Self-assembly of ABA triblock copolymers based on functionalized polydimethylsiloxane and polymethyloxazoline

This study describes the responsive behavior of modified amphiphilic ABA triblock copolymers of polymethyloxazoline-block-poly(methylhydrosiloxane-co-dimethylsiloxane)-block-polymethyloxazoline (PMOX-b-P(MHS-co-DMS)-b-PMOX) when subjected to compression on the water surface or to ions in the water bulk phase. The hydrophobic middle block was functionalized with spacers bearing methyl 2-hydroxybenzoate (Bz) or 18-crown-6 ether (Ce) groups. The behavior at the air-water interface was studied by measuring surface pressure versus mean molecular area (pi-mmA) isotherms, and atomic force microscopy (AFM) was employed to investigate the morphology of Langmuir-Blodgett (LB) films after transfer to solid supports. Ion-responsive self-assembly was followed using light microscopy and can be understood on a molecular level by employing H-1 NMR spectroscopy. The pi-mmA isotherm of PMOX-b-P(MHS-co-DMS)-b-PMOX-44Bz at the air-water interface had an extended pseudo-plateau at a surface pressure of ca 22 mN m(-1) reflecting the coil to loop transformation of the hydrophobic middle block which was absent for the crown ether-functionalized triblock copolymer. AFM images of LB films of PMOX-b-P(MHS-co-DMS)-b-PMOX-44Bz showed dewetting effects of the polymer film after transfer to a silicon wafer. LB films of PMOX-b-P(MHS-co-DMS)-b-PMOX-8Ce formed surface micelles having a size of ca 50-100 nm on the solid support. The ion sensitivities of the crown ether-derivatized copolymers in solution were investigated by exposing polymeric vesicles to potassium, sodium and magnesium ions. Exposure to K+ and Na+ led to vesicle rupture and the formation of micro-tubular structures, while Mg2+ had no effect on the vesicular structures as confirmed using light microscopy. Specific interactions between the crown ether-derivatized polymer and ions were further elucidated from H-1 NMR experiments that indicated that K+ coordinated with the crown ether causing the dense packing to subside and leading to solubilization of the polymer in water. (C) 2010 Society of Chemical Industry