Electrical characterization of the rectifying contact between aluminium and electrodeposited poly(3-methylthiophene)

A detailed investigation both of the DC and of the AC electrical properties of the Schottky barrier formed between aluminium and electrodeposited poly(3-methylthiophene) is reported. The devices show rectification ratios up to 2 x 10(4) which can be increased further after post-metal annealing. The reverse characteristics of the devices follow predictions based on the image-force lowering of the Schottky barrier, from which the doping density can be estimated, As the forward voltage increases, the device current is limited by the bulk resistance of the polymer with some evidence for injection limitation at the gold counter-electrode at high bias. In the bulk-limited regime, the device current is thermally activated near room temperature with an activation energy in the range 0.2-0.3 eV. Below about 150 K the device current is almost independent of temperature. Capacitance-voltage plots obtained at frequencies well below the device relaxation frequency indicate the presence of two distinct acceptor states, A set of shallow acceptor states are active in forward bias and are believed to determine the bulk conductivity of the polymer. A set of deeper accepters are active only for very small forward voltages and for all reverse voltages, namely when band banding causes the Fermi energy to cross these states. The density of these deeper states is approximately an order of magnitude greater than that of the shallow states. Evidence is presented also for the influence of fabrication conditions on the formation of an insulating interfacial layer at the rectifying interface. The presence of such a layer leads to inversion at the polymer surface and a modification of the I-V characteristics.

Controlling cell-material interactions with polymer nanocomposites by use of surface modifying additives

Polymer nanocomposites (NC) are fabricated by incorporating well dispersed nanoscale particles within a polymer matrix. This study focuses on elastomeric polyurethane (PU) based nanocomposites, containing organically modified silicates (OMS), as bioactive materials. Nanocomposites incorporating chlorhexidine diacetate as an organic modifier (OM) were demonstrated to be antibacterial with a dose dependence related to both the silicate loading and the loading of OM. When the non-antibacterial OM dodecylamine was used, both cell and platelet adhesion were decreased on the nanocomposite surface. These results suggest that OM is released from the polymer and can impact on cell behaviour at the interface. Nanocomposites have potential use as bioactive materials in a range of biomedical applications.

Studies on the surface properties of biodegradable polymer carriers in respiratory delivery of drug from Dry Powder Inhaler formulations

