J. Phys. Chem. B 1999, 103, 10591-10598
10591
Structure and Dynamics of Liquid Diphenyl Carbonate Investigated by Molecular Dynamics Simulations Hendrik Meyer,* Oliver Hahn, and Florian Mu1 ller-Plathe Max-Planck-Institut fu¨ r Polymerforschung, 55021 Mainz, Germany ReceiVed: May 26, 1999; In Final Form: September 16, 1999
We study liquid diphenyl carbonate (DPC) with molecular dynamics simulations in the temperature range from 350 to 600 K. The diffusion behavior as well as structural and relaxation properties of this complex liquid are analyzed in detail. To this end, an all-atom force field is developed for DPC. We studied two sets of partial charges and the influence of the torsion barrier at the carbonate group. We discuss radial distribution functions, orientation distribution functions, and orientation correlation functions of subgroups of the molecule. The flip frequencies of the dihedral angles of the phenyl rings and of the carbonate group are also considered. The internal flexibility and the nonspherical form of the molecule highly improve the diffusion process, whereas the height of the carbonate group torsion barrier is not a crucial parameter.
I. Introduction Diphenyl carbonate (DPC) is the main building block of the polycarbonate family. Since their first synthesis in 1955, polycarbonates have become industrially very important polymers, mainly because they combine several desirable properties, such as electrical insulation, high heat of distortion, transparency, and impact resistance.1 Consequently, considerable experimental and theoretical research has been carried out to understand the reason for this unique combination of material properties of polycarbonates. In particular, for the purpose of material design it would be highly desirable to be able to link the molecular structure and dynamics to these macroscopic quantities. In this respect, molecular dynamics (MD) simulations provide a unique tool to investigate these materials on the atomistic level. In the present piece of work, we address DPC, which is the common part of several important polycarbonate modifications such as bisphenol-A-polycarbonate and trimethylcyclohexanepolycarbonate. We, therefore, use simulations of liquid DPC to parametrize an all-atom force field that can be merged with already existing force fields to obtain a reliable set of force field parameters for the polycarbonate family.2 Existing force fields for polycarbonates always consider DPC as a building block, but there are no MD studies of the DPC melt itself. Existing force fields mostly rely on quantum calculations for parts of the DPC molecule.3,4 Improved computational power allows us to optimize the whole DPC molecule at the HF6-311G** level with higher precision consistent with recent independent density functional theory (DFT) calculations.5,6 Furthermore, there is considerable dispute in the literature on glassy polycarbonates6-8 as to whether the most important mechanism for the chain motion is conformational transitions (trans-trans f cis-trans) at the carbonate group or phenylene ring flips. Since it is impossible to study the diffusion of polycarbonate chains in a melt by means of an atomistic MD,9 the simulation of liquid DPC seems appealing to gain insight into the interplay between conformational transitions at the carbonate group and diffusion. In more general terms, this case study should be interesting on its own, since it allows the * Corresponding author. E-mail:
[email protected].
Figure 1. Diphenyl carbonate (DPC) with the atom names used in Table 1.
comparison of diffusion dynamics of strongly anisotropic molecules with internal degrees of freedom with that of rigid, nearly spherical molecules. II. Force Field and Simulation Details We report in this article extensive molecular dynamics (MD) simulations of liquid diphenyl carbonate (DPC) with an allatom force field using the MD code YASP.10 It allows for full flexibility of torsions and bond angles. Only bond lengths are kept constant. Compatible force fields for liquid phenol and benzene have already been developed and are described in the literature.11 A study of polycarbonates with a force field based on the parameters of this article will be published elsewhere.2 A. The Force Field. The form of the force field employed in this work, used to describe the potential energy experienced both within and between diphenyl carbonate molecules (see Figure 1), is given by
Epot )
1
1
∑ kθ(θ - θ0)2 + dihedrals ∑ 2kτ[1 - cos n(τ - τ0)] + angles 2 ∑
h-dihedrals
1 2
kδ(δ - δ0)2 +
∑ i
