R.H. Bradley

XPS and AFM surface studies of solvent-cast PS/PMMA blends

Films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) blends of two different thicknesses have been examined by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Blends with different compositions were spin-cast onto a mica substrate with chloroform as the mutual solvent. XPS measurements revealed surface enrichment of PMMA in all compositions. The thicker (66 nm) films exhibit a higher degree of PMMA surface enrichment than the thinner (17 nm) films. AFM imaging allows distinctions to be drawn between blends with differing compositions. The blend films with less than 50% PMMA bulk concentration generally exhibit pitted surfaces; the pit size varies with film thickness and bulk composition. When the PMMA bulk concentration is greater than 50%, the film surface changes to show island-like phase-separated structure. The surface segregation and morphology are explained in terms of solubilities of the two polymers in the solvent and dewetting of PMMA relative to PS. The phase domains on the film surface have also been resolved by frictional force microscopy (FFM) using hydrophilic tips bearing hydroxyl groups.

Surface characterization and ageing of ultraviolet–ozone-treated polymers using atomic force microscopy and x-ray photoelectron spectroscopy†

Ultraviolet–ozone (UVO) treatment of poly(ethyleneterephthalate) (PET) films and polystyrene (PS) dishes of up to 10 min exposure has been studied. Surface polarity, oxygen chemisorption and topographical change were analysed by contact angle measurement, x-ray photoelectron spectroscopy and atomic force microscopy. Studies of the treated surface reveal the chemistry behind the increasing oxygen content. The oxidation process is shown to proceed via different mechanisms for the two polymers. Polyethyleneterephthalate appears to undergo a Norrish-type chain depolymerization reaction, whereas PS undergoes a much more random chain scission attack. Atomic force microscopy analysis shows an increase in the surface roughness with increasing exposure to UVO for both polymers, with grains of low-molecular-weight oxidized material (LMWOM) forming at the surface. This material can be removed by washing. Surfaces that remain after washing have a higher concentration of oxygen species than the native surfaces. Analysis of aged surfaces shows that for oxidized PET a relaxation process occurs, lowering the levels of surface oxygen. This appears to occur due to the diffusion of LMWOM into the PET bulk. Relaxation of the oxidized PS surface is less thermodynamically favourable owing to the apolar nature of the PS. Copyright

Surface characterisation of ultraviolet-ozone treated PET using atomic force microscopy and X-ray photoelectron spectroscopy

The effects of ultraviolet-ozone (UVO) oxidation of polyethyleneterephthalate (PET) surfaces have been studied using atomic force microscopy and X-ray photoelectron spectroscopy. Surface oxygen increases from 26 at.% (untreated) to 37 at.% for the most oxidised surfaces produced and an increase in mean surface roughness and grain size is also observed. The larger grains appear to result from the formation of low molecular weight oxidised species by PET chain scission at the ester group and the material formed is therefore COOH/COOR rich. These species are mobile on the polymer surface and coalesce to form 200 nm grains which can be partially removed by water washing. The surface beneath is still rougher and more highly oxidised than untreated PET but the increase in stable oxygen is due to the formation of COH/COR groups.

Surface feature size of spin cast PS/PMMA blends

Thin films of polystyrene (PS)/polymethylmethacrylate (PMMA) blends have been spin cast on mica from chloroform solutions. When the concentration of PMMA in the casting solution is less than that of PS a pitted morphology is formed. The average sizes of the pits are shown to increase with both the total concentration of the casting solution and the relative concentration of PMMA. The change in pit size is explained in terms of incomplete dewetting of a PMMA solution from an underlying PS solution. For a given ratio of PMMA/PS the average pit diameters appear to increase linearly with the square of the film thickness, the gradient of which is dependent on the film composition.

Attachment of human primary osteoblast cells to modified polyethylene surfaces.

