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mercredi 10 décembre 2014 / Catégories: Sciences naturelles, Physique

The thermal glass transition as seen by temperature modulated refractometry (TMOR).

Annick Hoffmann

Initially, glass was obtained from a liquid that could not turn into its crystalline reference state because the cooling process was too
fast. If the cooling rate is adequate the liquid naturally evolves into its crystal configuration. Nowadays, these types of glass are called
quenched glasses because new types of glasses have been discovered. This new category of glasses, called the ideal glasses, do
not have a crystalline reference state. Their glassy state is the only solid-­‐state-­‐like configuration. This paper analyses the thermal
glass transition from a liquid into an ideal glass. Although glass is known for thousands of years, the mechanism of the glass transition is still not elucidated, mainly due to the overlapping of kinetic and dynamic effects in the material. Historically, glass has been seen as a liquid with very high viscosity whose shape variations can only be observed on very long time scales. In this regard,
glass would be a non-­‐ equilibrium liquid phase. Recently, the possibility of obtaining glass forming materials without crystalline reference state opened a new way of describing this glass state as a thermodynamically stable state. The dynamic effects can
thus be separated from the kinetic effects. In this work we explore the mechanisms involved in the glass transition taking advantage of the possibility of separating the dynamic from the kinetic effects. To this extent, two atactic polymers polyurethane (PU) and olyvinyl acetate (PVAc) will be studied. These polymers do not present a reference crystalline state. The experimental method used in this work will be the temperature modulated refractometry (TMOR) in which the temperature of the sample is varied step like and at
the same time it presents a sinusoidal modulation around the sample’s mean temperature. This new technique allows determining a
temperature dependent refractive index of the sample. A static thermal expansion coefficient can be retrieved from the experiments.
Also, through the modulation, a dynamic thermal expansion coefficient can be withdrawn. In this way we can access two aspects of the glass transition at the same time: the static and the dynamics.

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