Tracking of the physical ageing of amorphous pharmaceutical polymeric excipients by positron annihilation spectroscopy.

The structural changes observed in several amorphous polymers, commonly applied in the pharmaceutical technology to ensure conventional or controlled drug release, during relatively short storage periods are illustrated. The results suggest that the apparent structure formation of polymer molecules with water and, possibly, with other additives plays a significant role in the formation of such important physical and chemical parameters of tablets as drug release rate and solubility. Positron annihilation lifetime spectroscopy (PALS) was used to detect the changes of the free volume in the studied polymers under different storage conditions. Positrons react to the structural changes of amorphous polymers very sensitively, so the method can be recommended as useful means for stability tests during the development phase of dosage forms containing such excipients.