Monodisperse standards

Particle separation can be characterized by the separation factor, Rp, which is the ratio of eluant to particle elution volumes, or, by the difference in elution voliame, AV, between particle and eluant marker turbidity peaks. For polystyrene monodisperse standards, a linear relationship occ irs between the log of the particle diameter and AV, with a series of parallel lines resulting for different concentration of either salt or surfactant below its critical micelle concentration (IT>18,19) The separation factor has also been shown to be independent of eluant... 

Calculations for Rp as a function of the relevant experimental parameters (eluant ionic species concentration-including surfactant, packing diameter, eluant flow rate) and particle physical and electrochemical properties (Hamaker constant and surface potential) show good agreement with published data (l8,19) Of particiilar interest is the calculation which shows that at very low ionic concentration the separation factor becomes independent of the particle Hamaker constant. This result indicates the feasibility of xmiversal calibration based on well characterized latices such as the monodisperse polystyrenes. In the following section we present some recent results obtained with our HDC system using several, monodisperse standards and various surfactant conditions. 

Since the modified iterative method is completely numerical, data can be used directly from the monodisperse chromatograms to characterize the axial dispersion, eliminating the need for a specific axial dispersion function. The monodisperse standards were used to represent the spreading behavior for particle ranges as given in reference (27). 

An important by-product of the development of this approach is that Orthogonal Chromatography provides a direct method of estimating the shape of the chromatogram for extremely narrow molecular weight distributions. This shape function is fundamental information for axial dispersion evaluation and is not otherwise easily obtained. Even commercially available monodisperse standards synthesized by anionic polymerization are too polydisperse. 

Figure 20 shows an example of its use. As expected, the GPC 2 chromatogram of a fraction of a monodisperse standard (obtained by sampling it with GPC 2 at its peak) is narrower than the whole standard which in turn is narrower than chromatograms of slices of broad polystyrene distributions. In Figure 20, two examples of the latter show the difference obtained by improving resolution in GPC 1. 

Simple homopolymers, where monodisperse standards and suitable solvents are available, are easily characterized by SEC. Homopolymers for which no monodisperse standards are available additionally require some more elaborate detection system for transformation of the retention time into molecular weight. This can be done by, e.g., universal calibration. Alternatively, an absolute molar mass detector, like an on-line light scattering detector or mass spectrometer, can be used. 

Despite the above-mentioned features, ThFFF is not as widespread or popular as SEC. One possible reason is that calibration constants are only available for a very limited number of polymer-solvent systems. On the contrary, the SEC practitioner has available an extensive collection of Mark-Houwink constants for the different polymers, solvent types, and operating temperatures. In recent years, signihcant advances have been made in ThFFF calibration under isothermal conditions using a polydisperse standard as an alternative to a series of monodisperse standards. Recently, the calibration function was also made applicable to ThFFF field programming operation [32],... 

Represents the SEC chromatogram of monodispersed standards of diffenng molecular weight... 

Most of the work was done with the alkoxide TES and the alcohol ethanol as solvent. Lowering the temperature always results in larger particles. Sometimes the radius can become 4 times larger by lowering the temperature 30 °C (26). In the range of TES concentrations in which particles seem to be stable, larger particles generally tend to be more monodisperse. Standard deviations in particle size of only 2% have been observed. 

Another of SEC s serious imperfections is that it relies on calibration. However there are really only a handful of polymers for which monodisperse standards are available, and all of these are common polymers. A way around this is to use universal calibration, but this firstly relies on Mark-Houwink-Kuhn-Sakurada (MHKS) parameters being known, and secondly it requires that workers are aware of the need for this procedure - many evidently are not. Further, given that the aim of much research in polymer synthesis is to make new polymers for which, by definition, MHKS parameters will not be available, it follows that in many cases universal calibration will not be possible anyway. 

In reality, the universal calibration ctine is not quite universal. Certain exceptions exist, such as polymers with very rigid backbones. However, the equation does apply to a very large number of systems, and this means that it is possible to calibrate the chromatograph with one set of readily available monodisperse standards and to use the restilts to determine the real molar mass of a different polymer. 

The usual means of measuring molar mass distributions (MMDs) is gel permeation chromatography (GPC) which is described in Section 11.2.2.5. Denote the GPC trace as the signal G as a function of elution volume Veil it is assumed that the baseline has been subtracted. Appropriate calibration with monodis-perse standards yields the GPC calibration curve, which is the volumes at which monodisperse standards elute as a function of the molar mass M of the standard this is denoted V(A/ )... 

Before the introduction of MALDl, mass spectrometry was not a widely applicable method for the determination of MM of synthetic polymers. Since MALDl-TOE allows desorption and ionization of very large molecules, it is now possible to perform the direct identification of mass-resolved polymer chains and the measurement of MM in numerous high polymers. MM values up to 1.5 million Daltons have been measured for PS monodisperse standards. MALDI-TOF is therefore unique for the MM and MMD estimation in synthetic polymers by MS techniques. 

Figure 10.3 Chromatogram of bimodal mixture of polystyrene monodisperse standards of...

In order to standardize the GPC column set (ULTRASTYRAGEL), polystyrene standards and five selected polysilane samples were available. As the latter were not true monodisperse standards, calibration procedures were based on the molecular weights corresponding to the tops of he GPC peaks (Mp). 

The calibration curve, strictly speaking, applies only to the particular polymer, solvent, temperarnre, flow rate, and column for which it was established. Change any one, and the calibration is no longer valid. Most SEC calibrations are obtained with polystyrene, because the necessary monodisperse standards are readily available at reasonable cost. What do you do when you want to analyze another polymer If absolute values are not needed, the molecular weights can be reported with respect to the polystryene (or other polymer)... 

Ci2 Ci8 alkyl 3-mole ethoxysulfates (monodisperse standards) elution in order of increasing alkyl chain length... 

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