Elevated temperature nanoindentation and viscoelastic behaviour of thin poly(ethylene terephthalate) films

A commercial nanoindentation system fitted with a heating stage and heated indenter has been used to investigate how the elevated temperature nanoscale mechanical properties of poly(ethylene terephthalate) films vary with their processing history and crystallinity over the temperature range 60-110 degrees C. Three additive-free thin films were tested; an undrawn amorphous film, a uniaxially drawn film, and a commercial biaxially drawn Melinex film. A sharp decrease in mechanical properties was observed between 70 and 80 degrees C on the undrawn and uniaxial film consistent with the presence of a glass transition over this temperature range in agreement with literature values for bulk materials. In contrast, a gradual decrease in properties was observed over the same temperature range on the biaxially oriented film. The high crystallinity of the biaxial film could be beneficial in extending the operating temperature of the film. There is a minimum in the elastic recovery parameter around 80 degrees C on both the amorphous and biaxial film. This indicates that the elastic recovery parameter may be more sensitive to changes in mechanical properties occurring at/near the glass transition region than the hardness or modulus alone. A recently introduced dimensionless parameter for creep, A/d(0), was also found to be a promising way to characterise the increased time-dependent deformation around the glass transition region.