Die molekularen Grundlagen der anisotropen thermischen Ausdehnung von Paraffinen und Polyäthylen. Eine ramanspektroskopische Untersuchung am Beispiel des N-Tritriacontans

Abstract

The molecular background of the anisotropic thermal expansion generally observed for paraffins and polyethylene was explored in a study on n-C₃₃H₆₈ in the temperature range between 10 °C and 50 °C. The analysis was based on a generalized Grüneisen-theory, which connects the expansion coefficients along the a-,b- and c-axis of an orthorhombic crystal with its elastic constants and with the temperature coefficients of the anisotropic thermal pressure originating from the different groups of lattice vibrations. All necessary parameters can be derived from raman spectroscopic measurements. The compliance constant in chain direction S_ee, needed in addition to the known lateral compliance constants, followed from a precise determination of the frequencies of the longitudinal acoustical modes. The temperature coefficients of the thermal pressure can be related to the Grüneisen constants of the longitudinal and transverse acoustical modes and of the A_g-rotatory mode. Their values were obtained by a measurement of the temperature dependence of the respective frequencies. The investigation results in the conclusion, that the Grüneisen-theory is able to account correctly for the measured expansion coefficients. In particular it becomes clear, that the anisotropy in the lateral expansion coefficients (α_a >> α_b) is a consequence of the essentially uniaxial orientation of the thermal pressure associated with the rotational and torsional chain vibrations.