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

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

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

CNT composites for aerospace applications

Carbon nanotubes were synthesized by thermal arc plasma process after optimization of the synthesis parameters. These samples were then analysed by scanning and transmission electron microscopes (SEM and TEM), in order to establish the morphology of the nanostructures. Atomic force microscopy (AFM) and electron diffraction studies were also carried out before using the sample for the composite material preparation. Composites of epoxy resin with curing agent as well as a mixture of graphite and carbon nanotubes were prepared with varying proportions of the mixture. The electrical resistivity of the material was studied under varying pressure and voltage conditions. Preliminary results of these studies present interesting features which are reported here

Perdeuteration of cholesterol for neutron scattering applications using recombinant Pichia pastoris

Deuteration of biomolecules has a great impact on both quality and scope of neutron scattering experiments. Cholesterol is a major component of mammalian cells, where it plays a critical role in membrane permeability, rigidity and dynamics, and contributes to specific membrane structures such as lipid rafts. Cholesterol is the main cargo in low and high-density lipoprotein complexes (i.e. LDL, HDL) and is directly implicated in several pathogenic conditions such as coronary artery disease which leads to 17 million deaths annually. Neutron scattering studies on membranes or lipid-protein complexes exploiting contrast variation have been limited by the lack of availability of fully deuterated biomolecules and especially perdeuterated cholesterol. The availability of perdeuterated cholesterol provides a unique way of probing the structural and dynamical properties of the lipoprotein complexes that underly many of these disease conditions. Here we describe a procedure for in vivo production of perdeuterated recombinant cholesterol in lipid-engineered Pichia pastoris. Using flask and fed-batch fermenter cultures in deuterated minimal medium perdeuteration of the purified cholesterol was verified by mass spectrometry and its use in a neutron scattering study was demonstrated using neutron reflectometry

Predictive role of the p16 immunostaining pattern in atypical cervical biopsies with less common high risk hpv genotypes

P16 immunostaining is considered a useful surrogate of transcriptionally active high‐risk (hr) HPV infection. Only strong and widespread “block‐like” immunoreactivity is considered specific, whereas weak/focal p16 positive immunostaining is considered not specific, and follow‐up and HPV molecular detection is not indicated. The aim of the study was to evaluate the presence of HPV DNA and Ki67 immunostaining in 40 cervical atypical biopsies (CALs) with mild and focal histological features suggestive of HPV infection—20 cases with weak/focal p16 positive immunoreactivity and 20 cases negative for p16 expression. In 16/20 weak/focal p16 positive CALs (80%), the INNO‐LiPA HPV genotyping detected hrHPV genotypes (HPV 31, 51, 56, 59, 26, 53, 66, 73, and 82). Co‐infection of two or more hrHPV genotypes was often evidenced. HPV16 and 18 genotypes were never detected. Ki67 immunostaining was increased in 10/20 cases (50%). In 19/20 p16 negative CALs, hrHPV infection was absent and Ki67 was not increased. These results suggest that weak/focal p16 immunostaining represents the early stage of transcriptionally active infection, strongly related to the presence of less common hrHPV genotypes, probably with a slower transforming power, but with a potential risk of progression if the infection persists. HPV DNA genotyping and follow‐up could be useful in these cases to verify if they are able to evolve into overt dysplastic changes and to improve knowledge of less common hrHPV genotypes

Designing cascades of electron transfer processes in multicomponent graphene conjugates

A novel family of nanocarbon-based materials was designed, synthesized, and probed within the context of charge-transfer cascades. We integrated electron-donating ferrocenes with light-harvesting/electron-donating (metallo)porphyrins and electron-accepting graphene nanoplates (GNP) into multicomponent conjugates. To control the rate of charge flow between the individual building blocks, we bridged them via oligo-p-phenyleneethynylenes of variable lengths by β-linkages and the Prato-Maggini reaction. With steady-state absorption, fluorescence, Raman, and XPS measurements we realized the basic physico-chemical characterization of the photo- and redox-active components and the multicomponent conjugates. Going beyond this, we performed transient absorption measurements and corroborated by single wavelength and target analyses that the selective (metallo)porphyrin photoexcitation triggers a cascade of charge transfer events, that is, charge separation, charge shift, and charge recombination, to enable the directed charge flow. The net result is a few nanosecond-lived charge-separated state featuring a GNP-delocalized electron and a one-electron oxidized ferrocenium