The effect of bioglass addition on mechanical and physical properties of photoactive UDMA-TEGDMA resin composites

Light curable dimethacrylate resin composites undergo free radical photopolymerisation in response to blue light (wavelength 450-500 nm) and may offer superior handling and setting characteristics for novel hard tissue repair materials. The current investigation aims to determine the optimum formulation of bisphenol-A glycidyl methacrylate and triethyleneglycoldimethacrylate (bisGMA/TEGDMA) or urethane dimethacrylate (UDMA)/TEGDMA resin mixtures and the effect of Bioglass incorporation on the rate of polymerisation (RP), degree of conversion (DC) and flexural strength (FS) of light-curable filled resin composites (FRCs). Experimental photoactive resins containing a range of bisGMA, UDMA and TEGDMA ratios and/or filled with non-silanised irregular or spherical 45S5-Bioglass (50 μm; 5-40 wt%) and/or silanised silicate glass filler particulates (0.7 μm; 50-70 wt%) were tested. RP and DC were analysed in real-time using nearinfrared spectroscopy. FS of resins and FRCs were determined using three-point flexural strength tests. UDMA/TEGDMA resins exhibited increased DC compared with bisGMA/TEGDMA resins (p<0.05). The addition of spherical particles of Bioglass had a detrimental effect on the FS (p>0.05), whereas they increased DC of UDMA/TEGDMA resins (p<0.05). Addition of irregular shaped Bioglass particles increased the FS of UDMA/TEGDMA resins up to 20 wt% Bioglass (p<0.05). The flexibility and strength conferred by the urethane group in UDMA may result in enhanced physical and mechanical properties compared with conventional resins containing bulky (bisGMA) molecules. Addition of 45S5-Bioglass with specific filler content, size and morphology resulted in enhanced mechanical and physical properties of UDMA/TEGDMA composites. © (2014) Trans Tech Publications, Switzerland.

An XPS study of pulsed plasma polymerised allyl alcohol film growth on polyurethane

The growth of highly functionalised poly allyl alcohol films by pulsed plasma polymerisation of CH2 double bond; length as m-dashCHCH2OH on biomedical grade polyurethane has been followed by X-ray photoelectron spectroscopy (XPS) and contact angle measurements. Film thickness is observed to increase approximately linearly with plasma modification time, suggesting a layer-by-layer growth mode of poly allyl alcohol. Water contact angle measurements reveal the change in the surface free energy of wetting decreases linearly with plasma modification up to the monolayer point after which a constant limiting value of −24 mJ m−2 was attained. Films prepared at 20 W plasma power with a duty cycle of 10 μs:500 μs exhibit a high degree of hydroxyl (single bondOH) retention with minimal fragmentation of the monomer observed. Increasing the plasma power up to 125 W is found to improve single bondOH retention at the expense of ether formation generating films close to the monomer stoichiometry. Duty cycle plays an important role in controlling both film composition and thickness, with longer off times increasing single bondOH retention, while longer on times enhance allyl alcohol film growth.

Cellular uptake of ribonuclease A-functionalised core-shell silica microspheres

Analysis of protein function in a cellular context ideally requires physiologically representative levels of that protein. Thus conventional nucleic acid-based transfection methods are far from ideal owing to the over expression that generally results. Likewise fusions with protein transduction domains can be problematic whilst delivery via liposomes/nanoparticles typically results in endosomal localisation. Recently polymer microspheres have been reported to be highly effective at delivering proteins into cells and thus provide a viable new alternative for protein delivery (protein transduction). Herein we describe the successful delivery of active ribonuclease A into HeLa cells via novel polymer core-silica shell microspheres. Specifically, poly(styrene-co-vinylbenzylisothiouronium chloride) core particles, generated by dispersion polymerisation, were coated with a poly(styrene-co-trimethoxysilylpropyl methacrylate) shell. The resultant core-shell morphology was characterised by transmission electron, scanning electron and fluorescence confocal microscopies, whilst size and surface charge was assessed by dynamic light scattering and zeta-potential measurements, respectively. Subsequently ribonuclease A was coupled to the microspheres using simple carbodiimide chemistry. Gel electrophoresis confirmed and quantified the activity of the immobilised enzyme against purified HeLa RNA. Finally, the polymer-protein particles were evaluated as protein-transduction vectors in vitro to deliver active ribonuclease A to HeLa cells. Cellular uptake of the microspheres was successful and resulted in reduced levels of both intracellular RNA and cell viability.

