Over the last 20 years "composite" insulators have been increasingly used in high voltage applications as an alternative traditional materials. More recently, polydimethylsiloxane (PDMS) have been used as weather sheds on these composite insulators. The main attraction with PDMS is that the surface hydrophobicity can be recovered following pollution or surface discharges. Among the possible mechanisms for recovery the most likely is the migration of low molecular weight silicone oil (LMWS) from the bulk to the surface encapsulating pollutant particles. Although it is widely recognised that the migration of LMWS is the cause of this recovery of hydrophobicity, the mechanism of what actually occurs is not well understood. It is also not known for how long this process will continue. The main objective of this study program was to gain improved understanding of the surface hydrophobic recovery process that is unique to polydimethlysiloxane high-voltage insulators. Fundamental knowledge of this mechanism has been increased through the development of the Contact Angle DRIFT Electrostatic Deposition (CADED) novel analytical technique. This technique enabled study of the degradation of silicone elastomers subjected to high voltage environments by closely following LMWS migration from the bulk material to the surface and linking it to the contact angle measurements. The migration rate data showed that the aged material recovered faster that the virgin material. Differences in the rate and maximum surface levels of silicone were seen between materials from different manufacturers. This has significant implications for the life-time of these materials A model system has been developed to examine LMWS diffusion through the bulk material and into the interface of surface and pollutant. This was achieved by examining theoretical and empirically derived equations and using existing experimental data to better understand the mechanism of recovery. This diffusion was Fickian in the initial stages of recovery. X-ray photoelectron spectroscopy (XPS) and contact angle measurements were used to substantiate the degree of degradation in in-field silicone insulators by quantifying the levels of the major degradation products: silica and silica-like material and alumina.

L’environnement géostationnaire autour de la Terre présente des conditions complexes influençant les performances ainsi que la durée de vie des satellites. En vol et au cours du temps, les polydiméthylsiloxanes se dégradent ce qui se manifeste par une perte de souplesse, de transparence, ou encore une dégradation de l'état de la surface. Dans ce contexte, le but de cette thèse consiste à étudier l’évolution de la stabilité des polydiméthylsiloxanes en environnement géostationnaire simulé et d’autre part à proposer des solutions qui permettent de limiter la dégradation des propriétés d’intérêts technologiques, optiques en particulier. La stabilisation des polydiméthylsiloxanes soumises aux irradiations UV par l’incorporation de différentes structures d’additifs (Hindered Amine Light Stabilizers, absorbeurs UV, nanocristaux de cellulose) a été étudiée. Une autre approche pour augmenter la stabilité des polydiméthylsiloxanes aux rayonnements UV a été envisagée avec le remplacement du système catalytique actuellement utilisé (catalyseur de Karstedt) par l’emploi de dérivés organométalliques à base de Rhodium ou de Platine. Des solutions concernant la stabilisation de ces polydiméthylsiloxanes aux irradiations H+, comme l’ajout d’additifs tels que le polystyrène, les silsesquioxanes ont également été proposées. / The geostationary environment around Earth is complex which strongly influences the satellites performances and lifetime. In flight and over time, polydimethylsiloxanes exhibit degradations like a loss of flexibility and transparency, or a deterioration of the surface state. In this context, the aim of this work was to study the evolution of polydimethylsiloxanes stability in geostationary environment to find solutions to limit the degradation of interest technological properties, in particular the optical one. The stabilization of silicone resins under UV irradiation has been performed by the incorporation of different additives such as Hindered Amine Light Stabilizers, UV Absorbers and cellulose nanocrystals into the PDMS matrix. Generally, polydimethylsiloxanes networks are obtained by hydrosilylation with highly active Karstedt catalyst. Various organometallics derivatives based on Rhodium and Platinum were studied as alternative catalysts for the cross-linking of polydimethylsiloxane in order to improve the UV stability. Finally, different solutions concerning the stabilization of polydimethylsiloxane to proton irradiation, such as the addition of various additives like polystyrene or silsesquioxanes have been investigated.

