Influence of Secondary Components on the Synthesis of Self-Cross

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Langmuir 2005, 21, 545-551

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Influence of Secondary Components on the Synthesis of Self-Cross-Linked N-Isopropylacrylamide Microgels Jun Gao and Barbara J. Frisken* Department of Physics, Simon Fraser University, Burnaby, BC V5A 1S6, Canada Received June 7, 2004. In Final Form: September 23, 2004 This work presents systematic studies of cross-linker-free microgels formed by copolymerization of N-isopropylacrylamide (NIPAAm) and various secondary monomer components in water under standard reaction conditions. The sizes, solid densities, and volume phase transitions of these particles have been characterized through static and dynamic laser light-scattering experiments. We find that introducing a hydrophobic component, for example, styrene (St) or methyl methacrylate (MMA), leads to particles with smaller sizes and higher solid densities, while the volume phase transition shifts to lower temperatures. On the other hand, introducing a hydrophilic component such as acrylamide (AAm) or acrylic acid (AA) leads to larger particles with lower solid densities and a volume phase transition that shifts to higher temperatures and is broadened. The molar mass changes little in either case. Introducing a charged component such as sodium styrene sulfonate (NaSS) or poly(sodium styrene sulfonate) (PNaSS) leads to a sharp decrease in molar mass and particle size, and a very broad phase transition. These trends provide good guidance for synthesizing both self-cross-linked and cross-linked copolymerized microgels of different properties.

Introduction Poly(N-isopropylacrylamide) (PNIPAAm) microgels were first synthesized by Pelton and Chibante in 1986,1 and since then they have been the subject of various investigations. Not only are they members of an extremely useful class of colloidal particles that form stable dispersions in water,2 but, like PNIPAAm chains and gels, PNIPAAm microgels in an aqueous environment are extremely thermosensitive and are characterized by a reversible decrease in volume on heating. This property has been investigated extensively because of many potential applications, particularly to drug delivery and novel materials.3 Crucial for these applications is the ability to control particle size, swelling ratio, and chemical activity. This control can be accomplished by copolymerization of other components with NIPAAm during the polymerization process. Recent studies report, for example, the synthesis of pH-sensitive particles4 and particles with specific chemical activity.5-7 These results have been achieved by incorporation of a large range of chemical moieties including cationic groups,8-11 carboxylic groups,4,7,11-17 cyano groups,18 amine groups,19,20 1-vinylimidazole,6 * Corresponding author. E-mail: [email protected]. (1) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 20, 247. (2) Saunders, B. R.; Vincent, B. Adv. Colloid Interface Sci. 1999, 80, 1. (3) Pelton, R. Adv. Colloid Interface Sci. 2000, 85, 1. (4) Zhou, S. Q.; Chu, B. J. Phys. Chem. B 1998, 102, 1364. (5) Elmas, B.; Onur, M. A.; Senel, S.; Tuncel, A. Colloid Polym. Sci. 2002, 280, 1137. (6) Kazakov, S.; Kaholek, M.; Teraoka, I.; Levon, K. Macromolecules 2002, 35, 1911. (7) Choi, S. H.; Yoon, J. J.; Park, T. G. J. Colloid Interface Sci. 2002, 251, 57. (8) Meunier, F.; Elaissari, A.; Pichot, C. Macromol. Symp. 2000, 150, 283. (9) Zha, L. S.; Hu, J. H.; Wang, C. C.; Fu, S. K.; Luo, M. F. Colloid Polym. Sci. 2002, 280, 1116. (10) Pinkrah, V. T.; Snowden, M. J.; Mitchell, J. C.; Seidel, J.; Chowdhry, B. Z.; Fern, G. R. Langmuir 2003, 19, 585. (11) Ito, S.; Ogawa, K.; Suzuki, H.; Wang, B.; Yoshida, R.; Kokufuta, E. Langmuir 1999, 15, 4289. (12) Huang, J.; Wu, X. Y. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 2667.

