Molecular weight field-flow fractionation

Among the techniques employed to estimate the average molecular weight distribution of polymers are end-group analysis, dilute solution viscosity, reduction in vapor pressure, ebuUiometry, cryoscopy, vapor pressure osmometry, fractionation, hplc, phase distribution chromatography, field flow fractionation, and gel-permeation chromatography (gpc). For routine analysis of SBR polymers, gpc is widely accepted. Table 1 lists a number of physical properties of SBR (random) compared to natural mbber, solution polybutadiene, and SB block copolymer. 

Currently, there are several molecular weight separation techniques, such as OTHdC, PCHdC, SEC, thermal field flow fractionation (ThFFF), and sedimentation field flow fractionation (SdFFF). The molecular weight separation range... 

In conclusion one can say that SEC is a very powerful method for polymer characterization, especially in combination with other composition sensitive or absolute calibration methods. A big advantage is also that the sample amount is fairly small, typically 10 mg. For more complex polymers, such as polyelectrolytes, enthalpic effects often become dominant and also for rather high molecular weight polymers chromatographic methods such as field-flow fraction (FFF) techniques might be more suitable. For fast routine measurements linear columns are often used. 

Beckett, R., Jue, Z. and Giddings, J. C. (1987). Determination of molecular weight distributions of fulvic and humic acids using flow field-flow fractionation, Environ. Sci. Technol., 21, 289-295. 

Field-flow fractionation, commonly designated as FFF, is a versatile family of separation techniques able to separate and characterize an enormous assortment of colloidal-supramolecular species in a wide range of dimensions/molecular weights. Giddings is considered the inventor of this technique since he contributed to the development of theory, different techniques, instrumentation, methodology, and applications [1], even if studies on the theoretical fundamentals of fractionation under force and flow fields had appeared before and/or independently [2]. 

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. 

Lyven, B., Hassellov, M., Haraldsson, C. and Turner, D.R. (1997) Optimisation of on-channel preconcentration in flow field-flow fractionation for the determination of size distributions of low molecular weight colloidal material in natural waters. Anal. Chim.Acta, 357, 187-196. 

Thermal Field-Flow Fractionation Extension to Lower Molecular Weight Separations by Increasing the Liquid Temperature Range Using a Pressurized System, J. C. Giddings, L. K. Smith, and M. N. Myers, Anal. Chem., 47, 2389 (1975). Flow Field-Flow Fractionation A Versatile New Separation Method, J. C. Giddings, F. J. Yang, and M. N. Myers, Science, 193, 1244 (1976). 

Dense-Gas Chromatography of Nonvolatile Substances of High Molecular Weight, L. McLaren, M. N. Myers, and J. C. Giddings, Science, 159, 197 (1968). Nonequilibrium Theory of Field-Flow Fractionation, J. C. Giddings, J. Chem. Phys., 49, 81 (1968). 

Field-Flow Fractionation Extending the Molecular Weight Range of Liquid Chromatography to One Trillion, J. Chromatogr., 125, 3 (1976). 

Field flow fractionation (FFF) is an elution technique suitable for molecules with a molecular weight > 1000 up to a particle size of some 100 pm. Separating, driving, external field forces are applied perpendicular to a liquid carrier flow, causing different species to be placed in different stream lines (Fig. 26). Useful fields are gravity, temperature, cross flow, electrical charge, and others [128-131]. 

Myers MN, Chen P, Giddings JC (1993) Polymer separation and molecular-weight distribution by thermal field-flow fractionation. In Provder T (ed) Chromatography of polymers characterization by SEC and FFF. American Chemical Society, Washington, DC, pp 47-62... 

Determination of Molecular Weight and Size of Ultrahigh Molecular Weight Polymers Using Thermal Field-Flow Fractionation and Light Scattering... 

X HERMAL FIELD-FLOW FRACTIONATION (ThFFF) separates polymers according to their molecular weight and chemical composition. The molecular weight dependence is well understood and is routinely used to characterize molecular weight distributions (1-4). However, the dependence of retention on composition is tied to differences in the thermal diffusion of polymers, which is poorly understood. As a result, the compositional selectivity of ThFFF has not realized its full potential. How-... 

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