Flow behavior of well characterized polyethylene melts

Abstract

A careful characterization and rheological study of low density polyethylene (LDPE) reveals that long‐chain branching (LCB) plays a decisive role. At constant molecular weight (M̄w) higher LCB reduces the Newtonian viscosity ηo and the shear sensitivity, increases the activation energy Eo, and finally delays transition to pseudoplastic flow to higher shear rates and the onset of melt fracture to higher shear stresses (τd). The flow parameters ηo, \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma _{cr} $\end{document}cr, τd, and the derived flow relaxation times are uniquely correlatable by means of a modified molecular weight (gM̄w) incorporating the LCB effect. High density polyethylene are less shear sensitive than their low‐density counterparts, have a lower activation energy, fracture at higher shear stress levels and cannot be regarded as branchless LDPE's.

Linear and branched poly(butyleneterephthalate): Activation energy for melt flow

“…Neither the length of branches, )~rn, b, nor the degrees of branching/~, or/~w, or other parameters characterizing the branched structures seem to correlate in a clear way to E~,0. Nevertheless all the E~,0 values for branched samples are within the range 60-80 kJ/mol well above that for linear PBTP and are in agreement with the values of E~,0 reported by other authors [4][5][6][7].…”

“…Some other authors reported an increase in the flow activation energy with branching [5][6][7] for low-density polyethylene. For this polymer is usually assumed a randomly branched structure defined in terms of number of long and short branches per molecule as it is almost impossible to obtain reliable data about the distribution of the number and the length of the branches.…”

Prediction of rheological properties of well‐characterized branched polyethylenes from the distribution of molecular weight and long chain branches

“…A, = kl7 (7) where k is a constant to be determined from experimental values of J&). Experience with our own data of branched polyethylene, however, showed that the elastic properties were better predicted by assuming:…”

Structure and Rheological Function of Side Branches of Carbohydrate Polymers

“…Short branches generally do not affect the viscosity of polymer melts, rather influencing the morphology and solid-state properties of semicrystalline polymers, whereas long branches can have a remarkable effect on solution viscosity and melt rheology (Nielsen 1977;Nordmeier et al 1990). Branches which are long, but which are still shorter than those required for entanglements decrease the viscosity when compared to a linear polymer of the same molecular weight (Miltz and Ram 1973;Utracki and Roovers 1973). This is because the polymer molecules containing such branches are more compact than the linear molecules of the same molecular weight, leading to smaller molecular size.…”