MOLECULAR-WEIGHT CHARACTERIZATION OF SOLUBLE HIGH-PERFORMANCE POLYIMIDES .1. POLYMER-SOLVENT-STATIONARY PHASE INTERACTIONS IN SIZE-EXCLUSION CHROMATOGRAPHY

Soluble, fully cyclized m-amino phenyl acetylene terminated polyimides based on several anhydride/diamine monomers were prepared in N-methyl pyrrolidine (NMP) and cyclized by solution imidization to controlled m olecular weight. The polyimides and a polyamic acid precursor were suc cessfully analyzed by size exclusion chromatography (SEC) utilizing on line parallel coupled refractive index and differential viscometer det ectors. The calculated ($) over bar M(n) values were varied from 3,000 to 20,000 daltons. N-methylpyrrolidone (NMP), tetrahydrofuran (THF), and chloroform served as mobile phases for the cross-linked polystyren e gel packings. Normal retention behavior of the polyimides was observ ed in chloroform, THF, and NMP containing LiBr, or in NMP stirred over P2O5 before use. Values of Mark-Houwink-Sakurada exponents for narrow distribution linear polystyrene indicate that pure NMP and NMP with 0 .06 M LiBr are good solvents for polystyrene standards at 60 degrees C . In contrast, SEC behavior of polyimides in pure NMP leads to splitti ng of the peaks with the major portion observed to pass through the co lumns at the exclusion limit. In contrast to strong polymeric chain ex pansion of the polyamic acid in dilute solution, presumably due to a p olyelectrolyte effect, no increase of intrinsic viscosity of polyimide samples in pure NMP was observed. This exclusion effect of polyimides analyzed in NMP is discussed in terms of possible ion-exclusion from pores of the stationary phase. Differences in polystyrene calibration in NMP with of without additives and the temperature dependence of cal ibration curves in these mobile phases is discussed as well. (C) 1995 John Wiley & Sons, Inc.