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 prepa red by solution imidization to three controlled average molecular weig hts, were analyzed by size exclusion chromatography (SEC) using on-lin e parallel coupled refractometric and Viscometric detectors. N-methylp yrrolidone (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 N MP stirred over P2O5 were used as mobile phases for four of the polyim ides; NMP with 0.06 M LiBr tetrahydrofuran (THF) and chloroform served as mobile phases for the other two polyimides. For all the samples th e stationary phase in the SEC columns was cross-linked polystyrene bea ds. 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 diffe rent solvents confirms that the universal SEC calibrations are Valid f or these semiflexible polymers. There was good agreement with weight-a verage molecular weights obtained by low-angle laser light scattering (LALLS) performed in pure NMP. Intrinsic viscosity and molecular weigh t 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 wer e obtained by application of the Stockmayer-Fixman extrapolation proce dure to these data. (C) 1995 John Wiley & Sons, Inc.