New insights into the self-assembling of some hydrophobically modified polyacrylates in aqueous solution

Abstract

A series of hydrophobically modified sodium polyacrylates (NaPACn) grafted with linear alkyl side chains with 10 to 18 atoms of carbon (n) has been synthesized. FT-IR and 1H-NMR characterization proved that the alkyl grafts were attached to the polymer backbone via the amide link and the grafted amount was around 3 % (mol). The aqueous solutions of NaPACn were investigated by a score of methods including viscosity, fluorescence (steady-state and time-resolved), refractive index, and dynamic rheology. The results revealed that each NaPACn has a characteristic overlapping concentration (c*), which lowers as the grafted chain is more hydrophobic. Above c*, the viscosity deviates from that of sodium polyacrylate (NaPA) and solutions of high viscosity are obtained. The phenomenon is more evident as the alkyl chain increases in length and was attributed to the cross-association of the grafts. The NaPAC16 and NaPAC18 are viscoelastic solids, whereas the C10–C14 derivatives are viscoelastic liquids. The comportment of NaPAC16 and NaPAC18 is sustained by the positive yield stress and the high relaxation time values. Steady-state fluorescence unveiled intracoil hydrophobic microdomains (HMD), whereas dynamic fluorescence and refractive index showed the existence of both intra- and intercoil aggregates.