Impact of structural relaxation on mechanical properties of amorphous polymers

Fusion based methods, such as hot-melt extrusion, are a common way of preparing amorphous solid dispersions. Since the amorphous glass, however, is not in a configurational equilibrium, the molecular arrangement of the obtained material can differ in dependence of the preparation conditions. Although the changes in the configuration of an amorphous material, which are commonly referred to as structural relaxation or physical aging, are well investigated, the impact on mechanical properties of amorphous solid dispersions have widely been neglected so far. The presented study investigated copovidone as a model polymer commonly used in amorphous solid dispersions and revealed that structural relaxation was already introduced into the polymer during hot-melt extrusion while its degree was cooling rate dependent. The degree of structural relaxation significantly affected the mechanical properties of copovidone as assessed by diametral compression tests, macroindentation and nanoindentation. An increase in Young's modulus and indentation hardness was observable with a higher degree of structural relaxation, which, during tablet compression, translated into tablets with significantly lower tensile strength. Furthermore, evaluation of the force-displacement curves during tablet compression revealed a decreased proportion of irreversible deformation with higher degree of structural relaxation correlating well with the increased indentation hardness during macroindentation. Thus, understanding structural relaxation and its impact on material properties is of utmost importance to assess the processability and compaction performance of amorphous solid dispersions in dependence of their preparation conditions and thermal history.