SYNTHESIS, CHARACTERIZATION, AND RHEOLOGICAL BEHAVIOR OF POLYETHYLENEGLYCOLS END-CAPPED WITH FLUOROCARBON HYDROPHOBES

Two polyethylene glycols (PEG, M = 35 000) end-capped with short fluor ocarbon tails were synthesized and characterized. In aqueous solution, the fluorocarbon portions associate strongly to form micelle-like str uctures which are bridged by PEG chains to form a three-dimensional ne twork. As a result, these polymers in solution exhibit unusual rheolog ical properties as a function of fluorocarbon length, polymer concentr ation, and shear rate (frequency). Their zero-shear viscosity increase s with concentration, a common behavior of associating polymers. The v iscosity is dramatically enhanced by replacing the end hydrophobe C6F1 3 with C8F17, a consequence of the stronger association interaction of C8F17 in aqueous solution. The polymer with the longer end group exhi bits strong shear thinning once a critical shear rate is reached, wher eas for the C6F13 end-capped polymer, we cannot with our equipment rea ch the shear-thinning regime. Our data indicate that between 2 and 6 w t %, and perhaps over a wider range of concentrations, both systems ca n be characterized in terms of identical values of the plateau modulus G(N) degrees, implying a similar concentration of chains bridging mic elles in each system. The G(N) degrees values increase strongly with p olymer concentration, consistent with a larger fraction of bridging ch ains and a smaller fraction of looping chains at elevated concentratio n. The viscosity difference between the two polymers can be explained in terms of a slower exit rate of the longer fluorocarbon from its mic elle.