[Pphysical ageing of amorphous polymeric excipients II. Tracking possibilities]

In the course of physical ageing of amorphous polymers, both macro- and microstructural changes occur in the materials. Volume and enthalpy relaxation can be considered as macrostructural aspects of ageing, and can be tracked by dilatometry and calorimetry, respectively. In dilatometric measurements, volume changes are usually traced back to the changes in the length of the examined materials. Among calorimetric methods, differential scanning calorimetry (DSC) is the most widely used technique for monitoring enthalpy changes. Considering microstructural aspects of polymer ageing, free volume plays a critical role in the interpretation of the experimental results. Changes in the size and distribution of the free volume holes can be tracked by spectroscopic methods like positron annihilation lifetime spectroscopy (PALS), electron spin resonance spectroscopy (ESR) and fluorescence spectroscopy. Other techniques, as the use of photochromic probes and labels or small angle X-ray scattering (SAXS) can be informative, as well. The morphological aspects of ageing, on the other hand, can be studied by scanning electron microscopy (SEM).