Rheology control by modulating hydrophobic and inclusive associations of side-groups in poly (acrylic acid)

Abstract

In this article we demonstrated that the viscosities of modified poly (acrylic acid) (PAA) solutions were tunable by modulating the hydrophobic and inclusion association between alkyl groups and β-cyclodextrin (β-CD) groups grafted to PAA. The viscosity can be controlled by changing the host-guest molar ratio, alkyl chain length, polymer concentration, salt concentration, pH value, temperature, or addition of native β-CD. A viscosity maximum for inclusive polymer networks constructed by mixing hydrophobically modified PAA (HMPAA) and β-CD-modified PAA (β-CDPAA) appeared at the alkyl : β-CD molar ratio of 1 : 1, which implies the inclusion association between HM and β-CD grafts is binary. Longer side chain length or higher polymer concentration led to higher viscosity for aqueous HMPAA solution with only hydrophobic association or its mixture with β-CDPAA with inclusion association. Monotonically increasing the ionic strength or pH value resulted in a viscosity maximum due to the competition between electrostatic repulsion and hydrophobic or inclusive association. The hydrophobic interactions of alkyl groups could be masked by native β-CD, and the networks of HMPAA and β-CDPAA mixture deconstructed upon addition of native β-CD molecules. The storage modulus and loss modulus of hydrophobic HMPAA and inclusive HMPAA + β-CDPAA solutions obey time-temperature superposition. The horizontal and vertical temperature shift factors obeyed a simple-exponential Arrhenius relationship, where the activation energies for hydrophobic association system were found to be 3.4 and -12.1 kJ/mol, and for inclusive association system 53.9 and -2.9 kJ/mol, respectively. © 2009 Curtin University of Technology and John Wiley & Sons, Ltd.