Polymerisations in mesoporous environments

The solid acid supported aluminium chloride is an effective cationic initiator for the polymerisation of hydrocarbons. Reactions are highly dependent on the nature of the active sites and the Lewis/Bronsted acid balance in particular.

Novel starch-polyalkane composite materials

The polymerisation of dibromoalkanes adsorbed in potato starch leads to a novel class of hydrophobic starch-polyalkane composite materials with very high capacity for surface derivitisation.

Synthesis and evaluation of nanoparticle-polymer composites

This thesis describes the synthesis of functionalised polymeric material by variety of free-radical mediated polymerisation techniques including dispersion emulsion, seeded emulsion, suspension and bulk polymerisation reactions. Organic fluorophores and nanoparticles such as quantum dots were incorporated within polymeric materials, in particular, thiol-functionalised polymer microspheres, which were fluorescently labelled either during synthesis or by covalent attachment post synthesis. The resultant fluorescent polymeric conjugates were then assessed for their utility in biological systems as an analytical tool for cells or biological structures. Quantum dot labelled, thiol-functionalised microspheres were assessed for their utility in the visualisation and tracking of red blood cells. Determination of the possible internalisation of fluorescent microspheres into red blood cells was required before successful tracking of red blood cells could take place. Initial work appeared to indicate the presence of fluorescent microspheres inside red blood cells by the process of beadfection. A range of parameters were also investigated in order to optimise beadfection. Thiol-functionalised microspheres labelled successfully with organic fluorophores were used to image the tear film of the eye. A description of problems encountered with the covalent attachment of hydrophilic, thiol-reactive fluorescent dyes to a variety of modified polymer microspheres is also included in this section. Results indicated large microspheres were particularly useful when tracking the movement of fluid along the tear meniscus. Functional bulk polymers were synthesised for assessment of their interaction with titanium dioxide nanoparticles. Thiol-functionalised polymethyl methacrylate and spincoated thiouronium-functionalised polystyrene appeared to facilitate the attachment of titanium dioxide nanoparticles. Interaction assays included the use of XPS analysis and processes such as centrifugation. Attempts to synthesise 4-vinyl catechol, a compound containing hydroxyl moieties with potential for coordination with titanium dioxide nanoparticles, were also carried out using 3,4-dihydroxybenzaldehyde as the starting material.

Design, synthesis and thermal behaviour of a series of well-defined clickable and triggerable sulfonate polymers

In the printing industry, the exploitation of triggerable materials that can have their surface properties altered on application of a post-deposition external stimulus has been crucial for the production of robust layers and patterns. To this end, herein, a series of clickable poly(R-alkyl p-styrene sulfonate) homopolymers, with systematically varied thermally-labile protecting groups, has been synthesised via reversible addition-fragmentation chain transfer (RAFT) polymerisation. The polymer range has been designed to offer varied post-deposition thermal treatment to switch them from hydrophobic to hydrophilic. Suitable RAFT conditions have been identified to produce well-defined homopolymers (Đ, Mw/Mn < 1.11 in all cases) at high monomer conversions (>80% for all but one monomer) with controllable molar mass. Poly(p-styrene sulfonate) with an isobutyl protecting group has been shown to be the most readily thermolysed polymer that remains stable at room temperature, and was thus investigated further by incorporation into a diblock copolymer, P3HT-b-PiBSS, by click chemistry. The strategy for preparation of thermal modifiable block copolymers exploiting R-protected p-styrene sulfonates and azide-alkyne click chemistry presented herein allows the design of new, roll-to-roll processable materials for potential application in the printing industry, particularly organic electronics.

