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6.2.
LIGHT SCATTERING
6.2.1. General
Light scattering (LS) has become the very common method to determine
molecular weights of polymers in solution thanks to relatively simple but
effective commercially available instrumentation. LS is often combined with
SEC (sixe-exclusion chromatography), enabling not only the average
molecular weights, but also the molecular weight distributions.
Scattering of light can be easily observed by the naked eye if strong light is
sent through a slightly concentrated solution of large macromolecules or
colloidal particles. For example skimmed milk (diluted 10x with water) will
produce visible scattering using a simple laser pointer. The scattered light is
about 10
9
times weaker than the incident light. For practical use in dilute
polymer solutions a strong laser (typically 100 mW GaAs diode laser) is
needed in combination with sensitive detectors.
The blue sky observed on a clear day is observable scattering from molecules
(and particles) in the atmosphere. The sun is a fairly strong light source. The
blue colour is caused by the pronounced dependence on the wavelength.
A classical light scattering experiment simply involves preparing a
concentration series (polymer solutions with accurately known
concentrations). Each sample is entered into the instrument, either using flow
injection, separate glass vials, or pipetting the solutions into the optical cell of