Molecular weight dependence of near surface dynamical mechanical properties of polymers

We have made a detailed characterization of the molecular weight dependence of isothermal gold nanoparticle embedding into the surface of atactic polystyrene films. Polymers in the range 3 × 10³ < M_w < 1.2 × 10⁶ g mol^-1 were used within a temperature range that encompassed the bulk glass transition temperature, around which both M_w dependence and heterogeneous dynamics were measured. The range in M_w also allows for comparison to recent studies on molecular glass surfaces-essentially covering the entire range of possible polymer sizes. The results show that there is a different manifestation of enhanced surface mobility for large and small polymer molecules. The transition between the mechanisms provides some bound on the size of a surface region with enhanced surface dynamics on glassy polystyrene surfaces. © 2013 The Royal Society of Chemistry.