Thomas R. Gengenbach

Characterization of the Ageing of Plasma-deposited Polymer Films: Global Analysis of X-ray Photoelectron Spectroscopy Data

A protocol for global analysis of x-ray photoelectron spectra of plasma-deposited polymer films is presented. These materials are difficult to analyse because of the multitude of different chemical groups present. The combination of different primary and secondary binding energy shifts results in relatively broad, featureless photoelectron peaks. The protocol is based on fitting a series of spectra obtained by monitoring the surface composition of a plasma polymer film over extended periods of time after deposition. Information obtained from this first round of curve-fitting is used in the form of additional constraints for a second round of fitting. This leads to a self-consistent procedure which is akin to a global approach to curve-fitting. To illustrate application of this method, results of a study of ann-heptylamine plasma polymer are described. The spectral changes on ageing provided clear evidence for radical-initiated oxidation reactions. These reactions generated additional radicals, which sustained the oxidation process for many months and not only led to substantial incorporation of oxygen into the material (forming a variety of carbon–oxygen functionalities) but also to conversion of most of the existing carbon–nitrogen functionalities to an oxidized form.

Evolution of the surface composition and topography of perfluorinated polymers following ammonia-plasma treatment

Treatment of fluorinated ethylene propylene (FEP) and polytetrafluoroethylene (PTFE) in ammonia plasmas produced surfaces with very high wettability by water, but on storage in air at ambient temperature, the air/water contact angles increased markedly. The evolution of the surface composition and topography was studied by angle-dependent X-ray photoelectron spectroscopy (XPS), derivatization of amine groups with fluorescein isothiocyanate, scanning tunelling microscopy (STM), and atomic force microscopy (AFM). XPS demonstrated a continuous increase in the oxygen content over periods of weeks; this was assigned to oxidation of trapped radicals and subsequent secondary reactions. In addition, the fluorine content also changed markedly on storage; the XPS fluorine signal suggested that there was a substantial amount of fluoride in the freshly treated surfaces, and this component disappeared rapidly on storage. STM and AFM showed no changes in topography with aging but suggested surface hardening on plasma t...

Aging of 1,3‐diaminopropane plasma‐deposited polymer films: Mechanisms and reaction pathways

In the course of plasma deposition of organic–polymeric thin films, radicals are incorporated into the growing film. These radicals initiate spontaneous oxidation reactions that continue over many weeks when the plasma polymers are stored in air. These reactions and their products have been previously studied in detail for spectroscopically simple, hydrocarbon-based plasma polymers. In this investigation, the aging of 1,3-diaminopropane (DAP) plasma polymer samples was monitored by XPS and FTIR in order to study how the oxidative reaction pathways might differ in a plasma-deposited material that is initially rich in amine groups. The freshly deposited DAP plasma polymer consisted of a random hydrocarbon network with a considerable amount of unsaturation and a high concentration of nitrogen-containing functional groups, mainly primary/secondary amines and imines. These groups strongly influenced the aging reactions: in contrast to hydrocarbon-based material where hydrogen abstraction and reaction of carbon-centered radicals with in-diffusing oxygen result in a wide range of oxidative products, both XPS and FTIR identified a rather narrow range of products (mainly amides and similar groups) in DAP plasma polymers even after extensive aging for more than 2 years. Reaction routes based on oxidation and/or hydrolysis of nitrogen functional groups, and involving primary as well as secondary reactions, are proposed to account for the spectroscopic data. The structure of the aged DAP plasma polymer appeared to be stable, and did not undergo more extensive oxidation, in contrast to hydrocarbon plasma polymers. In particular, carboxylic acid groups and carbamates were not detected.

Glucose-assisted synthesis of the hierarchical TiO2 nanowire@MoS2 nanosheet nanocomposite and its synergistic lithium storage performance

A hierarchical nanocomposite of TiO2 nanowires decorated with molybdenum disulfide nanosheets (TiO2@MoS2) was synthesized by a facile and low-cost glucose-assisted hydrothermal approach. In this hierarchical nanocomposite, TiO2 nanowires served as an effective backbone for the nucleation and growth of few layered MoS2 nanosheets. Both glucose and the roughness of anatase-TiO2 (B) nanowires played important roles in the formation of the uniform TiO2 nanowire@MoS2 nanosheet (≤6 layers) nanocomposite. A synergistic effect was demonstrated on the nanocomposite of the TiO2 nanowire@MoS2 nanosheet. The one-dimensional robust TiO2 nanowire backbone provided a shortened and efficient pathway for electron and lithium ion transport and minimized the strain of the volume changes, while ultrathin MoS2 nanosheets offered high electrode/electrolyte interfacial contact areas, promoted rapid charge transfer and contributed to a high specific capacity. The favourable synergistic effect led to enhanced specific capacity, good cycling stability and superior rate capability of the nanocomposite, compared with either individual component. Such a TiO2 nanowire@MoS2 nanosheet nanocomposite is a promising anode material for high performance lithium ion batteries.

