Study On Controllable Crosslinking And Triboelectric Properties Of Poly (Arylene Ether) Containing Perfluorobiphenyl Structure

Fluorinated poly（arylene ether）s have attracted much attention in the fields of optical waveguide,gas separation,microelectronic manufacturing and proton exchange membrane due to their excellent thermal,mechanical,electrical and optical properties.Decafluorobiphenyl（DFBP）is a common monomer for the synthesis of fluorinated poly（arylene ether）s.The fluorine atoms at para position in DFBP are preferentially replaced by nucleophilic reagents in the nucleophilic substitution reaction owing to the strong electron absorption effect of fluorine atoms in adjacent benzene rings.However,the C-F bonds at the ortho and meta position in DFBP also exhibit nucleophilic substitution activity,which leads to branching,even cross-linking and other side reactions in nucleophilic polycondensation.Therefore,researchers have carried out a lot of researches on the polymerization conditions for preparing fluorinated poly（arylene ether）containing perfluorobiphenyl structure（FPAE）,and excellent progresses have been gained in polymerization control,copolymerization and functional modification of FPAE.Various functional FPAE films with excellent properties have been designed and developed.However,there are still some problems such as the studies on the structure analysis of polymer and the control mechanism of branching side reaction are not deep enough.Moreover,the research field and development of such materials are limited duo to the processing method that using casting solution to prepare films.In this dissertation,FPAE with high molecular weight and high linearity are prepared through the control and optimization of polymerization conditions.Furthermore,the number of active end groups in the polymer chain is reduced and high temperature melt stability is improved by using fluorine-containing monomers to block the end groups,realizing the melt processing FPAE.The cross-linking modification of FPAE is studied based on the nucleophilic substitution activity of C-F bond in FPAE,where the cross-linked FPAE films with excellent mechanical strength,high temperature resistance,low dielectric properties are successfully prepared by using small molecule phenate as crosslinking agent and grafting crosslinking group.Meanwhile,the triboelectric properties of FPAE and crosslinked FPAEs in the new field of triboelectric nanogenerator（TENG）are preliminarily explored.The details are as follows:1.FPAE with high molecular weight and linear molecular structure is successfully prepared by optimizing reaction conditions（reaction temperature,reaction time and the molar ratio of monomers）by using DFBP and BPAF as monomers,where the number average molecular weight（M_n）of FPAE is 70 900 Da.The structure of the FPAE is confirmed by ¹H NMR and ¹⁹F NMR,and it is proved that the polymer has a good linear molecular structure.FPAE can gain excellent solubility in different polar organic solvents and the FPAE films are prepared by solution casting.FPAE films obtain outstanding mechanical properties,when the tensile strength is 59 MPa and elongation at break is 92%.Meanwhile,the presence of fluorine elements and hexafluoroisopropyl groups in the molecular structure can reduce the polarizability of FPAE and the number of polarizing molecules per unit volume,and further make FPAE gain excellent low dielectric properties,in which the dielectric constant and the dissipation factor of FPAE respectively are 2.60 and 0.0038 at the frequency of 1 k Hz.The temperature stability of dielectric properties shows that the dielectric properties of FPAE films have good temperature stability before 180℃（near the glass transition temperature）,and the dielectric constant and dissipation factor of FPAE films always respectively maintain around 2.60 and 0.003 in the range of 30℃～180℃.The prepared FPAE with good solvent film-forming properties cannot be fully plasticized even at high temperature because of large molecular weight,so the prepared FPAE cannot be melt processed.2.The melt processing properties of FPAE are improved by reducing the molecular weight of FPAE based on above study.However,the decrease of molecular weight leads to the increase of the number of active end groups in the polymer chain,which can cause paste phenomenon due to the cross-linking reaction in the processing of FPAE.Fluorinated poly（aryl ether）（FPAE-4-S）with excellent melt stability is successfully prepared by using 4,4’-difluorodiphenyl sulfone to block the end groups in order to eliminate the influence of residual active end groups at the end of polymer molecular chains.The standard samples for testing the mechanical properties and tribological properties are gained by extrusion molding and injection molding of the prepared FPAE-4-S.The tensile strength and bending strength of the injection molded sample respectively are 43 MPa and 97 MPa.Compared with FPAE films,FPAE-4-S has reduced mechanical properties owing