Homogeneity, Modulus, and Viscoelasticity of Polyelectrolyte Multilayers by Nanoindentation: Refining the Buildup Mechanism

Abstract

Atomic force microscopy, AFM, and nanoindentation of polyelectrolyte multilayers, PEMUs, made from poly(diallyldimethylammonium), PDADMA, and poly(styrene sulfonate), PSS, provided new insight into their surface morphology and growth mechanism. A strong odd/even alternation of surface modulus revealed greater extrinsic (counterion-balanced) charge compensation for fully hydrated multilayers ending in the polycation, PDADMA. These swings in modulus indicate a much more asymmetric layer-by-layer growth mechanism than previously proposed. Viscoelastic properties of the PEMU, which may contribute to cell response, were highlighted by variable indentation rates and minimized by extrapolating to zero indentation rate, at which point the surface and bulk equilibrium moduli were comparable. Variations in surface composition were probed at high resolution using force mapping and the surface was found to be uniform, with no evidence of phase separation. AFM comparison of wet and dry films terminated with PSS and PDADMA revealed much greater swelling of the PDADMA-terminated PEMU by water, with collapse of surface roughness features in dry conditions. Dynamic and static contact angle measurements suggested less rearrangement for the glassy PSS surface.

Polyelectrolytes

“…Furthermore, Oleh et al [103] pointed out that the water is asymmetrically distributed along the three-dimensional structure of the multilayers. This has been recently confirmed for Ghostine et al [113] and Lehaf et al [212] in (PDADMAC + PSS) n multilayers. They found that the last layer play a key role in the hydration of the multilayers.…”

Influence of charge density and calcium salt on stiffness of polysaccharides multilayer film

“…Accordingly, the film surface is expected to be more "soft" when CHI adsorbs because of the larger excess of small counterions than in the films terminated in PEC [8][9][10]. For the CHI terminated films, we found unusual decrease in the electro-optical effect value at high field strengths (compared to the films ending in PEC) despite larger effects are measured for these films at low fields (Fig.…”

“…Another strategy for stiffening the polyelectrolyte multilayer films is to modify their internal composition and structure. According to Schlenoff et al, the mechanical properties of the films are strongly influenced by the presence of counterions, which must ensure charge balance within the film [8][9][10]. Multilayer films with extrinsic charge are much softer since counterions break ion pair cross-links between oppositely charged polyelectrolyte pairs [9].…”

Quasi-Spherical Cell Clusters Induced by a Polyelectrolyte Multilayer

“…From a mechanical properties arguments, these should be well adhered because PSS-terminated surfaces are stiff, with modulii in the range of 10 MPa. 34 In addition, the thickness of the thinner (0.15 M) PEMU allows the stiff (plastic) substrate to enhance the effective modulus the cells detect considerably. Using the estimates of Mehrotra et al, 15 who studied the relationship among thickness, stiffness, and cells on PDADMA/PSS multilayers and provided a model for an effective modulus based on finite difference analysis, the thin PEMU ( Figure 1B) is well within the range for strong substrate influence.…”