Comparison of surface mechanical properties among linear and star polystyrenes: Surface softening and stiffening at different temperatures

In a previous study, we reported both surface softening and stiffening for poly(α-methylstyrene) (PαMS) over a temperature range from room temperature to 21 C above the glass transition temperature (T_g) using the spontaneous particle embedment technique. In the present study, we have explored the surface mechanical responses of a linear polystyrene (PS), a 3-arm star PS, and an 8-arm star PS emphasizing the range of temperature from T _g -10 C to T_g +10 C, similar to that in which the prior study had shown a transition from surface softening (increased mobility) to stiffening (decreased mobility). We used atomic force microscopy for particle embedment depth estimation and the isochronal form of the Lee and Radok (LR) model for surface compliance determination. We observed both surface softening and stiffening for the linear PS and 3-arm star PS for the studied temperature range. However, surface softening was observed at all temperatures in this study for the 8-arm star PS. The surface compliance results for the different PS molecular architectures are compared with those for the linear PαMS from the previous study.