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Preparation of Thermosensitive Submicrometer Gel Particles with Anionic and Cationic Charges† Shoji Ito,‡ Kazuyoshi Ogawa,§ Hironori Suzuki,§ Benlian Wang,§ Ryo Yoshida,§ and Etsuo Kokufuta*,§ Department of Polymer Chemistry, National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan, and Institute of Applied Biochemistry, University of Tsukuba, Tsukuba, Ibaraki 305, Japan Received September 8, 1998. In Final Form: February 1, 1999 Aqueous redox polymerization using a surfactant was studied to prepare submicrometer-sized polyelectrolyte gel particles from a monomer solution containing N-isopropylacrylamide (NIPA), N,N′methylenebisacrylamide (Bis), and acrylic acid (AAc) or 1-vinylimidazole (VI). Sodium dodecylbenzenesulfonate and ammonium persulfate were used as the surfactant and the initiator, respectively. The gel particles purified via dialysis were characterized by photon correlation spectroscopy. Since the AAc and VI contents of the monomer solutions were controllable within 0-30 mol %, the diameter of the gel particles with 30 mol % of AAc or VI increased from 125 to 600 nm at 25 °C when converting the carboxyl or imidazolyl groups into the corresponding salt form. However, a complete elimination of charges from the anionic or cationic gel particles gave rise to a neutral gel, the size of which varied from 125 nm at 25 °C (swollen state) to 50 nm at temperatures >35 °C (fully collapsed state). These swelling behaviors were the same as those of NIPA-based ionic bulk gels. Detailed examinations of the gel particles with 30 mol % AAc by potentiometric titration and electrophoretic light scattering showed that almost all of the COOH groups in the pregel solution were incorporated into the particle interiors without concentration on the particle surface.
Introduction Submicrometer gel particles that thermally undergo reversible volume changes are of interest in both scientific and technical fields. Such thermosensitive gel beads based on N-isopropylacrylamide (NIPA) have been prepared and characterized in several previous studies.1-13 Synthetic techniques employed were the aqueous redox methods using different polymerization media such as surfactantcontaining water,1,2,12,13 surfactant-free water,3-7,11 and water suspended in oils.8-10 Usually, ammonium persulfate (AP) and potassium persulfate (KP) were employed as initiators, and N,N′-methylenebisacrylamide (Bis) was employed as the cross-linker. In general, the diameters of the gel particles (1 µm) obtained from surfactant-free systems. All of the preparations were * To whom correspondence should be addressed. † Presented at Polyelectrolytes ’98, Inuyama, Japan, May 31June 3, 1998. ‡ National Institute of Materials and Chemical Research. § Tsukuba University. (1) Ito, S.; Hirasa, O.; Fujishige, S. Bull. Res. Inst. Polym. Text. 1991, No. 167, 67-80 (in Japanese). (2) McPhee, W.; Tam, K. C.; Pelton, R. J. Colloid Interface Sci. 1993, 156, 24. (3) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 20, 247. (4) Pelton, R. H.; Pelton, H. M.; Morphesis, A.; Rowell, R. L. Langmuir 1989, 5, 816. (5) Kawaguchi, H.; Fujimoto, K.; Mizuhara, Y. Colloid. Polym. Sci. 1992, 270, 53. (6) Tam, K. C.; Ragaram, S.; Pelton, R. H. Langmuir 1994, 10, 418. (7) Makino, K.; Yamamoto, S.; Fujimoto, K.; Kawaguchi, H.; Ohshima, H. Colloid Interface Sci. 1994, 166, 251. (8) Tanaka, T.; Sato, E.; Hirokawa, Y.; Hirotsu, S.; Peetermans, J. Phys. Rev. Lett. 1985, 55, 2455. (9) Hirose, Y.; Amiya, T.; Hirokawa, Y.; Tanaka, T. Macromolecules 1987, 20, 1342. (10) Matsuo, E. S.; Tanaka, T. J. Chem. Phys. 1988, 89, 1695. (11) Crowther, H. M.; Vincent, B.Colloid Polym. Sci. 1998, 276, 46 (12) Lowe, T. L.; Tenhu, H. Macromolecules 1998, 31, 1590. (13) Zhou, S.; Chu, B. J. Phys. Chem. B 1998, 102, 1364.