Le but de ce travail est de comprendre l'influence des nanoparticules de silice sur les transitions physiques de matrices polymères de nature différente : l'alcool polyfurfurylique (PFA), le polytétrafluoroéthylène (PTFE) et le polydiméthylsiloxane (PDMS). Pour cela, les techniques d'analyse thermique conventionnelles (ATG, DSC, DMA) ont été couplées à des techniques atypiques (DSC multifréquence, FSC, UFSC).Dans le cas du PFA, les nanoparticules de silice ont entrainé une augmentation de la Tg ainsi qu'une amélioration des propriétés thermomécaniques. En outre, il a été démontré que la seule présence de silice suffit à favoriser les mécanismes de polymérisation. La cristallisation du PTFE à partir de l'état fondu a été étudiée pour la première fois sur une gamme de vitesse de refroidissement très large (jusqu'à 800 000 K.s-1). L'effet nucléant des nanoparticules de silice a également été mis en avant à faibles vitesses de refroidissement lors de l'étude de la cristallisation du PTFE chargé. Cependant, il s'est avéré qu'elle ralentit également la diffusion des chaines dans le milieu pour certaines vitesses. L'influence des nanoparticules de silice sur la transition vitreuse et la cristallisation du PDMS a finalement été étudiée. Les résultats ont montré que la silice n'induit pas d'effet significatif sur la transition vitreuse. D'autre part, la silice influence fortement la cinétique de cristallisation. Cet effet a été directement lié au fait que la silice favorise la nucléation sans influencer la diffusion des chaines. / The aim of this work is to understand the influence of silica nanoparticles on the physical transitions of different polymer matrix : polyfurfuryl alcohol (PFA), polydimethylsiloxane (PDMS) and polytetrafluoroethylene (PTFE). Thus, the conventionnal thermal analysis techniques (TGA, DSC, DMTA) were employed in correlation with atypical techniques (multifrequency DSC, FSC, UFSC). In the case of PFA, the silica nanoparticles led to an increase of the Tg and of the thermomechanical properties. Besides, it was demonstrated that the presence of silica is enough to realise faster the polymerization process. The crystallization from the melt of PTFE was investigated for the first time on a very large range of cooling rates (untill 800 000 K.s-1). The enhancement of the nucleation process at slow cooling rates due to the silica nanoparticles has been put in relief with the study of filled PTFE crystallization. Yet, it has been shown that it slows down the diffusion of the chains through the medium for some of the cooling rates. The influence of silica nanoparticles on PDMS glass transition and crystallization was finally studied. The silica nanoparticles don't induce any significative effect on the glass transition. It appeared that the nanoparticles strongly influence the crystallization kinetics. This effect is linked to the enhancement of nucleation process by silica without any effect on chain diffusion.

Estrogens, alkylphenols and bisphenol-A, enter the environment through waste water systems and waste disposal of manufactured products e.g. detergents, paints, polycarbonates and flameretardants. These analytes disrupt the endocrine function of living organisms affecting their reproductive health and those of future generations. Gas phase low molecular- mass aldehydes and amines are typically eye, nose, and throat irritants. Formaldehyde is classified as a probable human carcinogen. Given their negative impact on human health it is urgent to monitor pollutants at extremely low levels in both air and water. The aqueous pollutants are often concentrated using solid phase extraction cartridges or liquid-liquid extraction followed by derivatization. Methods that can most effectively and selectively pre-concentrate aldehydes and amines involve in situ derivatization. Unfortunately, the derivatizing reagents as well as their associated solvents or adsorbents, are responsible for problems encountered with these methods. Polydimethylsiloxane (PDMS) has emerged as the ideal concentration and reaction medium for trace analysis. However