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Langmuir 2005, 21, 7044-7051
Preparation of Monomodal Polyelectrolyte Complex Nanoparticles of PDADMAC/Poly(Maleic Acid-alt-r-methylstyrene) by Consecutive Centrifugation Martin Mu¨ller,*,† Bernd Kessler,† and Sven Richter‡ Leibniz-Institute of Polymer Research Dresden e.V., Hohe Strasse 6, D-01069 Dresden, Germany, and Institute of Physical Chemistry and Electrochemistry, Technical University Dresden, Mommsenstrasse 13, D-01062 Dresden, Germany Received March 17, 2005. In Final Form: May 4, 2005 We report on the refinement of anionic and cationic nanoparticles of nonstoichometric polyelectrolyte complexes (PEC) by consecutive centrifugation, which was studied by dynamic light scattering (DLS), atomic force microscopy (AFM), colloid titration and infrared spectroscopy (IR). PEC dispersions were prepared by mixing poly(diallyldimethylammonium chloride) (PDADMAC) and sodium poly(maleic acidalt-R-methylstyrene) (PMA-MS) at the monomolar mixing ratio of n-/n+ ) 1.50 (anionic PEC) and 0.66 (cationic PEC), respectively, and the polymer concentration of cPOL ) 0.002 M. The particle size (Rh), titrable charge amount, and IR spectra were determined for both dispersions in the original state, after the first centrifugation and after the second centrifugation. Freshly prepared PEC dispersions contained two different particle sizes: around 10-25 nm (small particles) and around 100 nm (large particles). Consecutive centrifugation of freshly prepared PEC dispersions resulted in the separation of highly charged excess polyelectrolyte (PEL) and small PEC particles from a low charged coacervate phase of the desired larger PEC particles. After the second centrifugation, the coacervate phase of both dispersions PEC-1.50 and PEC-0.66 consisted of monomodal particles sizing around 100 nm. These results were supported by AFM measurements on the respective dispersions deposited on glass plates. PEC-1.50 particles tended to adopt slightly smaller sizes (≈90 nm) in comparison to PEC-0.66 ones (≈110 nm). No significant influence of the PDADMAC molecular weight on the particle size was found. IR spectroscopy showed changes in the environment of the carboxylate groups of PMA-MS by consecutive centrifugation. The centrifuged PEC1.50 dispersions showed remarkable long-term stability over more than a year. The high macroscopic stability of the studied PEC dispersions is presumably due to repulsive electrostatic interparticle interactions and attractive hydrophobic intraparticle interaction. The introduced monomodal PEC particles might be projected as latex analogues or as nanocarriers for drugs and proteins.
Introduction Polyelectrolytes are classically used in applications such as colloid stabilization and flocculation for wastewater treatment,1-3 paper making,4-5 and as food thickeners. Currently, polyelectrolyte systems are under way to address novel application fields in nano- and biotechnology, especially for the sensor and biomaterial development. Beside their capabilities for surface modification, polyelectrolyte systems can be used to create functional polymer nanoparticles. Especially mixed systems are attracting much attention in that respect. Mixing oppositely charged polyelectrolyte (PEL) in the volume phase yields to the formation of polyelectrolyte complexes (PEC) as has been known for a long time from the work of Bungenberg de Jong, Michaels, Kabanov, and Dautzenberg.6-12 Empirically either a ladderlike8 or a scrambled * Author to whom correspondence should be addressed. Tel: +49(0)351-4658-405.Fax: +49-(0)351-4658-284.E-mail:
[email protected]. † Leibniz-Institute of Polymer Research Dresden e.V. ‡ Technical University Dresden. (1) Petzold, G.; Nebel, A.; Buchhammer, H. M.; Lunkwitz, K. Colloid Polym. Sci. 1998, 276, 125. (2) Buchhammer, H. M.; Oelmann, M.; Petzold, G. Melliand 2001, 82, E104-E105. (3) Petzold, G.; Lunkwitz, K.; Schwarz, S. Chem. Eng., Technol. 2003, 26, 48. (4) Wågberg, L.; Nygren, I. Colloids Surf., A. 1999, 159, 3. (5) Gernandt, R.; Wågberg, L.; Ga¨rdlund, L.; Dautzenberg, H. Colloids Surf., A 2003, 213, 15. (6) Bungenberg de Jong, H. G. Colloid Science; Kruyt, H. R., Ed.; Elsevier Publishing Company: Amsterdam, 1949; Vol. II, pp 335-384. (7) Michaels, A. S.; Miekka, R. G. J. Phys. Chem. 1961, 65, 1765. (8) Kabanov, V. A.; Zezin, A. B. Pure Appl. Chem. 1984, 56, 343.