found to swell or deswell with changes in temperature; such temperature-sensitive swelling-deswelling characteristics were similar to those of bulk NIPA gels in cylindrical, cubic, and other forms. It is well-known that the swelling degree and the temperature (Tν) bringing about the volume collapse for bulk NIPA gels dramatically increase when ionizable groups are incorporated within their polymer network.14 It would be of interest to prepare NIPA-based polyelectrolyte gel particles with ionizable groups and to investigate the temperature dependence of their particle sizes. Hirose et al.9 have demonstrated a suitable method for this purpose, in which neutral gel beads consisting of Biscross-linked NIPA and N-(acryloxy)succinimide (AOSI) chains were initially prepared using a water medium suspended in hexane and subsequently converted into a polyelectrolyte gel with COONa by alkaline hydrolysis. The second method for preparing ionic NIPA latexes was reported by Pelton et al.2,3 They conducted an aqueous redox polymerization of NIPA and Bis without ionic monomers in the absence and the presence of surfactant using KP or azobis(isobutylamidine) hydrochloride (AIBA‚ HCl) as an initiator. Since the ionizable groups originating from the initiator may covalently bind to many of the end groups of the NIPA chains within the network during the polymerization, they succeeded in preparing anionic and cationic latexes by use of the KP and the AIBA‚HCl initiator, respectively. Aqueous redox copolymerizations (third method) of NIPA with ionic monomers such as methacrylic acid (MAAc) in the presence of surfactant were also employed by Tenhu et al.12 and Chu et al.13 While the choice of method depends on the purpose for which the polyelectrolyte gel particles will be used, the inverse suspension polymerization employed by Hirose et al.9 generally requires great skill to obtain particles (14) Hirotsu, S.; Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1987, 87, 1392.
10.1021/la9811867 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/09/1999
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Table 1. Composition of Monomer Solutionsa,b designation
NIPAAm (g)
AAc (g)
VI (g)
A(0) A(5) A(10) A(30)
8.96 8.51 8.07 6.27
0 0.28 0.57 1.71
0 0 0 0
V(0) V(5) V(10) V(30)
8.96 8.45 8.12 6.28
0 0 0 0
0 0.37 0.75 2.28
a All of the monomer solutions contain 200 g of water, 0.12 g of Bis, 0.05 g of AP, and 0.7 g of NaDBS in addition to the above monomers. b The number in parentheses in the designation column denotes mol % of the corresponding ionic monomer.
with good reproducibilities regarding size and distribution. In Pelton and Chibante’s method3 it is difficult to introduce a large amount of charges into the gel beads from the initiator; this problem has already been pointed out by the authors themselves in another publication.2 The present paper reports the preparation and characterization of submicrometer microgel particles composed of a Bis-cross-linked NIPA network into which different amounts of acrylic acid (AAc) or 1-vinylimidazole (VI) were incorporated. We were able to control the AAc and VI contents of aqueous monomer solutions (pregel solutions) within 0-30 mol %; therefore, the particles obtained exhibit very large diameter changes in response not only to temperature but also to the treatment with acid and base. Experimental Methods Materials. All chemicals were obtained from commercial sources: NIPA from Kojin Chemical Co. (Tokyo, Japan); AAc, Bis, and AP from Wako Pure Chemical Co. (Tokyo, Japan); VI and sodium dodecylbenzene sulfonate (NaDBS) as a surfactant from Tokyo Chemical Industry Co. (Tokyo, Japan). The monomers were purified according to the usual methods. All pregel solutions were prepared with distilled water passed through a Milli-Q filter. Polymerization. The compositions of the pregel solution are shown in Table 1. The polymerization was carried out for 2 h using a usual 300 mL conical flask equipped with a cooler and a magnetic stirrer. The temperature (60 °C) was controlled using a water bath (these conditions were determined by carrying out several preliminary experiments according to a trial and error approach). After the polymerization was