Topology and Sequence-Dependent Micellization and Phase Separation of Pluronic L35, L64, 10R5, and 17R4: Effects of Cyclization and the Chain Ends.

The topology effects of cyclization on thermal phase transition behaviors were investigated for a series of amphiphilic Pluronic copolymers of both hydrophilic-hydrophobic-hydrophilic and hydrophobic-hydrophilic-hydrophobic block sequences. The dye solubilization measurements revealed the lowered critical micelle temperatures (TCMT) along with the decreased micellization enthalpy (ΔHmic) and entropy (ΔSmic) for the cyclized species. Furthermore, the transmittance and dynamic light scattering (DLS) measurements indicated a block sequence-dependent effect on the clouding phenomena, where a profound decrease in cloud point (Tc) was only found for the copolymers with a hydrophilic-hydrophobic-hydrophilic block sequence. Thus, the effect of cyclization on these critical temperatures was manifested differently depending on its block sequence. Finally, a comparison of the linear hydroxy-terminated, methoxy-terminated, and cyclized species indicated the effect of cyclization to be unique from a simple elimination of the terminal hydrophilic moieties.

Cyclization of PEG and Pluronic Surfactants and the Effects of the Topology on Their Interfacial Activity.

A series of cyclic surfactants were synthesized from a poly(ethylene glycol) (PEG) homopolymer and Pluronic surfactants L35, L64, P123, F68, 10R5, and 17R4, and their interfacial activity depending on the topology, chain ends, and block sequence was investigated. The cyclization was performed in a single step through etherification of the PEG homopolymer and the hydrophilic-hydrophobic-hydrophilic (ABA type) poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol) (PEG-PPG-PEG), while the hydrophobic-hydrophilic-hydrophobic (BAB type) PPG-PEG-PPG was cyclized via acetalization. The cyclized surfactants were rigorously characterized by nuclear magnetic resonance spectroscopy and size exclusion chromatography. Cyclization of the surfactants induced a significant decrease in the hydrodynamic volume, which was more pronounced than that of the PEG homopolymer. Surface tension (γ) measurements indicated that the interfacial activity of the cyclized surfactants is stronger than their corresponding linear precursors, due to the increase in the surfactant density at the air-water interface as a consequence of the decreased molecular occupational area (A) upon cyclization. In the case of the PEG homopolymer, A considerably decreased from 410 Å2 for the linear PEG prepolymer to 100 Å2 for the cyclized PEG product. While the effects of chain-end groups were found to be limited to surfactants of relatively small molecular weights, the influence of cyclization depended strongly on the hydrophilic/hydrophobic ratio; the higher the PEG composition the surfactants had, the larger the decrease in γ and A; in other words, stronger enhancement in the interfacial activity was observed.