Application of time–stress superposition to nonlinear creep of polyamide 66 filled with nanoparticles of various sizes

The long-term tensile creep of polyamide 66 and its nanocomposites filled with 1 vol.% TiO2 nanoparticles 21 and 300 nm in diameter is studied. It is assumed that the dominant mechanisms of creep deformation are of viscoelastic nature, while the contribution of plastic strains is not essential in the stress (< 0.6 of the ultimate stress) and time (about 100 hours) ranges considered. The creep isochrones obtained show that the materials exhibit a nonlinear viscoelastic behaviour and the degree of nonlinearity is reduced significantly by incorporation of the nanoparticles. The evolution of viscoelastic strains is less pronounced for the nanocomposite filled with smaller nanoparticles. Smooth master curves are constructed by applying the time–stress superposition (TSS). The Boltzmann–Volterra hereditary theory is used for the creep modeling. The nonlinearity of viscoelastic behaviour is taken into account by using the TSS principles and introducing a stress reduction function into an exponential creep kernel. The master curves are employed to predict the creep for time periods more than 60 times exceeding the test time. A comparison of relaxation spectra of the polymers shows that the incorporation of nanoparticles restricts the mobility of polymer chains. The smaller the nanoparticles, the greater the enhancement in the creep resistance. An empirical approach and a three-parameter law is also used for creep approximation. The efficiency of two models is evaluated by comparing their prediction validity.