Polyvinylpyrrolidone (PVP) Molecular Weight Determination with the BI-MwA

J. Bodycomb,

Polyvinylpyrrolidone (PVP) is a polar molecule and is used in a wide variety of applications. For example, since it simply passes through humans, it is a binder in pharmaceutical tablets. Because PVP binds to polar molecules quite well, it is also used as a paper coating. PVP can also be found in shampoo, toothpaste, paint, and adhesives. Rapid, reliable characterization is important to better understand manufacturing performance, processing, and structure-property relationships. With the correct choice of instrument, molecular weight determination of PVP is routine.

PVP obtained from Aldrich was dissolved in water and data were collected with a BI-MwA in batch mode. A significant problem in light scattering is dust and the BI-MwA design provides a solution with its sealed flow system and automated dust rejection algorithms. Both of these features eliminate the deleterious effects of dust. Values for Kc/ΔR were automatically calculated with the BI-MwA software.
Here, K is the Debye constant and it is proportional to the square of the refractive index increment, dn/dc. A value of 0.174 mL/g is used for dn/dc. c is polymer concentration, and ΔR is proportional to the excess scattered intensity.

To determine molecular weight, the software then generates a Zimm, Berry, or Debye plot and values for molecular weight, radius of gyration (except for the case of a Debye plot), and second virial coefficient are calculated and displayed. After extrapolation to zero angle and zero concentration, the quantity Kc/ΔR is equal to the reciprocal of the weight-average molecular weight (Mw).
The Mw determined by fitting is (5.490 +/- 0.030) x 104 g/mol.


The radius of gyration (Rg) determined by fitting is 18.1 nm. Note that this radius of gyration is much larger than one expects from such a low molecular weight polymer. Typically, random coil polymers with a molecular weight of less than 75,000 g/mol are Rayleigh scatterers since their Rg is less than 12 nm. The value of Rg determined by light scattering is the so-called z-average, an average weighted by the square of molecular weight. Since the PVP sample is poly-disperse, this explains the high value of Rg. The second virial coefficient (A2) obtained by fitting is 4.33 x 10-4 cm3 mol/g2. Since this value is positive, it indicates that water is a thermodynamically good solvent for PVP. For comparison, the second virial coefficient of molecular weight 55,000 g/mol PS in THF is 7 x 10-4 cm3 mol/g2.

Zimm Plot of PLA

With kind permission of Brookhaven Instruments Corp.