Dielectric Characterization Of Orientation Structure Evolution Of Polymer Molecular Chains And Carbon Fiber-reinforced Phase

The essence of polymers and their carbon fiber reinforced composites processing are formation,response,and evolution procedures,the multi-scale structures experienced under multiphysical coupling environment.Within,polymer molecular chains and reinforcing fibers have undergone a fairly complex orientation evolution under external fields.That is,the molecular chains and reinforcing fibers are oriented along a specific direction,and result in a ordered structure.This orientation structure significantly affects the mechanical,thermal,optical,and electrical properties of the final product.For example,higher orientation degree can increase the stiffness,strength,and toughness of the polymer by an order of magnitude.It can also cause spatial anisotropy distribution in optical product properties,leading to birefringence,optical distortion,etc.The existing polymer orientation measurement methods mainly include birefringence,infrared dichroism,wide-angle/small-angle X-ray diffraction,computed tomography,scanning electron microscope,etc.However,under actual production conditions,these methods have many drawbacks inclding failure in penetrating metal molds,inability to adapt actual working conditions,operation complexity,and high costs,so they can only be used for offline characterization of the the final product‘s orientation structure.It can be seen that there is currently no method for polymer orientation online characterization that can be used in actual processing environment.To this end,a novel in situ dielectric characterization method for the orientation evolution of polymer molecular chains and reinforcing carbon fibers was proposed and studied in this paper,which provided a new path for the modeling and precise regulation of the orientation evolution mechanism under processing conditions.Starting from analyzing dipole moment changes due to polymer molecular chains and fibers orientation,this paper analyzed and modelled the orientation evolution process,orientation structure,and dielectric response mechanism within.According to the generalized Clausius-Mosotti equation,the molecular chain orientation-dielectric quantitative relationship model for polymer solutions was deduced and established.The orientation state of fibers was characterized by orientation tensor,orientation probability distribution function,etc.,and based on the generalized Maxwell relation,a dielectric description equation of carbon fiber orientation in suspension was established.Based on the research of the polymer molecular chain/carbon fiber orientation-dielectric theory model,with the sensor array measurement strategy,a othorgonal weak electric field was constructed with the help of the interdigital electrode capacitor(IDEC)array,and the dielectric response(dielectric constant)anisotropy was used to characterize the orientation structure.In order to realize the dielectric characterization of the orientation structure,the penetrating electric field characteristics of the IDEC were analyzed,and the optimal design and manufacturing process of the IDEC sensor were also discussed.Through the modeling and simulation of the IDEC‘s penetration electric field,the quantitative relationship between the structure parameters(spatial wavelength,electrode finger number,metallization ratio,etc.)and measuring depth were analyzed with different dielectric properties.Two kinds of IDEC sensor types,rigid substrate and flexible substrate,were designed and manufactured.The factors that affect the measurement accuracy are analyzed from two aspects,the structure parameters and the surface contact conditions,which lays the foundation for the optimal design and installation of the IDEC sensor in a variety of material system under test.Based on the existing theoretical relationship between the strain of polymer materials and the orientation of molecular chains,a uniaxial tensile test was designed to verify the orientation dielectric characterization method proposed in this paper.Based on the polymer molecular chain orientation-dielectric model and the IDEC sensor designed and manufactured,the dielectric characterization and comparative verification of molecular chain orientation evolution in Couette flow were carried out.A in situ molecular chain dielectric characterization method was realized.A set of coaxial cylindrical barrel steady-state shearing platform for Couette flow was designed.The dielectric anisotropy response was measured with IDECs sensor,and orientation evolution of molecular chain in polymer solution systems with various solvents and solutes were analyzed.The experimental results not only accurately matched the elastic dumbbell model,but also was verified with other orientation characterization methods,such as rheological small-angle laser scattering,flow birefringence,and flow dichroism.And,the dielectric orientation characterization method has better advantages in opaque solutions or melts,with ease of installation.This orientation dielectric measurement methodology was also applied to realize the in situ characterization of molecular orientation during injection molding of polystyrene.The molecular orientation evolution during processing was analyzed,providing the preliminary verification of this method during injection molding.Since reinforcing carbon fibers had been widely used in polymer processing,this paper also studied the dielectric characterization of carbon fiber orientation evolution in Couette flow.The agglomeration and precipitation problems were solved with carbon fiber‘s surface modification.And,the rheological orientation-dielectric platform of the carbon fiber suspension for Couette flow was also constructed.In situ dielectric characterization experiments were conducted for different carbon fiber concentrations,aspect ratios,and solvents.The results showed that the concentration,aspect ratio and solvent viscosity of the fiber suspension had a positive correlation with orientation,which was consistent with the proposed fiber orientation-dielectric model.This paper proposed a method that applied dielectric response anisotropy of polymer molecular chain/carbon fiber orientation.Measuring the dielectric constant in different directions the orientation direction and degree can be sensed.With systematic theoretical modeling,sensor optimization design,and experimental research,in situ dielectric characterization of orientation evolution of molecular chains and carbon fiber reinforced phase were achieved.The novel orientation dielectric sensing method was helpful for the research and precise regulation of the orientation evolution during polymer processing,and had important scientific significance and application value for achieving high performance of polymer products.