Understanding the stiffness of Macromolecules: From single chains to semi flexible polymer brushes

DIPC Seminars

Kurt Binder (Institut fuer Physik, Johannes Gutenberg Universitaet, Mainz, Germany)
Donostia International Physics Center
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Understanding the stiffness of Macromolecules: From single chains to semi flexible polymer brushes Professor Kurt Binder Institut fuer Physik,Johannes Gutenberg Universitaet Mainz 55099 Mainz,Staudinger Weg 9,Germany ABSTRACT The key parameter to characterize the stiffness of polymers is their persistence length. Unfortunately,there is confusion in the literature on the proper definition of this length: Using the decay constant of an exponentially decaying orientation correlation of bonds along the chain contour works only for "phantom chains”. For real macromolecules in the long chain limit the decay is always a power law (with an exponent 3/2 for chains in the melt or Theta solutions,and an exponent related to the Flory exponent for polymers in dilute solutions under good solvent conditions).The validity of the Kratky- Porod model for chain extensions of semiflexible chains hence also is restricted (it fails in d=2 dimensions,a case relevant for adsorbed DNA,apart from the trivial rod-like regime ; in d=3 Gaussian behavior occurs only for a intermediate range of chain lengths).These results are exemplified by extensive Monte Carlo simulations of a lattice model using the pruned-enriched Rosenbluth method(PERM). A subtle problem is the extension of these concepts to "bottlebrush polymers",where flexible chains are grafted to a (flexible) backbone,and thus (on a coarse-grained level) a wormlike chain results.The dependence of the persistence length on the side chain length will be clarified,discussing also pertinent experiments.Finally,also the effects of chain stiffness for standard polymer brushes (where macromolecules are densely grafted by one chain end to a planar substrate) will be discussed,and evidence for a new kind of phase transition in compressed semiflexible polymer brushes is given.