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Current Research Topics

 

Effect of Copolymer Architecture on Microphase Separation

We study the structure and dynamics of tapered block copolymers, motivated by the experimental work in the group of Prof. Thomas H. Epps, III at the University of Delaware, with whom we have an ongoing collaboration. These are polymers with two pure blocks of A and B separated by a "tapered" region in which the composition changes statistically from A to B, as shown schematically below.

Using self-consistent field theory (SCFT), we found that tapering widens the gyroid region of the phase diagram for small and intermediate length tapers. Thus, these designer polymer architectures may make it easier to form such bicontinuous phases of interest for good mechanical properties and transport applications. Molecular dynamics (MD) simulations show structures consistent with the SCFT results and help us understand the individual polymer conformations and how they change over time. Below the SCFT phase diagrams are compared for typical diblocks (left, from Cochran et al. Macromolecules 2006, 39, 2449-2451) and for polymers with a linear taper of 30% of the length of the chain (right, Brown et al. ACS Macro Letters (2013) 2 (12) 1105-1109). Phases formed in MD are indicated with data points, and selected snapshots are also shown.

Ion Containing Microphase Separated Polymers

Solid, non-flammable polymer electrolytes are of interest to increase safety and in some cases also to lower the cost or increase the energy density of batteries. One strategy is to use block copolymers in which one block contains either ionic groups or added salt and the other is uncharged; such materials are of interest because one phase can be optimized for good ion conduction while the other phase would provide desirable mechanical properties. A major goal of our group is to understand of how charge is transported, and how charge transport may be improved, in such single-ion conducting or salt-doped microphase separating polymers. Our coarse-grained models allow us to simulate the time and length scales of interest in these systems while still capturing the basic physics of polymer connectivity and long-range Coulomb interactions.

We are currently working on MD simulations of ion conduction and on improved fluids density functional theory methods to calculate phase behavior and help guide our simulation work. A simulation snapshot below shows a salt-doped copolymer with conducting phase beads in transparent red, non-conducting beads in transparent blue, anions in yellow, and cations in green. We aim to find how precise control of the morphology via tapering or other adjustments to the polymer architecture could lead to better transport through them. Some of this work is in collaboration with Prof. Thomas H. Epps, III, The University of Delaware.

Ionic Aggregate Structure and Dynamics in Ionomer Melts

We use coarse-grained molecular dynamics simulations to understand how clusters of ions determine the properties of polymers with a small fraction of charged groups along the backbone. The snapshots below show how the molecular scale structure of these materials changes as they are deformed (top to bottom). The left snapshots show ionic aggregates (polymer is invisible) and the right shows three selected polymers (other polymers and all counterions are invisible); both the polymers and ionic aggregates are seen to line up. Some of this work is directly motivated by or in collaboration with Prof. Karen Winey's group at the University of Pennsylvania. We also collaborate closely with Prof. Vishnu Sundaresan, The Ohio State University in this area.

Modeling Encapsulation of Nanoparticles in Block Copolymer Micelles

In collaboration with Professors Jessica Winter and Barbara Wyslouzil at The Ohio State University, we are working to understand how to control the dispersion of nanoparticles inside block copolymer micelles. Snapshots from dissipative particle dynamics simulations of micelle formation are shown below. At left the entire micelles are shown (only water is invisible); at right the polymers are made transparent so that the nanoparticles within the micelles are seen more clearly.

Coarse-grained Modeling of Polymer Nanocomposites

Nanoparticles may be added to polymers to improve mechanical, thermal, or optical properties of the resulting nanocomposite; car tires are a classic example, as they typically contain carbon black and/or silica particles added to styrene-butadiene rubber. 

The large surface area of very small particles means that they can have a large effect on the polymer even at a low weight fraction. It also means that the state of dispersion of the particles and the properties of the polymer near the interface are crucial to determining the overall material properties. Thus, many scientific investigations focus on understanding the polymer structure and dynamics near the particle interface. One current goal of our group is to determine more clearly how the dynamics of nanoparticles in polymer melts or concentrated polymer solutions changes depending on the strength of adsorption of the polymer on the surface. Some of this work is in collaboration with Dr. Patrick Majors and Alex Trazkovich at the Cooper Tire and Rubber Company and Prof. Kurt Koelling at The Ohio State University.