ELONGATIONAL FLOW AND BIREFRINGENCE OF LOW-DENSITY POLYETHYLENE AND ITS BLENDS WITH ULTRAHIGH MOLECULAR-WEIGHT POLYETHYLENE

Via elongational flow opto-rheometry (EFOR), simultaneous measurements of tensile stress sigma(t) and birefringence Delta n(t) were conducte d on a low density polyethylene (LDPE) melt and its blends with an ult ra-high molecular weight polyethylene (UHMWPE) at 140 degrees C under transient elongational flow with constant tensile strain rate (epsilon ) over dot(0). The transient elongational viscosity eta(E)(t) = sigma( t)/(epsilon) over dot(0) of LDPE melt first gradually increases with t ime t following the linear viscoelasticity rule in that eta(E)(t) is 3 times the shear viscosity development, 3 eta(t), at low shear rate (g amma) over dot up to a certain critical strain, beyond which eta(E)(t) tended to increase rapidly with t. The behaviour was often referred t o as strain-induced hardening. For LDPE melt both sigma(t) and Delta n (t) versus tensile strain epsilon(t) (= (epsilon) over dot(0)t) curves were dependent on (epsilon) over dot(0) in such a manner that the str ess optical coefficient C(t) (drop Delta n(t)/sigma(t)) was independen t either of (epsilon) over dot(0), epsilon(t) or sigma(t). Addition of UHMWPE up to 10 wt% to LDPE melt increased the levels of both sigma(t ) and Delta n(t), but the tendency of strain-induced hardening was red uced. The C(t) was again independent either of (epsilon) over dot(0), epsilon(t) or sigma(t) and also essentially independent of molecular w eight (MW) and its distribution (MWD) or the blend ratio. For both LDP E and the blends the C(t) value roughly agreed with that (= 2.2 x 10(- 9)Pa(-1)) reported for shear flow experiments, thus confirming the val idity of the so far established stress optical rule. (C) 1998 Elsevier Science Ltd. All rights reserved.