Determination of the distribution of straight‐chain segment lengths in crystalline polyethylene from the Raman LAM‐1 band

It is shown how the shape of the longitudinal acoustic vibration observed in the low‐frequency region of the Raman spectra of crystalline polymers can be used to obtain a quantitative distribution of lengths of straight‐chain segments associated with polymer lamellas. The procedure is demonstrated for a “solid‐state” extrudate of polyethylene and for a bulk‐crystallized specimen of the same polymer. Equations relating the shape of the LAM‐1 band to the shape of the distribution curve are given. The low intensity observed for the LAM‐3 mode relative to LAM‐1 is explained quantitatively without recourse to end effects. LAM‐5 has been observed for bulk‐crystallized polyethylene. For the extrudate we find the distribution of lengths of straight‐chain segments to have a tail on the long‐length side which is not present for the bulk‐crystallized sample. The Raman technique is shown to provide new morphological data unattainable at present by other methods.