New Methods for Understanding and Controlling the Self-Assembly of Reacting Systems Using Coarse-Grained Molecular Dynamics

Abstract

Graduate College. dedicated to curiosity, tenacity and our families. iv ACKNOWLEDGMENTS I would like to thank my advisor Dr. Eric Jankowski for giving me the golden opportunity to work with him and setting me up on a path to success from day one. I want to especially thank him for showing me by example, how to be a good scientist, communicator and a strategist. I am grateful to Dr. Carla Reynolds for guiding me through this project and helping me get the practical perspective on ideas that I came up with. I am also thankful for her for helping me run many of the simulations on the Garcia cluster computer at Boeing and being the first beta tester for epoxpy. I also want to thanks our "American" cousins Sanju, Reshma, Vinu, Megha and our "American" aunt and uncle, Geetha and Dr. Mathew for being our Santa through our college life. ABSTRACTThis research aims at developing new computational methods to understand the molecular self-assembly of reacting systems whose complex structures depend on the thermodynamics of mixing, reaction kinetics, and diffusion kinetics. The specific reacting system examined in this study is epoxy, cured with linear chain thermoplastic tougheners whose complex microstructure is known from experiments to affect mechanical properties and to be sensitive to processing conditions. Mesoscale simulation techniques have helped to bridge the length and time scales needed to predict the microstructures of cured epoxies, but the prohibitive computational cost of simulating experimentally relevant system sizes has limited their impact. In this workwe develop an open-source plugin for the molecular dynamics code HOOMD-Blue that permits epoxy crosslinking simulations of millions of particles to be routinely performed on a single modern graphics card. Using these capabilities, we are able to use ensembles of epoxy processing pathways to obtain realistic bond kinetics and relaxation times that sensitively depend on stochastic bonding rates and a diffusive drag parameter respectively. This work also demonstrates the first implementation of fully customizable temperature-time curing profiles and the largest cross-linked structures obtained using molecular dynamics simulation. We evaluate coarse-grained models based on Dissipative Particle Dynamics (DPD) and compare with Lennard-Jones(LJ) models for their suitability to study glassy dynamics which is important for modeling epoxies or any other glassy material. We find that "hard" particle potentials such as the LJ potential are necessary to model glassy materials and characterize multiple viii methods for measuring the glass transition temperature (T g ) in simulations. We find that variations in temperature-time curing profiles result in significant differences in the final cured morphologies. Finally, we apply our general techniques to the specific DGEBA/DDS/PES system and validate our predicted glass transition temperatures against experiment.ix

General-Purpose Coarse-Grained Toughened Thermoset Model for 44DDS/DGEBA/PES

“…Unless otherwise noted, simulation parameters summarized in Table 2 are used throughout. Glass transition temperatures are calculated directly from coarse-grained simulation volumes as described in Section 4.3.3 of Reference [57]. Briefly, snapshots of simulations that have reached a specified degree of cure α are used to initialize new simulations that are instantaneously quenched across a range of temperatures to identify T g , below which the self-diffusion coefficient D vanishes (Figure 2).…”