THERMAL-DIFFUSION AND MOLECULAR-WEIGHT CALIBRATION OF POLY(ETHYLENE-CO-VINYL ACETATE)S BY THERMAL FIELD-FLOW FRACTIONATION

Copolymer characterization is accomplished with respect to measurement of thermal diffusion coefficient (D-T) and molecular weight determina tion by thermal field-flow fractionation. The examined copolymers are the eight poly(ethylene-co-vinyl acetate)s [P(E-V)] having different c ompositions of vinyl acetate ranging from 25 to 70% and the molecular weight from 110,000 to 285,000, and three polyvinyl acetate standards as component homopolymer. The carrier solvents are tetrahydrofuran, to luene, and chlorobenzene which have different viscosities and thermal conductivities. Measured D-T Values vary from 1.36 X 10(-8) to 5.97 X 10(-8) cm(2)/(s.K) which are dependent on the composition of copolymer s and types of carriers. These values increase linearly with the incre ase of weight percent of vinyl acetate. It is possible to estimate D-T values of polyethylene from the extrapolated intercept in the plots o f D-T vs. Vinyl acetate wt % of copolymer. Tetrahydrofuran is found to be the appropriate carrier solvent for the separation of P(E-V) copol ymers since D-T varies greatly with the increase of wt % in THF. Attem pts are made to correlate the measured retention data with molecular s izes of copolymers for the construction of the molecular weight calibr ation curve, Good correlations (r(2) greater than or equal to 0.931) a re found in which D/D-T values of polymers vary inversely with the pro duct of hydrodynamic volume by weight ratio of vinyl acetate. Based on this relationship, the unknown molecular weight of copolymer sample c an be determined from component homopolymers for which standards are r eadily available. (C) 1995 John Wiley & Sons, Inc.