The effect of temperature on the infrared absorption frequencies of polyethylene and ethylene–propylene copolymer films

Abstract

The frequency shifts of the six prominent infrared absorption bands were measured for films of polyethylene and ethylene–propylene copolymer as a function of temperature. Three bands (at 720, 731, and 1473 cm⁻¹) shifted to higher frequency, and three bands (at 1463, 2849, and 2918 cm⁻¹) shifted unexpectedly to lower frequency as the sample temperature was decreased. The greatest shift occurred with the CH₂ rocking band, which increased from 730.2 to 734.2 cm⁻¹ as the temperature was decreased from 313 to 22°K. The shift usually ceased in the temperature range from 40 to 110°K, probably because some kind of molecular motion ceased. Four mechanisms are discussed in an attempt to account for the different frequency shifts: bulk contraction with decreasing temperature, an increase in dispersion forces between chains, variation in the length and coupling of the vibrating chain molecule, and a change in the planar zigzag conformation of the chain molecule. Thermal contraction is sufficient to explain most of the observed frequency shifts. The CH₂ stretching modes (2849 and 2918 cm⁻¹) may be shifted to lower frequency by an increase in the dispersion forces between chains, caused by contraction. The displacement of the 1463 cm⁻¹ band‐shift curve is an indication of the sample density. The displacements of the 1473 and 731 cm⁻¹ band‐shift curves are indications of the proportion of propylene in the ethylene copolymer.