J. Phys. Chem. B 2007, 111, 10645-10650
10645
Molecular Dynamics Simulations of Polycarbonate Doped with Lemke Chromophores Qiong Zhang,†,‡ Yaoquan Tu,† He Tian,‡ and Hans Ågren*,† Department of Theoretical Chemistry, Royal Institute of Technology, AlbaNoVa UniVersity Center, S-106 91 Stockholm, Sweden and Laboratory for AdVanced Materials and Institute of Fine Chemicals, East China UniVersity of Science and Technology, Shanghai 200237, P.R. China ReceiVed: April 2, 2007; In Final Form: June 28, 2007
In the search for optimal electro-optical modulating materials we report in this work molecular dynamics simulations of polycarbonate doped with Lemke chromophores which is a promising candidate system for materials with such functionality. The simulations cover the electric field poling effects on the chromophore order at a temperature above the glass transition temperature (Tg) of the material, the cooling procedure from liquid state to the glass state in the presence of the poling field, and the back relaxation of the system after removal of the field. Our study shows that electric field poling results in a higher chromophore order and that the order is also maintained during the cooling procedure with the poling field applied. In the liquid state, the applied poling field has little effect on the structure of the material. However, after the cooling procedure, the structure changes significantly because the polymer matrix tends to become closely packed. Our study thus indicates that for the bulk material, the structure of the host matrix is very important in determining the performance of the material.
1. Introduction Nonlinear optical (NLO) polymeric materials have been extensively studied for potential use in future technologies as optical switches, high-speed image processing, and other photonic applications.1-3 Organic chromophores with large first hyperpolarizabilities embedded in amorphous guest-host polymers were the first NLO polymer systems investigated, due to the high NLO response, ease of fabrication, and wide range of operating frequencies.4-7 The magnitude of the macroscopic NLO responses in these systems is dependent on two factors: the molecular hyperpolarizability β, together with the density of NLO chromophores, and the degree of noncentrosymmetry. In guest-host polymer systems, noncentrosymmetry can be achieved by electric field poling methods performed at temperatures above the glass transition temperature Tg to facilitate the easy rotation of the dipoles of the chromophores. After the poling process, the system is cooled below Tg in the presence of the field, freezing the dipoles in a noncentrosymmetric fashion. To optimize the performance of the poled polymer devices, it is essential to understand the physical mechanism of these processes. Much effort has been devoted to investigation of the molecular dynamics of the processes mentioned above which are dominated by chromophore-chromophore interaction, the interactions between chromophores and host matrix, and the structure of the host matrix. Molecular dynamics (MD) simulations provide us with a powerful tool to model such detailed structure of an interaction system at the microscopic level. Kim and Hayden8 initiated fully atomistic molecular modeling to study static conformational properties, radial distribution functions, and polymer mobility in the poled and unpoled poly(methyl methacrylate)(PMMA)/N,N-dimethyl-p-nitroaniline* Corresponding author e-mail:
[email protected]. † Royal Institute of Technology. ‡ East China University of Science and Technology.
(DPNA) system, which were prepared with different densities corresponding to states above and below the glass transition temperature Tg. Makowska-Janusik et al.9 carried out MD simulations on three electric field poled host-guest systems. They applied simulated annealing to model the cooling process in the presence of the poling field and further investigated the orientational relaxation of the guest molecules following removal of the poling field in the glassy state using atomistic molecular modeling as well. Reis et al.10 calculated linear and nonlinear optical susceptibilities of poled guest-host polymer systems using structures obtained from molecular dynamics simulations. The concentration dependence of the electro-optic coefficient of two guest-host polymer composites was investigated by Leahy-Hoppa et al.11 By using Monte Carlo statistical mechanical simulations, Robinson et al.12,13 studied saturation effects at higher loadings. It is known that in molecular modeling, the simulation time required to reach equilibrium for the dipole orientation increases considerably with decreasing field strength for a given chromophore loading and with increased loading for a given field strength. Due to the limited computer resources, up to now, most simulations have been carried out at relatively large electric fields and/or with low loadings of the dopant chromophores. In our previous work,14 we carried out simulations, under conditions close to those in experiment, of an electric field poled guest-host system, with PMMA as host polymer matrix and dispersed red molecules as guest NLO chromophores. That effort offered the opportunity to explore the results from the simulations for practical applications. In this work, we apply MD simulations to study another electric field poled polymeric NLO system, with polycarbonate (PC) which is also a commonly used amorphous host polymer, doped with Lemke molecules which show large NLO response.15-19 The structures of the Lemke molecule and polycarbonate unit are shown in Figure 1. The simulations comprise the electric field poling effects on the chromophore order at a temperature
10.1021/jp072560r CCC: $37.00 © 2007 American Chemical Society Published on Web 08/18/2007
10646 J. Phys. Chem. B, Vol. 111, No. 36, 2007
Zhang et al. TABLE 1: Composition of the Three Samples Studieda
a
sample
1
2
3
nc weight %
16 11.05
32 19.90
64 33.20
nc is the number of chromophore molecules.
Figure 1. Structures of Lemke molecule and polycarbonate unit.
above Tg, the cooling procedure from liquid state to the glass state in the presence of the poling field and the back-relaxation of the system after the removal of the field. Based on the simulation results, the structures of the material in the liquid state and glass state and the role of the host polymer are investigated. 2. Computational Details Molecular dynamics (MD) simulation technique is the main tool that we used in this work. In these simulations, the intraand intermolecular interactions are modeled by a class I molecular mechanical force field with the following form:
E)
kr(r - req)2 + ∑ kθ(θ - θeq)2 + ∑ bonds angles
Vn
∑ [1 + cos(nφ - γ)] + ∑ dihedrals 2 i
