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

A series of hydrophobically modified sodium polyacrylates (NaPAC(n)) grafted with linear alkyl side chains with 10 to 18 atoms of carbon (n) has been synthesized. FT-IR and H-1-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 NaPAC(n) 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 NaPAC(n) 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 NaPAC(16) and NaPAC(18) are viscoelastic solids, whereas the C-10-C-14 derivatives are viscoelastic liquids. The comportment of NaPAC(16) and NaPAC(18) 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.