Low frequency Raman modes in solid amorphous polystyrene and polymethyl methacrylate

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Abstract
Measurements of the low frequency Raman spectra of solid amorphous polystyrene and polymethyl methacrylate, below about 100 cm−1, are reported, showing two previously unresolved broad bands in each case. For polystyrene the mode frequencies at room temperature are about 60 and 10 cm−1; for polymethyl methacrylate about 60 and 20 cm−1. The spectra of strained samples, showing no changes in these low frequency modes, and of crystalline isotactic polystyrene are also reported. The temperature dependence of the mode intensities in polystyrene is found not to deviate significantly from a first-order variation in the temperature range from 125 to 250 °K and there is no evidence of frequency shifts in this range, within the accuracy of the determination of peak frequencies (±7 cm−1). However, frequency shifts are found in both bands with variation in molecular weight. The frequency shift for the 10 cm−1 band between samples with molecular weights of 105 and 340 is 8 cm−1, both measurements being at 100 °K. For the 60 cm−1 band the corresponding shift is 11 cm−1. Possible causes for these shifts are discussed, including changes in the density of states functions for skeletal normal modes, derived from a simple chain model. This model is, however, found to be inadequate in describing both the low frequency Raman spectrum and the anomalous values of low temperature specific heat of polystyrene.