Isotropic Brillouin spectra of liquids having an internal degree of freedom

Abstract

Isotropic Brillouin spectra of the two chemically similar van der Waals glass forming liquids:1,1'-di(4-methoxy-5-methylphenyl)cyclohexane (BMMPC) and 1,1'-bis(p-methoxyphenyl)cyclohexane (BMPC) and ortho-terphenyl (OTP) were studied in a broad temperature and pressure range in order to characterize the effect of intra-molecular relaxations on damping of longitudinal phonons. Such relaxations are present in BMPC, while in BMMPC and OTP they are strongly hindered. We show that in BMPC (with strong internal relaxations) the damping (broadening) of longitudinal phonons (Brillouin peaks) is much stronger than in BMMPC and OTP (with weaker internal relaxations). The contributions of the intramolecular and structural relaxations to the phonon damping can be separated using high pressure, due to their very different pressure dependence. We show that internal relaxations strongly contribute to the damping of longitudinal phonons at all temperatures and should be taken into account in theoretical models describing the Brillouin spectra of supercooled liquids.

The effect of intramolecular relaxations on the damping of longitudinal and transverse phonons in polysiloxanes studied by Brillouin spectroscopy

“…Recently [4,5] we have shown, that in the case of molecules with weak internal relaxations (BMMPC, OTP) only the damping of the longitudinal phonons is affected, in agreement with the theoretical predictions for the weak coupling limit [3]. Strong internal relaxations (BMPC) contribute to damping of both longitudinal and transverse phonons.…”

“…5 and a corresponding dependence of the position ω B is shown in Fig.S3 in the supplement [24]. As already observed for low molecular weight liquids [5], the Brillouin width Γ B is decreasing with increasing pressure, reaching a high pressure plateau value. In order to obtain this value Γ 0 , the data was analyzed using the formula:…”

“…We have shown already that such a separation was possible in the case of low molecular weight supercooled liquids [4,5].…”

“…Later Fytas [16] showed in a study of siloxane oligomers that at least in the vicinity of maximum loss the single relaxation model is a good approximation. As we have shown previously [4,5], the most reliable way to separate the contributions of internal and structural relaxations to the broadening of the Brillouin lines and the damping of phonons, is to measure the pressure dependence of the Brillouin spectra.…”

“…This can be achieved by studying the pressure dependence of Γ B [4,5], since the pressure dependence of the structural relaxation times dlogτ s /dP∼5x10 -3 bar -1 [6] is much stronger than that of the internal relaxation time dlogτ int /dP∼2-5x10 -4 bar -1 [7,8,9]. The efficiency of the phonon damping depends mainly on the separation in frequency between the relaxation frequency 1/τ and the phonon frequency ω B .…”