Kinetic studies of the photopolymerisation of acrylamide in aqueous solution:effects of bromoform as a Chain transfer agent

The effects of adding bromoform (CHBr3) as a potential chain transfer agent in the photopolymerisation of acrylamide (AM) in aqueous solution have been studied both in terms of influencing the rate of polymerisation and the molecular weight of the polyacrylamide (PAM) formed. Using 4,4′-azo-bis(4-cyanopentanoic acid) (ACPA) as photoinitiator, two different CHBr3 concentrations as chain transfer agent were compared: 0.5 and 2.0 mol % (relative to AM), the higher of which was determined by the limit of CHBr3 water solubility. The results showed that CHBr3 was an effective chain transfer agent that could regulate the molecular weight of the PAM formed without seriously affecting the polymerisation rate. It is concluded that chain transfer to CHBr3occurs by both Br and H atom transfer although Br transfer is the more favoured due to the weaker C-Br bond. Furthermore, Br transfer leads to Br-terminated chains in which the terminal C-Br bond can re-dissociate leading to re-initiation and re-propagation of the same chain, thereby maintaining the polymerisation rate. Continuing studies into how this mechanism can be exploited in order to synthesize water-soluble block copolymers of potential biomedical importance are currently in progress.

Fullerene-capped copolymers for bulk heterojunctions: device stability and efficiency improvements

A fullerene end-capped polymer-compatibilizer based on poly(3-hexylthiophene) (P3HT) was synthesized and demonstrated to have a remarkable effect on both the stability and efficiency of devices made from exemplar P3HT and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). P3HT with ethynyl chain-ends and α-azido-ω-bromo-PS were prepared via Grignard metathesis (GRIM) and atom transfer radical polymerisation, respectively. “Click” chemistry resulted in the preparation of poly(3-hexylthiophene)-block-ω-bromo-polystyrene (P3HT-b-PS-Br), and subsequent atom transfer radical addition chemistry with fullerene (C60) yielded the donor–acceptor block copolymer P3HT-b-PS-C60. Both P3HT-b-PS-Br and P3HT-b-PS-C60 were considered as compatibilizers with P3HT/PCBM blends, with the study detailing effects on active-layer morphology, device efficiency and stability. When used at low concentrations, both P3HT-b-PS-Br (1%) and P3HT-b-PS-C60 (0.5%) resulted in considerable 28% and 35% increases in efficiencies with respect to devices made from P3HT/PCBM alone. Furthermore, P3HT-b-PS-C60 (0.5%) resulted in an important improvement in device stability.

Synthesis and characterisation of interfacial layers in organic solar cells

The quest for renewable energy sources has led to growing attention in the research of organic photovoltaics (OPVs), as a promising alternative to fossil fuels, since these devices have low manufacturing costs and attractive end-user qualities, such as ease of installation and maintenance. Wide application of OPVs is majorly limited by the devices lifetime. With the development of new encapsulation materials, some degradation factors, such as water and oxygen ingress, can almost be excluded, whereas the thermal degradation of the devices remains a major issue. Two aspects have to be addressed to solve the problem of thermal instability: bulk effects in the photoactive layer and interfacial effects at the photoactive layer/charge-transporting layers. In this work, the interface between photoactive layer and electron-transporting zinc oxide (ZnO) in devices with inverted architecture was engineered by introducing polymeric interlayers, based on zinc-binding ligands, such as 3,4-dihydroxybenzene and 8-hydroxyquinoline. Also, a cross-linkable layer of poly(3,4-dimethoxystyrene) and its fullerene derivative were studied. At first, controlled reversible addition-fragmentation chain transfer (RAFT) polymerisation was employed to achieve well-defined polymers in a range of molar masses, all bearing a chain-end functionality for further modifications. Resulting polymers have been fully characterised, including their thermal and optical properties, and introduced as interlayers to study their effect on the initial device performance and thermal stability. Poly(3,4-dihydroxystyrene) and its fullerene derivative were found unsuitable for application in devices as they increased the work function of ZnO and created a barrier for electron extraction. On the other hand, their parental polymer, poly(3,4-dimethoxystyrene), and its fullerene derivative, upon cross-linking, resulted in enhanced efficiency and stability of devices, if compared to control. Polymers based on 8-hydroxyquinoline ligand had a negative effect on the initial stability of the devices, but increased the lifetime of the cells under accelerated thermal stress. Comprehensive studies of the key mechanisms, determining efficiency, such as charge generation and extraction, were performed by using time-resolved electrical and spectroscopic techniques, in order to understand in detail the effect of the interlayers on the device performance. Obtained results allow deeper insight into mechanisms of degradation that limit the lifetime of devices and prompt the design of better materials for the interface stabilisation.

