Alison J. Beck

Plasma polymerisation for molecular engineering of carbon-fibre surfaces for optimised composites

Abstract This paper explores the relationship between fibre surface chemistry and interfacial bond formation in carbon-fibre/epoxy composites. Plasma copolymerisation of acrylic-acid/hexane, allyl-alcohol/hexane and allylamine/octadiene gas mixtures is used to obtain molecularly thin, conformai coatings on Type A carbon-fibre surfaces. Since the microporosity and chemical functionality of the untreated fibre surface can be concealed, any functionality incorporated into the film can be considered to provide the principal adhesion mechanism. The single-filament fragmentation test has been employed to estimate the adhesion of these modified fibres to an epoxy resin. Coatings of a hydrocarbon nature inhibit chemical interaction between the fibre and matrix. Improvements in the level of adhesion may be attributed to the introduction of oxygen- and nitrogen-containing functionalities which have known reactivity to epoxy groups. Thus, carboxylic acid and amine groups are shown to be more effective than hydroxyl groups.

Plasma copolymerization as a route to the fabrication of new surfaces with controlled amounts of specific chemical functionality

Abstract The plasma copolymerization of allyl amine with 1,7-octadiene and of acrylic acid with hexane has been investigated. Plasma copolymerization is shown to be a promising route to the fabrication of new surfaces with controlled concentrations of specific surface functionalities; in this case, amine and carboxylic acid.

Radiofrequency-induced plasma polymerisation of propenoic acid and propanoic acid

Inductively coupled, radiofrequency-induced plasmas of propenoic (‘acrylic’) acid and propanoic acid, operated at a low electrical power (1–10 W), have been investigated using a combination of mass spectrometry (MS) and deposition-rate measurements. Thin films of plasma polymers of both compounds were deposited onto silicon substrates and analysed by X-ray photoelectron spectroscopy (XPS). The positive-ion MS data obtained from both compounds indicate the presence of species of the form (M+ H)+, (2M+ H)+ and (3M+ H)+, where M represents the molecular weight of the starting material. No neutral oligomeric species were detected. XPS analysis reveals an inverse relationship between the electrical power supplied to the plasma and the degree of retention of the carboxylic acid functionality in the solid product. Comparison of the MS and XPS results suggests that the above cationic species are responsible for the carboxylic-acid functionalisation of the plasma-polymer product, whereas fragmentation processes lead to the introduction of other functional groups such as alcohol and ketone. The thin film which featured the highest degree of retention of carboxylic acid (65%) was obtained from a plasma of propanoic acid operated at 1 W, and was deposited at the lowest rate (0.90 ng s–1).

Interface molecular engineering of carbon-fiber composites

Abstract Adhesion of carbon fibers to epoxy and related resins is a complex subject requiring a fundamental understanding of: (a) the nature of the adhesive characteristics of the fiber; (b) the mechanisms involved; and (c) an appropriate quantification of adhesion. This paper discusses the application of plasma co-polymerization of acrylic acid/hexane, allyl alcohol/hexane and allylamine/octadiene, for the controlled functionalization of untreated type A carbon fiber through the deposition of thin, conformal coatings. The single-fiber fragmentation test has been used to estimate the influence of these pretreatments. The conventional data reduction technique and cumulative stress transfer function (CSTF) have been used as a measure of adhesion. The surface free energy of the coatings of allyl alcohol/octadiene deposited on glass plates, has also been estimated from the contact angle of polar and non-polar liquids. Hydrocarbon coatings resulted in a lower degree of adhesion than the parent untreated fiber. Increasing the concentrations of acrylic acid and allylamine promoted adhesion. This was attributed to the formation of covalent bonds between functional groups on the fibers and epoxy groups. The incorporation of allyl alcohol was less effective, because only dipole–dipole interactions were available. Contact angle measurements indicated that the polar component of surface free energy increased with the hydroxyl group concentration, despite no quantifiable increase in adhesion. The CSTF methodology has been shown to provide a precise estimate of adhesion.

Effects of “processing parameters” in plasma deposition: Acrylic acid revisited

In this article we explore the relationship between the plasma-phase chemistry and deposit in the plasma deposition of acrylic acid. This is achieved through systematic variation of some of the plasma deposition variables, termed “processing parameters.” The surface chemical composition of deposits has been investigated by means of x-ray photoelectron spectroscopy. Changes in the surface chemical composition with the ratio of plasma power-to-monomer flow rate, P/φ, substrate position (in the reactor), nature of the substrate, and washing and aging (of the deposit) are examined. Based on these, we provide a qualitative description of the deposits’ molecular structure, and how this changes with P/φ. The plasma-phase chemistry has been probed by means of mass spectrometry and attention given to the effect of P/φ on the neutral and ionic components of the plasma. The major plasma-phase reactions are elucidated, and a knowledge of these help to explain how the molecular structure of plasma deposits are affecte...

