Temperature responsive behavior of polymer brush/polyelectrolyte multilayer composites

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The complex interaction of polyelectrolyte multilayers (PEMs) physisorbed onto end-grafted polymer brushes with focus on the temperature-responsive behavior of the system is addressed in this work. The investigated brush/multilayer composite consists of a poly(styrene sulfonate)/poly(diallyldimethylammonium chloride) (PSS/PDADMAC) multilayer deposited onto the poly(N-isopropylacrylamide-b-dimethylaminoethyl methacrylate) P(NIPAM-b-DMAEMA) brush. Ellipsometry and neutron reflectometry were used to monitor the brush collapse with the thickness decrease as a function of temperature and the change in the monomer distribution perpendicular to the substrate at temperatures below, across and above the phase transition, respectively. It was found that the adsorption of PEMs onto polymer brushes had a hydrophobization effect on PDMAEMA, inducing the shift of its phase transition to lower temperatures, but without suppressing its temperature-responsiveness. Moreover, the diffusion of the free polyelectrolyte chains inside the charged brush was proved by comparing the neutron scattering length density profile of pure and the corresponding PEM-capped brushes, eased by the enhanced contrast between hydrogenated brushes and deuterated PSS chains. The results presented herein demonstrate the possibility of combining a temperature-responsive brush with polyelectrolyte multilayers without quenching the responsive behavior, even though significant interpolyelectrolyte interactions are present. This is of importance for the design of multicompartment coatings, where the brush can be used as a reservoir for the controlled release of substances and the multilayer on the top as a membrane to control the diffusion in/out by applying different stimuli.DFG, GRK 1524, Self-Assembled Soft-Matter Nanostructures at Interface

Short versus long chain polyelectrolyte multilayers: a direct comparison of self-assembly and structural properties

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Successful layer-by-layer (LbL) growth of short chain (∼30 repeat units per chain) poly(sodium styrene sulfonate) (PSS)–poly(diallyl dimethylammonium chloride) (PDADMAC) multilayers is presented for the first time and compared with the growth of equivalent long chain polyelectrolyte multilayers (PEMs). A detailed study performed by quartz crystal microbalance with dissipation (QCM-D) is carried out and three main processes are identified: (i) initial mass uptake, (ii) adsorption–desorption during layer equilibration and (iii) desorption during rinsing. In contrast to the high stability and strong layer increment of high molecular weight (HMW) PEMs, layer degradation characterizes low molecular weight (LMW) multilayers. In particular, two different instability phenomena are observed: a constant decrease of sensed mass during equilibration after PDADMAC adsorption, and a strong mass loss by salt-free rinsing after PSS adsorption. Yet, an increase of salt concentration leads to much stronger layer growth. First, when the rinsing medium is changed from pure water to 0.1 M NaCl, the mass loss during rinsing is reduced, irrespective of molecular weight. Second, an increase in salt concentration in the LMW PE solutions causes a larger increment during the initial adsorption step, with no effect on the rinsing. Finally, the mechanical properties of the two systems are extracted from the measured frequency and dissipation shifts, as they offer a deeper insight into the multilayer structures depending on chain length and outermost layer. The paper enriches the field of PE assembly by presenting the use of very short PE chains to form multilayers and elucidates the role of preparation conditions to overcome the limitation of layer stability.DFG, SPP 1369, Polymer-Festkörper-Kontakte: Grenzflächen und Interphase

Layer-by-layer formation of oligoelectrolyte multilayers: a combined experimental and computational study

For the first time, the combination of experimental preparation and results of fully atomistic simulations of an oligoelectrolyte multilayer (OEM) made of poly(diallyl dimethyl ammonium chloride)/poly(styrene sulfonate sodium salt) (PDADMAC/PSS) is presented. The layer-by-layer growth was carried out by dipping silica substrates in oligoelectrolyte solutions and was modeled by means of atomistic molecular dynamics simulations with a protocol that mimics the experimental procedure up to the assembly of four layers. Measurements of OEM thickness, surface roughness and amount of adsorbed oligoelectrolyte chains obtained from both approaches are compared. A good agreement between simulated and experimental results was found, with some deviations due to intrinsic limitations of both methods. However, the combination of information extracted from simulations to support the analysis of experimental data can overcome such restrictions and improve the interpretation of experimental results. On the other hand, processes dominated by slower kinetics, like the destabilization of adsorbed layers upon equilibration with the surrounding environment, are out of reach for the simulation modeling approach, but they can be investigated by monitoring in situ the oligoelectrolyte adsorption during the assembly process. This demonstrates how the synergistic use of simulation and experiments improves the knowledge of OEM properties down to the molecular scale

