Effect of Chain Stiffness on Dense Polyelectrolyte Systems - A Simulation Approach
Final Report Abstract
The first part of the project dealt with the numerical modeling of the experimental centrifugation process in the preparation of so-called compacted polyelectrolyte complexes or CoPECs created from poly(styrene sulfonate) (PSS) and poly(diallyldimethyl ammonium) (PDADMA), which are promising for various medical applications. Complementary to experimental characterizations that typically quantify the rheological properties of the material or its structure (porosity) on the micrometer scale, fully atomistic Molecular Dynamics (MD) simulations have been performed to assess the effect of the centrifugal force on the polyelectrolyte chains and the intermolecular coordinations (mainly to counterions) on the nanometer scale. Here, it turned out that the external force not only induces the experimentally observed formation of a composite structure consisting of a polymer matrix and well-defined water-containing pores, but also has a direct impact on the matrix itself. In particular, the network formed by PSS and PDADMA becomes significantly denser and is stiffened in this way. Simultaneously, the low-molecular counterions are expulsed from the complex. Basically these two effects lead to a slowing down of the polymer dynamics inside the complex, as the life time of the PSS-PDADMA coordinations is increased. The prestress expected from the anisotropic orientation of the PSS and PDADMA molecules in the polyelectrolyte matrix (likely to be most pronounced at the pore interfaces) is supposed to affect the rheological properties of the material significantly. Within the second part of the project, the disentanglement process of two initially overlapping polymer chains has been investigated. In this way, the study represents a minimal example for the entanglement picture typically invoked for the description of the polymer dynamics in dense solutions and melts. To address this issue, a combination of Monte Carlo simulations and MD simulations has been employed. A scenario was devised in which two long polymers are initially connected to each other by a labile bond. After the cleavage of this bond, the separation process was monitored for both non-crossing and intersecting polymer chains. It was demonstrated that the non-crossing constraint present for real polymers has a significant impact on the chain dynamics even in dilute systems under certain conditions, although this effect is commonly neglected in standard models such as the Zimm model. In how far the separation of a given configuration involves strict disentangling is governed by a delicate interplay of rather soft (intermolecular monomer contacts) or rigid topological constraints (windings and knots), the latter becoming increasingly important in the limit of long chains as estimated from theoretical scaling arguments. From a technological point of view, the separation of polymer chains is typically encountered in electrophoresis or microfluidics, so that a more profound understanding of the underlying molecular mechanisms might benefit from the implications derived in this part of the project. For these reasons, the results also suggest a rather elegant chain-separation experiment to assess the impact of the non-crossing constraint in real polymer systems.
Publications
- ACS Macro Letters, 2016, 5 (6), pp 740–744: “Disentanglement of Two Single Polymer Chains: Contacts and Knots”.
D. Diddens, N.-K. Lee, S. Obukhov, J. Baschnagel, A. Johner
(See online at https://doi.org/10.1021/acsmacrolett.6b00079) - Eingereicht bei Physical Review Letters, Febr 2017, 118, 067802 (5 S.): “Local Chain Segregation and Entanglements in a Confined Polymer Melt”
N.-K. Lee, D. Diddens, H. Meyer, A. Johner
(See online at https://doi.org/10.1103/PhysRevLett.118.067802)
