SHORT COMMUNICATION Influence of hydrophilic–lipophilic balance of nonionic emulsifiers on emulsion copolymerization of styrene and methacrylic acid Masayoshi Okubo & Amorn Chaiyasat & Masahiro Yamada & Toyoko Suzuki & Hiroshi Kobayashi Received: 2 May 2007 / Accepted: 24 August 2007 / Published online: 2 October 2007 # Springer-Verlag 2007 Abstract Emulsion copolymerizations of styrene and methacrylic acid (MAA) with various nonionic emulsifiers having a hydrophilic–lipophilic balance (HLB) range of 13.7–17.2 were performed to clarify the influence of emulsification state on polymerization. The emulsification state with a lower-HLB value emulsifier was worse than that with a higher one. In the lowest HLB value, MAA was predominantly polymerized over styrene in the early stage of the copolymerization, resulting in predominant (hetero- geneous) distribution of MAA units in the inside of the final polymer particles. In the higher-HLB emulsifiers, styrene and MAA were simultaneously copolymerized, resulting in a homogeneous MAA distribution. The percentage of incorporation of the nonionic emulsifier inside the particles was the highest (49% based on the total amount of the emulsifier) in the lowest HLB, whereas it was 1% in the highest HLB. Keywords Particle . Nonionic emulsifier . HLB . Emulsion polymerization . Carboxyl group Introduction Carboxylated polymer emulsion have been widely used in various industrial fields [1]. In recent years, such particles are applied as a carrier for bioactive protein to create bioreactors and biosensors [2–5]. In both cases, only functional groups at the particle surfaces are effective. Therefore, many fundamental studies have been conducted on the evaluation and control of the predominant distri- bution of carboxyl groups at the particle surfaces [6–12]. We have also carried out a series of investigations to distribute carboxyl groups predominantly at the particle surface [13–16]. Throughout these studies, it was found that styrene (S)–methacrylic acid (MAA) copolymer [P(S- MAA)] particles prepared by emulsion copolymerization with polyoxyethylene nonyl phenyl ether nonionic emul- sifier were changed to those having one or more hollows in their insides by the alkali treatment at a higher temperature than their glass transition temperature fol- lowed by the acid treatment and by the alkali treatment followed by cooling to room temperature, which were named the alkali/acid method [17] and alkali/cooling method [18], respectively. Effects of various characters of the particles (MAA content, molecular weight, and particle diameter) and treatment conditions (temperature, pH, time, degree of neutralization) on the formation of multihollow structure were clarified [19]. Moreover, surprisingly, it was found that nonionic emulsifier was incorporated inside particles [20, 21] prepared by the emulsion copolymerization. At the low stirring rate, MAA was more predominantly polymerized than that of S in the early stage of the copolymerization, resulting in particles having a MAA-rich copolymer in their insides, which greatly influenced the preparation of multihollow particles [22]. In this article, the influence of emulsification state, changed by using various nonionic emulsifiers having different hydrophilic–lipophilic balance (HLB) values, on the polymerization prepared by emulsion copolymerization will be studied. Colloid Polym Sci (2007) 285:1755–1761 DOI 10.1007/s00396-007-1770-1 DO01770; No of Pages Part CCLXXXIV of the series “Studies on suspension and emulsion”. M. Okubo (*) : A. Chaiyasat : M. Yamada : T. Suzuki : H. Kobayashi Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, Kobe 657-8501, Japan e-mail: okubo@kobe-u.ac.jp