Dry Powder Inhaler (DPI) technology has a significant impact in the treatment of various respiratory disorders. DPI formulations consist of a micronized drug (<5ìm) blended with an inert coarse carrier, for which lactose is widely used to date. DPIs are one of the inhalation devices which are used to target the delivery of drugs to the lungs. Drug delivery via DPI formulations is influenced by the physico-chemical characteristics of lactose particles such as size, shape, surface roughness and adhesional forces. Commercially available DPI formulations, which utilise lactose as the carrier, are not efficient in delivering drug to the lungs. The reasons for this are the surface morphology, adhesional properties and surface roughness of lactose. Despite several attempts to modify lactose, the maximum efficient drug delivery to the lungs remains limited; hence, exploring suitable alternative carriers for DPIs is of paramount importance. Therefore, the objective of the project was to study the performance of spherical polymer microparticles as drug carriers and the factors controlling their performance. This study aimed to use biodegradable polymer microspheres as alternative carriers to lactose in DPIs for achieving efficient drug delivery into the lungs. This project focused on fabricating biodegradable polymer microparticles with reproducible surface morphology and particle shape. The surface characteristics of polymeric carriers and the adhesional forces between the drug and carrier particles were investigated in order to gain a better understanding of their influence on drug dispersion. For this purpose, two biodegradable polymers- polycaprolactone (PCL) and poly (DL-lactide-co-glycolide) (PLGA) were used as the carriers to deliver the anti-asthmatic drug - Salbutamol Sulphate (SS). The first study conducted for this dissertation was the aerosolization of SS from mixtures of SS and PCL or PLGA microparticles. The microparticles were fabricated using an emulsion technique and were characterized by laser diffraction for particle size analysis, Scanning Electron Microscopy (SEM) for surface morphology and X-ray Photoelectron Spectroscopy (XPS) to obtain surface elemental composition. The dispersion of the drug from the DPI formulations was determined by using a Twin Stage Impinger (TSI). The Fine particle Fraction (FPF) of SS from powder mixtures was analyzed by High Performance Liquid Chromatography (HPLC). It was found that the drug did not detach from the surface of PCL microspheres. To overcome this, the microspheres were coated with anti-adherent agents such as magnesium stearate and leucine to improve the dispersion of the drug from the carrier surfaces. It was found that coating the PCL microspheres helped in significantly improving the FPF of SS from the PCL surface. These results were in contrast to the PLGA microspheres which readily allowed detachment of the SS from their surface. However, coating PLGA microspheres with antiadherent agents did not further improve the detachment of the drug from the surface. Thus, the first part of the study demonstrated that the surface-coated PCL microspheres and PLGA microspheres can be potential alternatives to lactose as carriers in DPI formulations; however, there was no significant improvement in the FPF of the drug. The second part of the research studied the influence of the size of the microspheres on the FPF of the drug. For this purpose, four different sizes (25 ìm, 48 ìm, 100 ìm and 150 ìm) of the PCL and PLGA microspheres were fabricated and characterized. The dispersion of the drug from microspheres of different sizes was determined. It was found that as the size of the carrier increased there was a significant increase in the FPF of SS. This study suggested that the size of the carrier plays an important role in the dispersion of the drug from the carrier surface. Subsequent experiments in the third part of the dissertation studied the surface properties of the polymeric carrier. The adhesion forces existing between the drug particle and the polymer surfaces, and the surface roughness of the carriers were quantified using Atomic Force Microscopy (AFM). A direct correlation between adhesion forces and dispersion of the drug from the carrier surface was observed suggesting that adhesion forces play an important role in determining the detachment potential of the drug from the carrier surface. However, no direct relationship between the surface roughness of the PCL or PLGA carrier and the FPF of the drug was observed. In conclusion, the body of work presented in this dissertation demonstrated the potential of coated PCL microspheres and PLGA microspheres to be used in DPI formulations as an alternative carrier to sugar based carriers. The study also emphasized the role of the size of the carrier particles and the forces of interaction prevailing between the drug and the carrier particle surface on the aerosolization performances of the drug.

Multi-wall carbon nanotube coating of fluorine-doped tin oxide as an electrode surface modifier for polymer solar cells

A controlled layer of multi-wall carbon nanotubes (MWCNT) was grown directly on top of fluorine-doped tin oxide (FTO) glass electrodes as a surface modifier for improving the performance of polymer solar cells. By using low-temperature chemical vapor deposition with short synthesis times, very short MWCNTs were grown, these uniformly decorating the FTO surface. The chemical vapor deposition parameters were carefully refined to balance the tube size and density, while minimizing the decrease in conductivity and light harvesting of the electrode. As created FTO/CNT electrodes were applied to bulk-heterojunction polymer solar cells, both in direct and inverted architecture. Thanks to the inclusion of MWCNT and the consequent nano-structuring of the electrode surface, we observe an increase in external quantum efficiency in the wavelength range from 550 to 650 nm. Overall, polymer solar cells realized with these FTO/CNT electrodes attain power conversion efficiency higher than 2%, outclassing reference cells based on standard FTO electrodes.

Atomistic simulation of surface functionalization on the interfacial properties of graphene-polymer nanocomposites

Graphene has been increasingly used as nano sized fillers to create a broad range of nanocomposites with exceptional properties. The interfaces between fillers and matrix play a critical role in dictating the overall performance of a composite. However, the load transfer mechanism along graphene-polymer interface has not been well understood. In this study, we conducted molecular dynamics simulations to investigate the influence of surface functionalization and layer length on the interfacial load transfer in graphene polymer nanocomposites. The simulation results show that oxygen-functionalized graphene leads to larger interfacial shear force than hydrogen-functionalized and pristine ones during pull-out process. The increase of oxygen coverage and layer length enhances interfacial shear force. Further increase of oxygen coverage to about 7% leads to a saturated interfacial shear force. A model was also established to demonstrate that the mechanism of interfacial load transfer consists of two contributing parts, including the formation of new surface and relative sliding along the interface. These results are believed to be useful in development of new graphene-based nanocomposites with better interfacial properties.

