Macromolecular Layer Research Articles

Stimuli-responsive polymers grafted to nanopores and other nano-curved surfaces: structure, chemical equilibrium and transport

This review discusses the properties of macromolecular layers on nano-curved surfaces, with special emphasis on the fast-growing field of polymer- and polyelectrolyte-modified nanopores and nanochannels. Organization in soft materials emerges from the competition between physical and chemical interactions acting on different length scales; the geometry of the system modulates this competition and, hence, dictates its structure and function. Theoretical studies of molecular organization in planar and curved geometries require the treatment of the coupling between interactions, chemical equilibrium and geometrical constraints. We analyse here how curvature affects the morphology of polymers on nano-curved surfaces and determines the apparent equilibrium constants of surface-confined chemical reactions, such as acid–base chemistry. We also review the practical implications of introducing soft materials into nanofluidic devices. In particular, we present and discuss recent theoretical studies and simulations of the transport of ions, solvent and large cargoes through polyelectrolyte-brush-modified pores. We believe that this basic understanding will guide experimental efforts towards the design of stimuli-responsive nanopore gates.

Excessive erythrocytosis compromises the blood–endothelium interface in erythropoietin‐overexpressing mice

Elevated systemic haematocrit (Hct) increases risk of cardiovascular disorders, such as stroke and myocardial infarction. One possible pathophysiological mechanism could be a disturbance of the blood-endothelium interface. It has been shown that blood interacts with the endothelial surface via a thick hydrated macromolecular layer (the 'glycocalyx', or 'endothelial surface layer'--ESL), modulating various biological processes, including inflammation, permeability and atherosclerosis. However, the consequences of elevated Hct on the functional properties of this interface are incompletely understood. Thus, we combined intravital microscopy of an erythropoietin overexpressing transgenic mouse line (tg6) with excessive erythrocytosis (Hct 0.85), microviscometric analysis of haemodynamics, and a flow simulation model to assess the effects of elevated Hct on glycocalyx/ESL thickness and flow resistance. We show that the glycocalyx/ESL is nearly abolished in tg6 mice (thickness: wild-type control: 0.52 μm; tg6: 0.13 μm; P < 0.001). However, the corresponding reduction in network flow resistance contributes <20% to the maintenance of total peripheral resistance observed in tg6 mice. This suggests that the pathological effects of elevated Hct in these mice, and possibly also in polycythaemic humans, may relate to biological corollaries of a reduced ESL thickness and the consequent alteration in the blood-endothelium interface, rather than to an increase of flow resistance.

Depth-resolved molecular structure and orientation of polymer thin films by synchrotron X-ray diffraction

An exemplary system suitable for optoelectronics applications, i.e. poly(3-hexylthiophene), hereinafter P3HT, deposited by spin casting onto silicon substrates functionalised by three selected molecules and then properly annealed, has been examined. Grazing Incidence X-ray Scattering (GIXS) measurements have been performed with 4-circle diffractometer, allowing for a fine control of sample axes movement.By choosing different grazing incident angles, diffraction patterns from different layers of polymeric thin films have been recorded. Both in-plane and out-of-plane geometries have been combined in order to obtain complementary structural information. In this way structural and orientational differences of the polymer along with the film thickness (⩽50nm) have been highlighted. For all P3HT films spun on functionalized Si wafer, macromolecular layers close to the substrate surface give some evidence of higher order and orientation than those outmost the surface, and this behaviour is pronounced to a different extent depending on the functionalized molecules used. Contrariwise P3HT layers deposited onto bare Si wafer display reduced orientation and decreased crystallite size, especially at buried interface.

