Polyethylene Oxide Layer Research Articles

Adsorption of polyethylene oxide on surface modified silica - stability of bare and covered particles in suspension

The adsorption of poly-ethylene oxide (PEO) on modified colloidal silica and the stability of the aqueous suspension was investigated. With octanol some silanol groups at the silica surface were replaced by octylgroups. The size of the modified silica particles and the charge and chemical groups on the surface were charaterized by ultracentrifugation, photon correlation spectrometry, polyelectrolyte titration and IR spectrometry. The adsorbed amounts of polyethylene oxides of different molar mass were determined on the modified silica in water. With photon correlation spectrometry (PCS) the hydrodynamic layer thickness of the PEO layers on the particles were measured. The dependences of the layer thicknesses on molar mass of the PEO, polymer concentration and adsorption time were determined. The aggregation of the suspended PEO coated and uncoated modified silica particles was examined with PCS by the time dependence of the diffusion coefficient at different salt concentrations. The influence of molar mass and concentration of PEO as well as of the age of the dispersion was explored. The measured dependences are discussed and compared with the behavior of unmodified silica- and latex-particles.

Surface phase separation in an amphiphilic block copolymer monolayer at the air-water interface

Deuterium labelled linear diblock copolymers of polyethylene oxide and polymethyl methacrylate were spread at the air-water interface and neutron reflectometry was used to determine the spatial organization of the copolymer. The mole fraction composition of the block copolymer was ca 0.5, but with some variation from copolymer to copolymer arising from the separate syntheses needed for each labelled copolymer. The neutron reflectometry data for two surface concentrations of the block copolymer (0.6 and 1.2 mg m −2) were analysed using the kinematic approximation. In addition to providing the composition and thickness of regions which are rich in one component or the other, this method also gives the separation between each of these regions. For the lowest surface concentration used the two blocks were mixed together near the surface with a Gaussian distribution of segments, the characteristic dimension of which was circa 8 Å. At the higher surface concentration of 1.2 mg m −2, there is a clear separation between the two blocks with the polyethylene oxide exploring the aqueous subphase and part of the polymethyl methacrylate block being expelled into the air phase. However, there is no evidence for the formation of a stretched brush like layer of polyethylene oxide at the surface. Attempts to analyse for such a layer are discussed and the care needed in such analyses is pointed out. For both concentrations the near surface distribution of water takes on a tanh distribution.

Dynamic Interactions between Polyethylene Oxide Layers Adsorbed on Latex Beads

Dynamic interactions between two polyethylene oxide layers adsorbed on polystyrene latex spheres have been studied by colloidal particle scattering, a method developed by us to measure surface forces. The adsorption layers were modeled as an elastic gel characterized by its elastic modulus and thickness. Both of them were determined experimentally, and the results agree with literature data. Furthermore, these two parameters provide us with information about polymer conformations on a latex surface at different adsorption concentrations, which is also consistent with previous theoretical and experimental findings.

A Total Internal Reflectance Fluorescence Nanoscale Probe of Interfacial Potential and Ion Screening in Polyethylene Oxide Layers Adsorbed onto Silica

This work investigated the effect of the ionic environment on the fluorescence signal from layers of fluorescein-tagged polyethylene oxide (FITC-PEO) adsorbed onto silica from aqueous solution. The fluorescence intensity from an adsorbed layer in a total internal reflectance fluorescence (TIRF) study was found to be highly sensitive to the fluid contacting the layer: Deionized water gave no signal at all, while the signal was strong for phosphate-buffered saline. The bulk solution fluorescence of FITC-PEO, however, was comparatively less sensitive to the presence of buffer as opposed to deionized water. This suggested that the fluorescent sensitivity to ionic conditions was an interfacial effect, possibly stemming from the pH sensitivity of fluorescein. Titrations of the fluorescence signal from the adsorbed layer as a function of salt concentration and as a function of phosphate concentration at nearly constant ionic strength revealed an increase in fluorescence with increasing concentration of the ionic species, with a sharp sigmoidal transition. This transition occurred at 0.0001Mfor the phosphate and near 1Mfor NaCl, demonstrating the phosphate-buffering species to be more effective in producing fluorescence compared with an unreactive salt. These experimental observations were best explained by the weak acid–base reaction of the fluorescein with the buffer and by the influence of the ionic strength on the debye length near the acidic silica surface. In the presence of buffer, the transition point was shown to be sensitive to the local potential, and the extent of the quench at low ionic strengths to the local pH.

