Molecular weight characterization of soluble high performance polyimides. 2. Validity of universal SEC calibration and absolute molecular weight calculation

Six different soluble high-performance aromatic polyimides, each prepared by solution imidization to three controlled average molecular weights, were analyzed by size exclusion chromatography (SEC) using on-line parallel coupled refractometric and viscometric detectors. N-methylpyrrolidone (NMP) with 0.06 M LiBr and NMP stirred over P₂O₅ were used as mobile phase for four of the polyimides; NMP with 0.06 M LiBr and NMP stirred over P₂O₅ were used as mobile phases for four of the polyimides; NMP with 0.06 M LiBr tetrahydrofuran (THF) and chloroform served as mobile phases for the other two polyimides. For all the samples the stationary phase in the SEC columns was cross-linked polystyrene beads. Molecular weight averages of the polyimides were calculated using universal SEC calibration with polystyrene standards in each solvent. The agreement of the calculated molecular weight averages in the different solvents confirms that the universal SEC calibrations are valid for these semiflexible polymers. There was good agreement with weightaverage molecular weights obtained by low-angle laser light scattering (LALLS) performed in pure NMP. Intrinsic viscosity and molecular weight data for a series of nine samples of one polyimide covering a M_w = 20,000–70,000 g mol^–1 interval were treated to obtain Mark-Houwink-Sakurada constants. Unperturbed chain dimensions of this polyimide were obtained by application of the Stockmayer-Fixman extrapolation procedure to these data. ©1995 John Wiley & Sons, Inc.