styrene,21-23 N-tert-butylacrylamide,24 and poly(ethylene glycol).25,26 However, a systematic study of the influence of secondary components on PNIPAAm microgel formation and on the size and solid density of the resultant particles is lacking. PNIPAAm microgels are typically made3 by precipitation polymerization of NIPAAm monomer in the presence of a cross-linker such as N,N′-methylenebisacrylamide (BIS), although several authors have indicated that it is possible to make stable particles in the absence of an added cross-linker.2,27,28 It appears that self-cross-linking occurs in this system through a chain transfer reaction at tertcarbon sites.29 We recently published results of systematic studies of PNIPAAm microgels made without added crosslinker.30 These results detailed the conditions necessary for the formation of stable particles and the relationship between particle size and solid density under different reaction conditions. In this work, we report studies of cross-linker-free particles made with different secondary components. We (13) Morris, G. E.; Vincent, B.; Snowden, M. J. J. Colloid Interface Sci. 1997, 190, 198. (14) Kratz, K.; Hellweg, T.; Eimer, W. Colloids Surf. 2000, 170, 137. (15) Kim, S.; Healy, K. E. Biomacromolecules 2003, 4, 1214. (16) Hoare, T.; Pelton, R. Langmuir 2004, 20, 2123. (17) Hoare, T.; Pelton, R. Macromolecules 2004, 37, 2544. (18) Zhou, G.; Elaissari, A.; Delair, T.; Pichot, C. Colloid Polym. Sci. 1998, 276, 1131. (19) Hu, Z. B.; Huang, G. Angew. Chem., Int. Ed. 2003, 42, 4799. (20) Xu, J. J.; Timmons, A. B.; Pelton, R. Colloid Polym. Sci. 2004, 282, 256. (21) Duracher, D.; Sauzedde, F.; Elaissari, A.; Perrin, A.; Pichot, C. Colloid Polym. Sci. 1998, 276, 219. (22) Hellweg, T.; Dewhurst, C. D.; Eimer, W.; Kratz, K. Langmuir 2004, 20, 4330. (23) Rossi, S.; Lorenzo-Ferreira, C.; Battistoni, J.; Elaissari, A.; Pichot, C.; Delair, T. Colloid Polym. Sci. 2004, 282, 215. (24) Yi, Y. D.; Bae, Y. C. J. Appl. Polym. Sci. 1998, 67, 2087. (25) Zhu, P. W.; Napper, D. H. Langmuir 2000, 16, 8543. (26) Gan, D. J.; Lyon, L. A. Macromolecules 2002, 35, 9634. (27) McPhee, W.; Tam, K. C.; Pelton, R. J. Colloid Interface Sci. 1993, 156, 24. (28) Kratz, K.; Hellweg, T.; Eimer, W. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 1603. (29) Gao, J.; Frisken, B. J. Langmuir 2003, 19, 5212. (30) Gao, J.; Frisken, B. J. Langmuir 2003, 19, 5217.

10.1021/la0485982 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/16/2004

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Table 1. Relative Molar Masses, Hydrodynamic Radii, and Apparent Solid Densities of Copolymerized PNIPAAm Gel Spheres at 25 and 40 °C in Water secondary component wt % mol %

batch

name

A B C D E F G H I J

none St MMA AAm AA NaA PNaSS-10 PNaSS-5 NaSS NaSS

a

0 4.2 7.9 7.9 7.7 3.8 7.9 7.9 7.9 2.5

0 4.5 8.8 12 12 4.5 1.6a 1.4a 4.5 1.4

Mw,app (g/mol)

〈Rh〉25°C (nm)

Fapp,25°C (g/cm3)

〈Rh〉40°C (nm)

Fapp,40°C (g/cm3)

1.5 × 109 9.0 × 108 1.2 × 109 6.6 × 108 1.7 × 109 1.6 × 108 4.5 × 107 2.0 × 107 3.2 × 106 2.2 × 106

390 270 340 450 670 190 110 90 70 54

0.010 0.018 0.013 0.003 0.002 0.009 0.013 0.011 0.004 0.006

133 97 119 153 120 73 49 41 49 29

0.25 0.39 0.29 0.07 0.38 0.16 0.15 0.12 0.011 0.036

Expressed as mol % NaSS.

