Paul J. Pigram

Contact angle measurement and surface energetics of sized and unsized paper

Abstract Determining the surface free energy of paper from contact angle data for various liquids is not straight forward and the results can sometimes be misleading. For insufficiently sized paper, errors may arise from rapid penetration of liquid probes into the sheet, which prevents accurate measurement of contact angles. For well-sized paper, errors may arise from factors associated with the roughness of the sheet. Despite the fact that some of the sources of error are well known, contact angle methods are still favoured for determining the surface free energy of paper. In this study, we focus on some factors associated with the topographical and chemical heterogeneity of paper (particularly sized sheets) that may cause contact angle data to lead to incorrect prediction of surface energetics. Whatman filter paper with and without calendering and AKD sizing treatment was used as a model system for studying the effect of sheet surface heterogeneity. To evaluate the sheet surface roughness at an interfibre pore level, the contact area between a water drop and the sized sheet surface was studied using confocal laser-scanning microscopy (CLSM). For well-sized sheets the actual contact area between the drop and the sheet was much smaller than the apparent contact area. The chemical heterogeneity of the sized filter paper sheets was studied using XPS and IGC. Both techniques again showed that the surfaces of well sized papers still possess acid and base polarities that are capable of interacting with liquid or vapour probes with acid and base functionality. Poor liquid–sheet contact area, however, acts as a factor that limits the number of acid–base adducts formed across the liquid–sheet interface. A possible way of correcting for the effect of sheet surface roughness on the apparent liquid–sheet contact angle using the Cassie–Baxter equation was tested.

Surface studies of low-temperature oxidation of bituminous coal vitrain bands using XPS and SIMS

Abstract Surface oxidation of samples from vitrain bands (low mineral content) in two Australian bituminous coals (Greta and Whybrow seams, Sydney Basin, NSW) were studied following thermal treatment in air at temperatures ranging from 20°C to 120°C for periods of up to 370 days. X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) studies indicate clear changes in oxygen function group concentration at the surface of coal organic matter under different atmospheric oxidation conditions. The oxidation products observed include hydroxyl groups or ether linkages, carbonyl groups, and carboxyl groups. Increases in oxidation temperature accelerate the process of carboxyl group formation. It is found that the surface concentration of carboxyl groups can be used as an indicator of the extent of oxidation in low sulfur-content bituminous coals. This may be useful in coal preparation research.

Surface modification of electrospun fibres for biomedical applications: A focus on radical polymerization methods.

The development of electrospun ultrafine fibres from biodegradable and biocompatible polymers has created exciting opportunities for biomedical applications. Fibre meshes with high surface area, suitable porosity and stiffness have been produced. Despite desirable structural and topographical properties, for most synthetic and some naturally occurring materials, the nature of the fibre surface chemistry has inhibited development. Hydrophobicity, undesirable non-specific protein adsorption and bacterial attachment and growth, coupled with a lack of surface functionality in many cases and an incomplete understanding of the myriad of interactions between cells and extracellular matrix (ECM) proteins have impeded the application of these systems. Chemical and physical treatments have been applied in order to modify or control the surface properties of electrospun fibres, with some success. Chemical modification using controlled radical polymerization, referred to here as reversible-deactivation radical polymerization (RDRP), has successfully introduced advanced surface functionality in some fibre systems. Atom transfer radical polymerization (ATRP) and reversible addition fragmentation chain transfer (RAFT) are the most widely investigated techniques. This review analyses the practical applications of electrospinning for the fabrication of high quality ultrafine fibres and evaluates the techniques available for the surface modification of electrospun ultrafine fibres and includes a detailed focus on RDRP approaches.

Factors determining the stability, size distribution, and cellular accumulation of small, monodisperse chitosan nanoparticles as candidate vectors for anticancer drug delivery: application to the passive encapsulation of [(14)C]-doxorubicin.

