Efficient Synthesis of Sterically Stabilized pH-Responsive Microgels of

Feb 23, 2006 - Brook Hill, Sheffield, South Yorkshire S3 7HF, UK. Paul Reeve. Rohm and Haas France, European Laboratories, 371 rue L. Van BeethoVen,...
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Langmuir 2006, 22, 3381-3387

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Efficient Synthesis of Sterically Stabilized pH-Responsive Microgels of Controllable Particle Diameter by Emulsion Polymerization Damien Dupin, Syuji Fujii, and Steven P. Armes* Department of Chemistry, Dainton Building, The UniVersity of Sheffield, Brook Hill, Sheffield, South Yorkshire S3 7HF, UK

Paul Reeve Rohm and Haas France, European Laboratories, 371 rue L. Van BeethoVen, 06560 Sophia Antipolis, France

Steven M. Baxter Rohm and Haas Company, Spring House Technical Center, P.O. Box 904, Spring House, Philadelphia, PennsylVania 19477-0904 ReceiVed December 1, 2005. In Final Form: January 19, 2006 Emulsion polymerization of 2-vinylpyridine (2VP) in the presence of divinylbenzene (DVB) cross-linker, a cationic surfactant, and a hydrophilic macromonomer, monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA), at around neutral pH and 60 °C afforded near-monodisperse, sterically stabilized latexes at approximately 10% solids. Judicious selection of the synthesis parameters enabled the mean latex diameter to be varied over an unusually wide range for one-shot batch syntheses. Scanning electron microscopy studies confirmed near-monodisperse spherical morphologies, with mean weight-average particle diameters ranging from 370 to 970 nm depending on the initiator, polymeric stabilizer, and surfactant concentrations. Particle sizing studies were also conducted using disk centrifuge photosedimentometry and dynamic light scattering and gave similar data. These lightly cross-linked latexes acquired cationic microgel character at low pH, as expected. The critical pH for this latex-to-microgel transition was around pH 4.1 at 1.0 wt % DVB, which is significantly lower than the pKa of 4.92 estimated for linear P2VP homopolymer by acid titration. 1H NMR and aqueous electrophoresis studies indicated that substantial swelling occurred at low pH due to protonation of the 2VP groups, while dynamic light scattering (DLS) studies indicated volumetric swelling ratios of up to 3 orders of magnitude, depending on the initial latex diameter. Systematic variation of the degree of cross-linking led to a monotonic decrease in the pKa values of the P2VP latexes (as judged by acid titration) and also the critical swelling pH (as judged by visual inspection). This was attributed to the increasingly branched nature of the P2VP chains in their swollen microgel form. Preliminary studies of the kinetics of acid-induced swelling were also conducted using the pH jump method in conjunction with a stopped-flow apparatus. These P2VP latexes swell significantly faster than P2VP latexes described in the literature and the characteristic time scales observed in the present study are much closer to those predicted by the Tanaka equation.

Introduction Increasing attention is being paid to the synthesis and applications of new stimulus-responsive microgels. One of the most well-documented microgel systems is based on poly(Nisopropylacrylamide) [PNIPAM], which is a thermo-responsive water-soluble polymer with a lower critical solution temperature (LCST) of approximately 32 °C.1-5 This LCST is unusually sharp for a water-soluble polymer, which is related to the coilto-globule transition for the PNIPAM chains at this temperature. Copolymerization of NIPAM in the presence of a bifunctional cross-linker such as bisacrylamide under aqueous dispersion polymerization conditions at 60-70 °C leads to near-monodisperse PNIPAM particles that hydrate and swell on cooling below the LCST.6-10 A number of applications have been suggested * Corresponding author. [email protected]. (1) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 20, 247. (2) Wu, X.; Pelton, R. H.; Hamielec, A. E.; Woods, D. R.; McPhee, W. Colloid Polym. Sci. 1994, 272, 467. (3) Tam, K. C.; Wu, X. Y.; Pelton, R. H. J. Polym. Sci., Polym. Chem. 1993, 31, 963. (4) Deng, Y.; Pelton, R. H. Macromolecules 1995, 28, 4617. (5) Snowden, M. J.; Vincent, B. Chem. Commun. 1992, 1103. (6) Yi, Y. D.; Oh, K. S.; Bae, Y. C. Polymer 1997, 38, 3471.

