Investigation of the Solution Behavior of Organosoluble Aromatic Polyimides

Abstract
Two high-molecular-weight organosoluble aromatic polyimides having the same diamine, 2,2′-(trifluoromethyl)-4,4′-diaminobiphenyl diamine (PFMB), but different dianhydrides, 2,2′-bis(trifluoromethyl)-4,4′,5,5′-biphenyltetracarboxylic dianhydride (HFBPDA) and 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), were prepared via one-step polycondensation reactions. Molecular weight fractionation of these two polymers was carried out and the polyimide fractions possess successively lower weight-average molecular weights. Intrinsic viscosity data from both sets of polyimide fractions gave the Mark-Houwink-Sakurada constants of these two polyimides, which reflect differing chain rigidity in tetrahydrofuran (THF) solutions. The MHS relations are [η] = 5.24 × 10−4 M0.65 and [η] = 3.77 × 10−5 M0.97 for 6FDA-PFMB and HFBPDA-PFMB, respectively. The physical meanings of these constants are discussed using the intrinsic viscosity data of these two polyimides in varying solvents. The persistence lengths are calculated based on the viscosity and molecular weight results using Bohdanecky' s approach and they were calculated to be 6.6 and 2.0 nm for HFBPDA-PFMB and 6FDA-PFMB, respectively. The experimental results indicate that the fully aromatic HFPBDA-PFMB is more rigid in THF solutions compared with 6FDA-PFMB. However, both of these polyimides are far from possessing true rigid-rod character. Plots of the weight-average molecular weight against the elution volume depict the differences in chain stiffness, and hence in hydrodynamic volume, between samples. Examination of the hydrodynamic volume of these polymers provides evidence that they both subscribe rather well to hydrodynamic theory at high molecular weights but that the stiffer HFBPDA-PFMB shows deviation at lower values.

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