Synthesis And Characterization Of High Performance Polyimides Incorporated With Heterocyclic Benzimidazole And Benzoxazole Units

Aromatic polyimides(PIs)are a class of high performance polymer materials,and have shown important applications in the field of aerospace industry and micro-electronics,aviation due to their exceptional array of properties such as excellent thermal,electrical,and mechanical stability,and,good chemical resistance.However,to meet the increasing demands of high-performance materials in the aerospace and electronics industries,there is a need to produce polyimide resins amenable to processing and cost-effective that can withstand at high temperatures over a prolonged time.Polybenzazole family as heterocyclic high performance polymers particularly polybenzimidazole and polybenzoxazole,has attracted much research interest because of superior mechanical properties,outstanding thermal and oxidative stability.However,the major drawback of aforementioned polybenzazoles as well as polyimides,is their extreme processing conditions owing to the poor solubility and infusibility on account of their structural regularity and rigid backbones.Therefore,main objective of the present research was to design and synthesize processable high performance polymers e.g.poly(benzimidazole-imide)s and poly(benzoxazole-imide)s.Adjusted degree of structural modification in monomers or polymers is believed to be an effective approach to optimize the balance of properties.Therefore,in this study the polymers based on phthalimide and benzazole units(i.e.benzimidazole and benzoxazole)are designed to integrate their excellent thermal and mechanical properties as well as structural modification approaches facilitating to induce the irregularity or inhibited chain packing in polymer structure,are adapted to enhance their processability.Firstly,two series of end-capped homo-and co-polyimides based on 3,3'-HQDPA were prepared,by varying the ratio of rigid 2-(4-aminophenyl)-5-aminobenzimidazole(BIA)and flexible 4,4'-oxydianiline(ODA)through one step polycondensation,aiming to extend the upper operating temperature range of melt processable PIs.The resulting copolyimides demonstrated excellent properties,such as good solubility,high Tg ranging from 354 to 328 0C and low melt viscosity in the range of 291 Pa-s to 935 Pa-s.Afterwards,this system was modified by copolymerization with different feed ratio of 4,4,-HQDPA isomer and produced another series of poly(benzimidazole-polyimide)s,that exhibited high Tg in the range of 315-330 0C with T5%of 502-543 0C and minimum complex viscosity in the range of 291-479 Pa·s.In the second part of work,two novel asymmetric diamine isomers,6-amino-2-(4-(4-aminophenoxy)phenyl)-1H-benzoimidazol and 6-amino-2-(3-(4-aminophenoxy)phenyl)-1H-benzoimidazol were synthesized and employed to react with several aromatic dianhydrides to yield two new series of poly(benzimidazole-imide)s(PBIPIs)through two-step and one step polymerization method,respectively.The effect of chain isomerism on molecular packing and physical properties of PBIPIs are investigated.Both series exhibited high Tg values(276-383 oC)with excellent thermal and mechanical properties.These asymmetric polyimides demonstrated improved solubility attributed to the flexible ether connecting groups with non-coplanarity.Furthermore,controlled molecular weight(Mw)PBIPIs derived from 3m demonstrated good solution and melt processability.In the third part,two asymmetric diamine monomers,incorporating aryl ether and non-liner benzoxazole group,5-amino-2-(4-(4-aminophenoxy)phenyl)benzoxazole(p-BOODA)and 5-amino-2-(3-(4-aminophenoxy)phenyl)benzoxazole(m-BOODA),were designed and synthesized.Two novel series of poly(benzoxazole-imide)s(PBOI)were prepared from these diamines via thermal imidization of commercial aromatic dianhydrides.The effect of chain isomerism on molecular packing and physical properties of PBOIs is investigated in detail.These PBOIs showed excellent mechanical properties with good thermal stability(T5%= 488-570 0C)and high glass transition temperatures(Tgs)up to 333 0C,The incorporation of asymmetric benzoxazole moiety with flexible ether connecting group,improved the solubility of PBOI system.Furthermore,controlled molecular weight(Mw)polymer derived from,m-BOODA,demonstrated excellent solution and melt processability with lowest complex viscosity of 44 Pa·s at 380?:Finally,a facile synthesis method,aromatic nucleophilic displacement was applied to synthesize poly(N-arylene benzimidazole ether imide)copolymers.So,bis-fluorophthalimide monomer was prepared and reacted with different mole ratio of bis-benzimidazole and bisphenol-A.The resulting copolymers demonstrated moderate to good solubility in polar aprotic solvents and high glass transition temperatures(Tgs)of 228-336 0C,good thermal stability with 10%weight loss temperatures in the range of 503-548 0C.All copolymer films showed good mechanical properties with tensile strengths of 101-130 MPa,tensile moduli of 3.3-4.0 GPa,elongations at break of 4.7-6.9%,and also presented low water uptake(0.15-0.34%).