Rheological and Structural Characterization of Hydrophobically Modified Polyacrylamide Solutions in the Semidilute Regime

Abstract

Semidilute solutions of two series of polyacrylamide (AM)-based copolymers containing hydrophobic di-n-propylacrylamide (DPAM) and di-n-octylacrylamide (DOAM) comonomers, namely P(DPAM-co-AM) and P(DOAM-co-AM), have been characterized using rheology and small angle X-ray scattering (SAXS). The relaxation time and plateau modulus obtained from rheology, and the correlation length (ξ) obtained from SAXS are compared for copolymers with different hydrophobe content (f) and hydrophobe block length (NH). Shear rheometry experiments revealed that P(DOAM-co-AM) copolymers formed gels characterized by a broad distribution of relaxation modes. In contrast, solutions of P(DPAM-co-AM) copolymers exhibited a near-Maxwellian response although additional fast modes contribute significantly to the dynamic shear modulus at high frequency. For both copolymers, the dynamic shear moduli obtained at different temperatures could be time−temperature superposed, with a shift factor described by the Williams−Landel−Ferry equation. For P(DPAM-co-AM) solutions, the sticker lifetime (determined from the fitting of the dynamic moduli to a Maxwell model) was found to increase with increasing f but to be insensitive to NH, in the range examined. However, the plateau modulus Go decreases with increasing NH, which is related to the increase in strand length between stickers. Similar to P(DPAM-co-AM) solutions, one relaxation time for the gels of copolymers with longer hydrophobes (in this case determined from the crossover in the elastic moduli) is more sensitive to f than to NH. The SAXS data for the P(DPAM-co-AM) solutions could be modeled using the structure factor for a polymer solution in a good solvent although enhanced fluctuations had to be considered for the polymers with higher hydrophobe content. The correlation length was found to decrease with increasing temperature for P(DPAM-co-AM) solutions, due an improvement of the solvent quality. But ξ did not depend on the temperature for P(DOAM-co-AM) samples, probably due to stronger associations between hydrophobes. Trends of ξ with NH and f are discussed.