High-temperature coupling of high-speed GPC with continuous viscometry. II. Ethylene–vinyl acetate copolymers
Abstract
An application of the high temperature coupling of a gel permeation chromatograph with a home-made continuous viscometric detector is described. It concerns the comprehensive characterization of ethylene–vinyl acetate copolymers. Suitable chromatographic conditions are chosen to enable a correct use of the universal calibration concept. Through analysis of poly(vinyl acetate) fractions and commercial polyethylene samples, a comparison is made with the results of classical measurements. Average molecular weights as well as intrinsic viscosity appear to be in good agreement within experimental error, which proves the system for the characterization of random ethylene–vinyl acetate copolymers. In attempting to obtain a reliable estimate of long chain branching frequency λ, a series of commercial samples has been selected, with the vinyl acetate weight fraction within the range 0–45%. As a rule, experimental viscosity law exhibits two parts, a straight line with a Mark–Houwink exponent 0.7 in the low molecular weight region and a curvature, well smoothed by a third-degree polynomial regression. Consequently, long chain branching does not appear before a limiting molecular weight of about 50,000. Beyond this limit, λ is 0.5 × 10−4, with no dependence on molecular weight, which resembles low density polyethylene.
