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Physicochemical Properties of Poly(N-isopropylacrylamide-co-4-vinylpyridine) Cationic Polyelectrolyte Colloidal Microgels V. T. Pinkrah,† M. J. Snowden,*,†,‡ J. C. Mitchell,† J. Seidel,§ B. Z. Chowdhry,†,‡ and G. R. Fern‡ School of Chemical & Life Sciences, and Medway Sciences, University of Greenwich, Medway University Campus, Chatham Maritime, Kent ME4 4TB, UK, and Institute of Physical Chemistry, Freiberg University, D-09596 Freiberg, Germany Received July 23, 2002. In Final Form: November 13, 2002 Aqueous colloidal poly(N-isopropylacrylamide-co-4-vinylpyridine) [poly(NIPAM-co-4-VP)] copolymer microgels have been synthesized using different percentages of 4-vinylpyridine (4-VP). A surfactant-free emulsion polymerization reaction using N-isopropylacrylamide (NIPAM) and 4-vinylpyridine comonomers cross-linked with N,N′-methylenebisacrylamide was utilized. The reaction was initiated using the cationic initiator 2,2′-azobis(2-amidinopropane) dihydrochloride. Transmission electron micrograph data show the copolymer microgels to be monodisperse spheres. The pH and electrolyte sensitivity of the copolymer microgels have been studied, as well as temperature sensitivity, since microgels undergo a reversible volume phase transition in response to heating and cooling. Changes in the hydrodynamic diameters in the system were monitored as a function of temperature (25-60 °C), pH (3-8), and ionic strength (10-3-10-1 mol dm-3 NaCl or NaClO4) using photon correlation spectroscopy. The hydrodynamic diameter of poly(NIPAM-co-4-VP) microgels increases with decreasing pH, as the vinylpyridine units become more protonated. However, the hydrodynamic diameter decreases with increasing ionic strength (over the pH range 3-8) and with increasing temperature (at pH 3 and pH 6). UV-visible spectrophotometry measurements showed a good correlation between the molar absorption and the percentage of vinylpyridine incorporated. Potentiometric titrations were used to determine the pKa values of the copolymer microgels.
1. Introduction Poly(N-isopropylacrylamide) [i.e. poly(NIPAM)] homopolymer is a nonionic, linear, water-soluble polymer. Colloidal poly(NIPAM) microgels, on the other hand, are intramolecularly cross-linked polymeric particles usually dispersed in an aqueous solvent, displaying hydrodynamic diameters in the range of ∼100 nm to 1 µm.1 Poly(NIPAM) microgels have attracted significant interest in the recent scientific literature,2-10 in part because such particles undergo a thermoreversible volume phase transition (VPT) at 34 °C in water.11 In fact, the physicochemical properties of microgels, based on poly(NIPAM) and various copolymer * Corresponding author address: Medway Sciences, University of Greenwich, Chatham Maritime, Kent ME4 4TB, UK. Tel: +44 (0) 208 331 9981. Fax: +44 (0) 1634 883 044. E-mail: m.j.snowden@ greenwich.ac.uk. † Medway Sciences, University of Greenwich. ‡ School of Chemical & Life Sciences, University of Greenwich. § Freiberg University. (1) Murray, M.; Snowden, M. J. Adv. Colloid Interface Sci. 1995, 54, 73. (2) Pelton, R. H.; Chibante, P. Colloid Surf. 1986, 20, 247. (3) Pelton, R. Adv. Colloid Interface Sci. 2000, 85, 1. (4) Mcphee, W.; Tam, K. C.; Pelton, R. J. Colloid. Interface Sci. 1993, 24, 156. (5) Saunders, B. R.; Vincent, B. Adv. Colloid Interface Sci. 1999, 80, 1. (6) Pelton, R. H.; Pelton, H. M.; Morphesis, A.; Rowell, R. L. Langmuir 1989, 5, 816. (7) Snowden, M. J.; Vincent, B. J. Chem. Soc., Chem. Commun. 1992, 1103. (8) Kawaguchi, H.; Fujimoto, K.; Mizuhara, Y. Colloid Polym. Sci. 1992, 270, 53. (9) Wu, X.; Pelton, R. H.; Hamielec, A. E.; Woods, D. R.; McPhee, W. Colloid Polym. Sci. 1994, 272, 467. (10) Snowden, M. J.; Marston, N. J.; Vincent, B. Coll. Polym. Sci. 1994, 272, 1273. (11) Murray, M.; Chowdhry, B. Z.; Snowden, M. J. Chem. Br. 1995, 12, 943.
