Knowledge Bank

The Raman spectra of highly crystalline polyethylene (Marlex 50 and hydrogenated Marlex 50), and of solid $n-C28H58$ and $n-C36H74$, have been obtained. On the basis of the space group $Vh16$ and unit cell determined by $Bunn,1$ the eight Raman-active unit cell fundamentals have been assigned as follows: $^{1}$061 (skel. def., $ag+b1g)$, 1131 (skel. def., $b2g+b3g$), 1168 ($CH2$) rock., $b1g+ag$ 1295 ($CH2$ twist., $b3g+b2g$), 1415 ($CH2$ was., $b2g+b3g$), 1440 ($CH2$ def., $ag+b1g$), 2848 (sym. CH str., $ag+b1g$), and $2883cm-1$ (asym. CH str. $b1g+ag$). No splitting was observed for any of these fundamentals. The infrared spectrum of Marlex 50, at room temperature and at $-180\circ C$, has been investigated with a Perkin-Elmer Model 112 spectrometer equipped with LiF, NaCl and CsBr prisms. In terms of the Raman-active fundamentals listed and the four infrared-active fundamentals assigned by previous workers, it has been possible to assign all of the 60 observed infrared bands, except for nine very weak ones. Several of the combination bands violate the u,g selection rule, indicating that they involve non-unit cell fundamentals. A few combination bands are interpreted in terms of fundamentals of the atmosphere phase. The infrared-active $CH2$ wagging fundamental (species $b1u$), which must be extremely weak, appears to be masked by a band of the amorphous phase. For this reason it has not been possible to determine its wave number from combination bands. The same is true of the twisting fundamentals of species $au+b1u$.