GPC Measurement Of Molecular Weight Of Poly (Amic-Acids) And Determination Of Mark-Houwink Constants

Content: Poly(amic-acids )(PAA), polyimide (PI) precursors,whose molecular weight data is very important to study thermal stability , chemical stability, mechanical and electrical properties of polyimide, and is closely related with the use characteristics and processability of polyimide materials. Gel permeation chromatography (GPC) is column chromatography with solvent as mobile phase, porous filling materials as separation medium. The monodisperse polystyrene (PS) with known molecular weight is used as standard, by which the relative molecular weight of polymer is calculated based on comparing the retention time of the polymer in separation column or the elution volume of leaching process with those of the standard polystyrenes. As a fast method of determining polymers'relative molecular weight, gel permeation chromatography (GPC) has characteristics of simple operation, short determining period, reliable data and good reproducibility. Therefore, GPC should be a good method for determining relative molecular weight of poly(amic-acids).This thesis mainly consists of the following three parts:1,Seven series of poly(amic-acids) with different molecular structure and intrinsic viscosity, were synthesized from 3,4,3',4'-biphenyltetracarboxylic dianhydride (BPDA), 4,4'-oxydiphthalic anhydride(ODPA), pyromellitic dianhydride (PMDA) and p-phenylenediamine(PDA), 4,4'-diaminodiphenyl-ether(ODA), 4,4'-biphenyldiamine (BPA) by adjusting the molar ratio of anhydride and diamine at -5℃.2,In GPC mobile phase DMF , poly(amic-acids) molecules are to some extent polyelectrolytes that affect elution time of sample and result in a poor reproducibility of GPC graph. Poly(amic-acids) more often deposit in the gel column which resulted in high column pressure and result in a high column pressure which may destroy the gel column. So low molecular weight electrolyte phosphoric acid(H₃PO₄)was added in mobile phase DMF to shield polyelectrolyte effect of poly(amic-acids). The calibration curve of molecular weight corresponding to elution volume was established by using monodisperse polystyrene(PS) with known molecular weight as standards. Taking poly(biphylene biphenyltetracarboxamic acid) with intrinsic viscosity 4.80 dl/g for example, the effect of concentration of H₃PO₄ or H₃PO₄ + anhydrous LiBr added in DMF on elution time, number-average molecular weight and weight-average molecular weight were discussed . The GPC operational condition for measuring the relative molecular weight of poly(amic-acids) was formed including mobile phase, DMF + 0.02 mol/L H₃PO₄; sample concentration, 2.0mg/mL; and injection volume,100μL.3,Seven series of poly(amic-acids) with different molecular structure and intrinsic viscosity were tested by using GPC the above-mentioned operational condition. The elution time of poly(amic-acids) shortened as the intrinsic viscosity increased, and the corresponding number-average molecular weight and weight-average molecular weight increased as the intrinsic viscosity increased. The constants of K andαin the Mark-Houwink equation [η]=KMαwere determined by measurements of [η] and molecular weight MGPC and by plots of lg[η] vs lg[M].