Composition and molecular weight analysis of styrene-acrylic copolymers using thermal field-flow fractionation

Thermal field-flow fractionation coupled with online multiangle light scattering, differential refractive index and quasielastic light scattering (ThFFF-MALS/dRI/QELS) was used to simultaneously determine the molecular weight (MW) and composition of polystyrene-poly(n-butyl acrylate) (PS-PBA) and polystyrene-poly(methyl acrylate) (PS-PMA) copolymers. The online measurement of the normal diffusion coefficient (D) by QELS allowed calculation of the copolymer thermal diffusion coefficient (D_T) of sample components as they eluted from the ThFFF channel. D_T was found to be independent of MW for copolymers with similar compositions and dependent on composition for copolymers with similar MW in a non-selective solvent. By using a solvent that is non-selective to both blocks of the copolymer, it was possible to establish a universal calibration plot of D_T versus mole fraction of one of the monomer chemistries comprising the copolymer. PS-PBA and PS-PMA linear diblock polymers were determined to vary in composition from 100/0 to 20/80wt% PS/acrylate and ranged in MWs between 30 and 360kDa. The analysis of a PS-PBA miktoarm star copolymer revealed a polydisperse material with a weight percent PBA of 50-75% and MW ranging from 100 to 900kDa. The presented ThFFF-MALS/dRI/QELS method allowed rapid characterization of polymers with MW and chemical distributions in a single analysis.