Phase Transformations Induced by Grinding: What is Revealed by Molecular Materials
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Phase Transformations Induced by Grinding: What is Revealed by Molecular Materials Marc DESCAMPS1, Jean FranÁois WILLART1, Emeline DUDOGNON1, Ronan LEFORT2, Sylvain DESPREZ1, and Vincent CARON1 1 Physique, University Lille1, UFR de Physique Bat P5, Villeneuve d'Ascq, 59655, France 2 GMCM, University Rennes1, Rennes, 35, France
ABSTRACT The purpose of this paper is to show some examples of phase transformations induced by grinding molecular materials. These materials are considered because they are extremely sensitive to external disturbances and are generally very good glass formers. This allows investigating more easily a broad range of the parameters which are open to influence the nature of the end product namely the temperature and intensity of grinding. Such an investigation has also a practical interest in pharmaceutical science. It is shown that the position of the grinding temperature with regard to the glass transition temperature of the compound is a key parameter. Comparison of the effects of temperature and intensity of grinding demonstrates that the driven material concept offers a framework to rationalize all the observed transformations. INTRODUCTION Contrary to metallurgy, systematic investigations of mechanically induced transformations of molecular compounds have not yet been done. However there are several specific properties of molecular compounds which have special relevance in mechanical activation [1]. They make this type of compounds very useful to gain further insight in the fundamental understanding of the physical behavior of materials under milling. The first original property of molecular solids is the sharp contrast existing between the strong intramolecular interactions and the rather weak intermolecular interactions (Van der Waals and hydrogen bonds) which leads to low elastic constants. This is at the origin of their low melting temperature and consequently also of the low glass transition temperature of their amorphous state. The second original aspect of molecular solids consists in their most frequent low molecular and low crystalline symmetries. This is especially true for the big therapeutic molecules newly synthesized. The crystallizations of such molecular compounds are difficult. Their kinetics of crystallization is often very slow. The consequence is that molecular compounds often show an ability to vitrify easily. When crystallization is avoided and when the route to vitrification opens up then the ìfragilityî of the intermolecular interactions is observed to provide transport coefficients in the amorphous state with specific temperature evolutions. For example viscosity exhibits a strongly non-Arrhenius temperature behavior with a dramatic increase on approaching the glass transition temperature (Tg) (ìfragile glass formers [2]î ). All these specificities lead to expect a strong sensitivity of molecular compounds to the temperature and intensity of milling. In this paper we present selected results of milling experiments performed for molecular compounds whi
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