[Physical ageing of amorphous polymeric excipients I. physicochemical principles]

Most of the polymeric excipients applied in pharmaceutical technology are amorphous, which, as a result of physical ageing, can lead to changes in the stability of these materials and dosage forms prepared from them. For the tracking of physical aging and understanding its consequences, a complex knowledge of the physicochemical properties and behaviour of amorphous polymers is necessary. In the case of these materials, three single-phase physical states can be distinguished: glassy, rubbery and viscous. The transition from glassy to rubbery state occurs at the glass transition temperature, the change of which as a function of the storage conditions provides information about the physical ageing of the material. The ageing process is usually accompanied by enthalpy and volume relaxation, which are considerably influenced by the presence of different plasticizers, e.g. water. These materials usually change the glass transition temperature of the polymer, which is a result of their effect on the free volume of the system. In view of the hygroscopic behaviour and water-uptake mechanisms of polymers, the probability and extent of structural changes caused by physical aging can be predicted.