have introduced small amounts of hydrophobic, hydrophilic, and charged components and have obtained information about the size, polydispersity, and solid density of the resultant particles through dynamic and static light-scattering experiments. To ensure random copolymerization, secondary monomers were chosen that have reaction rates similar to that of NIPAAm. The hydrophobic components used were styrene (St) and methyl methacrylate (MMA). The hydrophilic components used were acrylamide (AAm) and acrylic acid (AA). The charged components used were sodium acrylate (NaA), sodium styrene sulfonate (NaSS), and poly(sodium styrene sulfonate) (PNaSS). The results not only provide a better understanding of how secondary components influence particle size and density, but also provide guidance for the synthesis of different varieties of copolymerized PNIPAAm microgels. Because these are essentially lightly cross-linked microgel particles, the results should also be appropriate for particles made with added cross-linker. Materials and Method Synthesis of Microgels. NIPAAm (99%, Acros Organics) was recrystallized from hexane/acetone solutions. St (99%), MMA (99%), AA (99%), anhydrous AAm, NaSS‚xH2O (containing 10 wt % water), and potassium persulfate (KPS, reagent grade) were obtained from Aldrich-Sigma; the chemicals were then used as received. Macromonomers PNaSS-5 (containing 5 units of charged NaSS, number average molar mass Mn of 3500 g/mol) and PNaSS-10 (10 units NaSS, Mn of 6200) were synthesized by S. Holdcroft and colleagues at Simon Fraser University using pseudo-living free radical polymerization of NaSS followed by termination with divinylbenzene.31 A Milli-Q Plus water purification system (Millipore, Bedford MA) was used to provide purified water for these experiments; particulate matter was removed with a final 0.2 µm filter. Particle synthesis was performed as described previously.29 Water-soluble monomers were first dissolved in water and then heated in a nitrogen atmosphere in a reactor vessel to an incubation temperature of 70 °C. The reaction was initiated by adding KPS solution at a concentration of approximately 20 mg/mL. Nonwater soluble monomers were added to the reactor vessel immediately before the KPS solution. Total monomer and KPS masses were 1 g and 40 mg, respectively, in a final volume of 100 mL. Ten batches (A-J) were studied in detail; synthesis conditions and experimental results are summarized in Table 1. Laser Light Scattering. The apparatus used for lightscattering measurements was an ALV-5000 spectrometer/ goniometer equipped with a digital correlator and a heliumneon laser. Light-scattering measurements were made on dilute dispersions with concentrations of about 0.1 mg/mL. We used static light scattering (SLS) to measure the weight average molar mass Mw and the z-average root-mean-square radius of gyration Rg. For small particles, we used a standard (31) Ding, J. F.; Chuy, C.; Holdcroft, S. Adv. Funct. Mater. 2002, 12, 389.

Zimm plot analysis to find Mw and Rg from the Rayleigh ratio Rvv(q) determined from the time-averaged scattered intensity32

1 1 Kc = 1 + Rg2q2 + 2A2c 3 Rvv(q) Mw

(

)

(1)

where K ) 4π2n2(dn/dc)2/(NAλo4), q ) (4πn/λo) sin(θ/2) is the scattering wave vector, and A2 is the second virial coefficient, with NA, n, c, λo, and θ as Avogadro’s constant, the solvent refractive index, the concentration (g/mL), the light wavelength in a vacuum, and the scattering angle, respectively. For larger particles, we fit a function of the following form to the data

Rvv(q,R) ) KcMwP(q,R)

(2)

where P(q, R) ) [3/(q3R3)(sin(qR) - qR cos(qR))]2 is the form factor for uniform spheres with radius R. We used a dn/dc value of 0.167 mL/g, the value for pure NIPAAm in water at 25 °C, for all calculations.33 Because the amount of secondary component is so small, we do not expect large variations in dn/dc for different copolymers. We used dynamic light scattering (DLS) to measure the time correlation function of the scattered intensity g2(τ) as a function of the decay time τ. The hydrodynamic radius 〈Rh〉 and polydispersity of the samples were determined from analysis of g2(τ) using either CONTIN34 or cumulant analysis.35 z-Average particle size distributions fz(Rh) were determined from CONTIN analysis. The solid density of the particles was calculated from results of both static and dynamic light-scattering measurements. Previous studies have shown that PNIPAAm microgels are spherical.1 The narrow particle size distribution allowed us to calculate the solid density F from the simple equation Mw ) (4/3)πF〈Rh〉3NA. Turbidity. The turbidity τ of a dispersion is caused by both absorption and scattering of the incident light by the particles. For a system dominated by scattering, the turbidity is just the integral of the scattered intensity over all scattering angles and as such contains information about the size and solid density of the microgels. The turbidity was measured with a HewlettPackard UV-vis spectrometer at 532 nm in a quartz cell with inner dimensions of 10 mm × 10 mm × 42 mm.

