Aqueous Block Copolymer Systems. Self-association, Phase Behaviour and Partitioning of Biomolecules

Abstract: The phase and partitioning behaviour of aqueous block copolymer systems has been studied. The block copolymers used in this work were built up of three blocks. The middle block was poly(propylene oxide) (PPO) and hydrophobic. The two outer blocks constisted of poly(ethylene oxide) (PEO) chains and less hydrophobic. The amphiphilic property of this PEO-PPO-PEO block copolymers enable self-association into micelles and formation of lyotropic liquid crystalline (LLC) phases. The formation of micelles in aqueous solution may be induced either by an increase in polymer concentration (critical micelle concentration) or by an increase in temperature (critical micelle temperature). It was found that the phase behaviour of ternary PEO-PPO-PEO/dextran T500/water systems was strongly affected by the temperature, which, in turn was attributed to the temperature-triggered micelle formation of the PEO-PPO-PEO polymers. Theoretical calculations based on Flory-Huggins theory for polymer solutions were in good agreement with the experimental findings. Binary (PEO-PPO-PEO/water) phase behaviour were studied experimentally and theoretically. For example, the experimental phase diagrams dislayed both normal and reverse hexagonal phases LLC phases, an unique feature for these type of systems. By using a heterogeneous lattice model, the free energy for the different phases was detrmined and by extracting two-phase regions, binary phase diagram was constructed. Ternary (PEO-PPO-PEO/water/oil) systems were studied with a heterogeneous lattice model. Five ordered phases were considered: normal cubic, normal hexagonal, lamellar, reverse hexagonal and reverse cubic and ternary phase diagrams were constructed. The lamellar phase was studied in more detail and the results; domain spacings, interfacial area for the block copolymer and the apolar domain thickness were discussed and in some cases compared to experimental findings. The population of loop-like and bridging conformations of the block copolymer in the lamellar plane was also investigated. The effect of temperature on partitioning of biomolecules (amino acids, peptides and proteins) in PEO-PPO-PEO/dextran T500/water systems was studied. Hydrophobic biomolecules predominantly partitioned to the PEO-PPO-PEO-rich phase and their partitioning behaviour was in most cases strongly influenced by the temperature.