Physical aging behavior in amorphous PEN film as measured by creep

Characterization of the viscoelastic and aging responses of poly(ethylene naphthalate) (PEN) is of increasing interest as this material begins to see greater use in commercial applications. In typical aging experiments, one quenches the sample from above the glass transition to the final aging temperature and performs sequential experiments at increasing aging times to `probe' the changes in viscoelastic response due to the ongoing structural recovery. In the case of PEN, studies of the aging response are complicated by the presence of two viscoelastic mechanisms. Here we describe work in which creep measurements are used to probe the aging and temperature dependencies of the viscoelastic behavior of amorphous PEN. The results show that the large sub-glass β* transition has a large influence on the viscoelastic response and complicates the possibility of using classical time-temperature or time-aging time superposition for its description. At low temperatures and short aging times, one observes an almost power law behavior. However, as the aging times get longer, a second mechanism becomes evident and time-aging time superposition is not valid. In addition, as the temperature increases one also sees evidence for two mechanisms and lack of thermo-rheological simplicity.