Physics – Condensed Matter – Statistical Mechanics
Scientific paper
2007-07-24
Journal of Physics: Condensed Matter 19, 20 (2007) 205138
Physics
Condensed Matter
Statistical Mechanics
7 pages
Scientific paper
10.1088/0953-8984/19/20/205138
We have measured the linear dielectric susceptibility of two molecular glass formers close to Tg in order to estimate the size of the dynamically correlated clusters of molecules which are expected to govern the physics of glass formation. This size has been shown to be related to the dynamic dielectric susceptibility dEps(w)/dT (Eps : dielectric susceptibility, T : temperature, w: frequency). To allow for an accurate determination of the T derivative, we scanned the interval 192 < T < 232 K every 1 K for glycerol and 159 < T < 179 K every 0.5 K for propylene carbonate. The resolution on T variations was about 1 mK. The result for glycerol is that the number of correlated molecules increases by a factor 3 when T goes from 226 to 195 K. It has been shown that the non-linear susceptibility provides a direct measurement of dynamic correlations. To measure it, we used a standard Lockin technique yielding the third harmonic of the current flowing out of a capacitor. We obtained only an upper limit on the ratio of the third to the first harmonic, due to the non-linear response of standard electronics.
L'Hôte Denis
Ladieu Francois
Thibierge Caroline
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