egg structure7 was postulated for PEC, whereas morerecent theoretical studies on PEC formation were reported therein.13-16 Experimentally, after mixing semidilute PEL solutions (≈0.001 M) in the turbid dispersion, PEC particles due to different aggregation numbers and size regimes are formed. They are assumed to form a core/ shell structure, according to which in the particle core a 1:1 charge stoichometry prevails and in the shell the excess polyelectrolyte component is located, giving the particle the defined charge sign and colloid stability. There is experimental evidence that the internal structure and the size of these aggregated PEC particles is dependent on concentration, ionic strength, pH, molecular parameters of the used polyions, and the preparation protocol.17-20 However, the physical chemistry of PEC dispersions, (9) Philipp, B.; Dautzenberg, H.; Linow, K.-J.; Ko¨tz, J.;Dawydoff, W. Prog. Polym. Sci. 1989, 14, 91. (10) Dautzenberg, H.; Jaeger, W.; Ko¨tz, J.; Philipp, B.; Seidel, C.; Stscherbina, D. Polyelectrolytes: Formation, Characterization and Application; Carl Hanser Verlag: Munich, Vienna, New York, 1994. (11) Brandt, F.; and Dautzenberg, H. Langmuir 1997, 13, 2905. (12) Dautzenberg, H. Macromol. Chem. Phys. 2000, 201, 1765. (13) Nordmeyer, E.; Beyer, P. J. Polm. Sci., Part B: Polym. Phys. 1999, 37, 335-348. (14) Jonsson, M. and Linse, P. J. Chem. Phys. 2001, 115, 34063418. (15) Winkler, R. G.; Steinhauser, M. O.; Reineker, P. Phys. Rev. E 2002, 66, 21802. (16) Biesheuvel, P. M.; Cohen Stuart, M. A. Langmuir 2004, 20, 47644770. (17) Buchhammer, H. M.; Petzold, G.; Lunkwitz, K. Langmuir 1999, 15, 4306. (18) Buchhammer, H. M.; Mende, M.; Oelmann, M. Colloids Surf., A 2003, 218, 151.
10.1021/la050716d CCC: $30.25 © 2005 American Chemical Society Published on Web 06/22/2005
Monomodal Polyelectrolyte Complex Nanoparticles
especially concerning the formation, growth, and the involved interaction forces of aggregated PEC particles having a final more or less constant radius is still not resolved. Monomodal particles are among others of two particular interests. First, due to Ostwald polydisperse colloidal dispersions are ripening, so that smaller particles aggregate to form larger particles.21 Hence, if the particle ripening can be accelerated in the sense to end up more rapidly at monomodal particles, stable colloid dispersions can be obtained. In that context, refined PEC particles and their reproducible preparation protocol might be interesting for wide-spanning industrial applications such as for cosmetics, paints, and laundry products (e.g., ref 22) or as nanocarriers for drugs, proteins, and DNA analogously to polymer conjugates,23 liposomes,24,25 or hollow capsules.26 Second, there are surface-modification concepts based on the wet chemical adsorption of preformed PEC particles onto substrates (silicon wafer, silica suspension, membranes, textiles) in order to supply charge,27-29 mimic biological surfaces or generate surface nanostructures.30 For that, an effective adsorption process is desired. In earlier work, the unrefined PEC dispersion was used, which led often to incomplete coverage of the (especially planar) substrate.19 This was caused by smaller components such as the excess PEL or primary complexes (radius ≈ 5-25 nm), which are diffusing more rapidly to the oppositely charged surface compared to the slower larger particles (≈100 nm). If then a layer of the smaller particles of a certain charge has been formed, the larger particles are prevented to adsorb on top of the already formed layer of smaller like-charged particles. Recently, we found out that the composition, size, and polydispersity within a specific PEC system, which we have already introduced therein,19,20 could be influenced by consecutive centrifugation and redispersion cycles. It consists of the polycation poly(diallyldimethylammonium chloride) (PDADMAC) and the polyanion sodium poly(maleic acid-alt-R-methylstyrene) (PMA-MS). Herein, we report