The Determination of Weight-average Molecular Weight of Hydrophobically Associated Water-soluble Polymers

Intrinsic viscosity and strength of hydrophobic association of hydrophobically associated water-soluble polymers (HAWSP) were investigated in different solvents containing formamide and salt, through the capillary tube method and fluorescence probe technique. Weight-average molecular weight of HAWSP was measured by static light scattering method under suitable solvent conditions that could eliminate the hydrophobic association and polyelectrolyte effect. The experimental results showed that NaCl had the capability of eliminating the polyelectrolyte effect, but NaCl can not destroy hydrophobic association. And NaCl promoted hydrophobic interaction because of increasing the polarity of HAWSP solutions. Macromolecular aggregation would be greater at higher levels of hydrophobe, which explains the observed rise in molecular weight. As a result, the weight-average molecular weight of HAWSP measured in NaCl solvent was not the real molecular weight. However, hydrophobic association can be completely destroyed by formamide. Moleculars of HAWSP dispersed in the formamide solvent, and real molecular weight of HAWSP can be measured. The results showed that weight-average molecular weight of HAWSP can be measured accurately in 50% formamide solvent with 0.2 mol/L NaCl. When hydrophobic association was eliminated completely, the values of weight-average molecular weight were identical with increasing formamide concentration; when the polyelectrolyte effect was removed, altering the salt concentration would not change the results of weight-average molecular weight.