for such “smart” microgels, including viscosity modifiers, delivery vehicles, biosensors, and particulate emulsifiers.3,11,12 Over the past decade or so, several classes of pH-responsiVe microgels have been reported. These include the following: (i) methacrylic acid-based alkali-swellable latexes;12,13 (ii) Nisopropylacrylamide-based copolymer microgels containing either acidic or basic comonomers;14-16 (iii) acid-swellable latexes based on basic monomers such as 4-vinylpyridine (4VP), 2-vinylpyridine (2VP), or tertiary amine methacrylates such as 2-(diethylamino)ethyl methacrylate (DEA) or 2-(diisopropylamino)(7) Kawaguchi, H.; Fujimoto, K.; Mizuhara, Y. Colloid Polym. Sci. 1992, 270, 53. (8) (a) Snowden, M. J.; Thomas, D.; Vincent, B. Analyst 1993, 118, 1367. (b) Morris, G. E.; Vincent, B.; Snowden, M. J. J. Colloid Interface Sci. 1997, 190, 198. (9) Pelton R. H. AdV. Colloid Interface Sci. 2000, 85, 1. (10) Kiminta, D. M. O.; Luckham, P. F. Polymer 1995, 36, 4827. (11) Ngai, T.; Behrens, S. V.; Auweter, H. Chem. Commun. 2005, 3, 328. (12) Rodriguez, B. E.; Wolfe, M. S.; Fryd, M. Macromolecules 1994, 27, 6642. (13) Saunders, B. R.; Crowther, H. M.; Vincent, B. Macromolecules 1997, 30, 482. (14) Bradley, M.; Ramos, J.; Vincent, B. Langmuir 2005, 21, 1209. (15) Gan, G.; Lyon, L. A. J. Am. Chem. Soc. 2001, 123, 7511. (16) Jones, C. D.; Lyon, L. A. Macromolecules 2003, 36, 1988.

10.1021/la053258h CCC: $33.50 © 2006 American Chemical Society Published on Web 02/23/2006

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Figure 1. Schematic representation of the synthesis of the sterically stabilized lightly cross-linked poly(2-vinylpyridine) latexes via emulsion polymerization at 60 °C and their subsequent acid-induced swelling behavior in aqueous solution at 20 °C.

ethyl methacrylate (DPA).17-20 The present study is focused on poly(2-vinylpyridine)-based microgels. This system was first reported by Loxley and Vincent, who described the synthesis of near-monodisperse, charge-stabilized P2VP latexes at relatively low solids.18 In contrast, in this work we show that the use of a suitable reactive polymeric stabilizer allows the convenient synthesis of sterically stabilized P2VP latexes at much higher solids at neutral pH. These latexes acquire microgel character at low pH due to protonation of the pyridine residues. This latexto-microgel transition is investigated using dynamic light scattering and also by aqueous electrophoresis studies. Moreover, judicious adjustment of the synthesis parameters enables the mean particle diameter of these P2VP latexes to be varied over a surprisingly wide range, with narrow size distributions being obtained in each case. Preliminary kinetic data for the rate of acid-induced swelling of selected latexes using a stopped-flow apparatus is also presented. Experimental Section Materials. 2-Vinylpyridine (97%, 2VP; Aldrich) and divinylbenzene (80 mol % 1,4-divinyl content, DVB; Fluka, UK) were treated with basic alumina in order to remove inhibitor. Aliquat 336 (Aldrich, UK) and R,R′-azodiisobutyramidine dihydrochloride (97%, AIBA; Aldrich, UK) were used as received. Monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA) macromonomer (Mn ) 2000; Mw/Mn ) 1.10) was supplied by Cognis Performance Chemicals (Hythe, UK) as a 50 wt % aqueous solution. Doubly distilled deionized water (pH 6) was used in all the polymerizations. Latex Syntheses via Emulsion Polymerization. The Aliquat 336 surfactant (0.15-0.50 g) and the PEGMA stabilizer (0.90-1.00 g of 50 wt % aqueous PEGMA solution) were dissolved in doubly distilled, deionized water (38.50-38.95 g) in a 100 mL singlenecked round-bottomed flask. A comonomer mixture of 2VP (4.955.00 g) and DVB (0-0.05 g) was then added, causing the solution pH to increase to approximately pH 8.3. The flask was sealed with a rubber septum and the aqueous solution was degassed at ambient temperature using five vacuum/nitrogen cycles. The degassed solution was stirred at 250 rpm using a magnetic stirrer and heated at 60 °C with the aid of an oil bath, and then the initiator solution (0.05-0.10 g of AIBA dissolved in 5.0 g of water) was added after 20 min. The copolymerizing solution turned milky white within 10 min and stirring was continued for 24 h at 60 °C. (See Figure 1.) Purification. The latex particles were centrifuged at 8000 rpm for 40 min, followed by careful decantation of the supernatant, replacement with fresh water, and redispersion of the sedimented particles with the aid of an ultrasonic bath. This protocol was used to remove residual 2VP monomer, excess Aliquat 336 surfactant, and nongrafted PEGMA stabilizer. Purification was continued until (17) Ma, G. H.; Fukutomi, T. Macromolecules 1992, 25, 1870. (18) Loxley, A.; Vincent, B. Colloid Polym. Sci. 1997, 275, 1108. (19) (a) Amalvy, J. I.; Wanless, E. J.; Michailidou, V.; Armes, S. P.; Duccini, Y. Langmuir 2004, 20, 8992. (b) Hayashi, H.; Iijima, M.; Kataoka, K.; Nagasaki, Y. Macromolecules 2004, 37, 5389. (20) Mitsui Cyanamid KK, European Patent, EP0385627, 1990.

