Rheoviscosimetric Study of Poly(N-isopropylacrylamide) in Water and in the Solvent Mixture (Water-Ethanol), Evidence of High Huggin’s Coefficient

Abstract

This paper presents a rheoviscosimetric study of a thermo-sensitive polymer, poly(N-isopropyl acrylamide) PNIPA, with weight average mass Mw = 17774 Daltons whose LCST is 32 °C. The concentration regimes in water are identified. As solvent, we first used ultra pure water then we added an organic co-solvent (ethanol) with a mass up to 20%. Under shear, the dynamic viscosity of a PNIPA solution with water as solvent versus temperature increases from a critical temperature Tmin very close to the LCST until reaching a maximum at Tmax. Kinematic viscosity measurements do not show this increase in viscosity. It is believed that by deforming the chains of the polymer and bringing the hydrophobic end groups (isopropyl) each other, the shearing causes the formation of aggregates through an inter-chain association at the time of activation of the hydrophobicity of isopropyl groups at ~ 32 °C. The authors found that the gradual addition of ethanol decreases the quality of the solvent; the intrinsic viscosity shows a decrease in the volume occupied by the chain in the solvent mixture. The Huggins coefficient kH confirms this trend by largely positive values in zone I in the presence of ethanol, therefore the authors believe that two contributions (hydrodynamic and Brownian motion) can explain the highness of kH.