Dissolution and Swelling Behaviors of Random and Cross-Linked

Engineering, College of Engineering, Nanyang Technological University, ... The Dow Chemical Company, Asia-Pacific Technical Center, 16 Science Park Dr...
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Langmuir 2005, 21, 4191-4199

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Dissolution and Swelling Behaviors of Random and Cross-Linked Methacrylic Acid-Ethyl Acrylate Copolymers C. Wang and K. C. Tam* School of Mechanical and Aerospace Engineering & Division of Chemical & Biomolecular Engineering, College of Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798

C. B. Tan The Dow Chemical Company, Asia-Pacific Technical Center, 16 Science Park Drive, The Pasteur, Singapore 118227 Received November 15, 2004. In Final Form: January 20, 2005 The neutralization behaviors of random and cross-linked methacrylic acid-ethyl acrylate (MAA-EA) copolymers were examined as a function of degree of neutralization (R) using potentiometric titration and laser light scattering techniques. The random MAA-EA copolymers exhibit a conformational transition from a compact latex particle to a swollen randomly coiled aggregate upon neutralization over a certain range of R. With further addition of NaOH, the swollen aggregates dissociate into several smaller clusters. This conformational change is controlled by the balance between electrostatic repulsion within ionized MAA groups and hydrophobic attraction of EA. The cross-linked MAA-EA copolymers do not undergo a drastic conformational change during neutralization. The polymer latex particles swell slightly upon neutralization, and the extent of chain expansion is proportional to MAA molar composition and inversely proportional to cross-linked density. The electrostatic Gibbs energy (∆Gel) obtained from the potentiometric titration data indicates that a higher MAA portion is favorable for the deprotonization of both the random and cross-linked MAA-EA copolymers, suggesting that the dissociation is mainly dominated by polymer structure instead of the electrostatic attraction between H+ and -COO-. Moreover, static and dynamic light scattering results confirmed that the cross-linked latex particle exists as monodispersed hard sphere in the collapsed state, whereas in its swollen state the latex particle possesses a core-shell structure.

* To whom correspondence should be addressed. Fax: 65-67911859. E-mail: [email protected].

polyacids exhibit expansion upon neutralization, and they may form three-dimensional network structure (i.e., microgel or hydrogel) if the cross-linked density is sufficiently high.4-7 A swelling transition is also found in PMAA microgels where the microgel volume increases from 13.0 to 96.4 µm3 as the pH increases from 2.0 to 7.8, and the extent of swelling is dependent on the cross-linking density, functional groups grafted on the microgel, and ionic strength.4,5,7 Potentiometric titration is commonly used to characterize the dissolution (neutralization) behaviors of polyacids. The potentiometric titration curve is usually treated in terms of the negative logarithm of the apparent dissociation constant (pKa ) -log Ka). The dependence of pKa on degree of neutralization (R) for weak polyacids is quite different from that of low molar-mass weak acids, resulting from the strong Coulombic potential around the polyions.8 For low molar-mass weak acid, pKa increases slightly over a wide range of R and remains close to pK0 (the value of pKa at R ) 0). In the case of PAA and poly(2-methyleneglutaric acid) (PMGA), the values of pKa increase proportionately with R. In the case of polyacids such as PMAA and poly(glutamic acid) (PGA), the plot of pKa against R shows a maximum and a minimum. This nonmonotonic dependence of pKa on R is attributed to the discontinuous conformational transition of polymer particles over the course of neutralization. For example, PMAA, which

(1) Mandel, M.; Leyte, J. C. Electroanal. Chem. 1972, 33, 297. (2) Heitz, C.; Francois, J. Polymer 1999, 40, 3331. (3) Heitz, C.; Rawiso, M.; Francois, J. Polymer 1999, 40, 1637. (4) Eichenbaum, G. M.; Kiser, P. F.; Dobrynin, A. V.; Simon, S. A.; Needham, D. Macromolecules 1999, 32, 4867. (5) Eichenbaum, G. M.; Kiser, P. F.; Shah, D.; Simon, S. A.; Needham, D. Macromolecules 1999, 32, 8996.

(6) Poe, G.; McCormick, C. L. Polym. Prepr. (Am. Chem. Soc, Polym. Chem. Div.) 1999, 40 (2), 279. (7) Eichenbaum, G. M.; Kiser, P. F.; Simon, S. A.; Needham, D. Macromolecules 1998, 31, 5084. (8) Oosawa, F. Biopolymers 1968, 6, 135.

