Crystallization Weigh Loss And Surface Reactions In Fluorozirconate Glasses

Author : Chengbin Huang
Publisher :
ISBN 13 :
Total Pages : 195 pages
Book Rating : 4.:/5 (17 download)

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Download or read book Crystallization of Molecular Glasses and Effect of Polymer Additives on Crystallization in Amorphous Solid Dispersion written by Chengbin Huang and published by . This book was released on 2018 with total page 195 pages. Available in PDF, EPUB and Kindle. Book excerpt: Glasses are amorphous solids produced by cooling liquids, evaporating solutions, and condensing vapors while avoiding crystallization, with widespread applications ranging from optics to structural materials. There are many types of glasses, such as inorganic, polymeric, metallic and molecular glasses. Among them, molecular glasses have attracted increasing attention for applications in food, pharmaceutics, and organic electronics. In pharmaceutical science, molecular glasses are important for the delivery of poorly soluble drugs, because they are generally more soluble and bioavailable than their crystalline counterparts. Molecular glasses, however, readily crystallize. Therefore, a major task for developing these materials is to understand and avoid crystallization. This study concerns the stability of molecular glasses and the effect of polymer additives on this property. It is organized around four topics. The first is the role of surface mobility in the crystallization of molecular glasses. The surface diffusion of amorphous griseofulvin was measured for the first time and was found to outpace bulk diffusion by a factor of 108 at the glass transition temperature Tg. This fast surface mobility explains the fast crystal growth on free surfaces. The second topic is crystal nucleation, the first step of a crystallization process that controls many of its properties. Homogeneous nucleation rates were measured in four glass-forming molecular liquids using two methods. For the systems studied (polyalcohols), the rates of crystal nucleation span at least 10 orders of magnitude, while the rates of crystal growth are comparable at the same temperature relative to Tg. At large supercooling, the rates of nucleation and growth share a similar temperature dependence, which suggests a similar kinetic barrier for both processes. The Classical Nucleation Theory provides a reasonable description of our data, showing no sign of failure as a result of a growing length scale for corporative rearrangement in deeply cooled liquids, as recently suggested for lithium disilicate. The third topic focuses on a polymer's effect on the crystal growth and nucleation in molecular glasses. The rates of crystal growth were measured in many drug-polymer systems, and the results indicate a controlling role for the polymer's segmental mobility relative to host dynamics. The polymer effect on crystal nucleation was studied at temperatures at which the host molecules nucleate the fastest (slightly above Tg). The presence of a low-mobility polymer PVP in D-sorbitol effectively reduces the rate of crystal nucleation and the amount of reduction is similar to that seen for the growth rate. This argues that the decrease of molecular mobility by the polymer plays a controlling role. Our results, however, do not support the view that host-polymer hydrogen bonding controls the polymer's effect on crystallization. Finally, in the fourth topic, the structure of several polyalcohol liquids was studied by synchrotron total X-ray scattering to explore its relation to physical properties, such as crystallization tendency. The atomic pair-distribution functions from this work can be separated into intramolecular and intermolecular contributions to investigate the difference between the crystalline state and the liquid state in molecular structure and intermolecular hydrogen bonds. The findings of this thesis provide a better understanding of crystallization in molecular glasses and guide the selection of polymer additives to prevent the crystallization of molecular glasses.