrations were performed at 25 °C, in a titration vessel filled with 100 mL (10) (a) Bednar, B.; Morawetz, H.; Shafer, J. A. Macromolecules 1985, 18, 1940. (b) Koenig, J. L.; Angood, A. C.; Semen, J.; Lando, J. B. J. Am. Chem. Soc. 1969, 91, 7250. (11) Ravi, P.; Wang, C.; Tam, K. C.; Gan, L. H. Macromolecules 2003, 36, 173-179
of 0.01-0.1 wt % P(MMA-b-MAA) block copolymer solution subjected to constant stirring. A 1 M standard NaOH solution (from Merck) was used as titrant. One minute of lag time was allowed between each dosage, to ensure that the acid-base reaction has reached equilibrium. Because the acid-base reaction is a fast reaction, the pH reached an equilibrium value in less than 15 s. Laser Light Scattering (LLS). Dynamic light scattering (DLS) represents one of the more reliable techniques for studying the configuration and microstructure of polymeric micellar systems.12 The laser light scattering experiments were conducted using a Brookhaven laser light scattering system. This system consists of a BI200SM goniometer, BI-9000AT digital correlator, and other supporting data acquisition and analysis software and accessories. An argon-ion vertically polarized 488 nm laser was used as the light source. The G2(t) functions obtained from DLS were analyzed using the inverse Laplace transformation technique (REPES in our case) to produce the distribution function of decay times. The concentration of the polymer solutions investigated by light scattering is 0.01-0.1 wt %, which is in the dilute solution regime where the behavior of individual particles can be characterized. Several measurements were carried out at 90° for a given sample to obtain an average hydrodynamic radius, and the variation in the Rh values is small.13 Measurements of dynamic light scattering and static light scattering were performed at different scattering angles and sample concentrations. From dynamic light scattering (DLS), the hydrodynamic radii (Rh) of micelles were obtained, and from static light scattering (SLS), the molecular weights (Mw) and gyration radii (Rg) of micelles were measured by using Zimm, Berry, or Debye plots.8 Transmission Electron Microscopy (TEM). Observations by TEM (JEOL 2010, 200 kV) were performed on the polymeric solutions. One or two drops of a selected solution were dripped onto a carbon-coated copper grid, and the sample on the copper grid was then dried in a desiccator for 24 h before being characterized under the TEM instrument.
Results and Discussion Conductometric Titration.14 The systems being investigated contain the following ions: H+, Na+, OH-, Cl-, and macroion. The conductivity can be expressed as follows: 15
Λ ) CNa+λNa+ + CH+λH+ + COH-λOH- + CCl-λCl- + CPλP (1) where Ci is the concentration of free ion in solution and λi is the molar conductivity of the corresponding ion. During titration, the Cl- concentration remained constant, while the large macroions (denoted by “P”) do not contribute much to the conductivity; hence the conductivity curve reflects the concentration changes of H+, Na+, and OH-. Figure 1 shows the conductivity curve as a function of moles of NaOH titrated to 0.01 wt % of polymer MMA194-MAA114. The titration can be divided into three distinct regimes, based on the changes in the slope of the conductivity curve. Region 1 represents the neutralization reaction between excess HCl and NaOH, and the decrease in the conductivity is caused by the decrease of H+ concentration, since the mobility of H+ (λH+0 ) 350 S‚ cm2/mol, 25 °C) is much larger than that of Na+ (λNa+0 ) 50.5 S‚cm2/mol, 25 °C),14 even though the concentration of Na+ ion has increased. Region 2 corresponds to the reaction between MAA and NaOH, and the increase in (12) Stepanec, M.; Prochazka, K.; Brown W. Langmuir 2000, 16, 2502. (13) (a) Dai, S.; Tam, K. C.; Jenkins, R. D. Macromolecules 2001, 34, 4673. (b) Dai, S.; Tam, K. C.; Jenkins, R. D. J. Phys. Chem. B 2001, 105, 10189. (14) Nagasawa, M.; Murase, T.; Kondo, K. J. Phys. Chem. 1965, 69, 4005 (15) Kolthoff, I. M.; Laitinen, H. A. pH and electro titrations, 2nd ed.; John Wiley & Sons: New York, 1952.
