Simulation of vibrational spectra of polymers by molecular dynamics calculations

Abstract

SUMMARY Molecular dynamics is used to study the vibrational spectra (infrared and Raman) of polyethylene and polytetrafluoroethylene. The calculated frequencies are in good agreement with results of normal mode analyses. The calculated intensities lead to the conclusion that, while satisfactory relative intensities can be expected from simple models, quantitative determination of the intensities necessitates the use of complete sets of electro-optical parameters.

Predicting Raman Spectra of Condensed Polymer Phases from MD Simulations

“…Each contribution depends upon the bond type, orientation, and length. Bond polarizability models have been widely employed to represent the polarizability of single molecules, polymers, silica glasses, and nanotubes. ,,, For polymeric materials, only a few studies have focused on computing the Raman spectra of crystals and uses rather small molecular systems …”

“…The Raman intensity can be computed from the amplitude of molecular polarizability oscillations as the mode vibrates. The second approach is to obtain the time-varying molecular polarizability directly from a simulation trajectory and compute the power spectrum; this method has been applied only a few times, to crystalline systems. − The necessary polarizability can either be obtained either from ab initio calculations − or empirical models. , Ab initio methods such as density functional theory (DFT) are more accurate but require vastly more computational resources.…”

“…The bond polarizability constants are sometimes called electro-optical parameters in literature. − Values for bond polarizability constants can be obtained either by fitting to experimental Raman peak intensities of molecules containing the bonds of interest or by fitting molecular polarizabilities computed using DFT . The bond polarizability model has been used successfully to represent the molecular polarizability of hydrocarbons, ,,, small molecules, and silica polymorphs. , In our work, we will use the bond polarizability model to compute the Raman-activity of molecular vibrations in condensed phases.…”

“…In developing this method, we will simulate the Raman spectra of various condensed phases of PE and n -alkanes and compare our results with published experimental data. Some previous efforts have been made to simulate Raman spectra of PE crystals , and alkane liquids . However, there has been no comprehensive simulation study of the effect of phase structure on Raman vibrational signatures.…”