Library Polyvinylpyrrolidone (PVP) a Molecular Weight Determination with the BI-MwA
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/∆R 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/dc1. A value of 0.174 mL/g is used for dn/dc. c is polymer concentration, and ∆R 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/∆R 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 average, an average weighted by the square of molecular weight. Since the PVP sample is polydisperse, 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 -4cm3mol/g2.
These data show that the BI-MwA can be used to determine the molecular weight of PVP.