Influence of Biomacromolecules and Humic Acid on the Aggregation Kinetics of Single-Walled Carbon Nanotubes

The initial aggregation kinetics of single-walled carbon nanotubes (SWNTs) were studied using time-resolved dynamic light scattering. Aggregation of SWNTs was evaluated in the presence of natural organic matter [Suwannee River humic acid (SRHA)], polysaccharide (alginate), protein [bovine serum albumin (BSA)], and cell culture medium [Luria-Bertani (LB) broth] with varying solution concentrations of monovalent (NaCl) and divalent (CaCl(2)) salts. Increasing salt concentration and adding divalent calcium ions induced SWNT aggregation by screening electrostatic charge and thereby suppressing electrostatic repulsion, similar to observations with aquatic colloidal particles. The presence of biomacromolecules significantly retarded the SWNT aggregation rate. BSA protein molecules were most effective in reducing the rate of aggregation followed by SRHA, LB, and alginate. The slowing of the SWNT aggregation rate in the presence of the biomacromolecules and SRHA can be attributed to steric repulsion originating from the adsorbed macromolecular layer. The remarkably enhanced SWNT stability in the presence of BSA, compared to that with the other biomacromolecules and SRHA, is ascribed to the BSA globular molecular structure that enhances steric repulsion. The results have direct implications for the fate and behavior of SWNTs in aquatic environments and biological media.

Proteomics

Proteomics

Interaction of Fullerene (C60) Nanoparticles with Humic Acid and Alginate Coated Silica Surfaces: Measurements, Mechanisms, and Environmental Implications

The deposition kinetics of fullerene (C60) nanoparticles onto bare silica surfaces and surfaces precoated with humic acid and alginate are investigated over a range of monovalent (NaCI) and divalent (CaCl2) salt concentrations using a quartz crystal microbalance. Because simultaneous aggregation of the fullerene nanoparticles occurs, especially at higher electrolyte concentrations, we normalize the observed deposition rates by the corresponding favorable (transport-limited) deposition rates to obtain the attachment efficiencies, alpha. The deposition kinetics of fullerene nanoparticles onto bare silica surfaces are shown to be controlled by electrostatic interactions and van der Waals attraction, consistent with the classical particle deposition behavior where both favorable and unfavorable deposition regimes are observed. The presence of dissolved humic acid and alginate in solution leads to significantly slower deposition kinetics due to steric repulsion. Precoating the silica surfaces with humic acid and alginate exerts similar steric stabilization in the presence of NaCl. In the presence of CaCl2, the deposition kinetics of fullerene nanoparticles onto both humic acid- and alginate-coated surfaces are relatively high, even at relatively low (0.3 mM) calcium concentration. This behavior is attributed to the macromolecules undergoing complex formation with calcium ions, which reduces the charge and steric influences of the adsorbed macromolecular layers.

Observation of an Organic−Inorganic Lattice Match during Biomimetic Growth of (001)-Oriented Calcite Crystals under Floating Sulfate Monolayers

Macromolecular layers rich in amino acids and with some sulfated polysaccharides appear to control oriented calcite growth in living organisms. Calcite crystals nucleating under floating acid monolayers have been found to be unoriented on average. We have now observed directly, using in situ grazing incidence X-ray diffraction, that there is a 1:1 match between the monolayer unit cell and the unit cell of the (001) plane of calcite. Thus, sulfate head groups appear to act as templates for the growth of (001)-oriented calcite crystals, which is the orientation commonly found in biominerals.

Surface plasmon resonance as a tool for the estimation of adsorbed polymeric layer characteristics: Theoretical considerations and experiment

AbstractThe potential application of Surface Plasmon Resonance (SPR) spectroscopy in evaluating the thickness and volume fraction of adsorbed macromolecular layers is discussed in this work. The sensitivity of SPR spectroscopy to different layer concentration and to the layer extension normal to the surface (thickness) is theoretically illustrated. A new approach for the interpretation of SPR data is presented, which is applicable whenever the functional form of the density profile is known. The use of the proposed procedure for the fitting of experimental results from PS‐PEO brush self‐assembly on alumina surface has allowed the determination of the layer parameters, which have been found to be in accordance with theoretical mean‐field and scaling predictions, being also in good agreement with previous results from neutron reflectivity experiments. Furthermore, it has been confirmed that the dependence of the brush layer thickness d on the molecular weight Mw obeys the scaling law d ∼ Mw0.63. Since surface plasmon measurements can be acquired quite fast, it is suggested that under the present analysis scheme, the technique may be implemented to probe the average conformational properties of adsorbed macromolecular layers during their formation or under external stimuli. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2060–2070, 2007

