Applications of thermal FFF

Like SEC, the primary object of thermal FFF is to fractionate polymer components in a flow column so that they emerge at different times characteristic of polymer molecular mass. The time-based detection of the emergent polymer sample can then be translated into the molecular-mass characteristics of the polymer distribution. 

Earlier in this chapter we noted that an advantage of thermal FFF (actually of all forms of FFF) is the flexible control of retention by adjustments in the field strength, here represented by the temperature drop AT. By suitable adjustments in AT, polymers of all different mean molecular masses and with different extremes in the molecular mass range can be fractionated in the same channel system. We will now be more specific. 

Equation (8.3) shows that if the effective thickness of the polymer cloud I is to be maintained in the optimum range (- 1-10 /zm), then AT must be decreased as M is increased. Conversely, the temperature drop must be elevated for polymers of low molecular mass. The process of tuning AT to match the characteristics of the polymer sample requiring analysis can be illustrated by several examples from our laboratory. 

Thermal FFF experiments are usually carried out with AT ranging from 20-80°C. Such values give the effective retention and separation of polymer samples ranging from 10000 to 1000000 molecular mass. If the sample contains components of lower molecular mass for which high resolving power must be maintained, it is necessary to increase AT as discussed above. 

In a study designed to examine the means and effects of increasing AT, it was 

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The application of thermal FFF to a variety of colloids and particles has been demonstrated in both aqueous and organic carrier liquids. Figure 2 illustrates the dependence of retention on the surface composition of polystyrene particles. The three particles are similar in size, but the surface of one of the samples has been carboxylated, whereas another has been am-inated. The relative elution order of the three particles can be changed by modifying the carrier liquid [13]. 

The dependence of retention on thermodiffusion imparts an additional dimension to the thermal FFF separation that is not present in SEC. Although our understanding of thermodiffusion in solids and liquids is incomplete, certain aspects are clear. For example, thermodiffusion is very sensitive to the chemical composition of the polymer. As a result, thermal FFF is capable of separating components that differ in composition, even though they may have the same molecular-weight or diffusion coefficient. An example [4] of the separation of polystyrene and poly(methyl methacrylate) standards by thermal FFF is illustrated in Fig. 1. This separation cannot be accomplished with SEC because the diffusion coefficients (or hydrodynamic volumes) of the two materials are virtually identical. The ability of thermal FFF to separate materials by chemical composition has spurred additional research designed to increase our understanding of thermodiffusion [5], which, in turn, has led to the application of thermal FFF to polymer blends and copolymers [6]. 

Thermal field-flow fractionation was invented by Giddings. The universal applicability of thermal FFF for the analysis of various polymers had been already demonstrated in 1979. Several applications of TFFF to the analysis of polymers and colloidal particles were published (see Refs. for a review) but the contemporary TFFF channels have practically the same dimensions (roughly 50 X 2 X 0.01 cm) as those constructed at the very beginning. Giddings concluded, in 1993, that the miniaturization of the FFF channels could provide only some limited advantages. The experimental studydealt only with the effect of the reduced channel thickness on the performance of TFFF. 

Colloid characterization is not the classical application of Th-FFF. Nevertheless, Th-FFF was first applied to silica particles suspended in toluene testing a correlation between thermal diffusion and thermal conductivity [397]. Although a weak retention was achieved, no further studies were carried out until the work of Liu and Giddings [398] who fractionated polystyrene latex beads ranging from 90 to 430 nm in acetonitrile applying a low AT of only 17 K. More recently, polystyrene and polybutadiene latexes with particle sizes between 50 pm and 10 pm were also fractionated in aqueous suspensions despite the weak thermal diffusion [215] (see Fig. 30). Th-FFF is also sensitive to the surface composition of colloids (see the work on block copolymer micelles), recent effort in this area has been devoted to analyzing surfaces of colloidal particles [399,400]. 

The major limitation of thermal FFF occurs in the separation of low-molecular-weight materials. Thus, the technique is not widely applicable to molecular weights below about 10 g/mol. This limit can be reduced somewhat by the use of solvent mixtures. For example, polystyrene components as small as 2500 g/mol were resolved in a mixture of tetrahydrofuran and dodecane [4]. Even lower molecular weights than 2500 g/mol have been retained, but only through the use of special channels, which were highly pressurized in order to increase the temperature gradient without boiling the solvent. 