derivatives, have been widely investigated 1,2,4,11 in relation to the VPT changes that occur in response to a number of external stimuli such as pH,12 temperature,1,3 and ionic strength.12,13 The swelling ratio and the temperature at which the VPT occurs for bulk poly(NIPAM) gels dramatically increase when ionizable groups are incorporated within the polymer network.14 Microgels that are sensitive to pH can, on the other hand, be prepared via copolymerization by incorporating acidic or basic groups into the polymer network. Microgels based on homopoly(NIPAM) have the limitation that the thermally induced VPT occurs at a fixed temperature of ∼34 °C.14 This limitation can be overcome by the introduction of comonomers into the backbone of the polymer chain. Homopoly(NIPAM) does not show a polyelectrolyte-like solution response to increasing electrolyte concentration. Like all water-soluble polymers, however, the polymer-solvent interaction parameter (χ) will increase on going from water to electrolyte and the solvent quality will decrease. This will result in e.g. a lowering of the VPT value for poly(NIPAM). Other studies12 have investigated the effect of electrolyte on the swelling and dispersion stability of poly(NIPAM) microgels. The study considered a range of different electrolytes. The effect of increased electrolyte concentration generally resulted in a decrease in the hydrodynamic diameter of the microgel.12,13 Recently, thermosensitive copolymers of NIPAM and acrylic acid have also been prepared.12 The VPT at which (12) Snowden, M. J.; Chowdhry, B. Z.; Vincent, B.; Morris, G. E. J. Chem. Soc., Faraday Trans. 1996, 92, 5013. (13) Daly, E.; Saunders, B. R. Langmuir 2000, 16, 5546. (14) Hirotsu, S.; Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1987, 87, 1392.
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the particles change conformation is greatly influenced by the pH of their solvent environment. As a result of their VPT properties in different environments, such microgels offer opportunities for use in a wider range of applications, due to the increased range of transition temperatures and pH sensitivity when compared to the homopolymer poly(NIPAM). For example, copolymer microgels of NIPAM and acrylic acid have been investigated for the removal of heavy metals and nitrates from water supplies.15 The absorption and release of lead from the copolymer microgel was found to be very sensitive to changes in pH. Microgels have also been investigated for applications as diverse as drug delivery,16 rheology modification,17 optoelectric switches,18,19 and water purification.20 A wide range of techniques has been extensively used to characterize these gels, including photon correlation spectroscopy (PCS),12 transmission electron microscopy (TEM),21 differential scanning calorimetry,12,22 rheology, and nuclear magnetic resonance. Previous work in the scientific literature has reported data on similar copolymer microgels using cationic styreneco-2-vinylpyridine, prepared by the surfactant-free emulsion polymerization process used in this paper; the studies investigated the effect of the pH dependent swelling.21 Batich et al.23 have also examined copolymers of poly 4-(and 2-)vinylpyridine-co-styrene gel beads, prepared by suspension polymerization, with respect to the swelling behavior of the pH sensitive gels and the ionic strength dependency of the copolymers as a function of monomer composition. Other investigations on macrogels have also shown that the swelling behavior of copolymer gel containing hydrophobic monomers can be manupulated.24 The synthesis of [poly(NIPAM-co-4-VP)], together with an investigation of the electrochemical behavior of the copolymer coordinated with ruthenium (EDTA),25 has been previously reported. The aim of the work presented herein is to report the synthesis and physicochemical properties of cationic poly[(NIPAM-co-4-VP)] microgels, with varying percentage of 4-VP. The results of this investigation show these microgels to be pH, ionic strength, and temperature sensitive. In addition, the pKa of the pyridine groups within the microgel matrix were quantified on the basis of pH titrations. 