‘Click’ functionalised polymer resins : a new approach to the synthesis of surface attached bipyridinium and naphthalene diimide [2]rotaxanes

Herein we describe the design and synthesis of a series of solid-tethered [2]rotaxanes utilising crown ether-naphthalene diimide or crown ether- bipyridinium host guest interactions. TentaGel polystyrene resins were initially modified in a two-stage procedure to azide functionalised beads before the target supramolecular architectures were attached using a copper catalysed “click” procedure. The final assembly was examined using IR spectroscopy and gel-phase 1H High Resolution Magic Angle Spinning (HR MAS) NMR spectroscopy. The HR MAS technique enabled a direct comparison between the solid-tethered architectures and the synthesis and characterisation of analogous solution-based [2]rotaxanes to be made.

Hybrid polymer/metal nanoparticles with surface enhanced raman tracking for potential use in nanomedicine

This project was a preliminary step towards the development of novel methods for early stage cancer diagnosis and treatment. Diagnostic imaging agents with high Raman signal enhancement were developed based on tailored assemblies of gold nanoparticles, which demonstrated potential for non-invasive detection from deep under the skin surface. Specifically designed polymers were employed to assemble gold nanoparticles into controlled morphologies including dimers, nanochains, nanoplates, globular and core-satellite nanostructures. Our findings suggest that the Raman enhancement is strongly dependent on assembly morphology and can be tuned to adapt to the requirements of the diagnostic agent.

Temperature dependent free energy surface of polymer folding from equilibrium and quench studies

Langevin dynamics simulation studies have been employed to calculate the temperature dependent free energy surface and folding characteristics of a 500 monomer long linear alkane (polyethylene) chain with a realistic interaction potential. Both equilibrium and temperature quench simulation studies have been carried out. Using the shape anisotropy parameter (S) of the folded molecule as the order parameter, we find a weakly first order phase transition between the high-temperature molten globule and low-temperature rodlike crystalline states separated by a small barrier of the order of k(B)T. Near the melting temperature (580 K), we observe an intriguing intermittent fluctuation with pronounced ``1/f noise characteristics'' between these two states with large difference in shape and structure. We have also studied the possibilities of different pathways of folding to states much below the melting point. At 300 K starting from the all-trans linear configuration, the chain folds stepwise into a very regular fourfold crystallite with very high shape anisotropy. Whereas, when quenched from a high temperature (900 K) random coil regime, we identify a two step transition from the random coiled state to a molten globulelike state and, further, to a anisotropic rodlike state. The trajectory reveals an interesting coupling between the two order parameters, namely, radius of gyration (R-g) and the shape anisotropy parameter (S). The rodlike final state of the quench trajectory is characterized by lower shape anisotropy parameter and significantly larger number of gauche defects as compared to the final state obtained through equilibrium simulation starting from all-trans linear chain. The quench study shows indication of a nucleationlike pathway from the molten globule to the rodlike state involving an underlying rugged energy landscape. (C) 2010 American Institute of Physics. doi:10.1063/1.3509398]

Aromatic, microporous polymer networks with high surface area generated in Friedel-Crafts-type polycondensations

A series of novel, microporous polymer networks (MPNs) have been generated in a simple, acid catalysed Friedel-Crafts-type self-condensation of A(2)B(2)- and A(2)B(4)-type fluorenone monomers. Two A2B4-type monomers with 2,7-bis(N, N-diphenylamino) A or 2,7-bis [4-(N, N-diphenylamino) phenyl] D substitution of the fluorenone cores lead to MPNs with high S(BET) surface areas of up to 1400 m(2) g(-1). Two MPNs made of binary monomer mixtures showed the highest Brunauer-Emmett-Teller (BET) surface areas S(BET) of our series (SBET of up to 1800 m(2) g(-1)) after washing the powdery samples with supercritical carbon dioxide. Total pore volumes of up to 1.6 cm(3) g(-1) have been detected. It is observed that the substitution pattern of the monomers is strongly influencing the resulting physicochemical properties of the microporous polymer networks (MPNs).

