DETERMINATION OF LONG-CHAIN BRANCHING DISTRIBUTIONS OF POLYETHYLENES

The long-chain branching distribution (LCBD) of polyethylene was determined from measurements obtained from an on-line viscometer detector (VD) combined with size exclusion chromatography (SEC). The SEC was calibrated according to the universal calibration method, and the VD output was used along with the concentration measurement obtained from the refractive index detector of the SEC to determine the intrinsic viscosity distribution of the eluting polymers. The SEC and VD data were then combined to yield the molecular-weight distribution (MWD) and LCBD of a variety of polyethylene resins. The SEC-VD technique was shown to be useful for determining the LCBD across the entire MWD, except at very low MW (<3 x 10(4)) because of the insensitivity of the VD to low MW species. The LCBD of a large variety of commercial low-density polyethylene resins was constant across the MWD. This finding was confirmed by C-13 NMR spectroscopic measurement of the average LCB of narrow-MW fractions of one typical LDPE.