Preparation And Properties Of High-Performance Polyimide And Polyphenyl-Based Polymers

Over the past few decades,the synthesis of high-performance polymers with superior physical,chemical,and electrical properties has drawn much attention.In this dissertation research,we are interested in the development of high performance polymeric materials;in specific,we did some advance research on making high peormance polyimide and poly?p-phenylene?based materials.On the one hand,to overcome the processing difficuility of common polyimide materials,we synthesized a new dianhydride monomer?3,3?-RsDPA?for making processable?soluble and/or fusible?polyimide through rationally molecular design.The polyimides synthesized from 3,3?-RsDPA showed excellent thermal properties,good solubility and processing properties.The glass transition temperature?T_g?and melting temperature?T_m?of the polyimides can be readily adjusted by copolymerization reaction;and the resulting polyimide can be processed in to different shapes by 3D printing technology.Furthermore,owing to the good processing?melting?behavior,high performance electrospun nanofiber reinforced polyimide nanocomposites were prepared by co-electrospinning and hot-pressing techniques.On the other hand,given the fact that the common poly?p-phenylene??PPP?based materials are typically in low-molecualr weight or non-processible?particular the preparation of fibrous materials?,we synthesized a series of novel soluable PPP-based oligomers of phthalate-capped poly?2,5-benzophenone??PBPA?to prepare high performance PPP based nanofibers.In specific,the PBPA was first by Ni?II?complex-catalyzed cross-coupling reaction,and subsequently,it was used to prepare high molecular weight nanofibers through the combination of electrospinning and molecular coupling assemblytechniquesfollowedbyheattreatment.Theprepared poly?2,5-benzophenone?-pyrrolone/polyimide nanofiber belts?PBPY/PI?exhibited high mechanical strength,superior thermal stability and chemical resistance;hence,they could be used as filtration media under high-temperature and/or corrosive conditions,and they could also be used as separators in batteries and supercapacitors.It is important to note that,this is the first reported study on the preparation of PPP-based polymer nanofibers.The specific researches are described as follows:1.3,3?-?m-phenylene?dianhydride?3,3?-RsDPA?monomer was synthesized by using 3-chlorophthalic anhydride and 1,3-benzenediol as raw materials,and the reaction time and temperature were optimized for the synthesis.Subsequently,a series of polyimide?PI?molding powders were prepared by the polymerization of the3,3?-RsDPA with 4,4?-?1,3-phenylenedioxy?dianiline?TPEQ?,1,3-diaminobenzene?MPD?,4,4?-oxydianiline?4,4?-ODA?,and 1,4-diaminobenzene?PDA?,respectively,followed by chemical imidization.During the polymerization,phthalic anhydride?PA?was used as the blocking agent.The thermal and mechanical properties of the synthesized polyimides were characterized.Results showed that the polyimides exhibit excellent thermal stabilities with the T_5%being 525⁵31? and 526⁵38?,respectively,under air and nitrogen atmosphere.With the increased rigidity of the diamine monomer?TPEQ<4,4?-ODA<MPD<PDA?,the T_g increased from218? to 261?.When the diamine monomer was PDA,the PI?3,3?-RsDPA-PDA?showed obvious melting crystallization behavior,and the T_m was 327?.The excellent thermal property and good processability enables the synthesized polyimide powders have potential application in 3D printing technology.2.Using 3,3?-RsDPA,3,3?,4,4?-biphenyl tetracarboxylic dianhydride?BPDA?and4,4?-oxydianiline?4,4?-ODA?as monomers,phthalic anhydride?PA?as blocking agent,a series of thermoplastic co-polyimides were synthesized through the polymerization with different dianhydride ratio followed by chemical imidization.The chemical structure and performance of final products were characterized by FT-IR spectroscopy,Thermo-gravimetric analysis?TGA?,differential scanning calorimeter?DSC?,and X-ray diffraction?XRD?.Meanwhile,the mechanical properties of the sample were investigated.Results showed that when the ratio of 3,3?