Laser Light Scattering and Related Techniques

The electron clouds of a molecule irradiated with visible light will orient themselves to some extent in line with the electric vector of the light. If the light is plane polarised, scattered light is visible only in one plane, and if the dimensions of the molecule are small compared with the wavelength of the light, the Rayleigh equation, eqn. (4.21), holds  

The parameter Ir lf at a given value of 9 is called the Rayleigh ratio, Rq. Modern multi-angle laser light scattering equipment (MALLS) simultaneously measures scattered intensity at several values of 9. Therefore, a typical static light-scattering experiment involves the measurement at a matrix of values of 9 and c the polymer concentration. 

For an ideal solution of small particles, KcjRe, where c is the polymer concentration and is a system constant depending only on the solvent and 9, becomes the reciprocal of the weight-average molecular weight. For non-ideal solutions, molecular properties are expanded as a power series in concentration. Corrections can also be made for complications that arise when the size of the polymer becomes comparable to the wavelength of the light used (internal interference). 

Before computerisation, the weight-average molecular weight was obtained from the y intercept  

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