Introduction It has been observed that the viscosity of carboxylic latexes, such as copolymers of ethyl acrylate and methacrylic acid, changes upon partial or complete neutralization of carboxylic groups along the polymer chains. This property is of significant commercial importance since it provides a means to adjust the viscosity of polymeric latexes, for example, in paint and adhesive latex formulations. A number of studies have focused on developing an understanding on the dissolution and association behaviors of polyelectrolyte systems, such as poly(acrylic acid) (PAA), poly(methacrylic acid) (PMAA), and poly(methacrylic acid-co-methylenebisacrylamide) microsphere (PMAA microgel) using a number of different experimental techniques, such as potentiometry, X-ray scattering, viscometry, and electrophoretic mobility measurement.1-5 The conformation transition can be described by a change from a compact, polysoap-like conformation to an expanded polyelectrolyte-like coil conformation (globule to coil).2,3 Most of the studies focused on the dissolution behaviors of linear polyelectrolytes; however, the cross-linked polyelectrolytes, such as cross-linked PAA, PMAA, and poly(methacrylic acid-co-methylenebisacrylamide) microspheres, are less extensively studied. Cross-linked

10.1021/la047198b CCC: $30.25 © 2005 American Chemical Society Published on Web 03/22/2005

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Figure 1. Chemical structure of random and cross-linked MAA-EA copolymers.

exhibits a globule to coil conformational transition during neutralization, possesses a flex pKa curve.9-11 The detailed and direct information on the conformational transition can be extracted from pKa data. Some studies on the analysis of pKa had been reported, and most of them are concerned with determining thermodynamic parameters such as electrostatic Gibbs energy (∆Gel) from the pKa curve.11-15 The electrostatic Gibbs energy corresponds to the energy for extracting a proton from a charged site of the polyion.11-15 Zimm and Rice were the first to describe a method for determining ∆G values from the titration data of ionizable polypeptides. The polymer chains are randomly coiled in their charged state but transformed to a helical structure when the ionizable groups are not charged.16 Hermans calculated the free energy for the formation of an R helix structure for polypeptides based on the pKa data.17 Hirose and coworkers suggested that ∆Gel is related to the electrostatic potential on the surface of the polyelectrolyte chain if the free energy is only due to the electrostatic interaction.15 Dynamic and static light scattering examine the transport properties of polymer chains in solution, through which the hydrodynamic radius (Rh) and the radius of gyration (Rg) are determined and the conformation of the polymer can be evaluated. Characterization of polyelectrolytes using light scattering is always challenging because of the presence of long-range electrostatic interaction.18,19 Only a few light scattering studies were carried out to examine the conformation change of polycarboxylic acids during neutralization, and the nature of the conformation change is still not completely elucidated.2,3,18-23 Heitz et al. reported from X-ray scattering studies that PMAA exhibits a conformational transition from a compact sphere to a Gaussian coil in the course of neutralization.2,3 Our early light scattering studies on the hydrophobically modified MAA-EA copolymers also demonstrated that the polymers exhibit a conformational change from compact latex particles to hydrated swollen random coils and the transition is balanced by electrostatic repulsion and hydrophobic attraction.20,21 The present paper is an extension of our early studies which focused on the neutralization behaviors of various polyelectrolyte systems including hydrophobically modified MA-EA copolymers (HASE polymers), MAA-MMA block copolymers, and MAA-EA block copolymers.20-23

In this study, the main objective is to characterize and compare the neutralization behaviors of random and crosslinked MAA-EA copolymers containing different MAA molar fraction and cross-linked densities. The aggregation behaviors and conformational changes of these polymers were also evaluated as a function of degree of neutralization. Experimental Section Materials. The random and cross-linked MAA-EA copolymers were synthesized by Dow Chemicals using conventional emulsion polymerization technique, and the detailed polymerization procedure and characterization of these polymers have been reported previously.20,21 The cross-linker used in the synthesis is diallyl phthalate (DAP). The weight average molar mass of a single MAA-EA chain is approximately 200 000 to 250 000 g/mol as determined by intrinsic viscosity24 and static light scattering measurements.25,26 The chemical formulas of the polymers are depicted in structure A (random MAA-EA) and structure B (cross-linked MAA-EA) in Figure 1.Three series of MAA-EA copolymers were examined in this study: the first series is MAA-EA random copolymers with different MAA:EA molar ratio; the second series is MAA-EA copolymers cross(9) Muroi, S. J. Appl. Polym. Sci. 1966, 10, 713. (10) Leyte, J. C.; Mandel, M. J. Polym. Sci. 1964, 2, 1879. (11) Gregor, H. P.; Frederick, M. J. Polym. Sci. 1957, 23, 451. (12) Barone, G.; Di Virgilio, N.; Elia, V.; Rizzo, E. J. Polym. Sci. 1974, 44, 1. (13) Dubin, P.; Strauss, U. P. J. Phys. Chem. 1970, 74, 2842. (14) Dubin, P.; Strauss, U. P. J. Phys. Chem. 1973, 77, 1427. (15) Hirose, Y.; Sakamoto, Y.; Tajima, H.; Kawaguchi, S.; Ito, K. J. Phys. Chem. 1996, 100, 4612. (16) Zimm, B. H.; Rice S. A. Mol. Phys. 1960, 3, 391. (17) Hermans, J., Jr. J. Phys. Chem. 1966, 70, 510. (18) Chu, B.; Ying, Q.; Wu, C.; Ford, J. R.; Dhadal, H. S. Polymer 1985, 26, 1408. (19) Liu, T.; Rulkens, R.; Wegner, G.; Chu, B. Macromolecules 1998, 31, 6119. (20) Wang, C.; Tam, K. C.; Jenkins, R. D.; Bassett, D. R. Phys. Chem. Chem. Phys. 2000, 2, 1967. (21) Wang, C.; Tam, K. C.; Jenkins, R. D. J. Phys. Chem. B 2002, 106, 1195. (22) Ravi, P.; Wang, C.; Tam, K. C.; Gan, L. H. Macromolecules 2003, 36, 173. (23) Wang, C.; Ravi, P.; Tam, K. C.; Gan, L. H. J. Phys. Chem. B 2004, 108, 1621. (24) Guo, L.; Tam, K. C.; Jenkins, R. D. Macromol. Chem. Phys. 1998, 199, 1175. (25) Islam, M. F.; Jenkins, R. D.; Ou-Yang, H. D.; Bassett, D. R. Macromolecules 2000 33 (7), 2480. (26) Dai, S.; Tam, K. C.; Jenkins, R. D. Eur. Polym. J. 2000, 36, 2671.

