'In-Situ' Monitoring of the Non-Isothermal Crystallization of Polymers by Dielectric Spectroscopy

摘要

In recent years, experimental techniques based on microdielectrometry have attracted increasing interest for continuous monitoring, in a nondestructive way of the advancement of the reaction of thermoset resins under cure. The extension of this technique for "in-situ" monitoring of the crystallization of thermoplastics of the material and its viscoelastic properties was done. A dynamic rheometer equipped with a dielectric cell and an instrumented slit die was designed. The crystallization process is depicted by a conductivity drop and by the occurrence of a maximum in the real permittivity. The decrease of the specific conductivity results from a modification of the conductive paths, whereas the increase of the permittivity is due to an interfacial polarization phenomenon (so-called Maxwell Wagner Sillars effect) between the amorphous phase and the growing crystalline entities. The sudden variation of the dielectric response is envirsioned as a means for following the progression of the crystallization front within a mold. The description of the crystallization with Nakamura's model allowed the modeling of the crystallization inside a mold. The evolution of the temperature profile and of the crystallinity rate is modeled by a classical finite difference method. Dielectric measurements performed at controlled cooling rate eperiments are in good agreement with the numercial simulation results and confirm the usefulness of dielectric measurements for "in-situ" monitoring of the crystallization process of a polymer inside a mold.