ASTM D4001 Average Molecular Weight of Polymers By Light Scattering

Scope:

ASTM D4001 test method outlines how to use the light scattering method to determine the weight-average molecular weight M_w of polymers. The weight-average molecular weight, when combined with the average molecular weight, provides a useful estimate of the molecular-weight distribution’s breadth. This information on molecular weight also aids in the calibration of molecular weight standards for use in liquid-exclusion chromatography, as well as correlation with dilute-solution or melt-viscosity studies. ASTM D4001 also allows the determination of the second virial coefficient, A₂ (a measure of polymer-solvent interactions), and the radius of gyration, R_g (a measure of the dimensions of the polymer chain).

Procedure:

For the ASTM D4001 light scattering test, a high-intensity monochromatic light, generally a  laser, is launched into a solution containing macromolecules. At one or more angles, one or more detectors quantify the strength of scattering. For all macromolecules having a radius higher than  1-2% of the input wavelength, the angular dependency properly determines molar mass and size.  The measurements are taken at various angles in relation to the incident light direction. The radius of gyration R_g is then calculated by measuring the scattering intensity from various angles, while the second virial coefficient A₂ is obtained by measuring the scattering intensity for multiple samples of varying concentrations.

Specimen size

The specimen utilized in the ASTM D4001 test process must be homogeneous and free of all foreign impurities. It must be specifically treated from the point of synthesis to avoid exposure to or contamination with particle contaminants.

Data:

As per ASTM D4001, the weight-average molecular weight M_w is calculated as follows:

where, 

K = the calibration constant 

(C/∆R_θ )_{c = 0, θ = 0} = the intercept of Zimm plot

Conclusion:

ASTM D4001 test method outlines how to use the light scattering method to determine the weight-average molecular weight M_w of polymers.