Passage V

Size exclusion chromatography (SEC) is used to separate the components of a solution of polymer molecules. In SEC a sample solution of polymer molecules is injected into a flow of solvent. The sample is then carried through a column filled with beads that contain microscopic pores (see Figure 1).

Smaller molecules easily diffuse into the pores. Larger molecules do not as easily diffuse into the pores, or are larger than the pores. Therefore, smaller molecules spend more time in the column than do larger molecules, causing the components of the mixture to separate. As solvent containing a component of the mixture exits the column through the detector, a peak is plotted versus time (starting from injection). The portion of solvent corresponding to a peak is called a fraction. The time corresponding to the top of a peak is the fraction’s retention time (RT). The RT corresponds to the average, molecular mass (AMM) of the molecules in that fraction. The shape of the peak reflects the distribution of molecular masses of the molecules in the fraction.

Experiment 1

Polystyrene is a polymer made up of identical subunits. Five types of polystyrene (P1-P5)were dissolved together in a solvent. The AMM of P1-P5 is given in Table 1. The range of molecular masses for each of P1-P5 was ±amu of the AMM.

A sample of the solution was injected into an SEC apparatus. Each fraction was analyzed as it exited the apparatus (see Figure 2).

Chemists used 3 different methods to synthesize polystyrenes, following polystyrene mixtures Ml, M2, and M3, respectively. Each of M1-M3 was then analyzed as in Experiment 1 (see Figure 3).