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Thermodynamic, structural, and nanomechanical properties of a fluorous biphasic material.

Núñez E, Clark CG, Cheng W, Best A, Floudas G, Semenov AN, Fytas G, Müllen K

Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.

The dynamics of the amphiphilic semifluorinated F(CF2)12(CH2)12H (F12H12) alkane that undergoes two condensed phase transitions have been investigated by Brillouin light spectroscopy, shear rheometry, small- (SAXS) and wide-angle (WAXS) X-ray scattering, and thermodynamic PVT measurements. The solid (I)-solid (II) transition (Ts) is marked by a stronger temperature dependence of the sound velocity in phase II and by a 2 orders of magnitude drop of the shear modulus. Between the Ts and the melting transition (Tm), the presence of two phonons implies a coexistence of solid (II) and amorphous (liquid) regions in the submicrometer range at thermal equilibrium as revealed by the SAXS pattern of a single reflection superimposed on a very broad amorphous halo. This intriguing finding of a transient, very slow (over 10 h) solid/liquid coexistence within phase II is rationalized by a two-stage mechanism for melting of the smectic phase (II) of F12H12. A refinement of the known packing motifs for the two solid-state structures is proposed.

Published 26 May 2008 in J Phys Chem B, 112(21): 6542-9.
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