Ultra-high-molecular-weight polyethylene (UHMWPE) has a long history of use in medical devices, primarily for articulating surfaces due to its inherent low surface energy which limits tissue integration. To widen the applications of UHMWPE, the surface energy can be increased. The increase in surface energy would improve the adsorption of proteins and attachment of cells to allow tissue integration, thereby allowing UHMWPE to potentially be used for a wider range of implants. The attachment and function of human primary osteoblast-like (HOB) cells to surfaces of UHMWPE with various levels of incorporated surface oxygen have been investigated. The surface modification of the UHMWPE was produced by exposure to a UV/ozone treatment. The resulting surface chemistry was studied using X-ray photoelectron spectroscopy (XPS), and the topography and surface structure were probed by atomic force microscopy (AFM) and scanning electron microscopy (SEM), which showed an increase in surface oxygen from 11 to 26 atom % with no significant change to the surface topography. The absolute root mean square roughness of both untreated and UV/ozone-treated surfaces was within 350-450 nm, and the water contact angles decreased with increasing oxygen incorporation, i.e., showing an increase in surface hydrophilicity. Cell attachment and functionality were assessed over a 21 day period for each cell-surface combination studied; these were performed using SEM and the alamarBlue assay to study cell attachment and proliferation and energy-dispersive X-ray (EDX) analysis to confirm extracellular mineral deposits, and total protein assay to examine the intra- and extracellular protein expressed by the cells. HOB cells cultured for 21 days on the modified UHMWPE surfaces with 19 and 26 atom % oxygen incorporated showed significantly higher cell densities compared to cells cultured on tissue culture polystyrene (TCPS) from day 3 onward. This indicated that the cells attached and proliferated more readily on the UV/ozone-treated UHMWPE surfaces than on untreated UHMWPE and TCPS surfaces. Contact guidance of the cells was observed on the UHMWPE surfaces by both SEM and AFM. Scanning electron micrographs showed that the cells were confluent on the modified UHMWPE surfaces by day 10, which led to visible layering of the cells by day 21, an indicator of nodule formation. In vitro mineralization of the extracellular matrix expressed by the HOB cells on the modified UHMWPE surfaces was confirmed by SEM and EDX analysis; spherulite structures were observed near cell protrusions by day 21. EDX analysis confirmed the spherulites to contain calcium and phosphorus, the major constituents in calcium phosphate apatite, the mineral phase of bone. Overall cell attachment, functionality, and mineralization were found to be enhanced on the UV/ozone-modified UHMWPE surfaces, demonstrating the importance of optimizing the surface chemistry for primary HOB cells.

Use of Classical Adsorption Theory to Understand the Dynamic Filtration of Volatile Toxicants in Cigarette Smoke by Active Carbons

The ability of two very different active carbons, a polymer-derived carbon (with ultramicropores and supermicropores, and a large volume of “transport” pores) and a coconut shell-derived carbon (predominantly ultramicroporous), to reduce the levels of volatile toxicants in cigarette smoke has been measured and compared. The polymer-derived carbon was found to be approximately twice as effective in removing the majority of measured smoke vapour-phase toxicants compared to the coconut shell-derived carbon in three different cigarette formats and with two different smoking regimes. Single-component dynamic breakthrough experiments were conducted with benzene, acrylonitrile and 2-butanone at 298 K for beds of each carbon under dry (0% RH) and wet (60% RH) conditions. Longer breakthrough times were found with the polymer-derived carbon, and breakthrough times recorded under wet conditions were found to be up to 20% shorter than those obtained under dry conditions. Correlations between micropore volume, dynamic adsorption volume and filter bed breakthrough time have been demonstrated.

Chemical transformations resulting from pyrolysis and CO2 activation of Kevlar flocks

The chemical and structural transformations which result from the pyrolisis of Kevlar (poly[p-phenyleneterephthalamide]) fibres in argon, and from subsequent activations to differing levels of burn-off, under differing activation conditions in CO2, have been investigated using XPS, SEM and XRD. XPS indicates a marked decrease in surface oxygen after pyrolysis followed by the formation of oxygen functionalities at various levels depending upon the activation regime. Nitrogen levels appear relatively constant after pyrolysis indicating the condensation of stable aromatic nitrogen during pyrolysis. The corresponding oxygen and nitrogen chemistry obtained from chemical shift data is also discussed. SEM indicates that the lamellar structure of polyaramid is preserved after carbonization and activation which is consistent with the observations of other authors. XRD shows that the carbons produced are generally more ordered than those resulting from coal, other organic precursors and some ex-PAN fibres.

Polar and dispersion interactions at carbon surfaces: further development of the XPS-based model

