14692
2008, 112, 14692–14697 Published on Web 10/25/2008
Structural Ordering and Phase Behavior of Charged Microgels P. S. Mohanty* and W. Richtering Institute of Physical Chemistry, RWTH Aachen UniVersity, Landoltweg 2, 52056 Aachen, Germany ReceiVed: September 15, 2008; ReVised Manuscript ReceiVed: October 13, 2008
Recent theoretical phase diagrams for loosely cross-linked ionic microgels with a low monomer volume fraction (Gottwald; et al. Phys. ReV. Lett. 2004, 92, 068301) have predicted a re-entrant order-disorder transition (i.e., fluid-FCC-BCC-fluid) as a function of concentration and so far there has been no experimental verifications of these theoretical predictions. Here, we present experimental results on phase behavior of loosely cross-linked charged poly(N-isopropylacrylamide co acrylic acid) (PNIPAm-co-AAc) microgesls with a low monomer volume fraction (∼0.003) for a wide range of concentrations (0.02-0.6 wt %) using static and dynamic light scattering methods. These microgel dispersions exhibit a short-range liquid order at low concentration ( pKa) have been carried out to investigate the influence of soft and electrostatic interactions on the phase behavior of charged microgels using scattering techniques. To this end, we synthesized monodisperse loosely crosslinked charged microgel colloidal particles of poly(N-isopro 2008 American Chemical Society
Letters
J. Phys. Chem. B, Vol. 112, No. 47, 2008 14693
Figure 1. (A) Hydrodynamics radius (Rh in nanometer) plotted as a function of pH. Below pH < pKa ()4.25), the microgel is uncharged. Above pH > pKa ()4.25), the microgel is charged due to the dissociation of AAc groups. (B) Conductivity measured as a function of increasing 0.1 N HCl concentration in a conductometric titration. The concentration of microgel suspension is 0.044 wt %.
pylacrylamide-co-acrylic acid) (PNIPAm-co-PAA) with low monomer volume fraction (∼0.003) and investigated their structural ordering and phase behavior for a wide range of concentrations (between 0.02 and 0.6 wt %) under deprotonated state (at pH ) 6.2) using static and dynamic light scattering methods. The synthesis of microgel particles was carried out using dispersion polymerization and their purification was done by centrifugation followed by dialysis.33 The polydispersity of these microgel particles is less than 12%, as determined from dynamic light scattering. The crystallization behavior of these microgel particles at low concentration is itself an indication of low polydispersity. All the samples were prepared with highly deionized water (conductivity pKa, pH ) 6.2) at low salt concentration. We observed a liquid to FCC crystalline state above 0.03 wt %. As a function of increasing concentration, we found the structural ordering is density dependent and the first peak position, Qmax scales with concentration as F1/3. Surprisingly, at higher concentrated crystalline sample, we observed two coexisting Bragg peaks within a very narrow Q-range, which suggested a possible coexistence of FCC and BCC crystalline structures. These results clearly suggest that the fluid to crystal transition is governed by a screened Coulomb interaction and is in agreement with the theoretical phase diagram of ionic microgel below the overlap concentration. Furthermore, at very high concentration (∼0.57 wt %), we obtained a re-entrant disordered state that is also in agreement with theoretical predictions. This disordered state was confirmed to be a glass. There are several assumptions and approximations in which the theoretical model differs from the experiment. Therefore, at this point it is difficult to say whether the re-entrant disordered state described by theory is a liquid or glass.
