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Molekulare und chemische uneinheitlichkeit von copolymeren aus trübungstitrationen von GPC‐eluaten

1977; Wiley; Volume: 178; Issue: 9 Linguagem: Inglês

10.1002/macp.1977.021780918

0025-116X

M. Hoffmann, Heinrich Urban,

Analytical chemistry methods development

Abstract The distributions of molecular weights and of chemical compositions of a binary linear copolymer may be determined by gel permeation chromatography (GPC) followed by turbidimetric titration of the eluates. Eluate fractions of 5,0ml are automatically collected and titrated within 7 min. The intensity R of the scattered light at a wavelength λ 0 =940nm is measured at an angle of 19° to the primary beam and corrected for the change of solution volume V during titration. The solubilities of chemically different molecules of a mixture are influenced somewhat but not substantially by the components of the mixture. During the titration the turbidity of the eluates from graft copolymers often increases with time discontinuously, the points of inflection indicating the precipitation of a new species of molecules. If the dependence of solubility on chemical composition and concentration is known for the particular elution volume, the chemical composition of each species of the titrated molecules may be evaluated from these points of inflection. The elution volume V E and the chemical composition are used for obtaining the molecular weight M from plots of M vs. V E for different chemical compositions. The concentration of a certain species of molecules in the eluate cannot be derived from R itself as has been tried up to now because the turbidity continues to increase after complete precipitation of the polymer. However the rate of coagulation is proportional to the concentration c of precipitated polymer. This leads to a nearly linear dependence of R · V / V 0 on log V with a slope porportional to c 2 . The velocity constants of this coagulation are nearly independent of molecular weight and chemical composition. Therefore the concentration of the first precipitated component of a mixture may be determined absolutely with a relative error of about ±4% at a concentration of 50 mg/l of the mixture. The molecules of a second component of the mixture precipitated at a later stage of the titration are quickly adsorbed by the particles of the first component. After this the enlarged particles coagulate with the coagulation constant of the second component. The velocity of coagulation is now proportional to the sum of the concentrations of the first and second components. The distribution of chemical compositions of the molecules in an eluate may therefore be derived from the increase of turbidity with volume at different points of the titration curve. By plotting such distribution perpendicularly over the curves representing constant elution volume in the M ‐composition‐plane one obtains a three‐dimensional surface resembling a mountain range which characterises completely the molecular and compositional distribution of a binary copolymer.