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Figure 1. Potentiometric and conductometric titration curves of 0.01 wt % MMA194-MAA114: [, pH; 0, conductivity curve.
the conductivity is mostly caused by the increase in Na+ ion concentration, because in this region, the concentration changes of H+ and OH- as determined from pH measurement are small compared to changes in Na+ ion concentration. Thus, the conductivity curve in region 2 reflects the change in free Na+ concentration. Region 3 represents the excess NaOH, where the Na+ and OH- ions contribute to the large increase in the conductivity. In region 2, the conductivity curve shows an inflection at R of ∼0.4 (which corresponds to 0.022 mmol of NaOH), where the slopes (shown by dashed lines) at low R region are larger than the slope at high R. The conductivity in region 2 can be described by eq 2
Λ ) CNa+λNa+ + constant
(2)
where CNa+ is the concentration of free Na+ ion in solution. From eq 2, the linear increase in the conductivity is proportional to the increase in mobile sodium ions, where the slope corresponds to λNa+ and possesses a magnitude of 37 S‚cm2/mol (cf. λNa+0 ) 50.5 S‚cm2/mol, 25 °C). At low R, most of the Na+ ions added are mobile or free; however at larger R, such that the negative charge density on the micelles is sufficiently strong to attract the oppositely charged ions, counterion condensation may result. Thus, beyond a critical value where counterion condensation occurs, added Na+ ions will condense on the micelles, resulting in an overall reduction in free Na+ ion. This phenomenon can be identified by the inflection at R ∼ 0.4, which is the onset point for counterion condensation. From the ratio of the two slopes, the proportion of added sodium ions that are condensed can be determined using the expression
rcond ) 1 - khigh/klow
(3)
Here, rcond is the proportion of added Na+ ions that are condensed after the onset point, and khigh and klow are the larger and smaller slopes on the conductivity curve, respectively. The rcond value determined using eq 3 is about 0.64. When we compared the amounts of condensed Na+ ions and COO- groups at fully neutralized condition, the ratio, β (nNa+,condensed/nCOO-) is ∼0.21. Similar calculations were performed for the other three polymers, and the values of β are summarized in Table 1. We observed that the amount of condensed counterions is proportional to the polymer concentration. This phenomenon might be caused by the higher Na+ concentration involved in the neutralization of the polymer with higher concentration.
Manning theory16 can be used to predict the condensation of counterions on a charged surface or a streamline. The behavior of counterions surrounding a polyelectrolyte domain in aqueous solution can be described by this theory. Detailed description on the theory of counterion condensation can be obtained from the literature.16 A polyelectrolyte chain can be modeled as an infinite regular linear array of infinite fixed univalent point charges. A counterion may be considered to be strongly bound to any surface if the attraction energy is greater than the thermal energy, kT. Counterions surrounding the charged polyelectrolyte domains can be condensed on the charged sites, if the linear spacing between the charged sites is less than the Bjerrum length. From our study of conductivity curves, we observed that R ∼ 0.4 (i.e., ∼40% COOH groups are ionized as COO- groups) represents the critical value for the onset of counterion condensation. Potentiometric Titration. P(MMA-b-MAA) when dissolved in water can form micelles, where the shellforming MAA segments are anchored to the hydrophobic core and may be regarded as a convex polyelectrolyte brush. However, a linear PMAA chain does not resemble that of a typical polyelectrolyte observed for some polyelectrolytes, such as PAA. The conformational behavior of PMAA was studied by a number of research groups, and it was reported that the polymer chains undergo strong hypercoiling when the pH decreases from 6 to 5.10 Due to the Donnan equilibria, the apparent acidity of the PMAA shell is reduced, and the transition from a collapsed to a stretched chain configuration regime appears at a relatively high pH. Titration of polyelectrolyte micelles with alkali solution permits a fundamental characterization of the micellar shell. The acid dissociation equilibria of a weak polyacid polyelectrolyte may be expressed by the equilibrium equation shown below Ka
HA \ y z H+ + A where HA is the polyacid and H+ and A- are the hydrogen ion and anion, respectively. The equilibria can be quantified by defining the apparent acid dissociation constant, Ka, calculated using the measured pH and the degree of dissociation of the functionality, R, at equilibrium
(1 -R R)
pKa ) pH + log
(4)
pKa reflects the overall acid dissociation equilibrium and is affected by the polymer configuration. The degree of dissociation of the functionality, R, can be determined by the pH and the amounts of polyelectrolyte and the alkali added. For normal acids, such as CH3COOH, the pKa in a given environment is constant. But for polyacids, such as poly acrylic acid (PAA), the pKa is not constant, because the overall acidity depends on the ionization degree. The COO- groups on the polymer chain could prevent the dissociation of COOH groups, because H+ ions become more difficult to be abstracted from the polymer chain to the aqueous solution due to restriction of electrostatic attraction by COO- groups. Thus, the pKa curve provides (16) (a) Manning, G. S. J. Chem. Phys. 1969, 51, 924. (b) Manning, G. S. J. Phys. Chem. 1984, 88, 6654. (c) Winkler, R. D.; Gold, M.; Reineker, P. Phys. Rev. Lett. 1998, 80, 3731. (d) Brilliantov, N. V.; Kuznetsov, D. V.; Klein, R. Phys. Rev. Lett. 1998, 81, 1433. (e) Schiessel, H. Macromolecules 1999, 32, 5673. (f) Schiessel, H.; Pincus, P. Macromolecules 1998, 31, 7953. (g) Khokhlov, A. R.; Kramarenko, E. Y. Macromolecules 1996, 29, 681.