terminated by blowing air through the reactor, residual monomers and NaDBS were removed from the resulting reaction mixture through dialysis procedures with a dialyzing tube (Spectra/Por CLC500 with a molecular weight cut off 100 000) as described in detail in the following section. It is of great importance to examine the existence of un-crosslinked and dissolved polymers in our dialyzed samples, although this is a rather difficult problem with respect to experimental techniques. After several preliminary experiments were performed, we have found that a combination of turbidity analysis and colloid titration15 serves our purpose. The reasons for this are as follows: (i) Not only NIPA homopolymer but also NIPA copolymer with AAc or VI exhibits the lower critical solution temperature (LCST; ca. 32 ( 0.5 °C for all samples), and aqueous polymer solutions become turbid at temperatures > LCST under pH conditions where charges arising from AAc or VI are fully eliminated. (ii) The solution turbidity is detectable at polymer concentrations higher than 0.01 w/v %. (iii) Use of an appropriate precipitant (0.1 M HCl for the AAc system and 0.1 M NaOH for the VI system) enables us to aggregate and precipitate the ionic microgels as well as the corresponding copolymers at temperatures > LCST. (iv) A supernatant solution is easily obtained by filtration from the suspension including both microgel and (15) For the colloid titration method, see: Kokufuta, E. Macromolecules 1979, 12, 350.
polymer which had been precipitated and then subjected to the colloid titration. (v) A very slight amount (0.005 w/v %) of the copolymers with AAc or VI is detectable by the colloid titration under conditions where the copolymer was fully ionized. The homopolymer of NIPA and its copolymer with AAc or VI used in the above experiments were synthesized under the same conditions as those in Table 1, except for the use of the pregel solutions not containing the cross-linker (Bis). The polymer samples obtained were then purified by dialysis procedures which were just the same as those for the gel particles. A control sample was prepared by dissolving the polymer (0.01 w/v %) into the corresponding gel dispersion (0.5 w/v %). By use of such a control sample, we have succeeded in detecting the dissolved polymers in all preparations shown in Table 1. When the present analytical technique was applied to the sample dispersions purified through dialysis, however, it was found that there was no detectable amount (more than 2.5% in w/v) of the polymer. Size Measurement. The apparent Stokes diameters (ds) of the gel beads were estimated as a function of temperature by means of photon correlation spectroscopy (PCS, based on the Cumulants method). The measurements were performed by using an Otsuka model LPA 3000/3100 apparatus. Potentiometric Titration. The A(30) gel particles with COOH obtained via acidic dialysis were lyophilized for 1 day and then heated over phosphorus pentaoxide at 101 °C in a vacuum until a constant weight was reached. The sample dispersions were prepared by dissolving 0.1 g of the dry gel into 50 mL of the solvents (pure water and 0.05-0.2 M NaCl solutions). Each sample dispersion was titrated with the 0.1 M NaOH at 25 ( 0.1 °C under nitrogen using a pH meter and a microburet. Threetime-distilled carbonate-free water was used as the solvent for the preparations of the sample dispersions and NaOH titration. Electrophoretic Light Scattering (ELS). Two A(30) samples with COOH and COONa were used; the former and the latter were respectively obtained via acid and alkaline dialyses (see the following section). The sample with COOH was again dialyzed against 0.001 M NaCl solution whose pH was adjusted to 3.6 with 0.01 M HCl. The same NaCl solution was used in the dialysis for the gel with COONa, but its pH was adjusted to 8.6 with carbonate-free 0.01 M NaOH. The final concentration of the gel particles in both dispersions was ca. 1 mg‚mL-1. ELS measurements were made at fixed scattering angles of 8.7, 17.4, 26.0, and 34.7° using a Coulter DELSA 400 apparatus. The electric field was applied at a constant current of 0.3 mA. The temperature of the thermostated chamber was maintained at 25 °C.
