David Shirvanyants

Polymer Theory Group

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Molecular image analysis

Scanning force microscopy is an amazing technique that allows direct visualization of molecular size objects, as opposed to indirect methods such as ion scattering, ellipsometry, etc. However, this visual form of experimental output turns the routine of systematic analysis of observed data into a challenging task. Individual and collective properties of microscopic objects that appear on an image need to be measured and analyzed (e.g. averaged and compared).
Molecular image is a visual representation of the 2-dimensional array of data measured with sampling device of scanning microscope as it traverses across the sample surface. In collaboration with Sheyko Group (S.M.A.L.) the fast and reliable method was developed, that is capable to detect imaged molecules and to measure their characteristics. It has allowed us to study the static and dynamic, individual and collective properties of macromolecules on a surface.

Computer simulation of isolated macromolecules

Polymer solutions are the important class of polymeric liquids. They are used either directly (food and farmaceutical industries, production of motor oil, detergents, etc) or indirectly for polymer synthesis, reactions, characterization, etc.
One of the most surprising features of a polymer solution is the fact that seemingly tiny quantities of dissolved polymer can drastically change the viscosity (and some other properties) of the bulk liquid. The properties of polymer solution are in direct connection with the properties of dissolved polymer molecules. To study the details of internal structure of linear polymer molecules we simulate the behavior of individual chains using method of brownian dynamics. The results are now being prepared for publication.

Computer simulation of polymer melts

Macromolecules in a polymer melt are wound and tangled together, making analysis of polymer dynamics a very challenging problem. Motion of polymer chain in the melt is restricted by the surrounding molecules, since different molecules can not cross each other. Many analytical studies of such systems are based on the idea of so called reptation of polymer chain in a melt, proposed in 1971 by P.G. De Gennes. In this model polymer chain is assumed to perform the snake-like slithering motion in the framework formed by the immobile surrounding chains.
The theoretical description of polymer melt based on the De Gennes' reptation idea is still incomplete. The difficulty that reptating chain poses for the theoretical analysis have stimulated the development of numerical studies. We have implemented in computer code different numerical models of reptating chain, such as the Rubinstein's repton model, the Evans-Edwards' lattice model and the Rouse model of one-dimensional chain. This code was used to study the effect of the chain length fluctuations on the polymer dynamics. The results are now being prepared for publication.