Artificial Entanglements in Polymer Chains

• Main Author: Langeloth, Michael
• Format: Others
• Language: English; en
• Published: 2015
• Online Access: https://tuprints.ulb.tu-darmstadt.de/4600/1/dissertation_full.pdf; Langeloth, Michael <http://tuprints.ulb.tu-darmstadt.de/view/person/Langeloth=3AMichael=3A=3A.html> (2015): Artificial Entanglements in Polymer Chains.Darmstadt, Technische Universität, [Ph.D. Thesis]

We introduce a novel technique that facilitates entangled dynamics of coarse-grained polymer chains in Molecular simulations. Entanglements are very important for the dynamic properties of the chains and affect the rheology of the material. Molecular simulations often use a coarse-grained description of the molecules in order to reduce the time for computation. This is important especially for polymer simulations because of the enormous time and length scales of interest. Hence, a chain is not modeled by all its single atoms, but by only a few repeating units. These beads may represent several atoms, monomers or even entire segments of the chain. However, the nonbonded potential between two beads becomes softer with the level of coarse-graining in order to maintain the static properties of the chain. For highly coarse-grained models, the nonbonded potentials may not be strong enough to keep two chains apart. Then, the chains can simply pass through each other. As a consequence, they cannot entangle with their neighboring chains anymore. The absence of entanglements accelerates the chain dynamics. Important dynamic properties like diffusion or viscosity are overestimated compared to entangled chains. The viscoelastic behavior, a key property that distinguishes polymers from other types of material, is also gone with the entanglements. The present method inserts artificial bonds into the system in order to compensate for the loss of the entanglements. These bonds act like cross-links in a polymeric network. However, the position of the artificial bonds is not fixed, in contrast to cross-links. They may move along the chain in discrete steps and can be created or terminated at the chain ends. Hence, they are called "slip-springs". The slip-springs affect the chain dynamics in a way such that it becomes consistent with entangled dynamics. The static properties of the chains remain unaffected by the insertion and motion of the slip-springs. In contrast to other existing slip-link or slip-spring models, the present model employs dissipative particle dynamics. This simulation technique uses a special thermostat that ensures that momentum is locally conserved, which is a prerequisite for hydrodynamic interactions. The applicability of the "slip-spring" method has been verified for monodisperse and bidisperse melts as well as polymer solutions. The dynamic behavior is consistent with theoretical predictions or experimental observations. Hence, the model can describe Rouse, or Zimm behavior for unentangled chains, as well as reptation dynamics for entangled chains, including the transition between these regimes. The present model distinguishes itself from other slip-link or slip-spring approaches by the fact that the dynamics in all regimes is obtained a posteriori without the need of additional parameters. The present technique is two orders of magnitude faster than conventional Molecular Dynamics simulations that retain the chain uncrossability by excluded volume interactions of the beads. Therefore, the method is well-suited for a variety of applications with long, entangled polymer chains.