Vibrational Study of the Chain Conformation of the Liquid n-Paraffins and Molten Polyethylene

Abstract

A vibrational and attendant conformational analysis of the liquid n‐paraffins and molten polyethylene is presented. For the purposes of the analysis a valence force field was derived which is applicable to both planar and nonplanar chains. The force field was evaluated from observed frequencies of trans (T) and gauche (G) n‐C₄H₁₀; TT and GT n‐C₅H₁₂; TTT, GTT, and TGT n‐C₆H₁₄; and (T)_∞ polyethylene, all of whose infrared spectra were assigned in detail. Infrared spectra of the liquid‐n‐paraffins n‐C₄H₁₀ through n‐C₁₇H₃₆ were measured at room temperature and n‐C₄H₁₀ through n‐C₁₂H₂₆ also at a temperature just above their melting point. Frequencies and normal coordinates were calculated for the extended forms and for forms having one gauche bond of n‐C₄H₁₀ through n‐C₈H₁₈. These quantities were also calculated for the conformations of n‐C₅H₁₂ through n‐C₇H₁₆ having two gauche bonds and for the nonplanar but regular conformations (TG)_∞ and (G)_∞ of polyethylene. Some bands attributable to forms of n‐C₅H₁₂ and n‐C₆H₁₄ having two gauche bonds were found. In the case of n‐C₅H₁₂ the energy difference between the GT and TT states was found to be nearly the same as that between the GG and GT states. Bands in the region 1400–1300 cm⁻¹ were found to be characteristic of specific conformations involving sequences of five or fewer methylenes, such as —GTTG— (1338 cm⁻¹), —GTG (1368 and 1308 cm⁻¹), —GG— (1352 cm⁻¹), and terminal —TG groups (1344 cm⁻¹). All these bands together with two broader ones centered near 1270 and 1080 cm⁻¹ owe their intensity to the wagging of methylenes adjoining gauche bonds. An interpretation of the general features of the C–C‐stretching, methylene‐rocking, and methylene‐scissoring regions is given. Bands associated with molecules or chains having trans sequences involving at least four methylene groups are found. In the region 1300–1150 cm⁻¹ there are chain‐length‐dependent band progressions resembling those observed for the crystalline n‐paraffins. These indicate the presence of molecules with gauche bonds, but these gauche bonds are few in number and are located near the ends of the chains. It is shown that for certain kinds of vibrations, particularly totally symmetric C–C stretching and ∠CCC bending, there is very little change in frequency in going from a fully extended chain to one having one or even two or more gauche bonds. Hence, it is very difficult in the case of the longer n‐paraffins to distinguish spectroscopically between fully extended and almost fully extended conformations.