Insight into the Structure of a Comb Copolymer-Surfactant Coacervate from Dynamic Measurements by DOSY NMR and Neutron Spin Echo Spectroscopy
Abstract
ABSTRACT: Structure and dynamics of an anionic comb polyelectrolyte poly[methacrylic acid-stat-poly(ethylene glycol) methyl ether methacrylate] (PMAA-PEGMA) and a cationic surfactant N-dodecylpyridinium chloride (DPCl) in coacervate emulsion formed by electrostatic complexation of PMAA-PEGMA and DPCl were followed by a combination of diffusion-ordered and nuclear Overhauser effect NMR spectroscopies (DOSY and NOESY NMR), small-angle neutron scattering (SANS), and neutron spin echo (NSE) spectroscopy. The measurements were conducted for various charge ratios Z of the number of DPCl positive charges to the number of PMAA-PEGMA negative charges.
NMR spectra showed splitting of signals caused by partitioning of both PMAA-PEGMA and DPCl between the bulk aqueous phase and coacervate particles formed at Z > 0.7, with much slower diffusion of both the polymer and surfactant in the coacervate phase. SANS measurements revealed the formation of two types of DPCl micellar structures in the system: (i) Small elongated micellar aggregates forming the PE-S complex with PMAA-PEGMA chains.
These structures exhibited fast diffusion and weak NOESY contacts of DPCl heads with PEGMA side chains. (ii) Large aggregates of densely packed micelles forming the PE-S complex with PMAA-PEGMA chains. These structures exhibited slow diffusion and strong NOESY contacts of DPCl heads with PEGMA side chains.
The difference in dynamics between the two types of DPCl aggregates was confirmed by NSE, showing that apparent diffusion coefficients, D-app(q), in the high q region, 1.1-2.0 nm(-1), dominated by scattering from densely packed DPCl micelles were much lower (hydrodynamic radius, R(H) > 10(2) nm) than those in the low q region, 0.2-1.1 nm(-1), dominated by scattering from small elongated aggregates of DPCl micelles (R(H) of ca. 4 nm). NSE measurements thus confirmed that densely packed DPCl micelles were present in large aggregates, while the scattering contribution in 0.2-1.1 nm(-1) originated from small elongated micellar aggregates in the bulk solution.