the serum surface tension was close to that of pure water (71 ( 1 mN m-1). All dispersions were diluted using deionized water that had been ultrafiltered (0.20 µm filter) prior to use. The solution pH was adjusted by adding either HCl or NaOH. All measurements were undertaken on dispersions that had been equilibrated at the desired pH for at least 20-40 min. NaCl was used as background electrolyte in all experiments. In some cases (e.g., for aqueous electrophoresis studies) KBr was used instead of NaCl, but no significant differences were obtained. This suggests that no significant adsorption of silicic acid onto the latexes occurred on the time scale of our experiments.21 Particle Growth Kinetics. Selected latexes (with intensity-average diameters of 380, 640, and 830 nm respectively, see entries 1, 4, and 5 in Table 1) were re-synthesized and sampled regularly during the copolymerization in order to assess the particle growth kinetics. Aliquots of 0.50 mL were extracted via syringe under nitrogen flow and diluted with 5 mL of a 0.01 M aqueous hydroquinone solution at pH 8 in order to quench the copolymerization. Hydrodynamic diameters were measured at 25 °C using a Malvern Nanosizer ZEN 3600 instrument equipped with a 4 mW He-Ne solid-state laser operating at 633 nm. Backscattered light was detected at 173° and the mean particle diameter was calculated from the quadratic fitting of the correlation function over 30 runs of 10 s duration. All measurements were performed in triplicate on highly dilute dispersions. Disk Centrifuge Photosedimentometry. Weight-average particle diameters were calculated using a Brookhaven disk centrifuge instrument operating at 2500-6000 rpm. Typical run times ranged from 20 to 25 min. The mean P2VP density was determined for selected dried latexes using a helium pycnometer (AccuPyc 1330 instrument, Micromeritics, UK). These latex densities ranged from 1.07 to 1.13 g cm-3 with no obvious influence of particle size. Dynamic Light Scattering. Hydrodynamic particle diameters were measured at 25 °C using the same Malvern Nanosizer ZEN 3600 instrument setup described above for the particle growth experiments. All measurements were performed in triplicate on 0.01 wt % dispersions. Aqueous Electrophoresis. Zeta potentials were calculated from the measured electrophoretic mobilities using a Malvern Instruments Nanosizer ZEN 3600. Measurements were obtained as a function of pH on diluted dispersions (0.01 wt %) in 0.01 M NaCl by gradually adding HCl to induce the latex-to-microgel transition, starting from an initial pH of around 10. Zeta potentials were averaged over 20 runs. The variance was typically within the size of the data points shown. 1H NMR Spectroscopy. Selected dried latexes prepared in the absence of any DVB cross-linker (see entries 7 and 17 in Table 1) were dissolved in CD2Cl2 and 1H NMR spectra were recorded using a 250 MHz Bruker Avance DPX 250 spectrometer. Scanning Electron Microscopy. Micrographs were obtained using a JEOL JSM 6400 instrument operating at 15 kV. All samples were sputter-coated with a thin overlayer of gold prior to inspection to prevent sample-charging effects. Kinetic Studies of the Latex-to-Microgel Swelling Transition. Preliminary kinetics of swelling studies were carried out by (21) Routh, A. F.; Vincent, B. J. Colloid Interface Sci. 2004, 273, 435.

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Table 1. Effect of Varying the Synthesis Parameters on the Mean Diameters and Polydispersities of Poly(2-vinylpyridine) Latexes Prepared Using DVB Cross-linker, PEGMA Macromonomer, and a Cationic Aliquat 336 Surfactant. The wt % values in columns 2 to 5 are relative to the 2VP/DVB comonomer mixture.

entry number

PEGMA stabilizer (wt %)

Aliquat 336 surfactant (wt %)

DVB cross-linker (wt %)

AIBA initiator (wt %)

number-averagea diameter (nm)

weight-averageb diameter (nm)

intensity-average (hydrodynamic) diameterc (nm)

polydispersity indexc

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15d 16 17

10.0 10.0 10.0 9.0 10.0 10.0 10.0 10.0 9.0 9.0 9.0 9.0 9.0 9.0 0 0 0

10.0 5.0 4.0 3.0 10.0 10.0 10.0 10.0 3.0 3.0 3.0 3.0 3.0 3.0 0 10.0 0

1.0 1.0 1.0 1.0 1.0 1.0 0 0 0 0.4 0.6 0.8 1.5 2.0 1.0 1.0 0

1.0 1.0 1.0 1.0 1.7 2.0 1.0 2.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

