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

• Published in: Macromolecules Vol. 37; no. 4; pp. 1492 - 1501
• Main Authors: Castelletto, Valeria, Hamley, Ian W, Xue, Wei, Sommer, Cornelia, Pedersen, Jan Skov, Olmsted, Peter D
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 24.02.2004
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization; Solution and gel properties; Viscoelasticity; Semidilute solution; Acrylamide copolymer; Temperature effect; Hydrophobic group; Acrylamide derivative copolymer; Aqueous solution; Experimental study; Concentration fluctuation; Lateral group
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma035039d

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 (N H). 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 N H, in the range examined. However, the plateau modulus G o decreases with increasing N H, 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 N H. 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 N H and f are discussed.