to small molecular weight.There is a large mount of fluorine in the molecular chain of FPAE,and the surface of FPAE shows good hydrophobicity.The water contact angle is 103.1°（±0.79）,and the surface energy is 25.1 m N/m（±1.14）.Meanwhile,FPAE-4-S also showed good tribological properties,and the coefficient of friction and wear rate of the injection molding sample respectively are 0.44 and 4.99×10^-6 mm³/N·m.3.It is found that the fluorine atoms in the octafluorobiphenyl group of the FPAE chains are easily replaced by nucleophiles,which lays a foundation for the further functionalization and application of FPAE functional materials.Small molecule bisphenolate is used as the crosslinking agent when carrying out the controllable crosslinking of FPAE.The crosslinked FPAE films with high gel content are obtained by adjusting the amount of crosslinking agent,which effectively enhanced the glass transition temperature and thermal stability of the polymer.The glass transition temperature（T_g）of crosslinked FPAE films can reach 213℃,and the 5%thermal decomposition temperature(T_d5)in air atmosphere is 499℃.However,the dielectric constant of crosslinked FPAE films increases due to the existence of a small amount of residual bisphenolate and KF inorganic salt,which is unfavorable for the application of crosslinked FPAE films as low dielectric materials.Therefore,phenylethynyl groups is successfully introduced into the side chain of FPAE by the nucleophilic substitution reaction between 3-ethynylphenol and FPAE,and the cross-linked fluorinated poly（arylene ether）（C-FPAE-PEs）films are successfully prepared by the addition polymerization of phenylethynyl groups.The T_g of C-FPAE-PEs increases to 223℃,and then the tensile strength and the dielectric constant of C-FPAE-PEs respectively are63 MPa and 2.40（1 k Hz）.The results show that C-FPAE-PEs is a kind of high temperature resistant and low dielectric materials with excellent comprehensive properties.4.The triboelectric charges can be easily generated when fluoropolymers contact with different materials due to the difference of charge affinity between fluoropolymers and other materials.The prepared FPAE in this dissertation has high fluorine content.Therefore,the triboelectric properties of FPAE are studied,and its application in the field of triboelectric nanogenerators（TENG）is preliminarily explored.Due to the existence of a large number of benzene rings in the molecular structure of FPAE,the energy levels along the FPAE molecular chain are unevenly distributed,resulting in a large number of charge-trapping points.Hence,FPAE can effectively block the combination of triboelectric charges and induced charges and improve the stability of triboelectric charges,so the constructed TENG has higher triboelectric output performance.The output current is 2.4μA that is 2.4 times that of the TENG based on commercial PTFE film.Meanwhile,crosslinked FPAE（C-FPAE-PEs）are used to assemble TENG.With the increase of the degree of crosslinking,the output current of TENG based on C-FPAE-PEs increases to 10.7μA which is 10 times higher than that of TENG based on commercial PTFE film under the same test conditions.The molecular capture center formed by the crosslinking structure in the polymer can effectively limit the charge transmission.In other words,the introduction of crosslinked structure is also conducive to the stability of triboelectric charges and improves the triboelectric output performance of TENG.The main chain of FPAE is designed and different large conjugated structural units of bisphenol fluorene are introduced into the main chain of FPAE to further improve the output performance of TENG based on FPAE series polymers.The effect of different fluorene contents in the main chain of FPAE on the triboelectric output performance of TENG is explored.When the fluorene content is 50%,the output current of TENG can reach 6.5μA.However,the introduction of excessive fluorene structures can reduce the output performance of TENG.This may be because the introduction of fluorene structure leads to the decrease of the relative content of fluorine in polymer molecules,which reduces the charge affinity difference between triboelectric materials and is not conducive to the generation of triboelectric charges.It is worth noting that when PEEK and fluorene-containing FPAE series polymers are used to assemble TENG,the direction of TENG output current can be reversed with the increase of fluorene content,that is,when an appropriate amount of fluorene structure is introduced into FPAE,it can be converted from electron-accepting triboelectric materials to electron-donating triboelectric materials in TENG,which lays an experimental foundation for the molecular design control of triboelectric materials and further theoretical research.The above research work shows that FPAE has a good development potential in improving the output performance of TENG,which provides a new molecular design idea for the preparation of high performance TENG.