Question

A scientist wants to use GPC column to analyze copolymers of styrene (75%) with acrylonitrile (25%). Not having monodisperse calibration standards for the copolymer, the scientist goes ahead and use an existing polystyrene calibration. How and why will this affect their calculated average molecular weights?

Answer 1

Gel permeation chromatography seperates sample molecules by the difference in effective molecular size in solution.

this instrument allows the determination of weight average molecular weight, number average molecular weight, and dispersity of molecular materials.

there have two types of caliberation method to obtain weight averages. with polymers for which narrow molecular weight distribution standards are commercially available, eg polystyrene, a direct caliberation on the basis of molecular weight can be made. if standards are not commercially available they must either be prepared or an indirect method used.

if narrow molecular weight distribution standards are not exist, indirect method of caliberation can be applied. one of the indirect method can be applied is Q-factor method. from calulations molecular size averages will convert to molecular weight averages by a Q factor.

Limitations of Conventional GPC

1)Conventional GPC employing a single concentration detector is a comparative technique. During calibration, it is assumed that a series of polymer molecules elute from the GPC/SEC column at particular times based upon their molecular weights

2)Conventional GPC is relatively information poor. The column separates the polymer molecules on the basis of their size in solution, and the detector determines the concentration of the polymer molecules eluting from the column. No other information about the behavior of the polymers in solution is revealed, and their size, although crucial to the separation mechanism, is not measured in the analysis

It is possible to overcome the limitations of conventional GPC by performing multi-detector GPC