Broad molecular weight distribution calibration

An alternative approach to calibrating an SEC system has been to use a single broad molecular weight distribution calibrant. However, this method is not common. 

Nonlinear regression enables broad molecular weight distribution standards to be used for SBC calibration and permits simultaneous resolution correction euid calibration. Furthermore, it greatly increases the variety of equations 4iich can be fit to calibration curves, detector linearity data and chromatogreun shapes. 

Tn the previous papers of this series (1, 2, 3, 4) calibration and repro- ducibility of gel permeation chromatography (GPC) have been extensively examined. This paper describes the application of GPC to two selected samples of linear polyethylenes, one having a narrow molecular weight distribution (NMWD) and another a broad molecular weight distribution (BMWD). These samples were distributed by the Macro-molecular Division of IUPAC (5) for the molecular characterization of commercial polymers. The average molecular weights by GPC are compared with the data obtained from infrared spectroscopy, osmotic pressure, melt viscosity, and intrinsic viscosity. Problems associated with data interpretation are discussed. 

To determine the interdetector delay volume for a viscometer, a broad molecular weight distribution standard can be injected and a Mark-Houwink plot, that is, log [t ] versus log M, generated using universal calibration. The interdetector volume is adjusted until the expected Mark-Houwink exponent is obtained (36). 

With a light-scattering detector, a log M versus elution volume calibration curve is constructed from a series of narrow molecular weight distribution polymer standards. A broad molecular weight distribution standard is then injected, and an iterative procedure finds the interdetector volume that superimposes the broad MWD standard calibration curve onto the one established by the narrow standards (38). 

Because of its solubility in both THE and more polar solvents, PVAc has also been used to calibrate systems employing mobile phases that are nonsolvents for polystyrene. Gilding et al. (9) describes a technique for calibrating an SEC system in hexafluoroisopropanol (HPIP) using broad molecular weight distribution (MWD) PVAc standards. HPIP, a nonsolvent for polystyrene, is an excellent solvent for polyamides, polyesters, polyurethanes, and polyglycolic acid. 

Polydisperse polymers do not yield sharp peaks in the detector output as indicated in Fig. 9.14. Instead, broad bands are produced which reflect the polydispersity of synthetic polymers. Assuming that suitable calibration data are available, we can construct molecular weight distributions from this kind of experimental data. An indication of how this is done is provided in the following example. 

Previous methods of molecular weight calibration using broad MWD standards were of three basic types. Those which employ a broad MWD standard with known molecular weight distribution... 

The peak position and universal calibration methods rely on peak position calibration with known polymers of narrow molecular weight distribution. Several other calibration procedures requiring only a single broad moleculau weight standard have been proposed [77,439]. These procedures are quite c< plex and have a major drawback in that, unlike the peak position methods, instrumental peak broadening must be accounted for correctly if accurate results are to be obtained. 

The most straightforward method for calibrating the relationship between D and M is to measure both D and M for a set of monodisperse samples with different molecular weights. In reality, the monodisperse samples have to be replaced by narrowly distributed standards made available either by relevant living polymerization or by fractionation of a broadly distributed sample. However, only a few kinds of polymers, e.g. polystyrene and poly(methyl methylacrylate), can actually be prepared so as to have a sufficiently narrow molecular weight distribution (Mw/Mn 1.1), and the fractionation is very time consuming. Thus, the straightforward calibration of the D vs M relation is not always practical. 

Direct calibration of GPC-SEC columns requires well-characterized polymer standards of the same type of polymer one has to analyze. However, narrow molecular-weight distribution (MWD) standards are available for a limited number of polymers only, and well-characterized broad MWD standards are not always accessible. The parameter controlling separation in GPC-SEC is the size of solute in the chromatographic eluent. Therefore, if different polymer solutes are eluted in the same chromatographic system with a pure exclusion mechanism, at the same retention volume, molecules with the same size will be found. 

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