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ENTROPIC NETWORKS IN POLYMERS AND MICROEMULSIONS A. Zilman, S. A. Safran Weizmann Institute of Science, Rehovot, Israel 76100 email: sam.safran@weizmann.ac.il ABSTRACT Self-assembling networks and branched structures are common in both natural and synthetic materials and form under a variety of equilibrium and non-equilibrium conditions. Here, we review the theory of the structure and thermodynamic properties of equilibrium networks of cylindrical micelles [1], microemulsions [2] and selfassembling polymeric gels [3]. In all these systems, the networks consist of self-assembling, locally, onedimensional objects (e.g., polymer chains in the case of a gel, cylindrical micelles or microemulsions or chains of dipolar colloids [4]). The study of network phases is of both theoretical [5] and practical interest. From the practical point of view, network forming amphiphiles and sol-gel systems are at the core of many industrial, biological and bio-medical applications. The existence of junctions and their dynamics is important in understanding the rheology of worm-like micelles. Network formation may also be responsible for the unusual "closed loop" phase diagrams of microemulsions and dipolar liquids. Cryo-electron microscopy [1] shows clear evidence of coexisting network phases in dilute microemulsions. In many of the systems mentioned above, the chains themselves are self-assembled in an equilibrium manner from a large number of monomers. We begin with a simple estimate of the density and free energy contribution of ends and junctions in a system of self-assembling chains or tubes and show that the junction entropy introduces an effective attraction (negative second-virial coefficient) that can lead to phase separation. The relationship of this thermodynamic transition to the topological, connectivity transition (percolation or gelation) can be predicted in a unified manner using a modification of the n = 0 spin-model for polymers .The extension of these ideas to systems with crosslinks (e.g. actin gels) is also discussed and the theoretical phase diagrams are compared with recent experiments [6]. Finally, we show how entropic phase separation can also explain recent observations [7] in a combined system of telechelic polymers and microemulsions that form network structures. References 1. Berheim-Groswasser, A., Tlusty, T., Safran, S.A., Talmon, Y., 1999. Langmuir 15, 5448; Bernheim-Grosswasser, A., Wachtel, E., Talmon, Y., 2000. Langmuir 16, 4131. 2. Tlusty, T, Safran, S.A., Strey, R., 2000. Phys. Rev. Lett. 84, 1244. 3. Zilman, A. and Safran, S.A., submitted. 4. Tlusty, T, Safran, S.A., 2000. Science 290, 1328. 5. Drye, T., Cates, M.E., 1992. J. Chem. Phys. 96, 1367; Zilman, A., Safran, S.A., Phys. Rev. E, in press. 6. Tempel, M., Isenberg, G., Sackmann, E., 1996. Phys. Rev. E 54, 1802. 7. Filali, M., Ouazzani, M.J., Michel, E., Aznar, R., Porte, G. Appell, J., 2001. J. Phys. Chem B, 105, 10528.