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linked with 4 wt % of DAP, and the MAA:EA molar ratio varies from 20:80 to 80:20; the third series is also cross-linked MAAEA copolymers with a constant MAA:EA molar ratio, whereas the cross-linked density changes from 0.5 wt % DAP to 4 wt % DAP. These polymers were designated as MAx-EAy-z, where x and y correspond to the molar fractions of MAA and EA, respectively, and z refers to the weight percent of cross-linker. The values of hydrodynamic radius (Rh) and polydispersity index (PDI) for the random and cross-linked MAA-EA copolymers are given in Tables 1 and 3, respectively. The PDI varies from 1.2 to 1.4, suggesting that the size distributions of the synthesized polymers are narrow. Potentiometric Titration. An ABU93 Triburet titration system equipped with Radiometer pHG201 pH glass and Radiometer REF201 reference electrodes was used to conduct the potentiometric titrations. All the titrations were performed under constant stirring at 25 °C, in a titration vessel filled with 100 mL of 0.1 wt % polymer solution. A 1 M standard NaOH solution (from Merck) was used, and 1 min of lag time was allowed between two dosages to ensure that the reaction has reached equilibrium. Laser Light Scattering. The dynamic and static light scattering experiments were conducted using the Brookhaven laser light scattering system. This system consists of a BI200SM goniometer, BI-9000AT digital correlator, and other supporting data acquisition, analysis software and accessories. An argonion vertically polarized 488 nm laser was used as the light source. In dynamic light scattering (DLS), the time correlation function of the scattered intensity G2(t), which is defined as G2(t) ) I(t)I(t + ∆t) where I(t) is the intensity at time t and ∆t is the lag time, was 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 0.02 wt %, which is in the dilute solution regime where the behavior of individual particles can be characterized. All DLS measurements were conducted at a scattering angle of 90°; several measurements were performed at varying measurement angles for a given sample to obtain an average hydrodynamic radius. The variation in the Rh values was found to be small.

Results and Discussion Potentiometric Titration Studies of Random MAA-EA Copolymers. The degree of neutralization, R, of the carboxylic group is defined by the equation

R)

[BASE] + [H+] - [OH-] Cp

(1)

where [BASE], [H+], and [OH-] are the molarities of added base, free hydrogen ion, and hydroxide ion, respectively, and Cp is the concentration of methacrylic acid groups, expressed in moles per liter. The hydrogen and hydroxide ion concentration terms were calculated from the pH, where the activity coefficient in dilute solution is assumed to be unity. With this definition, R ) 1 at complete neutralization. The potentiometric titration of the polyelectrolyte solutions is usually treated in terms of the negative logarithm of the apparent dissociation constant (pKa), expressed by the Henderson-Hasselbalch equation

pKa ≡ pH + log

1-R R

(2)

where pKa is the sum of two terms

pKa ) pK0 + 0.4343

dGel RT dR

(3)

pK0 is the intrinsic dissociation constant independent of R, R is the gas constant, T is the absolute temperature,

Figure 2. Potentiometric titration behaviors of 0.1 wt % aqueous solutions of random MAA-EA copolymers: (a) pH and conductivity curves (b, O) MA70-EA30, (2, 4) MA50EA50, ([, ]) MA40-EA60, (9, 0) MA30-EA70 (filled symbols for conductivity, open symbols for pH); (b) pKa curves (O) MA70EA30, (4) MA50-EA50, (]) MA40-EA60, (0) MA30-EA70.

and Gel is the electrostatic Gibbs energy. It should be noted that Gel is not the free energy change of the neutralization process but the energy required in overcoming the strong electrostatic force to extract a proton from a charged polyion.1,7,20,21 The pK0 values were obtained by extrapolating the titration curves to zero neutralization degree (R ) 0). The electrostatic Gibbs energy (∆Gel) can be obtained from the graphical integration based on eq 4

