Electrospinning-Induced Fibrillated Distribution Of Powdery Nanofillers And Its Influence On Composites Performance

With the development of electronic communication technology,increasing demand has been put forward for composite materials with various functionality,such as high electrical conductivity,electromagnetic shielding,thermal conductivity,dielectric properties,etc.The structure of material determines its property.In order to obtain functional composites with excellent performance,precise structure regulation of functional fillers is very required.One-dimension（1D）fibrillated fillers or structures show outstanding advantages in the field of functional materials due to their high aspect ratio,which can construct electrically or thermally conductive networks easily as well as possess higher polarization ability.At present,the commonly used nano-functional fillers are mostly in powdered form and difficult to form continuous fibrillated structures.including 1D nanofillers like carbon nanotubes（CNT）and 2D nanofillers like boron nitride nanosheets（BNNS）.Therefore,regulating nanofillers into one-dimensional confined distribution to realize the fibrillation of powdered nanofillers is expected to prepare functional composites with better performance.Electrospinning,a common technique to produce fiber,has strong electric field stretching effect during electrospinning and can obtain micro-and nano-sized fibers.Meanwhile,electrospinning can fabricate various fibers and porous continuous fiber mats with diverse components and hierarchical structures by adjusting the parameters.Therefore,this thesis proposed the strategy of confining powdered nanofillers like CNT and BNNS into one-dimension distribution state via electrospinning and realize the fibrillation of powdered nanofillers.Meanwhile,according to the different requirements of functional composites such as electrical conductivity,thermal conductivity and dielectric property,the carriers of confined functional fillers and the polymer matrix were carefully selected and designed.and the electrospun porous continuous fiber mats were fully used to prepare functional composites with excellent performance.The relationship among structure regulation of electrospinning,the one-dimension confined fibrillated distribution of filler,and the properties of functional composites has been studied in depth,i.e.,the preparation-structure-property relationship.The main research contents and results are as follows:（1）The fibrillation of polymer is an important prerequisite for the one-dimensional fibrillated confinement of functional fillers.The fiber diameter of fiber is an important parameter and has a great influence on the microstructure inside fibers and the property of fiber mats.In this part,utilizing styrene-butadiene-styrene copolymer（SBS）and polyacrylonitrile（PAN）as examples,we controlled the fiber diameters by adjusting the spinning process and investigated the influence fiber diameter on structure and performance of fiber mats.Given that electrospun SBS fiber was difficult to refine in previous research,we proposed to reduce the fiber diameter by adding inorganic salt（Li Br）into SBS spinning solution.With the increasing Li Br concentration,a critical concentration（0.005 wt%）has been found for the growth of electrical conductivity of SBS spinning solution,and the similar critical concentration existed for the varying trend of mean fiber diameter with Li Br concentration,that is,when Li Br concentration below 0.005 wt%,the fiber diameter decreased significantly from 1.85μm（0 wt%）to 0.50μm（0.005 wt%）and when Li Br concentration above0.005 wt%,the average fiber diameter no longer decreased but fluctuated in the range of 0.4～0.5μm.Eventually,the SBS mat obtained the highest tensile strength of 5.7MPa at the critical concentration 0.005 wt%,which was 134%higher than that of pure SBS mat,thanks to the reduced fiber diameter and welded structure after introducing salt into spinning solution.In addition,the phase separation behavior of SBS fiber with different diameters was investigated,it turned out that larger phase domain and more distinct phase boundary existed in finer fibers.Secondly,we controlled the PAN fiber diameter by introducing ethyl acetate into spinning solution of PAN and obtained different carbonized products derived from PAN.The effect of fiber diameter on the pore structure,specific surface area and electrical conductivity of carbonized mats was also investigated.In this part,by adjusting the process parameter like adding inorganic salt and changing the solvent,the electrospun fiber diameter can be effectively regulated and the effect of fiber diameter on the fiber membrane properties and pore structure was analyzed in detail.This work not only lays the foundation for the preparation of high-quality electrospun fiber mats and the study of one-dimension confinement of nanofillers in the subsequent work,but also provides some guiding significance for the fiber refinement and mechanical enhancement of electrospun mats in practical applications.