Rheological properties of model alkali-soluble associative (HASE) polymers: Effect of varying hydrophobe chain length

The rheological properties of 1 wt % aqueous solutions (at pH between 8.7 and 9.5) of model associative (HASE) polymers are presented. These polymers are the polymerization product of methacrylic acid, ethyl acrylate, and macromonomers which contain hydrophobes with the alkyl chain ranging in length from C-12 to C-20. At high pH, the polymers form a network of temporarily associating hydrophobic junctions, resulting in an enhancement of the shear viscosity which increases with the hydrophobe chain length. Strain amplitude sweep results show that the strength of the hydrophobic association increases with the length of the hydrophobe. This is also reflected in the increase in the ratio of the elastic to viscous components of the linear viscoelastic properties. When sheared beyond its equilibrium state, the associative polymers display a terminal (second-order) viscoelastic behavior at higher frequencies as the network is increasingly being disrupted by higher applied stresses. The general behavior of the polymers changes from Zimm-like to Rouse-like, and to reptation-type with a crossover between the storage and loss moduli curves, as the alkyl chain of the hydrophobes increases from 12 to 16, and to 20, carbon atoms. It is believed that in the unstressed state, other relaxation processes with much longer times are involved, and it is these long relaxation times which are greatly curtailed as the network is disrupted by an applied stress.