Effects of molecular weight distribution and branching on rheological parameters of polyethylene melts. Part I. Unfractionated polymers

Abstract

synopsisIt is well known that the rheological behavior of polyethylene melts is affected by at least four variables: ( 1 ) molecular weight, (9) molecular weight distribution, (3) long-chain branching and its dietribution, and (4) short-chain branching. Of these, the first three appear to have the largest effects. I n the present paper an attempt is made to determine the effect of molecular weight distribution by rheological studies of polymers having similar molecular weights and degrees of branching, but varying considerably in their molecular weight distributions as determined by fractionation. The rheological parsmeters studied were melt recovery, non-Newtonian behavior, critical shear rate, and melt strength. It is shown that the melt recovery increases uniformly as the molecular weight distribution broadens. The degree of non-Newtonian behavior, as measured by the exponent n of the power law, also increases with distribution breadth and is particularly affected by the amount of low molecular weight polymer present.Critical shear rate is inversely related to the breadth of the molecular weight distribution and is particularly dependent on the molecular weight of the highest fractions. The log of the critical shear rate is inversely proportional to the melt index recovery.Melt strength increases in a similar manner.

Rheological Features and Flow‐Induced Crystallization of Branched Poly[ethylene‐co‐(1,4‐cyclohexanedimethylene terephthalate)] Copolyesters

“…As a matter of fact, enhanced melt elasticity was reported years ago for long chain branched polyethylenes, compared to linear samples. [45][46][47][48][49] More recently, the same effect has been recalled for isotactic and syndiotactic poly(propylene). [32,[49][50] According to these results, we assumed that long chain branching caused by the incorporation of PER to polymerization process produced the observed melt elasticity increase.…”

Viscoelastic properties of decrosslinked irradiation‐crosslinked polyethylenes in supercritical methanol

“…Generally, in a liquid‐like low‐frequency region known as a terminal region, the power law linear viscoelastic slopes of a monodisperse flexible homopolymer can be expressed as G ′ ∝ ω 2 and G ″ ∝ ω (and η* ∝ ω 0 ) 13. These slopes are very sensitive to changes in the molecular weight,14, 15 molecular weight distribution,16, 17 chain branching,18, 19 crosslinking,20 mesostructure of the polymers,21, 22 and filler incorporation 23, 24. The slope change of G ′ is more sensitive to the structures than that of G ″.…”

“…LDPE with a high molecular weight ( M w = 349,000 g/mol) can form tremendous entanglements, which can serve as physical crosslinking points 14, 15. The high molecular weight distribution ( M w / M n = 7.0) causes a broadening of the relaxation distribution of the polymer chains in the terminal region, thus enhancing the nonterminal elastic behavior 16, 17. However, the LDPE exhibited more liquid‐like properties than the decrosslinked XLPEs that contained a considerable residual network gel structure.…”

Structure and Properties of Reactive Extruded Ethylene-block-co-Polypropylene: Influence of Dicumyl Peroxide and Divinylbenzene

“…The melt strength (MS) was measured at 230 • C/2.16 kg according to the testing method reported in the literature for polyethylene [35] and the value of the MS was calculated by the following equation:…”