∆Gel ) 2.30RT

∫01 [pK(R) - pK0] dR

(4)

where pK(R) is the dissociation constant at any given R value and pK0 is the intrinsic dissociation constant.12-14,17 Figure 2a shows the pH and conductivity curves obtained from titrating 1 M NaOH into 0.1 wt % (-COOH concentration is 7.8 mM for MA70-EA30, 5.3 mM for MA50-EA50, 4.2 mM for MA40-EA60, and 3.1 mM for MA30-EA70) salt free aqueous solutions of random MAAEA copolymers with different MAA/EA molar ratio. The initial sharp increase of pH from 4 to 7 at low R (from 0 to 0.15) is caused by the neutralization of sulfate groups on the polymer chains introduced by the initiator.20,21 Thereafter, the pH remains essentially constant over a certain range of R which is dependent on the MAA/EA molar ratio. It was proven in our previous studies that the plateau region on the pH curves characterizes the multistep dissociation of polymer particles that involves a conformational transition;20-23 thus it is concluded that MAA-EA copolymers also exhibit a conformational change upon neutralization. Afterward the pH increases progressively and exhibits a well-defined equivalence point at R ) 1 when the complete neutralization is reached. It was reported previously that EA groups are sufficiently blocky to form hydrophobic associations.20,21 The hydrophobic attraction between EA blocks causes inter-

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polymer-chain associations that maintains the particle conformation, whereas the Coulombic repulsion between the charged carboxylate groups tends to expand the polymer particles. Consequently a transition region reflecting the conformational change of polymer particles caused by the competition between hydrophobic attraction and electrostatic repulsion is observed, which is expressed by the plateau in the pH curves. It is noted that the breadth of the plateau region is inversely proportional to the MAA content. The plateaus occur in the range of R ) 0.15 to 0.40, 0.15 to 0.56, 0.15 to 0.65, and 0.15 to 0.79 for MA70EA30, MA50-EA50, MA40-EA60, and MA30-EA70, respectively. This is caused by the shift in the balance between the electrostatic repulsion and hydrophobic attraction, which is controlled by the MAA/EA molar fraction. For MAA-EA copolymer with higher MAA/EA molar ratio, e.g., MA70-EA30, ∼40% of the neutralized carboxylic groups is sufficient to generate the necessary Coulombic repulsion to expand the polymer chains. However, for the polymer with lower MAA/EA molar ratio, e.g., MA30-EA70, at least ∼80% of the neutralized carboxylic groups is needed to generate the sufficient Coulombic repulsion in overcoming the hydrophobic attraction to expand the polymer particles. Thus, the polymers containing fewer MAA carboxylic groups but more EA groups exhibit a broader plateau region. It is also noted that the polymer containing more MAA groups (e.g., MA70-EA30) exhibits a lower pH in the plateau region but a higher pH after reaching the fully neutralization point (R ) 1). This may be attributed to the different ionic strength at the same degree of neutralization caused by varying amounts of carboxylate groups on the polymer chains (from the conductometric titration curves, note that the polymer containing more MAA groups possesses a higher conductivity at a given R). Our previous study on the effect of salt on the potentiometric titration behaviors of HASE polymers demonstrated that the pH is strongly dependent on ionic strength and a higher ionic strength results in a lower plateau.21 For the conductivity curves, two equivalence points were detected as shown in Figure 2a. The first equivalence point (indicated as equivalence point “A”) characterizes the condensation of sodium ions on the negatively charged carboxylate sites, which results in a decrease in the conductivity of the solution. It is noticeable that the R value corresponding to the equivalence point “A” increases from ∼0.4 to ∼0.85 as the MAA molar fraction of the polymer decreases from 70 to 30%. This reflects the fact that the counterion condensation is strongly dependent on the charge density of the polymer. For the polymer with lower MAA composition, that is, MA30-EA70, a higher degree of ionization is required to produce sufficient charge density to induce the condensation of counterions. This agrees with the prediction given by Manning that the condensation of counterions on polyelectrolytes takes place above a critical polymer charge density.27 The second equivalence point designated as equivalence point “B” at R ) 1 apparently corresponds to the complete neutralization of carboxylic groups along the polymer chains. Beyond this point, the conductivity increases proportionately to the excess NaOH. Figure 2b shows the pKa values plotted against R for MAA-EA copolymers. The pKa exhibits a nonmonotonic dependence on R. The pKa values increase with R and reach a maximum at R ) ∼0.15; thereafter they decrease to a minimum and subsequently increase again. Com(27) Manning, G. S. Ber. bunsen-ges. Phys. Chem. Chem. Phys. 1996, 100 (6), 909.

Wang et al. Table 1. Values of pK0, ∆Gel, Hydrodynamic Radius (Rh), and Polydispersity Index of Synthesized Random MAA-EA Copolymers MAA-EA copolymers MA70-EA30 MA50-EA50 MA40-EA60 MA30-EA70

pK0

∆Gel (kJ/mol)

