Within this paper we propose a versatile Brillouin technique which allows an easy determination of acoustic and opto-acoustic properties of thin films. This new technique is especially suitable for studying acoustic dispersion behaviour at hypersonic frequencies. Typical applications in the field of polymers have been used to demonstrate the efficiency of the proposed scattering technique.

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Frozen-in trans and gauche conformational disorder of symmetrical difluorotetra-chloroethane (DFTCE) produces the tendency of this plastic crystal to form an orientational glassy state on cooling below Tg=86 K. To throw light upon the freezing mechanism in DFTCE neutron diffraction and Brillouin scattering as well as infrared and dielectric measurements are employed. The overall picture of the results obtained can be characterized in terms of a classical glass-transition process with a relaxation frequency obeying the Vogel-Fulcher law. However, a more sophisticated picture of the freezing process is indicated by a quantitative analysis of the new Brillouin data, e.g. the temperature dependence of the sound velocity. The behaviour could be understood on the basis of a mode-coupling theory.

The polymer induced ali@ment (PIA) tcchniqne is presented as B new technique to grow single-crystalline films of linear telomers. Combining the PIA technique with special high-resolution Brillouin techniques. it is possible 10 determine the coefficienls of Ihe elastic stiffness tenwr of the crystalline state of linear lelomen. especially in their rotator phases. As model systems we have chosen the perfluoroalkane Ca~Fhz and the linear n-alkane C17H36. A comparison of elaslic data obtained from classically and PIA grown samples proves their equivdenee as well as their complemenlarity. Precursors of critical melting discovered xn the rataiotOr phase of C17Hy axe discussed in terms of the anomalous temperature dependence of cefiain elastic tensor coefficients and of related n-telomers.

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The rotator phase of C24F50 has been characterized in its calorimetric as well as in its elastic properties. To perform the elastic characterization we took advantage of a polytetraAuoroethylene-inducedalignment crystal-growth technique. Special interest is paid to the rotator phase and the premelting behavior therein as reAected by the elastic stifFness coefficients c33 and c44. The anomalous elastic behavior is interpreted in the frame of the Landau theory. The premelting process is related to molecular conformational defects.

Taking advantage of the surface-induced orientation of linear molecules on highly oriented nanostructured polytetrafluoroethylene, we were able to grow macroscopic single crystals of (pentacosane). Using high-performance Brillouin spectroscopy, we determined for the first time the complete elastic tensor of an n-alkane.

Large monocrystalline domains of pentacosane (C 25 H 52 ) have been prepared on nanostructured polytetrafluoroethylene films. The acoustic and thermal properties have been investigated around the various phase transitions of the crystalline state of pentacosane using high-performance Brillouin spectroscopy and modulated differential scanning calorimetry. In the crystalline orthorhombic phase the values of the elastic constants are c 22 ϭ7 GPa, c 33 ϭ35 GPa, c 44 ϭ2 GPa. In the R V orthorhombic rotator phase the elastic constants are c 22 ϭ3.35 GPa, c 33 ϭ3.67 GPa, c 44 Ͻ0.05 GPa. The evidenced thermal and acoustic anomalies are discussed in the frame of the accompanying structural changes. ͓S0163-1829͑97͒00737-6͔