Although Dj varies with the polymer-solvent system, it is independent of molecular weight in a given system, at least for random coil homopolymers. The separation of differing molecular-weight components is therefore based solely on differences in D, which means that the principles of universal calibration that are relevant to SEC are also applicable to thermal FFF. Thus, a calibration curve made with one polymer-solvent system can be applied to other systems, provided the two Dj values associated with each polymer-solvent system are available. Fortunately, accurate values... 

At the upper extreme of the molecular mass scale, AT must be lowered instead of raised, which simply requires an appropriate reduction in the heat input. Since thermal FFF appears to have some important advantages for ultrahigh molecular mass polymers (among them the lack of shear degradation), this subject is an important one in the context of thermal FFF applications. 

With the exception of the work reported on polyolefins, the applications described above have involved polystyrene samples. The reason for this restricted initial scope is that thermal FFF has evolved primarily in an academic setting where the availability of a wide range of narrow polystyrene standards was considered indispensable to the evolution and refinement of the mechanism and behaviour of thermal FFF systems. However, as thermal FFF increasingly enters the industrial laboratory, its applicability to other polymers becomes crucially important. Fortunately, we have done enough work with other polymer systems to understand the general behaviour of various polymer classes in thermal FFF systems. 

Another recent application uses thermal FFF in the radial direction of a circular channel and fluid shear generated by rotating one wall relative to the other. Polymer mixtures were continuously fed into the channel near the centre, and different molecular weight fractions were removed from the channel at points around the outer circular edge. 

Excluded from this list is sieving, to which the concept of selectivity is not applicable. For completeness, we have subdivided the FFF family into sedimentation FFF, thermal FFF, flow FFF, and steric FFF to show how the selectivity of each of these subtechniques compares to that of the other fractionation methods. The values reported here differ from S values reported elsewhere (12), which refer to mass rather than size selectivity. 

This review will introduce the basic principles, theory, and experimental arrangements of the various FFF techniques focusing on the most relevant for praxis Sedimentation-FFF (S-FFF),Thermal-FFF (Th-FFF) and Flow-FFF (Fl-FFF). In a second part,selected applications of these techniques both to synthetic and biological samples will illustrate applications under a variety of conditions, where problems and potential pitfalls as well as recent developments are also considered. 

The dimensionless retention parameter X of all FFF techniques, if operated on an absolute basis, is a function of the molecular characteristics of the compounds separated. These include the size of macromolecules and particles, molar mass, diffusion coefficient, thermal diffusion coefficient, electrophoretic mobility, electrical charge, and density (see Table 1, Sect. 1.4.1.) reflecting the wide variablity of the applicable forces [77]. For detailed theoretical descriptions see Sects. 1.4.1. and 2. For the majority of operation modes, X is influenced by the size of the retained macromolecules or particles, and FFF can be used to determine absolute particle sizes and their distributions. For an overview, the accessible quantities for the three main FFF techniques are given (for the analytical expressions see Table l,Sect. 1.4.1) ... 

The dependence of retention in Th-FFF on chemical composition of the polymers and solvent [84] also opens a wide field of application for Th-FFF, especially for copolymers. According to Eq. (42), retention in Th-FFF can be used to determine the thermal diffusion factor aT which was demonstrated for polystyrene in toluene [209]. Eater, this study was extended to other solvents (ethyl acetate, 2-butanone, p-dioxane, cyclohexane, dimethylformamide, chloroform, and ethylbenzene) [204]. 

Recent studies [111,214] indicate that Th-FFF can even be used to determine the relative chemical composition of two components in random copolymer and linear block copolymers whose monomers do not segregate due to solvent effects. However, this application is limited by the unpredictable nature of thermal diffusion. Nevertheless, combining information from Th-FFF with those derived on fractions by independent detectors selective to composition (such as an IR spectrometer) can yield further insight into the dependence of DT on the chemical composition. Even more powerful is the combination of Th-FFF with SEC as, here, the chemical composition (from Th-FFF) can be studied as a function of the molar mass (from SEC). This was demonstrated by van Asten et al. by cross fractionating copolymers and polymer blends with SEC and Th-FFF [358]. 

Kirkland et al. [359] reported the possibility of varying the retention behavior in Th-FFF by the application of a solvent mixture, later supported by other workers [58,360]. The retention enhancement so achieved was attributed to a synergistic effect involving the thermal diffusion of both polymer and solvent. 

Another application in which thermal FFF enjoys an advantage over SEC is the analysis of high-temper-ature polymers. The operating temperature is limited only by the degradation temperature of the spacer used to form the channel, which for polyimides can be as high as 600 K. In the analysis of high-molecular-weight polyethylene, for example, temperatures in excess of 400 K are required for the samples to be soluble. Under these conditions, column stability and separation efficiency limit the application of SEC. By contrast, such samples can be routinely analyzed with commercially available thermal FFF channels. 

