In-situ detection of phase transitions of semi-crystalline carbon fiber reinforced thermoplastics using dielectric analysis and its correlation to DMA and DSC

The mechanical performance of carbon fiber reinforced thermoplastics is strongly influenced by fiber volume fraction, fiber orientation, crystallization conditions, and in case of polyamides humidity. Commonly, the degree of crystallization is verified applying thermo-physical investigations after fabrication, which is often considered cumbersome and time-consuming, especially in a production site. Therefore, this work shows the potential of the in-situ detection of phase transitions such as glass-transition (T_g), melting and crystallization of polyamide-6 (PA6) reinforced by 45 vol.% of carbon fibers using dielectric analysis (DEA). Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to verify the observed phase changes measured by dielectric analysis. The glass transition temperature was determined using DEA, DSC and DMA during heating and cooling at a rate of 2 K/min. It can be seen that the glass transition temperature obtained from the analysis of the loss modulus E″ measured by DMA is in good agreement with the glass transition temperature determined from dielectric loss ε″ of the DEA. Consequently, the dielectric properties of carbon fiber reinforced semi-crystalline thermoplastics are strongly related to the molecular mobility changes at the glass transition temperature. This shows its potential as a high-sensitive inline measurement technique during the manufacturing of thermoplastic composites.