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Thermodynamic, Structural, and Nanomechanical Properties of a Fluorous Biphasic Material^†

E. Núñez^‡, C. G. Clark, Jr.^‡, W. Cheng^‡, A. Best^‡, G. Floudas^§, A. N. Semenov, G. Fytas*^‡ and K. Müllen^‡

Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany, Department of Physics, University of Ioannina, 451 10 Ioannina Greece and Biomedical Research Institute (BRI)–FORTH, Université Strasbourg 1, Institut Charles Sadron, CNRS UPR 22, 6 rue Boussingault, F67083 Strasbourg Cedex, France, and Department of Materials Science and F.O.R.T.H, P.O. Box 1527, 71110 Heraklion, Greece

J. Phys. Chem. B, 2008, 112 (21), pp 6542–6549

DOI: 10.1021/jp711945z

Publication Date (Web): May 7, 2008

†

Note Added in Proof: After this manuscript had been submitted, a new paper(22) has shown that n-alkanes premelt close to the interface giving rise to solid/liquid coexistence in agreement with the present investigation.

‡

Max Planck Institute for Polymer Research.

University of Ioannina.

Université Strasbourg 1.

* Corresponding author.

Department of Materials Science and F.O.R.T.H.

Abstract

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 (T_s) 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 T_s and the melting transition (T_m), 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.