Molecular weight characterization

Table 4. Molecular Weight Characterization Data for Polypropylene Samples ...

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). 

The most widely used molecular weight characterization method has been GPC, which separates compounds based on hydrodynamic volume. State-of-the-art GPC instruments are equipped with a concentration detector (e.g., differential refractometer, UV, and/or IR) in combination with viscosity or light scattering. A viscosity detector provides in-line solution viscosity data at each elution volume, which in combination with a concentration measurement can be converted to specific viscosity. Since the polymer concentration at each elution volume is quite dilute, the specific viscosity is considered a reasonable approximation for the dilute solution s intrinsic viscosity. The plot of log[r]]M versus elution volume (where [) ] is the intrinsic viscosity) provides a universal calibration curve from which absolute molecular weights of a variety of polymers can be obtained. Unfortunately, many reported analyses for phenolic oligomers and resins are simply based on polystyrene standards and only provide relative molecular weights instead of absolute numbers. 

Table 1. Molecular Weight Characterization of Polymer Sample...

The lithium naphthalene was prepared in THF from lithium metal and a 25 mole % excess of naphthalene to minimize formation of the dilithium naphthalene dianion (18,19). A sample of a,w-dilithiumpolystyrene was quenched with methanol for molecular weight characterization (see Table II). 

Table 1 along with the corresponding supplier molecular weight characterization data. 

GPC Analysis. Molecular weight characterizations were carried out using a Waters 840 Gel Permeation Chromatograph equipped with both an ultraviolet (UV) (Model 481) and a refractive index (RI) detector (Model 410). Two Ultrastyragel columns, in the running order of 1,000 and 10,OOOA pore... 

Characterization was achieved by using a Waters Gel Permeation chromatograph. The Mn molecular weight characterization was achieved by membrane osmometry. Table IV summarizes the... 

The solution behavior of poly(amic acids) was until recently, probably the least understood aspect of the soluble polyimide precursor. However, the advent of sophisticated laser light scattering and size exclusion chromatography instrumentation has allowed elucidation of the solution behavior of poly(amic adds). In the early days of polyimide chemistry, when most molecular weight characterization was based on viscosity determinations, a decrease in viscosity was associated with molecular weight degradation [15, 28, 29]. Upon combination of the two monomers an increase in the viscosity to the stoichiometric equivalence point is observed, followed by a decrease in the solution viscosity as a... 

Molecular Weight Characterization of Resole Phenol—Formaldehyde Resins... 

Dase-catalyzed phenol-formaldehyde resins polymerized with a mole ratio of formaldehyde to phenol greater than one pose an interesting molecular weight characterization problem. This system is a dynamic one with active methylol end groups. Branched and crosslinked structures are formed, and in general, the separation of the resin from the reaction mixture is difficult. Figure 1 illustrates the chemical nature of a resole resin. 

Most molecular weight characterization now is done by size-exclusion chromatography (sec), also known as gel-permeation chromatography (gpc). 

Konas M, Moy TM, Rogers ME, Shultz AR Ward TC, McGrath JE (1995) Molecular weight characterization of soluble high performance polyimides. 1. polymer-solvent-stationary phase interactions in size exclusion chromatography, J. Polym Sci, Part A Polym Phys 33 1429... 

Molecular weight characterization of modacrylic fibers is difficult because of the limited number of solvents available and inhomogeneties in composition between individual polymer chains that affect solution properties, particularly if the comonomers are ionic in character. Di-methylformamide and dimethylacetamide are suitable for measurement of molecular weight of polyacrylonitrile, but errors are introduced when copolymers are analyzed (126). Bortniak et al. (127) have analyzed modacrylic fibers quantitatively in microgram quantities by using pyrolysis gas chromatography. 

As noted previously by Senich et al. (8), the usefulness of the DSA technique in following the cure of epoxy curing systems stems from the fact that a relative maximum Is produced in the composite loss tangent when the viscosity of the resin reaches a predetermined value. In a previous study (23) which considered a series of polystyrenes of varying molecular weights characterized... 

While the radical chain length as the ratio of the propagation and chain-breaking rates refers to conditions existing at the moment, the degree of polymerization and the molecular weight characterize a polymer that may have been produced over a span of time during which the conditions varied. Here, only some idealized situations with conditions held constant can be described in any detail. 

We have previously reported on the coupling of an SEC to a mass spectrometer operated in the electrospray mode of ionization and its application to the molecular weight characterization of octylphenoxy-poly(ethoxy)ethanol oligomers (20). The analysis of nonionic surfactants... 

After receiving his doctorate, he joined the Monsanto Company in St. Louis as a senior research chemist and carried out research on the characterization and material properties of exploratory polymers and composites. While he was at Monsanto, his research interests focused on molecular weight characterization, particularly by size-exclusion chromatography. Recently, his research has focused on size-exclusion chromatography, particle size distribution analysis, cure chemistry and physics, and the application of computers in the polymer laboratory. He is the author of more than 100 publications, is credited with three patents, and has edited or co-edited 10 volumes in the ACS Symposium Series and co-edited two volumes in the Advances in Chemistry series. 

Gel Permeation Chromatography. A Waters (model 150-C ALC/GPC) liquid chromatograph with a refractive index detector was used for molecular weight characterization. The unit was equipped with two sets of silanized Dupont blmodcLl columns (Zorbax PSM 60S/100S), with molecular weight range 2 x 10z to 10° and total retention volume of 24 ml. Samples were cut across the plaque thickness as with DSC. Polymer solutions were prepared 1n N,N -dimethyl -formamide, DMF, at 0.1%... 

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