Synthesis And Characterization Of Melt-processable Semi-crystalline Polyimides Based On 1,4-bis(3,4-dicarboxyphenoxy) Benzene Dianhydride

Semicrystalline polyimide is one of the most important research areas of polyimide due to its excellent heat resistance,melt-processability and high-temperature mechanical properties.However,large scale industrial applications of semicrystalline polyimide are restricted because of its poor recrystallization ability from melt,slow melt crystallization rate or high melting point.Therefore,it needs molecular design to develop novel melt-processable semicrystalline polyimide to solve these problems.Two flexible ether linkages of 1,4-bis(3,4-dicarboxyphenoxy)benzene dianhydride(HQDPA)could improve the mobility of molecular chains.It makes HQDPA based polyimides have excellent melt crystallization ability and melt-processability.In this thesis,a series of semicrystalline homo-and co-polyimides derived from HQDPA were synthesized.Their thermal stability,crystallization properties and melt-processability were systemically investigated to find the relationship between their structures and properties.The main contents and conclusions are listed as below:(1)Semicrystalline homopolyimides derived from HQDPA.The influencing facts of semicrystalline polyimides derived from HQDPA were investigated from chemical structure,molecular weight and polymerization method.A series of semicrystalline homopolyimides were synthesized from HQDPA with six different ether-containing aromatic diamines.The studies shown that ether-containing aromatic diamines are most favorable for the formation of melt-processable semicrystalline polyimides;with the increase of molecular weight,the melting points of polyimides are gradually increased,and the original double melting behavior becomes a single melting peak;the polymerization method would have impacts on the crystallinity of polyimide.The melt crystallinity of polyimide synthesized by thermal imidization decreased obviously,while the melt crystallinity of polyimides obtained by chemical imidization,solution imidization and one pot method are similar.It was found that polyimides based on HQDPA/TPER(T_m=331 ~oC and 350~oC)and HQDPA/TPEQ(T_m=387 ~oC)have excellent recrystallization ability from melt,fast crystallization rate and good melt processability.These semicrystalline polyimides might become a new generation of high performance engineering plastics in the future.(2)Semicrystalline copolyimides derived from HQDPA with other diamines.A series of semicrystalline copolyimides were synthesized from1,3-Bis(4-aminophenoxy)benzene(TPER)and 1,4-bis(4-aminophenoxy)benzene(TPEQ),two isometric diamines,with HQDPA.It was found that the copolymers exhibited a unique double melting behavior,with the increase of the content of TPEQ,the double melting points and melt viscosity of the copolymers increased gradually,the thermal stability increased slightly.The introduction of a small amount of diamine into HQDPA/TPER would reduce the melting point of polymer.When the content of co-diamine is 50%,the more rigid of diamine is,the more obvious the melting points of the copolymers increase.(3)Semicrystalline copolyimides derived from HQDPA with other dianhydrides.Two series of molecularly controlled copolyimides were prepared by introducing3,3',4,4'-biphenyltetracarboxylic dianhydride(BPDA)and pyromellitic dianhydride(PMDA)into HQDPA/TPER homopolyimide,respectively.It was found that with the increasing of the PMDA content in copolymers,both of the T_ms and T_gs of copolymers increase,while thermal stabilities decreased slightly,the melt crystallinity decreased obviously.It was found that with the increasing of the BPDA content,the thermal stabilities of copolymers were improved,and the T_ms decreased first then increased,while the melt fluidity become poorer.By introducing appropriate proportion of BPDA,it can achieve a semicrystalline polyimide with excellent melt crystallinity and melt flowability,as well as high melting point.When the content of BPDA in the copolyimde is 85%,the polymer has high heat resistance(T_m=367°C),excellent melt repeatability and fast melt crystallization rate,good melt flowability with a wide melt processing window at the same time.These properties might make it a novel of high performance semicrystalline engineering plastics in the future.(4)Semicrystalline engineering plastics of copolyimide based on HQDPA and BPDA.One of the copolyimides based on HQDPA and BPDA with excellent comprehensive performance was focused.The feasibility of using this copolymer as engineering plastic was studied.It was found that the melt-processed samples have excellent themal stability,melt recrystallization ability,mechanical properties,solvent resistance,flame retardancy and friction properties etc.,which could be an attractive candidate for the applications of high-performance engineering plastics and composite materials.