Friction and transfer layer formation in polymer-steel tribo-system: Role of surface texture and roughness parameters

In the present investigation, various kinds of surface textures were attained on the steel plates. Roughness of the textures was varied using various grinding or polishing methods. The surface textures were characterized in terms of roughness parameters using an optical profilometer. Then experiments were conducted using an inclined pin-on-plate sliding apparatus to identify the role of surface texture and its roughness parameters on coefficient of friction and transfer layer formation. In the experiments, a soft polymer (polypropylene) was used for the pin and hardened steel was used for the plate. Experiments were conducted at a sliding velocity of 2 minis in ambient conditions under both dry and lubricated conditions. The normal load was varied from 1 to 120 N during the tests. The morphologies of the worn surfaces of the pins and the formation of a transfer layer on the steel plate surfaces were observed using a scanning electron microscope. Based on the experimental results, it was observed that the transfer layer formation and the coefficient of friction along with its two components, namely adhesion and plowing, were controlled by the surface texture of the harder mating surfaces and were less dependent of surface roughness (R(a)) of the harder mating surfaces. The effect of surface texture on the friction was attributed to the variation of the plowing component of friction for different surfaces. Among the various surface roughness parameters studied, the mean slope of the profile, Delta(a), was found to most accurately characterize variations in the friction and wear behavior. (C) 2011 Elsevier B.V. All rights reserved.

Polymer template-assisted microemulsion synthesis of large surface area, porous Li2MnO3 and its characterization as a positive electrode material of Li-ion cells

Lithium-rich manganese oxide (Li2MnO3) is prepared by reverse microemulsion method employing Pluronic acid (P123) as a soft template and studied as a positive electrode material. The as-prepared sample possesses good crystalline structure with a broadly distributed mesoporosity but low surface area. As expected, cyclic voltammetry and charge-discharge data indicate poor electrochemical activity. However, the sample gains surface area with narrowly distributed mesoporosity and also electrochemical activity after treating in 4 M H2SO4. A discharge capacity of about 160 mAh g(-1) is obtained. When the acid-treated sample is heated at 300 A degrees C, the resulting porous sample with a large surface area and dual porosity provides a discharge capacity of 240 mAh g(-1). The rate capability study suggests that the sample provides about 150 mAh g(-1) at a specific discharge current of 1.25 A g(-1). Although the cycling stability is poor, the high rate capability is attributed to porous nature of the material.

Dispersion of polymer grafted nanoparticles in polymer nanocomposite films: Insights from surface x-ray scattering and microscopy

Dispersion of nanoparticles in polymer nanocomposite films determines the application potential of these systems as novel materials with unique physical properties. Grafting polymers to, mostly inorganic, nanoparticles has been suggested as an effective strategy to enhance dispersion and hence the efficacy of materials. In this review, we discuss the various parameters which control dispersion of polymer grafted nanoparticles in polymer nanocomposite films. We discuss how surface x-ray scattering and microscopy can provide complementary and unique information in thin polymer nanocomposite films to unravel the subtle interplay of entropic and surface interactions, mediated by confinement, that leads to enhanced dispersion of the nanoparticles in these films. (C) 2014 AIP Publishing LLC.

Spontaneous formation of highly ordered nanostructures: thermal instability and mode selection in surface-capped polymer films.

We demonstrate a controllable formation process of wave-like patterns in thermally unstable surface-capped polymer films on a rigid substrate. Self-ordered wave-like structures over a large area can be created by applying a small lateral tension to the film, whereupon it becomes unstable. A clear mode selection process which includes creation, decay and interference between coexisting waves at different annealing conditions has been observed, which makes it possible to restrain the patterns which are formed finally. Our results provide a clear and new evidence of spinodal behaviour in such a film due to thermal instability. Furthermore, we show that the well-controlled patterns generated in such a process can be used to fabricate nanostructures for various applications.