Thermal Expansion of the Polyethylene Unit Cell

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.