PDI of synthesized latice particles

Rh (nm) of synthesized latice particles

6.62 6.49 6.44 6.52

3.95 4.28 4.93 4.97

1.42 1.33 1.33 1.28

45.9 42.3 39.0 35.1

parison between Figure 2a and 2b demonstrates that the negative slope between the two inflection points on the pKa curves coincides with the plateau region on the pH curves, both of which are assigned as the “transition region” that characterizes the conformational change of the polymer particles.9-11,20,21 Moreover, the MAA-EA molar fraction significantly affects the conformational transition region, as it becomes narrower with the increase of MAA/EA molar ratio. The polymer with lower MAA/EA molar ratio (e.g., MA30-EA70) possesses a weaker electrostatic repulsion between the MAA groups but a relatively stronger hydrophobic attraction because of more EA associations. Therefore, highly ionized polymer chains are required to accumulate sufficient Coulombic repulsion in overcoming the hydrophobic attraction between EA segments to swell the polymer particles. Hence, the completion of the conformational transition, which is characterized by the second inflection point on the pKa curves, occurs at a higher R (∼0.8 for MA30-EA70). On the other hand, for polymers with higher MAA/EA molar ratio, moderately charged polymer chains may have sufficient electrostatic repulsion to swell, and the conformational change is completed at lower R (∼0.4 for MA70-EA30). This feature is identical with that of the polymer with lower MAA/EA molar ratio exhibiting a broader plateau region in pH curves. By extrapolating pKa curves and performing graphical integration using eq 4, the pK0 and ∆Gel values can be determined. The pK0 and ∆Gel values are summarized in Table 1. It is found that the pK0 values are identical with each other and have an average value of 6.52, suggesting that the spontaneous dissociation of the carboxylic sites of the polymers is independent of the MAA/EA molar ratio of polymers. On the other hand, the value of ∆Gel decreases from 4.97 to 3.95 kJ/mol as the molar fraction of MAA groups increases from 0.3 to 0.7, indicating that less work is required to extract proton from the polymers containing more carboxylic groups. Thus, we concluded that the dissociation of the MAA-EA copolymers is generally controlled by the polymer structure instead of the electrostatic attraction between H+ and -COO-. Thus, highly charged polymer chains promote the expansion of the polymer particles and favor the deprotonization of the polymers. Dynamic Light Scattering Studies of Random MAA-EA Copolymers. The relaxation time distribution functions measured at 90° during the neutralization process of MA70-EA30 and MA30-EA70 are given in parts a and b of Figure 3, respectively. The trends of the distribution functions over the course of the neutralization for these two polymers are identical. The distribution function at R ) 0 is monomodal over a narrow range of relaxation times, and the amplitude is high, corresponding to the compact structure of insoluble polymer particles before neutralization. With increasing R, the relaxation time shifts to the right, indicating the expansion of the polymer particles driven by electrostatic repulsion between carboxylate groups. When R reaches a critical value, the

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Figure 3. Relaxation time distribution functions for 0.02 wt % aqueous solutions of random MAA-EA copolymers at different R: (a) MA70-EA30; (b) MA30-EA70.

Figure 4. Dependence of Rhapp on R for 0.02 wt % aqueous solutions of random MAA-EA copolymers: (b) MA70-EA30; (O) MA30-EA70.

relaxation time decreases with R and the amplitude of the distribution function decreases drastically, denoting the disintegration and dissolution of the swollen polymer particles. The apparent hydrodynamic radii (Rhapp) were calculated from the relaxation times using the Stokes-Einstein equation. The plots of Rhapp versus R for MA30-EA70 and MA70-EA30 are shown in Figure 4. The trend of the Rhapp curves resembles that of HASE polymers, and it suggests that the MAA-EA copolymers also undergo expansiondissociation behavior upon neutralization.20,21 However, the extent of expansion and the R value corresponding to the maximum particle size are different and depend on MAA/EA molar ratio. As shown in Figure 4, the Rhapp of

MA70-EA30 increases from ∼45 nm at R ) 0 to its maximum of ∼280 nm at R ) 0.4 and, thereafter, it decreases to ∼190 nm; while the Rhapp of MA30-EA70 increases from ∼35 nm at R ) 0 to the maximum of ∼160 nm at R ) 0.84, afterward it decreases to a minimum of ∼94 nm. It is known that the expansion of the polymer particles is controlled by the competition between the electrostatic repulsive force of charged MAA groups and the hydrophobic attractive force of EA segments; consequently, the change of MAA/EA composition alters the process of swelling and the particle size. High MAA/EA molar ratio results in strong electrostatic repulsion and relatively weak hydrophobic attraction; therefore the particle size of MA70-EA30 is larger and the R value corresponding to the dissociation of swollen particles, that is, the R corresponding to the maximum Rhapp, is lower compared with MA30-EA70. Moreover, comparison of the Rhapp curves in Figure 4 with the potentiometric titration curves in Figure 2 indicates that the increase of particle size coincides with the plateau region on the pH curves and the negative slope on the pKa curves. This further confirms that the “transition region” characterized by the potentiometric titration corresponds to the conformational change, that is, the gradual expansion of polymer from compact insoluble particles to swollen and hydrated coils upon neutralization. Potentiometric Titration Studies of Cross-Linked MAA-EA Copolymers. Figure 5a shows the pH and conductivity curves obtained from titrating 1 M NaOH into the aqueous solutions of 0.1 wt % MAA-EA copolymers cross-linked by 4 wt % DAP. The pH values increase consistently with R and do not exhibit any plateau over the entire course of neutralization, indicating that the

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Figure 5. Potentiometric titration behaviors of 0.1 wt % aqueous solutions of cross-linked MAA-EA copolymer with different MAA/EA molar ratio: (a) pH and conductivity curves (b, O) MA70-EA30-4, (2, 4) MA50-EA50-4, ([, ]) MA40EA60-4, (9, 0) MA30-EA70-4 (filled symbols for conductivity, open symbols for pH); (b) pKa curves (O) MA70-EA30-4, (4) MA50-EA50-4, (]) MA40-EA60-4, (0) MA30-EA70-4.