2. Experimental Section 2.1. Materials. All chemicals were obtained from commercial suppliers and used without further purification. The chemical structures of the monomers, cross-linker, and cationic initiator are shown in Figure 1. 2.2. Microgel Preparation. Poly(NIPAM-co-4-VP) microgel particles were prepared by a surfactant-free emulsion polymerization reaction in deionized water at 70 °C, under a nitrogen atmosphere. 2,2′-Azobis(2-amidinopropane) dihydrochloride (0.5 g dm-3), a cationic free radical initiator (Aldrich), (15) Morris, G. E.; Vincent, B.; Snowden, M. J. J. Colloid. Interface Sci. 1997, 190, 98. (16) Peppas, N. A. J. Bioactive Compat. Polym. 1991, 6, 241. (17) Quadrat, O. Chem. Listy 1990, 84, 496. (18) Quadrat, O. Snuparek. J. Prog. Org. Coatings 1990, 18, 207. (19) Sawai, T.; Shinohara, H.; Ikariyama, Y.; Aizawa, M. J. Electronal. Chem. 1991, 297, 399. (20) Sawai, T.; Yamazaki, S.; Ikariyama, Y.; Aizawa, M. Macromolecules 1991, 24, 217. (21) Loxley, A.; Vincent, B. Colloid Polym. Sci. 1997, 275, 1108. (22) Murray, M.; Rana, F.; Haq, I.; Cook, J.; Chowdhry, B. Z.; Snowden, M. J. J. Chem. Soc., Chem. Commun. 1994, 1803. (23) Batich, C. D.; Yan, J.; Bucaria, C., Jr.; Elsabee, M. Macromolecules 1993, 26, 4675. (24) Siegel, R. A.; Firestone, B. A. Macromolecules 1988, 21, 3254. (25) Iwaku, M.; Haseba, T.; Tatsuma, T.; Oyama, N. J. Electroanal. Chem. 1998, 442, 27.
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Figure 1. Structural representation of the chemicals used for copolymer microgel synthesis. was placed in a 1-L three-necked round-bottomed flask and stirred continuously at ∼120 rpm. Predissolved N-isopropylacrylamide (NIPAM; Aldrich) and 4-VP (Aldrich) were added to the reaction vessel. 4-VP was added at levels of 15, 25, 35, 45, and 55 wt %. The cross-linking agent N,N′-methylenebisacrylamide (0.5 g dm-3; Aldrich) was also added and the reaction allowed to proceed for 6 h. The copolymer microgel was then cooled, filtered through glass wool, and repeatedly dialyzed against deionized water (to remove any unreacted monomer and ionic species), until the conductivity of the dialysate was 1 µS cm-1. Typical reaction yields were on the order of 85 ( 2%. The final volume of the reaction mixture was ∼910 mL, and dry weight analysis of the microgels showed the dispersions in the order of 0.52 (w/w %). 2.3. Transmission Electron Microscopy (TEM). Transmission electron micrographs of the microgel particles were obtained using a JEOL JEM 200CX TEM operated at either 120 or 200 kV. The colloidal particles were supported on carboncoated copper grids, by micropipetting ∼10 µL of a suspension onto the grids and allowing them to dry. Measuring between 20 and 30 particles per micrograph and taking a mean value allowed the determination of the average particle diameter. 2.4. Dynamic Light Scattering. Microgel dispersions of 0.10 (w/w %) were prepared by dilution of the stock dispersions in clean stoppered vials. The pH was then adjusted by the addition of small quantities (µLs) of 0.1 mol dm-3 hydrochloric acid, perchloric acid, or sodium hydroxide. Sodium perchlorate stock solutions were then used in appropriate dilutions to produce the required ionic strength. All samples were sonicated in an ultrasonic water bath for 5-10 min to ensure thorough mixing. The hydrodynamic diameter of each microgel dispersion was determined at 25 °C using a Malvern Instrument Zetasizer 3000 instrument, fitted with a 5 mW He-Ne laser (λ ) 632.8 nm) with a detector at 90°. 