Remarkable Structure and Elasticity Relaxation Dynamics of Poly(diallyldimethylammonium chloride)–Poly(acrylic acid) Multilayer Films

Remarkable mechanical and structural properties of out-of-equilibrium poly(diallyldimethylammonium chloride) (PDADMAC)–poly(acrylic acid) (PAA) multilayer films are elucidated from in situ atomic force microscopy and spatially resolved Raman spectroscopy analyses complemented by density functional theory (DFT) computations. Surprisingly, fresh exponentially grown (PDADMAC–PAA)n polyelectrolyte films behave as glassy materials with Young moduli as large as 2 MPa. Their organization is governed by a competition between PDADMAC–PAA electrostatic interactions and water stabilization of PAA charges that limits association between polycationic and polyanionic chains. At pH 3 where PAA is weakly deprotonated, this competition leads to the formation of water-free PDADMAC–PAA polyelectrolyte complexes within well-defined donut-like structures (2–12 μm in diameter, 100–200 nm in height) that confer upon the film a mechanical rigidity comparable to that classically achieved for linearly growing films. The relaxation of (PDADMAC–PAA)n films to equilibrium occurs over 5 days and is marked by a gradual disappearance of all donut-like structures, resulting in a 3-fold decrease of the Young modulus. This mechanical softening of the film is significantly accelerated by increasing the diffusion rate of PDADMAC and PAA chains upon heating: the morphological and mechanical features of the 5-day old, naturally aged films are recovered after 2 h heating treatment at 60 °C. In combination, this invokes a transition from intrinsic to extrinsic film charge compensation; i.e., the tightly compacted polyelectrolyte complexes progressively change to coacervates that are loosely associated by electrostatics. It is shown that such atypical structure transition of exponentially grown films can be used for reversible laser-assisted printing applications at microscales.