Determination of MWD

Common methods for the experimental determination of the molecular mass of polymeric materials include  

Gel permeation chromatography is a very popular method that gives a full account of the MWD of a polymer expressed in intervals of chain length values, by separating the molecules according to their chain length. 

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In this work there is proposed approach which allows using broad standar ds for calibration and system suitability test. The approach is based on main principle of using of standards i.e. maximal closeness of tested sample and standar d. It has been shoved that the approach allows achieving essential improvement of robustness of method for determination of MWD of dextrans. 

The broad pore size distribution of Sepharose makes it well apt for the analysis of broad molecular mass distributions of large molecules. One example is given by the method for determination of MWD of clinical dextran suggested in the Nordic Pharmacopea (Nilsson and Nilsson, 1974). Because Superose 6 has the same type of pore size distributions as Sepharose 6, many analytical applications performed earlier on Sepharose have been transformed to Superose in order to decrease analysis time. However, Sepharose is suitable as a first try out when no information about the composition of the sample, in terms of size, is available. 

The uncomplicated packing procedure of traditional media to yield efficient columns is demonstrated by the reduced plate heights of 2 or slightly below as reported for Sephadex and for the qualification of Sepharose columns for the determination of MWDs. 

Aqueous SEC is widely used for the determination of MWDs of a variety of synthetic and naturally occurring water soluble polymers, as well as for separations of small molecules. The column requirements for aqueous SEC are very demanding to eliminate ionic and hydrophobic effects. 

GPC is extremely valuable for both analytic and preparative work with a wide variety of systems ranging from low to very high molecular weights [2], The method can be applied to a wide variety of solvents and polymers, depending on the type of gel used. GPC has been used for routine polymer characterization and quality control, particularly in determinations of MWD and for characterizing low polymers and small molecules, e.g., for prepolymers in resins and for polymer additives [7,8]. 

GPC has become especially useful in polymerization reaction engineering since it permits comparatively rapid and precise determination of MWD. As shown in Figure 5, Xw obtainable from the MWD in a batch reactor can be used to determine the initiation rate constant as well as kp2/kt assuming the pseudo-steady state, the absence of chain transfer, and termination by combination. Similar relationships with CSTR s are available. 

Not all process variables are measurable on-line most end-use properties are not. Some controlled variables affecting product quality are measurable or they can be estimated many are not. Even when the technology to measure these variables on-line exists, the cost of such sensors may be prohibitive. On-line gel permeation chromatography for the determination of MWD is an example. Almost all controlled variables specifying operating conditions are measured. Most manipulated variables are known because they are either measured or set by a control system (or both). As discussed above, disturbances may be measured or unmeasured. 

A primary application of SEC is for the determination of MWD (31). In this approach the raw data chromatogram obtained as output from the concentration detector is divided into a number of time slices of equal width, as shown in Figure 10. 

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