Design of high mobility DPP-based polymer semiconductors for organic thin film transistors

π-Conjugated polymers are the most promising semiconductor materials to enable printed organic thin film transistors (OTFTs) due to their excellent solution processability and mechanical robustness. However, solution-processed polymer semiconductors have shown poor charge transport properties mainly originated from the disordered polymer chain packing in the solid state as compared to the thermally evaporated small molecular organic semiconductors. The low charge carrier mobility, typically < 0.1 cm2 /V.s, of polymer semiconductors poses a challenge for most intended applications such as displays and radio-frequency identification (RFID) tags. Here we present our recent results on the dike topyrrolopyrrole (DPP)-based polymers and demonstrate that when DPP is combined with appropriate electron donating moieties such as thiophene and thienothiophene, very high charge carrier mobility values of ~1 cm2/V.s could be achieved.

Density of trap states in a polymer field-effect transistor

We report a more accurate method to determine the density of trap states in a polymer field-effect transistor. In the approach, we describe in this letter, we take into consideration the sub-threshold behavior in the calculation of the density of trap states. This is very important since the sub-threshold regime of operation extends to fairly large gate voltages in these disordered semiconductor based transistors. We employ the sub-threshold drift-limited mobility model (for sub-threshold response) and the conventional linear mobility model for above threshold response. The combined use of these two models allows us to extract the density of states from charge transport data much more accurately. We demonstrate our approach by analyzing data from diketopyrrolopyrrole based co-polymer transistors with high mobility. This approach will also work well for other disordered semiconductors in which sub-threshold conduction is important.

Enhanced interfacial thermal transport across graphene–polymer interfaces by grafting polymer chains

Thermal transport in graphene-polymer nanocomposite is complicated and has not been well understood. The interfacial thermal transport between graphene nanofiller and polymer matrix is expected to play a key role in controlling the overall thermal performance of graphene-polymer nanocomposite. In this work, we investigated the thermal transport across graphene-polymer interfaces functionalized with end-grafted polymer chains using molecular dynamics simulations. The effects of grafting density, chain length and initial morphology on the interfacial thermal transport were systematically investigated. It was found that end-grafted polymer chains could significantly enhance interfacial thermal transport and the underlying mechanism was considered to be the enhanced vibration coupling between graphene and polymer. In addition, a theoretical model based on effective medium theory was established to predict the thermal conductivity in graphene-polymer nanocomposites.

Assessment of the effects of CT dose in averaged x-ray CT images of a dose-sensitive polymer gel

The signal-to-noise ratio achievable in x-ray computed tomography (CT) images of polymer gels can be increased by averaging over multiple scans of each sample. However, repeated scanning delivers a small additional dose to the gel which may compromise the accuracy of the dose measurement. In this study, a NIPAM-based polymer gel was irradiated and then CT scanned 25 times, with the resulting data used to derive an averaged image and a "zero-scan" image of the gel. Comparison between these two results and the first scan of the gel showed that the averaged and zero-scan images provided better contrast, higher contrast-to- noise and higher signal-to-noise than the initial scan. The pixel values (Hounsfield units, HU) in the averaged image were not noticeably elevated, compared to the zero-scan result and the gradients used in the linear extrapolation of the zero-scan images were small and symmetrically distributed around zero. These results indicate that the averaged image was not artificially lightened by the small, additional dose delivered during CT scanning. This work demonstrates the broader usefulness of the zero-scan method as a means to verify the dosimetric accuracy of gel images derived from averaged x-ray CT data.

Finite element modeling of carbon fibre reinforced polymer (CFRP) strengthened steel tubes under axial impact

Steel hollow sections used in structures such as bridges, buildings and space structures involve different strengthening techniques according to their structural purpose and shape of the structural member. One such technique is external bonding of CFRP sheets to steel tubes. The performance of CFRP strengthening for steel structures has been proven under static loading while limited studies have been conducted on their behaviour under impact loading. In this study, a comprehensive numerical investigation is carried out to evaluate the response of CFRP strengthened steel tubes under dynamic axial impact loading. Impact force, axial deformation impact velocities are studied. The results of the numerical investigations are validated by experimental results. Based on the developed finite element (FE) model several output parameters are discussed. The results show that CFRP wrapping is an effective strengthening technique to increase the axial dynamic load bearing capacity by increasing the stiffness of the steel tube.