Development of parameters for the fabrication of nanosized vectors is pivotal for its successful administration in therapeutic applications. In this study, homogeneously distributed chitosan nanoparticles (CNPs) with diameters as small as 62 nm and a polydispersity index (PDI) of 0.15 were synthesized and purified using a simple, robust method that was highly reproducible. Nanoparticles were synthesized using modified ionic gelation of the chitosan polymer with sodium tripolyphosphate. Using this method, larger aggregates were mechanically isolated from single particles in the nanoparticle population by selective efficient centrifugation. The presence of disaggregated monodisperse nanoparticles was confirmed using atomic force microscopy. Factors such as anions, pH, and concentration were found to affect the size and stability of nanoparticles directly. The smallest nanoparticle population was ∼62 nm in hydrodynamic size, with a low PDI of 0.15, indicating high particle homogeneity. CNPs were highly stable and retained their monodisperse morphology in serum-supplemented media in cell culture conditions for up to 72 hours, before slowly degrading over 6 days. Cell viability assays demonstrated that cells remained viable following a 72-hour exposure to 1 mg/mL CNPs, suggesting that the nanoparticles are well tolerated and highly suited for biomedical applications. Cellular uptake studies using fluorescein isothiocyanate-labeled CNPs showed that cancer cells readily accumulate the nanoparticles 30 minutes posttreatment and that nanoparticles persisted within cells for up to 24 hours posttreatment. As a proof of principle for use in anticancer therapeutic applications, a [14C]-radiolabeled form of the anticancer agent doxorubicin was efficiently encapsulated within the CNP, confirming the feasibility of using this system as a drug delivery vector.

X-ray photoelectron emission microscopy and time-of-flight secondary ion mass spectrometry analysis of ultrathin fluoropolymer coatings for stent applications.

Fluoropolymer plasma coatings have been investigated for application as stent coatings due to their chemical stability, conformability, and hydrophobic properties. The challenge resides in the capacity for these coatings to remain adherent, stable, and cohesive after the in vivo stent expansion, which can generate local plastic deformation of up to 25%. Plasma-coated samples have been prepared by a multistep process on 316L stainless steel substrates, and some coated samples were plastically deformed to mimic a stent expansion. Analyses were then performed by X-ray photoelectron spectroscopy (XPS), X-ray photoelectron emission microscopy (X-PEEM), and time-of-flight secondary ion mass spectrometry (TOF-SIMS) to determine the chemical and physical effects of such a deformation on both the coating and the interfacial region. While XPS analyses always showed a continuous coating with no significant effect of the deformation, TOF-SIMS and near-edge X-ray absorption fine structure (derived from X-PEEM) data indicated the presence of a certain density of porosity and pinholes in all coatings as well as sparse fissures and molecular fragmentation in the deformed ones. The smallness of the area fraction affected by the defects and the subtlety of the chemical changes could only be evidenced through the higher chemical sensitivity of these latter techniques.

Comparative analysis of Ti3SiC2 and associated compounds using x-ray diffraction and x-ray photoelectron spectroscopy

Ti3SiC2 exhibits a unique combination of ceramic and metallic properties suitable for both electrical and mechanical application. With high-temperature stability, high electrical and thermal conductivity and resistance to oxidation, Ti3SiC2 has proven promising as a contact layer for high power SiC semiconductors. However, until recently, synthesis of this material has proven difficult without appreciable quantities (<2 vol{%}) of impurity phases, namely TiC1-x and Ti5Si3Cx. As such, many properties of this compound are as yet unknown. In this paper, a comparable analysis of Ti3SiC2 and associated compounds, TiC and Ti5Si3Cx has been performed using both x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). Assessing impurity sensitivities for each technique, XRD was shown to readily identify impurities of TiC and Ti5Si3Cx within Ti3SiC2 at <2 wt{%}. Although XPS could not independently resolve these impurities, its use resulted in the detection of a complex oxide structure on Ti3SiC2. It was speculated that it was composed of mixed C-Ti-C-O and Si-Ti-C-O bond chemistries. In a comparison of TiC, Ti5Si3Cx and Ti3SiC2, differences in oxide states suggest that oxidation is chemically dissimilar for all the three compounds. However, upon etching, the binding energies of Ti3SiC2 and Ti5Si3Cx were shown to be very similar. It may be concluded that a concurrent analysis of both XRD and XPS was essential for identifying the overall surface chemistry of Ti3SiC2.

