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PubMed2005

Single molecule nanoparticles of the conjugated polymer MEH-PPV, preparation and characterization by near-field scanning optical microscopy.

We have developed a straightforward method for producing a stable, aqueous suspension of hydrophobic, fluorescent pi-conjugated polymer nanoparticles consisting primarily of individual conjugated polymer molecules. Features of the method are the facile preparation, purity, unique optical properties, and small size (approximately 5-10 nm) of the resulting nanoparticles. The results of TEM, scanning force microscopy, and near-field scanning optical microscopy of particles cast from the suspension indicate that the particles are single conjugated polymer molecules. The NSOM results yield estimates of the optical cross-sections of individual conjugated polymer molecules. The UV-vis absorption spectra of the nanoparticle suspensions indicate a reduction in conjugation length attributed to deformations of the polymer backbone. Fluorescence spectra of the aqueous nanoparticle suspensions indicate interactions between segments of the polymer chain and intramolecular energy transfer.

Journal Article↗

PubMed2005

Conformational fluctuations and large fluorescence spectral diffusion in conjugated polymer single chains at low temperatures.

The fluorescence of single chains of the conductive polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) was studied by means of single-molecule spectroscopy at 15 K. MEH-PPV was deposited onto a surface from a toluene solution and covered with a polymer cap layer of poly(vinyl alcohol) spin-coated from an aqueous solution for protection against air. Because MEH-PPV is insoluble in water, such sample preparation guarantees that MEH-PPV chains do not mix with the cap polymer. We found that this "host matrix free" environment results in substantially stronger fluorescence spectral diffusion than that observed for conjugated polymer single chains embedded into polymer matrices. The average spectral diffusion range was 500 cm(-1), and the maximum registered value reached 1100 cm(-1), which is approximately 6 times larger than the values reported before. We analyzed spectral diffusion by observation of temporal evolution of the fluorescence intensity, the position of the maximum, and the width of fluorescence spectra. We propose that the transition energy shifts are caused by the differences of the London dispersive forces in slightly different polymer chain conformations. Such conformational changes are possible even at low temperatures because the MEH-PPV single chains in our samples have more freedom for fluctuations than in the usual "in host" arrangement.

Journal Article↗

PubMed2006

Photo-induced relaxation and proton transfer in some hydroxy naphthoic acids in polymers.

Photophysical and photochemical properties of 3-hydroxy-2-naphthoic acid [3,2-HNA] and 1-hydroxy-2naphthoic acid [1,2-HNA] in different aprotic, protic, and ion exchange (Nafion) polymers have been described in this article. In both molecules, intramolecular hydrogen bond (IMHB) exists between OH and COOH functional groups. Both 3,2-HNA and 1,2-HNA form different emitting species in different polymeric media. Fluorescence characteristic of 3,2-HNA is found to depend on its concentration, nature of the microenvironment, and wavelength of excitation, while 1,2-HNA is less susceptible to these changes. 3,2-HNA exhibits dual fluorescence band (normal and large Stokes shifted) in aprotic and only a single large Stokes shifted fluorescence in protic polymers, while 1,2-HNA shows a single fluorescence band along with weak phosphorescence emission in these polymers. Both excited-state inter and intramolecular proton transfer (ESPT) take place in 3,2-HNA in aprotic and protic polymers, resulting in large Stokes shifted emission band. A competition between ESIPT and excimer formation is observed by the appearance of rise time on increasing the concentration of 3,2-HNA in protic polymer. In Nafion film, 3,2-HNA is present as a cationic as well as neutral species. The presence of extra protons in Nafion film facilitates excited-state intramolecular proton transfer (ESIPT) in the neutral species of 3,2-HNA and gives large Stokes shifted emission (10 500 cm(-1)). No such effect is observed in 1,2-HNA doped in Nafion film. It is observed that, depending on the position of the IMHB ring, the electronic spectra get modified and the strength of IMHB is affected by the micro-environment of the polymer which alters the photophysics of these molecules.

Journal Article↗

PubMed2005

A dendronized polymer is a single-molecule glass.

