The Synthesis and Self-Asgembly of ABA Amphiphilic Block Copolymers Containing Styrene and Oligo(ethylene glycol) Methy Ether Methacrylate in Dilute Aqueous Solutions: Elevated Cloud Point Temperatures for Thermoresponsive Micelles

A series of ABA amphiphilic triblock copolymers possessing polystyrene (PS) central hydrophobic blocks, one group with short PS blocks (DP = 54-86) and one with long PS blocks (DP = 183-204) were synthesized by atom transfer radical polymerization. The outer hydrophilic blocks were various lengths of poly(oligoethylene glycol methyl ether) metbacrylate, a comb-like polymer. The critical aggregation concentrations were recorded for certain block copolymer samples and were found to be in the range circa 10(-9)) mol L-1 for short PS blocks and circa 10(-12) mol L-1 for long PS blocks. Dilute aqueous solutions were analyzed by transmission electron microscopy (TEM) and demonstrated that the short PS block copolymers formed spherical micelles and the long PS block copolymers formed predominantly spherical micelles with smaller proportions of cylindrical and Y-branched cylindrical micelles. Dynamic light scattering analysis results agreed with the TEM observations demonstrating variations in micelle size with PS and POEGMA chain length: the hydrodynamic diameters (DH) of the shorter PS block copolymer micelles increased with increasing POEGMA block lengths while maintaining similar PS micellar core diameters (D-C); in contrast the values of D-H, and D-C for the longer PS block copolymer micelles decreased. Surface-pressure isotherms were recorded for two of the samples and these indicated close packing of a short PS block copolymer at the air-water interface. The aggregate solutions were demonstrated to be stable over a 38-day period with no change in aggregate size or noticeable precipitation. The cloud point temperatures of certain block copolymer aggregate solutions were measured and found to be in the range 76-93 degrees C; significantly these were similar to 11 degrees C higher in temperature than those of POEGMA homopolymer samples with similar chain lengths. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polyrn Chem 46: 7739-7756, 2008