Results and Discussion Methods for Purification and Sample Preparation. To demonstrate the effectiveness of the present preparation method, it is necessary to reply to the following two questions: (i) Were the surfactant molecules completely removed from the surfaces and/or bodies of the gel particles during dialysis? (ii) Were the ionizable monomers actually incorporated within the polymer network? The former question, in particular, seems to be the most important because (i) DBS molecules bind to a NIPA gel (bulk gel) to convert it into an ionic gel (see refs 16 and 17) and (ii) DBS may form a complex with VI as the cationic monomer. Since the presence of minute amounts of ionizable groups within the cross-linked polymer network is very sensitive to the swelling degree of polymer gels, we may reply to the above questions by carefully examining the temperature dependence of ds. For this purpose, we purified the gel samples through the dialysis method shown in Figure 1. The dialysis against NaOH solution (alkaline dialysis) was expected to result in COONa groups for the NIPA-AAc gel and in dN- groups for the NIPAVI gel (see eqs 1 and 2). By dialysis against HCl solution (16) Kokufuta, E.; Nakaizumi, S.; Ito, S.; Tanaka, T. Macromolecules 1995, 28, 1704. (17) Kokufuta, E.; Suzuki, H.; Sakamoto, D. Langmuir 1997, 13, 2627.
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Figure 1. Purification scheme for Bis-cross-linked NIPAAAc and NIPA-VI gel particles.
Figure 2. Temperature dependence of diameters (ds) for neutral NIPA gel particles obtained at different stages of the dialyzing procedures in Figure 1: open circles, dialysis against pure water; open squares, acidic dialysis; open triangles, alkaline dialysis.
(acidic dialysis), on the other hand, the gels with COOH groups and tNH+-Cl groups would be obtained.
Prior to the study of the ionic gel particles by the above dialysis method, the neutral gel particles, A(0) or V(0), were purified and subjected to size measurements in order to examine the suitability of the present method. The swelling curves for the samples from the alkaline and acidic dialyses are shown in Figure 2. Also shown in this figure is the swelling curve for the sample obtained by dialyzing the reaction mixture against pure water before the alkaline and acidic dialyses. Three neutral gels obtained at different stages of the dialysis exhibit the same temperature dependence of ds within an experimental error ((10 nm). In each swelling curve, a very sharp decrease in ds was observed around 32 °C, corresponding to the Tν for bulk NIPA gels as well as the LCST of NIPA polymer. These results suggest that the present purification method is appropriate for removing the residual monomers and the surfactant from the resulting reaction mixture. In the Experimental Section, we have mentioned that all of the purified samples were free from dissolved polymers (sol) within the accuracy (less than 2.5%) of our analytical technique. Temperature Dependence of Particle Size for the Ionic Gels. Figure 3 shows the temperature dependence
Figure 3. Temperature dependence of diameters (ds) for NIPA-AAc gel particles obtained via (a) alkaline dialysis and (b) acidic dialysis: open circles, A(0); open squares, A(5); closed circles, A(10); open triangles, A(30).
of ds for the NIPA-AAc gel particles with COOH and COONa. A considerably rapid but continuous decrease in ds for A(5) and A(10) with COONa was observed when increasing temperature. The A(30) gel exhibited little or no temperature dependence of ds at the temperatures 6 × 1018 cm-3 and at U ) -3.63 × 10-4 cm2‚V-1‚s-1. One might assume that this problem would be solved by employing a model of Ohshima et al.,7 who have succeeded in accounting for electrophoretic behaviors of latex particles covered with NIPA gel layers. However, we should note that our gel particles are distinct from their specimens with respect to preparation method. In addition, their theoretical treatment of mobility (as a function of ionic strength) is based on the two-parameter model, that is, a parameter (∼ Ns in eq 4) due to the charge density and another parameter (∼b-1 in eq 4) of a thickness of polymer layer bound to an inner “hard core” of the particle. Even when our mobility data successfully fit with a calculation curve obtained by choosing a set of both parameters, several difficulties would arise in the discussion about whether their choice is reasonable. In other words, we would not make clear a “microstructure” of submicrometer gel particles without any assumption. On the Differences in the Properties of the Present and Previous Preparations. It would be interesting to compare the present and previous methods for the preparation of ionic NIPA gel particles with respect to the size and charge content. The ds of the NIPA gel particles with COONa prepared by Hirose et al. changed from 814 to 880 nm in pure water at 22 °C when increasing the (21) Suzuki, H.; Wang, B.; Yoshida, R.; Kokufuta, E. Potentiometric Titration Behaviors of a Polymer and Gel Consisting of N-Isopropylacrylamide and Acrylic Acid. Langmuir 1999, 15, 4283. (22) Huizenga, J. R.; Grieger, P. F.; Wall, F. T. J. Am. Chem. Soc. 1950, 72, 2636. (23) Buscall, R.; Corner, T.; McGowan, I. J. In Effect of Polymers on Dispersion Properties; Tadros, Th., Ed.; Academic Press: New York, 1982; p 379.
