SEC Columns and the Separation Mechanism

In SEC, the polymer molecules are separated as a function of their size in solution. The column pack ings are porous and the separation is achieved according to the degree of access of the polymer molecules to the pores.

The solvent in the column packing pores can be considered as the stationary phase and the interstitial solvent as the mobile phase. A distribution coefficient is established by which the time spent in the pores, or stationary phase, is dependent upon the solvated size of the polymer molecules and on the pore geometry. Since the larger molecules are more restricted in the pore volume available to them, the larger molecules spend less time in the stationary phase and are eluted first; smaller molecules are effectively retarded and elute later.

It is fundamental to SEC than no other separation mechanism (e.g. adsorption) is occurring. As a consequence, the maximum elution volume available to achieve separation is the total pore volume. Molecules which are so large that they are totally excluded from the pores will elute at a volume corresponding to the interstitial or 'void' volume while molecules which can permeate all of the pores will elute at a volume corresponding to the void volume plus the pore volume. Anything eluting at a greater volume than this must be retarded by some additional mechanism.

With consideration to the above requirements, the materials used for SEC column packings are mainly selected for their pore geometry and lack of other interaction with the polymer molecules. In many solvent systems, the potential for adsorption limits the applicability of inorganic packings and polymeric packings are the more usual choice. The bulk of all SEC work with synthetic polymers is carried out using column packings produced from polystyrene, cross-linked with divinylbenzene. In the production of these materials, the pore geometry is manipulated to give variation in pore size and hence 'tailored' to suit particular molecular mass ranges.

In addition to pore size and inertness, the normal chromatographic relationships between packing size and efficiency are applicable but, as noted later, there are limitations to using too small a packing size with the higher molecular weight polymers.

SEC column packings are normally produced with a limited polymer molecular weight applicability but it has been normal practice to use a number of different columns (with specific pore sizes) in series to give an acceptably wide molecular weight range. Typically, to cover a molecular weight range of 2000 to 2000000, a bank of columns such as 1 x 103 A, 1 x 104 A, 1 x 105 A and 1 x 106 A would be used.

Figure 1 An overlay plot of the computed molecular weight distributions for a range of extrusion grades of polystyrene from various producers.

(The identification of pore size expressed in angstroms is common practice but potentially misleading, since the dimension refers to maximum extended polymer chain length, rather than actual pore size.)

There is potential for using an inappropriate combination of column packings and it is now becoming more common to obtain single columns which the manufacturer has filled with different porosity packings to give a wide molecular weight range. This

'mixed-bed' approach also gives an optimal combination of packing porosities such that a 'linear' calibration is obtained (see later). Even with the mixed-bed approach, a number of columns will normally need to be combined to achieve an acceptable peak capacity.

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