Ordering transitions of fluoroalkyl-ended poly(ethylene glycol): Rheology and SANS

Aqueous solutions of associative polymers consisting of poly(ethylene glycol) (PEG) (6K or 10K g/mol) terminated at both ends with hydrophobic fluoroalkyl segments, -(CH₂)₂C_nF_2n+1 (n = 6, 8, or 10), exhibit ordering transitions with increasing concentration. The hydrophobic cores of the micelle-like aggregates order into a body-centered-cubic (bcc) structure, as observed by small-angle neutron scattering. The aggregation number of the hydrophobic core N_ag is determined by the length of the hydrophobic end group and is insensitive to polymer concentration or temperature. Ordering is enhanced by reducing PEG length for a given end group (hence, similar N_ag) or by increasing end group length (larger N_ag) for a given PEG length. This micelle packing effect is manifested in changes in the viscoelastic properties. Specifically, the single-relaxation behavior in the dynamic moduli changes upon ordering as a new low-frequency elastic plateau appears, and in creep a linear response changes to yielding behavior.