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Figure 2. Titration curves of 0.04 wt % MMA6-MAA54: O, pKa; 9, Rh. The pKa curve shows a flat region at 0.1 < R < 0.3, which means a structural transition from a relatively ordered to random orientation. The Rh curve reveals the swelling of the micelle, and the majority of the chain expansion is attributed to the uncoiling of MAA chain at 0.1 < R < 0.3.
useful information on the polymer structure, such as the charge density on the polymer chain. Several research groups7,9,12 have reported on the micelle structure of PS-b-MAA, where both the density profile and degree of dissociation depend strongly on the distance from the core/shell interface. There exists a relatively dense and fairly nonpolar layer of collapsed section of nondissociated MMA chains close to the core and a diffuse peripheral region of the shell formed by partially stretched and ionized chains. The shell remains almost a concentric double-layer system. Hence the effective pKa not only is the apparent value dependent on the average ionization degree and ionic strength but also represents an average of the pKa profile across the shell. Figure 2 shows a curve with the pKa data plotted against the degree of ionization (R) obtained from titrating 1 M NaOH into an aqueous solution of 0.04 wt % MMA6MAA54 solution. At the un-ionized state (R ) 0), where the electrostatic potential is negligible, the pKa of MAA with different molecular weights and salt concentrations is ∼4.8 (cf. pKa of PAA is ∼4.3).17 A clear transition occurs at R ∼ 0.3, where the slope is small at 0.1 < R < 0.3, and becomes larger at R > 0.3. During neutralization, the acidity of polymer deceases due to the higher charge density restricting the dissociation of COOH groups, which results in the increase of pKa. The R-methyl groups on the MAA chain and the hydrophobic interactions between the R-methyl groups produce a compact chain configuration at low ionization degree. The polymer chain possesses a compact structure, where the R-methyl groups and the COOH/COO- groups are located on the interior and exterior of the compact structure, respectively. The sharp increase in the pKa at R < 0.1 is attributed to the swelling where the compact structure is not destroyed. The COOH and COO- groups impart a relatively high surface charge density to the micelles, and the acidity of PMAA decreases faster than PAA that does not contain R-methyl groups. Further neutralization increases the charge density of polymer chains, such that the hydrophobic forces could not retain the structure, and the distance between COOgroups is increased by electrostatic repulsion, which destroys the compact structure. The “flat region” on the pKa curve at 0.1 < R < 0.3 corresponds to the region where the structure is being destroyed. Through the rearrange(17) Crescenzi, V.; Quadrifoglio, F.; Delben, F. J. Polym. Sci., Part A-2 1972, 10, 357
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Figure 3. Effect of NaCl concentration on pKa of 0.014 wt % MMA22-MAA175: 0, 0.001 M; O, 0.1 M; ], 1 M. At higher salt concentration, the pKa decreases and the transition region shifts to higher R.