370 460 540 620 800 960 380 940 670 620 580 570 550 550 150 370 360

370 470 530 630 810 970 390 950 690 630 590 580 560 570 150 370 370

380 480 560 640 830 1010 400 970 700 650 600 590 570 570 160 380 380

0.08 0.06 0.09 0.02 0.08 0.09 0.07 0.17 0.12 0.07 0.03 0.08 0.01 0.06 0.06 0.17 0.17

a Estimated by scanning electron microscopy. b Measured by disk centrifuge photosedimentometry at 20 °C. c Measured by dynamic light scattering at 20 °C. d Prepared by surfactant-free emulsion polymerization at 1% solids content.

monitoring the turbidimetry changes associated with a pH jump using a commercial stopped-flow apparatus (SFM 300, Bio-logic Science Instruments, France) combined with a microprocessor (MPS 60, Bio-logic Science Instruments, France) that allows accurate mixing. Aqueous dispersions comprising 0.01 wt % P2VP latex in 0.01 M NaCl were prepared at pH 10. Microgel swelling was induced in the former solution by rapid mixing (within 0.1 ms) with an acetate buffer at pH 3.8. The latex-to-microgel swelling transition was followed by monitoring the optical absorbance at a fixed wavelength of 500 nm, recording at 500 µs intervals. Three kinetic runs were recorded for all experiments and in each case excellent reproducibility was observed.

Results and Discussion Sterically stabilized microgels were of particular interest to us since in principle this approach should offer the best prospect for microgel syntheses at high solids. In this context, it is noteworthy that the charge-stabilized P2VP microgels reported by Loxley and Vincent18 were synthesized at only 1% solids, which is much too low to be feasible on an industrial scale. The synthesis parameters for the P2VP microgels prepared at approximately 10% solids using the DVB cross-linker are summarized in Table 1. In principle, significantly higher solids (>20%) should be readily achievable using monomer-starved feeds. The water used in these syntheses had an initial pH of approximately 6. On addition of the basic 2VP monomer, the solution pH increased to around pH 8.3. Under these conditions, most of the 2VP is immiscible with water and its polymerization therefore proceeded under emulsion conditions. According to a technical data sheet supplied by a manufacturer of 2VP (Reilly, IN), the water solubility of this monomer is approximately 2.8 g/L. This may well be sufficiently high to allow homogeneous nucleation in aqueous solution, rather than the heterogeneous nucleation that characterizes the polymerization of more hydrophobic monomers such as styrene.22 The final solution pH in these emulsion polymerization syntheses was around pH 6.3, which is somewhat lower than the initial solution pH of 8.3. Presumably, this is because the P2VP microgel is significantly less basic than 2VP monomer. Inspecting Table 1, if the amount (22) Lovell, P. A.; El-Aasser, M. Emulsion Polymerization and Emulsion Polymers; Wiley: New York, 1997.

of Aliquat 336 cosurfactant is reduced (see entries 1-3), the mean particle diameter increases, as expected. Two P2VP syntheses (entries 5 and 6) were carried out at higher initiator concentrations. By empirical variation of essentially just these two parameters, we were able to control the mean latex diameter over a wide range (see Table 1). Emulsion polymerizations typically yield latexes with mean diameters ranging from 100 to 500 nm,22-24 with larger latexes usually requiring either seeded growth25 or monomer swelling techniques.26,27 Thus, it is quite unusual that our latex formulation produces mean particle diameters of almost 1 µm with relatively narrow size distributions at 10% solids using a simple “one-shot” technique.28 Three typical particle growth curves obtained using DLS to monitor the latex syntheses conducted at 60 °C are shown in Figure 2. There is reasonably good reproducibility between the final intensity-average diameters indicated in this figure and the three corresponding P2VP latexes shown in Table 1 (see entries 1, 4, and 6). For example, the 370 nm diameter latex in Table 1 had a final diameter of approximately 410 nm in Figure 2. Both this latex and the 610 nm latex were prepared using 1.0 wt % AIBA initiator (based on 2VP monomer) and no further increase in particle diameter was observed after 2 h in both cases. This was taken to indicate cessation of the polymerization. In contrast, the P2VP latex prepared using 2.0 wt % AIBA required around 3 h to reach its final diameter of approximately 1050 nm. Thus, these batch emulsion polymerization syntheses are reasonably efficient, as expected. The pH-responsive PDEA and PDPA latexes reported earlier19a had glass transition temperatures (Tg) below ambient temperature and hence tended to form films on the sample stubs. This made imaging individual latex particles extremely difficult. In contrast, the Tg of P2VP is greater than 100 °C, which leads to relatively hard, rigid latexes that are well-suited to electron microscopy (23) Fitch, R. M. Polymer Colloids; Academic Press: San Diego, CA, 1997. (24) Cairns, D. B.; Armes, S. P.; Chehimi, M. M.; Perruchot, C.; Delamar, M. Langmuir 1999, 15, 8059. (25) (a) Chung-Li, Y.; Goodwin, J. W.; Ottewill, R. H. Prog. Colloid Polym. Sci. 1976, 60, 163. (b) Yamada, Y.; Sakamoto, T.; Gu, S.; Cono, M. J. Colloid Interface Sci. 2005, 281, 249. (26) Okubo, M.; Shiozaki, M.; Tsujihiro, M.; Tsukuda, Y. Colloid Polym. Sci. 1991, 269, 222. (27) Ugelstad, J.; Kaggerud, K.; Hansen, F.; Berge, A. Macromol. Chem. 1979, 180, 737.

