Interdigital Capacitor Detection Method For Oriented Polymer Structure

The mechanical and optical properties for polymers and composites strongly depend on the molecular chains/fillers orientation.However,the common methods such as birefringence,X-ray scattering,infrared dichroism,etc.,are difficult to realize online orientation characterization due to limited penetration,expensive equipment and complicated operation.Aiming at this scientific problem,this paper proposes a new orientation detection method based on interdigital capacitor.The following work was carried out in terms of interdigital capacitor design,orientation model verification and practical application.Firstly,the optimal analytical model between interdigital capacitance and dielectric constant,the Rui model,was selected by finite element simulation.Based on the Rui model,the sensitivity and penetration depth of the interdigital capacitor were analyzed.Then an interdigital electrode with metallization ratio of 0.5 and finger number less than 50 was selected to get higher dielectric constant response sensitivity while ensuring the penetration depth.Secondly,the dielectric-orientation model of fiber reinforced composites was established.Based on the Rui model,the relationship between interdigital capacitance and fiber orientation was obtained.By measuring the interdigital capacitance of carbon fiber reinforced polyether-ether-ketone sheets with different orientation angles,the feasibility of this orientation detection method in static measurement was verified.The determination coefficient between the measured results and the fitted values is 0.99.The average error of the orientation angle measurement is less than 3%.Further,a rheo-dielectric experimental platform was built and the capacitance response of the carbon fiber/silicone oil suspension in steady shear flow was measured.After analyzing the orientation tensor and its influencing factors in fiber suspension,it is found that the trend over time of interdigital capacitance and orientation tensor is consistent,which verifies the feasibility of this orientation detection method for complex fluid.Finally,the interdigital capacitor sensor was applied in the injection molding process.After the static calibration,the variation of dielectric property of polystyrene was studied by short-shot experiments.The interval in which the melt flow leads to changes in dielectric property was determined.By changing the injection speed,it is found that the dielectric anisotropy of polystyrene increases with the increasing of injection speed,which is consistent with the trend of shear orientation of molecular chain.It is preliminarily indicated that the sensor can be used for the shear orientation characterization of polymer melt.