The molecular architecture of dendronized polymers can be tuned to obtain nanoscale objects with desired properties. In this paper, we bring together experiments and computer simulations to study the thermodynamic and dynamic properties of a single dendronized polymer chain. We find that, upon changing certain architectural features, dynamic correlations characterizing backbone conformational fluctuations of a dendronized polymer exhibit dynamics akin to glass-forming bulk liquids. Thus, a dendronized polymer chain is a novel macromolecule that is a single-molecule glass. Over a range of conditions that lead to glassy dynamics, there does not appear to be any thermodynamic singularities. We discuss how a dendronized polymer is a molecular system that can directly test different models of glassy dynamics. We also show that defect densities characteristic of typical synthesis conditions do not alter the material properties of dendronized polymers.

Journal Article↗

PubMed1996

Quantification of insulin release from implantable polymer-based delivery systems and augmentation of therapeutic effect with simultaneous release of somatostatin.

Insulin injections control diabetes mellitus but do not reproduce physiologic regulation. Polymer-based controlled-release technology has enabled us to demonstrate: that the controlled release of insulin from polymer matrices can indeed be used to control diabetes mellitus but does so at the expense of hyperinsulinemia and hypoglycemia; and that somatostatin can be delivered in similar fashion, so as to provide glucose homeostasis in a more physiologic range, at lower insulin levels and at somatostatin doses below those used in intermittent infusion studies; and, that microgram quantities of a drug can be delivered successfully in vivo with intact biological function and in a manner that can be monitored continuously. In the present study the simultaneous polymer-matrix-controlled release of insulin with somatostatin extended glycemic control in diabetic rats. Eleven rats received subcutaneous polymer matrix implants containing insulin alone and 11 rats received implants containing insulin and somatostatin. Plasma and urinary glucose control were improved in both groups. Glucose concentrations in the insulin alone group remained depressed for 5 days until insulin release from the matrices declined below 11.6 units/kg/day. When somatostatin was delivered at 0.75-1.1 micrograms/kg/day together with insulin, plasma glucose control persisted for 12 days until insulin release decreased below 3.6 units/kg/day. It is our hope that further experiments regarding the potential role of both controlled-release devices and somatostatin will be performed to provide continuing therapeutic alternatives to the insulin-dependent diabetic. This is also the first in vivo demonstration of the simultaneous release of two biologically active peptide hormones from polymer matrices. The use of the polymer matrix systems may not only have profound effects on the ambulatory care of diabetes but might also permit the investigation of the synergistic effects of other families of compounds.

Animals↗

PubMed2004

Reduction of friction at oxide interfaces upon polymer adsorption from aqueous solutions.

Reduction of the interfacial friction for the contact of a silicon oxide surface with sodium borosilicate in aqueous solutions has been accomplished through the adsorption of poly(L-lysine)-graft-poly(ethylene glycol) on one or both surfaces. Spontaneous polymer adsorption has been achieved via the electrostatic attraction of the cationic polylysine polymer backbone and a net negative surface charge, present for a specific range of solution pH values. Interfacial friction has been measured in aqueous solution, in the absence of wear, and on a microscopic scale with atomic force microscopy. The successful investigation of the polymer-coated interfaces has been aided by the use of sodium borosilicate microspheres (5.1 microm diameter) as the contacting probe tip. Measurements of interfacial friction as a function of applied load reveal a significant reduction in friction upon the adsorption of the polymer, as well as sensitivity to the coated nature of the interface (single-sided versus two-sided) and the composition of the adsorbed polymer. These measurements demonstrate the fundamental opportunity for lubrication in aqueous environments through the selective adsorption of polymer coatings.

Journal Article↗

PubMed2004

Molecular structure of an alkyl-side-chain polymer-water interface: origins of contact angle hysteresis.

A new and direct approach to verify surface heterogeneity as the microscopic origin of contact-angle hysteresis is demonstrated. IR-visible sum-frequency-generation spectroscopy (SFG) was used to selectively probe the molecules at the interface of an alkyl-side-chain polymer [poly(vinyl n-octadecyl carbamate-co-vinyl acetate)] with water. The spectra indicate that in contact with water, the polymer surface is heterogeneous (having areas of differing surface energies). This evidence of surface heterogeneity supports the hysteresis observed in the advancing and receding contact angles of the polymer surface with water. The same measurements made for the chemically and structurally similar surface of an octadecyltrichlorosilane self-assembled monolayer indicates a homogeneous surface at the water interface. In this case, contact-angle hysteresis measurements implicate surface roughness as the cause of hysteresis. Atomic force microscopy measurements of roughness for these surfaces further support our conclusions. The polymer-water interface was probed using SFG at above-ambient temperatures, and an order-to-disorder transition (ODT) of alkyl side chains at the interface was observed, which closely follows the melting of crystalline side chains in the bulk. This transition explains the increased wettability of the polymer, by water, when the temperature is raised above the bulk melting temperature. Furthermore, the irreversibility of this ODT suggests that the disordered polymer-water interface is the thermodynamic equilibrium state, whereas the before-heating structure of this interface is a kinetically hindered metastable state.

