While using the combination of synthetic polymers like Carbopol with naturally derived polysaccharides, I came across to the fact that the viscosity has been going down despite it should have the increment in the viscosity. I just want to know, the reason behind this unusual trend in terms of viscosity seen. Your input would be appreciated....Thank you..
One reason we have observed for sudden decreases in viscosity on mixing two polymer solutions is that the polymers may interact and form an insoluble precipitate. The precipitate may be so finely divided that you don't see it as such, but the drop in viscosity is a clue that it may be occurring.
Similar or same to Dr. Cote's explanation is, your polymer chain dimension in the blend may be smaller than in the neat Carbopol and neat polysaccharide. For the same chain length, your viscosity will scale with chain dimension (larger = higher viscosity), and the chain dimension can change with solvent quality (which changes with what solvent you use -- in this case, the solvent of a polymer in a blend can be the 'other' polymer)
According to the other explanations: carbopol is in fact poly(acrylic acid). So carboxyl groups of this polymer can form interpolymer complexes with proton donating groups from polysaccharides (e.g carbonyl). Formation of interpolymer complexes decreases size of the macromolecules and this influences solution viscosity. One have to keep in mind that this phenomenon is pH sensitive.
Viscosity of mixtures of polymer solutions can be reduced by forming a smaller number of associates in the mixture as compared to the original system, or by forming associates with less strength. Most pairs of polymers - are incompatible, and you observed phenomenon occurs quite frequently. It is advisable to obtain flow curves for the starting polymers (shear rate - shear stress) and for mixtures of different composition. If the curves have a yield-point- stress, then compare their values or compare the shear stress characterizing the switch-over to a Newtonian flow. Look at articles:
Chuppina S.V. Anti-Icing gradient organosilicate coatings/ARTICLE in GLASS PHYSICS AND CHEMISTRY 33(5):502-509 · OCTOBER 2007
DOI: 10.1134/S1087659607050136
S. V. Chuppina · V. A. Zhabrev · V. S. Baragunova. Structural-mechanical properties of organosilicate composites with introduced curing agents
Glass Physics and Chemistry 02/2009; 35(1):67-73. DOI:10.1134/S1087659609010106 ·
There's another effect (the polyelectrolyte effect) that causes reduction in viscosity with increase in concentration. If you're using pure water, then the ionic strength may be low enough to see this. A clue here would be adding salt -- if the trend then reverses (increased concentration gives increased viscosity), you may be seeing the polyelectrolyte effect. Shear thinning is another possibility, so the advice from previous comments all applies.
