Abstract
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)2CnF2n+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 Nag 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 Nag) or by increasing end group length (larger Nag) 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.
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