the expensive commercial devices such as SPME and SBSE both require the samples to be returned to the laboratory for concentration. Due to the open tubular nature of the PDMS multichannel trap (MCT), developed in our laboratory, it is ideally suited for on-site and online sampling. The MCTs have a high analyte capacity owing to the large volume of PDMS available for concentration. The derivatization reaction can be performed in situ providing a “onepot concentration and reaction device”. This allows for reduced risk of contamination of / or losses of the sample and a sampling method that can cater for both air and water samples. To demonstrate the versatility of the PDMS MCT, two approaches for concentration in PDMS were investigated in this study, namely, 1) the on-line concentration and in situ derivatization of volatile polar analytes from air followed by REMPI-TOFMS detection, and 2) the concentration of phenolic lipophilic analytes from water requiring derivatization prior to analysis by GC/MS. 1) Analyte and derivatizing reagent were simultaneously introduced into the PDMS trap using a ypress- fit connector. The reaction occurs in situ followed by thermal desorption using a thermal modulator array alone or in conjunction with a thermal desorption unit. The aldehydes and amine derivatives were successfully detected by the REMPI-TOFMS. Reaction efficiencies were determined at room temperature without catalysts. Formaldehyde yielded a low reaction/concentration efficiency of 41 % with phenylhydrazine in PDMS, while acetaldehyde, acrolein and crotonal displayed much improved values of 92, 61 and 74 % respectively. Both propylamine and butylamine yielded 28 % reaction/concentration efficiency with benzaldehyde in the PDMS matrix. Detection limits obtained with this technique were significantly lower than the permissible exposure limits set by the Occupational Safety and Health Administration. It should be noted that the detection limits were not determined by actual measurement but by extrapolation from a larger signal. 2) Aqueous analytes were concentrated in the PDMS MCT using a gravity flow rate of ~50 ìl/min. The trap was dried and 5 ìl derivatizing reagent added. At room temperature and without the presence of a catalyst, the reaction of alkylphenols with trifluoroacetic acid anhydride in the PDMS matrix was 100% complete after 5 minutes. Bisphenol-A reacted less than 50 % to completion during this period, but the amount of derivative formed remained constant. This study revealed that extraction efficiencies of the alkylphenols and bisphenol-A off the PDMS trap have poor batch-tobatch repeatability indicating that the PDMS matrix was not homogenous. For two different PDMS batches: tert-octylphenol displayed an extraction efficiency of 70 and 79%, nonylphenol displayed 84 and 43% while Bisphenol-A displayed 10 and 26% respectively. The thermally desorbed derivatives were analysed by GC/MS. Despite background contamination in the desorption unit, detection limits were at the ppt level. Detection limits were not determined by actual measurement but by extrapolation from a larger signal. / Thesis (PhD)--University of Pretoria, 2009. / Chemistry / unrestricted

This thesis aims to develop a new type of highly stretchable strain sensor to measure large deformation of a specimen subjected to dynamic loading. The sensor was based on the piezo-resistive response of carbon nanotube(CNT)/polydimethysiloxane (PDMS) composites thin films, some nickel particles were added into the sensor composite to improve the sensor performance. The piezo-resistive response of CNT composite gives high frequency response in strain measurement, while the ultra-soft PDMS matrix provides high flexibility and ductility for large strain measuring large strain (up to 26%) with an excellent linearity and a fast frequency response under quasi-static test, the delay time for high strain rate test is just 30 μs. This stretchable strain sensor is also able to exhibit much higher sensitivities, with a gauge factor of as high as 80, than conventional foil strain gauges.

