Mechanical Activation of Chemical Bonds at Polymer Interfaces

Polymer brushes are a specific class of polymer thin films in which each chain is tethered to the underlying substrate via one chain end. In sufficiently densely grafted assemblies, the individual polymer chains are forced into an extended chain conformation and form a brush-like structure, colloquially referred to as polymer brush. Upon incubation in a good solvent, polymer brushes experience swelling which leads to a tension at the polymer brush - substrate interface. This tension is believed to mechanically facilitate cleaving reactions at the tethering point resulting in the detachment of polymer chains, a process which is known as degrafting. Degrafting, and in the case of crosslinked polymer brushes delamination, has been reported for a large variety of polymer brushes and surface materials. Even though this phenomenon is already known for more than two decades, in depth understanding of the underlying mechanism is lacking and quantitative description of the process is rare. This thesis aims to gain further understanding of the phenomenon by quantitative description of the degrafting process. Further, swelling induced mechanical activation at the tethering point of the polymer to the underlying substrate is discussed in the context of polymer mechanochemistry and presented as a tool to selectively facilitate mechanophore cleavage at the interface.

Chapter 1 provides a short historical review on the development of polymer mechanochemistry and gives an overview of commonly applied techniques in the field. The concept of polymer mechanochemistry is then extended to polymer brushes and the discussion results in a number of issues and research questions which are addressed in the subsequent chapters.

The preparation and characterization of polymer brushes is not trivial. In Chapter 2 limitations and challenges with focus on experimental reproducibility and thin film characterization are discussed. Chapter 3 discusses the influence of a number of factors, i.e. grafting density and molecular weight of the polymer brush, solvent quality and salt concentration in the incubation medium, on the degrafting process of hydrophilic brushes in aqueous media. Further, the degrafting behavior of polymer brushes prepared from different monomers is compared. It is attempted to correlate degrafting behavior with polymer brush swelling.

Contradicting results concerning the actual cleaving point of chains in polymer brushes are found in literature. In Chapter 4 presents structures incorporating only one single potential cleaving point. However, non-specific degrafting could not completely be suppressed. Further, a disulfide bond and oxy-norbornene moiety, commonly used mechanophores, were introduced at the anchoring point.

In Chapter 5 the limitation of unwanted background hydrolysis was circumvented by investigating degrafting processes in dry organic media. Specific cleavage of the two mechanophores, disulfide and oxy-norbornene, could be demonstrated.

Functional polymer brushes via surface-initiated controlled radical polymerizations

Stefano Tugulu

The term polymer brush refers to a well defined arrangement of polymer chains, which are tethered with one end to an interface and usually the surface of a solid substrate. Due to steric repulsion the polymer chains furthermore adopt a defined, stretched chain conformation, which significantly differs from the random walk conformation of free polymer chains in solution or in conventionally solution casted polymer coatings. From a structural point of view polymer brushes can therefore be regarded as the most defined type of polymer coating that can currently be realized. Especially the combination of surface-initiated polymerizations (SIP) with modern, controlled polymerization methods thereby allows tailoring the structure of the polymer brushes almost on the molecular level. Specifically precise control over the thickness, composition, chain architecture, grafting density and via the use of lithographic techniques also over the topography of the brushes can be achieved. This high degree of control over the structural and implicated (physico-)chemical properties of the brush allows at the same time tailoring and fine tuning the interfacial properties of the brush. This thesis describes the systematic exploitation of well-defined polymer brushes as functional coatings with fine-tuned physicochemical properties to direct and control the interaction with and the response of specific biological, bioanalytical and even reactive chemical environments. In detail the use of surface-initiated atom transfer radical polymerization (SI-ATRP) for the fabrication of functional polymer brushes will be presented. These brushes were used as highly defined coatings in various applications ranging from biomedical and bioanalytical applications to biomimetic materials fabrication (Figure 1). Figure 1 Numerous synthetic approaches for the preparation of polymer brushes have been described in the literature. Chapter 2 will therefore present a summary of the most important types of surface-initiated polymerizations, focusing especially on controlled SIP methods and SI-ATRP. Chapter 3 describes a chemoselective and specific immobilization strategy to decorate intrinsically bioinert polymer brushes with proteins of interest in a defined orientation and density. Specifically the resulting substrates represent attractive candidates for the realization of protein microarrays. In view of this application protein-small molecule and protein-protein interactions as well as posttranslational modifications were analyzed within a feasibility study. Due to their well-defined structure polymer brushes represent also attractive candidates for the realization of molecularly defined cell adhesion substrates for tissue engineering or as highly defined biophysical model system to analyze cell adhesion and mechanics. Chapter 4 describes the decoration of intrinsically bioinert polymer brushes with small peptide ligands with defined density to induce integrin specific cell adhesion on the brushes. Peptide functionalized polymer brushes could be readily endothelialized and adhering cell layers have shown to preserve their homeostatic ability to respond to an applied mechanical stimulus in a physiological manner. In Chapter 5 the limitations of high density polymer brushes in biomedical applications were assessed by analyzing their stability under in vitro cell culture conditions. Intrinsically bioinert high density polymer brushes were found to degraft from glass or silicon substrates upon swelling in good solvents, which lead to deterioration of the bioinert properties of the substrate. The degrafting of the brushes might be connected to swelling induced mechanical activation of chemical bonds, which link the brushes to the substrate. Chapter 6 describes the transfer of the developed SIP approach onto flexible PDMS substrates by exploiting the formation of interpenetrating networks of ATRP-initiator siloxane and PDMS. The modification of the PDMS substrates with hydrophilic brushes effectively reduces the unspecific protein adsorption on the substrates. This approach might therefore represent an attractive and facile route for the surface modification of miniaturized PDMS devices for biomedical or bioanalytical applications. In Chapter 7 the systematic development of a reliable and robust protocol for the aqueous SI-ATRP of sodium methacrylate is presented, which gives access to poly(methacrylic acid) (PMAA) brushes with defined molecular weight and grafting density. The resulting brushes were assessed for their ability to direct the mineralization of calcium carbonate. Chapter 8 describes the use of photolithographically patterned poly(methacrylic acid) brushes as biomimetic, acidic macromolecular matrix to fabricate microstructured calcite thin films that are an exact 3D replica of the PMAA brush. The presented strategy relies on three key elements: (i) the use of photolithographic techniques to prepare microstructured PMAA brushes; (ii) the ability of PMAA brushes to stabilize amorphous calcium carbonate (ACC) and (iii) the possibility to convert the metastable ACC phase into a polycrystalline calcite film via a thermal treatment.

