Polymer molecular weight characterization

Lee HC, Chang T. Polymer molecular weight characterization by temperature gradient high performance liquid chromatography. Polymer 1996 37 5747. 

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...

Size exclusion chromatography (SEC) polymer elution profiles yield information regarding the molecular size distributions of polydisperse macromolecules. Polymer molecular weight distribution (MWD) represents an intrinsic property which provides direct correlation with many end-use physical properties and a universal criterion for polymer characterization (1). In order to convert elution profiles or chromatograms into MWD information proper calibration methods are required. SEC molecular weight calibration techniques represent experimental approaches for transformation of polymer elution profiles into MWD information and are dependent upon instrumentation, columns, and the polymer/solvent system under study. 

In polymer science we typically do not measure viscosity directly, but rather look at relative viscosity measures by determining the flow rate of one material relative to that of a second material. Viscosity is one of the most widely used methods for the characterization of polymer molecular weight because it provides the easiest and most rapid means of obtaining molecular weight-related data that requires minimal instrumentation. A most obvious characteristic of polymer solutions is their high viscosity, even when the amount of added polymer is small. This is because polymers reside in several flow planes (Figure 3.19b), acting to resist the flow of one plane relative to the flow of another plane. 

Polymerizations with initiators (II) and (III) are characterized by good molecular weight control and monodispersity of polymer molecular weights (Table I). Also, it is possible to prepare polymers with the same or different end groups in this manner. 

Thin-layer chromatography (TLC) is a common laboratory technique for separating complex mixtures of solutes, usually by an adsorbtion mechanism. Several laboratories have applied the technique to the separation of polymer fractions and characterization of polymer molecular weight distributions. This work reviews the experimental results and theoretical approaches to the fractionation mechanisms. 

Table III. Molecular Weight Characterization Data for PMMA Whole Polymers and Fractions...

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. 

For a dilute solution of polymer A in a low molecular weight solvent B, the polymer molecules are modeled as beadspring chains. Resistance in the motion of beads is characterized by a friction coefficient . As the number of beads is proportional to the polymer molecular weight M, we have... 

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. 

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. 

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