ON THE CONTRIBUTION OF LONG CHAIN BRANCHING TO POLYETHYLENE MELT RHEOLOGY

The sensitivity of rheological behavior to the presence of long chain branched (LCB) molecules in a matrix of ostensibly linear high density polyethylene (HOPE) is examined. Branching is introduced by gamma irradiation of the HOPE just short of the gel dose. The irradiated and unirradiated polymers are blended to obtain various concentrations of LCB from 1 to 7.2 LCB/ 10,000 carbon atoms. Dramatic changes in viscoelasticity and temperature dependencies far in excess of those expected from molecular weight changes are detected. While the (Mw) over bar observed by low angle laser light scattering (LALLS) and size exclusion chromatography (SEC) increases by a factor of only about 1.2, the dynamic viscosity at low frequencies and 190 degrees C increases by a factor of 14. Furthermore, flow activation energy values increase from 8 to 40 kcal/mole.