Synthesis, Characterization, and Rheological Behavior of Polyethylene Glycols End-Capped with Fluorocarbon Hydrophobes

Two polyethylene glycols (PEG, M = 35 000) end-capped with short fluorocarbon tails were synthesized and characterized. In aqueous solution, the fluorocarbon portions associate strongly to form micelle-like structures which are bridged by PEG chains to form a three-dimensional network. As a result, these polymers in solution exhibit unusual rheological properties as a function of fluorocarbon length, polymer concentration, and shear rate (frequency). Their zero-shear viscosity increases with concentration, a common behavior of associating polymers. The viscosity is dramatically enhanced by replacing the end hydrophobe C 6 F 13 with C 8 F 17, a consequence of the stronger association interaction of C 8 F 17 in aqueous solution. The polymer with the longer end group exhibits strong shear thinning once a critical shear rate is reached, whereas for the C 6 F 13 end-capped polymer, we cannot with our equipment reach the shear-thinning regime. Our data indicate that between 2 and 6 wt %, and perhaps over a wider range of concentrations, both systems can be characterized in terms of identical values of the plateau modulus G N °, implying a similar concentration of chains bridging micelles in each system. The G N ° values increase strongly with polymer concentration, consistent with a larger fraction of bridging chains and a smaller fraction of looping chains at elevated concentration. The viscosity difference between the two polymers can be explained in terms of a slower exit rate of the longer fluorocarbon from its micelle.