Results and Discussion Evidence of Particle Formation. Dispersions with varying turbidities rather than transparent solutions were obtained for all batches reported in this Article, indicating particle formation. When concentrated, dispersions consisting of particles of sufficient size showed brilliant iridescent colors consistent with the crystalline structures (32) Zimm, B. H. J. Chem. Phys. 1948, 16, 1099. (33) Gao, J.; Wu, C. Macromolecules 1997, 30, 6873. (34) Provencher, S. W. Makromol. Chem. 1979, 180, 201. (35) Chu, B. Laser Light Scattering; Academic Press: Boston, 1991.

Synthesis of Self-Cross-Linked NIPAAm Microgels

Figure 1. (a) Zimm plot of SLS data for PNIPAAm-co-NaSS microgels (batch I) dispersed in water at 25 °C at concentrations of 5.04 × 10-5 (4), 9.87 × 10-5 (0), 1.64 × 10-4 (3), and 2.12 × 10-4 (*) g/mL. The data were measured at scattering angles between 20° and 150°. The O and b circles are the results of extrapolating the data to zero concentration and zero angle, respectively. Values of 3.2 × 106 g/mol and 52 nm for the molar mass and radius of gyration were obtained using eq 1. (b) The solid curve is a fit of eq 2 to SLS data (b) for PNIPAAm microgels (batch A) dispersed in water at 25 °C at a concentration of 6.41 × 10-5 g/mL. The molar mass and static radius were found to be 1.5 × 109 g/mol and 340 nm, respectively.

that can be obtained with close-packed near-monodisperse particles.36,37 Results for dynamic and static light scattering confirmed particle formation. Particle distributions were generally near-monodisperse. Values obtained for the ratio of the radius of gyration to the hydrodynamic radius Rg/〈Rh〉 ranged from 0.5 to 1, consistent with the particles having a spherical structure. The Zimm plot showed linear behavior for small particles even for samples with high charge content, indicating that, in dilute dispersions, electrostatic interactions between particles could be neglected. Typical results for PNIPAAm-co-NaSS microgels (batch I) are shown in Figure 1a. Molar masses measured at 25 and 40 °C were consistent for all but one sample, indicating that most of the secondary components selected were incorporated into the PNIPAAm microgels.30 For larger particles, for example, pure NPNIAPM microgels from batch A as shown in Figure 1b, eq 2 was fit to the data to determine molar mass and particle size. Particle Size and Volume Phase Transition: Influence of Neutral Secondary Components. Figure 2a shows the particle size distributions as measured by DLS for samples made with a neutral secondary component. Introduction of both hydrophobic components, St and MMA, and hydrophilic components, AAm and AA, were investigated. The b symbols show the data for pure PNIPAAm microgels made under the same reaction conditions. Introducing a hydrophobic secondary group led to a smaller particle size, while introducing a hydrophilic secondary group led to a larger particle size as compared to pure PNIPAAm particles. In general, the particle sizes are narrowly distributed, although incor(36) Senff, H.; Richtering, W. J. Chem. Phys. 1999, 111, 1705. (37) Gao, J.; Hu, Z. B. Langmuir 2002, 18, 1360.

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Figure 2. The influence of a neutral secondary component on (a) the hydrodynamic radius distributions fz(Rh) measured at 25 °C and (b) the volume phase transitions of copolymerized PNIPAAm microgels in water: PNIPAAm-co-St (0, batch B), PNIPAAm-co-MMA (4, batch C), PNIPAAm-co-AAm (], batch D), and PNIPAAm-co-AA (3, batch E). Results for pure PNIPAAm particles (b, batch A) are shown for comparison. Lines are included as a guide for the eyes. DLS was performed at a scattering angle of 30° for dilute water dispersions with a concentration of ∼10-4 g/mL.