Novel polymer synthesis methodologies using combination of thermally and photochemically-induced nitroxide mediated polymerisation

The combination of thermally- and photochemically-induced polymerization using light sensitive alkoxyamines was investigated. The thermally driven polymerizations were performed via the cleavage of the alkoxyamine functionality, whereas the photochemically-induced polymerizations were carried out either by nitroxide mediated photo-polymerization (NMP2) or by a classical type II mechanism, depending on the structure of the light-sensitive alkoxyamine employed. Once the potential of the various structures as initiators of thermally- and photo-induced polymerizations was established, their use in combination for block copolymer syntheses was investigated. With each alkoxyamine investigated, block copolymers were successfully obtained and the system was applied to the post-modification of polymer coatings for application in patterning and photografting.

Synthesis of biodegradable polymer-mesoporous silica composite microspheres for DNA prime-protein boost vaccination

DNA vaccines or proteins are capable of inducing specific immunity; however, the translation to the clinic has generally been problematic, primarily due to the reduced magnitude of immune response and poor pharmacokinetics. Herein we demonstrate a composite microsphere formulation, composed of mesoporous silica spheres (MPS) and poly(d,l-lactide-co-glycolide) (PLGA), enables the controlled delivery of a prime-boost vaccine via the encapsulation of plasmid DNA (pDNA) and protein in different compartments. Method with modified dual-concentric-feeding needles attached to a 40 kHz ultrasonic atomizer was studied. These needles focus the flow of two different solutions, which passed through the ultrasonic atomizer. The process synthesis parameters, which are important to the scale-up of composite microspheres, were also studied. These parameters include polymer concentration, feed flowrate, and volumetric ratio of polymer and pDNA-PEI/MPS-BSA. This fabrication technique produced composite microspheres with mean D[4,3] ranging from 6 to 34 μm, depending upon the microsphere preparation. The resultant physical morphology of composite microspheres was largely influenced by the volumetric ratio of pDNA-PEI/MPS-BSA to polymer, and this was due to the precipitation of MPS at the surface of the microspheres. The encapsulation efficiencies were predominantly in the range of 93-98% for pDNA and 46-68% for MPS. In the in vitro studies, the pDNA and protein showed different release kinetics in a 40 day time frame. The dual-concentric-feeding in ultrasonic atomization was shown to have excellent reproducibility. It was concluded that this fabrication technique is an effective method to prepare formulations containing a heterologous prime-boost vaccine in a single delivery system.

Dewatering of microalgal culture for biodiesel production : exploring polymer flocculation and tangential flow filtration

Background: Conventional biodiesel production relies on trans-esterification of lipids extracted from vegetable crops. However, the use of valuable vegetable food stocks as raw material for biodiesel production makes it an unfeasibly expensive process. Used cooking oil is a finite resource and requires extra downstream processing, which affects the amount of biodiesel that can be produced and the economics of the process. Lipids extracted from microalgae are considered an alternative raw material for biodiesel production. This is primarily due to the fast growth rate of these species in a simple aquaculture environment. However, the dilute nature of microalgae culture puts a huge economic burden on the dewatering process especially on an industrial scale. This current study explores the performance and economic viability of chemical flocculation and tangential flow filtration (TFF) for the dewatering of Tetraselmis suecicamicroalgae culture. Results: Results show that TFF concentrates the microalgae feedstock up to 148 times by consuming 2.06 kWh m-3 of energy while flocculation consumes 14.81 kWhm-3 to concentrate the microalgae up to 357 times. Economic evaluation demonstrates that even though TFF has higher initial capital investment than polymer flocculation, the payback period for TFF at the upper extreme ofmicroalgae revenue is ∼1.5 years while that of flocculation is ∼3 years. Conclusion: These results illustrate that improved dewatering levels can be achieved more economically by employing TFF. The performances of these two techniques are also compared with other dewatering techniques.

Time-independent charge carrier mobility in a model polymer:fullerene organic solar cell

The technique of photo-CELIV (charge extraction by linearly increasing voltage) is one of the more straightforward and popular approaches to measure the faster carrier mobility in measurement geometries that are relevant for operational solar cells and other optoelectronic devices. It has been used to demonstrate a time-dependent photocarrier mobility in pristine polymers, attributed to energetic relaxation within the density of states. Conversely, in solar cell blends, the presence or absence of such energetic relaxation on transport timescales remains under debate. We developed a complete numerical model and performed photo-CELIV experiments on the model high efficiency organic solar cell blend poly[3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene] (PDPP-TNT):[6,6]-phenyl-C71-butyric-acid-methyl-ester (PC70BM). In the studied solar cells a constant, time-independent mobility on the scale relevant to charge extraction was observed, where thermalisation of photocarriers occurs on time scales much shorter than the transit time. Therefore, photocarrier relaxation effects are insignificant for charge transport in these efficient photovoltaic devices.

