Titration of tetraethylenepentamine (TEPA) and its phenyl isocyanate reaction products: A model correction factor for determination of TEPA grafted to carbon surfaces

Ex-PAN carbon fibers (high strength, Celion G30-500, 3K-HTA-7C-N501, BASF Structural Materials, Inc.) were oxidized in 70% nitric acid at 115 degrees C for various times and then reacted with tetraethylenepentamine (TEPA) at 190-200 degrees C, thereby grafting TEPA to the fiber through amide bonds. To determine the resulting quantity of surface amino groups present, HCl uptake experiments were conducted. However, charge-charge repulsion prevents complete protonation of all the amino groups at suitable pH values for accurate measurements. Therefore, model compounds were prepared by reacting TEPA with phenyl isocyanate (PhNCO). These compounds were used to model the protonation of amine sites in the TEPA grafted onto nitric-acid oxidized carbon fiber surfaces. TEPA and the model compounds were independently titrated with HCl solution to measure the fraction of amino groups in these compounds which are protonated at various pH values. This titration mimics the titration of TEPA amino groups which would be grafted onto oxidized carbon fiber surfaces. Protonation of TEPA was not complete at pH 1, and protonation of surface-bound TEPA was certainly not complete at pH 2.8, the lowest pH for accurate determination of basic functions on carbon fiber surfaces. Therefore, direct measurements of HCl uptake were lower than the true quantity of amino groups present in TEPA molecules. A correction procedure has been established, based on the titration results from the model TEPA/phenyl isocyanate reaction products, to correct the direct HCl uptake measurements on fiber-bound TEPA or TEPA in solution. (C) 1997 Elsevier Science Ltd.