Immobilization of dendrimers on Si–C linked carboxylic acid-terminated monolayers on silicon(111)

Poly(amidoamine) dendrimers were attached to activated undecanoic acid monolayers, covalently linked to smooth silicon surfaces via Si–C bonds. The resulting ultra-thin dendrimer films were characterized by X-ray photoelectron spectroscopy (XPS), X-ray reflectometry (XR) and atomic force microscopy (AFM). XPS results suggested amide bond formation between the dendrimer and the surface carboxylic acid groups. XR yielded thicknesses of 10 Å for the alkyl region of the undecanoic acid monolayer and 12 Å for the dendrimer layer, considerably smaller than the diameter of these spherical macromolecules in solution. This was consistent with AFM images showing collapsed dendrimers on the surface. It was concluded that the deformation arose from a large number of amine groups on the surface of each dendrimer reacting efficiently with the activated surface, whereby the dendrimers can deform to fill voids while spreading over the activated surface to form a homogeneous macromolecular layer.

A simple atomic force microscopy method for the visualization of polar and non-polar parts in thin organic films

Here we present a scanning probe microscopy method that allows for the identification of regions of different polarity (i.e. hydrophilicity) in thin organic films. This technique is based on the analysis of the difference between phase images generated at different applied bias voltages in tapping-mode atomic force microscopy. We show that, without any chemical modification of the microscope tip, it is possible to investigate surface properties of complex macromolecular layers, yielding new insight into the functional properties of the photosynthetic electron transport macromolecular complex, Photosystem I.

Plasma‐enhanced deposition of silver nanoparticles onto polymer and metal surfaces for the generation of antimicrobial characteristics

AbstractSilver nanoparticle thin layers were deposited onto formaldehyde‐radio frequency (RF)‐plasma‐functionalized medical‐ and food‐grade silicone rubber, stainless steel, and paper surfaces. The silver deposition was carried out under ex situ plasma conditions using the Tollen's reaction. Results from survey and high‐resolution electron spectroscopy for chemical analysis, scanning electron microscopy (SEM), atomic force microscopy (AFM), and energy dispersive X‐ray spectroscopy investigations confirmed the presence of thin silver layers on the plasma‐exposed and subsequently modified substrate surfaces. In addition, SEM and AFM demonstrated the nanoparticle‐based morphology of the deposited layers. Our results showed that thin macromolecular layers bearing aldehyde functionalities can be deposited onto silicone rubber, stainless steel, and paper surfaces. The bactericidal properties of the silver‐coated surfaces were demonstrated by exposing them to Listeria monocytogenes. No viable bacteria were detected after 12 to 18 h on silver‐coated silicone rubber surfaces. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1411–1422, 2004

Flame‐retarded polyolefin systems of controlled interphase

AbstractThe principle of multilayer interphases was proposed earlier for modifying the mechanical properties and UV stability of various multicomponent polymer systems. This paper focuses on the applicability of this principle for improving the performance of intumescent flame‐retardant systems using melamine‐treated ammonium polyphosphate, silicone modified polyol + ammonium polyphosphate, and silicone modified nanoparticles in polypropylene. The structure–property relationship of the formed systems was studied. A melamine layer of 1.45 nm thickness was formed around ammonium polyphosphate in order to improve the hygrothermal stability, but this layer was not shear‐resistant enough. An interphase formed using a special silicone additive is more stable and acts with the intumescent flame‐retardant system synergistically. The advantageous interfacial structure is quite complex in this case: polyphosphate particles are surrounded with a macromolecular layer formed from polyol, silicone, and reactive surfactant in order to ensure good stability, efficiency and compatibility. AFM, XPS and a Cone Calorimeter were used for determining the structure and flame‐retardancy of these systems. Nanocomposites combined with silicone‐containing intumescent system were developed in order to avoid dipping at ignition in the vertical position. SAXS and µ‐TA methods were used for determining the structure of this material. Copyright ­© 2003 John Wiley &amp; Sons, Ltd.