MEMBRANE FUSION: RESULTS FROM SIMULATION AND SELF-CONSISTENT FIELD THEORY Michael Schick Department of Physics, University of Washington, Box 351560, Seattle, WA 98195-1560, U.S.A. email: schick@mahler.phys.washington.edu ABSTRACT Our group has been studying self assembly in systems of block copolymer and of biological lipids, as there is much similarity in the phase behavior of the two. Our studies have focussed on defects in these systems. In the first part of my talk, I will discuss various grain boundaries in lamellar phases of block copolymers, work which was accomplished utilizing self-consistent field theory only, and in the Fourier representation. In the second part, I will discuss our work on the mechanism of membrane fusion, which was carried out utilizing both Monte Carlo simulations and self-consistent field theory in real-space representation. Grain Boundaries Grain boundaries in smectics, such as the lamellar phases of block copolymers, are easier to study than those in crystalline solids due to the lack of in-plane order in the smectics. Matsen first showed how scft in the Fourier representation could be applied to symmetric-tilt grain boundaries. A notable feature was the occurrence of a symmetry-breaking transition as a function of tilt angle. We have extended these studies to include twist grain boundaries and, most recently, T-junctions. Symmetric tilt grain boundaries and Tjunctions both can connect lamellar grains of different orientations. Presumably the frequency with which one observes one or the other depends upon their relative free energy. Our motivation was a series of experiments which showed that T-junctions did not occur very often in a system consisting of block copolymer only. However the addition of homopolymer significantly increased their occurrence. Our studies showed that in the system of copolymer only, there was only a small range of angles over which the free energy of the T-junction was less than that of the symmetric tilt grain boundary. However this range of angles increases appreciably with the addition of homopolymer. Of additional interest is the appearance of a morphological change as the angle between grains approaches 180 degrees. This change is rather similar to that which occurs in the symmetric tilt boundaries.Membrane FusionAlthough the fusion of membranes is an enormously important biological process, it is poorly understood. One knows that specialized proteins are needed to bring two fluctuating membranes close to one another, a process which puts the membranes under tension. Beyond this, little is known. Almost all theories begin with an initial state consisting of a cylindrically symmetric stalk, a localized region in which the lipids of each of the proximate lipid layers rearrange to form a continuous hydrophobic region joing the hydrophobic cores of the two bilayers. The final fusion pore, whose hydrophilic region traverses both bilayers, is also cylindrically symmetric. It has been assumed by all theories that all intermediate states in the fusion process were equally symmetric. We investigated this process by Monte Carlo simulation and found the process took a completely different symmetry-broken route. Knowing the form of the intermediate, we have used self consistent field theory to calculate the free energy barriers along previously assumed paths and the one we found. Our calculations help us understand why the path we did see is the one we should have seen.
Page 1 and 2: Frontis - Wageningen International
Page 3 and 4: Prof. dr. Hans Fraaije Leiden Unive
Page 5 and 6: Prof. Ullrich Steiner University of
Page 7 and 8: Preface Self-consistent field model
Page 9 and 10: covered by polymers), and R. Rajago
Page 11 and 12: FIRST-ORDER WETTING TRANSITION AT F
Page 13 and 14: MOLECULAR SCF MODELLING OF PORES IN
Page 15 and 16: COLLAPSE OF POLYMER BRUSHES GRAFTED
Page 17 and 18: SURFACE FORCES, SUPRAMOLECULAR POLY
Page 19 and 20: SWEET BRUSHES: SYNTHESIS AND INTERF
Page 21 and 22: EFFECT OF MIXING OF TWO CHARGED PHO
Page 23 and 24: Figure 2 show a semi-log comparison
Page 25 and 26: MODELING OF STATIONARY POLYMER DIFF
Page 27 and 28: THEORY AND SIMULATION OF BILAYER ME
Page 29 and 30: Figure 1. Time evolution of the she
Page 31 and 32: SALT EFFECT TO RAYLEIGH DROPLETS OF
Page 33 and 34: MACROMOLECULES AT INTERFACES, A FLE
Page 35 and 36: APPLICATION OF HETEROGENEOUS LATTIC
Page 37 and 38: 8. Linse, P., 1993b. Phase behavior
Page 39 and 40: MODELING INTERFACIAL EFFECTS ON THE
Page 41 and 42: Using this patterned electrode, we
Page 43 and 44: span the gap between the electrodes
Page 45 and 46: desirable to use a probe with a muc
Page 47: span the gap between the electrodes
Page 51 and 52: the transition temperature T * when
Page 53 and 54: PROTEIN ADSORPTION ON SURFACES WITH
Page 55 and 56: MODELLING STRUCTURE AND ADHESION AT
Page 57 and 58: References 1. Fischel, L.B. and The
Page 59 and 60: MULTIDOMAIN CONFORMATIONS OF RANDOM
Page 61 and 62: SUPRAMOLECULAR COORDINATION POLYMER
Page 63 and 64: INFLUENCE OF POLYMERS ON THE BENDIN
Page 65 and 66: SPINODAL DECOMPOSITION IN BINARY PO
Page 67 and 68: PERSISTENCE LENGTH OF WORM-LIKE MIC
Page 69 and 70: For small particles with radius R <
Page 71: MEAN-FIELD THEORY FOR THE ADSORPTIO

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