Molecular weight measurement methods

What molecular weight measurement methods could be used practically to determine the following ... 

The severity of the compositional inhomogeneity, even at very low conversions, renders molecular weight measurement methods invalid on these samples. Additional experiments will be required at the azeotropic composition (for DMAEA) or in a continuous reactor. 

The measurement techniques most frequently used are derived from Raoult s and Van t Hoff s laws applied to cryometry, ebulliometry, osmometry, etc. They are not very accurate with errors on the order of ten per cent. Consequently, the molecular weight is often replaced by correlated properties. The mean average temperature or viscosity can thus replace molecular weight in methods derived from ndM. 

Molecular Weight. Measurement of intrinsic viscosity in water is the most commonly used method to determine the molecular weight of poly(ethylene oxide) resins. However, there are several problems associated with these measurements (86,87). The dissolved polymer is susceptible to oxidative and shear degradation, which is accelerated by filtration or dialysis. If the solution is purified by centrifiigation, precipitation of the highest molecular weight polymers can occur and the presence of residual catalyst by-products, which remain as dispersed, insoluble soHds, further compHcates purification. 

The molecular stmcture of the copolymers is also important. Molecular-weight measurements (osmometry, gpc) and functional group analysis are useful. Block copolymers require supermolecular (morphological) stmctural information as well. A listing of typical copolymer characterization tools and methods is shown in Table 6. 

ASTM D-2502 is one of the most accurate methods of determining molecular weight. The method uses viscosity measurements in the absence of viscosity data, molecular weight can be estimated using the TOTAL correlation. 

Thus, the slope of a plot of ln[T] vs ln[M] will yield the transfer constant. This method does not rely on molecular weight measurements. 

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. 

Gel permeation chromatography (GPC) or size exclusion chromatography (SEC) has been routinely used to estimate die molecular weight of die polymers. The molecular weight measured by GPC is relative to a polymer standard, typically polystyrene GPC is dius a relative method rather than an absolute one. For those polymers whose structure is very different from polystyrene, GPC molecular weight values could significantly differ from the real ones. In those cases, GPC values should only be regarded as a reference. 

Molecular weights measurable by the osmotic method is about 106. 

In this chapter, we restrict our discussion to a chromatographic technique normally used for molecular weight measurements. The chromatographic concept can also be used for direct size (instead of molecular weights) measurement in the case of rigid particles, as we illustrate in our description of field-flow fractionation methods in Chapter 2. 

Mass chromatography has several advantages (2) over conventional methods of molecular weight measurement such as cryoscopy, ebulli-ometry, vapor density, and osmometry. For example, these methods require a pure sample and are dependent on ideal solution behavior or extrapolation to infinite dilution. 

Methods used to obtain conformational information and establish secondary, tertiary, and quaternary structures involve electron microscopy, x-ray diffraction, refractive index, nuclear magnetic resonance, infrared radiation, optical rotation, and anisotropy, as well as a variety of rheological procedures and molecular weight measurements. Extrapolation of solid state conformations to likely solution conformations has also helped. The general principles of macromolecules in solution has been reviewed by Morawetz (17), and investigative methods are discussed by Bovey (18). Several workers have recently reexamined the conformations of the backbone chain of xylans (19, 20, 21). Evidence seems to favor a left-handed chain chirality with the chains entwined perhaps in a two fold screw axis. Solution conformations are more disordered than those in crystallites (22). However, even with the disorder-... 

The coordination number can sometimes be established by coordinated physical measurements such as conductance, molecular weight measurement, infrared, UV-VIS-near IR, and emission spectroscopy [16]. The coordination numbers of solid complexes can be obtained by X-ray diffraction methods. Infrared spectroscopy and the conductance methods were used in the determination of the coordination numbers of Nd(NCh)3 4DMSO and Dy(N03>3 3DMSO complexes [17]. 

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