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COONa content from 0 to 4.4 mol % (AOSI amount in the pregel solution).9 Also, the particle with 4.4 mol % COONa collapsed to ds ∼ 200 nm at temperatures above 45 °C. Pelton et al.2-4 studied the preparations of NIPA particles with sulfate plus carboxyl groups under a variety of conditions. Their particles exhibited a change in ds from 500 to 150 nm in 1 mM KCl solution with a rise in temperature from 20 to 50 °C.4 Such a temperatureinduced decrease in ds brought about an increase in mobility from -0.1 × 10-4 cm2‚V-1‚s-1 (ds ∼ 500) to -3.0 × 10-4 cm2‚V-1‚s-1 (ds ∼ 150) due to a reduction in the friction factor. Although most of the charges were located in the particle interiors, the total charge was an order of µequiv‚g-1. Such a low charge density was due to the reliance on the KP free radical initiator for the introduction of charges into the network. Indeed, Pelton et al. used a large amount of KP, the concentration of which was more than two times that in usual preparations of NIPA gels. One may claim that the third method (see Introduction) in which ionic monomers are copolymerized with NIPA would be superior to the above two methods with respect to the possibility of incorporating a large amount of anionic or cationic changes in the particle interiors. The present study has demonstrated that the use of a high concentration of DBS (>cmc) and a low concentration of AP makes it possible for us to prepare NIPA gel particles that undergo volume changes similar to those of the corresponding bulk gels. The reason for this was found to be due to a quantitative incorporation of each monomer into the network during the particle formation. Tenhu et al.12 and Chu et al.13 have used the third method for preparing ionic NIPA microgels with COOH, the diameter of which varied from 100 nm (COOH) to 850 nm (COONa) depending on the content of COOH groups. They used MAAc as the ionic monomer under a consideration of its hydrophobicity. However, the polymer of MAAc undergoes a pH-induced conformational change as the ionization of the COOH proceeds.13 This would arise
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as a complication when one studies the particle size as a function of pH. We could not compare their preparations with ours with respect to whether the majority of charges are located on the surface or in the interior, because they did not conduct the titration and electrophoresis. Conclusions We have attempted to prepare thermosensitive submicrometer gel particles with incorporated acidic or basic groups. The aqueous redox polymerization initiated by AP was carried out at 60 °C in the presence of NaDBS as the surfactant. The pregel solutions contained NIPA, Bis, and AAc or VI; the concentration of both ionic monomers were varied from 0 to 30 mol %. The results obtained are summarized as follows: (i) The photon correlation spectroscopy showed that the diameter of the gel particles with 30 mol % of AAc or VI increased from 125 to 600 nm at 25 °C when converting the carboxyl or imidazolyl groups into the corresponding salt form. (ii) A complete elimination of charges from the anionic or cationic gel particles gave rise to a neutral gel, the size of which varied from 125 nm at 25 °C (swollen state) to 50 nm at temperatures > 35 °C (fully collapsed state). (iii) These swelling behaviors were the same as those of NIPA-based ionic bulk gels. (iv) Examinations of the gel particles with 30 mol % AAc by potentiometric titration and electrophoretic light scattering demonstrated a quantitative incorporation of the COOH groups in the gel without their localization on the particle surface. Therefore, we may say that the present method is suitable for introducing a large amount of anionic or cationic charges into the NIPA-based gel particles. Acknowledgment. This research was supported by a grant to E.K. from the Ministry of Education of Japan (No. 09875232). LA9811867
