Synthesis and surface characterization of well-defined amphiphilic block copolymers containing poly [oligo(ethylene glycol) methacrylatel segments

A series of well-defined amphiphilic block copolymers containing poly [oligo (ethylene glycol) methacrylate] (POEGMA)) segments were synthesized by the sequential anionic copolymerization of styrene and trialkylsilyl-protected oligo(ethylene glycol) methacrylates followed by deprotection. OEGMA monomers possessed terminal OH groups on the pendant side chains and included the esters of ethylene glycol, di(ethylene glycol), and tri(ethylene glycol) to change the polarity of hydrophilic polymethacrylate segments. The corresponding methyl ethers of oligo(ethylene glycol) methacrylates (MOEGMA) were also employed to synthesize the amphiphilic block copolymers of polystyrene (PS) and PMOEGMA. Microphase separated structures of the block copolymers were analyzed by small-angle X-ray scattering (SAXS). The block copolymers containing POEGMA segments displayed strongly segregated lamellar structure with sharp interfaces; however, the PMOEGMA counterparts showed either weakly segregated lamellar or disorder structures. Angle-dependent X-ray photoelectron spectroscopic (XPS) measurement of the films of block copolymers with POEGMA segments revealed that hydrophobic polystyrene segments exclusively covered the outermost surfaces, as expected. By contrast, enrichment of oligo(ethylene glycol) units was unexpectedly observed in the block copolymers of PMOEGMA types. Contact angle measurement also indicated that the film surface was almost completely covered with hydrophilic PMOEGMA segments rather than polystyrene segments. The nature of the terminal moiety on the oligo(ethylene glycol) side chain, OH or OCH3, played a very important role in determining the surface structure of the amphiphilic block copolymer films. The terminal moiety had a dominant influence either on the interaction parameter with PS or on the interaction with the surface, whereas the number of ethylene glycol units primarily determined the solubility of polymethacrylate segments in water.