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College Park, Maryland      June 6 - 10 , 2004

W2-B4 (2:30 PM): Neutron Spin-Echo Study of Dynamics of Hydrophobically Modified Polymer Doped Surfactant Bilayers

R. K. Prud’homme (Department of Chemical Engineering, Princeton University, Princeton, NJ 08544-5263), B.-S. Yang (Bristol-Myers Squibb, One Squibb Drive, New Brunswick, NJ 08903), J. Lal (Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, IL 60439), M. Mihailescu, M. Monkenbusch, D. Richter (Institut fur Festkorperforschung-Forschungszentrum Julich D-52425 Julich, Germany), J. S. Huang (Department of Chemical Engineering, Princeton University, Princeton, NJ 08544-5263), J. Kohn (Department of Chemistry, Rutgers University, Piscataway, NJ 08854), W. B. Russel (Department of Chemical Engineering, Princeton University, Princeton, NJ 08544-5263)

We characterize the effect of adsorbed hydrophobically modified polymers (hm-polymers) on the dynamics of surfactant bilayers by small-angle neutron scattering (SANS) and neutron spin-echo (NSE) spectroscopy. Two kinds of hm-polymers, (a) hydrophobically modified poly(acrylate) (hmPAA) with tetradecyl (C14) sidegroups randomly grafted to the poly(acrylate) backbone and (b) poly(PEG6k-lysine-stearylamide) (hmPEG) with equally spaced hydrophobes, are added to bilayers of penta(ethylene glycol) dodecyl ether (C12E5) and hexanol. Both bare and polymer doped membrane exhibit a stretched exponential relaxation in the form of I(q) @ S(q)exp(-Gqt) 2/3, where S(q) is the static structure factor and Gq is the relaxation rate. The relaxation rate depends subtly on the surface-coverage of polymer. At high surface coverage, polymer slows down the relaxation by 20 %; whereas at low surface coverage, polymer enhances the rate relative to the bare membranes. Hindered flow of solvent through the adsorbed polymer layer at high coverage appears to explain retardation. The faster dynamics at low polymer coverage may be due to stiffening of the membrane or lateral diffusion of the dilute adsorbed polymer chains. The q range covered by the static and dynamic measurements coincide. The slowest relaxation rate occurs at length scales (i.e. q vectors) of the diffraction peak corresponding to interlamellar spacings. The quality of the NSE data available invites the development of new theories of the dynamics of lamellar phase liquid crystalline fluids.

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