Influence of polymer architecture, ionization, and salt annealing on the stiffness of weak polyelectrolyte multilayers

The layer-by-layer deposition of polyelectrolyte multilayers (PEMs) is a versatile and widely used technique of forming nanoscale polymer films with controlled properties. Yet, the influence of polymer architecture and assembly conditions on the mechanical properties of PEM films is not well understood. In this paper, we compare the growth and mechanical properties of all-linear PEM films <i>versus</i> all-star (8-arm) PEM films assembled at varied assembly pH. The properties of these PEM systems, composed of linear and 8-arm weak polyelectrolytes poly(2-aminoethyl methacrylate) (PAMA) and poly(methacrylic acid) (PMAA), are affected by the assembly pH, leading to differences in internal ionization, film growth rates, swelling, and Young's modulus. For films assembled using either linear or star polyelectrolytes in acidic conditions - where PMAA has low ionization - we show slow, linear growth with reduced swelling and similar Young's moduli of the as-deposited PEM films. However, a striking difference in the mechanical behavior of dry PEM films made from linear and star polymers was found for the films showing nonlinear growth (<i>i.e.</i>, assembled at neutral and slightly alkaline conditions). Specifically, while all-star films demonstrated relatively high, thickness-independent Young's moduli, the stiffness of all-linear PEM films strongly decreased with film thickness, reflecting the overall weakening of the network of ionic connections. Finally, we show that the ductility of all-star films was more affected by salt annealing than all-linear films, which agrees with previous reports of faster salt-induced diffusion of polyelectrolytes in PEM films composed of star polymers.