Calibration narrow molecular weight

Noting GPC is a relative molecular weight method, such instrumentation needs to be calibrated. Narrow molecular weight distribution, anionically synthesized polystyrenes are used most often for the purpose. Other polymers used for calibration include poly(methyl methacrylate), polyisoprene, polybutadiene, poly(ethylene oxide), and the sodium salt of poly(methacrylic acid). Molecular weight ranges available start at low oligomers of only a few hundred g/mol, up to 20,000,000 g/mol. In aU cases, use of narrow molecular weight distribution standards is preferred. 

Determinarion of MW and MWD by SEC using commercial narrow molecular weight distribution polystyrene as calibration standards is an ASTM-D5296 standard method for polystyrene (11). However, no data on precision are included in the 1997 edition of the ASTM method. In the ASTM-D3536 method for gel-permeation chromatography from seven replicates, the M of a polystyrene is 263,000 30,000 (11.4%) for a single determination within the 95% confidence level (12). A relative standard deviation of 3.9% was reported for a cooperative determination of of polystyrene by SEC (7). In another cooperative study, a 11.3% relative standard deviation in M, of polystyrene by GPC was reported (13). 

The log Mn vs. count calibration curve is shown on Figure 5. This is a fairly linear calibration curve, but it covers only a relatively narrow molecular weight range of 145,000 to 317,000 g/mole. Although we have sought to prepare higher MW samples for this purpose, we inadvertently obtained polymers with bimodal MWD s and did not use them for this calibration. 

Figure 5. Calibration curve obtained with relatively narrow molecular weight samples of poly(dichlorophosphazene)...

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. 

Novolac molecular weights were measured in THF at 35°C by high pressure size exclusion chromatography using a Waters Model 510 pump (flow rate=1.0 ml/min), 401 differential viscometer detector and a set of Dupont PSM 60 silanized columns. A universal calibration curve was obtained with a kit of 10 narrow molecular weight distribution, linear polystyrene standards from Toya Soda Company. Data acquisition and analysis were performed on an AT T 6312 computer using ASYST Unical 3.02 software supplied with the Viscotek instrument. 

GPC calibration curves are established based on the radius of gyration of known-molecular-weight polymers, such as well characterized, narrow-molecular-weight distribution polystyrene. Branched polymers have a lower radius of gyration for their molar mass than the corresponding linear molecule. Thus, as branching increases the GPC numbers become less and less accurate and so should only be used for trends, and not exact calculations as some authors have done. 

Narrow Molecular Weight Triacetate Calibration. A linear relationship was found when log against the elution volumes of various cellulose triacetate fractions was plotted. For narrow molecular weight distribution triacetate fractions, the GPC experimental average molecular weight, termed can be expected to conform... 

Determining Calibration Curves frcm Polydisperse Samples. In conventional SEC interpretation, narrow molecular weight distribution standards are needed for calibration purposes. Nonlinear regression has enabled polydisperse scimples to be used. A variety of methods... 

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. 

Calibration. The accepted method of calibrating a GPC system was used. Narrow molecular-weight distribution high-density polyethylene polymers were characterized by light scattering, osmometry, and sedi-... 

In the Introduction it has been mentioned that the immediate reason for the development of phase distribution chromatography was the search for a fast and exact method to determine very narrow molecular weight distributions of polystyrenes obtained from anionic polymerization. The long way to reach this goal became evident only in the course of the investigations shown in the previous sections and representing a basis for the computation of such MWDs. In fact, not only exact measurements are necessary, but also mathematical expressions for the measured calibration curves and for the spreading of the injected concentration profile in the column must be stated, which cannot be done empirically. 

One of the most important advantages of gel chromatographic separations could not be used—namely, the calibration of the column using narrow-molecular-weight standard substances to determine the a... 

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. 

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. 

A major interest in narrow distribution polymers is for research and molecular weight calibrations in gel permeation chromatography. Narrow-molecular-weight polystyrenes are made by initiation with alkali metal alkyls that are particularly effective in this application but that only polymerize conjugated monomers like styrene or butadiene. 

As expected, the higher molecular weight n = 50 oligomer with a larger hydrodynamic volume elutes before (19 min) the smaller n == 35 oligomer (20 min) and n == 20 oligomer (22 min). Such data can be used to calibrate the SEC and unrelated calibrants such as narrow molecular weight polystyrenes can be avoided (20). 

Narrow dispersed standards are the most commonly used. This involves the use of well characterised polymers standards of narrow molecular weight/size distribution and known molecular weight. A variety of polymer types are commercially available, polystyrene being the most widely used. These standards are run either individually or in well resolved combinations, alongside the polymer solutions under analysis. The elution volumes of the standards are then recorded and a calibration graph of log(molecular weight) versus elution volume constructed (similar to Figure 9.2) whose slope is equal to... 

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