The interaction between sodium dodecylsulfate and the cationic-nonionic random copolymer (3-(2-methylpropionamide)propyl) trimethylammonium chloride-acrylamide of two different charge densities studied using dynamic light scattering and rheometry

The effects of complex formation between sodium dodecylsulfate (SDS) and th e positively charged (3-(2-methylpropionamide)propyl)trimethyl-ammonium chl oride-acrylamide (MAPTAC-AM) copolymer have been studied in dilute and semi dilute aqueous solution in the presence of 10 mM NaCl. Two different charge densities of the copolymer have been used in the study: 0.31 and 0.66, cor responding to the proportion of MAPTAC units. Dynamic light scattering (DLS ) and rheometry (static low-shear and capillary viscometry) have been perfo rmed on the systems at; different charge ratios, i.e., SDS/MAPTAC molar rat ios, I. Regarding the phase behavior, the maximum binding ratio prior to pr ecipitation differs between the copolymers. A 1.0% w/v solution of SDS/31% MAPTAC-AM is soluble at r = 0.4, while an SDS/66% MAPTAC-AM solution of 1.0 % w/v shows phase separation at this ratio. With excess surfactant, the com plex in the former system is resolubilized at r = 2.0, whereas the latter s ystem is still phase-separated at r = 5.0. DLS results show that, for both copolymers, the hydrodynamic radius, R-h, Of the single-chain copolymer-sur factant complex decreases as a function of r, but then increases slightly p rior to phase separation. The corresponding hydrodynamic virial coefficient , k(D), changes in the same manner as R-h,. The light-scattering data also show that the formation of larger structures is promoted as the polymer con centration is increased from 0.2 to 1.0% w/v. This is shown by the increase in the relative aggregate-to-single coil peak areas in the relaxation time distributions. Both systems have this common trend. The results from rheol ogical measurements support the results from DLS. A reduction in intrinsic viscosity, [eta], is observed on increasing r up to phase separation. The m ajor part of the static low-shear measurements showed Newtonian behavior fo r both systems at different copolymer concentrations (27.6-138 mM), and at different r. These systems, partially ionic polymer/oppositely charged surf actant, present very interesting rheological behavior at relatively high po lymer concentrations and at low r values. Their behavior is similar to thos e of hydrophobically modified polyelectrolytes.