Concentration dependent effects of urea binding to poly(N-isopropylacrylamide) brushes: a combined experimental and numerical study

The binding effects of osmolytes on the conformational behavior of grafted polymers are studied in this work. In particular, we focus on the interactions between urea and poly(N-isopropylacrylamide) (PNIPAM) brushes by monitoring the ellipsometric brush thickness for varying urea concentrations over a broad temperature range. The interpretation of the obtained data is supported by atomistic molecular dynamics simulations, which provide detailed insights into the experimentally observed concentration-dependent effects on PNIPAM-urea interaction. In particular, in the low concentration regime (c(u) = 2 mol L-1, the lower T-tr is explained by the favorable replacement of water molecules by urea, which can be regarded as a cross-linker between adjacent PNIPAM chains. Significant effects of the concentration-dependent urea binding on the brush conformation are noticed: at c(u) <= 0.5 mol L-1, although urea is loosely embedded between the hydrated polymer chains, it enhances the brush swelling by excluded volume effects. Beyond 0.5 mol L-1, the stronger interaction between PNIPAM and urea reduces the chain hydration, which in combination with cross-linking of monomer units induces the shrinkage of the polymer brush.DFG, EXC 310, SimulationstechnikDFG, SFB 716, Dynamische Simulation von Systemen mit großen TeilchenzahlenDFG, GRK 1524, Self-Assembled Soft-Matter Nanostructures at Interface

Reflectometry Reveals Accumulation of Surfactant Impurities at Bare Oil/Water Interfaces

Bare interfaces between water and hydrophobic media like air or oil are of fundamental scientific interest and of great relevance for numerous applications. A number of observations involving water/hydrophobic interfaces have, however, eluded a consensus mechanistic interpretation so far. Recent theoretical studies ascribe these phenomena to an interfacial accumulation of charged surfactant impurities in water. In the present work, we show that identifying surfactant accumulation with X-ray reflectometry (XRR) or neutron reflectometry (NR) is challenging under conventional contrast configurations because interfacial surfactant layers are then hardly visible. On the other hand, both XRR and NR become more sensitive to surfactant accumulation when a suitable scattering length contrast is generated by using fluorinated oil. With this approach, significant interfacial accumulation of surfactant impurities at the bare oil/water interface is observed in experiments involving standard cleaning procedures. These results suggest that surfactant impurities may be a limiting factor for the investigation of fundamental phenomena involving water/hydrophobic interfaces

Organische und inorganische Beschichtungen auf Polymerbürsten : gegenseitige Effekte auf die Struktur und auf die stimuli-sensitiven Eigenschaften

