Unconventional structures and related physical properties of the semifluorinated triblock alkane ${\mathrm{C}}_6$${\mathrm{F}}_{13}$-${\mathrm{C}}_4$${\mathrm{H}}_8$-${\mathrm{C}}_6$${\mathrm{F}}_{13}$

Single crystals of the symmetrically fluorinated triblock n-hexadecane ${\mathrm{C}}_6$ ${\mathrm{F}}_{13}$-${\mathrm{C}}_4$ ${\mathrm{H}}_8$-${\mathrm{C}}_6$ ${\mathrm{F}}_{13}$ have been grown from cyclohexane solution at 303 K (β modification). At 310 K, they undergo a monotropic phase transition to a plastically crystalline modification (α phase), with a melting temperature of 323 K. The α phase has a layer structure similar to that found in alkanes, apart from the large tilt angle of 42°. According to an x-ray structure analysis, the β-phase structure stands out from normal alkanelike structures because the molecular layers defined as usual contain two subsets of molecules with different orientations of their long axes, the angle between the long axes being 55°. Optical and elastic properties of both modifications have been measured. Special Brillouin techniques have been introduced for the determination of the elastic properties. The results are discussed in view of the crystal structures. For the β structure we conclude that it should be considered as composed of molecular ‘‘strata,’’ the long axes of the constituent molecules being parallel to the strata. The unconventionally large morphological difference between the α and the β phase is responsible for the occurrence of an intermediate metastable phase that can be frozen in.