Currently, we are working in details to explore this re-entrant phase both by scattering and by confocal laser scanning microscopy. Acknowledgment. We gratefully acknowledge financial support by the Alexander von Humboldt Foundation, Germany. References and Notes (1) Yethiraj, A. Soft Matter 2007, 3, 1099. (2) Mohanty, P. S.; Kesavamoorthy, R.; Matsumoto, K.; Matsuoka, H.; Venkatesan, K. A. Langmuir 2006, 22, 4552. (3) Mohanty, P. S.; Tata, B. V. R.; Yamanaka, J.; Sawada, T. Langmuir 2005, 21, 11678. (4) Ballauff, M.; Borisov, O. Curr. Opin. Colloid Interface Sci. 2006, 11, 316. (5) Tata, B. V. R.; Mohanty, P. S.; Valsakumar, M. C. Phys. ReV. Lett. 2004, 88, 018302. (6) Tata, B. V. R.; Mohanty, P. S.; Valsakumar, M. C. Solid State Commun. 2008, 147, 360. (7) Asher, S. A.; Guisheng, P.; Kesavmoorthy, R. Nonlinear Opt. 1999, 21, 343. (8) Marta, K.; Igor k, L.; Alexander, M.; Kesavmoorthy, R.; Asher, S. A. AdV. Funct. Matter 2003, 13, 774. (9) Asher, S. A.; Serban, P.; Reese, E.; Xiang, L.; David, F. Anal. Bioanal. Chem. 2002, 373, 632. (10) Pusey, P. N.; van Megen, W. Phys. ReV. Lett. 1987, 59, 2083. (11) Pusey, P. N.; van Megen, W. Nature 1986, 320, 348. (12) Tan, B. H.; Tam, K. C. AdV. Colloid Interface Sci. 2008, 136, 25. (13) Das, M.; Zhang, H.; Kumacheva, E. Annu. ReV. Mater. Res. 2006, 36, 117. (14) Berndt, I.; Pedersen, J. S.; Lindner, P.; Richtering, W. Langmuir 2006, 22, 459. (15) Berndt, I.; Pedersen, J. S.; Lindner, P.; Richtering, W. Angew. Chem. 2006, 45, 1737. (16) Berndt, I.; Pedersen, J. S.; Lindner, P.; Richtering, W. J. Am. Chem. Soc. 2005, 127, 9372. (17) Senff, H.; Richtering, W. J. Chem. Phys. 1999, 313, 1705. (18) Pyett, S.; Richtering, W. J. Chem. Phys. 2005, 122, 034709. (19) Stieger, M.; Pedersen, J. S.; Lindner, P.; Richtering, W. Langmuir 2004, 20, 7283. (20) Eckert, T.; Richtering, W. J. Chem. Phys., in press. (21) Wu, J.; Zhou, B.; Hu, Zu. Phys. ReV. Lett. 2003, 90, 0483041. (22) Brijitta, J.; Tata, B. V. R.; Kaliyappan, T. arXiV 2008, 0806.1658; J. Nanosci. Nanotechnol., in press. (23) Alsayed, A. M.; Islam, M. F.; Zhang, J.; Collings, P. J.; Yodh, A. G. Science 2005, 309, 1207. (24) Crassous, J. J.; Witteman, A.; Siebenbu¨rger, M.; Schrinner, M.; Drechsler, M.; Ballauff, M. Colloid Polym. Sci. 2008, 286, 805. (25) Karg, M.; Pastoriza-Santos, I.; Rodriguez-Gonza´lez, B.; von Klitzing, R.; Wellert, S.; Hellweg, T. Langmuir 2008, 24, 6300. (26) Lopez-Leon, T.; Ortega-Vinuesa, J. L.; Bastos-Gonzalez, D.; Elarssari, A. J. Phys. Chem. B 2006, 110, 4629. (27) Denton, A. R. Phys. ReV. E 2003, 67, 011804. (28) Gottwald, D.; Likos, C. N.; Kahl, G.; Lo¨wen, H. Phys. ReV. Lett. 2004, 92, 068301.
Letters (29) Gottwald, D.; Likos, C. N.; Kahl, G.; Lo¨wen, H. J. Chem. Phys. 2005, 22, 074903. (30) Meng, Z.; Cho, J. K.; Debord, S.; Breedveld, V.; Lyon, L. A. J. Phys. Chem. B 2007, 111, 6992. (31) John, A.; Breedveld, V.; Lyon, L. A. J. Phys. Chem. B 2007, 111, 7796. (32) St. John, Ashlee N.; Breedveld, Victor; Lyon, L. Andrew J. Phys. Chem. B 2007, 111, 7796. (33) NIPAm (1.4 g) and 0.03 g of cross-linker (BIS) were dissolved in 100 mL of water. The mixture was transferred into a 250 mL round-bottom flask and was heated at 70 °C under a gentle nitrogen stream for 45 min. AAc (0.08 g in 10gm of water) was added to the above mixtures. Finally, the initiator, potassium perox-disulfate (KPS) (0.05 g dissolved in 10 g of water) was added to initiate the polymerization. The reaction mixture was kept at 70 °C under nitrogen at least for 6 h to complete the reaction. Finally, the suspensions were purified by dialysed followed by centrifugation under high rate to remove the unreacted monomers, linear chains and ionic impurities.
J. Phys. Chem. B, Vol. 112, No. 47, 2008 14697 (34) Fernandez-Nieves, A.; Fernandez-Barbero, A.; Vincent, B.; de las Nieves, F. J. Macromolecules 2000, 33, 2114. (35) Mohanty, P. S.; Dietsch, H.; Richtering, W. 7th Liquid Matter Physics Conference 2008 (Abstract). (36) Hansen, J. P.; MacDonald, I. R. Theory of Simple liquids, 3rd ed.; Academic Press: Amsterdam, 2006. (37) The effective volume fraction, φeff ()0.0376) was obtained from polymer concentration (0.00026 g/mL), the swelling ratio (∼(Rh,20)3/(Rh,45)3 ) 160.5) and the density14 of the polymer at collapsed state (∼1.1 g/cm3) at 45 °C. Rh,20 is the hydrodynamic radius at 20 °C and pH ) 6.2. Rh,45 is the hydrodynamic radius at 45 °C at pH ) 3.2. It was taken into account that in the collapsed state, the microgel particle contains 50% water. (38) Okubo, T.; Kiriyama, K.; Nemoto, N.; Hashimoto, H. Colloid Polym. Sci. 1996, 274, 93. (39) Xue, J.-Z.; Pine, D. J.; Milner, S. T.; Wu, X.-l.; Chaikin, P. M. Phys. ReV. A 1992, 46, 6550. (40) Urban, C.; Schurtenberger, P. J. Colloid Interface Sci. 1998, 207, 150.
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