Analysis by size exclusion chromatography of the graft terpolymer present in MBS

Abstract

The molar mass distribution (MMD) and branching distribution (BD) of the graft terpolymer present in MBS (methyl methacrylate-butadiene-styrene, MMA-B-St) were estimated by size exclusion chromatography (SEC) with a series of three detectors: a UV photometer, a specific viscometer (SV), and a differential refractometer (DR). A solvent extraction procedure was employed to isolate the main MBS constituents: unreacted polybutadiene (PB), a PMMA-PS free copolymer (FC), and the PB-g-(PMMA-PS) graft terpolymer (GT). The FC was analyzed by UV + DR, yielding an almost uniform St mass fraction. The analysis of the GT involved the following assumptions: (i) the composition of the grafted branches coincides with that of the FC; (ii) the volumetric contractions of the molecules are represented by the Zimm–Stockmayer equations for randomly branched homopolymers; (iii) the low molar mass fraction exhibits a single branch per molecule; and (iv) the Mark–Houwink–Sakurada (MHS) parameters of the hypothetical homologous linear terpolymer are estimated by weighting the MHS constants of the PB and FC according to their mass fractions in the GT. The data treatment employed also enabled the branching exponent (e) that relates the geometric and hydrodynamic contraction factors to be estimated, yielding e ≈ 1.45.