Gel Permeation Chromatography and Its Use in the Development of Resins

Because polymeric materials are mixtures of molecules of various sizes and weights, average molecular weight is used to describe them, but this can be misleading because polymers of the same average weight may have different weight distributions. Molecular weight distribution affects tensile strength, elongation, impact strength, and elasticity. Molecular weight distribution can be determined for any polymer sample that is soluble by several methods: fractional precipitation, equilibrium, centrifugation, sedimentation, column fractionation, and others. While these techniques are expensive and time consuming, gel permeation chromatography is a fast and precise method of determining molecular weight distribution. In GPC, a sample is transported by a suitable solvent through a column packed with porous material and molecules are separated by size according to their varying mobility. Once the sample is resolved, the solvent carries it out of the column, and eluted solution is collected in small fractions (0.25 m1-5.0 ml); the fractions are analyzed by various techniques to obtain the chemical compositions of the components. The fractions contain enough material for obtaining an infrared spectrum; NMR, x-ray emission spectrography, radioisotope counting, or C, H, and N analyses can also be used in certain cases. A molecular size distribution curve can he obtained by plotting the difference in refractive index between the solvent and each fraction against the elution volume of the fraction. GPC chromatograms describing urea-formaldehyde and phenol-formaldehyde resins are given. This technique allows close scrutiny of how a resin is formed and what charges takes place in its molecular size distribution. This information can make it possible to produce resins with a wide variety of physical properties.

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