LIGHT-SCATTERING AND SIZE EXCLUSION CHROMATOGRAPHY OF SEMIFLEXIBLE POLYIMIDE PRECURSORS

Measurements of the molecular weight, radius, and molecular weight distribution of polyimide precursors (amic acids and esters) is quite difficult to strong polymer-solvent interactions, electrostatic interactions, and the possibility of partial imidization in solution. The large quantities of solvent and high dilutions necessary in liquid chromatography make this technique the most sensitive to these effects. The distortions in the chromatogram from these effects can make assessment of the molecular weight distribution from a polystyrene calibration very tenuous. Use of a light-scattering detector permits direct measurement of the scattering intensity (and thus the molecular weight), as the sample elutes from the column. For the esters, anomalous peaks and unexpectedly large apparent values for both the molecular weight and molecular weight distribution are shown to be due primarily to partial imidization in solution catalyzed by basic solvent impurities. Chromatographic results obtained in neutralized solvent are in agreement with light-scattering results obtained at higher concentrations.