Unusual Surface Mechanical Properties of Poly(α-methylstyrene): Surface Softening and Stiffening at Different Temperatures: Surface softening and stiffening at different temperatures

We report results from experiments based on the spontaneous particle embedment technique to determine the surface compliance of poly(α- methylstyrene) (PαMS) for temperatures from room temperature to T _g + 21 C. Both submicrometer and nanometer diameter particles were investigated. Atomic force microscopy (AFM) measurements were used for the particle embedment determination, and the Lee and Radok (LR) model was applied to calculate the surface shear compliances using the embedment depth of the particles for the temperatures of interest. The adhesion force between the particles and the PαMS was used as the driving force for the embedment in the LR model calculation. Results were compared with macroscopic viscoelastic data for PαMS obtained from the literature and with new data from the present study. We observed surface softening for the PαMS in the temperature range from room temperature to T_g -21 C. This was followed by a crossover to a surface mechanical response that was stiffer than the macroscopic material for temperatures from T_g -18 C to T _g + 21 C. These novel findings were observed for both 199 and 60 nm diameter silica particles.