The separation principle in size-exclusion chromatography (SEC) is determined by the selective permeation of the polymers into and out of the mobile-phase filled pores of the column packing. The elution time of the polymer is governed by the time that it expends in the pores; hence, larger molecules that expend less time in the pores elute first, and smaller molecules elute later. In Figure 1, the series of events are shown that occur during a size-exclusion separation. In the first column, the large and small molecules are injected and form a narrow band. This narrow band starts to separate in the second step "B", in which the larger molecules start moving faster down the column. In step "C", the larger molecules are eluted from the column, while the smaller molecules elute in step "D". It is accepted; therefore, that the principal factor affecting the size-exclusion chromatography of a polymer is the hydrodynamic volume and not the molecular weight. The apparent molecular weight of polymer samples is determined by comparison of the chromatogram of a sample to that of standards with known molecular weight. The relationship between molecular size and molecular weight depends on the conformation of the dissolved solute molecules; however, for any soluble conformation (such as random coil, rigid rod, or hard sphere), the molecular size increases with molecular weight. The rate at which the molecular size increases with molecular weight varies for the different possible solute conformations.

Figure 1 Illustrative description of separation of SEC. (From Introduction to Modern Liquid Chromatography, 2nd edition by Snyder and J.J. Kirkland, C1979 by John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.)

The SEC technique is capable of the separation of high molecular weight materials. The information obtained from the separation makes it easy to determine distribution of polymer molecular weights and molecular weight averages. The information generated can predict physical properties of polymers. It is possible to determine this information not only in linear polymers, but also in some cross-linked materials like superabsorbent polymers (1), after the cross-links are broken. Even in the event that cross-links cannot be broken, useful information can be obtained from the soluble polymers that did not cross-link. In the case of polyelectrolytes, the polymer properties in solution and characteristics of their electrostatic nature have to be taken into consideration.

Some examples of the size exclusion capabilities at Impact Analytical are shown below:

Dendrimers Organopolysiloxanes General Purpose and High Impact Polystyrene (GPPS, HIPS) Acrylonitrile-Butadiene-Styrene (ABS) Polymer Polycarbonate Polyurethane (PU,TPU) Acid Copolymers Hydrophilic Polymers Polyethylene Glycol Cellulose Ethers Polyacrylic Acid Linear and Cross-linked

Polyacrylamide Slightly Hydrophilic Polymers Sodium Polystyrene Sulfonates Alkali Soluble Latex Hydrophobic Polymers Underivatized Cellulose Polyallylamine Hydrochloride Hindered Amine Light Stabilizers Heavy Mineral Oil in Superabsorbent Polymers Polymer in Vinylbenzyl Chloride