Aggregation behaviour of well defined amphiphilic diblock copolymers with poly(N-isopropylacrylamide) and hydrophobic blocks Markus Nuopponen, Jussi Ojala, Heikki Tenhu * Laboratory of Polymer Chemistry, Department of Chemistry, University of Helsinki, A.I. Virtasen aukio 1, PB 55, Helsinki FIN-00014 HY, Finland Received 1 December 2003; received in revised form 23 March 2004; accepted 26 March 2004 Abstract Series of amphiphilic diblock copolymers with poly(N-isopropylacrylamide) as a hydrophilic block and a hydrophobic block consisting of either polystyrene or poly(tert-butyl methacrylate) were synthesised using RAFT polymerisations. Differential scanning calorimetry showed the chemically different blocks being phase separated in dry polymers. Light scattering and microcalorimetry studies were performed on aqueous solutions to investigate the phase behavior of the diblock copolymers. By carefully transferring the polymers from an organic solvent to water, either micellar particles or large aggregates were obtained depending on the relative lengths of the blocks. Large aggregates collapsed upon heating, whereas collapse occurred slowly within a broad temperature range in the case of micelle like structures. However, microcalorimetrically the collapse of the PNIPAM chains was observed to take place in all samples, suggesting that the shells of the micellar particles are crowded in a way which hinders the compression of the poly(N-isopropylacrylamide) chains. q 2004 Elsevier Ltd. All rights reserved. Keywords: Diblock copolymers; Polymeric micelles; Controlled radical polymerization 1. Introduction Much effort has recently been focused on the synthesis of tailor-made amphiphilic block copolymers [1–3]. These polymers are interesting for a range of potential applications because of their property to self-assemble into aggregates and micelles when dispersed in selective solvents. Amphi- philic water-soluble polymers can undergo a conformational change in response to an external stimuli [4–6]. The simplest type of aggregation is exhibited by diblock copolymers, which can self-assemble when the hydrophobic core is surrounded by a shell of the solvated hydrophilic part [7,8]. In aqueous solutions, block copolymers with blocks of different hydrophilicities may form random aggregates, micelles, or even more organised structures like, e.g. liposomes. Well-known examples of self-assembling in water are phospholipid cell membranes, surfactants in aqueous solutions, and block copolymers in a way similar to that of surfactants. Self-organisation may also take place in a dry polymer leading to a spontaneous formation of nanoscaled structures due to competing interactions [9]. During the past decades, considerable progress has been made in understanding of the melt phase behavior of block copolymers. Supramolecules self-organise in the form of structure-within-structure morphologies, with a large length scale structure and microphase separation [10,11]. Several syntheses of well-defined block copolymers with predetermined molar masses as well as other polymers with complex molecular architectures have been reported, where the polymers have been prepared using controlled radical polymerisation. Reversible addition-fragmentation chain transfer (RAFT) [12–15] polymerisation is often a method of choice when control on molecular weight and molar mass distribution is needed. RAFT mediated by thiocarbonylthio compounds is an effective and versatile process, applicable to a wide range of vinyl monomers without a need for protecting group. RAFT reactions typically require much less stringent reaction conditions than ionic polymerisation and offer the most of the advantages of conventional free radical polymerisation. With RAFT, the number of easily accessible amphiphilic polymers increase. Poly(N-isopropylacrylamide) [16] (PNIPAM) is one of the most studied thermally responsive polymers. It has a lower critical solution temperature (LCST) around 32 8C in 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.03.083 Polymer 45 (2004) 3643–3650 www.elsevier.com/locate/polymer * Corresponding author. Tel.: þ 358-9-19150334; fax: þ 358-9- 19150330. E-mail address: heikki.tenhu@helsinki.fi (H. Tenhu).