We report x-ray scattering, rheological, and freeze-fracture and polarizing microscopy studies of a liquid crystalline hydrogel called L(α,g). The hydrogel, found in DMPC, pentanol, water, and PEG-DMPE mixtures, differs from traditional hydrogels, which require high MW polymer, are disordered, and gel only at polymer concentrations exceeding an 'overlap' concentration. In contrast, the L(α,g) uses very low-molecular-weight polymer-lipids (1212, 2689, and 5817 g/mole), shows lamellar order, and requires a lower PEG-DMPE concentration to gel as water concentration increases. Significantly, the L(α,g) contains fluid membranes, unlike L(β), gels, which gel via chain ordering. A recent model of gelation in L(α) phases predicts that polymer-lipids both promote and stabilize defects; these defects, resisting shear in all directions, then produce elasticity. We compare our observations to this model, with particular attention to the dependence of gelation on the PEG MW used. We also use x-ray lineshape analysis of scattering from samples spanning the fluid-gel transition to obtain the elasticity coefficients κ and B; this analysis demonstrates that although B in particular depends strongly on PEG-DMPE concentration, gelation is uncorrelated to changes in membrane elasticity.
Bibliographical noteFunding Information:
This work was partially supported by National Science Foundation Grant DMR-9624091 and the Petroleum Research Fund (31352-AC7). J. A. Zasadzinski was supported by National Science Foundation Grant 9319447. S. L. Keller was supported by a University of California President’s postdoctoral fellowship. The Materials Research Laboratory at Santa Barbara is supported by National Science Foundation Grant DMR-96-32716. Synchrotron Experiments were carried out at Stanford (SSRL) which is supported under the United States Department of Energy.
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