2.5. Effect of Counterions. The influence of Cl- concentration on the microgel (25%) dispersion properties was investigated over a range of pH (3-8). A constant ionic strength of 0.05 mol dm-3 was maintained, by adding an appropriate ratio of NaCl/NaClO4 to the fixed dispersion volume. Microgel dispersions of 0.05 (w/w %) were prepared by dilution of the stock solution The solutions were then adjusted to the required pH using 0.1 mol dm-3 HClO4 and 0.1 mol dm-3 NaOH solutions. The ionic strength of the samples was adjusted after setting the pH value by adding the appropriate amounts of NaClO4 and NaCl to adjust the ionic strength to 0.05 mol dm-3. The hydrodynamic diameters were then measured by dynamic light scattering. 2.6. Potentiometric Titration. Potentiometric titrations were performed at 25 °C using a digital pH-meter (model 7065, Scientific Instruments Limited, UK) and a pH-electrode (model 1058 Scientific Laboratory Supplies, UK). The pH-electrode was calibrated against standard buffer solutions of pH ) 3.00, 7.00, and 10.00 (Aldrich Hydrion Buffers) before each titration. CO2free water was used for preparing all solutions. Microgel
[Poly(NIPAM-co-4-VP)] Colloidal Microgels
Figure 2. Absorbance (AU) versus percentage of increasing 4-VP, at 25 °C, with wavelength at 257 nm: (1) 0%, (2) 15%, (3) 25%, (4) 35%, (5) 45%, (6) 55%. dispersions of 0.05 (w/w %) for pH-titrations were prepared by dilution of stock dispersions and adding appropriate amounts of NaClO4 to adjust the ionic strength to 0.01 and 0.1 mol dm-3, respectively. A 13.1-mL aliquot of the dispersions was titrated with 0.01 mol dm-3 HClO4 in steps of 50 µL to a final pH < 3 in order to achieve full protonation of the available pyridine sites. The pH values were recorded after an equilibration period of at least 2 min. The 0.01 mol dm-3 HClO4 titrant was standardized by using a 0.01 mol dm-3 NaOH solution. The perchlorate anion was used as a counterion to control ionic strength, because of its low ability to form complexes or ion pairs.26 2.7. UV-Vis. UV-vis analysis was carried out at 25 °C, on the microgel samples. The samples were prepared at particle concentrations on the order of 0.006 (w/w %) by dilution of the stock dispersion. Water was used as the reference solvent in each case. The molar absorption of the samples was measured between 190 and 500 nm using a Cary 100 UV-vis spectrophotometer to collect data.
3. Results and Discussion An increase in the absorbance of the 4-VP moiety with increasing percentage incorporation was observed from the UV-vis measurements at 257 nm; all samples were very dilute and at the same particle concentration of 0.006 (w/w %). These data provide strong evidence that an increase in the concentration of 4-VP in the reaction mixtures correlates with a higher incorporated portion in the resultant microgel. These data are shown in Figure 2. A portion of the microgel dispersions at the end of each reaction was centrifuged and the supernatant analyzed by means of UV-vis spectroscopy to determine the presence of any unreacted 4-VP; none was found. A transmission electron micrograph of a copolymer microgel containing 25% of 4-VP is shown in Figure 3. The TEM image shows the microgels to be monodisperse spheres, it also appears to suggest the presence of a coreshell-like structure. Table 1 shows a comparison of the particle diameters obtained from the TEM measurements with the hydrodynamic diameters obtained from PCS measurements. The values obtained by PCS are larger than those determined from the TEM micrographs. This discrepancy has been noted by others21 and may be attributed to the use of a dried sample under vacuum for TEM measurements and does not reflect a true measurement for the collapsed state. In the case of the light scattering measurements, the hydrodynamic diameters reflect the true dimension of the hydrated polymeric particles more accurately. The PCS data for the copolymer microgels show a trend of decreasing hydrodynamic (26) Lewis, E. A.; Barkley, T. J.; Reams, R. R.; Hansen, L. D. Macromolecules 1984, 17, 2874.