porating a secondary component does broaden the particle size distribution. Using the hydrodynamic radius of the fully swollen spheres and the molar mass from SLS measurements, the solid densities of the fully swollen gel spheres (at T ) 25 °C) were calculated to be 1.8%, 1.3%, 1.0%, 0.3%, and 0.2% g/cm3, for PNIPAAm-co-St, PNIPAAm-co-MMA, pure PNIPAAm, PNIPAAm-co-AAm, and PNIPAAm-co-AA particles, respectively. Results at 25 and 40 °C are summarized in Table 1. The temperature dependence of the particle size expressed relative to the size at 40 °C is shown in Figure 2b for all five batches. All five samples show a distinct volume phase transition. Three of the copolymer samples show swelling ratios 〈Rh〉25°C/〈Rh〉40°C of 2.8-3 similar to that of the pure PNIPAAm particles, while PNIPAAm-co-AA particles show a significantly larger ratio of 5.6. The volume phase transition temperature TVPT shifts about 1 °C lower as hydrophobic components are incorporated and ∼3 °C higher as hydrophilic secondary components are incorporated. The trends in size and changes of the volume phase transition are consistent with the relative hydrophobicity or hydrophilicity of the secondary components. Incorporating the hydrophobic monomers, St and MMA, results in denser particles that have smaller swelling ratios. These particles also show a decrease in the temperature at which the volume phase transition occurs, consistent with recent studies of PNIPAM-co-polystyrene particles22 and PNIPAM/polystyrene core-shell particles.23 At elevated temperatures, the PS or PMMA chain segments may form hydrophobic sites inside the particles that act as hydrophobic attraction centers and facilitate the “coil-to-globule” transition of the PNIPAAm chain segments.33 In contrast, introducing a hydrophilic component leads to bigger particles with smaller solid densities. The volume phase transition broadens and occurs at obviously higher

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Figure 3. The influence of a charged secondary component on (a) the hydrodynamic radius distributions and (b) the volume phase transitions of copolymerized PNIPAAm microgels in water: PNIPAAm-co-NaA (*, batch F) and PNIPAAm-co-NaSS (O, batch I). Results for pure PNIPAAm particles (b, batch A) and PNIPAAm-co-St (0, batch B) are shown for comparison. DLS was performed at a scattering angle of 30° for all samples except for PNIPAAm-co-NaSS, which was performed at 90°. Other experimental parameters were the same as those in Figure 1.

temperatures. These observations are consistent with the particles being more hydrophilic. The swelling ratio for the PNIPAAm-co-AAm particles is lower than expected because even at 40 °C these particles have not yet reached their fully shrunken state. The extreme swelling ratio observed for PNIPAAm-coAA indicates that copolymerization of NIPAAm with acrylic acid is a special case. We believe that this is due to the fact that the dissociation of AA is strongly dependent on pH. Microgel synthesis occurs in an acidic environment due to the decomposition of the initiator KPS. In fact, after synthesis, the pH of the particle dispersion was found to be 2.9. Furthermore, upon dilution of the dispersion to concentrations suitable for DLS characterization, the pH was found to be 4.5. Using a pKa value of 4.25 for AA,38 the calculated ratio of dissociated and nondissociated AA groups was found to be 0.05 and 1.8, respectively, under these two very different conditions. Thus, in the synthesis environment, we believe that AA behaves like a basically neutral monomer that is easily incorporated throughout the microgels as well as at or near their surfaces. Upon dilution, the increase in the ionization of AA results in a significantly larger swelling ratio. A similar increase of the swelling ratio has been observed recently for PNIPAAm-co-vinylacetaic acid microgels as a result of increase in ionization.17 Particle Size and Volume Phase Transition: Influence of Charged Secondary Components. Figure 3a shows that introducing NaA or NaSS comonomers results in obviously smaller particles. Data for pure PNIPAAm and PNIPAAm-co-PS particles are included for comparison. Introducing a charged component also increases TVPT, broadens the volume phase transition, and reduces the shrinking ratio as seen in Figure 3b. In fact, (38) Lide, D. R., Ed. Handbook of Chemistry and Physics, 75th ed.; CRC Press: Boca Raton, FL, 1994-1995; pp 8-45.