Journal Article↗

PubMed2004

Formation and photoluminescence of silver nanoparticles stabilized by a two-armed polymer with a crown ether core.

Silver nanoparticles were synthesized by the use of a two-armed polymer with a crown ether core [poly(styrene)]-dibenzo-18-crown-6-[poly(styrene)] based on the flexibility of the polymer chains and the complex effect of crown ether with Ag(+) and Ag. The size of silver nanoparticles could be tailored by controlling the initial concentrations of the polymer and Ag(+), and the molecular weight of the polymer. The emission of silver nanoparticles was blue-shifted, and the intensity of the photoluminescence of silver nanoparticles stabilized by the polymer was significantly increased due to the complex effect between the crown ether embedded in the polymer and the silver nanoparticles.

Journal Article↗

PubMed2004

Inkjet printing of well-defined polymer dots and arrays.

Inkjet printing represents a highly promising polymer deposition method, which is used for, for example, the fabrication of multicolor polyLED displays and polymer-based electronics parts. The challenge is to print well-defined polymer structures from dilute solution. We have eliminated the formation of ring stains by printing nonvolatile acetophenone-based inks on a perfluorinated substrate using different polymers. (De)pinning of the contact line of the printed droplet, as related to the choice of solvent, is identified as the key factor that determines the shape of the deposit, whereas the choice of polymer is of minor importance. Adding 10 wt % or more of acetophenone to a volatile solvent (ethyl acetate)-based polymer solution changes the shape of the deposit from ring-like to dot-like, which may be due to the establishment of a solvent composition gradient. Arrays of closely spaced dots have also been printed. The size of the dots is considerably smaller than the nozzle diameter. This may prove a potential strategy for the inkjet printing of submicrometer structures.

Journal Article↗

PubMed2004

Controlled nanoparticle assembly by dewetting of charged polymer solutions.

In this paper, we present an alternative approach for controlled nanoparticle organization on a solid substrate by applying dewetting patterns of charged polymer solutions as a templating system. Thin films of charged polymer solutions dewet a solid substrate to form complex dewetting patterns that depend on the polymer charge density. These patterns, ranging from polygonal networks to elongated structures that are stabilized by viscous forces during dewetting, serve as potential templates for two-dimensional nanoparticle organization on a solid substrate. Thus, while nanoparticles dried in pure water undergo self-assembly to form close-packed arrays, addition of charged polymer in the dispersion leads to the formation of open structures that are directed by the dewetting patterns of the polymer solution. In this study, we focus on the application of elongated structures resulting from dewetting of high-charge-density polymer solutions to align nanoparticles of silica and gold into long chains that are several micrometers in length. The particle ordering process is a two-step mechanism: an initial confinement of the nanoparticles in the dewetting structures and self-assembly of the particles within these structures upon further drying by lateral capillary attractions.

Journal Article↗

PubMed2005

Effect of hydrophobically modified polymers on shear-induced multilamellar vesicles.

The effects of polymer concentration, polymer molecular weight, and hydrophobe substitution level of modified poly(acrylic acid) polymers on the formation, size, and viscoelastic properties of shear-induced multilamellar vesicles (onions) are studied by rheology and light diffraction. The onions are close-packed, space-filling vesicles formed by shearing aqueous lamellar phases of C12E5 surfactant to produce phases with sufficient order and size uniformity (O(1-3 microm)) to diffract light. The addition of hydrophobically modified polymers enhances the rate of formation, uniformity, and stability independent of hydrophobe substitution level. Onion size decreases with increasing shear rate as observed for pure surfactant onion systems, but the shear-rate dependence is changed by the polymer. The onion phase has a plateau modulus that increases with polymer concentration but is independent of hydrophobe substitution level or molecular weight. The model presented by Panizza et al. that relates the plateau modulus of the onion phase to membrane rigidity and the compression modulus is consistent with independent measurements of membrane properties from SANS.

