Rheological properties of transient networks formed from copolymers of sodium acrylate and methacrylates substituted with amphiphiles: Comparison with sodium 2-(acrylamido)-2-methylpropanesulfonate copolymers

Rheological properties of aqueous solutions of the random copolymer of sodium acrylate (NaAA) and a methacrylate substituted with HO(CH(2)CH(2)O)(m)C(12)H(25) (C(12)E(m)), where m = 2, 6, or 25 (DEmMA), were panesulfonate (NaAMPS) and DEmMA of similar molecular weights focusing on the effect of the length of ethylene oxide (EO) spacer (i.e., the number of m in the DEmMA unit). The pendant C(12)E(m) moieties in these copolymers form micelles through intra- and interchain associations, and hence, polymer chains are cross-linked by the micelles, forming a network structure. Steady-shear and oscillatory rheological measurements were performed for the NaAA/DEmMA and NaAMPS/DEmMA copolymers with the DEmMA contents of 12 and 10 mol %, respectively, in 0.1 M NaCl aqueous solutions of pH 10 at varying polymer concentrations (C(p)). For the NaAA/DEmMA.copolymer, the network was formed more favorably when the EO spacer was shorter whereas the trend was completely opposite in the case of the NaAMPS/DEmMA copolymer. For m = 2, values of plateau modulus (G(0)) and rheological relaxation time (lambda) for the NaAA-based copolymer were larger than those for the NaAMPS-based copolymer by more than 4 and 3 orders of magnitude (at C(p) = 40-50 g/L), respectively. The NaAA copolymer of m = 2 exhibited gellike behavior with G(0) and lambda on the order of 10(2) Pa and 10(2) s, respectively (at C(p) = 40 g/L), solutions showing shear thinning over the shear rate range studied (10(-2)-10(3) s(-1)). In contrast, solutions of the NaAMPS copolymer of m = 2 were much less viscous with G(0) and lambda on the order of 10(-2) Pa and 10(-1) s, respectively (at C(p) = 40 g/L). For m = 6, the differences in G(0) and lambda between the two copolymers decreased to ca. 2 orders and 1 order of magnitude, respectively. When m = 25, however, G(0) for the NaAMPS copolymer was found to be larger than that for the NaAA copolymer by more than 1 order of magnitude and lambda values for the two copolymers were on the same order of magnitude. These striking differences in the rheological properties between the NaAA-based and NaAMPS-based copolymers were attributed to a much stronger tendency for interchain association in the NaAA-based copolymers.