Polymer Theory Group

Department of Chemistry

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Research in Rubinstein Group

Overview

The research of our group is in the field of polymer theory and computer simulations. The unique properties of polymeric systems are due to the size, topology and interactions of the molecules they are made of. Our goal is to understand the properties of various polymeric systems and to design new systems with even more interesting and useful properties. Our approach is based upon building and solving simple molecular models of different polymeric systems. The models we develop are simple enough to be solved either analytically or numerically, but contain the main features leading to unique properties of real polymers. Computer simulations of our models serve as an important bridge between analytical calculations and experiments.

Current Projects

Molecular Dynamics Simulation on Polymer Bottle-Brushes

The primary focus of my research has been to compare numerical results of molecular dynamicsc simulations with theoretical predictions and experimental results regarding polymer bottle-brushes. The scission of carbon-carbon bonds along the backbones of adsorbed bottle-brush polymers has been observed experimentally in systems such as poly (butanoate-ethyl methacrylate)-graft-poly(n-butyl acrylate) on mica. See http://www.chem.unc.edu/people/faculty/sheikoss/sssgroup/ for more details. We are interested in the effects of phase separation as well as the effects of variations in side-chain lengths, monomer density, solvent quality, and the spreading parameter on tension. By better understanding complex polymer architectures, new materials can be made with exciting and unique properties.

Theory on Neutral and Charged Polymer Brushes

The long term goal of this project is to develop a molecular model of the lining in human lungs called Airway Surface Layer (ASL). It is proposed that many aspects of the biological functioning of the ASL is strongly dependent on a major structural constituent called brush which is essentially a bunch of long molecules densely grafted onto a surface.
Currently we are trying to understand the neutral and charged polymer brushes. By using self consistent field approximation and scaling analysis, we are trying to predict how two brushes would interact in terms of the normal and shear stresses generated as a result of the macromolecular interactions when they come into contact.

Shear and Normal Forces between Neutral and Charged Polymer Brushes

A polymer brush is obtained when long chains are attached to a surface at one end and the interaction between chains allow them to extend away from the surface. Recently friction coefficients between two grafted charged polymers were measured to be much lower than between grafted layers of uncharged polymers at identical polymer volume fraction (Klein, J. et al. Nature 1994, 2003). If the interpenetration of the two grafted layers is not strong enough for chains to, e.g. entangle, the friction coefficient will be lower. MD simulation techniques are employed to help understand about the mechanism of shear/lubrication between grafted neutral and charged polymers (see figure).

Polymer Conformations

We studied conformations of isolated linear macromolecule in theta-solvents and in the melts. We have found that polymer conformations do not obey the classical theory predictions (see the right figure: long range correlations in a polymer). The bond vector correlations decay as a power of the separation along the chain, and not exponentially, as expected. Polymer mean-square size is not proportional to the degree of polymerization (N) at theta-temperature (or any other temperature), as predicted by classical theory The new theory has been developed, which takes into account long-range correlations and predicts the dependence of polymer size on (N). More interestingly, new theory suggests a new experimental method to determine the theta-temperature.

Polymer Melt Dynamics

We study the dynamics of entangled polymer systems within framework of the tube model (see the right figure) and reptation theory. Our goal is construction of a simple model, which can be used to predict the stress relaxation in the entire frequency range. In this project we focus on the effect of tube length fluctuations. Our data suggest that their contribution to the stress relaxation is much larger than included in the currently available theories. The tube length fluctuations effect might be sufficient to describe the behavior of the loss and storage moduli in the intermediate frequency range.