Thermal Expansion of the Polyethylene Unit Cell

Abstract

The 719–730‐cm⁻¹ doublet in the infrared spectrum of polyethylene (PE) is known to shift to higher frequencies with decreasing temperature. In this work we have measured these peak positions as a function of temperature down to 10°K for three bulk polyethylene samples of three different degrees of crystallinity, as well as solution‐grown single polyethylene crystals both treated and untreated by fuming nitric acid. All these samples were found to behave similarly. X‐ray diffraction measurements were performed for PE single crystals at four temperatures: 10°, 80°, 20°, and 300°K. The unit cell dimensions of polyethylene at these temperatures were found to correlate linearly with the infrared peak positions. On the basis of this correlation, a specific volume–temperature plot was made for the PE unit cell, from which thermal expansion coefficients were obtained as a function of temperature. The latter data can be fitted by the Debye–Grüneisen equation. Values of Grüneisen parameters were estimated and were found to increase monotonically with decreasing temperature.