（2）Carbon nanotube（CNT）has been commonly used as conductive fillers for conductive composites.CNT tends to curl and agglomerate due to their extremely high specific surface area and inert surface properties and is difficult to form a continuous conductive pathway.How to regulate the CNT into more accurate restricted distribution state and obtain high conductivity at low filler content is still a difficult issue to address.In this part,we proposed to confine CNT into one-dimension carbon nanofiber mats via electrospinning and make full use of the porous continuous fiber network as lightweight electromagnetic shielding materials.We first prepared PAN-based composite fiber mats with different contents of CNT,and obtained highly conductive hybridized carbon nanofiber mat（C@CNT）after high-temperature heat treatment.The confined state of CNT in the fibers was carefully observed,and the effects of heat treatment temperature,pressure applied during heat treatment,and CNT addition on the structure and conductive properties of the carbon films were investigated.As a result,C@CNT carbon mats consisted of continuous fiber.In C@CNT hybrid fiber,CNT was fully confined,homogenously dispersed,straightly aligned along fiber-axis.CNT promoted the development of graphite microcrystals in the C@CNT hybrid fiber and enhanced the connection among graphite microcrystals,leading to a strengthened conductive network within the fiber.The conductivity of C@CNT achieved 565 S cm^-1 at a bulk density of 0.78 g cm^-3,C@CNT exhibited excellent electromagnetic shielding effectiveness,achieving the highest value of 84d B at 120μm and obtained an extraordinary specific shielding effectiveness of 14000d B cm²g^-1.attributed to the extremely high conductivity,porosity and specific surface area of the carbon nanofiber mats.This work demonstrates that the electrospinning can confine CNTs into one-dimensional fibers,where CNTs orient axially and eventually overlap inside fibers.The continuous fiber network structure from electrospinning can build a continuous conductive network and obtain high conductivity and shielding effectiveness,inspiring the next research on highly thermally conductive fibrillated skeletons.（3）Based on the study of continuous carbon fiber membranes in the last part,we propose a 1D-constrained fibrillated structure of BNNS.To meet the high thermal conductivity and low dielectric requirements of electronic packaging materials in 5G era,we selected alumina（AO）as the confined carrier.We confined BNNS into continuous one-dimensional AO fiber skeleton via electrospinning,and investigated the fibrillated confined distribution of BNNS and its effect on the thermal conductivity of the composite material.A novel fibrillated hybrid skeleton（f-AO@BNNS）was fabricated by electrospinning and sintering,where long-range phonon transferring highway was successfully constructed by both macroscopically interconnected hybrid fiber and microscopically oriented and overlapped BNNSs inside AO@BNNS hybrid fiber.In addition,it was found that high-temperature sintering transformed the amorphous AO within the skeleton into crystalline AO and generated aluminum borate transition phase at the BNNS-AO interface,which greatly reduced the interfacial thermal resistance between the filler and the filler inside the skeleton.Prepared polybenzoxazine/f-AO@BNNS composites exhibited the highest TC of 3.24W m^-1K^-1 at a skeleton loading of 6.9 vol%.The higher TC was achieved at lower filler content when compared with other reported skeletons,demonstrating the superiority of f-AO@BNNS skeleton.Finally,we compared the properties of composite of one-dimension confined fibrillated distribution of BNNS with those of uniformly distributed BNNS,demonstrating that the fibrillated structure of BNNS was easier to construct thermally conductive network at low filler content.The reduced filler-matrix interface allowed the resulted composites to maintain the original low dielectric and high heat resistance properties of PBOZ,which guaranteed its practical application in the 5G era.The work demonstrates that the one-dimensional confined fibrillated distribution of BNNS and obtained continuous fiber network by electrospinning are beneficial to build thermal conductivity network,and provides new ideas for the preparation of high thermally conductive composites.