Positively charged amphiphilic polymers based on poly(N-isopropylacrylamide):: Phase behavior and shear-induced thickening in aqueous solution

The phase behavior and rheological properties of hydrophobically modified copolymers based on a thermosensitive N-isopropylacrylamide backbone are reported. These polymers bear a rather high fraction (5-25 mol %) of octyl, dodecyl, or octadecyl side groups, and their solubility in water is ensured by positive charges, introduced to the same unit as the alkyl side group. Their phase behavior is governed by the competition between the hydrophilic character of the charges and the hydrophobic character of the alkyl groups. Thus, the cloud point temperature, TCP, and the turbidity level above T-CP depend strongly on the alkyl length, the modification degree, and the polymer concentration. From the rheological point of view, the dodecyl- and octadecyl-modified polymers are powerful shear thickening materials in aqueous solution. The sharpness elf this shear-induced thickening phenomenon and the critical shear rate at which it occurs depend on various factors: length of the alkyl groups, modification degree, polymer concentration, and temperature. Possible mechanisms responsible for this behavior are discussed.