Introduction Polymer emulsion produced by emulsion polymerization has wide industrial applications in film form, such as for paints, coatings and adhesives. The polymer emulsion has a minimum film forming temperature (MFT) below which film is not formed. The MFT has been discussed for a while in relation to the glass transition temperature (Tg) of the base polymer in a dry state (Tg D ), which is measurable. Although Tg in an emulsion state (Tg E ) should be more closely related to the MFT, its mea- surement had, until recently, not been possible. Recently, Lee and coworkers reported that Tg E could be measured with a power compensation-type highly sensitive differential scanning calorimeter (PC- DSC) [1], which has been developed for measuring the thermal behaviors of nucleic acids and proteins. In their report, Tg E was obviously lower than Tg D because of the plasticization with water. This result is a great incentive to us, because Tg E seems to have a strong influence on the preparation of multi-hollow polymer particles by the alkali/acid [2, 3, 4, 5, 6, 7, 8] and alkali/cooling [9, 10, 11] methods, and of nano- sized particles by the particle dissolution method [12, 13, 14, 15, 16] that we proposed. However, most of the polymer emulsions measured by Lee et al. were multi- component polymers (such as terpolymers) having functional groups. This led us to consider the need for further fundamental study. In this study, fundamental data on Tg E for three kinds of poly(methyl methacrylate) based-copolymers with different hydrophilicities, which were prepared by emulsion copolymerizations with various amounts of ethyl acrylate, n-butyl methacrylate and methacrylic acid, will be shown in order to clarify the effect of polymer hydrophilicity on Tg E in comparison with Tg D . Masayoshi Okubo Mayuko Inoue Toyoko Suzuki Masaya Kouda Effect of hydrophilicity of polymer particles on their glass transition temperatures in the emulsion state Received: 6 August 2003 Accepted: 7 November 2003 Published online: 16 January 2004 Ó Springer-Verlag 2004 Abstract The glass transition tem- peratures (Tg) of three kinds of poly(methyl methacrylate)-based copolymer emulsions having wide polymer compositions, which were prepared by emulsifier-free emul- sion copolymerizations of methyl methacryalate with ethyl acrylate, n-buthyl methacrylate and metha- crylic acid, were measured with a power compensation-type highly sensitive differential scanning calorimeter. The Tg values of the copolymers in their emulsion state (Tg E ) were always lower than those in their dry states (Tg D ), and the difference between Tg E and Tg D increased with an increase in copolymer hydrophilicities. Keywords Glass transition tempera- ture Æ Differential scanning calorim- eter Æ Emulsion polymerization Æ Plasticization Æ Hydrophilicity Colloid Polym Sci (2004) 282: 1150–1154 DOI 10.1007/s00396-003-1038-3 SHORT COMMUNICATION Part CCLIII of the series ‘‘Studies on Sus- pension and Emulsion’’ M. Okubo (&) Æ M. Inoue Æ M. Kouda Graduate School of Science and Technology, Kobe University, 657-8501 Kobe, Japan E-mail: okubo@cx.kobe-u.ac.jp Tel.: +81-78-803-6161 Fax: +81-78-803-6161 M. Okubo Æ T. Suzuki Department of Chemical Science and Engineering, Faculty of Engineering, Kobe University, 657-8501 Kobe, Japan