Raman band profiles and mobility of polymethylene chains

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Abstract
In order to elucidate quantitatively the effects of the conformational order as well as of the chain mobility on the scattering profile of the CH2 antisymmetric stretching (νa) Raman band, the temperature dependence of the half-width and the integrated intensity was investigated on single crystals of urea inclusion adduct of n-C20H42, liquid n-alkanes of n-C6H14 through n-C18H38, and linear-low-density polyethylene (LLDPE). In the case of the high-temperature (hexagonal) phase of urea-n-alkane adduct where the paraffin molecules undergo a vigorous rigid rotational motion, a significant motional broadening of the νa band was observed with a rise of temperature, keeping the integrated intensity unaltered. The trans νa band of molten n-paraffins became broader as temperature increases. These results indicate that the νa band is broadened by the conversional motion between the trans and gauche forms as well as the rigid rotation of the molecule as a whole. In the case of LLDPE, with decreasing temperature the integrated intensity of the crystalline νa band at 2883 cm−1 increased very sharply in the vicinity of the glass transition temperature, Tg. The integrated intensity of this band was concluded to be proportional to the amount of the CH2 groups which are linked to trans sequences having less mobility and are accommodated in both the crystalline and noncrystalline phases.