Abstract
The phase behavior of amphiphilic Pluronic block copolymers in aqueous solution is of importance for a broad spectrum of practical applications but has not been fully exploited yet. Here, the phase behavior of the mixture of the Pluronic P65 and P105 triblock copolymer, (which have the same composition of PEO and PPO but the different molecular weight) and organic derivative, 5-methyl salicylic acid (5mS), in aqueous solution has been investigated by using small angle neutron scattering (SANS). According to the temperature and the 5mS concentration, SANS measurements showed that the P65-5mS mixtures sequentially transform into a random coil, sphere, vesicle, cylinder, and vesicle again, while the P105-5mS mixtures form spherical particles with two different sizes without any topological phase transition. Upon heating, the formation of two different kinds of the vesicle structure of amphiphilic block copolymer in aqueous solution is very unusual. This phase behavior was explained as the coupled effect of the simultaneous increase of the hydrophobicity of the polymer and the solubility of 5mS molecules upon heating. This result gives fundamental information for the practical use of Pluronic polymers in nano- and bio-science and it provides a simple route for the fabrication of the nanostructure without a complicated procedure.
Highlights
Pluronic triblock copolymers (PEOmPPOnPEOm), which are one of representative amphiphilic molecules, have been of great interest for a large variety of phase behaviors and the self-assembly into various micellar structures, such as sphere, cylinder, or lamellae in aqueous solution
The 5-methyl salicylic acid (5mS) molecule consists of the salicylic acid with the methyl group and it is slightly soluble in aqueous solution
This indicates that the P105-5mS mixture solution with the 5mS concentration of 0.175% and 0.2% is insensitive to temperature elevation, even though the spherical particle size is very unusual, where we expect that it is directly related to the origin of the phase behavior of Pluronic polymer arising from the control of the external conditions such as temperature and additives
Summary
Pluronic triblock copolymers (PEOmPPOnPEOm), which are one of representative amphiphilic molecules, have been of great interest for a large variety of phase behaviors and the self-assembly into various micellar structures, such as sphere, cylinder, or lamellae in aqueous solution. It is well known that Pluronic triblock copolymers self-assemble into various micellar structures according to their molecular weight and composition ratio of PEO and PPO, which have different size, shape, critical micelle concentration (CMC), and temperature dependence in aqueous solution [14,15,16,17,18,19,20] In this sense, when an additive (which can lead to the control of the effective geometrical molecular shape of block copolymers without their chemical synthesis) is mixed with the Pluronic triblock copolymers, it is expected that the phase behavior of the polymer-additive mixtures is dependent with their molecular weight or composition ratio. We expect that the formation of vesicle structures in the specific temperature range can be applied to smart drug delivery systems for an organ with specific temperature, and this study can provide the fundamental information for practical use of Pluronic polymers into various building blocks as well as a simple way for their fabrication
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