The thesis presents the preparation and characterization of smart coatings based on polymer brushes, polyelectrolyte multilayers (PEMs), aluminium oxide and their combination as multicompartment systems. The latter are highly relevant, as they extend the properties of each subsystem. For instance, they carry multiple sensitivity to external stimuli, hierarchical structures which can be selectively activated, enhanced robustness and thermal stability. However, the interaction between the constituent parts might alter their individual responsiveness to the surrounding environment, therefore nature and strength of such a synergy is a crucial knowledge to achieve the desired structure and properties of multicompartment coatings. The combination of optical methods and scanning probe techniques allowed to characterize structure, morphology and responsiveness of the systems. In addition, these studies provided information on the interaction and the consequent mutual effects governing the behavior of the composites. The first part of the thesis presents the successful assembly of short-chain polyelectrolytes into multilayers. The development of an appropriate preparation protocol is fundamental to achieve a systematic layer-by-layer growth from the balance between the characteristic layer instability, leading to degradation, and enhanced mass uptake. In the second part, the interaction between polymer brushes and osmolytes is analyzed for different co-solute concentration and temperature. In particular, the synergistic use of experiments and simulations clarify direct and indirect effects on the conformational behavior of the polymer chain. In the third part, multicompartment systems are presented, in which homo- and block copolymer brushes are used as substrates for the deposition of PEMs and aluminium oxide films. In the first case, strong interactions in the brush/PEM composites is observed, leading to stable complexes in the inner part of the brush which alter but not suppress the responsive properties of the system towards humidity and temperature. The results suggest a crucial role of brush thickness and charge density on the diffusion and interaction of the polyelectrolytes, which represents a fundamental tool to tune the properties of the resulting composites. Finally, a novel procedure allows to coat a polymer brush with aluminum oxide, where the quality of the coating is determined by the wettability of the polymer substrate. Significant mutual effects on morphological, mechanical and swelling properties of the films are found, meaning that brush/alumina composites can successfully exploit the properties of both organic and inorganic materials. This thesis analyzes fundamental aspects for the design of complex smart coatings, where the interaction between the constituent parts govern their structure and responsive behavior under different conditions.In der vorliegenden Dissertation wird die Herstellung und die Charakterisierung “intelligenter” Beschichtungen, basierend auf Polymerbürsten, Polyelektrolyt-Multischichten (PEMs) und Aluminiumoxid, sowie deren Kombination in Multikompartimentssystemen diskutiert. Letztere sind von besonderer Bedeutung, da sie die Eigenschaften jedes Untersystems erweitern. Dies führt zu hierarchischen Strukturen, deren unterschiedliche Sensitivität zu externen Stimuli separat aktiviert werden können, bei zusätzlicher Erhöhung der mechanischen und thermischen Stabilität. Folgerichtig ist für das Erreichen der gewünschten Strukturen und Eigenschaften die Kenntnis über die Wechselwirkung der einzelnen Bestandteile in Bezug auf Stärke und Natur von essentieller Bedeutung, da diese die Antwort des Gesamtsystems auf dessen Umgebung entscheidend beeinträchtigen können. Es wird eine Kombination optischer, sowie Raster-Methoden verwendet, welche Aufschluss über Struktur, Morphologie sowie Reaktionsfähigkeit der betrachteten (Komposit-) Systeme geben. Zusätzlich erlauben diese Methoden Einblick in die Wechselwirkungen und damit deren Effekt auf das Gesamtverhalten der Komposite. Im ersten Teil der Dissertation wird die Anordnung kurzkettiger Polyelektrolyte zu Multischichten präsentiert. Dies beinhaltet die Entwicklung eines geeigneten Präparationsprotokolls für das Layer-by-layer Wachstum, zur Findung der optimalen Balance zwischen charakteristischer Schichtstabilität, welche den Zerfall begünstigt, und erhöhter Stoffaufnahme. Im nächsten Abschnitt werden die Wechselwirkungen von Polymerbürsten mit verschiedenen Osmolyten in Abhängigkeit deren Konzentration sowie der Temperatur charakterisiert. Insbesondere gibt die Synergie experimenteller Methoden mit Simulationen Aufschluss über indirekte Effekte auf die Konformation der Polymerketten. Im dritten Teil geht es um Multikompartimentsysteme, in denen Homo- sowie Block Copolymerbürsten als Substrate für die Beschichtung mit PEMs und Aluminiumoxidfilmen dienen. Im Fall von PEM, führen die starken Wechselwirkungen zu stabiler Komplexierung im Inneren der Bürste, welche das Gesamtreaktionsvermögen verändert, jedoch nicht unterbinden in Bezug auf Temperatur und Luftfeuchte. Die Ergebnisse legen eine essentielle Rolle der Bürstendicke und der Ladungsdichte für die Diffusion und Wechselwirkung der Polyelektrolyte nahe, welche von besonderer Bedeutung für die finalen Kompositstrukturen sind. Zuletzt wird eine neue Methode zur Beschichtung der Polymerbürsten mit Aluminiumoxid entwickelt, dessen Qualität durch das Benetzungsverhalten bestimmt wird. Die signifikanten Effekte bezüglich Morphologie, mechanische, sowie Schwellungseigenschaften verleihen diesen Bürsten/Aluminiumoxid-Kompositen, Eigenschaften sowohl inorganischer als auch organischer Materialien. Diese Dissertation dient der Analyse fundamentaler Aspekte für das Design von “intelligenten” und komplexen Beschichtungen, wobei deren Struktur und Verhalten unter verschiedensten Bedingungen entscheidend durch die Wechselwirkungen der verwendeten Bestandteile im Komposit bestimmt wird.DFG, SPP 1369, Polymer-Festkörper-Kontakte: Grenzflächen und InterphasenDFG, IRTG 1524, Self-Assembled Soft-Matter Nanostructures at Interface

