SEC with Light Scattering Detection

Light scattering has long been used as a stand-alone technique for the absolute measurement of the weight average molecular weight of polymers. The classical approach to this technique was to measure the scattered light for a range of different angles and different solution concentrations such as to produce a Zimm plot which would allow determination of both the molecular weight and the shape of the molecule in solution. In the early 1970s, the introduction of laser light scattering instruments allowed the optical cell to be reduced in size to be compatible with a chromato-graphic detector cell and to work at a low enough angle to avoid the need for extrapolation to zero angle. These Low-Angle Laser-Light Scattering (LALLS) instruments allowed the stand-alone measurement to be carried out far more efficiently but were immediately used for direct combination with SEC such that the 'absolute' measurement of the molecular weight distribution was practical for the first time.

More recently, other instruments have been introduced which measure the scattered light at a range of angles simultaneously; these are known as Multi-Angle Laser-Light Scattering (MALLS) detectors. Instruments have also been introduced which only monitor the light scattered at a right angle (RALLS).

In addition to providing information on the absolute molecular weight distribution, SEC-light scattering is used to obtain information on branching

(basically by comparing the absolute molecular weight with the apparent molecular weight for the linear polymer calibration). The response of the light scattering detector increases dramatically with molecular weight and SEC-light scattering systems are very good at examining any variation at the high molecular weight end of a distribution.

In SEC-light scattering, the solution concentration is an important parameter in the calculation and it is necessary to have accurate information of the differential refractive index for the polymer/solvent (this is a squared term in the calculation). This requirement for information on the differential refractive index is problematic for examination of copolymers.

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