Identification of the degradation mechanisms of organic solar cells:active layer and interfacial layers

Organic Solar Cells (OSCs) represent a photovoltaic technology with multiple interesting application properties. However, the establishment of this technology into the market is subject to the achievement of operational lifetimes appropriate to their application purposes. Thus, comprehensive understanding of the degradation mechanisms occurring in OSCs is mandatory in both selecting more intrinsically stable components and/or device architectures and implementing strategies that mitigate the encountered stability issues. Inverted devices can suffer from mechanical stress and delamination at the interface between the active layer, e.g. poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM), and the hole transport layer, e.g. poly(3,4-ethylenedioxythiophene):poly(p-styrene sulfonate) (PEDOT:PSS). This work proposes the incorporation of a thin adhesive interlayer, consisting of a diblock copolymer composed of a P3HT block and a thermally-triggerable, alkyl-protected PSS block. In this context, the synthesis of poly(neopentyl p-styrene sulfonate) (PNSS) with controlled molar mass and low dispersity (Ð ≤ 1.50) via Reversible Addition-Fragmentation chain Transfer (RAFT) polymerisation has been extensively studied. Subsequently, Atomic Force Microscopy (AFM) was explored to characterise the thermal deprotection of P3HT-b-PNSS thin layers to yield amphiphilic P3HT-b-PSS, indicating that surface deprotection prior to thermal treatment could occur. Finally, structural variation of the alkyl protecting group in PSS allowed reducing the thermal treatment duration from 3 hours (P3HT-b-PNSS) to 45 minutes for the poly(isobutyl p-styrene sulfonate) (PiBSS) analogous copolymer. Another critical issue regarding the stability of OSCs is the sunlight-driven chemical degradation of the active layer. In the study herein, the combination of experimental techniques and theoretical calculations has allowed identification of the structural weaknesses of poly[(4,4’- bis(2-ethylhexyl) dithieno [3,2-b:2’,3’-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5’-diyl], Si-PCPDTBT, upon photochemical treatment in air. Additionally, the study of the relative photodegradation rates in air of a series of polymers with systematically modified backbones and/or alkyl side chains has shown no direct correlation between chemical structure and stability. It is proposed instead that photostability is highly dependent on the crystalline character of the deposited films. Furthermore, it was verified that photostability of blends based on these polymers is dictated by the (de)stabilising effect that [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) has over each polymer. Finally, a multiscale analysis on the degradation of solar cells based on poly[4,4' bis(2- ethylhexyl) dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-[2,5 bis(3 tetradecylthiophen 2-yl)thiazole[5,4-d]thiazole)-1,8-diyl] and PCBM, indicated that by judicious selection of device layers, architectures, and encapsulation materials, operational lifetimes up to 3.3 years with no efficiency losses can be successfully achieved.

Quitooligosacáridos bifuncionales: relación entre características químico-físicas y propiedades biológicas

As the degraded products of chitosan, chitooligosaccharides (COS) have recently been produced by several methods, such as enzymatic an acidic hydrolysis. Chitosans are a family of biocompatible and biodegradable biopolymers obtained by N-deacetylation of chitin, the most abundant natural polymer after cellulose, consisting of two monomeric units, N-acetyl-2- amino-2-deoxi-D-glucose (A units) and 2-amino-2-deoxi-D-glucose (D units) linked by β (1→4) links. The degraded products COS, have a smaller molecular weight and therefore have better solubility and lower viscosity under physiological conditions because of shorter chain lengths and free amino groups in D-glucosamine units. The study of COS has been increasing not only because they come from a natural source, but also because of their biological compatibility and effectiveness. There are numerous reports on the biological activities of COS and their potential applications in food industry, pharmacy, agricultural or biomedicine. Nevertheless, in these studies it is difficult to find well defined COS in terms of physicochemical parametres, because these samples are usually poorly characterized. This makes it difficult to compare the results and to understand their mecanism of action. Degradation of the O-glycosidic linkages of chitosan by different methods, results in COS with different numbers and sequences of A and D units as well as different degrees of polymerisation (DP). Over the past few years, several technological approaches have been taken in preparing COS, including acid hydrolysis or enzymatic methods, among others. Therefore, in order to obtain COS with different physicochemical properties, different preparation methods of COS have been developed in this work. Then, the study of the relationship between physicochemical properties of these COS and their biological activities such as natural antioxidants, antibacterial agents, mucoadhesive and anti-inflammatory effects have been studied...

