Raman spectroscopy is useful for probing conformational disorder and detecting rotator phases in n-alkanes and polyethylene. Raman spectra do not give direct information about molecular conformations or phase structure; instead, conformations and phase are inferred from spectral signatures of previously identified phases. Here, we use MD simulations to predict Raman spectra, so that both spectral and structural information can be obtained simultaneously. Central to our method is the Bond Polarizability Model (BPM), which represents molecular polarizability as a sum of individual bond polarizabilities. We apply BPM to simulation trajectories of polyethylene to compute the time-dependent polarizability; its power spectrum is the time-averaged Raman spectrum. Our spectra captures the qualitative features of experimental spectra for orthorhombic, rotator, and melt phases of polyethylene. However, simulated peak frequencies are sensitive to the applied forcefields. We can use this method to predict the Raman spectra of hypothetical ordered structures, such as rotator phases in isotactic polypropylene, which presently are not well characterized.

Predicting Raman spectra of condensed polymer phases from simulations

253rd National Meeting (2017)

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