Figure 6. Effect of MAA molar fraction on ∆Gel for the crosslinked and un-cross-linked MAA-EA polymers: (9) un-crosslinked MAA-EA; (0) cross-linked MAA-EA.

cross-linked MAA-EA copolymers may not undergo a conformational transition upon neutralization. It should be noted that a less distinct flat region from R of 0.18 to 0.26 on the pH curve for the cross-linked polymer containing 70% MAA (i.e., MA70-EA30-4) is observed, which may suggest that the strong Coulombic repulsion caused by the higher fraction of carboxylate groups may be sufficient to expel some polymer chains and thereby alter the polymer conformation. However, this change in the conformation is appreciably weaker and less identifiable compared to the un-cross-linked polymers. As shown in the figure, the values of conductivity increase proportionately to R; however, the slopes of the conductivity curves are gentler compared with those of un-cross-linked systems. For the cross-linked polymers, the polymer chains

Wang et al.

Figure 7. Potentiometric titration behaviors of 0.1 wt % aqueous solutions of cross-linked MAA-EA copolymer with different cross-linking density: (a) pH and conductivity curves (]) 4% DAP, (O) 2% DAP, (4) 1% DAP, (0) 0.5% DAP, (b) 0% DAP; (b) pKa curves (]) 4% DAP, (O) 2% DAP, (4) 1% DAP, (0) 0.5% DAP, (b) 0% DAP. Table 2. Values of pK0 and ∆Gel of Cross-Linked MAA-EA Copolymers Obtained from Potentiometric Titration MAA-EA copolymer

pK0

∆Gel (kJ/mol)

MA70-EA30-4 MA50-EA50-4 MA40-EA60-4 MA30-EA70-4

6.29 6.19 6.03 6.15

9.43 10.51 11.43 11.94

are chemically bonded with each other and they cannot diffuse freely as linear polymer chains; thus the mobility of the polyanions is considerably hindered, resulting in the gentler slope on the conductivity curves. It is also noticeable that the values of the conductivity over the course of neutralization (from R ) 0 to 1) are fairly identical, suggesting that the polymer particles have analogous diffusion velocity that is independent of MAA/ EA molar ratios. In other words, the polymer particles have comparable size and molar mass. The pKa curves of cross-linked MAA-EA copolymers with different MAA/EA molar ratio are given in Figure 5b. The values of pK0 and ∆Gel are summarized in Table 2. The pKa curve of MA70-EA30-4 is the only one resembling the pKa curves of un-cross-linked polymers, which exhibits a less recognizable negative slope between two inflection points at R ) 0.18 and 0.26 corresponding to the plateau region on the pH curve. This feature may characterize a less identifiable conformational change in this narrow range of R. The pKa curves of the other three polymers containing lower MAA content (i.e., MA50EA50-4, MA40-EA60-4, and MA30-EA70-4) are either flat or slightly declining in the course of neutralization, which suggests that (i) no conformational transition is present during neutralization and (ii) the deprotonization of the polymers is dominated by the polymer structure instead of the proton/carboxylate Coulombic attraction.

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Figure 8. Relaxation time distribution functions at 90° of 0.02 wt % (a) MA20-EA80-2 and (b) MA20-EA80-4 aqueous solution at different R. Table 3. Values of pK0, ∆Gel, Hydrodynamic Radius (Rh), and Polydispersity Index of Synthesized MA20-EA80 Copolymers Cross-Linked by Different Amounts of DAP MAA-EA polymer

amt of diallyl phthalate (wt %)

MA20-EA80 MA20-EA80-0.5 MA20-EA80-1 MA20-EA80-2 MA20-EA80-4

0 0.5 1 2 4

pK0

∆Gel (kJ/mol)

PDI of synthesized latice particles

Rh (nm) of synthesized latice particles

6.59 6.53 6.63 6.58 6.58

6.79 7.93 8.27 9.36 11.75

N/A 1.26 1.26 1.28 1.22

N/A 33.9 32.0 32.5 35.5

This deduction is furthermore confirmed by examining the effect of MAA/EA molar ratio on the ∆Gel values of cross-linked polymers as shown in Figure 6. The ∆Gel of cross-linked polymers decreases from 11.94 to 9.43 kJ/ mol as the MAA molar fraction increases from 30 to 70%, indicating that a higher charge density is favorable for the deprotonization of cross-linked polymers. For the crosslinked polymers, the polymer chains are chemically bonded, which considerably decreases the affinity between the carboxylic groups inside the polymer particles and NaOH. When the polymers are gradually neutralized and ionized by NaOH, the accumulated electrostatic repulsion between the carboxylate groups expands the polymer particles and thus allows NaOH molecules to penetrate into the polymer particles and react with the carboxylic groups within the particle. However, the electrostatic repulsion is insufficient to disintegrate the cross-linked polymer chains and induce a conformational change of the particles. As shown in Figure 6, a higher charge density of the polymers enhances the affinity between the carboxylic groups and NaOH molecules and thus favors the deprotonization of both the cross-linked and un-cross-

