Synthesis and Characterization of Charged Polystyrene-Acrylic Acid Latex Particles P. S. Mohanty, † R. Kesavamoorthy, ‡ Kozo Matsumoto, † Hideki Matsuoka,* ,† and K. A. Venkatesan § Department of Polymer Chemistry, Kyoto UniVersity, Kyoto 615-8510, Japan, Materials Science DiVision, Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, India, and Fuel Chemistry DiVision, Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, India ReceiVed NoVember 7, 2005. In Final Form: February 18, 2006 Novel, monodisperse charged colloidal particles of polystyrene cross linked with divinylbenzene and surface- grafted with acrylic acid were synthesized by emulsion polymerization and were characterized by estimating the dissociable surface charge by conductivity titration, the particle effective charge by conductivity verses particle concentration, and the particle size by dynamic light scattering and atomic force microscopy. The structural ordering and dynamics were investigated as a function of the volume fraction of the particles using static and dynamic light scattering, respectively. Furthermore, from the electrophoresis measurements, these particles are found to have a high salt tolerance due to increases in charge as a function of salt concentration. Introduction The most fascinating aspect of charged colloidal suspensions is the appearance of long-range order. 1-3 These structures, commonly known as colloidal crystals, exhibit iridescence arising from the Bragg diffraction of visible light. 1 These colloidal crystals are very important because of their use as Bragg-diffraction devices 3 , nanoswitches, 4,5 chemical sensors, 6 and templates for preparing photonic band gap materials. 7,8 Recently, these colloidal crystalline arrays (CCAs) have been immobilized into an expandable transparent hydrogel to obtain a gel-sensing device. 7-11 Apart from crystalline order, 1,12 these colloidal suspensions also exhibit gas, 12-14 liquid, 12,14,15 and even glasslike 12,16,17 order. The structural ordering and phase transitions in charged colloidal suspensions can be easily tailored by tuning the range and strength of the interparticle interaction energy U. 12 This interaction energy U can be varied over a wide range by changing the Debye screening parameter κ, which can be achieved by changing the particle concentration n p , the charge on the particle Z, or the salt concentration C s . Most of the past studies on structural ordering, dynamics, and phase transitions have been investigated on charged polystyrene particles having a strong acid group such as sulfonate. Because of the complete dissociation of the sulfonic groups, these particles in general carry more charges than particles having weak acid group. Hence, they can be ordered at lower volume fraction 1 compared to the latter one. However, in the case of a strong acid group, the particles are not sensitive to pH, and pH control is very important for a drug delivery system. In the case of a weak acid group, the particles will have high charge at high pH and a low charge at low pH. Therefore, these particles can be effectively used as drug carriers in a drug delivery systems. One can design the drug release mechanism at both low and high pH, but this is not possible in the case of a polystyrene particle having a strong sulfonate group. Apart from this, these particles can also be self-assembled at higher volume fraction and at higher pH to form colloidal crystals, which is also very useful for preparing photonic band gap materials. So far, there have been no reports on the synthesis, characterization, and study of structural ordering and dynamics using static and dynamic light scattering. This motivates us to synthesize polystyrene latex particles having COO - ions on their surfaces and to study their basic properties. In this article, we will report the synthesis and characterization of polystyrene-divinyl benzene-acrylic acid copolymer colloidal particles. The synthesis is carried out according to the general procedures used by Asher and co-workers. 18-20 They have used a sulfonated co-monomer such as sodium 1-allyloxy-2-hydroxy- propane sulfonate (COPS1) to charge the particles. Here we have used acrylic acid as the co-monomer. We carried out conductivity titration to determine the dissociable surface charge number (Z t ) and conductivity verses particle concentration to estimate the effective particle charge (Z). The size distribution was estimated using dynamic light scattering and atomic force * Corresponding author. E-mail: matsuoka@star.polym.kyoto-u.ac.jp. † Kyoto University. ‡ Materials Science Division, Indira Gandhi Centre for Atomic Research. § Fuel Chemistry Division, Indira Gandhi Centre for Atomic Research. (1) Mohanty, P. S.; Tata, B. V. R.; Yamanaka. J.; Sawada, T. 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