Knowledge Bank

Polyethylene (PE) shows in its FIR-absorption spectrum a very strong band at 73 $cm-1$ at room temperature, which was assigned to the $B1u$ transverse lattice vibration by Tasumi and $Krimm.1$ The integrated absorption depends on the degree of crystallinity as well as the annealing conditions. The absorption shows anisotropic behavior when samples are oriented by $stretching,2$ The temperature dependence of this band for linear and branched PE was investigated from $15\circ K$ to the melting point using a Fourier FIR spectrometer. The second transverse lattice mode $(B2u)$, predicted also in (1) was observed below $100\circ K$ with very weak intensity at 110 $cm-1$. On cooling the samples the integrated absorption increases and the maximum absorption frequency shifts to higher frequency. Above $360\circ K$ a characteristic decrease in absorption is observed at the higher frequency flank of this band.