Modular design of profluorescent polymer sensors

A simple modular strategy for the synthesis of profluorescent nitroxide containing polymers is described. The incorporation of an epoxide as a pendant functionality on a polymer backbone synthesized using ATRP and subsequent nucleophilic ring-opening with sodium azide gave hydroxyl and azide functionality within a 3-bond radius. Orthogonal coupling chemistry then allowed the independent attachment of fluorophore and nitroxide groups in close proximity, giving rise to a profluorescent polymer. Validation of the viability of these materials as fluorescent sensors is demonstrated through efficient fluorescence switch-on observed when the materials are exposed to a model reductant or carbon-centred radical source.

Characterisation of thin layer polymer cement mortared concrete masonry bond

Masonry bond is affected by many parameters such as the type of mortar used, the techniques of dispersion of mortar and the surface texture of the concrete blocks. Additionally it is understood from the studies on conventional masonry that the bond characteristics are also influenced by the curing methods as well as the age of the bond at the time of testing. These effects on thin layer mortared masonry employing polymer cement mortars are not well understood. Therefore, the effect of curing methods and age to the bond strength and deformation of masonry containing thin layered polymer cement mortar was investigated as part of an ongoing research program at the Queensland University of Technology. This paper presents an experimental investigation of the flexural and shear bond characteristics of the thin layer mortared concrete masonry. The parameters examined include the effects curing and ageing to the bond development over a period from 14 days to 56 days after fabrication. The results exhibit that dry cured thin layer mortared masonry exhibits higher bond strength and Young’s and shear moduli compared to the wet cured specimens.

Synthesis, characterization and applications of graphene oxide-polymer nanocomposites

Graphene has emerged as one of the most exciting materials of the 21st century due to its unique properties which have demonstrated great potential for applications in energy storage, flexible electronics and multifunctional composites. This thesis has established a new technique for investigating the structure-property relationship of graphene-polymer nanocomposites at micro and nanoscales. The outcomes can help gain a fundamental understanding of the toughening mechanism in these novel nanocomposites and benefit the development of broad graphene based materials and devices.

Effect of covalent functionalisation on thermal transport across graphene-polymer interfaces

This paper is concerned with the interfacial thermal resistance for polymer composites reinforced by various covalently functionalised graphene. By using molecular dynamics simulations, the obtained results show that the covalent functionalisation in graphene plays a significant role in reducing the graphene-paraffin interfacial thermal resistance. This reduction is dependent on the coverage and type of functional groups. Among the various functional groups, butyl is found to be the most effective in reducing the interfacial thermal resistance, followed by methyl, phenyl and formyl. The other functional groups under consideration such as carboxyl, hydroxyl and amines are found to produce negligible reduction in the interfacial thermal resistance. For multilayer graphene with a layer number up to four, the interfacial thermal resistance is insensitive to the layer number. The effects of the different functional groups and the layer number on the interfacial thermal resistance are also elaborated using the vibrational density of states of the graphene and the paraffin matrix. The present findings provide useful guidelines in the application of functionalised graphene for practical thermal management.

Magnetic nanoparticles boosting the osmotic efficiency of a polymeric FO draw agent: Effect of polymer conformation

Poly sodium acrylate (PSA)-coated Magnetic Nanoparticles (PSA-MNPs) were synthesized as smart osmotic draw agent (SMDA) for water desalination by forward osmosis (FO) process. The PSA-coated MNPs demonstrated significantly higher osmotic pressure (~ 30 fold) as well as high FO water flux (~ 2–3 fold) when compared to their polymer (polyelectrolyte) counterpart, even at a very low concentration of ~ 0.13 wt.% in the draw solution. The PSA polymer chain conformation – coiled to extended – demonstrates a significant impact on the availability of the polymer hydrophilic groups in solution which is the driving force to attain higher osmotic pressure and water flux. When an optimum concentration of the polymer was anchored to a NP surface, the polymer chains assume an extended open conformation making the functional hydrophilic groups available to attract water molecules. This in turn boosts the osmotic pressure and FO water flux of the PSA-MNP draw agents. The low concentration of the PSA-MNP osmotic agent and the associated high water flux enhances the cost-effectiveness of our proposed SMDA system. In addition, easier magnetic separation and regeneration of the SMDA also improves its usability making it efficient, cost-effective and environment-friendly.

Developing starch-based polymer composites

This project aim was to replace petroleum-based plastic packaging materials that pollute the environment, with biodegradable starch-based polymer composites. It was demonstrated that untreated sugar cane bagasse microfibres and unbleached nanofibres significantly improved the physical, mechanical and chemical properties of starch films, while thermal extrusion of starch with alcohol improved the stiffness and the addition of aconitic acid cross-linked the film making it moisture resistant and extensible.