Farah, Karim Frédéric
Interphase Formation During Curing Simulated by Reactive Molecular Dynamics.
Technische Universität, Darmstadt
[Ph.D. Thesis], (2011)
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|Item Type:||Ph.D. Thesis|
|Title:||Interphase Formation During Curing Simulated by Reactive Molecular Dynamics|
The present PhD thesis is part of a six year project initiated to tackle the issue of interphase formation in hybrid materials characterized by polymer-solid contacts. Interphases are regions “near” the solid surface boundaries where the polymer properties are modified in comparison to more distant regions. Experimental investigations on the nature and length scales of interphases have been hampered by the challenging task of setting up apparatus that spatially resolve polymer properties. Thus, unresolved questions on the role of interphases in polymer-based hybrid materials still remain. While a broad class of polymer-solid surface systems is currently being investigated in the framework of this six year project, our task has been to setup a simulation tool to tackle the question of interphase formation during curing. The final goal of the project is to simulate experimentally relevant polymer-solid systems. The present PhD thesis deals with the first three years of the project devoted to the development of the simulation tool, to the investigation of interphase formation at a generic level and to the search of suitable methods to develop interaction potentials to model experimental systems. Curing denotes the formation of a polymer network from a reactive system which consists of components of lower molecular weight. It is an important class of reactions as many technical adhesives such as epoxy polymers are formulated as reactive liquids of monomers and curing agents. The polymer adhesives are generated from the reactive liquids in the presence of the adherend surfaces under specific thermodynamic conditions. Thus, a suitable modelling tool should allow the dynamics and chemical reactions occurring in the systems to be investigated under the desired thermodynamic conditions. These requirements can be satisfied by the use of Reactive Molecular Dynamics methods (RMD) reviewed in the first chapter of the thesis. The time scale accessible to atomistic RMD simulations is in some cases not sufficient to observe reactive events. At a coarser level, reactive simulations allow the treatment of bigger systems using simplified reaction schemes. Currently, the formation of an interphase during curing can not be simulated at the atomistic level in a reasonable amount of time. Thus, our systems are described at a coarser level with a simplified reaction scheme. Each monomer and polymer repeat unit is a coarse-grained (CG) bead that is formed by merging several atoms into “super-units”. In the second chapter we present our RMD approach. The possibility to perform reactive simulations with material-specific CG potentials has been exemplified by growing polystyrene chains from ethylbenzene as a model of styrene monomers. Iterative Boltzmann Inversion (IBI) [Reith, D.; Pütz, M.; Müller-Plathe, F.; J. Comput. Chem. 2003, 24, 1624.] has been employed to develop these potentials. Many properties of the samples grown with the RMD approach have been compared to the results of equilibrium molecular dynamics simulations on identical polymer samples. An agreement between RMD-based and equilibrium simulation results has been found. Our RMD algorithm is based on alternating between bond formation steps driven by a reaction cutoff parameter and diffusion periods defined by a delay time parameter. We have performed a number of simulations varying the RMD parameters to understand their role and to gain insight into polymerization processes. We have correlated the final degree of polymerization distributions to the reaction conditions. Our reactive scheme is inspired by a RMD model proposed approximately ten years ago, which was however not material-specific [Akkermans, R. L. C.; Tøxvaerd, S.; Briels, W. J.; J. Chem. Phys. 1998, 109, 2929.]. The third chapter deals with the issue of material-specific force-fields for the simulation of experimental adhesive systems. From the investigations of the first chapter, IBI appeared as a suitable method to obtain material-specific potentials. To understand the challenges involved in developing such force-fields we have investigated the temperature transferability of IBI potentials optimized for liquid hexane. Temperature transferability is the capability of a CG force field to describe the system at a temperature far away from where it has been parameterized. This study has been a simplified way to gain experience on transferability issues of CG potentials before tackling the complex case of adhesive systems and surfaces. In this study we have also experienced the use of scaling factors that are currently a rather common solution to generate CG potentials suitable for different thermodynamic states and we have developed a novel, more robust scheme. The fourth chapter presents generic simulations of curing processes in the presence of idealized surfaces. Here, the reactive liquid mixtures are first equilibrated in the presence of the surfaces prior to the curing process. The constituents of the reactive liquids are bifunctional beads and tetrafunctional curing agents. The surface interactions with the different constituents are tunable. In this way, preferential adsorption at the surfaces of one of the constituents can be promoted. In fact, the purpose was to investigate the impact of surface-induced segregation on interphase formation. From the RMD simulations it appears that densities and average bond orientation profiles are not affected by the segregation processes. In fact, the perturbations of these two properties remain similar with or without preferential surface interactions. In contrast, properties which depend on the spatial distribution of the curing species such as the average length of the polymer segments between two branching points (i.e. curing agents with more than two bonds) are affected by segregation processes. In fact, the average length of the polymer segments increases with increasing distance from the surfaces for a preferential adsorption of the curing agents. The opposite scenario is observed for a preferential adsorption of the bifunctional monomers. In the absence of segregation processes, the average length of polymer segments is rather constant in the polymer samples. The fifth chapter summarizes some of the future steps in connection with the simulations of interphase formation. The possibility to extend the present RMD tool to simulate other reactive processes such as the growth of grafted polymer brushes is briefly discussed.
|Place of Publication:||Darmstadt|
|Classification DDC:||000 Allgemeines, Informatik, Informationswissenschaft > 004 Informatik
500 Naturwissenschaften und Mathematik > 530 Physik
500 Naturwissenschaften und Mathematik > 540 Chemie
|Divisions:||07 Fachbereich Chemie > Physical Chemistry|
|Date Deposited:||19 Dec 2011 11:00|
|Last Modified:||07 Dec 2012 12:03|
|Referees:||Müller-Plathe, Prof. Dr. Florian and van der Vegt, Prof. Dr. Nico|
|Refereed:||31 October 2011|