Narrow molecular weight distribution polymer standards

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). 

Since the calibration of a GPC is dependent upon the effective size in solution of the sample molecules, the type (structure) of molecules used for the calibration is important. The ideal case is to calibrate with a standard sample(s) of the material of interest. However, this is not always possible. In those instances, arbitrary standards are chosen. The arbitrary standards are used to construct a size calibration where the molecular size is calculated from the standard. For polymer analysis, these standards are often polystyrene of narrow molecular weight distribution. These standards may be purchased from a variety of suppliers. 

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. 

To date, the standard samples of narrow molecular weight distribution polymers available commercially are mainly polystyrenes. These samples have polydispersity indexes that are close to unity and are available over a wide range of molecular weights. For determining molecular weights of polymers other than polystyrene, however, the molecular weights obtained from these samples... 

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. 

SRM 705 is the narrow molecular-weight-distribution sample the other SRM 706, by contrast, is a broad molecular-weight-distribution polymer, the result of a thermal polymerization which more closely resembles commercially available polymers. Both of these were supplied by the Dow Chemical Co. (Certain commercial equipment, instruments, or materials are identified in this paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose.)... 

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. 

The retention volume in SEC increases with increasing flow rate [3]. This is attributed to nonequilibrium effects, because polymer diffusion between the intrapores and extrapores of gels is sufficiently slow that equilibrium cannot be attained at each point in the column. With a decreasing flow rate, the efficiency and the resolution are increased. Bimodal distribution of a PS standard (NBS706) with a narrow molecular weight distribution was clearly observed at the lower flow rate. 

Figure 4.26 Gel permeation chromatography elution eurves for polymer standards having very narrow molecular weight distribution.

The SEC columns were calibrated using narrow molecular weight distribution water soluble polymer standards sodium polystyrene sulfonate, NaPSS (Pressure Chemical Company, Pittsburgh, PA) and dextrans (Pharmacia Fine Chemicals, Piscataway, NJ). The universal calibration curve proposed by Grubisic et al. (13) was found not to be applicable for these polymer standards. The alternative calibration procedure of Coll and Prusinowski (J ) which incorporates excluded volume effects in the numerical value of the Flory parameter was found to be applicable for this system ( ). The calibration curve for the two-column network system described above is shown in Figure 3. The Kyyy parameter used on the abscissa of Figure 3 is a normalized elution volume parameter defined by Equation 1,... 

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