Chain conformation of poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] in aqueous sodium chloride solutions

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 (R_H) of PMTAC initially decreased with increasing salt concentration (C_s) and then leveled off at C_s = 0.5-1.0 M. The values of z-average root-mean-square radius of gyration (〈S²〉_z^1/2) simply increased with a decrease in C_s for M_w > 2 × 10⁴ g/mol. In contrast, the values of 〈S²〉_z^1/2 for M_w < 2 × 10⁴ g/mol increased with an increase in C_s probably because of a change in local conformation of PMTAC chain. To study local chain conformation of PMTAC, scattering functions and M_w dependences of 〈S²〉_z^1/2 and R_H 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.