Synthesis of polyaniline-type thin layer structures under low-pressure RF-plasma conditions

Macromolecular thin layers were deposited under continuous and modulated aniline-RF-plasma conditions, and by Ar-, NH 3, and I 2-plasmas-enhanced crosslinking of pre-deposited N-phenyl-1,4-phenylenediamine (PPD) films on optically smooth quartz substrates. Survey and high-resolution ESCA, Fourier transform infrared spectroscopy (FT-IR), UV–VIS analyses were used for the investigation of the chemical structure of the deposited films. The thermal behavior of deposited (undoped and doped) plasma-treated PPD films was characterized by TG/DTA and DSC analysis and their electrical conductivities were evaluated by using a commercial four-probe system. It was found that gas phase aniline-plasma environments induce intense molecular fragmentation processes, which result in the formation of structures that are significantly different in comparison to that of conjugated polyaniline. The plasma-mediated crosslinking approach allows the synthesis of macromolecular layers, which retain most of the structure of the starting component, and their doped versions exhibit electrical conductivities comparable to the electrical conductivities of doped conventional polyaniline.

Low-Temperature Synthesis and Properties of Silver Nanoparticles Stabilised by Acrylic Polymers

A cryochemical synthesis of silver nanoparticles stablised by poly(2-dimethylaminoethyl methacrylate) has been performed. It has been shown that precursors which do not possess optical properties of colloidal silver are present in the freshly prepared organodispersion of silver in 2-(dimethylamino)ethyl methacrylate (DMAEMA) on the early stages of nanoparticle growth. Sols prepared with cryochemically synthesised silver nanoparticles in water, acetone, and toluene were found to be sterically stabilised by macromolecular poly-DMAEMA layers formed at the surface of the nanoparticles. Dynamic light scattering data show that the thickness of these layers depends on the nature of the solvent.

Atomic Force Microscopy Study of Structural Organization of Carbosilane Liquid Crystalline Dendrimer

Structural organization of a carbosilane liquid crystalline dendrimer of fifth generation with cyanobiphenyl mesogenic groups in the 25−130 °C range was examined by atomic force microscopy (AFM). Studies were performed on the dendrimer films of different thicknesses, from single macromolecular layers to films several micrometers thick. Packing motifs of spherical macromolecules (5.5 nm in diameter) were revealed in submicrometer AFM images of the dendrimer films. Rectangular and hexagonal lattices were found in the single layers on different substrates. With an increase of film thickness, first smectic layers are formed, and then domains consisting of aggregates of flat-on lying and edge-on standing smectic layers were found. In thick films, the domains exhibit layered structures of different thicknesses and with different orientations. AFM images recorded at different temperatures demonstrated height changes of different domains caused by anisotropic changes of lattices of the smectic layers.

Deposition of high fluorine content macromolecular thin layers under continuous-flow-system corona discharge conditions

High fluorine content macromolecular layers were deposited on polyethylene (PE) film surfaces under an originally designed, continuos-flow-system plasma reactor conditions. Survey and angle resolution, ESCA data and ATR-FTIR results indicate that the plasma-created films are thin and have a fairly uniform structure. The fluorinated layers have a 60% relative fluorine atomic concentration, and are mainly composed of CF2-CF, and C-CF3 groups. AFM images collected from virgin and plasma-exposed PE surfaces show a significantly rougher surface of the plasma-treated substrates. Applications can be envisaged for creating Teflon-like coatings on various polymeric film surfaces using a continuous plasma process.

