THE INFLUENCE OF POLYMER MOLECULAR-WEIGHT IN LAMELLAR GELS BASED ON PEG-LIPIDS

We report x-ray scattering, rheological, and freeze-fracture and polar izing microscopy studies of a liquid crystalline hydrogel called L-alp ha,L-g. The hydrogel, found in DMPC, pentanol, water, and PEG-DMPE mix tures, differs from traditional hydrogels, which require high MW polym er, are disordered, and gel only at polymer concentrations exceeding a n ''overlap'' concentration. In contrast, the L-alpha,L-g uses very lo w-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-alpha,L-g contain s fluid membranes, unlike L-beta' gels, which gel via chain ordering. A recent model of gelation in L-alpha phases predicts that polymer-lip ids both promote and stabilize defects; these defects, resisting shear in ail directions, then produce elasticity. We compare our observatio ns to this model, with particular attention to the dependence of gelat ion on the PEG MW used. We also use x-ray lineshape analysis of scatte ring from samples' spanning the fluid-gel transition to obtain the ela sticity coefficients kappa and B; this analysis demonstrates that alth ough B in particular depends strongly on PEG-DMPE concentration, gelat ion is uncorrelated to changes in membrane elasticity.