Gao and Frisken

Figure 4. The influence of charged secondary components of different size on (a) the hydrodynamic radius distributions and (b) the volume phase transitions of copolymerized PNIPAAm microgels in water: PNIPAAm-co-NaSS (O, batch I), PNIPAAmco-PNaSS-5 (g, batch H), and PNIPAAm-coPNaSS-10 (pentagons, batch G). Results for pure PNIPAAm particles (b, batch A) and PNIPAAm-co-St (0, batch B) are shown for comparison. DLS was performed at a scattering angle of 30° for pure PNIPAAm and PNIPAAm-co-St and at 90° for all other samples. Other experimental parameters were the same as those in Figure 1.

the incorporation of charged groups has a much stronger effect on the broadening of the volume phase transition than the incorporation of neutral hydrophilic components. The density of the particles in both the swollen and the nonswollen state is lower than that of pure PNIPAAm particles. The weak-acid salt NaA, which has a smaller effective charge, has a smaller influence on these properties. The most obvious result of introducing charged groups is a large decrease in particle size. It is apparent that the size of the particles is much more sensitive to the addition of a charged monomer than to the addition of a hydrophobic component. In this case, the charged monomer acts as a polymerizable surfactant that decreases the particle size in the same way as an ionic surfactant.27 However, unlike a surfactant, a charged monomer is chemically bound to the surface of the particles and cannot be removed through dialysis. The decreased density and increased transition temperature are consistent with these particles having an increased hydrophilicity. The charged groups that have been incorporated have a strong affinity to water, and this increases the swelling of the particles, hence decreasing their density. They also increase the resistance of the particles to shrinking, resulting in an increase in TVPT. The incorporation of charged groups also has a profound effect on the swelling ratio, reducing it significantly below that for pure PNIPAAm particles. For PNIPAAm-co-NaSS particles, 〈Rh〉25°C/〈Rh〉40°C is only 1.5; however, the transition is so broad that the particles continue to shrink above 40 °C (data not shown), with an 〈Rh〉52°C/〈Rh〉40°C of 0.90. Particle Size and Volume Phase Transition: Influence of the Size of the Charged Secondary Components. Figure 4 compares the results for three batches of particles obtained upon copolymerization of

Synthesis of Self-Cross-Linked NIPAAm Microgels

NIPAAm with 7.9 wt % NaSS and two macromonomers, PNaSS-5 and PNaSS-10, containing 5 and 10 units of NaSS, respectively. The results for pure PNIPAAm and PNIPAAm-co-St are also included for easy reference. Introducing a charged macromonomer also results in smaller particles and a broader phase transition at a higher temperature. However, the monomer NaSS has the largest influence on all of these properties. The particles made with PNaSS-10 have larger radii, but their swelling factor is smaller than the particles made with PNaSS-5. Like charged monomers, charged macromonomers can also be considered to be polymerizable surfactants. When the synthesis precedes in a polar dispersion agent, such as water, the charged monomers or macromonomers will prefer to stay on the particle surfaces and the repulsion due to the final surface charge density will determine the final size of the particles. Thus, we would expect the size of the particles to decrease as the quantity of charge increases: this affect will be discussed in the last section. It also appears that the size of the charged secondary component can influence the size of the particles. For example, 〈Rh〉25°C for PNIPAAm-co-NaSS particles made with an initial charge content of 0.12 mmol/g (batch J), PNIPAAm-co-PNaSS-5 particles (0.12 mmol/g, batch H), and PNIPAAm-co-PNaSS-10 particles (0.14 mmol/g, batch G) were 54, 90, and 110 nm, respectively. The macromonomers with longer chain lengths may tend to stretch out into the water phase once they are chemically anchored to the particle surfaces, spreading out the charges in a larger region and decreasing the actual surface charge density. As a result, PNaSS-10 macromonomer, at the same molar charge concentration, has the weakest effect on the decrease of the particle size, while NaSS monomer has the strongest. The swelling ratio of the particles, on the other hand, does seem to be strictly related to the charge content. Contrast between the Effects of Charged Components on Microgels and on Bulk Gels. The observation of smaller swelling ratios reported here for microgels made by copolymerization of NIPAAm with charged monomers is in contrast to what has been observed in PNIPAAm-co-NaA bulk gels39 and microgel beads synthesized through inverse emulsion polymerization.40 We attribute these differences to different charge distributions. The bulk gels and microgel beads were synthesized at room temperature, below the phase transition temperature. We believe that, in this case, the charges are uniformly distributed during the gelation process. However, the syntheses of the microgel particles discussed here occurred at temperatures higher than the TVPT, which should lead to a nonuniform distribution of charges with the charges preferentially located on the surface of the particles. On heating, the shrinkage will be confined to the neutral PNIPAAm-enriched core, leading to a smaller swelling ratio and a more continuous shrinking. However, the large swelling ratio observed for PNIPAAm-co-AA microgels, shown in the inset of Figure 2b, is consistent with that of ionic bulk gels and with our speculation that the charge is distributed throughout these particles. Relative Molar Mass and the Solid Density of the Particles. Figure 5 is a bar graph comparing the molar masses of particles from the nine main batches. The shaded area in the first bar to the left shows the range of molar masses around that of pure PNIPAAm observed as (39) Hirotsu, S.; Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1987, 87, 1392. (40) Hirose, Y.; Amiya, T.; Hirokawa, Y.; Tanaka, T. Macromolecules1987, 20, 1342.

