Synthesis And Properties Of Novel Biphenyl Poly(Aryl Ether Sulfone)

As a kind of special engineering plastic with excellent performance,Poly（aryl ether sulfone）（PAES）has been widely used in the fileds of aerospace,automotive,medical and energy.However,with the advancement of technology,the heat resistance grade,solvent resistance and mechanical strength of traditional PAES resins have been unable to meet the stringent requirements for material performance in some cutting-edge fields.Therefore,it is concerned widely to synthesize more novel PAESs with higher grade of heat resistance and better mechanical properties by the design of molecular structure.The molecular chain structure of polymer is the main factor affecting its performance.As a rigid group with a simple structure,the biphenyl group has excellent stability and good symmetry.Therefore,it is often introduced into aromatic polymers to improve the rigidity of their molecular chains,which can enhance the heat resistance,mechanical properties,and friction properties of polymer.Polyphenylene sulfone（PPSU）is a typical commercial polymer with biphenyl structure in the molecular segment among several traditional PAES resins.In addition to the high temperature resistance of polysulfone（PSU）and polyethersulfone（PES）,it also has higher impact strength.This is mainly because there are more flexible ether bonds in PPSU（the ratio of ether/sulfone groups is 2:1）,and the introduction of biphenyl groups can effectively maintain the stiffness of the polymer,which can ensure the great mechanical and thermal properties of PAES resin.Therefore,the impact of the introduction of higher proportions of biphenyl groups on the comprehensive performance of PAES resins also requires us to conduct in-depth research.In addition to the molecular structure,the aggregate structure of the polymer also has an significant effect on performance.As the main molecular group constituting the molecular chain structure of PAES,the sulfone group is an asymmetric tetrahedral structure with a highly irregular spatial configuration.The large gap between the bond angle of C-SO2-C（105°）and C-O-C（124°）destroys greatly the regularity of the PAES molecular chain,which makes the traditional PAES resins are all amorphous polymers.Although PAESs have many excellent properties,they are still insufficient in solvent resistance and mechanical properties compared with PEEK and other crystalline special engineering plastics.Therefore,if the regularity of the PAES molecular chain can be further improved,or even to synthesize a PAES with crystallizing ability by the design of molecular structure,it would not only improve the solvent resistance,but also enhance its heat resistance and mechanical properties.In order to investigate the effects of the introduction of high proportion biphenyl groups on the properties of PAES,we synthesized a monomer named BDS with biphenyl structure by the Suzuki-Miyaura coupling reaction.Furthermore,two kind of PAESs containing a high proportion of biphenyl groups were successfully synthesized by a nucleophilic substitution polycondensation reaction.By comparing with commercially available PPSU,the effects of the addition of biphenyl structure on the properties of PAES were comprehensively investigated.The results show that the two novel biphenyl PAESs（BDS-PES,BDS-PDBES）both have high heat resistance rating,whose glass transition temperatures（Tg）are as high as 276℃and 300℃respectively.And their 5%weight loss temperatures are both as high as 526℃.The mechanical properties of the two novel PAESs have also been enhanced.Compared with PPSU,the tensile strength of BDS-PES and BDS-PDBES increased by 9%and12%,and the tensile modulus increased by 18%and 30%,respectively.In addition,two novel PAESs also have better high-temperature mechanical properties,whose storage modulus are both higher than that of PPSU,and can be maintained at higher temperatures.Thermal conductivity analysis shows that the thermal conductivity of the three PAESs is positively correlated with temperature.Compared with PPSU,two novel PAESs have better thermal conductivity.Solubility tests show that the two novel PAESs can still be dissolved in most organic solvents,and the solvent resistance has not been greatly improved.In order to further improve the regularity of molecular chain of PAES and enhance its solvent resistance,the synthesized BDS above and more structured4,4’-difluorobenzophenone were selected as the reaction monomer by the design of molecular structure.A series of biphenyl PAESKs(BD-PESK13)with different molecular weights were successfully synthesized by nucleophilic substitution polycondensation reaction.Thermal analysis shows that BD-PESK133 has excellent heat resistance,whose 5%weight loss temperature are all above 530℃.With the increase of molecular weight,the Tg can reach 240℃at highest.BD-PESK133 has excellent solvent resistance,which can only be dissolved in the strong polar protonic solvent concentrated H2SO4 at room temperature.BD-PESK133 powder shows good crystallization performance after annealing treatment at 320℃,and their crystallinity are 29%,24%and 16%,respectively.With the increase of annealing temperature,the melting point and the crystallinity of BD-PESK1 gradually increases,and the crystallization is the most perfect when the annealing temperature rises to 320℃.Meanwhile,BD-PESK also has excellent mechanical properties and high temperature mechanical properties.The tensile strength and storage modulus are both higher than PPSU.The mechanical properties of BD-PESK are improved after annealing,but it does not show the unique performance advantages of crystalline polymers.This may be due to the effect of melt processing on the crystalline properties of the polymer.In this paper,the crystallization properties of BD-PESK after melt processing are studied in depth.From DSC tests on polymer samples treated in different ways,it is found that BD-PESK can form crystal under the induction of diphenylsulfone solvent,and the crystallization can be more perfect after annealing treatment.However,the melting process can destroy the crystal structure of BD-PESK,which cannot be reconstructed even annealing again.In view of this problem,we think the main reason is that the molecular chain of high molecular weight BD-PESK has poor regularity,high viscosity,and difficult segment movement,which is not conducive to folding and forming crystals.Therefore,we investigated the effect of the molecular weight of BD-PESK on its crystallization properties after high temperature processing,and found that when the viscosity of BD-PESK is as low as 0.51 dL·g-1,the polymer after high temperature melt processing still has good crystallization ability.Due to its too small molecular weight,complete polymer splines cannot be obtained for further applications.But in theory,BD-PESK that has undergone the melting process can be recrystallized,which provides ideas for future work.How to further induce high molecular weight polymers to form crystals is the main research content in the future.