As we know, with the addition of filler into polymer matrix, the elastic property of polymer composites melt would decrease generally. However, when I am using the PTT model (a viscoelastic constitutive model) to simulate the polymer composites, I find that the simulated results regarding the elastic flow behaviors (like die swell ratio) increased with the filler addition which means the simulated results are not trusted.

So does anyone knows why the simulated results when using the PTT model could overpredicted the elastic properties of polymer composites? Is it possible that PTT model is not suitable for simulating the polymer composites flow behaviors?

Thanks a lot.