Temperature Dependence of Chain Conformation and Local Rigidity of Isomerized Polyimides in Dimethyl Formamide

Temperature dependence of chain conformation and local rigidity of two soluble isomerized polyimides (PIs), poly(hexafluorodianhydride/3,3'-dimethylbenzidine)[poly(6FDA/3,3'-DMB)] and poly(hexafluorodianhy-dride/2,2'-dimethylbenzidine)[poly(6FDA/2,2'-DMB)] were investigated by dilute solution viscosity, size exclusion chromatography(SEC) coupled with multi-angle laser light scattering, viscometer, and refractive index detector in dimethylformamide(DMF) with either 0.1 mol/L LiBr or 3.1 mmol/L tetrabutylammonium bromide(TBAB) in the temperature range of 30 to 50 ℃. The scaling relationships of[η]=K_ηM^α and R_g=K_gM^v obtained are employed to investigate the temperature dependence of chain conformation for the two polyimides. The values of α and v are in the range of 0.66―0.69 and 0.53―0.56, respectively, for poly(6FDA/3,3'-DMB), meanwhile they are in the range of 0.64―0.68 and 0.53―0.56, respectively, for poly(6FDA/2,2'-DMB). These results reveal that random coil conformations for both PIs are not affected visibly with increasing temperature from 30 ℃ to 50 ℃. However, values of more exact intrinsic viscosity from dilute solution measurement indicate there is only tiny coil extension or shrinkage for both PIs with temperature rising. Parameters related to chain flexibility of polymer, including persistence length l_p, shift factor M_L(relative molecular weight per unit contour length) and backbone diameter d are estimated from the relationship between intrinsic viscosity and molecular weight for the continuous wormlike cylinder model, which indicates that two samples are flexible chains, only the chain of poly(6FDA/3,3'-DMB) is stiffer than that of poly(6FDA/2,2'-DMB) slightly.