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Figure 2. Particle growth kinetic data obtained for three poly(2vinylpyridine) latexes prepared at 60 °C using the following formulations (see also Table 1): (9) 1.0 wt % AIBA initiator, 10.0 wt % Aliquat 336 surfactant, and 10.0 wt % PEGMA stabilizer; (2) 2.0 wt % AIBA initiator, 10.0 wt % Aliquat 336 surfactant, and 10.0 wt % PEGMA stabilizer; ([) 1.0 wt % AIBA initiator, 3.0 wt % Aliquat 336 surfactant, and 9.0 wt % PEGMA stabilizer.

studies. Near-monodisperse spherical morphologies were observed for selected sterically stabilized latexes by scanning electron microscopy; see Figure 3. The mean diameters estimated from these images are generally lower than those obtained from DLS studies (see Table 1). There are several reasons for this apparent discrepancy. First, electron microscopy reports numberaverage diameters, whereas DLS reports intensity-average diameters. Thus, particle size distributions with finite polydispersities are always oversized by DLS, especially with the backscattering detector employed in our Malvern instrument. Second, DLS is also sensitive to the solvated PEGMA chains surrounding each P2VP particle, whereas this steric stabilizer layer contributes negligible thickness under the high vacuum conditions required for SEM. It is perhaps worth emphasizing that the linear, non-crosslinked P2VP latexes (e.g., entries 7 to 9 in Table 1) prepared at pH 8.3 simply dissolVed in acidic solution: on returning to pH 9, the soluble P2VP chains precipitated from solution. This control experiment confirmed that cross-linking is essential for the protonated P2VP chains to “remember” their former latex form. In our preliminary P2VP syntheses we examined the use of divinylbenzene (DVB), ethylene glycol dimethacrylate, and poly(propylene glycol) diacrylate as cross-linkers. However, the latter two cross-linkers tended to produce varying amounts of coagulum, whereas our syntheses with DVB were invariably coagulumfree. The reason for this difference may be related to the lower water solubility of DVB compared to those of the diacrylate and dimethacrylate cross-linkers, which should ensure better partitioning of DVB within the 2VP monomer droplets. Determination of the PEGMA stabilizer contents of the P2VP latexes prepared using 1.0% DVB cross-linker by 1H NMR spectroscopy proved difficult because the high viscosities of the swollen microgel solutions led to very broad NMR signals and hence poorly resolved spectra. This unexpected problem was observed for a range of NMR solvents (including DCl/D2O in the presence of added salt). Thus, several P2VP latexes were prepared under identical conditions in the absence of any DVB cross-linker (see entries 7-9 in Table 1). These non-cross-linked P2VP particles completely dissolved in organic solvents such as CD2Cl2, which allowed much better quality NMR spectra to be obtained. Figure 5 shows the 1H NMR spectra recorded in CD2Cl2

Dupin et al.