Abstract Enthalpy of immersion (Δ H i ) in water has been measured for a series of ozone oxidised non-porous carbon blacks and, as in our previous studies been found to correlate directly with the total surface oxygen level [O] T measured by X-ray photoelectron spectroscopy. An equation that allows calculation of either parameter from the other is given and shown to describe behaviour for a wide range of carbon black surfaces which contain ozone-generated or native oxygen functional groups. Using this approach, the surface polarity and the relative hydrophilic character of such surfaces can be predicted. A molar enthalpy for the polar interaction between water and surface oxygen atoms of 17 kJ mol −1 is obtained by assuming a 1:1 co-ordination between water molecules and carbon surface oxygen atoms. The data lead to a predicted value of 37.5 mJ m −2 for the immersion of oxygen-free carbon black external surface into water. This equates to a value of 2.5 kJ mol −1 for the non-specific dispersion interaction between water and an oxygen-free carbon black surface when a molecular area of 10.5×10 −20 m 2 for water is assumed. The same carbon black when oxidised using nitric acid gives a different enthalpy of immersion to the ozone-treated and native oxide materials, this is attributed to differing chemistry of the two surface types, this aspect is discussed. The nitric acid treated carbons do, however, give the same value as the ozonated and native oxide carbons (37.5 mJ m −2 ) for the immersion of an oxygen-free carbon surface into water. A correlation between the point of zero charge (pH PZC ) of the carbons and Δ H i or [O] T is also presented. The results from these measurements show extremely good agreement with data from other groups who have used TPD to assess surface oxygen concentration. This gives a firm basis for confident prediction of the thermodynamic properties of carbon surfaces from single measurement techniques.

Recent Developments in the Physical Adsorption of Toxic Organic Vapours by Activated Carbons

Active, or microporous, carbons are established and effective adsorbents for toxic and other organic vapour-phase compounds. Their porosity results predominantly from slit-shaped pores of molecular dimensions (and hence widths < 2 nm) formed by spaces between graphene layers in a turbostratic structure. They are used in a wide range of separation and recovery processes where adequate performance is often routinely achieved using relatively cheap, generic active carbons (ACs) produced from precursors such as coal, nutshells and wood. However, many current, or new, applications require more refined properties, such as greater selectivity or specific surface chemistry, before the full potential of ACs can be realized. Typical examples occur in separations associated with toxic organic compounds, for example in industrial and military respiratory protection or effluent clean-up, where the removal of highly toxic or noxious vapour species, sometimes present at very low concentrations, is required from atmospheric air streams which contain near-saturation pressures of water vapour. In this instance, a hydrophobic carbon with a pore structure which is optimized to the target species is required to solve this problem, but other applications, for example to control other emissions to the environment (SOx/NOx/Hg) or in energy storage (H2/CH4) and electrochemical double-layer capacitance (EDLC) usage, may require quite different properties. Against this background, it is not surprising that there is currently great interest in active carbons with controllable properties and increased adsorption performance to meet these new challenges. Research workers are now looking beyond conventional ACs to materials with very specific properties which are tailored to meet these more exacting high-performance applications. However, in turn, achieving these goals places greater emphasis on both accurate structure/property characterization and also upon methods by which adsorption properties can be manipulated. In this review, the basic structure and properties of ACs are presented in relation to their adsorption characteristics; this is accompanied by an overview of adsorption theories which underpin quantitative equilibrium isotherm analysis (Polanyi–Dubinin–Stoeckli, Sing-αS, DFT) and allow the derivation of detailed physical and chemical information about carbon adsorbents, and comparison and understanding of the adsorption behaviour of target vapour species. Emphasis is placed on the types of interactions which occur between vapour molecules and the carbon surfaces, and the relative energy of these interactions. Consideration is given to the non-specific (London-type) forces which occur between vapour molecules and graphene basal planes, and the enhancement of these forces, between these planes and within micropores (width < 2 nm), leading to the Type I isotherms characteristic of these carbons. The size of vapour molecules relative to the (slit) width of pores is also discussed, as is the effect of the presence of wider (> 2 nm) pores. In contrast, specific interactions occurring at polar sites, and therefore dominated by the surface chemistry of the carbon structure, are also reviewed; critical in this respect are the interactions which occur between surface oxygen-containing groups — which are mainly present on the edge sites of the carbon planes — and water molecules, the competitive adsorption of which may seriously limit the adsorption of target species in many separation applications which are carried out in humid environments. Discussion of these aspects is followed by some comments on adsorption kinetics and dynamic filtration and, to emphasize the theoretical points which arise, examples are given of how manipulation of carbon characteristics can begin to be used to control and optimize adsorption for a range of more challenging applications.

Activated Carbons for the Adsorption of Vapours from Cigarette Smoke

The main requirements for the adsorption of cigarette smoke vapours using active carbons and the methods currently being used to characterise and select carbons for this application are reviewed. Emphasis is placed on the total volume of smoke, the pulsed characteristics and relatively short challenge times, the compounds in the smoke and the environment in which adsorption takes place. Using experimental data, the types of carbons most suitable for cigarette filter application are considered, as are the adsorption kinetics required to meet this challenge. The concept of carbon ageing and the effects of water within the smoke are also reviewed.