Molekulare Deutung der Umwandlungswärmen bei den Modifikationsübergängen des n-Tritriacontans

Abstract

Models are developed, which permit a discussion of the heats and entropies of transition observed for the three solid-solid phase transitions of n-tritriacontane. They are based upon the qualitative picture suggested by the experiments. At the first phase transition from modification A to B chains start to perform 180°-jumps about their long axes. The jumps occur in a cooperative way so that an orientational short range order is preserved. The transition can be discussed by making reference to the known properties of the two-dimensional Ising lattice and introducing some slight modifications. At the next phase transition to modification C longitudinal jumps of the chains are superimposed, resulting in a strong perturbation of the lamellar interfaces. The structural changes around the lamellar interfaces can be described in a "hole-layer model". Using this model it is possible to decompose the transition process into several successive steps and to estimate for each step the enthalpy and entropy change. The agreement with the experimental result is satisfactory. The last transition to modification D, known as "rotatorphase", is accompanied by a strong decrease in the lateral packing density. The jump-like motions characteristic for the modifications B and C change to a more continuous form and intramolecular kink defects appear. The measured increase in enthalpy and entropy can be understood as resulting from the volume change and the creation of kink defects.