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Figure 5. A comparison of the influence of the secondary component on the molar mass of copolymerized PNIPAAm microgels. The hatched area shows the variation of Mw for different neutral secondary components A-E, which is smaller than the decrease of Mw observed as charged groups F, G, H, and J were introduced. The gray bars show the influence of the length of the charged secondary component introduced at similar charge content.

different neutral components are introduced. Charged secondary components have a much more dramatic effect on the molar mass, with the size of the charged secondary component again playing a role. In Figure 5, samples G, H, and J made with similar charge content but different chain lengths show that the shortest charged component has the largest effect on the decrease of the molar mass. The solid densities of the particles studied are summarized in Table 1. It is interesting to note that, although the incorporation of charged monomers effectively results in smaller particle sizes with lower molar masses, this does not always yield the particles having the lowest solid densities. The particles incorporating the neutral hydrophilic components, AA and AAm, have the lowest solid densities, measured as 0.002 and 0.003 g/cm3 at 25 °C, respectively. Because the solid density is an indication of the degree of particle swelling, the lower solid densities observed for these particles may indicate a difference in the spatial distribution of the secondary components within the particle. As discussed above, we believe that these neutral hydrophilic components are distributed more evenly throughout the particle as compared to the charged components, which should be distributed primarily at or near the particle surface, and that this more uniform distribution results in more uniform swelling of the particles and hence lower density. We were surprised to observe such low solid densities. In our previous work on self-cross-linking of pure PNIPAAm microgels,30 we found that the solid density above TVPT had to be greater than a critical value for selfcross-linking sufficient for stable particle production to occur. We judged whether sufficient cross-linking had occurred by comparing the molar mass measured at 25 and 40 °C; an increase in molar mass at high temperature implies that free linear chains, present in solution because of insufficient cross-linking, have aggregated with the particles. In our previous study, only PNIPAAm microgels with solid densities greater than 0.30 g/cm3 at 40 °C appeared to be well cross-linked. All of the particles reported in Table 1, except for PNIPAAm-co-AAm particles, have molar masses above and below the volume phase transition that are consistent, even 2 years after their initial preparation. The consistency of the molar masses suggests that these particles are sufficiently self-cross-linked, even batches F-J that incorporate charged secondary components and have densities as low as 0.011-0.16 g/cm3 at 40 °C. It may be that these particles have a denser core, sufficient for self-

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Figure 6. The dependence of the hydrodynamic radius 〈Rh〉, molar mass Mw, solid density F, and radii ratio Rg/〈Rh〉 on the feed content of charged macromonomer PNaSS-5 in copolymerized PNIPAAm microgels. DLS and SLS were performed at 25 °C in a dilute water dispersion with a C of ∼10-4 g/mL.

