HEAT-CAPACITIES AND TRANSITIONS IN N-PERFLUOROALKANES AND POLY(TETRAFLUOROETHYLENE)

Heat capacities and transitions of perfluorododecane (C12F26), perfluo rotetradecane (C14F30), perfluorohexadecane (C16F34), perfluoroeicosan e (C20F42) and perfluorotetracosane (C24F50) were measured by adiabati c calorimetry and differential scanning calorimetry from 5K up to the melt. The heat capacities of the homologous series of perfluoroalkanes from n-octafluoropropane to poly(tetrafluoroethylene) in the solid st ate are analyzed using the Advanced THermal Analysis System (ATHAS) de veloped for linear macromolecules. All of the n-perfluoroalkanes show solid-mesophase transitions before the melting transition. The mesopha ses of short chain-length molecules can be identified as plastic cryst alline (CF4 to C3F8). After a group of intermediate length (C4F10 to C 12F26), the entropies of the solid-mesophase and isotropization transi tions of the n-perfluoroalkanes approach the value of poly(tetrafluoro ethylene) when expressed per mole of carbon atoms. Based on the measur ed heat capacities, it is possible to predict also the thermodynamic p roperties of the n-perfluoroalkanes not measured. Both disordering tra nsition and isotropization temperature in the n-perfluoroalkanes with more than 12 carbon atoms can be expressed in functional form (T(d) = 295(n - 1.55)1(n + 7.55) and T(i) = 605(n - 3.56)/(n + 2.75), both in kelvin).