A series of poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMTAC) in aqueous NaCl solutions with a weight-average molecular weight (M w) of between 6.0 × 103 and 1.5 × 10 5 were characterized by synchrotron radiation small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS) measurements at 298 K. The hydrodynamic radius (RH) of PMTAC initially decreased with increasing salt concentration (Cs) and then leveled off at Cs = 0.5-1.0 M. The values of z-average root-mean-square radius of gyration (〈S2〉z1/2) simply increased with a decrease in Cs for Mw > 2 × 104 g/mol. In contrast, the values of 〈S2〉z1/2 for Mw < 2 × 104 g/mol increased with an increase in Cs probably because of a change in local conformation of PMTAC chain. To study local chain conformation of PMTAC, scattering functions and Mw dependences of 〈S2〉z1/2 and RH for PMTAC were analyzed in terms of the helical wormlike chain (HW) model which is a simple continuous elastic wire model with bending and torsional energies. The characteristic helix of the HW model in potential energy minimum for PMTAC clearly indicates that the local conformation is strongly influenced by the ionic strength of the solution.
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