extensively on the colloidal characterization of the phases formed after each step of this refinement process. With that experimental study, both a further fundamental understanding of the PEC aggregation process and a reproducible preparation protocol for monomodal PEC particles, which is of technological relevance, is addressed. For the particles size and distribution characterization, dynamic light scattering (DLS) was performed in combination to atomic force microscopy (AFM). Furthermore, colloid titration (PCD) was applied to get information on charge state, and infared (IR) spectroscopy was used to obtain new structural information on PEC particles. Materials and Methods Polyelectrolytes. The polycation PDADMAC (Figure 1) with the molecular weight Mw ) 250 000-350 000 g/mol (PDADMAC300000) was obtained from Aldrich Chemical Co., Inc., Milwau(19) Reihs, T.; Mu¨ller, M.; Lunkwitz, K. Colloids Surf., A 2003, 212, 79-95. (20) Reihs, T.; Mu¨ller, M.; Lunkwitz, K. J. Colloid Interface Sci. 2004, 271, 69-79. (21) Ostwald, W. Z. Phys. Chem. 1897, 22, 289. (22) Scha¨dler, V.; Mukherjee, P. Macromol. Rapid. Commun. 2005, 26, F43 (special issue). (23) Ringsdorf, H. Polym. Sci. Polym. Symp. 1975, 51, 135-153. (24) Lasic, D. D. Medical applications of Liposomes; Papahadjopoulos D., Ed.; Elsevier Science: Amsterdam, 1998. (25) Antonietti, M.; Fo¨rster, S. Adv. Mater. 2003, 15, 1323-1333. (26) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davies, S. A.; Mo¨hwald, H. Angew. Chem., Int. Ed. 1998, 37, 2201. (27) Petzold, G.; Schwarz, S.; Buchhhammer, H. M.; Lunkwitz, K. Angew. Makromol. Chem. 1997, 253, 1-15. (28) Buchhammer, H. M.; Kramer, G.; Lunkwitz, K. Colloids Surf., A 1994, 95, 299-304.
Langmuir, Vol. 21, No. 15, 2005 7045
Figure 1. Structural formula of the polyelectrolytes poly(diallyldimethylammonium chloride) (PDADMAC) (A) and poly(maleic acid-alt-R-methylstyrene) (PMA-MS) (B).
Figure 2. Preparation of PEC particles including mixing, centrifugation, decantation, and redispersion. kee, WI, as a 20% (w/w) aqueous solution and that with Mw ) 37 000 g/mol (PDADMAC-37000) from Katpol Chemie GmbH, Bitterfeld, Germany. The anionic alternating copolymer of maleic acid with R-methylstyrene (PMA-MS) (Figure 1) was prepared by hydrolysis of the corresponding anhydride with a solution of NaOH in equimolar amounts. The polyanhydride with Mw ≈ 25.000 was obtained from Leuna AG, Leuna, Germany, and supplied by V. Steinert (IPF Dresden). The initial PEL solutions (cPOL ) 0.002 mol/L) were prepared by dissolving PDADMAC and PMA-MS, respectively, in deionized and purified water (Millipore-Q reagent grade water system, Millipore GmbH, Eschborn, Germany) and adjusting them to pH 6 with HCl solution. Afterward, the solutions were characterized by colloid titration (particle charge detector (PCD), see below) before use. Preparation of Stable PEC Dispersions. PEC dispersions were prepared by direct mixing of the solutions of the oppositely charged PEL according to Figure 2. The calculated amount of the non-excess PEL solution was added slowly to the stirred solution of the oppositely charged excess PEL solution in the mixing beaker by a dosage system. The total polymer concentration cPOL of the solution was 0.002 mol/L and the molar mixing ratio was n-/n+ ) 1.50 and 0.66, respectively. After further stirring for a defined time, the initial PEC dispersions (50 mL, pH ≈ 6) denoted as PEC-1.50-0× or PEC-0.66-0× in the whole text according to their mixing ratios were obtained, for which solid contents SC ≈ 0.062 were determined by gravimetry (residual water could not be fully evaporated under vacuum). These raw dispersions were then centrifuged at 11 000 rpm for 20 min (Centrifuge 5416, Eppendorf AG, Hamburg, Germany). Three fractions were obtained after centrifugation from top to bottom: a supernatant (SUP) fraction (≈48 mL) with only a weak turbidity (i), a small milklike coacervate (COAC) fraction (≈2 mL) (ii), and a solid insoluble rubberlike precipitate (