Figure 4. Titration curves of random copolymer of poly(methacrylic acid-ethyl acrylate) (P(MAA-EA)) containing 40 wt % MAA and 60 wt % EA, 0.1 wt % aqueous solution: [, conductivity; 9, l pKa. The pKa exhibits a minimum at R ) 0.65, and conductivity possesses an inflection at R ) 0.7.
ment of COO- groups on the polymer chain, the charge density is kept relatively low, which maintains a constant acidic condition, as depicted by the flat region on the pKa curve. In this region, Rh increases from ∼23 to 32 nm, and it becomes constant thereafter suggesting that the increasing degree of ionization is balanced by the charge neutralizing effect of counterion condensation. Figure 3 shows the effect of salt on the pKa of 0.014 wt % MMA22-MAA175. At high salt concentration, the electrostatic repulsion is screened by Na+ ions, resulting in a lower degree of swelling during ionization. The compact structure is relatively more difficult to destroy; hence the transition region is shifted to a higher ionization degree. The attraction between COO- and H+ is also screened by salt, and this causes the acidity to increase. Thus, in high salt solutions, the pKa is smaller and the transition region shifts to higher R. Comparison between the block copolymers and random copolymer of poly(methacrylic acid-ethyl acrylate) (P(MAA-EA)) containing 40 wt % MAA and 60 wt % EA was conducted. The pH and conductivity titrations were performed on a 0.1 wt % aqueous polymer solution. From Figure 4, we observed that the pKa exhibits a minimum at R ) 0.65, and conductivity exhibits an inflection at R ) 0.7. The random copolymer forms complicated aggregate structure that contains multiple hydrophobic domains, due to the blocky runs of MAA and EA segments. The MAA segments form compact coil at low ionization degree.
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Figure 5. Evolution of Rh distribution functions (measured at 90°) during the neutralization for MMA6-MAA54 and MMA22MAA175. The two polymers have similar MMA/MAA ratio, but exhibit different aggregation behavior.
When the random copolymer is ionized, MAA segments swell due to electrostatic repulsion, and the random microstructure can swell much more than block copolymer micelles, because it contains multiple domains and, hence, a larger degree of freedom for chain expansion on neutralization. The decrease in the pKa at low ionization degree is due to the decrease in the charge density caused by the large expansion of the aggregate, and the chain expansion breaks the large aggregate into small ones.18 When the aggregate cannot be further separated by ionization, the charge density has to increase, and this corresponds to the inflection point on the pKa curve at R ) 0.64. Since COOH groups on the random copolymer are distributed along the backbone, it produces a lower charge density compared to a block copolymer containing COOH groups. Thus, the inflection point for pKa and conductivity of random copolymer occurs at a relatively higher R. The counterion condensation ratio determined at fully neutralized condition is ∼0.12, which is lower than that of block copolymer. This suggests that the degree of counterion condensation depends on the charge density of system. Laser Light Scattering On the basis of the analysis of potentiometric and conductometric titration curves, we observed that the polymeric aggregate undergoes configuration changes during the neutralization process. Laser light scattering experiments can provide additional information on the morphology of this system. Similar MMA/MAA Ratio with Different MAA Chain Length. Parts A and B of Figure 5 show the evolution of the distribution functions during the ionization of MMA6MAA54 and MMA22-MAA175 respectively. Comparison of these two polymers yields information on the aggregation behavior of copolymers with similar MMA/MAA ratio but different MAA chain length. The evolution of distribution functions for MMA6MAA54 shows one major peak of large particles and one small peak of small particles at all R values. The large particles are believed to be related to the micelles, while the small particles are contributed from smaller clusters (18) (a) Wang, C.; Tam, K. C.; Jenkins, R. D. J. Phys. Chem. B 2002, 106, 1195. (b) Wang, C.; Tam, K. C.; Jenkins, R. D., Bassett, D. R. Phys. Chem. Chem. Phys. 2000, 2, 1967.