linked MAA-EA polymers. Another noteworthy characteristic of Figure 6 is that the ∆Gel values of cross-linked polymers are much higher than those of un-cross-linked ones. This is expected because the compact structure of the clusters formed by the cross-linked polymer chains hinders the carboxylic groups from reacting with NaOH; thus a larger amount of work is required to transfer H+ from the polyion to the bulk solution. The potentiometric titration curves of MA20-EA80 copolymers cross-linked by different amounts of DAP are given in Figure 7a. With increase of cross-linked density, the plateau region is gradually replaced by a continuous increasing pH while the conductivity curve levels off, indicating the disappearance of the conformational transition and the reduction in the particle mobility when the polymer chains are highly cross-linked. Figure 7b shows the pKa curves of MA20-EA80 series copolymers with varying cross-linked density, and the relevant parameters derived from pKa curves are given in Table 3. The pKa curves show a sharp increase from R ) 0 to R ) 0.2 and then exhibit a small decrease in the slope that becomes less negative with increasing cross-linked

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density. As discussed earlier, such a negative slope represents the expansion of polymer particles during neutralization. It is known that Coulombic repulsion between the carboxylate groups is insufficient to disintegrate the cross-linked polymer particles and change the polymer conformation; however, it still swells the particles. Moreover, the osmotic pressure created by counterions trapped within the volume of the microgel also contributed to the expansion of the polymer particles. The extent of swelling is balanced by the electrostatic repulsive force and the cross-linked density. As shown in Figure 7b, the polymers with a lower DAP content (e.g., MA20-EA800.5) exhibit the largest swelling with an apparent negative slope in the pKa curve. This negative slope becomes gentler with the increasing cross-linked density, and the pKa curve completely levels off when the amount of DAP reaches 4 wt %, suggesting that the swelling is substantially reduced. It is evident that the area under the pKa curve increases with cross-linked density, indicating that a higher ∆Gel is needed to deprotonate carboxylic groups of highly crosslinked polymers. Dynamic and Static Light Scattering Studies of Cross-Linked MAA-EA Copolymers. The relaxation time distribution functions measured at 90° in the course of neutralization for cross-linked MAA/EA copolymers, MA20-EA80-2 and MA20-EA80-4 are given in Figure 8a and 8b, respectively. The dynamic light scattering behaviors of cross-linked polymers are completely different from those of the un-cross-linked MAA-EA copolymers. As shown in Figure 3, the distribution functions of uncross-linked polymers exhibit a maximum in the relaxation time at certain R and the scattering intensity decreases continuously with R, characterizing the pH-induced conformational change and the dissolution of insoluble latex particles at high pH. However, the distribution functions of cross-linked polymers progressively shift to higher relaxation times and the scattering intensities remain high over the course of neutralization, suggesting that the polymer particles retain their compact coiled conformation and the solution is still opaque even at complete neutralization. Figure 9a demonstrates the dependence of Rhapp on R for MA20-EA80 series polymers with different crosslinked density. Unlike the un-cross-linked polymers which exhibit a maximum on the Rhapp curves representing the swelling-dissociation of polymer particles (refer to Figure 4), the Rhapp of cross-linked polymers increases continuously with R, which characterizes the steady expansion of particles induced by the strengthening electrostatic repulsion between the carboxylate groups, which further confirms that cross-linked MAA/EA copolymers do not exhibit a pH-induced conformational change. It is noted that the expansion is not as significant as the un-crosslinked polymers because the polymer chains are chemically bonded by DAP. Figure 9a also demonstrates that the extent of expansion decreases with increasing cross-linked density. In the course of neutralization, the Rhapp of MA20EA80-0.5 increases from 33.9 to 74.0 nm, whereas the Rhapp of MA20-EA80-4 increases from 35.5 to 55.2 nm. This is illustrated in the inset of the figure, where the swelling ratios (RhR)1/RhR)0) of the polymers were plotted against the weight fraction of cross-linker. The swelling behavior is strongly affected by the cross-linked density. The polymer cross-linked with 0.5 wt % DAP (i.e., MA20EA80-0.5) exhibits a volume increase of ∼10.8 times, whereas the polymer cross-linked by 4 wt % DAP (i.e., MA20-EA80-4) exhibits a volume increase of only ∼3.8 times. The neutralization behaviors of cross-linked MAA-EA copolymers were also investigated by static light scat-

Wang et al.

