Measuring mechanical properties of polyelectrolyte multilayer thin films: Novel methods based on AFM and optical techniques

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Abstract

Mechanical properties of polyelectrolyte multilayers are of great importance for their various applications like their use as biocompatible surface coatings or for encapsulation and release. Measuring these properties poses considerable experimental problems, since the films of interest are freestanding and/or of nanoscale thickness. We report here on recent method developments based on atomic force microscopy and/or microinterferometry techniques that allow for quantitative measurements on various polyelectrolyte multilayer systems. The results form the basis for a quantitative understanding of structure property relations for these systems, but are as well of interest for other thin film/membrane systems where similar experimental challenges are met.

Introduction

Polyelectrolyte multilayers (PEMs) were originally developed by Decher [1] and Decher and Hong [2] as surface coatings using the layer-by-layer electrostatic self-assembly technique. The films are built up by alternate dipping of charged substrates into aqueous solutions of positively and negatively charged polyelectrolytes which adsorb onto the substrate surface. During each adsorption cycle only a layer of molecular thickness is built up and the surface charge is reversed, which allows repeating the assembly process as often as desirable. The thickness of a PEM can thus be adjusted well and since only electrostatic (or in some cases other non-covalent) interactions are involved in the buildup process, it is applicable for a broad variety of polyelectrolytes including proteins (see [3] for a recent overview). In terms of substrates, not only flat surfaces are suitable, but multilayer coating can be carried out on colloidal particles as well. Donath et al. [4] demonstrated that even hollow PEM capsules can be produced in a two-step process: first, colloidal particles are coated using layer-by-layer electrostatic self-assembly; then, the particles are dissolved under conditions that do not destroy the multilayer. Provided the dissolution products can permeate the multilayer, capsules with walls consisting of the original particle coating remain after the dissolution procedure. Usually, films are impermeable for large particles and polymers but permeable for entities with molecular weight below 1 kDa. This high versatility explains the broad interest in PEMs and their applications for various tasks.

In recent years, there was an increasing interest in characterizing the mechanical properties of PEM films. This may be explained by the fact that mechanical properties are truly important in a wide number of applications ranging from coating of biomaterials (for resistance to various stresses in vivo), drug delivery using capsules (the wall stiffness will influence the durability in the blood stream) and understanding the mechanics of cell adhesion. In addition, measurements of mechanical properties can provide insight into intermolecular interactions in the films and are thus of fundamental interest. However, mechanical measurements on PEM films or membranes pose special experimental challenges. The films are usually of submicron thickness and/or freestanding like in the case of PEM capsules. Thus, macroscopic mechanical tests often fail and new methods have to be developed. Among the techniques used are the atomic force microscopy (AFM) employed in force spectroscopy mode [5], [6], [7], [8], the reflection interference contrast microscopy (RICM) [9], [10] and micro-traction tests [11], [12] The two first techniques can be applied to films whereas micro-traction experiments have to be performed on self-standing films (i.e. on membranes). We will here focus on RICM and AFM that we have used for the characterization of polysaccharide and polypeptide multilayer films made of poly(l-lysine)/hyaluronan (PLL/HA) and chitosan/hyaluronan (CHI/HA) and (in combination) for PEM capsules. In the following, we will use the convention that (X/Y)n denotes a multilayer of components X and Y consisting of n pairs of layers.

Section snippets

Viscoelastic properties of (PLL/HA) films evidenced by RICM and confocal laser scanning microscopy (CLSM)

RICM can be used for the investigation of the surface potential of a colloidal bead (typically latex beads of 10 μm in diameter) by monitoring their Brownian height fluctuations [13]. In conventional RICM, monochromatic light is incident on the bead under study, which typically hovers over the polymer film deposited on a glass substrate in a liquid medium. The incident light is reflected from the glass/buffer interface and again from the buffer/bead interface. These two reflected rays interfere

AFM nano-indentation experiments on (PLL/HA) and (CHI/HA) films

Our first AFM nano-indentation experiments dealt with native and cross-linked (PLL/HA) films and were performed at relatively high approach velocity (2 μm/s) [6], i.e. the piezodrive bearing the film moved at 2 μm/s toward the spherical indenter. For experimental force versus indentation curve fitting, we used a pure elastic model, namely the Hertz model corrected for the finite film height h, according to Dimitriadis [16]. Then, E, the Young's modulus, can be deduced from the force, F, versus

AFM/RICM combination for probing (PAH/PSS) microcapsule mechanics

Multilayers formed from poly(allylamine) hydrochloride and poly(styrene)sulfonate show pronounced differences to the systems discussed above with regard to their polymer mobility. While molecules in PLL/HA multilayers are highly mobile [20], self-diffusion coefficients typical for polymeric glasses (10−14 to 10−16 cm2/s) were found in PAH/PSS [21]. For this reason, this system is used with great success for the production of hollow PEM capsules, which require a low mobility of the multilayer,

Conclusions

Polyelectrolyte multilayers offer exciting possibilities as ultrathin surface coatings and membranes and their mechanical properties are highly relevant for applications and fundamental understanding of these systems. In this contribution, we have summarized novel experimental developments that help to overcome difficulties in measuring these properties. In the first two parts, we have focused on solid supported films, where problems arise due to the extremely small thickness of the films,

Acknowledgements

We would like to thank the CNRS and the German Science Foundation for supporting this project with the French-German network “Complex-Fluids-From 3 to 2 dimensions”.

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    Present address: Laboratoire de Dynamique des Interactions Membranaires Normales et Pathologiques, CNRS UMR 5235, 34 095 Montpellier Cedex 05, France.

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