title: Development and Application of Multiscale Methods for Simulating Polymer Properties
Creators: name: Schneider, Jurek
Publisher: name: UNSPECIFIED
Year: 2021

description: A full inspection of the motion and relaxation of soft matter systems, such as polymer melts, solutions, and networks, spans from the quantum mechanical derivation of bond fluctuations to the macroscopic viscous modes of rubbers. The simulation of all properties at once is impossible, and crucial simplifications have to be made. For example, atoms can be viewed as rigid objects, and multiple atoms, monomers, or chain segments can be displayed by a single "bead". A generic polymer chain, stripped of its chemical details, might be modeled by a number of beads that are connected by springs, in a so-called coarse-grained, mesoscale description. This mesoscale chain does no longer obey all physical laws of its more detailed parent model, and they must be reintroduced as artifacts from a different scale. One of the multiscale methods capable of doing so is the slip-spring dissipative-particle-dynamics method presented here. Dissipative particle dynamics is a mesoscale method that allows a description of polymers by a simple bead-and-spring model. The interactions of beads are repulsive and soft, so that multiple beads can occupy the same point in space, and chains can pass through each other. In a real system, a polymer chain in a melt can be imagined as being confined in a tube by its neighboring chains. Its translational degrees of freedom are reduced to a creeping, "reptating" motion along its main axis. In regular dissipative-particle-dynamics simulations, this behavior is no longer obeyed, and the chain can escape its tube in a lateral motion. Restoring the correct physics means to confine this motion, which is done by slip springs, that is, artificial, mobile bonds that travel along the chain by their own dynamics. The slip-spring dissipative-particle-dynamics method has been proposed some years ago and was successfully applied to study polymer melts and solutions. Here, it is extended to further systems where a crucial role of topological interactions is anticipated. By correctly applying the slip-spring dissipative-particle-dynamics method, the role of entanglements in these systems can be studied. A first objective is to model flash nanoprecipitation by slip-spring dissipative particle dynamics. Flash nanoprecipitation is an experimental technique to produce nanoparticles based on the rapid mixing of a polymer solution with a nonsolvent. As solvent and nonsolvent mix, polymer chains collapse, and the polymer solution precipitates. Here, two studies are presented that investigate the collapse pathway of a single, isolated chain, and the precipitation of a polymer solution. In the first study, a single, long chain is quenched from solvent into nonsolvent conditions. The collapse pathway exhibits three stages: first, the chain collapses into blobs along its backbone, which are initially connected by bridges of slack polymer. Second, the bridges are absorbed by the blobs, until the chain resembles a pearl necklace. Third, the so-far undeformed chain backbone collapses. For each of the collapse stages, a characteristic scaling with the chain length is identified. Both the collapse pathway as well as the scaling are in excellent agreement with a theoretical model, and discrepancies can be readily explained by the mergence of different dissipative mechanisms. In the case of precipitating polymer solutions, the collapsing chains interact with each other and form a network-like structure, where blobs are spanned by highly stretched chains. The study's working hypothesis is that the stability of this intermediate structure would be altered by varying numbers of slip springs if topological interactions played a role. However, no influence of slip springs is found, and the intermediate network structure quickly dissolves into spatially separated nanoparticles. In a third study, the slip-spring dissipative-particle-dynamics method is extended to study the topological interactions of elastomers. In these network structures, slip springs successfully recover the topological effects of neighboring network strands. The resulting change in the entanglement contribution to the shear modulus of the inspected networks is in excellent agreement with reference simulations. Moreover, the dynamic storage- and loss moduli of experimental polyisoprene rubbers can be predicted by cross-linking a polymer melt mapped onto polyisoprene. The successful extension of the slip-spring dissipative-particle-dynamics method to networks will allow future investigations of systems of scientific and industrial interest, such as network defects and filled elastomers.