The effect of the ionic strength on the rheological behavior of hydrophobically modified polyacrylamide aqueous solutions mixed with sodium dodecyl sulfate (SDS) or cetyltrimethylammonium p-toluenesulfonate (CTAT)

Two hydrophobically modified polyacrylamides (HMPAMs) with different molar contents of t-octylacrylamide as hydrophobic comonomer (1 and 1.5%molar) were synthesized by aqueous micellar copolymerization in water. The molecular structure was characterized in detail by light scattering and viscometric measurements using automatic continuous mixing (ACM) techniques in dilute solution. There was no significant difference between the weight average molecular weight (Mw) of homopolyacrylamide and the copolymers. The elucidation of polymer microstructure by 1H NMR showed that the hydrophobe content in the polymer backbone is almost the same as the hydrophobe content in the feed. This study reveals that branched t-octyl alkyl chains are not hydrophobic enough to induce significant associating properties, but can exhibit strongly magnified thickening properties in the presence of surfactant. The effect of NaCl addition to the solutions of the prepared HMPAMs was also explored as well as their interactions with sodium dodecyl sulfate (SDS) or cetyltrimethylammonium p-toluenesulfonate (CTAT). A direct comparison of the influence of spherical surfactant micelles and worm-like micelles on the rheological properties of the same polymer samples was made. For a given HMPAM copolymer at a fixed concentration in deionized water, the solutions zero-shear viscosities tend to increase upon adding surfactant, then they go through a maximum and then decrease (in the case of CTAT the viscosity after the maximum tends to slowly increase with CTAT concentration). These well known effects are due to the hydrophobic interactions between HMPAM and SDS or CTAT. The maximum viscosity occurs in HMPAM/SDS deionized water solutions. However, we demonstrate that by the addition of salt, the HMPAM/CTAT solutions can exhibit much higher viscosities (two orders of magnitude) than HMPAM/SDS solutions at low CTAT concentrations and high ionic strengths (larger than 1.5 M NaCl). This pronounced increased in viscosity is attributed to the growth of the CTAT worm-like micelles as the ionic strength increases, which can interact with the hydrophobic groups present in the HMPAMs and can form an entangled interacting network in solution.