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Figure 3. Transmission electron micrograph of the copolymer microgel containing 25% 4-VP. Table 1. Hydrodynamic Diameters of Poly(NIPAM)-co-4-VP Microgel Particles in the Unswollen State Determined Using TEM and PCS, at 25 °C and pH ∼6 % of 4-VP 0 15 25
diameter (nm) TEM PCS 200 ( 10 170 (15 120 ( 20
% of 4-VP
768 ( 10 374 ( 6 274 ( 5
35 45 55
diameter (nm) TEM PCS 120 ( 40 180 ( 10 140 ( 30
244 ( 5 211 ( 7 196 ( 8
diameter with increasing 4-VP content. This observation is again consistent with work reported by others23 and has been attributed to the increased levels of the more hydrophobic component, in this instance 4-VP, and also the effect of particle nucleation. Above its pKa value a decrease in the overall microgel-solvent interaction is observed, with increasing amounts of 4-VP, and as a result the microgel adopts a more collapsed conformation. The apparent pKa of the 4-VP monomer, pKa ) 5.39, was determined using the computer software ACD/pKa (Advanced Chemistry Development Inc., Toronto, Canada).27 The potentiometric titration results for microgels containing different percentages of 4-VP at ionic strength I ) 0.01 and 0.1 mol dm-3 are summarized in Figures 4 and 5. To quantify the total concentration of protonatable sites and the pKa values characterizing the different microgels, the following thermodynamically based charge-balance equation28,29 for the dispersion was fitted to the experimental data by using a standard nonlinear leastsquares procedure provided by MATLAB (MathWorks Inc.) n
CH+ - COH- +
∑1
(
CHBi+0
10pKai-pKW
)
10pKai-pKW + 10pH-pKW CHClO4 ) 0 (1)
with
CH+ )
10-pH 10pH-pKW mol L-1 and COH- ) mol L-1 γH+ γOH-
In eq 1, γH+ and γOH- denote the activity coefficients of hydrogen and hydroxide ions (mean activity coefficients (27) pKa values for the 2- and 4 -vinylpyridine monomers were calculated using the computer software AC/pKa (Advanced Chemistry Development Inc, Toronto, Canada). (28) Borkovec, M.; Rusch, U.; Cernik, M.; Koper, G. J. M.; Westall. J. C. Colloid Surf. A. 1996, 107, 285. (29) Fukishima, M.; Tanaka, S.; Hasebe, K.; Taga, M.; Nakamura, H. Anal. Chim. Acta 1995, 302, 365.
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Pinkrah et al. Table 2. Titration Curve Fitting Parameters for Poly(NIPAM)-co-4-VP at Different Ionic Strengths (25 °C) CHB1+0 (mmol dm-3)
CHB2+0 (mmol dm-3)
∑CHB+0 (mmol dm-3)
I ) 0.01 mol dm-3 4-VP 5.51 1.023 poly(NIPAM)-co-4-VP 15% 5.34 4.00 0.131 poly(NIPAM)-co-4-VP 25% 5.08 3.87 0.199 poly(NIPAM)-co-4-VP 35% 5.36 3.81 0.286 poly(NIPAM)-co-4-VP 45% 5.42 3.85 0.307 poly(NIPAM)-co-4-VP 55% 5.42 3.79 0.396
0.298 0.466 0.786 0.93 1.195
0.429 0.665 1.072 1.237 1.591
I ) 0.1 mol dm-3 4-VP 5.61 poly(NIPAM)-co-4-VP 15% 5.69 4.52 poly(NIPAM)-co-4-VP 25% 5.79 4.47 poly(NIPAM)-co-4-VP 35% 5.81 4.45 poly(NIPAM)-co-4-VP 45% 5.89 4.36
0.381 0.588 0.919 1.170
0.526 0.803 1.304 1.616
pKa1
Figure 4. Experimental and fitted pH-titration curves of microgels with different percentage content of 4-VP at 25 °C; solid lines (-) are fitted curves: (O) 0.01 mol dm-3 NaClO4, (b) 15%, (0) 25%, (9) 35%, (4) 45%, (2) 55%, (3) 1 mmol dm-3 4-vinylpyridine.
Figure 5. Experimental and fitted pH-titration curves of microgels with different percentage content of 4-VP at 25°C; solid lines (-) are fitted curves: (O) 0.1 mol dm-3 NaClO4, (b) 15%, (0) 25%, (9) 35%, (4) 45%, (3) 4-vinylpyridine.
of HClO4 and NaOH at I ) 0.1 mol dm-3 were used),30 KW is the dissociation constant of water, Kai is the acid constant of the ith site, CHBi+0 is the total concentration of protonatable site i in mol L-1, and CHClO4 is the concentration of added perchloric acid in mol L-1 in the dispersion. Adjustments were made in the calculation algorithm for the volume changes due to the injection of the titrant to the sample. Excellent curve-fitting of the microgel titration results could be obtained (solid lines in Figures 4 and 5) with a minimum of two different protonatable sites (n ) 2 in eq 1) in the microgels. The fitting process has been repeated with different start values of pKa and site concentrations in order to prove that the iteration resulted in the optimum solution set. The use of more than two different protonatable sites did not significantly improve the quality of the fit. This result is consistent with the polyelectrolyte effect in a polymer containing one type of ionizable group and can be understood by recognizing that neighboring ionizable groups may interact strongly via Coulombic repulsion. Hence, at the beginning of the titration only ionic sites that are several (30) Lide, D. R. (Ed.) CRC Handbook of Chemistry and Physics, 75th ed.; CRC Press: Boca Raton, 1994.
