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

The designation “glass” is commonly used to describe a transparent and brittle solid made mainly of silica like windows or jar. However, the word “glass” has a larger meaning in science. Whereas a crystal has a 3-­‐dimensional translational order a glass as it was seen 50 years ago lacks this 3-­‐dimensional order and has an amorphous structure. In the last decades, glass also started to describe solids, which may have a translational order but differ from crystals in the way that the orientation of the molecules within the structure is disordered. 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.