Molecular Dynamics Simulation of the Glass Transition of Ortho-Terphenyl in Bulk and Thin Films
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0924-Z03-21
Molecular Dynamics Simulation of the Glass Transition of Ortho-Terphenyl in Bulk and Thin Films Jayeeta Ghosh, and Roland Faller Chem Eng & Mat Sci, UC Davis, 1 Shields Ave, Davis, CA, 95616 ABSTRACT The glass transition temperature in thin film depends strongly on film thickness and interaction with the substrate and it is normally a priori not clear which way it deviates from the bulk value. This causes new challenge in the technological advancement of smaller and smaller electronic devices. In this study molecular dynamics simulations of a low-molecular weight organic glass former, ortho-terphenyl (OTP), are carried out in bulk and freestanding films. The main motivation is to provide insight into the confinement effect without interface interactions. Based on earlier models of ortho-terphenyl we developed an atomistic model for bulk simulations. The model reproduces the literature data from simulations as well as experiments. After characterizing the bulk model using 800 OTP molecules, we form a freestanding film. This film gives us the opportunity to study the dynamical heterogeneity near the glass transition by inplane mobility and reorientation dynamics. We also develop a structurally coarse-grained model for this glass former based on our atomistic model to study bigger systems for longer times in the future. At present we are using 800 molecules for the mesoscale system as well. INTRODUCTION Ortho-terphenyl is a small organic glass former and an excellent model system to study the glass transition. The molecule consists of three benzene rings with two rings in ortho positions of the central one. There are no directional forces, which makes it a fragile glass former. In recent years a lot of effort and various approaches [1-11] have been undertaken to study confinement effects, which include confining glass-forming liquids within a porous medium, semi-crystalline polymers, or micro-phase separated block copolymers, leading to thin films with or without explicit interfaces. With the advent of advanced microprocessors simulations play an increasing role [12]. Free surfaces of amorphous glassy polymers are important for a wide variety of technological applications involving protective polymeric coatings such as microelectronic devices [13]. Simulations often show a liquid-like behavior at the surface and a glassy region at the middle of the freestanding film [12]. Spatial domains, i.e. heterogeneity within the bulk, leading to stretched exponential relaxation functions near the glass transition temperature, are abundant [14-17]. In this work we study a model of ortho-terphenyl in bulk and freestanding film to examine the differences in behavior due to confinement effects. We address the heterogeneity in different layers depending on distance from the film center. We moreover develop a structurally coarse-grained model of OTP to study the system for longer times especially below the glass transition temperature.
SIMULATIONS Molecular Model of Ortho-terphenyl We perform atomistic molecular dynamics
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