Gel Permeation or Size-Exclusion Chromatography GPC, SEC

FIGURE 5.8 Molecular weight distribution of Example 5.5 plotted in reduced form. 

Until fairly recently, the approximate molecular weight distribution of a polymer could be determined only by laborious fractionation of the sample according to molecular weight followed by determination of the molecular weights of the individual fractions with one of the techniques previously discussed. The bases for these fractionation techniques are discussed in Chapter 7. Suffice it to say that they tend to be difficult and time-consuming and so, in general, are avoided whenever possible. 

A concentration-sensitive detector is placed at the outlet of the column. The most common detector is a differential refractometer (which measures the difference in refractive index between the pure solvent and that of the eluting fluid, containing the polymer), but ultraviolet (UV) or infrared (IR) detectors are also useful as long as the polymer has a functional group that can be detected in these light ranges. Regardless of the type of detector, it is essential that it measure some quantity Q that is proportional to the mass concentration of polymer at the column outlet Q must be independent of M)  

a GPC curve consists of a plot of Q (usually in arbitrary scale divisions, but related to the quantity of chains eluting at a particular time) versus v, the volume of solvent that has passed through the detector since sample injection, called elution or retention volume. The x-axis can easily be converted to time by dividing the elution volume by the volumetric flow rate of the solvent. 

The total area under a chromatographic peak is proportional to the amount (mass) of polymer eluted. Thus, for polymer mixfiires, it is possible to use GPC to estimate the weight fractions of different components, as long as their peaks do not overlap on the elution (x) axis. 

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