Figure 9. Light scattering measurements of MA20-EA80 series polymers with different cross-linking density at different R: (a) dependence of Rhapp on R (]) 4% DAP, (O) 2% DAP, (4) 1% DAP, (0) 0.5% DAP, (b) swelling ratio; (b) dependence of Rg on R (]) 4% DAP, (O) 2% DAP, (4) 1% DAP, (0) 0.5% DAP; (c) dependence of F(Rg/Rh) on R (]) 4% DAP, (O) 2% DAP, (4) 1% DAP, (0) 0.5% DAP.

tering. The radius of gyration (Rg) of MA20-EA80 series polymers obtained from static light scattering measurements were plotted as a function of R in Figure 9b. The dependence of Rg on R resembles that of Rh shown in Figure 9a, in which the particle sizes are identical at R ) 0 and they increase consistently with R upon neutralization. Combination of Rg and Rh provides structural information on the polymer particles upon neutralization. This is illustrated in Figure 9c, where the ratios of Rg to Rh (i.e., F ) Rg/Rh) for the cross-linked polymers were plotted against R. The morphology of the polymer particles may be determined from the values of F. The theoretical F value for hard spheres with constant density is 0.774, and 1.501 for random coils or Gaussian chains.28,29 As shown in Figure 9c, the F value varies between 0.72 and 0.88 from R ) 0 to R ) ∼0.3, indicating that the particles of unneutralized or partially neutralized (R e 0.3) cross-

Dissolution and Swelling of Cross-Linked Copolymers

linked polymers are hard spheres. With further increase of R, F decreases and reaches its minimum value of approximately 0.65 at R ) ∼0.6; thereafter it remains essentially constant. The F value at high R is significantly lower than that of hard sphere; hence it may correspond to a “core-shell” structure of polymer particles.30-32 Thus, we hypothesize that the majority of the cross-linkers are in the nucleus of the polymer particles to form a dense core, which is surrounded by a layer of loosely cross-linked MAA-EA chains. The shell layer with low density has negligible contribution to the radius of gyration, and Rg is mainly determined by the size of the dense core of higher mass distribution, which is not significantly affected during neutralization. On the other hand, the loose shell layer where the polymer chains are less cross-linked is hydrated and expands significantly upon neutralization, which increases considerably the hydrodynamic radius in the course of neutralization as shown in Figure 9a. Moreover, the F values are identical for the four polymers as demonstrated in the figure, indicating that even slightly cross-linked polymer (0.5 wt % DAP) forms a stable coreshell structure, which is essentially insensitive to the crosslinked density (up to 4 wt %). Conclusions The neutralization and association behaviors of random and cross-linked MAA-EA copolymers were examined as a function of R using potentiometric titration and laser light scattering techniques. For the linear random MAA(28) Wu, P.; Mohammad, S.; Chen, H.; Qiang, D.; Wu, C. Macromolecules 1996, 29, 277. (29) Fo¨rster, S.; Hermsdorf, N.; Bo¨ttcher, C.; Linder, P. Macromolecules 2002, 35, 4096. (30) Gila´nyi, T.; Varga, I.; Me´sza´ros, R.; Filipcsei, G.; Zrı´nyi, M. Phys. Chem. Chem. Phys. 2000, 2, 1973. (31) Varga, I.; Gila´nyi, T.; Me´sza´ros, R.; Filipcsei, G.; Zrı´nyi, M. J. Phys. Chem. B. 2001, 105, 9071. (32) Lee, A. S.; Bu¨tu¨n, V.; Vamvakaki, M.; Armes, S. P.; Pople, J. A.; Gast, A. P. Macromolecules 2002, 35, 8540.

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EA copolymer, the most striking feature of the pH curves is that a plateau was detected over a certain range of R. In this range of R, the negative logarithm dissociation constant (pKa) curve exhibits a negative slope between two inflection points, and the hydrodynamic radius (Rh) curve shows an increasing trend. These features characterize the conformational transition of the polymers from a compact latex particle to a swollen randomly coiled aggregate upon neutralization. With further addition of NaOH, the swollen aggregate dissociates into several smaller clusters, which is reflected by the decrease in Rh at higher R. The expansion-dissociation of the polymer is driven by the electrostatic repulsion between ionized MAA groups, whereas the hydrophobic attraction between EA segments opposites the swelling of the latex particle. For cross-linked MAA-EA copolymers, the pH curve does not show a plateau, the pKa curve is rather flat compared to the un-cross-linked system, and the Rh curve exhibits a gradual increasing trend with R. These features suggest that the cross-linked polymers do not undergo a drastic conformational change in the course of neutralization. The latex particles only expand slightly upon neutralization, where the extent of expansion is proportional to the MAA:EA molar ratio and inversely proportional to crosslinked density. Furthermore, combination of static and dynamic light scattering suggests that the cross-linked latex particle exists as monodispersed hard sphere in the collapsed state (Rg/Rh < 0.8), whereas in its swollen state, the latex particle possess a core-shell structure (Rg/Rh < 0.6). Acknowledgment. We appreciate the support and enthusiasm of Dow Chemicals (especially Dr. Richard Jenkins) in this research collaboration with NTU. The research is funded by Ministry of Education (MOE) and the Agency for Science, Technology and Research (A*STAR). LA047198B