Simplifying the Delivery of Melanocytes and Keratinocytes for the Treatment of Vitiligo Using a Chemically Defined Carrier Dressing

Obtaining pigmentary function in autologous skin grafts is a current challenge for burn surgeons as is developing reliable robust grafting strategies for patients with vitiligo and piebaldism. In this paper, we present the development of a simple methodology for delivering cultured keratinocytes and melanocytes to the patient that is of low risk for the patient but also user friendly for the surgeon. In this study, we examined the ability of keratinocytes and melanocytes to transfer from potential cell carriers under different media conditions to an in vitro human wound bed model. The number of melanocytes transferred, their location within the neoepidermis, and their ability to pigment were evaluated as preclinical end points. Two inert substrates (polyvinyl chloride and silicone sheets) and three candidate plasma-polymerized coatings with controlled surface chemistry deposited on these substrates were explored. Two media for expansion of cells, Greens, currently used clinically (but which contains fetal calf serum), and a serum-free alternative, M2 (melanocyte medium), were explored. Reproducible transfer of physiologically relevant numbers of melanocytes capable of pigmentation from the coculture of melanocytes and keratinocytes was obtained using either Greens medium or M2 medium, and a silicone carrier pretreated with 20% carboxylic acid deposited by plasma polymerization.

The role of ions in the continuous-wave plasma polymerisation of acrylic acid

Ion flux, mass spectral and mass deposition rate measurements have been made in radiofrequency-induced continuous-wave plasmas of acrylic acid. At 1 W input power, an ion flux of 0.05±0.1×1018 ions m-2 s-1 was measured for acrylic acid. At this power, ions corresponding to (2M+H)+ and (3M+H)+ were prominent in the positive-ion mass spectrum. When this spectrum was corrected for the transmission function of the quadrupole mass spectrometer (conservatively taken as intensity∝m-1), it was evident that the cationic portion of plasma contained many ions of high m/z, as opposed to small fragments of acrylic acid. The m/z of the ‘average’ ion was calculated as 115. The mass of ions arriving at a solid surface in the centre of the plasma was then calculated by multiplying the flux by the average mass to give 9.6 µg m-2 s-1. This value represents a significant fraction of the total mass deposited, determined by means of a quartz crystal mass balance (45.5 µg m-2 s-1). Repeating the calculation for a 5 W plasma yields an ion mass flux of 39.6 µg m-2 s-1 (measured mass deposition of 57.3 µg m-2 s-1). At 15 W, the calculated mass deposited (based on ion flux) exceeds that measured by the quartz mass balance. The ‘average’ ion mass decreased as plasma input power increased.Based on these data, and XPS measurements of the solid-phase deposit we make a first attempt at describing semi-quantitatively the possible role of ions in deposit formation.

Development of a plasma-polymerized surface suitable for the transplantation of keratinocyte-melanocyte cocultures for patients with vitiligo.

The purpose of this study was to develop a convenient methodology for the coculture of autologous melanocytes and keratinocytes for grafting of patients with vitiligo. While grafting of pure melanocytes may achieve repigmentation, the inclusion of keratinocytes ensures rapid reepithelialization. Previously we have used confluent sheets of keratinocytes (with melanocytes present) to transfer cells. However, we found that as the keratinocyte density increased, melanocyte number and function were downregulated. Accordingly in this study we explored combinations of three culture surfaces and three media, seeking to achieve subconfluent culture of primary keratinocytes with a reasonable density of melanocytes, using cells immediately after isolation from skin. For this in vitro study, the surfaces studied were uncoated glass coverslips, and glass coverslips coated with collagen I or a nitrogen-containing plasma polymer. The results show that both the substrate surface and the medium composition influence the proliferation and survival of melanocytes. Keratinocytes and melanocytes could be successfully cocultured on a chemically defined plasma polymer substrate using a serum-free medium.

Mass spectrometric study of the radiofrequency-induced plasma polymerisation of styrene and propenoic acid

The specificity of plasma gas-phase reactions, in low-power plasmas, has been investigated. Quadrupole mass spectrometry has been used to detect the gas-phase species in the inductively coupled, radiofrequency-induced 1–10 W plasmas of propenoic acid and styrene. The mass spectra have been compared with those for plasmas of the individual compounds. Both ionic and neutral species, present in the plasma, can be sampled through a 50 µm aperture which provides an interface between the plasma reactor (10-2 mbar) and the spectrometer (10-7 mbar). In styrene plasmas, oligomeric species in the form of radical cations, nM+, were observed in the positive ion mass spectrum. Oligomers were also observed in the positive ion spectrum of the propenoic acid plasmas, but in this case they were in a protonated form (nM+H)+. Oligomers were not observed as neutral species in the plasmas of either compound. The mass spectra of plasmas containing the vapour of both monomers showed no significant new peaks. It is argued that the two monomers do not react with each other in the plasma to produce stable new species. This implies a hitherto unexpected specificity in the plasma gas-phase reactions.

Chemistry and aging of organosiloxane and fluorocarbon films grown from hyperthermal polyatomic ions

Polyatomic ions can be used to deposit thin films, modify the phases of interfaces, dope trace elements into interface regions, impact specific chemical functionalities to a surface, and create micron- and nanometer-scale interface structures. This article demonstrates the broad flexibility over the modified surface properties allowed by variation of the incident ion chemical structure and kinetic energy. Organosiloxane (OS) films are deposited here on Al from mass-selected 15–100 eV Si2O(CH3)5+ ions. Monochromatic x-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry are used to determine the OS film chemistry. The OS films vary from essentially organic (lightly cross-linked network) at 15 eV deposition to a more inorganic (highly cross-linked “silica like”) at 100 eV. XPS is also used to examine the aging of fluorocarbon films deposited on polystyrene by 25–100 eV CF3+ and C3F5+ ions. These films undergo only minor changes during aging, forming only a few percent oxyg...