Effect of molecular weight distribution and branching on the viscosity of polyethylene melts

Abstract

The effects of molecular weight distribution and branching on the viscosity of polyethylene melts were studied by making mixtures of paraffin wax with 0 to 100% of various types of high and low density polyethylenes of known weight‐average molecular weights, M̄w and measuring their Newtonian melt viscosity at 150°C. The range of molecular weights from about 400 to 106 gave about a billionfold range in the viscosities (10−2 to 10+7 poises). The results showed that the melt viscosity did not vary with M̄w3.4, since samples with the same M̄w might differ by up to 100,000‐fold in viscosity. An analysis of the flow process in melts suggested that the viscosity‐average molecular weight M̄v might be more significant than the M̄w. Within the accuracy of our knowledge of the initial molecular weight, the viscosities of all mixtures having viscosities above about 10 poises fit the equation where w1 and w2 are the weight fractions, and M1 and M2 are the molecular weights of the paraffin and polyethylene, respectively, and M̄v is a viscosity‐average molecular weight of the mixture. Theory and experiment show that the value of α may be obtained without any knowledge of the initial molecular weight, from the slope of the curve for log η0 versus log concentration at small concentrations of wax. Polyethylenes with large amounts of long‐chain branching had low values of α (down to about 0.25), while polymers with relatively little long‐chain branching had values of α up to about 0.8. Where the comparisons could be made, the value of α determined from a polymer melt was surprisingly close (∼ +0.15) to the value of α in the intrinsic viscosity equation for the same polyethylene dissolved in paraffin wax.

Melt Rheology of Thermoplastic Starch

“…Data of Lai and Kokini (31) show that at constant temperature, moisture content, and shear rate, high amylose starch melts have a higher viscosity than waxy maize starch melts. Similar branching effects have been observed for polyethylene (32) and polystyrene (29) melts. The shear strength of expanded starch products decreases with increasing amylopectin content (33).…”

Rheology of polysaccharide systems

“…In the past, several approximation techniques have been suggested for determining H(!..) from dynamic measurements [295,296], as well as from steady state viscosity data [297,298], most of which involve taking derivatives of the rheological functions. In particular, the methods of obtaining relaxation spectrum from loss modulus data have been widely used.…”

Effect of molecular structure on polyethylene melt rheology. I. Low‐shear behavior

“…The resulting qa and IT*\ curves are shown in Figure 4. Again the agree- More commonly, the effect of LCB in various polymer systems has been studied coupled with changing molecular weight.2v4, 6,[42][43][44] In such studies it has been conflictingly reported that branched polymers always have lower 70)s than their linear analogs of the same molecular weight or that at higher molecular weights the branched polymers may have higher l o values than the linear analogs, and that plotting log 70 versus log ( g M ) , where g is the branching index, reduces the data for branched and linear polymers to a single straight line (which requires the above first reported result to be true since g 5 1). Figure 6 and are compared to the linear polyethylene result ( g = 1 for linear polymer).…”