Orderly Assembly Of Micro-/Nano-Fibers In PDMS And Its Dielectric Properties

High dielectric polymer materials are widely used in flexible wearable electronic devices.Due to the low dielectric constant of the polymer itself,high-dielectric fillers are often added to improve its dielectric properties.It is believed that the dielectric constant of the material is optimal when the filler reaches an osmotic threshold in the polymeric matrix（when the filler forms a continuous channel in the composite）.The permeability threshold of inorganic fillers is very high,generally more than 50 vol.%,thus nagtively influencing the mechanical properties of the final composite materials、flexible properties and light transmittance of composite materials.Therefore,constructing a continuous channel and reducing the permeation threshold of the filler in a composite is essential for improving the dielectric performance.In addition,for implantable electronic devices,dielectric materials need to be flexible,transparent and biocompatible.In this study,PDMS featuring good flexibility was selected as the matrix,microscale sodium carboxymethylcellulose（CMCNa）with natural biocompatibility and nanoscale ceramic halloysite were used as fillers,and the composite materials with high dielectric constant,flexibility and transparency were fabricated by electric field induction method.The main contents of current study are shown as follows:First,the study of the orderly assembly process of cellulose sodium/PDMS mixture under the action of DC electric field.The effects of DC field strength,time and concentration on the orientation of CMCNa in the blend were investigated via online observation using optical microscope.The results showed that when the field strength,energization time and concentration was 400v/mm,180s and 15wt.%,respectively,the most penetrating chain structure was formed,and this was the optimised condition for electric field driven orientation of CMCNa in the blend.The changes of viscosity and response time of CMCNa in CMCNa/PDMS mixture under different field strengths,concentrations and shear rates were studied by the current variation test so as to reflect the arrangement of CMCNa,and the results showed that shear stress and shear viscosity increased with the increase of electric field strength at lower shear rates,while the chain structure formed by electrophoretic effect was damaged at higher shear rates,and shear stress and shear viscosity were no longer affected by field strength.The time-viscosity curves of the two shear rates 1s^-1 and 0.01 s^-1 under different field strengths were further compared,indicating that the stability of the CMCNa chain structure was favorable at low shear rates.When the shear rate is 0.01 s^-1,the relationship among field strength,concentration and response time is:at a same concentration,the larger the field strength,the shorter the response time of current change;Under a same field strength,the higher the concentration,the shorter the path to form the chain structure,and the response time gradually decreases with the increase of concentration.With the field strength of400V/mm,the viscosity difference before and after the energization of the blend at a concentration of 15wt.%was the highest,implying that the CMCNa-PDMS solution at this concentration formed the most chain structure under the action of electric field,and the structure was compact.Second,the preparation and performance of CMCNa/PDMS ordered materials.On the basis of the above study,the performance of the composite films after applying electric field orientation and the unoriented was compared:the surface and cross-section of the film were analyzed by polarizing microscopy,SEM and EDS elemental analysis,confirming that CMCNa was oriented in the composite film driven by electric field especially at a concentration of 15 wt.%,and the chain structure running through the poles was the most.The transmittance of the film after applying the electric field was higher than that of the film without applying the electric field,and the transmittance of the composite film with a concentration of 15wt.%increased the most from 75.73%to 78.23%.The permittivity of CMCNa composite film after applying electric field was significantly higher than that of composite film without applying electric field（22.29 vs 6.85）at 1KHz,and the dielectric loss increases from 0.101 to 0.924,but still less than 1.Although the orientation of cellulose in the film after applying the electric field has little effect on the flexibility of the film,the storage modulus of the composite film increased by 17.25 times.Additionally,both the thermal conductivity of the oriented composite material became better.Third,the sequential assembly process of halloysite/PDMS under the action of alternating electric field and the properties of its composite materials.Green and natural ceramic nanofiber halloysite was selected as the filler and PDMS as the substrate.Firstly,halloysite was modified by silanization using KH560 and confirmed by TGA,FT-IR and XRD characterizations.The modified halloysite was well dispersed in PDMS substrate,indicating that the silanization had no effect on the dielectrophoresis effect of halloysite.The modified halloysite can be well oriented in PDMS solution driven by AC electric field,especially under the conditions with a frequency of 100Hz,a field strength of 700V/mm,and concentration of7wt.%.The oriented halloysite was maintained in final composite film along the film thickness supported by optical microscopy,SEM and EDS elemental analysis.The transmittance of the composite film increased from 5.098%to 20.31%with the addition of 3 wt%halloysite.The dielectric constant of the composite film fabricated with the electric field treatment was increased,while the dielectric loss was still low,for instance,the dielectric constant of the composite film with a concentration of 7wt.%increased from 4.48 to 8.36 at 1KHz,but the dielectric loss value was much less than 1.The orientation of halloysite has little effect on the flexibility of the film.The storage modulus of the composite film with a concentration of 7wt.%after applying electric field is increased by 60%compared with that without electric field.