Synthesis, structures and multifunctional sensory properties of poly[2,7-(9,9-dihexylfluorene)]-block-poly[2-(dimethylamino)ethyl methacrylate] rod-coil diblock copolymers

Abstract

The synthesis, structures and multifunctional sensory properties of amphiphilic poly[2,7-(9,9-dihexylfluorene)]-block-poly[2-(dimethylamino)ethyl methacrylate] (PF-b-PDMAEMA) rod-coil diblock copolymers are reported. The new copolymers, with PDMAEMA coil lengths of 31, 45, 93 and 185 repeating units, were synthesized by atom transfer radical polymerization. The surface structures and photophysical properties of the synthesized polymers were studied through the variation of solvent composition (water–THF), temperature, and pH. The PF7-b-PDMAEMA45 structure changed from spheres to separate cylinders, bundles of cylinders and spiral-shaped micelles as the solvent composition changed from 0 to 90 wt% water in THF. However, the long-range order structure of spiral-shaped loops was not observed at a long coil length. The micellar aggregates of PF7-b-PDMAEMA45 in water showed a reversible surface structure transformation from cylinder-bundles to spheres on heating from 25 to 75 °C. The variation of the micelle size with temperature was judged to be similar from both atomic force microscopy (AFM) and dynamic light scattering (DLS) measurements. The intermolecular PF aggregations led to fluorescence quenching and a blue-shift in the absorption spectra of the block copolymer as the water content increased. The photoluminescence (PL) intensity of PF7-b-PDMAEMA45 in water was thermoreversible based on its lower critical solution temperature. The PL characteristics suggested the new copolymers behave as an on/off fluorescence indicator of temperature or pH, with a reversible “on–off” profile at an elevated temperature in water: the pH–fluorescence intensity profile switched from “off–on” to “on–off” as the temperature increased. The present study suggests that PF-b-PDMAEMA copolymers have potential applications as multifunctional sensory materials toward solvent, temperature, and pH.