Lyotropic behavior in water of amphiphilic ABA triblock copolymers based on poly(propylene sulfide) and poly(ethylene glycol)

The behavior in water of block copolymers of poly(ethylene glycol) and polypropylene sulfide) (PEG-PPS-PEG) has been investigated using polarized light optical microscopy (POM), rheometry, freeze-fracture transmission electron microscopy (FFTEM), and surface pressure isotherm measurements. These copolymers are amphiphilic and, due to the presence of sulfur atoms instead of oxygen in the hydrophobic block, show an increased lipophilicity when compared to their analogous block copolymers based on polypropylene oxide) hydrophobes (Pluronics). The marked difference in hydrophilicity between the blocks favors the formation of stable lyotropic mesophases in water (a solvent for PEG but not fur PPS) over a wide range of composition. Lamellar phases were identified by POM and confirmed by FFTEM. Upon dilution, the lamellar gel evolved into a suspension of vesicles that could be stored for months at room temperature without loss of stability. The gel-state of copolymer/water mixtures and their stability was investigated by dynamic shear rheometry. A comparison between monolayer properties at the air-water interface and membrane morphology was made to predict favored triblock conformation in the vesicle (U-or I-shaped). The size of the hydrophobic PPS block acts as a limiting factor when its size is over 50 units and when the PEG chains are able to adopt a brush conformation upon compression. PEG-PPS-PEG copolymer vesicles are potential candidates for controlled drug delivery systems due to their stability and ease of preparation.