-RsDPA and BPDA was 6:4,the obtained co-polyimide?Co-PI?has good performance,which has the T_g,T_m and T_5%of 252?,326?,and 546?,respectively.In addition,the tensile strength was up to 124±5.3 MPa,and the bending strength was 175±6.2 MPa.XRD analysis showed that the Co-PI has certain crystallization behavior.Moreover,the polyimide also can be printed into different shapes by 3D printing technology.Therefore,the as-synthesized polyimide can be used as high-performance plastics and composite substrates due to the excellent heat resistance,outstanding mechanical properties,and good process ability.3.High-performance fiber-reinforced polyimide homogeneity nanocomposites were fabricated by hot pressing aligned electrospun composite PI nanofiber belts,which prepared from binary co-electrospinning of two kinds of polyamic acids?PAAs?,a thermosetting BPDA-PDA?BP-PAA?and a thermoplastic HQDA-ODA?HO-PAA?,followed by thermal imidization.During the hot pressing process,the thermoplastic PI nanofibers were melted to form the polymer matrix,while the thermosetting PI nanofibers retained their fibrous structure served as the reinforcement.Due to the uniform dispersion and high orientation of nanofibers,as well as the good compatibility between the two PIs,the as-fabricated fiber-reinforced PI nanocomposites demonstrated excellent mechanical strength and modulus.Specifically,the PI nanocomposites?BP/HO-8/2?containing 80 wt.%of BP-PI nanofibers showed the highest tensile strength and modulus of 957±18 MPa and12.32±0.32 GPa,respectively,which were higher than both neat PIs.Comparing to the polymer matrix?i.e.,neat HO-PI?,the strength and modulus of BP/HO-8/2increase by 551%and 608%,respectively.In addition,these values also increased by 355%and 148%when comparing to the counterpart of solution-cast blend-PI?BP/HO-8/2?film;and increased by 245%and 396%when comparing to the corresponding as-electrospun composite PI nanofiber belt.Therefore,this method of co-electrospinning a thermoplastic polymer and a thermosetting polymer,followed by hot-pressing,should be an effective strategy for preparing high-performance fiber-reinforcing composites.4.Four soluble PPP-based oligomers of PBPA with varied molecular weights were synthesized via Ni?II?complex-catalyzed cross-coupling reaction of2,5-dichlorobenzophenone and 4-bromophthalic acid dimethyl ester;subsequently,the PBPY/PI blend nanofiber belts were prepared by the combination of electrospinning and molecular coupling assembly techniques followed by heat treatment.In specific,by using PBPA as assembling unit,DAB as coupling agent,and PAA as carrier polymer,PBPA/DAB/PAA precursor nanofiber belts were electrospun from the mixture solution of PBPA/DAB/PAA in DMAc;within these nanofibers,the PBPA and DAB molecules were coupling-assembled through ammonium bonds.Upon heat treatment,the PBPY/PI nanofiber belts were obtained.The chemical structure of PBPA and nanofibers were characterized by FT-IR and ¹H-NMR,the results show that the PBPY/PI nanofibers were prepared successfully wherein the use of PAA as carrier polymer for assisting electrospinning is crucial for successful preparation of the nanofibers.5.Since the heat-treatment temperature and the DAB amount had significant influences on the mechanical and thermal properties of final PBPY/PI nanofiber belts,they were investigated and then optimized,respectively.The results showed that the optimal molar ratio of PBPA/DAB was 1:0.7 at 330?,the PBPY/PI nanofibers belts exhibited superior thermal and mechanical properties;for example,the T_g values were up to 348?,the T_5%were higher than 500? in both air and nitrogen atmospheres,the mechanical strengths were as high as 360 MPa,and the storage moduli could be retained above 4 GPa at the temperature up to 280?.Furthermore,the PBPY/PI nanofiber belts also demonstrated excellent chemical resistance under various harsh environments such as being soaked in boiling water,as well as in 7.14M H₂SO₄ or 6 M KOH solutions at elevated temperatures,the tensile strength of the PBPY/PI nanofiber belts still maintain 80%.