A novel ceramic-supported polymer membrane for pervaporation of dilute volatile organic compounds

A novel asymmetric ceramic-supported polymer (CSP) pervaporation membrane was developed using free-radical graft polymerization of polyvinyl acetate (PVAc) onto a porous tubular silica substrate. The resulting membrane was characterized by pervaporation removal of trichloroethylene (TCE) and chloroform from dilute aqueous solutions. PVAc was chosen since it has a high affinity for TCE and chloroform and a low affinity for water, thus making the membrane permselective toward these solutes. This study has shown, for the first time, that pervaporation is possible even with a large substrate starting pore size (∼500 Å) and where the active separation phase is a macromolecular layer of terminally anchored chains. Performance of the CSP membrane was assessed at various hydrodynamic conditions and feed concentrations. Resistance of the PVAc/CSP membrane was found to be negligible compared with the concentration boundary layer resistance. The enrichment factor, defined as the ratio of total solute concentration in the permeate to that in the feed, increased with polymer graft yield. For a given graft yield, the enrichment factor approached an asymptotic plateau value, as the tube-side Reynolds number was increased due to increased water flux in response to the rise in transmembrane pressure. Preliminary analysis suggests that the CSP pervaporation membrane performance could be substantially increased by optimizing reaction conditions to produce a higher polymer graft density.

Synthesis and Surface Functionalization under Cold-plasma Conditions.

Controlled, plasma-mediated, surface modification of various substrates including inorganic and organic polymers is essential for the creation of miniaturized advanced materials. Many of these approaches have focused on the surface-implantation of specific chemical groups which can be employed for subsequent chemical reactions where molecular recognition plays a significant role (e, g, immobilization of biomolecules; preparation of catalysts, etc.). The deposition of predesigned-structure thin macromolecular layers on various substrate surfaces is another branch of surface modification processes. By creating high specificity surface layers (e. g. chemically inert films, electrically insulating layers, etc.) on commodity materials will open up routes for the development of economical future technologies. In this contribution some of the recent results, generated at the Center for Plasma-Aided Manufacturing, University of Wisconsin-Madison, in the field of RF-plasma-mediated surface deposition and functionalization processes are discussed.

Microvascular blood flow resistance: role of endothelial surface layer.

Observations of blood flow in microvascular networks have shown that the resistance to blood flow is about twice that expected from studies using narrow glass tubes. The goal of the present study was to test the hypothesis that a macromolecular layer (glycocalyx) lining the endothelial surface contributes to blood flow resistance. Changes in flow resistance in microvascular networks of the rat mesentery were observed with microinfusion of enzymes targeted at oligosaccharide side chains in the glycocalyx. Infusion of heparinase resulted in a sustained decrease in estimated flow resistance of 14-21%, hydrodynamically equivalent to a uniform increase of vessel diameter by approximately 1 micron. Infusion of neuraminidase led to accumulation of platelets on the endothelium and doubled flow resistance. Additional experiments in untreated vascular networks in which microvascular blood flow was reduced by partial microocclusion of the feeding arteriole showed a substantial increase of flow resistance at low flow rates (average capillary flow velocities < 100 diameters/s). These observations indicate that the glycocalyx has significant hemodynamic relevance that may increase at low flow rates, possibly because of a shear-dependent variation in glycocalyx thickness.

Refractive Index Measurements to Study the Structural Behavior of Anisotropic Macromolecules Confined between Two Mica Surfaces

A study was performed with a surface force apparatus (SFA) in which the automation of a mica surface displacement at very low speed and the use of multiple beam interferometry yield force profiles and variations of the refractive index of the macromolecular layers adsorbed between the two mica plates of the device. The performances of such an SFA are illustrated by the identification of the two molecular types of the plant lectin concanavalin A (Con A) when the anisotropic dimer Con A is converted into an isotropic tetramer form. Thus, the SFA can be a good model device for studying macromolecular interactions submitted to a variable compression stress in a confined space.