Synthesis and characterization of electron deficient heterocycles as building blocks for electronic materials

This thesis describes the synthesis of a new electropolymerizable viologen derivative. A reasonably high-yielding route is reported, and a preliminary investigation of its polymerisation is described. The viologen and its precursors were examined by 1H NMR, MS, IR and elemental analysis. The energies of the band gap for the materials have been calculated using UV-vis spectroscopy, and cyclic voltammetry was also used to estimate the oxidation and the reduction potentials and to calculate the HOMO and LUMO energies. Theoretical calculations were performed using DFT. The attempted synthesis of a new flavin-functionalised phenanthroline derivative is described. Unfortunately, the protocol used failed to provide the desired compounds.

The interplay between neuronal activity and actin dynamics mimic the setting of an LTD synaptic tag

Persistent forms of plasticity, such as long-term depression (LTD), are dependent on the interplay between activity-dependent synaptic tags and the capture of plasticity-related proteins. We propose that the synaptic tag represents a structural alteration that turns synapses permissive to change. We found that modulation of actin dynamics has different roles in the induction and maintenance of LTD. Inhibition of either actin depolymerisation or polymerization blocks LTD induction whereas only the inhibition of actin depolymerisation blocks LTD maintenance. Interestingly, we found that actin depolymerisation and CaMKII activation are involved in LTD synaptic-tagging and capture. Moreover, inhibition of actin polymerisation mimics the setting of a synaptic tag, in an activity-dependent manner, allowing the expression of LTD in non-stimulated synapses. Suspending synaptic activation also restricts the time window of synaptic capture, which can be restored by inhibiting actin polymerization. Our results support our hypothesis that modulation of the actin cytoskeleton provides an input-specific signal for synaptic protein capture.

Engineering the transition from non-living to living matter

Re-creating and understanding the origin of life represents one of the major challenges facing the scientific community. We will never know exactly how life started on planet Earth, however, we can reconstruct the most likely chemical pathways that could have contributed to the formation of the first living systems. Traditionally, prebiotic chemistry has investigated the formation of modern life’s precursors and their self-organisation under very specific conditions thought to be ‘plausible’. So far, this approach has failed to produce a living system from the bottom-up. In the work presented herein, two different approaches are employed to explore the transition from inanimate to living matter. The development of microfluidic technology during the last decades has changed the way traditional chemical and biological experiments are performed. Microfluidics allows the handling of low volumes of reagents with very precise control. The use of micro-droplets generated within microfluidic devices is of particular interest to the field of Origins of Life and Artificial Life. Whilst many efforts have been made aiming to construct cell-like compartments from modern biological constituents, these are usually very difficult to handle. However, microdroplets can be easily generated and manipulated at kHz rates, making it suitable for high-throughput experimentation and analysis of compartmentalised chemical reactions. Therefore, we decided to develop a microfluidic device capable of manipulating microdroplets in such a way that they could be efficiently mixed, split and sorted within iterative cycles. Since no microfluidic technology had been developed before in the Cronin Group, the first chapter of this thesis describes the soft lithographic methods and techniques developed to fabricate microfluidic devices. Also, special attention is placed on the generation of water-in-oil microdroplets, and the subsequent modules required for the manipulation of the droplets such as: droplet fusers, splitters, sorters and single/multi-layer micromechanical valves. Whilst the first part of this thesis describes the development of a microfluidic platform to assist chemical evolution, finding a compatible set of chemical building blocks capable of reacting to form complex molecules with endowed replicating or catalytic activity was challenging. Abstract 10 Hence, the second part of this thesis focuses on potential chemistry that will ultimately possess the properties mentioned above. A special focus is placed on the formation of peptide bonds from unactivated amino acids, despite being one of the greatest challenges in prebiotic chemistry. As opposed to classic prebiotic experiments, in which a specific set of conditions is studied to fit a particular hypothesis, we took a different approach: we explored the effects of several parameters at once on a model polymerisation reaction, without constraints on hypotheses on the nature of optimum conditions or plausibility. This was facilitated by development of a new high-throughput automated platform, allowing the exploration of a much larger number of parameters. This led us to discover that peptide bond formation is less challenging than previously imagined. Having established the right set of conditions under which peptide bond formation was enhanced, we then explored the co-oligomerisation between different amino acids, aiming for the formation of heteropeptides with different structure or function. Finally, we studied the effect of various environmental conditions (rate of evaporation, presence of salts or minerals) in the final product distribution of our oligomeric products.