Identification of inorganic nitrogen in an Australian bituminous coal using x-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOFSIMS)

Abstract Evidence has been found indicating that inorganic nitrogen species may also occur in middle rank coal in addition to anthracite and semi-anthracite. The sample studied in this work is a bituminous coal from the Young Wallsend seam, New South Wales, Australia. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOFSIMS) surface studies indicate that pyrrolic and pyridinic functionalities are the dominant forms of nitrogen in both bright and dull bands of the coal. XPS spectra for the dull bands of the coal show an additional nitrogen component peak, the position of which (401.7 ± 0.5 eV, after charge correction) is consistent with reported values for ammonium species. In addition, the charging properties of this nitrogen component suggest that it is associated with clay mineral elements, such as Si, Al and K. TOFSIMS mass spectra from the dull bands of the coal include peaks corresponding to ammonium ions (NH 4 + ) and ammonium-containing fragment ions (e.g. C 8 H 17 NH 3 + ). TOFSIMS ion imaging of the coal surfaces shows a spatial association between the ammonium-containing ions and elements contained in clay species (i.e. Si, Al and K). The discovery of inorganic nitrogen matter in bituminous coal implies that NH 4 -bearing illite is not necessarily a parameter uniquely related to the absolute age of coalification.

A comparative study between the adsorption and covalent binding of human immunoglobulin and lysozyme on surface-modified poly(tert-butyl methacrylate).

The adsorption and covalent immobilization of human immunoglobulin (HIgG) and lysozyme (LYZ) on surface-modified poly(tert-butyl methacrylate) PtBMA films have been evaluated using x-ray photoelectron spectroscopy (XPS), ellipsometry and atomic force microscopy (AFM). Surface modification of PtBMA (UV irradiation) afforded surfaces suitable for both the physical and covalent attachment of proteins. The XPS and ellipsometry results showed good correlation in terms of variable-dense/thickness protein layer formation between physisorbed and covalently bound proteins. The amount of physisorbed HIgG ranged from 23.0 +/- 1.6 ng mm(2) on PtBMA, with corresponding film thicknesses 17.0 +/- 1.2 nm. Covalent immobilization mediated through 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC)/N-hydroxysulfosuccinimide (sulfo-NHS) coupling chemistry, afforded 5.6-8 ng mm(2) of HIgG with a corresponding thickness of 5.9 +/- 0.6 nm on PtBMA. The attachment of LYZ to modified PtBMA surface was similarly translated, where adsorption yielded up to 15 ng mm(2), while covalent immobilization afforded typically 7-8 ng mm(2). The thickness of the adsorbed LYZ protein layer was 11.0 +/- 3.2 nm (PtBMA), suggesting the greater portion of protein adsorbs on surface-modified PtBMA.

Surface studies of quinhydrone pH sensors

Abstract A polymer-modified non-glass electrode was fabricated using quinhydrone (QH) in a poly(vinyl chloride) (PVC) matrix supported by a low porosity carbon rod. The electrode has a Nernstian response with a slope of 34.1 ± 7.3 mV/pH (25°C), a linear working range of pH 3 to 10 and an average response time of 5 to 7 min. Electrode stability was maintained over a period of 36 days without the need of pre-treatment prior to use or immersion in solution when not in use. X-ray photoelectron spectroscopy (XPS) was used to probe the composition and surface characteristics of the electrode in attempt to understand the chemical basis of electrode performance. Angle resolved XPS showed that electrodes with a poor performance had an oxygen enhanced surface attributed to increased surface concentration of oxygen containing QH. This electroactive species is readily oxidised when in contact with solution. Electrodes with a high performance had a chlorine enhanced surface, indicating a PVC-rich surface. The PVC serves both as a matrix and a protective layer for the embedded QH. The comparison of electrochemical and XPS data indicates that electrode composition and performance are closely related to fabrication techniques.

The influence of hydroxyl group concentration on epoxy–aluminium bond durability

The influence of hydroxyl group (OH) concentration on the durability of adhesive bonds formed between an epoxy resin and aluminium adherend has been examined. Initially, surface analysis in combination with chemical derivatisation was employed to characterise the OH and epoxy functional groups present in FM-73, a structural epoxy adhesive. Bulk FM-73 indicated a higher degree of cure than the surface of FM-73 present at the interface of an epoxy–aluminium adhesive joint. Plasma and water treatment of the aluminium adherend was employed to alter the metal oxides surface OH concentration. Despite a several-fold difference of aluminium surface OH concentrations for the different metal pre-treatments, there was no significant variation in the adhesive joint fracture toughness in a humid environment, G Iscc. In contrast, grit-blasting the aluminium prior to bonding increased G Iscc almost 15-fold. Simple calculations indicate that the aluminium surfaces used in the bonding experiments would have a large excess of OH groups available to react with a standard epoxy resin and that the influence of surface roughness on adhesion durability is not insignificant.