End-functional poly(ether amic acid)s and poly(siloxane imide) multiblock copolymers, comprised of 2,2'-Bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BPADA) / meta-phenylene diamine (MPDA) and hexafluoroisopropylidene-2-bis(phthalic acid anhydride) (6FDA) / meta-phenylene diamine (MPDA) polyimide segments, have been prepared and characterized to explore possibilities for controlling interface properties. Incorporation of polydimethylsiloxane (PDMS) components into polyimide backbone structures can yield advantageous properties such as low energy surfaces and low stress interfaces. End-functional BPDA/MPDA poly(amic acid) salts and poly(siloxane amic acid) salts were prepared in methanolic or aqueous tripropylamine solutions. The polymeric salts formed stable water solutions (or dispersions) and imidized in less than 10 minutes at 260°C. The water solubility and rapid imidization times are ideal for on-line processing. Thus, these materials can be used as sizing and interface toughening agents for fiber reinforced composite manufacturing. Epoxy-polyimide networks prepared from the amine functionalized polyimide with DER 331 epoxy resin and diamino diphenylsulfone showed microphase separation (100-300 nm inclusions) by transmission electron microscopy. Slight toughening of the cured epoxy with 9 weight % imide was observed with the imide as the included phase. Epoxy bilayer films of polyimide (amine end-functional and commercial Ultem™) and poly(siloxane imide) multiblock copolymers were prepared to evaluate the polymer-matrix interphase region. Atomic force microscopy (AFM) analysis of the bilayer films showed diffusion at the interphase for the bilayers prepared with the polyimides and the BPADA/MPDA block copolymers containing polyimide continuous phases. Poly(siloxane imide) multiblock copolymers comprised of 6FDA/MPDA polyimide structures are ideal candidates for controlling interfacial properties between silicon substrates layered with thin films for microelectronic applications. These high Tg materials offer an approach for obtaining reduced moisture absorption and low stress interfaces. Evaluation of the refractive indices of the block copolymer films showed a decrease with increasing siloxane content thus suggesting the possibility of lower dielectric constants. The polymer-metal interfacial properties were investigated for films cast on titanium and tantalum substrates. The results suggested a correlation between the surface hydroxyl concentration of the metal oxide layer with the interfacial properties of the cast poly(siloxane imide) block copolymer films. The surface hydroxyls were thought to hydrogen bond with the PDMS component of the block copolymer. Since the titanium substrate has a higher surface hydroxyl concentration than the tantalum, higher silicon concentrations were observed. The melt imidized end-functional polyimides and poly(siloxane imide) block copolymers produced thermally stable materials with 5% weight loss temperatures well above 400°C. However, the block copolymers showed slightly lower 5% weight loss temperatures as a function of siloxane content with a significant increase in char formation. Correlation of the upper glass transition temperatures with the imide segment length was consistent with findings noted for other phase separated randomly segmented block copolymers. Incorporating PDMS into the polyimide backbone structure has an effect on the bulk and surface properties. The bulk properties of the poly(siloxane imide) block copolymers were characterized using TEM. The morphologies were consistent with classical block copolymers. Surface properties of the block copolymer films as a function of PDMS content were investigated using angular dependent X-ray photoelectron spectroscopy at take-off angles of 15, 30, and 45°. Surface enrichment of PDMS content over that of the bulk was observed at all three sampling depths. Further evidence of this siloxane enrichment in the surface was demonstrated with water contact angle analyses. With as little as 5 weight % PDMS (<Mn> = 5000 g/mol) in the block copolymer there was over a 25% increase in the water contact angle over the polyimide control. The surface topography was influenced by the degree of phase separation and was characterized using AFM. The roughness factor was used to represent the data. It was found that the surface roughness increased with increasing PDMS content. / Ph. D.

La catalyse de polycondensation des silicones, et notamment des élastomères vulcanisables à froid bicomposants, constitue un enjeu scientifique, environnemental et économique. Le dilaurate de dibutyl étain (DLDBE), classé CMR2, très largement utilisé dans l'industrie du silicone sera prochainement interdit par la nouvelle réglementation REACH. L'objectif de cette étude est donc d'étudier de nouveaux systèmes catalytiques des réactions de polycondensation. L'effet des différents constituants de la formulation (masse molaire du PDMS réactif, concentration en réticulant, concentration en catalyseur) sur la cinétique de réticulation et sur les propriétés finales a été étudié par combinaison des techniques de rhéologie, RMN et taux de gonflement. Principalement, ce travail consiste en l'étude approfondie de deux catalyseurs : une guanidine et un complexe de zinc et les résultats montrent que la construction des réseaux est plus ou moins différente selon la catalyse. En