Salt Dependence of the Chain Stiffness and Excluded-Volume Strength for the Polymethacrylate-Type Sulfopropylbetaine in Aqueous NaCl Solutions

A series of zwitterionic polyelectrolytes poly[3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate] (PMAPS) with a wide range of weight-average molecular weight (M_w) between 5.5 × 10³ and 1.6 × 10⁶ g mol^-1 with narrow molecular weight distribution (M_w/M_n = 1.07-1.19) were thoroughly characterized in aqueous NaCl solutions for salt concentration (C_s) over a range from theta C_s (0.074 M) to 1.0 M by synchrotron radiation small-angle X-ray scattering (SAXS), light scattering, and viscometry at 25 °C. To determine the chain stiffness parameter (λ^-1) and the excluded-volume strength (B) of PMAPS in an aqueous NaCl solution, SAXS profiles and the M_w dependences of the radius of gyration, the second virial coefficient, the interpenetration function, the hydrodynamic radius, and the intrinsic viscosity for PMAPS were analyzed on the basis of the (un)perturbed cylindrical wormlike chain model. The experimental λ^-1 value for PMAPS chains in aqueous NaCl solutions barely decreased but was almost constant with the increasing C_s, whereas the value of B was increased gradually with the increasing C_s. Thus, the dominant factor for the chain dimension of PMAPS in aqueous NaCl solutions was the long-range interaction (i.e., B) than the short-range interaction (i.e., λ^-1). The observed C_s dependences of λ^-1 and B for PMAPS chains in aqueous NaCl solutions were fairly described by the theories of the polyampholyte with the nonrepulsive, the repulsive, and the attractive electrostatic interactions.