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

Abstract

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.

21st Century Advances in Fluorescence Techniques to Characterize Glass‐Forming Polymers at the Nanoscale

“…By using nanobubble inflation to measure creep compliance of free‐standing poly(vinyl acetate) and polystyrene films in the rubbery regime, O'Connell and McKenna revealed a stiffening of modulus with decreasing thickness for sub‐200 nm films . Karim and McKenna later used nanoparticle embedding techniques to measure stiffness at the free surface in poly(α‐methylstyrene) (PαMS) . They noted a softening of the surface well into the glassy state, consistent with previously mentioned confined polymer studies .…”

“…Several techniques have been developed to measure the mechanical properties of ultrathin polymer films and have largely been used to elucidate deviations from bulk stiffness and residual stress . The experimental temperatures at which these studies were performed proved important in revealing different confinement behaviors below and above bulk T g : glassy softening and rubbery stiffening .…”

“…Films in the rubbery regime exhibited a significantly weaker free‐surface deviation and a larger perturbation length scale of the substrate influence than those in the glassy state. These differences could explain the differences in surface and average film behavior between the two states.…”

Mechanical properties of pentaerythritol tetranitrate(PETN) single crystals from nano‐indentation: Depth dependent response at the nano meter scale

“…Similar observations of an indentation depth dependent modulus have been observed by other groups in experiments on amorphous glassy polymers, where it has been reported that the mechanical response at small scales can be different from those of the macroscopic material. In those cases the surface has been reported to be different from the bulk, i.e., softer or stiffer , and sometimes the same. A full understanding remains elusive, though it seems that the pressure dependence of the glass transition temperature is important .…”

“…In those cases the surface has been reported to be different from the bulk, i.e., softer or stiffer , and sometimes the same. A full understanding remains elusive, though it seems that the pressure dependence of the glass transition temperature is important .…”