Journal Article↗

PubMed2005

Influence of added particles on the phase behavior of polymer solutions. Analysis by mean-field lattice theory.

The influence of added colloidal particles on the phase stability of polymer solutions is investigated theoretically. The polymer has an affinity to the particle surface. A mean-field lattice theory based on the Flory-Huggins theory is used to calculate the phase behavior in solutions containing a single polymer component and particles. The particles are described in two different ways. The first approach considers the surface free energy associated with added solid particles and the mixing entropy of the particles. In the second approach, the particles are simply modeled as large polymers. Both ways of describing the added particles show that the added particles decrease the stability of the polymer solution when the polymer-particle attraction is strong. A higher particle concentration enhances the effect. Experiments where polystyrene latex particles are added at different concentrations to aqueous dispersions of ethyl(hydroxyethyl)cellulose (EHEC) support the theoretical findings.

Journal Article↗

PubMed2006

Selective protein adsorption on a phase-separated solvent-cast polymer blend.

Polymer-based biomedical devices are growing increasingly sophisticated as compositions evolve toward copolymers and blends in order to satisfy complex design criteria. Such polymers afford opportunities for both micro- and macrophase separation at nano- and micro-length scales and raise questions concerning the role of heterogeneous surface morphology on protein adsorption. Adsorbed protein layers play a critical role in mediating the interaction of cells with polymer surfaces, and both understanding and controlling protein adsorption is assuming greater significance in the development of surfaces with enhanced physiological compatibility. Here we study the short-time adsorption of ferritin, a model protein highly resistant to denaturation and easily imaged in the transmission electron microscope (TEM), onto a phase-separated homopolymer blend of polycaprolactone (PCL) and a polycarbonate derived from desaminotyrosyl-tyrosine dodecyl ester (PDTD). At physiological pH, ferritin selectively adsorbs onto the PDTD phase at a surface density approximately three times greater than that on the PCL phase. By decreasing the pH below ferritin's isoelectric point so its average charge becomes positive, the selective adsorption disappears and the surface density of adsorbed ferritin becomes independent of the phase separation. We attribute the selectivity to the electrostatic repulsion between ferritin and hydrolytically charged PCL, both of which will have a net negative charge at physiological pH. To perform these experiments, we solvent-cast ultrathin polymer films onto dissolvable salt substrates, and we characterize the morphology by TEM imaging and quantitative spatially resolved electron energy-loss spectroscopy (EELS). We find that the film morphology depends strongly on such processing-related variables as the solvent evaporation rate and the nature of the surface in contact with the polymer film during casting. The adsorption of ferritin depends on whether the film is phase-separated as well as to which surface of the film the protein solution is exposed, and these findings suggest that seemingly small variations in polymer processing that influence both the bulk and surface morphology can have a profound effect on the short-time protein adsorption.

Adsorption↗

PubMed2006

Friction of single polymers at surfaces.

Atomic force microscope (AFM) single molecule force spectroscopy has been used to investigate the friction coefficient of individual polymers adsorbed onto a solid support. The polymer chains were covalently attached to an AFM tip and were allowed to adsorb on a mica surface. Different polymers (ssDNA, polyallylamine) were chosen to cover a range of friction coefficients. During the experiment, the AFM tip was retracted in- and off-plane which results, depending on the chosen conditions, in a desorption of the polymer from the surface, a sliding across the surface, or a combination of both. Thus, the obtained force-extension spectra reveal detailed information on the mobility of a polymer chain on a surface under experimentally accessible conditions. This study demonstrates that absorbed polymers with comparable desorption forces may exhibit drastically different in plane mobility.

Adenylosuccinate Synthase↗

PubMed1998

Direct Observation of Polymer Network Structure in Macroporous N-Isopropylacrylamide Gel by Raman Microscopy.

The Raman microscopy technique is used to characterize the temperature-induced evolution of the pore structures of a macroporous N-isopropylacrylamide (NIPA). The gel is synthesized using a suspension of 45 wt % toluene and 55 wt % pregel NIPA solution. The intensity of the band due to the CH2 bending vibration, centered at 1445 cm-1, is used to monitor the distribution of the polymer chain density in the lateral plane. It is found that the macroporous gel consists of water-rich areas, which can be interpreted as the pores, and polymer-rich areas. At room temperature, the average sizes of the pores and the width of polymer-rich areas are 75 µm and 20 µm, respectively. Both the pores and their surrounding polymer-rich areas have random geometry, as demonstrated by the Raman microimaging. With increased temperature the size of the pores decreases. This process is accompanied by a narrowing of the polymer-rich areas. At higher temperatures polymer chains bunch together and this process accelerates rapidly near the volume phase transition temperature (34 degreesC). Above 36 degreesC, the pore sizes become too small to be resolved using Raman microscopy.