Historically, the application of FFF to colloids and particles has been limited to flow and sedimentation FFF. However, the thermal FFF channel in not only capable of separating these materials, it is simpler in design and can be used with both aqueous and organic solvents. Furthermore, the dependence of retention on chemical composition presents unique opportunities for the separation of such materials. 

Thermal FFF (thermal field-flow fractionation) is an elution-type separation technique applicable to the characterization of various synthetic organic polymers with molecular weights higher than about 10 [1], In thermal FFF, a dilute solution of polymer sample is injected into a thin ribbon-shaped flow channel across which an external field (in the form of a temperature gradient) is applied. Under the influence of the temperature gradient, different components of the sample are carried down the channel at different velocities, leading to the elution of different components at different times and separation is achieved. 

It is noted, however, that for polymers of molecular weight below about 1 X 10" Da, SEC may be more useful than thermal FFF. Analysis of such low-molecular-weight samples using thermal FFF requires the application of a very high AT that may require the use of a pressurized system to avoid boiling of the solvent. 

Historically, thermal FFF has been applied primarily to lipophilic polymers. The technique has not found wide applicability to hydrophilic polymers because thermodiffusion, and therefore retention, is very weak in water. Although a few hydrophilic polymers have been separated by thermal FFF, they generally must have a high molecular weight (>10 g/mol) in order to be adequately retained for characterization by thermal FFF. Alternatively, an aprotic solvent such as dimethyl sulfoxide can be used to separate hydrophilic polymers with lower molecular weights. However, flow FFF is more suited to the characterization of hydrophilic polymers. 

The open FFF channel is especially suited to fragile materials, and thermal FFF has found a definite niche in its application to ultrahigh-molecular-weight polymers. Furthermore, because samples need not be filtered, thermal FFF is the technique of choice for analyzing gels, rubbers, and other materials that tend to plug SEC columns [7]. Even particles can be analyzed... 

Advances in our understanding of thermodiffusion in polymer solutions have led to the application of ther-mal FFF to copolymers. With random copolymers, for example, the dependence of on chemical composition is now predictable [6], so that compositional information can be obtained from retention measurements. With block copolymers, thermal FFF can still be used to separate components according to molecular weight, branching, and composition, but independent measurements on the separated fractions must be made in order to get quantitative information, except when special solvents are used. Special solvents yield a predictable dependence of Dj on composition even for block copolymers [6]. Different solvents are special for different copolymer systems. 

The ability of a single thermal FFF channel to be used for the separation of lipophilic polymers, gels, rubbers, and particles makes it a very useful tool for polymer and colloid analysis. The versatility, however, comes with a price. Because aU of the various applications cannot be implemented with a single field strength or carrier liquid, the user must have more than just a basic familiarity with the technique. To use thermal FFF efficiently, while taking advantage of its versatility, the user must understand the fundamentals behind the separation mechanism and assimilate a certain amount of experience. 

Different FFF subtechniques result from the application of different types of fields or gradients. To date, the methods that have been employed are sedimentation, electrical, thermal, and flow FFF. 

List the types of substances to which each of the following separation methods is most applicable (a) supercritical-fluid chromatography (b) thin-layer chromatography (c) capillary zone electrophoresis (d) thermal FFF (e) flow FFF... 

For context, we point out here that FFF is a broad family of techniques applicable to macromolecules, colloids, and particles of diverse types extending over a mass range from a few hundred to 10 dalton [3-8]. FFF has perhaps become best known for high-resolution colloid separation and characterization [5,7,9]. For this task, a specific subtechnique termed sedimentation FFF is most often utilized [10]. For polymer analysis, another subtechnique, thermal FFF, is most commonly employed [11,12]. 

While the above characteristics and advantages have been discussed specifically in reference to thermal FFF, much the same can be said in regard to the other FFF subtechniques, including sedimentation FFF, electrical FFF, and flow FFF. All of these are flexible techniques subject to ready optimization. Their application to polymers will be described after establishing the necessary theoretical framework for FFF. 

The application of programmed thermal FFF to a broad molecular mass range of polymer standards is illustrated in Figure 8.11. For this figure the... 

Thermal field-flow fractionation (FFF) is an elution-type separation technique applicable to the characterization of various synthetic organic polymers with molecular weights higher than about In thermal FFF, a... 

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