No abstract

The synthesis of thermoresponsive graft copolymers based on a carboxymethylcellulose (CMC) backbone is reported. Thermal responsive properties are introduced by grafting the CMC sample with amino-terminated poly(N-isopropylacrylamide) (PNIPAM) side chains of a relatively low molecular weight. Turbidity measurements in dilute copolymer solutions showed that, due to the hydrophilic CMC backbone, macroscopic phase separation by increasing temperature above the lower critical solution temperature (LCST) of PNIPAM is not allowed for pH ≥ 3. Pyrene fluorescence probing studies in aqueous solutions revealed the formation of hydrophobic microdomains above the LCST of PNIPAM. In semidilute solution these microdomains interconnect the polymer chains, leading to the thermally induced formation of a physical network. The macroscopic result is the observation in semidilute solutions of a pronounced thermally induced viscosity enhancement. This thermothickening phenomenon is almost irrespective of pH, and it remains very important even at pH values as low as 3.

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 TCP 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 of 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.