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

Abstract

A series of zwitterionic polyelectrolytes poly­[3-(<i>N</i>-2-methacryl­oyloxy­ethyl-<i>N</i>,<i>N</i>-dimethyl)­ammonato­propane­sulfonate] (PMAPS) with a wide range of weight-average molecular weight (<i>M</i>_w) between 5.5 × 10³ and 1.6 × 10⁶ g mol^–1 with narrow molecular weight distribution (<i>M</i>_w/<i>M</i>_n = 1.07–1.19) were thoroughly characterized in aqueous NaCl solutions for salt concentration (<i>C</i>_s) over a range from theta <i>C</i>_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 (<i>B</i>) of PMAPS in an aqueous NaCl solution, SAXS profiles and the <i>M</i>_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 <i>C</i>_s, whereas the value of <i>B</i> was increased gradually with the increasing <i>C</i>_s. Thus, the dominant factor for the chain dimension of PMAPS in aqueous NaCl solutions was the long-range interaction (i.e., <i>B</i>) than the short-range interaction (i.e., λ^–1). The observed <i>C</i>_s dependences of λ^–1 and <i>B</i> 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