Correlation of the Nitrogen 1s and Oxygen 1s XPS Binding Energies with Compositional Changes During Oxidation of Ethylene Diamine Plasma Polymers

In the course of plasma deposition of organic-polymeric thin films, radicals are incorporated into the growing film. These radicals initiate spontaneous oxidation reactions that continue over many weeks when the plasma polymers are stored in air. The resultant changes to the composition with time were monitored by XPS for ethylene diamine plasma polymer samples in order to improve understanding of the products of the oxidative reactions. The broadness of all the peaks, and the multitude of functional groups expected to be present, precluded the obtainment of detailed compositional information by curve fitting for components, but shifts in the binding energies (BEs) of the N 1s and O 1s peaks with time provided useful evidence of compositional changes. As the oxygen content of the plasma polymer increased upon oxidation, the binding energy of the N 1s photoelectrons increased from 399.1 to 399.8 eV. Concurrently, the amount of nitrogen in relation to carbon (N/C) decreased from 0.42 to 0.34. As the nitrogen content of the plasma polymer decreased, the binding energy of the O 1s photoelectrons increased from 531.1 to 531.8 eV. The BE values and their shifts with time/compositional changes suggested that the oxidation process predominantly caused oxidation of the carbon atoms that had amine groups attached to them, leading to the formation of amide groups and perhaps also some imides.

Post-deposition ageing reactions differ markedly between plasma polymers deposited from siloxane and silazane monomers

Abstract Plasma polymer coatings deposited from hexamethyldisiloxane (HMDSO) and hexamethyldisilazane (HMDSA) were monitored by XPS, FTIR and contact angle (CA) measurements as they aged in air after fabrication. Of particular interest was the influence of the monomer structure on the long term properties of the plasma deposited materials: in conventionally synthesized organosilicon materials, the siloxane unit provides very good long-term stability whereas the silazane structure is prone to hydrolytic attack. During plasma deposition of both coatings, abstraction of methyl groups was the major activation mechanism and the monomer structure was retained to a substantial extent. In the case of plasma polymerised (pp) HMDSA, however, other reactions such as Si–N bond cleavage resulted in considerably more structural diversity. During storage in air, the ppHMDSO film underwent minor chemical changes such as incorporation of additional siloxane cross-links and a small extent of loss of methyl groups. The chemical structure of both the freshly deposited material and the aged coating were unusually homogeneous, compared with the broad range of chemical structures typically found in most other plasma polymers. The structure of the ppHMDSA coating, in contrast, changed dramatically on ageing: almost all silazane moieties were lost after one year and substantial amounts of oxygen incorporated, mainly in the form of siloxane links. In spite of the initial chemical differences, the two materials became more similar over time, with the final structures of the aged materials based on a cross-linked siloxane backbone. The wettability data reflected the structural differences between the two materials. However, correlations between structures and surface properties were not predictable. The overall wettability of these surfaces was determined by a complex balance of several factors such as chemical structure, topography and mobility.

Interactions of lens care with silicone hydrogel lenses and effect on comfort.

Purpose. The purpose of this study was to investigate the effect of lens care products on short-term subjective and physiological performance silicone hydrogel lenses. Methods. Ten subjects wore either lotrafilcon B or galyfilcon A silicone hydrogel contact lenses soaked in a lens care product containing either Polyquad/Aldox or PHMB or control lenses inserted directly from the pack. Subjects wore the lenses for 6 h. Ocular comfort (graded on a 1 to 10 scale) and ocular physiology were assessed. Unworn but soaked lenses were analyzed for metrological changes, release of excipients into phosphate buffered saline, and changes to their surface chemical composition. Results. None of the lens metrology measures or clinically observed conjunctival or limbal redness changed. Corneal staining was significantly (p < 0.008) raised, albeit to low levels, after 6 h wear for either lens type when soaked in the PHMB solution compared with the control lens (lotrafilcon B 0.4 to 0.9 ± 0.7 to 0.4 vs. 0.1 to 0.4 ± 0.3 to 0.5; galyfilcon A 0.2 to 0.3 ± 0.2 to 0.4 vs. 0.0 ± 0.0). For lotrafilcon B lenses, there were decreases in comfort (p = 0.002), increases in burning/stinging (p = 0.002) after 1 h of wear, and increases in lens awareness on lens insertion (p = 0.0001) when soaked in PHMB. However, lotrafilcon B lenses soaked in Polyquad/Aldox showed increases in burning/stinging after 1 and 6 h (p < 0.008) of lens wear. For galyfilcon A lenses, most significant (p ≤ 0.002) changes to symptomatology occurred after soaking in Polyquad/Aldox solution. More PHMB was released from lotrafilcon B lenses, and more MPDS material was released from galyfilcon A lenses. The surface of galyfilcon A lenses changed but irrespective of lens solution type, whereas the changes to the lens surface was dependent on solution type for lotrafilcon B lenses. Conclusions. Lens care products can change corneal staining and comfort responses during wear. These changes may be associated with release of material soaked into lenses or changes to the lens surface composition.

