Tunability and molecular weight dependence of the physical aging behavior of polystyrene and poly(methyl methacrylate) films

Abstract

This study investigated the physical aging behavior of commodity polymers, specifically polystyrene (PS) and poly(methyl methacrylate) (PMMA), across various molecular weights (MWs). Using ellipsometry, we examine the physical aging behavior of bulk polymer films supported on silicon across a wide range of quench depths below glass transition temperature. At shallow quench depths, where the physical aging behavior is thermodynamically controlled, we observe that the physical aging rate for both PS and PMMA increases with decreasing MW. The MW dependence of the physical aging rate in the thermodynamic‐control region is hypothesized to originate from the MW dependence of the fractional free volume. In support of this hypothesis, we observe that the difference in thermal expansivity between the rubbery state and the glassy state (αrubbery − αglassy), which has been demonstrated to be positively related to fractional free volume, exhibits a similar MW dependence as the physical aging behavior. This correlation lends support to the free volume model of physical aging. Building on the free volume model, we propose a method to tune the physical aging behavior, demonstrating that incorporating flexible chain ends alters the physical aging behavior in a manner akin to reducing MW.Highlights Polystyrene and poly(methyl methacrylate) have maximum physical aging rates at specific quench depths. Physical aging behavior is thermodynamically or kinetically controlled. With thermodynamic control, the aging rate increases with a reduction in molecular weight. With thermodynamic control, the aging rate correlates with free volume. Aging behavior can be tuned by blending with low MW polymer with highly flexible chain ends.