EPFL2007

Highly branched polymer/montmorillonite clay nanocomposites

Marlene Rodlert

The aim of this thesis has been to assess the potential of branched polymers as novel exfoliants for layered silicates and their application in polyurethanes. Three different types of –OH functional branched polymers have been studied: dendrimers, hyperbranched polymers (HBPs) and star branched polymers (SBPs), with a range of structures and molecular weights. The dendrimers and their HBP analogues were synthesized in-house, while the other HBPs and SBPs were obtained from commercial suppliers. Various types of montmorillonite (MMT) layered aluminosilicate clays have been used as modifiers. The different branched polymers were either melt processed or solution processed with the MMT, using water and THF as dispersants. Whether intercalated or exfoliated nanocomposites were obtained depended on the choice of MMT, the presence of solvent during processing as well as the characteristics of the interface between the polymer and the MMT, with exfoliation being favored when the polymer-MMT interactions were strong. Very high degrees of exfoliation were obtained from dendrimers and HBPs processed in water at loadings as high as 20 wt% unmodified Na+MMT, with intercalation only becoming dominant at higher loadings. The MMT layer spacing in the intercalated HBP/Na+MMT nanocomposites depended directly on the pseudo-generation number, that is, on the size of the HBP, at intermediate loadings, consistent with adsorption of a monolayer of unperturbed HBP molecules at the layer surface in aqueous suspension. Dispersions of the HBPs with a range of organically modified MMTs in THF, on the other hand, showed mainly intercalation on drying, with layer spacings of the order of 3.8 nm at intermediate clay contents, which were apparently independent of the pseudo-generation. The difference between the final MMT layer spacing and its value prior to mixing was found to increase significantly with the polarity of the organic modifier. The contrasting behavior of Na+MMT and the organically modified MMTs was partly attributed to the tendency of Na+MMT to exfoliate in aqueous dispersion. The HBPs and dendrimers are then able to coat the individual MMT layers and stabilize the exfoliated structure during drying. The organically modified MMTs, on the other hand, did not exfoliate in THF dispersions and intercalation consequently dominated. Predominantly intercalated nanocomposites were also obtained with the SBPs, although increasing degrees of exfoliation were observed as the strength of the interactions between the SBP and the clay increased, as a result of systematic modification of the polarity of the SBP. The rheological properties of a range of exfoliated and unexfoliated highly branched polymer/MMT nanocomposites were investigated in small strain oscillating shear. The exfoliated nanocomposites showed a sharp increase in shear viscosity at very low MMT contents, accompanied by a transition from Newtonian behavior to solid-like behavior, characterized in terms of a limiting shear modulus at low frequencies and a limiting viscosity at high frequencies, both of which were strongly dependent on MMT content. Unexfoliated nanocomposites showed a more gradual increase in viscosity with MMT content, although they showed qualitatively similar behavior to the exfoliated nanocomposites at sufficiently high loadings. The rheological behavior was analyzed in terms of models for conventional filled polymers, leading to estimates of the effective particle aspect ratio that were approximately consistent with direct morphological observations by transmission electron microscopy.To investigate the influence of the MMT on solid state properties, the HBP/MMT and SBP/MMT nanocomposites were incorporated into model polyurethane formulations. In the case of the HBP/MMT, a reactive low molar mass polyol or an un-reactive solvent (THF) was used to reduce the viscosity sufficiently for room temperature processing. Exfoliated HBP/Na+MMT led to a 60% increase in the rubbery plateau modulus relative to that of the corresponding unfilled matrix at clay contents as low as 0.5 vol%, whereas more modest relative increases were observed with unexfoliated or partly exfoliated MMT. The same materials also showed a 2-fold increase in stress at break and elongation at break in the presence of 0.5 vol% exfoliated Na+MMT, although no improvement was observed for the corresponding intercalated polyurethane nanocomposites obtained in the absence of HBP. Much higher exfoliated clay contents led to processing problems owing to their high viscosities. Polyurethanes containing unexfoliated MMT, such as obtained by using SBPs rather than HBPs as the matrix, could be prepared up to relatively high MMT contents, but the improvements in mechanical properties were modest at any given MMT content. The semi-empirical Halpin-Tsai model and Mori-Tanaka's average stress theory, which are both based on a classical approach to particle reinforcement, accounted well for the evolution of the mechanical properties of the polyurethanes as the MMT content was varied and they led to estimates of the particle aspect ratio that correlated well with those inferred from the rheological models and direct observations. This in turn allowed correlations to be established between the degree of exfoliation, the shear viscosity of the nanocomposite precursors and the mechanical properties of the final polyurethanes.

