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Multiscale Simulations of Ortho-Terphenyl in Bulk and Freestanding Films

Jayeeta Ghosh and Roland Faller. UC Davis, One Shields Ave, Davis, CA 95616

One of the most challenging unsolved problems in condensed matter research is the glass transition under confinement. There have been many approaches to understand it using experiments or simulations to for polymeric and small organic glass formers under different confining conditions. The glass transition temperature under confinement depends strongly on the extent of confinement and the interaction with the substrate. It is difficult to predict which way it deviates from the bulk value. This has a broad impact on thin film applications such as lubricants, adhesives, and photo resists. In this work we study two molecular models of ortho-terphenyl, a detailed atomistic and a mesoscale one, both in bulk and freestanding film to examine the differences in behavior due to confinement effects. Based on earlier models of ortho-terphenyl we developed an atomistic model with 18 interaction sites, one for each carbon without any electrostatics. In the meso-scale model of Ortho-terphenyl each benzene ring is replaced by a single interaction center. For the non-bonded potential we use iterations based on the potential of mean force (PMF) obtained by Boltzmann inversion of the radial distribution function between different sites leading to a numerical potentials. Both models are used to study bulk and freestanding films. We model our films of 7 nm thickness sandwiched between large vacuum layers to avoid self interaction allowing us to use periodic boundary conditions in all directions. The model reproduces literature data for the bulk. Comparing to experimental results we see that in the simulation, the thermal expansion coefficient for the glass is slightly higher whereas that of the liquid is slightly lower. We study the mean square displacements to elucidate the mechanisms of molecular motion and to calculate the diffusivity. The properties calculated from the mesoscale simulation have been compared with the atomistic ones. The freestanding film gives us the opportunity to study the dynamical heterogeneity near the glass transition by in-plane mobility and reorientation dynamics. At the bulk glass transition the freestanding film is no longer glassy and becomes unstable. We find that the glass transition temperature is reduced for a freestanding film of OTP. The inner layers behave bulk-like as expected and outer layers show a higher mobility.