Investigation of the solid state behaviour of a semi-fluorinated n-alkane by means of NMR, calorimetric and dielectric techniques

The solid-state behavior of the semifluorinated n-alkane C8F17-C16H33 (F8H16) has been characterized by temperature-modulated calorimetry, dielectric spectroscopy, and H-1 and F-19 nuclear magnetic resonance. Three solid-solid phase transitions were observed below the melting point (325 K) at about 258, 275, and 297 K. Wide-line H-1 and F-19 NMR suggests that the first transition is related to the onset of a dynamic process occurring in the hydrocarbon chain, which is, however, mainly involved in the central transition. Contributions from the fluorinated segment to the transitions at 275 and 297 K can be revealed. NMR relaxation times, temperature-modulated calorimetry, and dielectric spectroscopy agree in suggesting that the motional processes activated at the phase transitions remain quite slow in the whole solid range and are characterized by maximum frequencies of the order of some mHz.