Clinical observations of biofouling on PEO coated silicone hydrogel contact lenses

Silicone hydrogel contact lenses, which have been a major advance in the field of vision correction, require surface modification or coatings for comfort and biocompatibility. While current coatings show adequate clinical performance, advanced coatings may improve the biocompatibility of contact lenses further by reducing biofouling and related adverse clinical events. Here, we have produced coatings on Lotrafilcon A contact lenses by deposition of a thin film of allylamine plasma polymer (ALAPP) as a reactive interlayer for the high density grafting of poly(ethylene oxide) dialdehyde (PEO(ALD)(2)), which had previously shown complete resistance to protein adsorption in vitro. The performance of these contact lenses was evaluated in a controlled clinical study over 6h using Focus Night and Day (also known as Air Optix Night & Day) contact lenses as control lenses. Surface modified lenses were characterised by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) before and after wear. Clinical data showed a high level of biocompatibility of the PEO coated lenses equivalent to control lenses. Surface analysis of worn contact lenses demonstrated that the high density PEO coating is effective in reducing biofouling in vivo compared to control lenses, however small amounts of protein deposits were still detected on all worn contact lenses. This study highlights that elimination of biofouling in vivo can be much more demanding than in vitro and discusses issues that are important for the analysis of worn contact lenses as well as the design of improved contact lenses.

Deposition conditions influence the postdeposition oxidation of methyl methacrylate plasma polymer films

Plasma polymer films were deposited from methyl methacrylate (MMA) vapor under various plasma conditions and XPS and FTIR used to study the changes to the compositions of the films as they were stored in air for longer than 1 year. The plasma power input per monomer mass unit (W/FM) markedly affected the composition of the freshly deposited MMA plasma polymers. A low value of W/FM led to a high degree of retention of the original monomer structure, whereas a high value of W/FM resulted in substantial monomer fragmentation and the formation of a partially unsaturated material considerably different to conventional PMMA. As the MMA plasma coatings were stored in ambient air after fabrication, all showed spontaneous oxidative changes to their composition, but the extents and reaction products differed substantially. Deposition at low W/FM led to moderate oxidative changes, whereas high power led to a pronounced increase in the oxygen content over time and resulted in a wide range of carbon–oxygen functionalities in the aged material. As the initial compositions/plasma deposition conditions thus influenced the oxidative postdeposition reactions, MMA plasma polymers deposited under different conditions not only varied in their initial composition but then became even more diverse as they aged.

Hydrophobic radiofrequency plasma-deposited polymer films : dielectric properties and surface forces'

Abstract Advancing and receding contact angles for various liquids placed on the surface of thin n -hexane plasma polymer films have confirmed that the surface is hydrophobic; comparable to polyethylene. A waveguide technique has been employed to determine the refractive indices of the plasma polymer films in the visible and near infrared regions of the dielectric spectrum. Along with ESCA data that provide polymer group functionality, this information has been used to estimate the n -hexane plasma polymer density and to construct a simple dielectric permeability function (ϵ(iξ), the dielectric constant at imaginary frequency iξ) for the polymer. The dielectric permeability function and Lifshitz theory have been used to calculate the Hamaker function (including non-retarded Hamaker constants) which should characterise the van der Waals interaction in systems that contain n -hexane plasma polymers. The atomic force microscope (AFM) has been employed to measure the force of interaction between n -hexane plasma polymer films deposited on muscovite mica flat plates and either uncoated silicon nitride AFM tips or n -hexane plasma polymer coated glass spheres. The force versus separation curves have been compared with DLVO theory. The homointeraction between n -hexane plasma polymer coatings in water initially displays a long-range attraction additional to van der Waals interaction. There is evidence that after prolonged immersion in water the thin n -hexane plasma polymer films swell.