Chain Conformation and Local Rigidity of Isomerized Polyimides in Dimethyl Formamide by Size Exclusion Chromatography Coupled with Multi-Detectors

Chain conformation and local rigidity of two isomerized polyimides (PIs), poly(6FDA/3,3′-DMB) and poly(6FDA/2,2′-DMB) in dimethyl formamide (DMF) with either 0.1 M LiBr or 3.1 mM tetrabutylammonium bromide at 35 °C, are investigated. Size exclusion chromatography (SEC) coupled with multi-angle laser light scattering detector, viscometer, and differential refractive index detector was used. The scaling exponents α and ν related to conformation are estimated from the above results. The values of α and ν for poly(6FDA/3,3′-DMB) are 0.68 ± 0.01 and 0.54–0.55 ± 0.01, respectively. The values of α and ν for poly(6FDA/2,2′-DMB) are 0.65 ± 0.02 and 0.55 ± 0.01, respectively, which indicate that both PIs in DMF have a random coil conformation. In particular, poly(6FDA/3,3′-DMB) exhibits more extended conformation than that of poly(6FDA/2,2′-DMB). Parameters related to chain flexibility of polymers, including persistence length l _p, shift factor M _L (relative molecular weight per unit contour length), and backbone diameter d are evaluated from the relationship between intrinsic viscosity and molecular weight based on the wormlike continuous cylinder model. The three parameters (l _p, M _L, and d) indicate that the two samples are flexible chains with local rigidity, and poly(6FDA/3,3′-DMB) is slightly stiffer than poly(6FDA/2,2′-DMB). In addition, influence of salt types on the parameters is also discussed.