（4）Based on the results in the last part,we confined BNNS in higher thermally conductive carbon fiber and realized macroscopic orientation structure of the fibrillated hybrid fiber.We first prepared electrospun PAN/BNNS composite fibers and then carbonized PAN to obtain the fibrillated skeleton（f-C@BNNS）,and investigated the effect of BNNS contents on the morphology of skeleton and the thermal and electrical conductivity of the composites（PDMS/f-C@BNNS）.It was found that the f-C@BNNS skeleton consisted of long-range continuous fibers and BNNSs in skeleton tightly stacked and aligned parallelly in axial direction due to the strong stretching effect during spinning.Besides,the in-situ formed carbon derived from PAN possessed high intrinsic TC and was considered to have good interfacial bonding with BNNSs.Consequently,PDMS/f-C@BNNS achieved excellent TC and the highest TC of 4.09 W m^-1 K^-1 was obtained at 42 vol%BNNS in the skeleton,which was about 16 times higher than that of pure PDMS.After analyzing the morphology of skeleton and the electrical conductivity of the composite,double-network thermal conductive mechanism was proposed for PDMS/f-C@BNNS,i.e.,the thermal conductive pathway formed by overlapped BNNS in the skeleton and the thermal conductive pathway composed of carbon contributed together to the highest TC at this optimal BNNS content.In addition,anisotropic thermal conductivity was achieved by preparing composite films with uniaxial parallelly aligned and vertically aligned f-C@BNNS fiber via drum receiver.In addition,we took advantage of the one-dimensional property of f-C@BNNS hybrid fibers and used a rolling receiver to prepare films with uniaxial parallelly and vertically aligned fibers,to achieve exceptional anisotropic thermal conductivity,including high out-of-plane TC.The work prepares f-C@BNNS continuous fibrillated skeletons,where BNNS are confined into one-dimensional carbon fibers with high aspect ratio,constructs long-range continuous thermal conductivity pathway,and then use the one-dimensional characteristics of f-C@BNNS fibers to realize the macroscopic orientation of f-C@BNNS fibers to obtain excellent anisotropic thermal conductivity.（5）In the previous parts,we found that fibrillation of functional filler such as CNT and BNNS achieved significant synergistic effects of thermal and electrical performance.In order to broaden the applied scope of fibrillation structure regulation of electrospinning,in this part we choose two functional components with complementary property,that is,compounding BNNS with barium titanate（BT）.The influence of 1D-confined distributing structure of BNNS on the performance of dielectric materials was investigated.First,we used electrospinning to confine BNNS with high insulation and breakdown strength into barium titanate fibers（BTNF）to obtain hybrid fibers（BT-BN NF）,where BNNS was embedded among the BT grain inside fiber,Using BT-BN NF hybrid nanofiber as filler,resulted polyvinylidene fluoride（PVDF）-based nanocomposite exhibited simultaneously enhanced dielectric constant and breakdown strength and obtained a much larger energy storage density(U_e≈15.25 J cm^-3)than conventional nanofiller-incorporated composite,which was3.07 times of PVDF.Compared with the randomly dispersed BNNS,the 1D-confined BNNS can directly enhance the BTNF and obtain higher enhancement efficiency,and the BT-BN NF has more comprehensive dielectric properties than the conventional high dielectric filler.This phenomenon was ascribed to the comprehensive performance of hybrid BT-BN NF,where the introduced BNNS improved the insulation and hindered the growth of electric tree and conduction of leakage current,thus reducing the failure probability at the weak point inside filler and on the filler-matrix interface.This work analyzes the effect of the distribution structure of 1D-confined distributed BNNS on the dielectric property of the composites,and demonstrates that the restricted distribution of BNNS can improve the insulation of the hybrid filler,hinder the growth of electric trees and leakage current conduction in the filler,and directly enhance the high dielectric filler,which provides a new idea for the subsequent structural tuning of dielectric composites.