Relating fractional free volume to physical aging: A study of tunability and molecular weight dependence of physical aging behavior of polystyrene and poly(methyl methacrylate) films

Ellipsometry is used to investigate the physical aging behavior of bulk polystyrene (PS) and poly(methyl methacrylate) (PMMA) films supported on silicon over a relatively large range of molecular weight (MW) and aging quench depth. Here, the aging quench depth is defined as the difference between aging temperature and the glass transition temperature. At shallow quench depth, for both PS and PMMA, ultralow MW films show higher aging rates compared to high MW films. However, at deep quench depth, there is no significant MW dependence of aging rate for PMMA whereas low MW PS shows a lower aging rate compared to high MW PS. The difference of aging behavior in deep and shallow quench depth may be attributed to the interplay of polymer chain mobility and thermodynamic driving force. In the shallow quench depth, we observe correlated MW dependence of fractional free volume and physical aging rate based on which we demonstrate that the physical aging rate of PS can be tuned to be significantly higher by incorporating flexible chain ends.