for a PEGMA-stabilized, non-cross-linked P2VP latex (entry 7 in Table 1; spectrum A) and a charge-stabilized non-cross-linked P2VP latex (entry 17 in Table 1; spectrum B). The additional signal at δ 3.6 ppm in the former spectrum is assigned24 to the oxyethylene protons of the grafted PEGMA stabilizer; comparison of this peak integral with that of the signals at δ 6.0-8.5 ppm due to the four aromatic protons of the 2VP residues enabled the PEGMA stabilizer content of this latex to be calculated. Two other linear PEGMA-stabilized P2VP latexes (entries 8 and 9 in Table 1) were also analyzed by this method. In each case the PEGMA stabilizer was estimated to be around 2.0 wt %. If it is assumed that the DVB cross-linker does not influence the PEGMA grafting efficiency, the cross-linked P2VP microgels should have essentially the same PEGMA contents as the corresponding linear P2VP latexes. This assumption is supported by qualitative FT-IR spectroscopy studies, which confirmed the presence of the PEGMA stabilizer in P2VP particles prepared with (microgel) and without (linear latex) DVB cross-linker (see Supporting Information). Using the equation As ) 3/FR (where As is the surface area per unit mass, F is the density of P2VP, and R is the particle radius) and assuming that all of the PEGMA stabilizer is located on the outside of the latex particles, the adsorbed amount, Γ, of PEGMA in mg m-2 can be calculated for each of the three linear P2VP latexes (see Table 1). Such Γ values ranged from 1.3 to 3.9 mg m-2, which are similar to the Γ values calculated for the PEGMA-stabilized poly(tert-amine methacrylate) microgels reported by Amalvy and co-workers.19 Another advantage of the linear P2VP latexes is that their molecular weight distributions can be assessed. Latex dissolution in CHCl3 allowed GPC analyses to be undertaken using polystyrene calibration standards, which indicated an Mn of approximately 79000 and an Mw/Mn of 3.6 for entry 9 in Table 1. This Mn corresponds to a mean degree of polymerization of around 750. Assuming that the primary chain length is not affected by the addition of cross-linker, the nominal 1.0 wt % DVB (which corresponds to an actual degree of cross-linking of 0.80%) used in most of the microgel syntheses is sufficient to ensure that essentially all the primary chains are cross-linked within the microgel, i.e., that there is no significant fraction of soluble P2VP chains. Disk centrifuge photosedimentometry (DCP) is an excellent particle sizing technique for particles that behave as “hard spheres”. However, for sterically stabilized latexes it is necessary to assume that the stabilizer thickness has a negligible effect on the effective particle density.24 Fortunately, this is a reasonable approximation for the smallest P2VP latex (where the PEGMA stabilizer thickness of approximately 5 nm is less than 2% of the particle diameter) and negligible errors are incurred for the larger latexes. The weight-average particle size distributions obtained from DCP confirmed that a range of latexes prepared using 1.0% DVB cross-linker (entries 1-6 in Table 1) were near-monodisperse; moreover, adjusting the synthesis parameters allows the entire size distribution to be shifted (see Figure 4). Acid-induced swelling of these P2VP latexes was monitored by DLS using HCl to lower the solution pH. Each of the 1.0% cross-linked microgels (entries 1-6 in Table 1) exhibited a critical latex-to-microgel swelling transition between pH 4.0 and 4.5 (see Figure 6). This is significantly lower than the pKa of 4.92 for linear P2VP homopolymer (ex. Aldrich) determined by acid titration. One possible interpretation is that a significantly higher degree of protonation is required to cause microgel swelling, as compared to dissolution of the linear P2VP homopolymer. However, acid titration (data not shown) of the P2VP microgel indicated a pKa of approximately 4.1, which correlates well with

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Figure 3. Scanning electron micrographs of selected near-monodisperse PEGMA-stabilized poly(2-vinylpyridine) latexes (see entries 1-6 in Table 1) dried from aqueous solution at ambient temperature. The scale bar is 2 µm in all cases.

Figure 4. Disk centrifuge photosedimentometry curves of selected poly(2-vinylpyridine) latexes (entries 1-6 in Table 1) prepared by emulsion polymerization at 60 °C.

the critical swelling pH range indicated by the DLS studies. This suggests that there is a shift in the pKa due to the branched microgel nature of the P2VP chains. Similar observations have been recently described by Sherrington and co-workers, who reported suppression of the so-called “polyelectrolyte effect” for branched poly(methacrylic acid) relative to linear poly(methacrylic acid).29 To test this hypothesis, several additional microgels were synthesized using other degrees of cross-linking (see entries 9-14 in Table 1). Further acid titration studies revealed an approximately linear relationship between the pKa

of the P2VP chains and their degree of cross-linking (see Figure 7). Thus, addition of 2.0% DVB cross-linker (of which only 80 mol % is the active 1,4-divinyl isomer) to the emulsion polymerization of 2VP is sufficient to lower the pKa (and also the critical swelling pH) of the resulting P2VP particles from around 4.75 to 3.85. Presumably, the branched nature of the microgel allows more degrees of freedom for the polyelectrolytic chains in aqueous solution, which in turn lessens the intrachain electrostatic repulsion between neighboring protonated 2VP residues. The intensity-average diameters of the swollen microgels at low pH are at least 3 times larger than those of the nonsolvated latexes at high pH. This corresponds to volumetric swelling factors of around 27. However, the two largest P2VP latexes exhibit volumetric swelling factors of up to 64. Although the reliability (28) One reviewer of this paper has suggested that our tentative hypothesis of a “homogeneous nucleation” mechanism is not correct since this should lead to a larger number of particles with smaller size, rather than the larger latexes that are actually observed. The same reviewer postulated that either “nucleation within monomer droplets” or the “collision of droplets due to their incomplete stabilization” are more likely explanations. It is clear that this formulation is worthy of more detailed kinetic studies, but unfortunately this is beyond the scope of the current manuscript. (29) Graham, S.; Cormack, P. A. G.; Sherrington, D. C. Macromolecules 2005, 38, 86.