effet, un dérivé de l'étain conduit à une construction en deux étapes d'hydrolyse et de condensation, dont l'hydrolyse est d'abord favorisée. En comparaison, les complexes de zinc tendent à se comporter d'une façon très similaire, bien que l'hydrolyse des alcoxysilanes soit plus importante. Au contraire, une catalyse basique favorise très largement la condensation par rapport à l'hydrolyse. La guanidine réagit très rapidement avec les fonctions silanols. Ensuite, l'hydrolyse des alcoxysilanes se produisant dans une moindre proportion, le réseau tridimensionnel se construit. Ces travaux ont donc montré que le complexe de zinc étudié est un bon remplaçant du DLDBE / Silicones condensation catalysis, and more especially room temperature vulcanizable compounds, is a very important scientific, environmental and economic issue. The catalysis species, derivated from tin dilaurate, which are now classified as a CMR2 compound (toxic for reproduction), have been widely used in the silicone industry. The new REACH regulation is going to forbid its use very soon. This study aims to work on new catalytic systems. The influence of the components of the formulation on the kinetics and mechanical properties has been study by different methods: rheology, NMR and swelling measurements. More precisely, this work focuses on two catalysts: one guanidine and zinc complexes. Results have shown that the network construction depends on the catalysis. In fact, while using a tin compound, the construction occurs in two steps, in which hydrolysis is the most important. As a comparison, zinc complexes present the same behaviour as tin compounds even though the hydrolysis step is much more important. On the contrary, it’s known that basic catalysis enhances the condensation step. Then, the guanidine quickly reacts with the silanol functions. Then, in a second step, the alkoxysilanes hydrolysis occurs in a smaller proportion. The tridimensional network is getting built. This work mainly showed that zinc complexes can easily replace tin compounds

Dans l’optique de développement de matériaux performants avec une approche respectueuse de l’environnement, l’obtention de copolymères à blocs de type hard-soft avec une forte proportion de polydiméthylsiloxane (PDMS) en utilisant le procédé d’extrusion est une étape vers des élastomères thermoplastiques d’intérêt. Afin de s’affranchir de la faible réactivité des extrémités de chaînes des longues macromolécules, la voie originale mise en avant consiste en la réalisation de copolymères ayant une partie centrale PDMS courte puis en l’allongement de celle-ci selon les propriétés visées. L’étape critique d’allongement est effectuée à l’aide de bases phosphazènes comme agents de polymérisation de décaméthylcyclopentasiloxane (D5). Dans un premier temps, une approche chemio-rhéologique de la polymérisation du D5 à l’aide de ces superbases a été réalisée. L’acquisition des données intrinsèques de cette réaction permet de mettre au point la modélisation de l’évolution de viscosité du système en cours de réaction, vérifiant ainsi sa compatibilité avec l’utilisation de l’extrusion réactive. Dans un second temps, l’utilisation d’une architecture modèle de PDMS fonctionnalisé en bout de chaîne par des groupements chimiques volumineux de type naphtyl valide l’hypothèse d’allongement du chaînon central par insertion de D5 selon cette catalyse. Finalement, cette approche a été appliquée à des architectures macromoléculaires de type poly(styrène-b-diméthylsiloxane-b-styrène). Dans ce cas, les résultats sont, à cette heure, moins probants. Ceci est potentiellement dû à l’aspect procédé de nos manipulations. Cette dernière observation révèle l’intérêt de l’extrusion dans ce type de synthèse. / Nowadays the development of performing new materials using an environmental friendly route is a challenge. To produce hard-soft block copolymers based on a high polydimethylsiloxane (PDMS) content using reactive extrusion process is a milestone to reach thermoplastic elastomers. Because of the low reactivity of high molecular weight macromolecule chain ends an original route is described. It consists in the synthesis of copolymers containing low central PDMS and then increasing the molecular weight of this central part. This crucial step is performed using phosphazene bases as polymerization agents of decamethylcyclopentasiloxane (D5). Firstly, the polymerization of D5 by phosphazene bases has been investigated by chemiorheological means. To define intrinsic data of this reaction allows modelling the viscosity change during the chemical reaction. Thus, it is observed this polymerization system is compatible with reactive extrusion. Secondly, we investigate the hypothesis of increasing the molecular weight of a short central PDMS part in a triblock copolymer by D5 insertion using the catalysis system previously described. Naphtyl end-chain functionalized PDMS was used as a model. So we confirmed this route as an interesting one to achieve the targeted macromolecular architectures. Finally, we tried to produce poly(styrene-b-dimethylsiloxane-b-styrene) through this way. In this case, early investigations are not so convincing. This may come from the experimental device used. This last observation stresses out the great potential of extrusion process to implement such a route to reach thermoplastic elastomers based on high polysiloxane content.

