THE EFFECT OF PROCESSING ON RHEOLOGICAL AND MOLECULAR CHARACTERISTICSOF A LOW-DENSITY POLYETHYLENE

The viscoelastic responses of some molten polymers, and particularly o f low density polyethylene (LDPE), are known to vary with processing h istory. Reasons for the variations include the effects of shear histor y on morphological states of the polymer, or on its molecular weight p arameters. A typical low density polyethylene has been used to test th e shear-history dependence concept following a variety of processing s teps. The polymer was sheared in single-screw and twin-screw extruders , and in a high speed melter/mixer (Gelimat). Samples also were precip itated from very dilute solutions in trichlorobenzene and in p-xylene. GPC analyses showed that, in general, these procedures did not affect the various moments of molecular weight. An exception was the Gelimat -mixed sample, for which mild reductions in M(n) and M(w) were noted. In contrast, melt viscosity and elasticity readings, the former from l ow shear evaluations and the latter from extrudate swelling, were affe cted by the various procedures. A drop in melt viscosity and in elasti city was observed, being most pronounced for precipitated and twin-scr ew extruded versions of the LDPE. Reductions also were observed in the specimen sheared in the Gelimat instrument. Following conditioning at the test extrusion temperature (170-degrees-C), viscous and elastic r esponses tended to revert to those of the unsheared control sample, th e exception again being the sample sheared in the Gelimat melter/mixer . Of the various mechanisms proposed in the literature to account for transient property changes such as those reported, temporary changes i n the degree of chain entanglement appear the most satisfactory explan ation. Irreversible alterations in viscoelastic properties may be asso ciated with changes in molecular weights due to processing at high she ar.