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.