Lipopolysaccharides at Solid and Liquid Interfaces : Models for Biophysical Studies of the Gram-negative Bacterial Outer Membrane

Lipopolysaccharides (LPSs) are a constitutive element of the cell envelope of Gram-negative bacteria, representing the main lipid in the external leaflet of their outer membrane (OM) lipid bilayer. These unique surface-exposed glycolipids play a central role in the interactions of Gram-negative organisms with their surrounding environment and represent a key element for protection against antimicrobials and the development of antibiotic resistance. The biophysical investigation of a wide range of different types of in vitro model membranes containing reconstituted LPS has revealed functional and structural properties of these peculiar membrane lipids, providing molecular-level details of their interaction with antimicrobial compounds. LPS assemblies reconstituted at interfaces represent a versatile tool to study the properties of the Gram-negative OM by exploiting several surface-sensitive techniques, in particular X-ray and neutron scattering, which can probe the structure of thin films with sub-nanometer resolution. This review provides an overview of different approaches employed to investigate structural and biophysical properties of LPS, focusing on studies on Langmuir monolayers of LPS at the air/liquid interface and a range of supported LPS-containing model membranes reconstituted at solid/liquid interfaces

Organized Hybrid Molecular Films from Natural Phospholipids and Synthetic Block Copolymers: A Physicochemical Investigation

International audienceIn the last few years, hybrid lipid-copolymer assemblies have attracted increasing attention as possible two-dimensional (2D) membrane platforms, combining the biorelevance of the lipid building blocks with the stability and chemical tunability of copolymers. The relevance of these systems varies from fundamental studies on biological membrane-related phenomena to the construction of 2D complex devices for material science and biosensor technology. Both the fundamental understanding and the application of hybrid lipid-copolymer-supported bilayers require thorough physicochemical comprehension and structural control. Herein, we report a comprehensive physicochemical and structural characterization of hybrid monolayers at the air/water interface and of solid-supported hybrid membranes constituted by 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and the block copolymer poly(butadiene-b-ethyleneoxide) (PBD-b-PEO). Hybrid lipid-copolymer supported bilayers (HSLBs) with variable copolymer contents were prepared through spontaneous rupture and fusion of hybrid vesicles onto a hydrophilic substrate. The properties of the thin films and the parent vesicles were probed through dynamic light scattering (DLS), differential scanning calorimetry (DSC), optical ellipsometry, quartz crystal microbalance with dissipation monitoring (QCM-D), and confocal scanning laser microscopy (CSLM). Stable, hybrid lipid/copolymer systems were obtained for a copolymer content of 10−65 mol %. In particular, DSC and CSLM show lateral phase separation in these hybrid systems. These results improve our fundamental understanding of HSLBs, which is necessary for future applications of hybrid systems as biomimetic membranes or as drug delivery systems, with additional properties with respect to phospholipid liposomes

Pressure-Induced Phase Transitions of Nonionic Polymer Brushes

While the temperature responsive behavior of nonionic polymers has been extensively studied and is nowadays one of the key mechanisms of smart materials, the pressure response of thin films remains basically unexplored. We investigate the conformational transition of nonionic brushes and semidilute solutions induced by hydrostatic pressure and temperature variations. Interestingly, the pressure–temperature phase diagram for the coil-to-globule transition of brushes, probed by neutron reflectometry, nearly coincides with that in semidilute solutions. We also show that the phase behavior can be understood and predicted with simple thermodynamic concepts employed so far for the denaturation of proteins. Fully atomistic molecular dynamics simulations provide molecular insight into the pressure-responsive behavior. Combining all three approaches allows us to demonstrate that pressure-induced hydration of nonionic polymers at low pressure is universal as it is dictated by water and is polymer-independent. In contrast, the pressure-induced dehydration at high pressure is strongly polymer-specific. The outcomes apply to a wide class of nonionic polymers and can aid the design of responsive coatings with the desired pressure-responsive behavior