The determination of segment density profiles of polyethylene oxide layers adsorbed at the air-water interface

The adsorption of polyethylene oxide of molecular weight 17 800 and 87 000 at the air/water interface has been studied by neutron reflection and surface tension measurements. Over the concentration range 10 −4−10 −1 wt% of polymer the surface excesses determined by neutron reflection were consistent with the results derived from the surface tension using the Gibbs equation. Reflectivity measurements were made on three samples of different isotopic composition at a fixed concentration of 0.1 wt%. The adsorbed polymer layers were found to be essentially completely immersed in the water, contrary to earlier suggestions that a significant fraction of the polymer protrudes into the vapour phase. The distribution profiles of the polymers with respect to the surface normal direction are composed of two major blocks, one constituting the layer at the top surface and the other forming a diffuse region extending into the bulk solution. Over 90% of the homopolymer is in the first region with a thickness of 18 ± 2 Å, and the remaining diffuse region has a thickness of 35 ± 5 Å. Alternatively, the volume fraction profile of the polymer can be described as a 1 : 1 sum of two half Gaussians of widths 10 and 35 Å. Change of molecular weight and addition of salt do not change either the surface coverage or the segment density profiles significantly.

Coagulation and flocculation measurements by photon correlation spectroscopy ? colloidal SiO2 bare and covered by polyethylene oxide

With photon correlation spectrometry (PCS) the diffusion coefficients, average diameters and polydispersities of colloidal particles can be determined in dilute aqueous suspensions. In this study PCS is used to follow the coagulation and flocculation of silica particles. Electrolyte solution added to suspensions of bare particles and of particles covered with adsorbed polyethylene oxide layers induces aggregation. The rate constants of aggregation are evaluated by the second-order Smoluchowski theory with the assumptions of spherical aggregated particles and volume proportional light-scattering amplitude. Adsorbed PEO layers of molar mass lower thanMw=160000 decrease the critical flocculation concentration and the flocculation states and rate constants for bare and covered particles are the same at high electrolyte concentrations. Polymer layers of high molar mass (Mw=325000, 900000) reducved at full coverage the rate constants and stabilize the suspensions even at high electrolyte concentrations. At low coverage adsorption of high molar mass polymers results in the same values as of low molar mass PEO. The correlation between rate constants and hydrodynamic PEO layer thicknesses demonstrates the steric influence of the tails of the adsorbed macromolecules on stability and flocculation.

Protein—surface interactions in the presence of polyethylene oxide: II. Effect of protein size

Polyethylene oxide (PEO) surfaces exhibit low protein adsorption. PEO surface—protein interactions are examined theoretically as a function of surface density and chain length of PEO and variation in the size of the protein (assumed to be a sphere). Recent studies suggest that the PEO surface may have a small hydrophobic character. We study the effect of surface density of PEO and protein size and deduce the PEO surface density conditions for optimal protein resistance. For small proteins ( R ∼ 20 A ̊ ), D should be small (∼ 10 Å), while for large proteins ( R ∼ 60–80 A ̊ ), D should be larger (∼ 15 Å), where R is the protein radius and D is the average distance between end-attached PEO chains. These results evolve from the trade-offs between steric repulsion and the assumed weak hydrophobic interaction between the PEO layer and the protein. The longest chain length of PEO at optimum surface density appears best for protein resistance. As a number of assumptions and estimates are involved in the model, the results can be taken only as qualitative trends at this time. The trends should be helpful in the design and evaluation of surfaces resistant to protein adsorption.