Polydimethylsiloxan (PDMS)-basierte Materialien finden Verwendung als Schirmmaterial für Verbundisolatoren in der Hochspannungstechnik. Diese Schirmmaterialen benötigen im Außeneinsatz eine herausragende Hydrophobie, um Spannungsüberschläge durch leitfähige Elektrolytfilme zu vermeiden. Im Außeneinsatz widerfährt ein Schirmmaterial unterschiedliche Witterungsbedingungen wie z.B.: Regen oder Betauungsvorgänge. Unter Wirkung hoher elektrischer Felder können dann auf der Oberfläche Tropfenteilentladungen auftreten, die zu einer Hydrophilisierung der Oberfläche führen. Ziel der Arbeit ist es die Materialparameter zu beleuchten, die einen Einfluss auf die Hydrophobiebeständigkeit von Schirmmaterialien besitzen. Kommerziell erhältliche Silikon-Komposite besitzen neben dem eigentlichen Silikonnetzwerk auch Füllstoffe in unbekannter Art und Konzentration, wodurch eine Ursachenfindung für den Hydrophobieverlust erschwert ist. Aus diesem Grund wurde auf eigens synthetisierte Silikonelastomere zurückgegriffen. Diese wurden durch platinkatalysierte Hydrosilylierungsreaktion aus vinylterminierten PDMS (vPDMS) und dem tetrafunktionalem Vernetzer Tetrakis(dimethylsiloxysilan) (TDSS) hergestellt. Durch Variation der Kettenlängen des vPDMS und durch Variation des stöchiometrischen Verhältnisses konnte ein vPDMS-TDSS-Modell-Silikonsystem entwickelt werden, worin Netzwerkparameter (Netzwerkdichte und Sol-Anteil) gezielt justiert werden konnten. Die hergestellten Silikonelastomere wurden anschließend hinsichtlich ihrer Hydrophobie untersucht und im Anschluss gegenüber ihrer Beständigkeit gegenüber Tropfenteilentladungen getestet. Zur Simulation der kombinierten elektrischen und elektrolytischen Beanspruchung wurden dynamische Tropfentests durchgeführt, um die Hydrophobiebeständigkeit zu untersuchen. Im Rahmen der Arbeit wurde festgestellt, dass insbesondere die Steifigkeit der Materialien einen wesentlichen Einfluss auf die Hydrophobiebeständigkeit von Silikonen hat. Zusätzlich beeinflusst die Rauheit eines Materials die Ausfallzeiten im dynamischen Tropfentest maßgeblich. Darüber hinaus führt ein überlagerter Ölfilm auf der strukturierten Oberfläche dazu, dass sich das Abgleitverhalten von Wassertropfen im Laufe der Zeit verändert.

Roughly 123 million people worldwide are affected by conditions such as epilepsy, dementia, and cardiovascular diseases. Wearable electrodes are currently used to monitor these conditions short-term. Long-term monitoring would allow for predicting seizures and could be used as a preventive treatment. As opposed to the currently used electrodes, wearables that are intended for long-term use must be soft and flexible in order not to cause harm or discomfort for the user. The electrodes should also have high resolution, meaning that the electrode paths should be as narrow as possible without negatively affecting the performance of the electrode. In this thesis, soft and flexible electrode grids based on silicones are developed using UV laser patterning. Two different methods are evaluated: laser curing of silicones with the addition of a photoinitiator, and laser ablation of conductive composite. The results found in this thesis are that photocuring silicones gives a too low resolution to be useful for patterning soft electrode grids. UV laser ablation on the other hand showed high resolution while the electrodes retained stretchability. / <p>Examensarbetet är utfört vid Institutionen för teknik och naturvetenskap (ITN) vid Tekniska fakulteten, Linköpings universitet</p>