Aggregation number distributions and mesoglobules in dilute solutions of diblock and triblock copolymers

– We investigate the aggregation number and size distributions for inter-molecular clusters of amphiphilic diblock and triblock copolymers in poor solvent at very low concentrations. Diblocks and triblocks with hydrophilic ends are shown to possess narrow distributions corresponding to formation of monodispersed mesoglobules. Diblocks with hydrophobic ends are found to produce inter-cluster multimers due to bridging by the hydrophilic middle blocks, resulting in polydisperse distributions. Implications of these observations for the preparation of monodispersed nanoparticles and, potentially, for the understanding of the quaternary structure of proteins are discussed. Introduction. – The phenomena of coil-to-globule transition for a single polymer molecule and phase separation in polymer solutions are foundations of the modern polymer science [1, 2]. In recent years there were numerous studies of the behaviour of water soluble polymers near the lower critical solubility temperature (LCST). Typical systems include poly-Nisopropylacrylamide (PNIPAM) homopolymer and block copolymers [2] of the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), or PEO-PPO systems briefly [3]. These copolymers are widely used in pharmaceutical, agricultural and food industries due to their low toxicity and surfactant characteristics (EO is hydrophilic and PO is hydrophobic). Block and more complex random heteropolymers in solutions and melts at higher concentrations exhibit a variety of ordered micro-phase separated and disordered glassy phases [4, 5]. Block copolymers also play the role of surfactants in ternary mixtures of two otherwise immiscible liquids such as water and oil, and these mixtures also produce micelles and lamellae [6]. There are two types of triblock copolymers having rather distinct properties. First, a PEO-PPO-PEO polymer with hydrophilic ends and a hydrophobic middle is the standard Pluronic [3, 7], which forms stable micelles. Second, a polymer with the inverse structure would tend to network hydrophobic end “stickers” by hydrophilic bridges. These are so-called telechelic associating polymers forming physical gels with nontrivial rheological properties at higher concentrations [8]. The aggregation number distribution during micellisation has been a matter of intensive studies for amphiphilic molecules [7]. The classical situations, in which there is a clear phase separation transition from Mcl = 1 to Mcl = Mmax, or where one could introduce notions of CMC (critical micelle concentration), and possibly also CAC (critical aggregation (∗) E-mail: [email protected] http://darkstar.ucd.ie