Protein—surface interactions in the presence of polyethylene oxide: I. Simplified theory

The protein resistance character of polyethylene oxide (PEO) chains terminally attached to a hydrophobic solid substrate is theoretically studied. Steric repulsion, van der Waals attraction, and hydrophobic interaction free energies are considered. The results are dependent on the chain length and surface density of PEO. The protein approaches the PEO surface by diffusion and is affected by the van der Waals attraction between the PEO surface and protein through water. Further approach of the protein initiates the compression of PEO chains, which induces a steric repulsion effect; an additional van der Waals attraction becomes important between the substrate and protein through the water solvated PEO layer. The van der Waals component with the substrate decreases with increasing surface density and chain length of terminally attached PEO chains. Other synthetic polymers were also studied, indicating that the protein resistance character is related to the refractive index, with PEO having the lowest refractive index of the common water-soluble synthetic polymers. The osmotic and elastic constants of steric repulsion for terminally attached PEO were estimated as ∼0.007 and 0.02, respectively, from literature data for PEO adsorbed to mica. The steric repulsion free energy and the combined steric repulsion and hydrophobic interaction free energies were calculated as a function of surface density and chain length of PEO. The free energy calculations as a function of surface density and chain length of PEO reveal that a high surface density and long chain length of terminally attached PEO should exhibit optimal protein resistance, with the attainment of high surface density of PEO being more important than long chain length. These theoretical results should be helpful in the design and development of materials resistant to protein adsorption.

Study of PEO on LTI carbon surfaces by ellipsometry and tribometry

Proteins absorb to almost all surfaces during the first few minutes of exposure. Surfaces that show minimal protein absorption are important in many biomedical applications. Moreover, patient discomfort due to poor lubricating action between tissue and various medical devices, especially contact lenses, is a serious medical problem. An effective polymer for protein- resistant surfaces and improved lubrication properties appears to be polyethylene oxide (PEO). Here we report a study of PEO films on low temperature isotropic (LTI) carbon surfaces, including preparation using a photochemical reaction, characterization of the thickness of the PEO layer by ellipsometry and measurement of coefficient of friction with a custom built tribometer.

A Solid‐State Thin Film Cell for In Situ Transmission Electron Microscopy of Electrodeposited Silver on Gold

A cell to permit in situ imaging of the electrodeposition of silver on gold <100> from a polymer electrolyte by TEM is described. The extremely thin electrochemical cells were composed of a layer of polyethylene oxide containing a silver salt between evaporated silver and gold electrodes. Cyclic voltammetry experiments in larger cells show the deposition and stripping behavior of silver on polycrystalline gold from the polymer electrolyte as a function of temperature. Observations in preliminary in situ TEM experiments include the nucleation and growth, and coalescence of silver islands oriented epitaxially to the gold electrode.

Techniques for the Study of the Initial Stages of Electrocrystallization Using TEM

ABSTRACTA cell to permit in-situ imaging of the electrodeposition of silver on gold &lt;100&gt; from a polymer electrolyte by TEM is described. The extremely thin electrochemical cells were composed of a layer of polyethylene oxide containing a silver salt between evaporated silver and gold electrodes. Observations from preliminary in-situ TEM experiments include the nucleation and growth, and coalescence of silver islands oriented epitaxially to the gold electrode.

Forces between two adsorbed polyethylene oxide layers immersed in a good aqueous solvent

Recent measurements of the forces acting between two polystyrene layers adsorbed onto mica surfaces, immersed in a poor solvent for the polymer, show strong initial attraction as the surfaces approach, followed by ultimate repulsion1. This has been attributed to attractive osmotic interactions between the adsorbed layers, and possibly to the effect of bridging2,3. We have extended these measurements to the case of polyethylene oxide (PEO) layers adsorbed on mica, immersed in an aqueous 0.1 M KNO3 medium at pH 6 (a good solvent for PEO). Using monodispersed polymer of two molecular weights, we find that an equilibrium force–distance profile is indicated. As the surfaces bearing the adsorbed PEO approach, repulsive forces commence at a surface separation D ≃ 6±1Rs (unperturbed radius of gyration of the respective polymers) and increase monotonically on approach; on subsequent separation the forces decrease monotonically to zero. We find no evidence for attraction or adhesion between the adsorbed layers in this system.