Evolution of iPP Relaxation Spectrum during Crystallization

The linear viscoelastic response, usually described through the moduli G′(ω) and G″(ω), is widely employed to investigate the material properties because it is strictly related to the microstructure of thermoplastics. In a semi-crystalline polymeric material both the amount (degree of crystallinity) and the morphology of the crystalline phase strongly influence the polymer rheological behavior. In order to obtain information about the effect of crystallinity on the linear viscoelastic functions, the parameters of the linear multi-mode Maxwell equation have been determined by fitting literature data of G′(ω) and G″(ω) collected at different crystallinity degrees. The analysis of the resulting spectra, at least in the considered frequency and crystallinity range, clearly shows that the relaxation times of all modes increase with crystallinity in the same way. On the other hand, the parameters Gi of faster modes do not depend upon the crystallinity, whereas the parameters Gi increase with crystallinity only for the slowest modes. These results are very relevant to the rheology evolution during solidification: it is not sufficient to analyze only one viscoelastic function during crystallization, the relaxation time for instance; also the moduli change, and their increase seems concentrated to the modes having the largest relaxation times.