Polymer Molecular Weight Measurement

Maria Guadalupe Neira-Velazquez, Maria Teresa Rodriguez-Hernandez, Ernesto Hernandez-Hernandez, and Antelmo R. Y. Ruiz-Martinez 

Despite the tremendous potential impact of his discovery, chromatography was not revived until 1931, under the stimulus of widespread research on the separation of carotenoids from several natural sources by adsorption analysis on fibrous alumina [4]. Since the cited work was published, chromatography application has been extended over practically all areas of chemistry. In polymer science, chromatography was used for the first time by Moore in 1963 to determine the MWD of polymers [5]. 

James Waters, industry pioneer and entrepreneur, had founded Waters Associates (WA) in 1958 in order to invent instruments for others. He worked with five employees in the rented basement of a police station. In 1961 John Moore required from WA to develop a 0.1 ml volume flow cell, which would enable him to develop an instrument using gel columns to analyze the MW of polymers (natural and synthetic macromolecules). After experimental work by Moore, followed by negotiations between Dow and WA and additional hard work to scale up the synthesis of the polymeric gel used in the columns, the invention of the GPC was completed, becoming a major breakthrough for WA. 

Handbook of Polymer Synthesis, Characterization, and Processing, First Edition. Edited by Enrique Saldivar-Guerra and Eduardo Vivaldo-Lima. 2013 John Wiley Sons, Inc. Published 2013 by John Wiley Sons, Inc. 

In 1963, Waters obtained an exclusive license to Dow s patent [5] for GPC and introduced Waters first liquid chromatography (LC) system, the GPC 100, which was larger than a refrigerator. 

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At the theta temperature a polymer in solution is on the threshold of precipitation. A solvent, or a mixture of solvents, for which the theta temperature is close to room temperature (or whatever temperature is to be employed) is a theta solvent for the particular polymer. Such a solvent neither expands nor contracts the macromolecule, which is then said to be in its unperturbed state. The diameter of the polymer chain random coil in solution is then equal to the diameter it would have in the amorphous bulk polymer at the same temperature. Use of theta solvents enables the experimenter to obtain polymer molecules that are unperturbed by solvent but separated from each other far enough not to be entangled. However, theta solvents or solutions under theta conditions are not normally used for polymer molecular weight measurements because the solute is on the verge of precipitation. 

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. 

Moonej Viscosity. This is a measurement of the viscosity of the polymer that is commonly used ia the mbber iadustry. Mooney viscosity values typically range from 25 to 100. Mooney viscosity generally relates to polymer molecular weight, with the lower Mooney viscosity polymers providing improved flow and processiag characteristics and the higher Mooney NBRs providing improved physical properties. 

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. 

Mixed solvents are generally unsatisfactory for use in the determination of polymer molecular weights owing to the likelihood of selective absorption of one of the solvent components by the polymer coil. The excess of polarizabilit f of the polymer particle (polymer plus occluded solvent) is not then equal to the difference between the polarizabilities of the polymer and the solvent mixture. For this reason the refractive increment dn/dc which would be required for calculation of K, or of i7, cannot be assumed to equal the observed change in refractive index of the medium as a whole when polymer is added to it, unless the refractive indexes of the solvent components happen to be the same. The size Vmay, however, be measured in a mixed solvent, since only the dissymmetry ratio is required for this purpose. 

Utilization of a microfabricated rf coil and gradient set for viscosity measurements has recently been demonstrated [49]. Shown in Figure 4.7.9 is the apparent viscosity of aqueous CMC (carboxymethyl cellulose, sodium salt) solutions with different concentrations and polymer molecular weights as a function of shear rate. These viscosity measurements were made using a microfabricated rf coil and a tube with id = 1.02 mm. The shear stress gradient, established with the flow rate of 1.99 0.03 pL s-1 was sufficient to observe shear thinning behavior of the fluids. 

However, the synthesis process, depicted in scheme 5, is rather idealized. In reality, the chemistry appears to be quite complex, resulting in a partially cross-linked rubber and the evolution of gaseous species other than chloromethane. Dietrich et al.17 reported that the progress of the polymerization at 116°C, as measured by gas evolution and polymer molecular weight, significantly slowed at around 50% conversion. The reaction could, however, be driven further forward by increasing the temperature to > 150°C. 

It is not possible to determine from A atr ) alone whether the polymerization will be controlled fast activation and more importantly fast deactivation are required to achieve good control over polymer molecular weights and molecular weight distributions. Therefore, precise measurements of the activation (kj and deactivation (kj rate constants should be used for correlation with catalyst, alkyl halide, and monomer structures. 

Since the polymer-solvent interaction changes with concentration, Molecular weight measurements by VPO, must be conducted within controlled concentration range. If the concentration is too high, significant condensation will take place on the solution droplet, thus reducing the difference in vapour pressure between the solution and the solvent. 

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