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

Heat capacities and transitions of perfluorododecane (C12F26), perfluorotetradecane (C14F30), perfluorohexadecane (C16F34), perfluoroeicosane (C20F42) and perfluorotetracosane (C24F50) were measured by adiabatic 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 state are analyzed using the Advanced THermal Analysis System (ATHAS) developed for linear macromolecules. All of the n-perfluoroalkanes show solid-mesophase transitions before the melting transition. The mesophases of short chain-length molecules can be identified as plastic crystalline (CF4 to C3F8). After a group of intermediate length (C4F10 to C12F26), the entropies of the solid-mesophase and isotropization transitions of the n-perfluoroalkanes approach the value of poly(tetrafluoroethylene) when expressed per mole of carbon atoms. Based on the measured heat capacities, it is possible to predict also the thermodynamic properties of the n-perfluoroalkanes not measured. Both disordering transition 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).