pKa2
1.030 0.145 0.215 0.385 0.446
VP residues apart become protonated, while the remaining sites, i.e., in immediate proximity to each other, protonate at lower pH values. This leads to an affinity distribution that could, in this case, be approximated to two discrete pKa values. In contrast, the experimental titration data for the pure monomer 4-VP could be described, as expected, using one ionizable site. The results of the calculations are summarized in Table 2. The pH titration results confirm that the number of sites available for protonation increases with the percentage of 4-VP in the initial monomer mixture. However, there is also an increase of the concentration of protonatable sites and a reduction of acidity with increasing ionic strength. These results can be understood through weaker electrostatic interactions due to charge screening at higher electrolyte concentration and by conformational changes induced by ionic strength reflected in the hydrodynamic diameter of the gel particles. The first pKa value of the microgels is close to the pKa of the 4-VP monomer, suggesting that polymerization only alters the acid-base properties of the pyridine group slightly. In addition, the sum over all protonatable sites (last column in Table 2) allows an estimation of the final composition of the copolymer in relation to the composition of the initial monomer mixture. On the basis of the results at high ionic strength and assuming that all protonatable sites are pyridine groups, the final 4-VP content corresponds to approximately 70% of the theoretical value calculated from the initial composition of the monomer mixture. This confirmed that a significant proportion of the 4-VP in the initial mixture is incorporated into the polymer network. Table 3 shows an estimated calculation of the composition based on the 4-VP molecules accessible to the acid for protonation. This concentration is dependent on the ionic strength, as determined experimentally. Figure 6 shows the change in hydrodynamic diameter for the microgels containing 15, 25, 35, 45, and 55 wt % of 4-VP as a function of pH. For the 25% 4-VP at low pH (ca. 3), the microgel environment is very acidic [i.e., it is at a pH below the apparent pKa (pKa of 4-VP is 5.39)27 of the monomer] and the hydrodynamic diameter of the microgel particles is 470 ( 5 nm in their swollen state. Under acidic environmental conditions the nitrogen on the pyridine group is protonated, giving rise to internal charge repulsion between neighboring protonated pyridine groups, and this leads to an expansion in the overall dimensions of the microgel. As the pH (ca. 6) increases, above the pKa of the 4-VP, the nitrogen groups become less ionized, and the charge repulsion is greatly reduced, giving rise to an increase in polymer-polymer interaction,
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Table 3. Calculated Composition for Poly(NIPAM)-co-4-VP Microgels at 0.01 and 0.1 mol dm-3 of NaClO4a initial % 4-VP
initial composition 4-VP (mmol/g)
concn of dispersion (g/L)
composition at I ) 0.01 mol dm-3 (mmol/g)
% relative to initial composition
composition at I ) 0.1 mol dm-3 (mmol/g)
% relative to initial composition
15 25 35 45 55
1.46 2.43 3.34 4.37 5.34
0.52 0.52 0.48 0.51 0.54
0.83 1.28 2.23 2.42 2.95
56.8 52.7 66.7 55.4 55.2
1.01 1.54 2.72 3.17
69.2 63.4 81.4 72.5
a Example of calculation (15%-VP): initial composition in mmol 4-VP/g of polymer ) 0.15 g of 4-VP/103 g mol-1 × 1000 ) 1.46 mmol/g composition at I ) 0.01 mol dm-3 ) 0.429 mmol/L (from Table 2)/0.52 g/L ) 0.83 mmol/g percentage relative to initial composition ) 0.83/1.46 × 100% ) 56.8%.
Figure 6. Hydrodynamic diameters of copolymer microgel particles with varying percentage of 4-VP as a function of pH (using HCl and NaOH to control the pH) at 25°C: (0) 15%, (b) 25%, (4) 35%, (1) 45%, and ())55%.
