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Internal Structure of Polyelectrolyte Multilayers on Nanometer Scale

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Internal Structure of Polyelectrolyte Multilayers on Nanometer Scale INTERNAL STRUCTURE OF POLYELECTROLYTE MULTILAYERS ON NANOMETER SCALE Inauguraldissertation zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften der Mathematisch-Naturwissenschaftlichen Fakult¨at der Ernst-Moritz-Arndt-Universit¨atGreifswald vorgelegt von Oxana Ivanova geboren am 06. April 1980 in Sankt-Petersburg (Russland) - Greifswald, 2010 - Dekan: Prof. Dr. Dr. Klaus Fesser 1. Gutachter: Prof. Dr. Christiane A. Helm 2. Gutachter: Prof. Dr. Monika Sch¨onhoff Tag der Promotion: 15. Oktober 2010 Ich m¨ochte mich ganz herzlich bei allen Mitarbeitern des Instituts f¨urPhysik f¨urzahllose fachverwandte sowie fachfremde Unterst¨utzungbedanken. Contents I Introduction to the Field of Research 7 1 Introduction 9 1.1 Polyelectrolytes and their Properties . 9 1.2 Layer-by-Layer Assembly of Polyelectrolytes . 10 1.3 Polyelectrolyte Multilayers . 13 1.4 Objectives . 15 2 Physical Background 17 2.1 Electrostatic and Secondary Interactions . 17 2.1.1 Electrostatic Interactions: Electric Double Layer . 18 2.1.2 Effect of Salt on Formation of PEMs . 21 2.1.3 Classification and Range of Intermolecular Forces . 22 2.1.4 Temperature Effect: Hydrophobic Interactions . 23 II Materials and Methods 25 3 Materials and Sample Preparation 27 3.1 Chemicals . 27 3.2 Fabrication of Multilayered Nanofilms . 29 4 Characterization Methods 33 4.1 X-ray and Neutron Reflectivity . 33 4.1.1 Basic Principles of X{ray and Neutron Reflection . 35 4.1.2 Reflection from Ideally Smooth & Rough Interfaces . 37 4.1.3 Refractivity Set{up . 41 4.2 UV-Vis Absorption Spectroscopy . 43 4.2.1 Interaction of Light with Matter . 43 4.2.2 Beer-Lambert Law . 44 4.2.3 Optical Properties of Metallic Nanoparticles . 44 4.3 Atomic Force Microscopy . 48 4.3.1 Basic Principles . 49 4.3.2 Tip Convolution and Surface Reconstruction . 51 5 Data analysis 53 5.1 Reflectivity Data Analysis . 53 5.1.1 Dynamic Approximation (Parratt Algorithm) . 53 5.1.2 Kinematic Approximation for Superlattice PEM Structure . 55 5 III Results 59 6 Immobile Water and Proton{Deuterium Exchange 61 6.1 Introduction . 61 6.2 Swelling of Polyelectrolyte Multilayers . 62 6.3 Determination of Tightly Bound Immobile Water . 66 6.4 Proton{Deuterium Exchange in PAH Monomers . 81 6.5 Discussion . 82 6.6 Conclusion . 84 7 Thickness and Composition of PEMs 87 7.1 Introduction . 87 7.2 Effect of Temperature on PEMs . 89 7.2.1 The Onset of the Temperature Effect . 89 7.2.2 Influence of Temperature during Formation of PEMs . 92 7.2.3 Influence of Temperature after Preparation of PEMs . 95 7.3 Effect of Ions: Hofmeister Series . 102 7.4 Discussion . 107 7.5 Conclusion . 108 8 Tuning of Lateral Structure 109 8.1 Introduction . 109 8.2 Silver Nanoparticles Embedded into PEM . 110 8.2.1 Optical Properties of Silver Monolayer . 112 8.2.2 Aggregation of Silver Nanoparticles . 115 8.2.3 Rearrangement of Nanoparticles beneath a PEM . 120 8.3 Discussion . 126 8.4 Conclusion . 126 IV Thesis Summary 129 Summary 131 Symbols and Abbreviations . 135 Bibliography . 136 Acknowledgements . 147 Curriculum Vitae . 149 Publications . 151 Conference Contribution . 153 Declaration . 155 Part I Introduction to the Field of Research 7 Chapter 1 Introduction The idea to create materials with specific properties is as old as mankind. Besides the knowledge how to create the material, the knowledge how to manipulate it in a proper way is essential. This requires deep understanding of background physics and chemistry, especially of surface phenomena at the nanoscale. Development of new experimental methods makes the nano{cosmos accessible as a tool for advanced materials. Increasing importance of functionalized surfaces and nanocomposite systems places their properties in the foreground of scientific and technical interest. Flexible composition control of such systems can be gained by use of multilayer technology. The polyelectrolyte multilayers are a new class of organic thin films that allow precise control of film thickness within a few nanometers. Furthermore, it is possible to manipulate molecular architectures composed of a number of functional materials and study physical phenomena on the molecular level. 