Molecular Weight of Polyethylenes

Melt index of polyethylene may be measured on an instrument called an extrusion plastometer. (Photo courtesy of Tinius Olsen.) 

Another value is also measured on the plastometer at 190 °C, but under a load of 21.6 kg (ASTM D 1238-04C Condition 190/21.6). This is called the high load melt index (HLMI) and is also reported in g/10 min or dg/min. HLMI is often used for polyethylenes that have very high molecular weights. Because it may be difficult to measure the small amount of extrudate from the standard conditions for what are called fractional MI products (typically resins that have Mis 1), HLMI may be a more accurate measure of molten flow. 

Dividing HLMI by MI affords the melt index ratio (MIR), a dimensionless number which gives an indication of breadth of molecular weight distribution. As MIR increases, MWD broadens. 

In ASTM D 1238-04c, a term called the flow rate ratio (FRR) for polyethylene is defined as flow rate at Condition 190/10 divided by flow rate at Condition 190/2.16, abbreviated Like MIR, the flow rate ratio is dimensionless 

The term melt flow rate (MFR) is sometimes applied (erroneously) to polyethylene. ASTM suggests melt flow rate be applied to other thermoplastics and 

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A similar situation is observed when studying the effect of temperature on inhibition of thermal destruction of polyethylene by fiber glass of varying composition (Table 6). The molecular weight of polyethylene is practically unchanged when exposed over a period of 6 hours at 350°C with 30% of fiber glass containing 16%... 

Na20 content Filler content Time Molecular weight of polyethylene... 

The same research group proved the applicability of PEO-silicas to the separation of ribonucleic acids and studied how the log k vs m slopes are affected by the molecular weight of polyethylene oxide, the type of salt used in eluent... 

With the aid of elaborate refinements, including a 20-junction thermocouple. Pay was able to measure boiling point elevations with a precision of 0.0002°C and molecular weights of polyethylene up to 35,000 with an error not exceeding 10 percent. 

It may be interesting that chain termination with hydrogen, which is utilised in Ziegler-Natta polymerisations, does not operate in polymerisation systems with Phillips catalysts no influence of hydrogen to control the molecular weight of polyethylene in the latter case was achieved [37],... 

G1 Gaylord, N. G., and S. Rosenbaum Intrinsic viscosity and molecular weight of polyethylene terephthalate. J. Polymer Sci. 39,545 (1959). 

Direct comparisons between melt index and molecular weight of polyethylene should be made with caution. Such comparisons are only appropriate when the polymers have similar histories (made using the same catalyst, by the same process, at near identical densities, etc.). An example of the relationship between melt index and molecular weight for a series of LLDPEs with similar histories is... 

Vila-Jato JL, Blanco J, Alonso MJ. The effect of the molecular weight of polyethylene glycol on the bioavailability of paracetamol-polyethylene glycol solid dispersions. / Pharm Pharmacol 1986 38 126-128. 

Rezaei H, Sakr A. Effect of molecular weight of polyethylene glycol binders on acet aminophen tablets low shear wet granulation. Pharm Ind 2001 63(9) 974-84. 

Molecular Weight of Polyethylenes and Viscosity of Their Hot Melts What are... 

Higher molecular weight of polyethylene often improves the physical properties of products (though, see below). However, increasing of the molecular weight, hence, the viscosity of the polymer, usually increases the extrusion pressure and torque on... 

P. and Boileau, S., J. Polym. Sci. Polym. Symp., 62, 51 (1978)] to be 7.0xl0 in THF, 1.1x10 in THF+ [2.2.2]cryptand and 7.5x10 - in hexamethylphospho-ramide (HMPA). (a) Calculate the fraction of free ions in the three cases with NK concentration 10 mol/L. (b) Calculate the molecular weights of polyethylene oxide initiated by NK in the three cases at 90% conversion of the monomer of initial concentration 1.4 mol/L. (c) What is the number of initiator residue per molecule of the polymer formed ... 

The first degradable carbon-chain polymer was synthesised by Brubaker of the Dupont Company as early as 1950. This was a copolymer of ethylene and carbon monoxide (E-CO) which has since been extensively studied by photochemists, notably by J. E. Guillet and his co-workers at Toronto University. It was seen in Chapter 3 (Scheme 3.6) that macro-molecular ketones are formed by peroxidation of polyolefins and by subsequent photolysis they play an important role in the reduction of molecular weight of polyethylene during environmental exposure. 

Industry practice breaks the molecular weight of polyethylene into four distinct classifications that are ... 

K. Sask, L. Berry, A. Chan, J. Brash, Modification of polyrrrethane strrface with an antithrombin-heparin complex for blood contact influence of molecular weight of polyethylene oxide used as a linker/ spacer, Langmuir 28 (4) (2012) 2099-2106. 

As the temperature of a semicrystalline polymer is raised above the melt temperature, it may imdergo a flow transition and become liquid. Polyethylenes with a molecular weight of less than 500,000 g/mol can be observed to undergo such a flow transition (Tf). However, when the molecular weight of polyethylene increases above 500,000 g/mol, the entanglement of the... 

They demonstrated that the catalyst can be immobilized on silica. The product yields ultrahigh molecular weight polyethylene. Increased polymerization temperature resulted in higher activity, but lower molecular weight of polyethylene. 

Figure 12.1.37. Effect ofviscosity on molecular weight of polyethylene wax obtained by thermolysis in the presence of phenylether at 370 C. [Adapted, by permission, from L Guy, B Fixari, Polymer, 40, No.lO, 2845-57 (1999).]...

Because of their ductility, polyolefin parts must be carefully assembled using mechanical fasteners. These assembly methods are normally used on the materials having higher modulus such as high molecular weights of polyethylene and polypropylene. 

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