cross-linking. However, our molar mass-based criterion may not be sufficient for these samples; highly charged chains may not collapse onto the particles even above the VPT. We have not done a quantitative study of the particles and their supernatant for the presence of linear chains due to the difficulty of separating chains from particles. We are also not able to distinguish between “gel spheres” and “hyper-branched polymers” for those particles with high charge contents that have ultralow densities at 40 °C and molar masses as low as ∼106 g/mol, as in batches I and J in Table 1. The Influence of the Quantity of Charged Groups Introduced. We varied the concentration of both NaSS and PNaSS-5 in different particle syntheses to study the influence of the quantity of charge incorporated on the properties of the microgels. The results are shown in Figures 6 and 7. In Figure 6, the average hydrodynamic radius, molar mass, solid density, and the ratio of Rg/〈Rh〉 are plotted as functions of the weight fraction of PNaSS-5. The initial charge content is shown on the top axis; in these samples, the charge content was varied from 0 to 0.2 mmol/g. Both 〈Rh〉 and Mw (Figure 6a and b) decrease very sharply when a small amount of PNaSS-5 is introduced and continue to decrease gradually as further macromonomer is included. The decrease in particle size with increased charge content is consistent with the trend observed in emulsion polymerization when an ionic surfactant is added. The solid density (Figure 6c) at 25 °C in water first increased from 0.010 for pure PNIPAAm particles to 0.019 g/cm3 for 0.73 wt % of PNaSS-5 copolymerized particles, and then decreased gradually to 0.010 for 15 wt % PNaSS-5 particles. The ratios of Rg/〈Rh〉 in Figure 6d for all particles containing PNaSS-5 range from 0.47 to 0.62, implying some level of core-shell structure with a denser core. Other results of Rg/〈Rh〉 for particles containing charged secondary components also lie in this range. Results for particles made with increasing amounts of NaSS, with initial charge content ranging from 0 to 1.30 mmol/g, are shown in Figure 7. Particles with lower initial

Gao and Frisken

Figure 7. The dependence of 〈Rh〉, Mw, FF, and turbidity τ on the feed content of charged monomer NaSS in copolymerized PNIPAAm microgels. DLS and SLS were performed at 25 °C in a dilute water dispersion with a C of ∼10-4 g/mL. The turbidity of the original, undiluted dispersions was measured at 532 nm and 22 °C.

charge content, in the range from 0 to 0.2 mmol/g, have particle sizes and molar masses that show the same trends as observed in Figure 6a and b. However, as the charge content is increased further, the particle size and molar mass appear to increase. At higher charge content, we expect the accompanying increase in the ionic strength to screen the surface charge on the particles, thus minimizing the effect of further charge incorporation on particle size. The solid density (Figure 7c) decreases monotonically, as expected, because these highly charged particles should swell considerably upon dilution for DLS measurements. The turbidity of the original, undiluted dispersions is consistent with the variation in particle size observed. Figure 7d shows values for the turbidity measured for the PNIPAAm-co-NaSS particles. The turbidity first decreases sharply and then, above an initial IEC of ∼0.2 mmol/g, increases gradually, consistent with the size of the particles shown in Figure 7a. Conclusions Cross-linker-free NIPAAm microgels incorporating different comonomers have been synthesized using the method of precipitation polymerization developed for selfcross-linked PNIPAAm microgel synthesis.29 The resultant microgels were characterized by static and dynamic laser light scattering. When neutral components are incorporated, the swelling properties and narrow particle size distributions observed suggest that neutral components are incorporated uniformly into the particles. Incorporating a neutral hydrophobic component results in denser and smaller particles with a lower phase transition temperature than that of pure PNIPAAm gel spheres, while incorporating a neutral hydrophilic component results in less dense and larger particles with the phase transition shifting to higher temperatures. Introducing a

Synthesis of Self-Cross-Linked NIPAAm Microgels

charged monomer or macromonomer dramatically decreases the size and molar mass of the particles but also broadens the size distribution and the phase transition temperature range. The charged monomer or macromonomer behaves like an ionic surfactant; these are known to have the same effects on the particle size distribution. All components lead to a smaller swelling ratio with charged components having the largest effect, except for the special case of AA. The charge properties of this monomer change with pH so that the monomer is actually introduced as a basically neutral component at the low pH conditions that exist during synthesis. We believe that this leads to the charge being distributed throughout the particles, rather than being primarily localized on the surfaces, so that greater swelling is eventually possible.

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This shows that it is possible to engineer particles with charges located either at the surface or inside the particle. In general, these different trends provide information about the influence of a secondary component on the size, molar mass, and phase transition behavior of PNIPAAmbased microgels that should be useful in various applications. Acknowledgment. We are grateful to Prof. S. Holdcroft and his group for generously providing us with the macromonomer PNaSS samples. We thank David Lee for helpful discussions. This work was supported by the Natural Science and Engineering Research Council of Canada. LA0485982