The complexation of oppositely charged polymers, or polyelectrolytes, in aqueous solution produces viscoelastic material, commonly termed polyelectrolyte complexes, or PECs, with interesting properties. The reversible doping of PECs to a specific salt concentration sheds light on the mechanical properties of the material. Using dynamic mechanical analysis along with ion diffusion studies, the viscoelastic responses of a series of poly-([3-(methacryloylaminomethacryloylamino)propyl) trimethylammonium chloride) (PMAPTAC) and poly-(sodium methacrylate) (PMA) coacervates with a wide range of matched molecular weights of narrow molecular weight distribution were recorded. An unusually strong dependence of viscosity on molecular weight was found. On the other hand, ion diffusion measurements were used to correlate the dynamics of NaCl ions, that dope the complex, to the segmental relaxation times of the PMAPTAC/PMA PEC. A simple pair exchange mechanism was proposed to explain the mechanical relaxation in PECs and an expression free of any adjustable fit parameters was derived to explain the non-exponential relaxation of PECs over a wide range of temperatures. The equation was also exceptional in fitting the relaxation of three neutral polymers. Studying the dynamics in PECs can help in designing saloplastics with novel applications, and in understanding complex physical processes such as the sluggish relaxation of supercooled liquids.

Dissecting dynamics in polyelectrolyte complexes and coacervates

ACS Spring 2022

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