Energy dissipation and transport in polymeric switchable nanostructures via a new energy-conserving Monte-Carlo scheme
by Marcel Simon Langenberg
Date of Examination:2018-04-09
Date of issue:2018-04-13
Advisor:Prof. Dr. Marcus Müller
Referee:Prof. Dr. Marcus Müller
Referee:Prof. Dr. Reiner Kree
Referee:Prof. Dr. Cynthia A. Volkert
Referee:Prof. Dr. Matthias Krüger
Referee:Prof. Dr. Annette Zippelius
Referee:Prof. Dr. Stefan Klumpp
Files in this item
Name:eDISS_MLangenberg.pdf
Size:20.0Mb
Format:PDF
Description:Dissertation
Abstract
English
Polymers are materials that are widely used in industry. The advantages of polymers are their comparatively simple possibility of processing, e.g, injection moulding processes, 3D printers, and self-assembly. Polymers are macromolecule chains and are composed of (simple) molecular repeating units. From a certain abstraction level, polymers have a universal behaviour that can be described by a few properties only. The most important features, which are required are (i) a length scale given by the mean end-to-end distance of a polymer , (ii) an isothermal compressibility that controls the strength of the non-bonded interactions, and (iii) an invariant degree of polymerisation . In recent years, polymers also had a high impact on high-tech sectors, such as the semiconductor industry or battery research. In this context, particularly interesting issues arise, e.g., which mechanisms in polymers can be used in order to be able to use energy transport or thermal transport properties inexpensively and efficiently. A promising mechanism is the self-assembly in microphase structures. These microphase structures have many interfaces. These interface structures create a thermal resistance for an energy flow that propagates through the material, so interfaces are a possibility to control thermal transport processes by assigning directional preferences. This work focuses on the thermal transport properties of these self-assembled microphase structures. A major difference from previous theoretical approaches is a new energy Monte-Carlo scheme eMC. This eMC scheme allows polymer systems to be studied on large time and length scales. Existing methods, e.g., energy conserving dissipative particle dynamics eDPD, need a very small time increment due to numerical instabilities, so that a study of thermal transport processes on the same scale as eMC would require too much computing time. For the first time, eMC enables to address scales, which until now have been reserved for continuum models. However, a fundamental difference of the eMC method to continuum models is that local properties of the underlying molecular structure, e.g., specific heat capacity, polymer chain confirmations and density differences can be resolved without significant additional effort. In the present thesis the properties of homopolymer melts, diblock copolymers, star polymers and soft cubic crystals, are examined and qualitatively related to experimental observations. In particular, it is shown that the eMC method provides easy access to effects on thermal conductivity with respect to formed interfaces, density differences, and molecular architecture. However, it shall be emphasised that the change in the solubility parameter of a star polymer in a homopolymer mixture, is a switching process that allows controlling the thermal relaxation of the star polymer. A further part of this thesis is the determination of a phonon density of states (DoS) of a microcanonically regarded soft polymer melt. Determining the DoS of polymer melts is extremely difficult, since it has been found that the behaviour of polymer melts is primarily diffuse. In the last section, a possible control mechanism for energy transport at a higher abstraction level is discussed. Starting from the observation that the thermal conductivity can be controlled by interfaces and polymer lengths, photoswitches are used to control the self-assembly of microphase-separated polymer melts.
Keywords: acceptance criterion, coarse-grained molecular dynamic simulations, energy transport, energy Monte-Carlo, density of states, diblocks, homopolymer, internal degree of freedom, Kapitza resistance, reverse non-equilibrium molecular dynamic simulation, star polymer, soft cubic crystal