Figure 7. Hydrodynamic diameter of copolymer microgel particles with 25% 4-VP as a function of temperature at pH 3 [(O) heating, (9) cooling] and pH 6 [(2) heating, and (3) cooling].
and the overall hydrodynamic diameter of the microgel particles is reduced to 270 ( 10 nm. Figure 7 shows the change in the hydrodynamic diameter of the microgels as a function of temperature for the 25% 4-VP microgel, at pH 3 and 6. In both instances the temperature-driven swelling and deswelling of the microgel is seen to be reversible. Both the microgels are swollen at 25 °C, pH 3 (470 ( 5 nm), pH 6 (300 ( 3 nm) and shrunk at 60 °C, pH 3 (320 ( 4 nm), pH 6 (177 ( 2 nm). The hydrodynamic diameter of the microgel is greatest at pH 3 and 25 °C. Increasing the environmental temperature results in a greater reduction in size for the most swollen microgel (pH 3). This is not surprising, as
Figure 8. The effect of increasing chloride counterion concentration on the hydrodynamic diameters of the copolymer microgel (25% 4-VP) dispersion at constant pH and at a constant ionic strength ) 0.05 mol dm-3: (1) pH 3, (0) pH 4, (4) pH 6, (b) pH 7, (b) pH 8.
Figure 9. Hydrodynamic diameter of the microgel particles at 25% incorporation of 4-VP as a function of increasing NaClO4 concentration; pH adjusted using HClO4, at 25 °C: (3) pH 3, (9) pH 4, (4) pH 6, (b) pH 7, (b) pH.
the microgel at pH 6 has a much more compact conformation at 25 °C as a result of less charge repulsion and therefore has less scope for conformational reduction. The results of the counterion effect on the microgel dispersion (25%) at constant ionic strength of 0.05 mol dm-3 maintained by supplementing with NaClO4 is shown in Figure 8. Between pH 6 and 8 there is little difference between the hydrodynamic diameters of the microgel, which indicates a nonspecific counterion effect. At lower pH values, the perchlorate ions appear to have a more pronounced effect on the hydrodynamic size than the
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chloride ion at the same ionic strength. This result was a little surprising, as the Hoffmeister series31 suggests that the chloride ion is more strongly salting out than the perchlorate ions. We would therefore expect from a consideration of this series that any observed effects would be contrary to those reported. We can only speculate that chloride ions are giving rise to a dehydration of the gel, possibly via an osmotic pressure effect. We intend to investigate this behavior more comprehensively in a future publication. Figure 9 shows the hydrodynamic diameters of the microgels with 25% 4-VP as a function of increasing sodium chloride concentration, in different pH environments (pH 3-8, pH adjusted using HClO4/NaOH) at a temperature of 25 °C. At all pH values, the hydrodynamic diameters of the microgels decrease as the electrolyte concentration increases; comparable observations for similar systems23 have been made. This is a result of the electrolyte screening the repulsion between near pyridine neighbors within the microgel backbone. Not surprisingly, the influence of electrolyte is greatest at pH 3, where the pyridine groups are protonated and the intramolecular charge repulsion is at a maximum value. 4. Concluding Remarks A variety of different physical and chemical approaches have been used here to interpret the properties of poly-
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(NIPAM-co-4-vinylpyridine) cationic microgel, which displays temperature, pH, and electrolyte sensitivity. The most consistent observation made is that the degree of ionization of the polyelectrolyte system has a significant effect on the hydrodynamic diameters of the microgels. The absolute swelling behavior of these gels was affected by the increased level of 4-vinylpyridine, due to the increased amount of hydrophobic groups. Ionic strength and counterion effects showed a dramatic change on the hydrodynamic diameter at a maximum pH, due to the effective binding of the chosen counterions. Using potentiometric titration, it was possible to calculate the concentration of the protonatable sites and the range of the characteristic pKa values. Acknowledgment. The authors would like to thank Prof. Anthony Beezer for helpful discussions and assistance in interpreting the effect of chloride counterion on the microgel dispersions. A Marie Curie Fellowship (J.S.) of the European Community program IHP under contract numbers HPMF-CT-2001-01446 has supported part of this research. LA026283L (31) Shaw, D. J. Colloid and Surface Chemistry, 4th ed.; ButterworthHeinemann: Woburn, MA, 1992.
