Structural Characterization of Semifluorinated n-Alkanes. 2. Solid–Solid Transition Behavior

A series of semifluorinated n-alkanes, F(CF2)n(CH2)mH, have been observed to undergo a solid–solid phase transition below the crystal melting point. Characterization of this transition has been achieved by a combination of differential scanning calorimetry, Raman spectroscopy, small-angle X-ray scattering, and wide-angle X-ray diffraction. For compounds where n = 12 and m = 2, 4, or 6 the molecular axes are tilted with respect to the crystal surface normal prior to the transition. For compounds where n = 12 and m = 8, 10, or 12 evidence for a tilted bilayered crystal structure below the transition temperature was found. The transition involves a translation of the molecules along their axes via a simple screw motion whereupon they enter a rotator phase prior to melting similar to that found in n-alkanes. © 1986, American Chemical Society. All rights reserved.