Molecular weight determination polyethylene

Table II. Precision in Molecular Weight Determination Polyethylene...

In very few cases solid-state NMR has been used to determine molecular weights for polyethylene [99]. In general solid-state NMR is not so suitable due to the long relaxation times of the end groups, which lead to long measurement times. The strength of NMR is in chemical structure characterization and also the possibility to determine the chain branching, tacticity or to obtain further details of the microstructure. 

Mid-IR spectroscopy, alongside gravimetric and molecular weight determinations, has also been used to analyse the biodegradation by a thermophilic bacterium (isolated from soil) of an LDPE film [44], The mid-IR studies were undertaken using the ATR sampling technique on control samples, samples that had been UV irradiated, and samples that had been UV irradiated then incubated with bacteria. The study showed that the particular bacterial strain was capable of utilising standard and photo-oxidised polyethylene as the sole carbon source. 

Proton NMR relaxation parameters have also been determined for polyethylene ( ) and polyethylene oxide (39) in the melting region. The apparent contradiction between the proton spin-lattice relaxation parameter for a high molecular weight linear polyethylene sample at its melting point, with the relaxation measurements, has previously been pointed out. (17) This discrepancy is still maintained with the more detailed results reported here for both types of polyethylene. For the proton relaxation a small, but distinct, discontinuity is reported at the melting teirperature. (38)... 

W 5 — Molecular weight determination in polyethylene terephthalate. Nature (Lond.) 180, 141 (1957). 

Comparison of polyethylene molecular weights determined by light scattering in 1 chloronaphthalene and tetralin. J. Polymer Sci. 2A, 4875 (1964). 

GENERAL INFORMATION The Surlyn brand of ionomers consists of copolymers of ethylene with methacrylic acid, partially or wholly neutralized with a variety of metals, including sodium, zinc, and lithium.The neutralization process drastically increases the melt viscosity and decreases the solubility, making molecular weight determinations of the final product impossible. However, the metal ions can be removed by treatment with acids, and the unneutralized copolymer examined by methods similar to those used for low density polyethylene (LDPE) and copolymers thereof. In certain cases, the properties of the ionomer resemble LDPE where applicable, these values are given in italics. About twenty grades of Surlyn plastics exist. Here we report on two representative samples sodium (Na) neutralized and zinc (Zn) neutralized. Where experimental conditions are described by a "D-" number, these refer to test procedures of the American Society for Testing Materials. 

Molecular weight of polyethylene glycol can be calculated by dividing the measured hydroxyl number by 34 (Molecular weight = 34/OH) A typical calibration ranges from 31-137 OH number with an SEE of 0.15 and anr of 0.998. Samples were run on a filter instrument in a65°C thermostated transflectance cell. Table 27.2 shows lab versus Infra Alyzer-predicted results for polyethylene glycol molecular weight determination and OH determination. 

Lab vs. InfraAlyzer Predicted Results for Polyethylene Glycol Molecular Weight Determination and OH Determination... 

In the polyethylene industry, the molecular weight of polyethylene is measured by determining a Melt Index (MI) value. Melt Index is the amount of molten polyethylene that flows in ten minutes through a specific orifice maintained at 190°C with a weight of 2.16 Kg, positioned directly on the molten polyethylene sample. The test method is ASTM D-1238-condition E and the test value is referred to as the Melt Index, MI or T or simply T. 

However some of the weight loss can be masked by the incorporation of oxygen into the material. To make residual weight loss measurements more accurate and convenient, the solvent extraction techniques for the retrieval of polymer incubated in soil have been applied. However, for some polyethylene based materials these techniques give erroneous results because of PE crosslinking (partially insoluble in 1,2,4-trichlorobenzene). Ideally, different measurements for biodegradation should correlate molecular weight determination when applied to the same polymer [120]. 

GRI-GG8, Standard Test Method for Determination of the Number Average Molecular Weight of Polyethylene Terephthalate (PET) Yams Based on a Relative Viscosity Rate. 

Table 4. Characteristic temperatures and molecular weights evaluated from H and (of quaternary carbons) relaxation data of linear polymers [33, 35, 139, 140]. PE, polyethylene PIP, polyisoprene PIB, polyisobutylene PS, atactic polystyrene PTHF, polytetrahydrofuran PDMS, polydimethylsiloxane. Literature data for critical molecular weights determined with rheology are also listed for comparison and completeness...

The bulk viscosity control parameter for CSM, as with other elastomers, is molecular weight M and molecular-weight distribution (MWD). Mooney viscosity for CSM is determined by selection of the polyethylene precursor. 

Polyethylene and polypropylene are semitransparent plastics made by polymerization. They are produced from ethylene and propylene in a variety of grades. Their mechanical properties are determined mainly by density (degree of crystallinity) and molecular weight, characterized by the Melt Index (MI). 

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