Nano-scale temperature dependent visco-elastic properties of polyethylene terephthalate (PET) using atomic force microscope (AFM)

Abstract

Visco-elastic behaviour at the nano-level of a commonly used polymer (PET) is characterised using atomic force microscopy (AFM) at a range of temperatures. The modulus, indentation creep and relaxation time of the PET film (thickness=100 mum) is highly sensitive to temperature over an experimental temperature range of 22-175 degrees C. The analysis showed a 40-fold increase in the amount of indentation creep on raising the temperature from 22 degrees C to 100 degrees C, with the most rapid rise occurring above the glass-to-rubber transition temperature (T(g)=77.1 degrees C). At higher temperatures, close to the crystallisation temperature (T(c)=134.7 degrees C), the indentation creep reduced to levels similar to those at temperatures below T(g). The calculated relaxation time showed a similar temperature dependence, rising from 0.6s below T(g) to 1.2s between T(g) and T(c) and falling back to 0.6s above T(c). Whereas, the recorded modulus of the thick polymer film decreases above T(g), subsequently increasing near T(c). These visco-elastic parameters are obtained via mechanical modelling of the creep curves and are correlated to the thermal phase changes that occur in PET, as revealed by differential scanning calorimetry (DSC).