'ISOMERIC EFFECT' ON THE CRYSTALLIZATION BEHAVIOR OF POLYIMIDE FROM ODPA AND ODA

Abstract

The isomeric effects of dianhydride on the crystalline properties of polyimide were reported,it was a continuation of isomeric polyimides studies in our group.A series of molecular-weight-controlled isomeric polyimides powders based on three dianhydride isomers,3,3′,4,4′-oxydiphthalic dianhydride(4,4′-ODPA),2,3′,3,4′-oxydiphthalic dianhydride(3,4′-ODPA) and 2,2′,3,3′oxydiphthalic dianhydride(3,3′-ODPA),and 4,4′-oxydianiline(ODA) were prepared by traditional two-step method in DMAc,and phthalic anhydride(PA) was used as end-capper.Wide-angle X-ray diffraction(WAXD) and differential scanning calorimetry(DSC) were used to investigate the crystallization behavior of isomeric polyimides.The annealing temperature,annealing time,external force shearing and molecular weight effects on the crystallization of isomeric polyimides were studied.The results proved that 4,4′-ODPA/ODA-based polyimide powder was amorphous at room temperature and could crystallize during annealing or shearing process above T_g.The crystallization of(4,4′-ODPA/ODA-based) polyimide was enhanced by increasing the annealing temperature and time,decreasing molecular weight or pushing shearing above T_g.Two melting endo-peaks appeared evidently when the annealing temperature was below 300℃.The two melting peaks tended to be close to each other with the increase of annealing temperature,and only one melting peak was found when annealing temperature reached 370℃.The higher melting temperature of(4,4′-ODPA/ODA-based) polyimide was aroud 370～380℃,however,this polyimide was difficult to recrystallize once melted.This polyimide was annealed at 370℃ for different times,the amorphous as-prepared samples had crystallized after annealing at 370℃ only in 1 min,besides the crystallinity was increased with the increase of the annealing time,but no crystalline form change was found.The polyimide with low molecular weight was easy to crystallize.It can be seen that the polyimide with η_(inh) of 0.34 dL/g showed the highest endo-peak,the polyimide with the η_(inh) of 0.64dL/g showed the smallest endo-peak,while the polyimide with the η_(inh) of 1.1 dL/g exhibited completely amorphous.Pushing shearing at 250,260 and 300℃ for 100 min with a shear rate of 2 s~(-1) indicated that the shearing process indeed enhanced the crystallization of polyimide above 260℃,although the effect was not so obvious.While at 250℃ or lower temperatures the shearing almost did not effect at all.The isomeric polyimides based on 3,4′-ODPA/ODA and 3,3′-ODPA/ODA were synthesized with the comparable inherent viscosity with 4,4′-ODPA/ODA.They were tested after the same treatment as we had did for 4,4′-ODPA/ODA,such as different annealing temperature,annealing time,shearing,etc.,however,the two isomeric polyimides just as expected remained amorphous.