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Figure 5. 1H NMR spectra (CD2Cl2) of (A) a PEGMA-stabilized poly(2-vinylpyridine) latex non-cross-linked using 1.0 wt % DVB (entry 7 in Table 1); (B) charge-stabilized linear poly(2-vinylpyridine) latex prepared in the absence of any PEGMA stabilizer (entry 17 in Table 1). The additional signal at 3.6 ppm in spectrum (A) is assigned to the oxyethylene protons of the PEGMA stabilizer.

Figure 6. Variation of mean hydrodynamic diameter with solution pH for 0.01 wt % aqueous solutions of selected PEGMA-stabilized poly(2-vinylpyridine) latexes (entries 1-6 in Table 1) in the presence of 0.01 M NaCl: 370 nm diameter (0), 480 nm diameter (9), 560 nm diameter (b), 640 nm diameter (2), 830 nm diameter (f), and 1010 nm diameter (4).

of DLS diameters exceeding 3 µm is uncertain,30 this observation may indicate nonuniform distribution of the DVB cross-linker within the particles.31 Aqueous electrophoresis studies were carried out as a function of pH (see Figure 8). The latexes exhibited isoelectric points (IEP) ranging from pH 5.8 to pH 6.7. In each case no precipitation was observed at the IEP, presumably due to the stabilizing effect (30) According to the Malvern manual provided with the Malvern Nanosizer ZEN 3600 instrument, the nominal upper limit for particle sizing using DLS is 3 µm. However, this size limit is based on the probability that larger particles sediment on time scales that are comparable to the analysis time; thus, the particle motion is influenced by gravitational forces as well as Brownian motion. In the case of the acid-swollen microgel particles described in the present study, their effective particle density is so close to that of water that gravitational effects are likely to be negligible even for relatively large microgels. Thus, it is likely that DLS remains a valid particle sizing technique beyond its normal upper size limit for such particles. (31) Hoare, T.; Pelton, R. H. Polymer 2005, 46, 1139.

Figure 7. Influence of the degree of cross-linking of the poly(2vinylpyridine) latexes on their respective pKa values, as determined by acid titration studies. Excellent agreement was obtained between these pKa values and the critical swelling pH observed by visual inspection during the titrations. In each case the degree of crosslinking has been corrected to allow for the 80 mol % purity of the DVB cross-linker (see ref 32).

Figure 8. Electrophoretic mobility vs pH curves obtained for 0.01 wt % aqueous solutions of PEGMA-stabilized poly(2-vinylpyridine) latexes (entries 1-6 in Table 1) in the presence of 0.01 M NaCl: 370 nm diameter ([), 480 nm diameter (2), 560 nm diameter (×), 640 nm diameter (0), 830 nm diameter (b), and 1010 nm diameter (9).

of the outer layer of grafted nonionic PEGMA chains. As expected, protonation of the 2VP residues led to strongly cationic character at low pH for each of the 1.0% cross-linked microgels shown in Figure 8, with zeta potentials ranging from +22 to +32 mV.

Synthesis of Sterically Stabilized Microgels

Figure 9. Swelling behavior of 0.01 wt % aqueous dispersions of a PEGMA-stabilized poly(2-vinylpyridine) latex (entry 1 in Table 1), an Aliquat-stabilized poly(2-vinylpyridine) latex (entry 15 in Table 1), and a surfactant-free charge-stabilized poly(2-vinylpyridine) latex (prepared according to the protocol described by Loxley and Vincent in ref 18; entry 16 in Table 1) as monitored by turbidimetry at 500 nm using the pH jump technique in conjunction with stoppedflow apparatus. In each case the initial dispersion was at pH 10 and latex-to-microgel swelling was induced using an acetate buffer at pH 3.8.

