Effect of Copolymerization Structure and Molecular Weight on Melt Fluidity and Thermal Properties of Thermoplastic Polyimide Resins

A series of molecular-weight controlled thermoplastic polyimide(TPI) resins was synthesized based on 4,4'-(hexafluoroisopropylidene)diphthalic anhydride/ p-phenylenediamine(6FDA/ p-PDA) endcapped with phthalic anhydride(PA),in which the aromatic diamides with rigid backbone and large free volume, i. e., m-phenylenediamine m-PDA),2,2'-bis(trifluoromethyl)benzidine(TFDB)and 9,9-bis (4-aminophenyl)fluorene(BAFL),were incorporated respectively. The effects of copolymerization structure and molecular weight on the melt fluidity and thermal properties of TPI resins were investigated. The correla. tion between the aggregation structures of TPI resins and their melting performance was constructed. The me. chanical properties of TPI resins were evaluated at ambient and elevated temperatures. The results indicated that incorporation of aromatic diamines with large free volume as copolymerization structure can effectively re. duce the molecular chain stacking density and increase the free volume of polymer,as a result,providing the resin good melting performance. The melt processability of resins can be further improved by reducing their de. signed molecular weight. These TPI resins with rigid backbone have excellent thermal stability and mechanical properties. The glass transition temperatures of these resins were in the range of 308-338 oC. TPI-C-25k and TPI-D-25k resins with the copolymerization structure of TFDB and BAFL,respectively,revealed high strength and toughness. They gave the tensile strength and flexural strength exceeding 120 MPa and 190 MPa,respec. tively,and showed the elongation at break over 8.2%. These TPI resins also exhibited good thermal stability at 250..