Journal Article↗

PubMed2000

Expression of heat shock protein 70 (HSP70) at the interface of polymer-implants in vivo.

Synthetic polymer meshes are widely applied in the modern surgical approach for repairing abdominal wall defects. The implanted material is often observed leading to post-operative complications such as deficient abdominal wall mobility and adhesion formation with the abdominal cavity and/or abdominal organs. However, the functioning of the implant is primarily affected by the wound healing process guided by inflammatory events occurring at the tissue-material interface. This could presumably be influenced by the physicochemical properties of the polymer. With regard to it, the cellular and molecular processes involved in the successful restoration of the abdominal wall function are poorly understood. The present in vivo study, therefore, exemplary investigated in a rat model, the commercially available polymer-meshes Prolene (polypropylene, PP), Mersilene (polyester, PE) and Vicryl (polyglactin 910), as well as new mesh variants consisting either of PP (EB) or a combination of PP and polyglactin 910 (A plus or Vypro). The implanted material was evaluated by light and electron microscopy, immunohistochemistry as well as morphometry over an implantation period of 90 days. The data show that polymers induce heat shock protein (HSP)70, and its expression at the interface correlates inversely with the activity of the inflammatory reaction in vivo. Further, an ascent in HSP70 expression parallels the increasing implantation period and evolving foreign-body granulomas. Accordingly, a major role for HSP70 in modulating the local acceptance of polymers and as an additional marker for in vivo testing of polymers is suggestive.

Journal Article↗

PubMed2002

Active fluidization of polymer networks through molecular motors.

Entangled polymer solutions and melts exhibit elastic, solid-like resistance to quick deformations and a viscous, fluid-like response to slow deformations. This viscoelastic behaviour reflects the dynamics of individual polymer chains driven by brownian motion: since individual chains can only move in a snake-like fashion through the mesh of surrounding polymer molecules, their diffusive transport, described by reptation, is so slow that the relaxation of suddenly imposed stress is delayed. Entangled polymer solutions and melts therefore elastically resist deforming motions that occur faster than the stress relaxation time. Here we show that the protein myosin II permits active control over the viscoelastic behaviour of actin filament solutions. We find that when each actin filament in a polymerized actin solution interacts with at least one myosin minifilament, the stress relaxation time of the polymer solution is significantly shortened. We attribute this effect to myosin's action as a 'molecular motor', which allows it to interact with randomly oriented actin filaments and push them through the solution, thus enhancing longitudinal filament motion. By superseding reptation with sliding motion, the molecular motors thus overcome a fundamental principle of complex fluids: that only depolymerization makes an entangled, isotropic polymer solution fluid for quick deformations.

Actins↗

PubMed2001

Molecularly imprinted polymer sensors for pesticide and insecticide detection in water.

Antibodies, peptides, and enzymes are often used as molecular recognition elements in chemical and biological sensors. However, their lack of stability and signal transduction mechanisms limits their use as sensing devices. Recent advances in the field of molecularly imprinted polymers (MIPs) have created synthetic materials that can mimic the function of biological receptors but with less stability constraints. These polymers can provide high sensitivity and selectivity while maintaining excellent thermal and mechanical stability. To further enhance the advantages of the traditional imprinted polymer approach, an additional fluorescent component has been introduced into these polymers. Such a component provides enhanced chemical affinity as well as a method for signal transduction. In this type of imprinted polymer, binding of the target analyte invokes a specific spectral signature from the reporter molecule. Previous work has provided molecularly imprinted polymers that are selective for the hydrolysis products of organophosphorus species such as the nerve agents sarin and soman. (A. L. Jenkins, O. M. Uy and G. M. Murray, Anal. Chem., 1999, 71, 373). In this paper the direct imprinting of non-hydrolyzed organophosphates including pesticides and insecticides is described. Detection limits for these newly developed MIP sensors are less than 10 parts per trillion (ppt) with long linear dynamic ranges (ppt to ppm) and response times of less than 15 min.

Insecticides↗