Only weakly anionic character was observed at high pH, which is consistent with the sterically stabilized nature of the microgel particles. In contrast, significantly more negative zeta potentials (-15 mV) were obtained above pH 8 for the charge-stabilized P2VP latexes. Swelling Kinetics for the Latex-to-Microgel Transition. The turbidity of the diluted P2VP particles changed significantly on switching from high pH (milky, nonsolvated latex) to low pH (almost transparent swollen microgel). This change in physical appearance can be used to monitor the kinetics of swelling/ deswelling using the classical pH jump method. Latex-to-microgel swelling was induced by rapid mixing (within 0.1 ms) of a P2VP latex initially at pH 10 with an acetate buffer at pH 3.8 using a stopped-flow apparatus. Loxley and Vincent reported that their charge-stabilized P2VP latexes responded surprisingly slowly to changes in pH.18 According to these workers, the Tanaka equation τ ) a2/D (where a is the swollen diameter and D is the diffusion coefficient of the gel), which was originally derived for macroscopic gels,32 predicts swelling on millisecond time scales for their P2VP microgels, which had mean diameters of around 200 nm. However, Loxley and Vincent observed swelling times on the order of 60 s, for levels of DVB cross-linker varying from 0.25 to 1.5%.18 In contrast, we obtained much faster swelling behavior within a time scale of around 100 ms for our 370 nm diameter sterically stabilized P2VP latex cross-linked with 0.80% DVB (see entry 1 in Table 1 and Figure 9).33 These latter particles are almost twice as large as the charge-stabilized P2VP latexes studied by Loxley and Vincent,18 so in principle a slower rate of swelling had been expected. To investigate whether the colloid (32) Tanaka, T.; Filmore, D. J. J. Chem. Phys. 1979, 70, 1214. (33) It is worth noting that the purity of the DVB cross-linker can vary significantly depending on the grade and the supplier. The DVB grade used by Loxley and Vincent18 comprised only 55 mol % of the bifunctional 1,4-substituted isomer, and these authors took this purity into account when calculating their target degrees of cross-linking. In contrast, the DVB grade used in the present study comprised 80 mol % DVB; thus, a nominal target degree of cross-linking of 1.0% gives an actual degree of cross-linking of 0.8% (assuming complete incorporation of the DVB comonomer within the latex). It is emphasized that these differences are irrelevant to our comparison of the acid-induced swelling kinetics in Figure 9 since the same DVB grade was used at the same target degree of cross-linking (i.e., a 1.0% nominal degree of cross-linking but 0.8% actual degree of cross-linking based on the DVB purity) for the sterically stabilized, the charge-stabilized, and the surfactant-stabilized P2VP particles (see entries 1, 15, and 16 in Table 1).

Langmuir, Vol. 22, No. 7, 2006 3387

stabilization mechanism influenced the kinetics of swelling, a 150 nm diameter charge-stabilized P2VP latex was synthesized with 0.80% DVB cross-linker using essentially the same protocol as that described by Loxley and Vincent.18 In our hands, the latex-to-microgel swelling transition for this latter P2VP latex occurred within 200 ms, which again is much faster than that reported by the Bristol group. We are unable to account for this discrepancy, but it is noteworthy that this much shorter characteristic swelling time is significantly closer (but not identical) to that predicted by the Tanaka equation. Finally, we examined a surfactant-stabilized P2VP latex prepared using 0.80% DVB cross-linker in the presence of Aliquat 336 but in the absence of any PEGMA stabilizer (see entry 8 in Table 1). This latter latex had a particle diameter comparable to that of the sterically stabilized P2VP latex (see entry 1) and also exhibited a latexto-microgel swelling transition on a time scale of 100-150 ms. In future studies we plan to systematically examine the kinetics of swelling and deswelling of selected sterically stabilized P2VP latexes (e.g., entries 1-6 in Table 1) in order to examine whether the Tanaka equation is valid for microgels.

Conclusions A series of sterically stabilized, lightly cross-linked, pHresponsive poly(2-vinylpyridine)-based latexes have been synthesized at 10% solids by emulsion polymerization at pH 8 using a cationic Aliquat 336 surfactant and a monomethoxy-capped poly(ethylene glycol) monomethacrylate macromonomer as a reactive polymeric stabilizer. Variation of the concentrations of the PEGMA stabilizer, cationic surfactant, and cationic initiator allowed the mean particle diameter of the P2VP latexes to be controlled over a surprisingly wide range (370-970 nm). In each case narrow size distributions were obtained, as judged by both disk centrifuge photosedimentometry and scanning electron microscopy. The critical pH for the latex-to-microgel swelling transition of these P2VP particles varied systematically from 4.55 to 3.85, depending on their degree of cross-linking. Acid titration studies of linear (i.e., non-cross-linked) P2VP latex indicated a pKa of approximately 4.75, which suggests that the branched nature of the microgel causes a significant shift in the pKa of the P2VP chains. Dynamic light scattering studies indicated volumetric swelling factors of up to 3 orders of magnitude and aqueous electrophoresis studies confirmed that the swollen microgels became highly cationic at low pH, as expected. Preliminary studies of the kinetics of swelling of these sterically stabilized (and also related charge-stabilized) pH-responsive latexes using stopped-flow apparatus combined with the pH jump technique indicated significant discrepancies with previously published data. It is emphasized that the much faster rates of swelling observed for these new poly(2-vinylpyridine) latexes are much closer to the millisecond time scales predicted by the Tanaka equation. Acknowledgment. Rohm and Haas (France) is thanked for funding a PhD studentship for D.D. and also for permission to publish this work. Cognis Performance Chemicals (Hythe, UK) is thanked for donating the PEGMA stabilizer. Supporting Information Available: FT-IR spectra of lightly cross-linked and linear PEGMA-stabilized P2VP latexes. This material is available free of charge via the Internet at http://pubs.acs.org. LA053258H