The use of amorphous model substances to study mechanically activated materials in the solid state

Abstract

Spray drying of aqueous solutions of sucrose and lactose produced amorphous solids that differed greatly from their crystalline counterparts in morphology, X-ray diffraction patterns, DSC scans and their water vapour uptake behaviour. Amorphous sucrose and lactose, being in high energy states, are thermodynamically and physically unstable. When exposed to high humidity, they took up moisture to a certain critical uptake level, depending on temperature, then crystallised and released the sorbed moisture. The crystallisation was described as a deactivation process as it ordered the constituent molecules as well as reduced the total energy content of the system. Water vapour uptake of amorphous sucrose and lactose reduced the glass transition temperature (Tg). The Tg was also affected by the heating rate during the DSC scan. When Tg was reduced to, or below, the operating temperature, transformation of the amorphous solids from the glassy state to the rubbery state occurred. The transformation set up conditions for crystallisation to occur. The time delay for crystallisation depended on the temperature difference between the operating temperature and Tg.