consisting of several entangled polymer chains at low R or unimers at high R. For MMA22-MAA175, the distribution function shows an obvious peak of large particles at R < 0.3, and this peak disappears at R ) 0.36. In the same R region, a fast mode corresponding to small particles increases both in size and magnitude. The large particles may be contributed from large compound micelles formed by entanglements of several core shell micelles by MAA chains, while the small particles are related to core-shell micelles. During ionization, large compound micelles are transformed into core-shell micelles through the uncoiling of MAA chains. The formation of larger compound micelles for MMA22MAA175 suggests that the entanglement at low ionization degree can produce such complicated microstructure due to the long MAA chains. Scheme 1 shows the aggregation behavior of the P(MMA22-b-MAA175) during the ionization process. At low ionization degree, entanglements of long MMA chains lead to large compound micelles that are in equilibrium with core-shell micelles. Increasing ionization produces stronger electrostatic repulsion, which destroys the entanglements to produce simpler core-shell micelles. These two polymers have a similar MMA/MAA ratio, but exhibit quite different aggregation behavior, and the controlling parameter is the length of the MAA chain, since the entanglement of MAA chains controls the morphology of the aggregates during the neutralization process. Similar MAA Chain Length with Different MMA Chain Length. Figure 6 shows the evolution of distribution functions during the ionization for MMA10-MAA102 and MMA194-MAA114. The evolution of MMA10-MAA102 (Figure 6A) and MMA6-MAA54 (Figure 5A) peaks is similar, where two peaks of different particle size are observed through out the neutralization process. This suggests that when the amount of MAA units changes from 54 to 102, the entanglement of MAA102 chains retain the morphology of the aggregates. The evolution of MMA194-MAA114 shows very complicated structural changes during the neutralization process. We observed that the size of large and small particles is enhanced between R of 0 and 0.71, and the proportion of small particles is increased at the expense of larger
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Figure 6. Evolution of Rh distribution functions (measured at 90°) during the ionization of MMA10-MAA102 and MMA194-MAA114. The two polymers have similar MAA content. The difference in aggregation behavior is controlled by MMA chain length. Scheme 1. The Aggregation Behavior of MMA22-b-MMA175 during the Neutralization Process
ones. This suggests that with increasing degree of ionization, large compound micelles become swollen and dissociate to form smaller aggregates. When the ionization
degree exceeds 0.95, two distinct peaks are present, representing simple core-shell micelle (Rh ) 39 nm) and compound micelle (Rh ) 84 nm), which are stabilized by
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reported, and well-defined core-shell structures were obtained. However, in the present study, solution preparations were carried out by the addition of NaOH, which may not be sufficient to disentangle the long MMA and MAA chains; thus large aggregates could be present. Thus, the dissociation of MMA194-MAA114 aggregates at different neutralization conditions tends to be more complex than originally thought. However, for polymers with short MMA chains, micelle solutions can be prepared by directly dissolving the copolymer in alkaline aqueous environment. Transmission Electron Microscopy (TEM). To examine the entanglement effects of MAA segments at low ionization degree, we performed TEM measurements on MMA22-MAA175 solution at pH ) 5 (Figure 7A). The MMA segments form compact cores as represented by several dark domains in the TEM micrograph (Figure 7B) and these domains associate to form larger aggregates, which corresponds to the large compound micelle. The lower magnification micrograph shown in Figure 7A reveals the presence of several large compound micelles. This evidence confirmed the microstructure determined from light scattering and titration experiments. Conclusions
Figure 7. The morphology of MMA22-MAA175 solution at pH ) 5. The TEM pictures reveal multiple dark domains representing the MMA cores, suggesting that entanglements of MAA segments at low ionization degree lead to large compound micelles.
entanglements of long MMA chains. The two polymers MMA10-MAA102 and MMA194-MAA114 contain about the same MAA units; hence the difference in the aggregation behavior is mainly controlled by the MMA chain length. We should note that the sample preparation may affect the association behavior, especially for MMA194-MAA114. The preparation of PS-MAA micellar solutions using dialysis from 1,4-dioxane-water mixtures7,9 had been
From the results of potentiometric and conductometric titrations and laser light scattering, several conclusions can be drawn regarding the aggregation behavior of P(MMA-b-MAA) in aqueous solution. The MAA chains possess a hypercoil configuration at low ionization degree due to R-methyl groups on the backbone. They swell and uncoil at R of 0-0.3, and counterion condensation commences at R ∼ 0.4. The MAA chain becomes fully ionized at large R, which produces extended ionized MAA chains. When we dissolved the polymer with long MMA chains in alkaline aqueous medium, it produces a complicated microstructure as entanglement between MMA and MAA chains may occur. Electrostatic repulsion may not be sufficient to destroy such entanglements, even at high ionization condition; hence large particles in the form of large compound micelles can still exist. Acknowledgment. Y.J. and P.R. wish to acknowledge the financial support for the postdoctoral fellowship provided by the Singapore-MIT Alliance (SMA). We wish to thank the reviewers for their constructive comments, some of which were incorporated in the revised manuscript. LA0355343