EPFL2005

Interactive Surfaces Based on Polymer Brushes

Dusko Paripovic

While the bulk structure of a material determines its mechanical properties, its surface plays a key role when interactions with the environment are important. Nowadays materials in bio- and nanotechnology are required to possess very complex functionalities and to interact specifically to the stimuli they receive from their surroundings. Thus, their surface modifications are becoming very demanding. In recent years surface initiated controlled radical polymerization techniques have emerged as very versatile strategies that provide a powerful toolbox to tackle the challenges that are set to material scientists. The attractiveness of these methods lies in the possibility to create well defined surface arrangements of polymer chains, which we refer to as polymer brushes, with precise control over thickness, composition and architecture. In addition, these techniques can be applied to very challenging substrates with respect to the chemical composition and geometry, which also includes porous materials. This Thesis, therefore, exploits the possibilities that surface-initiated controlled radical polymerization techniques offer to finely tune interfacial properties of materials and render them capable to interact with chemical and biological species in a desired way. Its paramount aim is to demonstrate how the functionalities and geometry of a polymer brush matrix can be employed to influence inorganic film deposition using synthetic, biomimetic and purely biological mechanism or enhance polymer brushed stability in aqueous media. This Thesis is divided into five distinct chapters, which will be briefly summarized in following paragraphs. Chapter 1 describes the preparation of polymer brushes and strategies that are used to control their chemical composition and architecture. Additionally, it will give an overview on the use of brushes as cell adhesive surfaces as well as templates for the production of nanoparticles and thin inorganic films. Chapter 2 explores a new strategy to guide the chemical solution deposition of thin, microstructured metal films. The proposed strategy is based on the use of poly(2-(methacryloyloxy)ethyl ammonium chloride) (PMETAC) brushes grown via surface-initiated atom transfer radical polymerization as a template. Thin films are prepared by first loading the PMETAC brushes with an appropriate precursor, followed by its reduction by NaBH4 and finally an oxygen plasma treatment to remove the stabilizing polymer brush matrix and generate the desired thin metallic film. Of particular interest is the question whether the thickness and lateral dimensions of the deposited metallic films can be controlled by micropatterning the polymer brush template and varying the brush thickness. In addition, this concept will be extended to the preparation of bimetallic (gold/palladium) films using a single polymer brush matrix. A similar strategy is explored in Chapter 3 to control the deposition of thin calcium carbonate films using a biomimetic process. The process is characterized by the stabilization of amorphous calcium carbonate, the geometry and localication of which can be controlled by the poly(methacrylic acid) brush template. A special focus of this Chapter is to examine the influence of the polymer brush thickness on the mineral deposition and to determine to which extent is the mineral phase is integrated into the polymer brush film. The aim of Chapter 4 is to develop a general approach for the modification of implantable materials with a polymer brush based coating that promotes bone formation. The proposed coatings are prepared via surface-initiated copolymerization of 2-hydoxyethyl methacrylate and 2-(methacryloyl)ethyl phosphate, which results in a thin hydrogel layer that mimics the negatively charged matrix macromolecules involved in natural bone formation. To further emulate the properties of the natural bone extracellular matrix, the coatings will be functionalized with the cell adhesive GGGRGDS peptide. The ability of the proposed coatings to induce bone formation is investigated in in vitro experiments on MC3T3-E1 pre-osteoblasts and evaluated using an ALP and alizarin red assays. Finally, Chapter 5 focuses on studying the parameters that could prevent the hydrolytic cleavage of Si-O bond, which tethers the polymer brushes grafted from SiOx in aqueous media. In the first part of the study, the length of the organosilane initiator's alkyl spacer will be varied, in order to evaluate to which extent it can insulate the site that anchors the hydrophilic polymer brush to the substrate. In the second part, a short, hydrophobic block will be introduced between the silicon oxide substrate and the hydrophilic polymer in order to more efficiently protect the underlying Si-O bond from the surrounding aqueous media and prevent the chain detachment.

EPFL2011