Polyethylene, longitudinal acoustic modes

The infrared spectrum of partially deuterated polyethylene has been examined as a function of temperature 323). Bands associated with trans-trans (tt) bond pairs and trans-gauche (tg) bonds have been identified so it is possible to follow the changes in these isomers as a function of temperature. As the temperature increased, the intensity of the tg band increased, while the intensity of the tt band decreased. It is hypothesised that the extinction coefficients for the two bands are essentially equal and the nearly constant total intensity of the two bands seems to confirm this hypothesis. Thus it appears that each time a tt sequence disappears a tg sequence is created. Correlations of this infrared result with the intensity of the Raman longitudinal acoustic mode, suggest that the new thermally induced tg sequences occur mainly as point dislocations in the crystal 323). 

One of the main uses of Raman spectroscopy in morphological analysis is the measurement of the longitudinal acoustic mode (LAM) in polyethylene which goes back to relatively early work (1949) on paraffins. The LAM frequency (u(LAM- )) can be converted to all-tmns chain length or crystal thickness (Lc) according to the following formula ... 

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