A Molecular Dynamics Simulation Study of Dynamic Processes in Molecular Glass-Forming Liquids

• Main Author: Henritzi, Patrick
• Format: Others
• Language: German; en
• Published: 2014
• Online Access: http://tuprints.ulb.tu-darmstadt.de/4025/1/Dissertation_Patrick_Henritzi_2014.pdf; Henritzi, Patrick <http://tuprints.ulb.tu-darmstadt.de/view/person/Henritzi=3APatrick=3A=3A.html> : A Molecular Dynamics Simulation Study of Dynamic Processes in Molecular Glass-Forming Liquids. Technische Universität, Darmstadt [Ph.D. Thesis], (2014)

In this molecular dynamics simulation study we ascertain the dynamics of the glass transition for polymer melts and ionic liquids. Moreover, we systematically investigate changes of dynamical behavior by varying the molecular weight of the polymers and investigate the interpretation of experimental results, in particular of nuclear magnetic resonance data. The glass transition occurs in a wide range of systems and upon, e.g., cooling and increasing pressure. Its description is thereby commonly kept universal so that the understanding of the glass transition is closely connected to the understanding of common features for the different systems and control parameters. We find that various scaling approaches, which incorporate different control parameters, e.g., temperature, volume and entropy, describe the dramatic slowdown of dynamics approaching the glass transition for an ionic liquid and four polymer systems. The breakdown of the Stokes-Einstein relation is found for a variety of glass formers and related to other features of the glass transition, e.g., the properties of spatially het- erogeneous dynamics. Thereby, the fractional Stokes-Einstein exponent, obtained as a result from the breakdown, is found to be a temperature- independent key feature of glass formers. The growth of regions with spatially heterogeneous dynamics has inspired the most promising theories of the glass transition. Length scales are thereby a critical component and not clearly defined. Recent studies discuss a description of length scales for confinement systems, where region sizes and confinement sizes intersect. In further analyses we, hence, test the Adam-Gibbs and Random First Order Transition (RFOT) theory via a determination of dynamic and static length scales. For this purpose we study poly(propyleneoxide) in a neutral confinement of four geometries and find length scales, which describe the evolution of bulk dynamics approaching the glass transition. These results and findings in a complementary study of water carried out by F. Klameth [1] support the predictions of the RFOT theory. Experimental measurements of the diffusion and structural relaxation is a fundamental prerequisite for the study of the glass transition. Recent studies have developed new approaches to study diffusion coefficients and structural relaxation times by way of nuclear magnetic resonance spectroscopy for wide time ranges and long times. Hence, the key aspects in the interpretation of spin-lattice relaxation times from nuclear magnetic resonance experiments are evaluated from a molecular dynamics simulation perspective. Therein, we find the investigated approaches to determine diffusion coefficients and relaxation times acceptable within sometimes more stringent criteria. Furthermore, we provide a detailed analysis on the origin of deviations between the theoretic framework of polymer dynamics and experimental results. We find that intermolecular contributions, inherent to the method, and fast end groups may result in deviations of the experimental data from theoretical predictions. The simulation approach enables us to identify contributions unaccounted for in common interpretation and theoretic consideration and quantify them. The present study provides new insight into differences and similarities between a wide range of glass formers, the application of theoretic models of the glass transition, application of confinements to investigate the glass transition, methodical understanding of NMR observables, and deviations from predicted regimes in polymer dynamics.