Shear-Induced Rearrangement of Self-Assembled PEG-Lipids Structures in Water

A number of nonionic PEG-lipids were synthesized via anionic polymerization of ethylene oxide using 1,3-didodecyloxypropan-2-ol as a starting material. The two aliphatic chains as well as the PEG moieties are linked to the glycerol skeleton via ether linkages. By varying the ethylene oxide polymerization time, PEG moieties of degrees of polymerization ranging from 30 to 92 were constructed. The PEG-lipids were characterized by 1H nuclear magnetic resonance and gel permeation chromatography. In aqueous solution they formed spontaneously micellar and possibly lamellar aggregates. The latter were seen in the relaxation time distribution spectra as low-amplitude diffusive slow modes. The particles responsible for the slow mode behavior were eliminated by centrifugation, and light scattering in an extended temperature interval was carried out. At elevated temperatures particles with increasing density were formed. The solutions were found to be sensitive to shear forces as revealed by comparing the relaxation time distribution spectra of vigorously stirred, filtered, or extruded solutions with those of the starting solutions. Objects identified as bilayer structures were found in the extruded solutions. They were visualized by cryogenic transmission electron microscopy. The micellization regions were investigated by a fluorescence probing technique.