1.1 Polyelectrolytes and their Properties The term polyelectrolyte denotes a class of macromolecular compounds or polymers with high molecular weight whose repeating units bear an electrolyte group that dissociates in a suitable polar solvent (usually water) and acquires a large number of elementary charges. In Fig. 1.1 a general schematic of the polyelectrolyte structure is shown. The polyelectrolyte ionizable groups, whose dissociation in solution makes the polyelectrolyte charged, leave ions of one sign bound to the chain and mobile counterions in solution. The charged groups contribute substantially to the chain stiffness and conformational degrees of freedom. The properties of polyelectrolytes are similar to both: electrolytes (they exhibit electrolytic con- ductivity and interionic interaction) and neutral polymers (their solutions are often viscous). Polyelectrolytes are classified according to their origin as either natural or synthetic. Many biological macromolecules are polyelectrolytes. The most important examples are DNA and RNA molecules, which dissociate in solution forming a strongly negatively charged polyion surrounded by an atmosphere of small mobile counterions, like K+, Na+, Ca+2 and Mg+2. Common synthetic polyelectrolytes are especially interesting for industrial applica- tions because they dissolve in polar solvents like water, instead of most neutral hydrocarbon polymers, which are only soluble in organic solvents [G¨ornitz1997].Some natural polyelec- trolytes, such as glue and gum, have been used from time immemorial as thickeners. The use of gum as a protective colloid is as old as the use of India ink, which is finely ground 10 CHAPTER 1. INTRODUCTION soot, suspended in water and stabilized with gum arabic [Overbeek1976]. Despite their widespread presence and use, polyelectrolytes remain among the least un- derstood materials in soft condensed matter. They are difficult to understand because of the entwined correlations of chain configuration and polyelectrolyte charges. In analogy to acids, one can distinguish weak and strong polyelectrolytes according to the degree of their dissociation in solution. The strong polyelectrolytes completely dissociate in solution and their ionic groups are present in a large pH{window (1<pH<13) in contrast to weak polyelec- trolytes that only partially dissociate at intermediate pH (6{8). Since weak polyelectrolytes are not fully charged in solution their fractional charge can be modified by changing the solution pH or ionic strength (measure of the ion concentration in solution). Figure 1.1: Schematic of the polyelectrolyte macromolecule structure. Note that structure may vary for different polyelectrolytes. Idea of the picture is adapted from A. Laschewsky tutorial on polyelectrolytes. 1.2 Layer-by-Layer Assembly of Polyelectrolytes In 1991, G. Decher and J.{D. Hong presented a new layer-by-layer (LbL) sequential assembly technique for depositing ultrathin films [Decher1991]. This technique was first suggested by R. K. Iler with regard to the concept of creating layers of oppositely charged colloidal particles [Iler1966]. Each time that an initially charged substrate is exposed to a so- lution of oppositely charged polyions, a definite amount of polyelectrolyte (PE) is adsorbed. Provided that each adsorption step leads to charge inversion of the surface, the subsequent de- position finally results in a layered complex, stabilized by strong electrostatic forces, so{called self{assembled polyelectrolyte multilayer (PEM) [Sch¨onhoff2003a].Schematic illustration of the first two adsorption steps and resulting polyelectrolyte multilayer system, consisting of several layers deposited onto planar solid support is shown in Figure 1.2. It is important to note that with the exception of the conventional solution{dipping method developed by Decher (a technique also used in this thesis) spraying and spin self{assembly methods were introduced more recently [Schlenoff2000], [Cho2001]. These techniques provide much shorter processing times for multilayer build{up, which is important for PEMs, consisting of a large number of layers; however, they have certain disadvantages. Thus, difference in film thick- 1.2. LAYER-BY-LAYER ASSEMBLY OF POLYELECTROLYTES 11 ness between dipping and spraying is not fully understood. Also, the spin self{assembly method is restricted with respect to the substrate size and planarity. Figure 1.2: Schematic of the layer-by-layer assembly (top) resulting in ultra{thin polyelec- trolyte multilayer film adsorbed onto solid substrate (bottom). Here different colors represent positively and negatively charged polyelectrolytes (polycations and polyanions) respectively. Charge of the substrate is shown in gray circles. In contrast to other thin film deposition techniques that have been reported in the liter- ature, an alternative approach - LbL adsorption of polyanions and polycations - is far more general. This technique is independent on the size, shape and topology of the substrate and has been extended to other materials such as proteins or colloids [Decher1997]. Indeed, the well known vacuum (physical vapor) deposition technique requires specialized expensive in- strumentation and specific type of substrates [Mattox2003]; spin coating is restricted to the macro- and nanofilms, in this case contamination by dust is likely and film thicknesses on the order of one molecular layer cannot
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