This work aims to investigate the vapor sensing behavior of conductive polymer composites (CPCs). In connection with the protection of the environment and human beings, sensing of different kinds of chemical vapors is of increasing importance. At the moment, four kinds of vapor sensors are widely investigated and reported, namely semiconducting metal oxide sensors (MO), conjugated polymer sensors, carbonaceous nanomaterial based sensors, and CPC based sensors. Due to their unique component systems, the different sensor types are based on different sensing mechanisms resulting in different potential application ranges. In consideration of cost and processability, CPC based vapor sensors are promising owning to their low cost, excellent processability, and designable compositions. In terms of vapor sensing behavior of CPC sensors, the interaction between the polymer and the organic vapor is a decisive factor in determining the sensing performance of CPCs. Ideally, the chosen polymer matrix should be able to swell without dissolving during vapor exposure so that the conductive network within the matrix can be disconnected, giving rise to the resistance change of CPCs. In some reported cases, polymers such as PLA and polycaprolactone (PCL) are degradable polymers, which are not durable when being exposed to environmental conditions for a long time. Therefore, it is necessary to make sure whether the selected polymers are resistive to vapors or not. There are two options for the polymer selection. One is to select a polymer that is only swellable in a specific or few organic solvents; another one is to select a polymer that is swellable to a variety of solvents. Since CPC sensors are used for detecting as many as possible hazardous chemicals to human beings or environment, the second case is more desired because of its broader window of detection. The solubility parameter is effective to characterize the interaction of polymers and organic solvents/vapors, which was firstly proposed by Charles Hansen. Initially, the Hansen solubility parameter (HSP) was used to predict the compatibility between polymer partners, chemical resistance, permeation rates, and even to characterize the surface of fillers. Liquids with similar solubility parameter (δ) are miscible, and polymers will dissolve in solvents whose δ is similar to their own value. This behavior is recognized as “like dissolves like”. Based on the description above, CPCs that can be used as liquid/vapor sensor materials should meet the following two requirements: 1) the chosen polymer should be swellable to vapors; 2) the CPCs as sensor materials have to be electrically conductive. Therefore, the relationship between conductive network and vapor sensing behavior of CPCs was investigated from the following aspects: 1) According to the previous studies, CB/polymer composites exhibit poor reversibility in cyclic vapor sensing tests because of the susceptible conductive network formed by CB particles. Thus, there is a need to improve the reversibility and increase the relative resistance change (Rrel) of CPCs. MWCNTs, as 1-dimensional carbon fillers with high aspect ratio, have excellent electrical and mechanical properties. Therefore, a hybrid filler system (MWCNT and CB) was utilized and incorporated in polycarbonate (PC) via melt compounding. PC was selected as the polymer matrix of CPCs because it showed high affinity with many commercial organic solvents/vapors as well as high and fast volume change upon organic solvents/vapors. In order to discuss the effect of conductive network formation on the vapor sensing behavior of PC/MWCNT/CB composites, two MWCNT contents were selected, which were lower and higher than the electrical percolation threshold of the PC/MWCNT composites. In the following, three CB contents were selected for the mixtures with MWCNT. The conductive networks composed of either MWCNT or hybrid CB/MWCNT are compared. The morphology of CPCs with different hybrid filler ratios was observed and investigated using SEM and OM. Moreover, to quantify the vapor sensing behavior of CPCs, some organic solvents were chosen and characterized by Flory-Huggins interaction parameter to demonstrate the polymer-vapor interaction. Afterwards, the cyclic vapor sensing was applied to illustrate the vapor sensing behavior of CPCs with different conductive network formations. 2) At moment, the filler dispersion is still a big challenge for MWCNT filled polymer composites due to the fact that the strong Van der Waals force among nanotubes makes them easily to entangle with each other resulting in the formation of agglomerates. A good filler dispersion state is desirable to achieve CPCs with low φc and. In order to reduce the φc of CPCs, immiscible polymer blend systems are introduced, which can have different blend microstructures by adjusting the polymer component ratios. In the second section, an immiscible polymer blend system based on two amorphous component, namely PC and polystyrene (PS), was chosen aiming to explain the influence of the blend morphology on the sensing performance of CPCs. PC/PS blends with different compositions filled with MWCNT were fabricated by melt mixing. The selective localization of MWCNTs in the blends was predicted using the Young’s equation. Moreover, the composite morphology, filler dispersion, and distribution were characterized by SEM and TEM. In the following, three kinds of CPCs ranging from sea-island structure to co-continuous structure were selected for the cyclic sensing measurement. The relationship between composite microstructure and resulting vapor sensing behavior was evaluated and discussed. 3) The poor reversibility of CPCs towards good solvent vapors is still a problem that hinders the cyclic use of CPC sensor materials. As an important class of polymer, crystalline polymers are rigid and less affected by solvent penetration because of the well-arranged polymer chains. Therefore, the effect of polymer crystallinity on the vapor sensing behavior of CPCs is imperative to be studied. In the third section, poly(lactic acid) (PLA), a semi-crystalline polymer, was selected to melt-mixed with PS and MWCNTs with the aim to improve the sensing reversibility of CPCs towards organic vapors, especially good solvent vapors. Thermal annealing was utilized to tune the PLA crystallinity and the polymer blend microstructure of CPCs. The electrical, morphological, and thermal behavior of CPCs after different thermal annealing times is discussed. In the following, the effect of crystallinity on the vapor sensing behavior of the CPCs was studied in detail. Besides, the different sensing performances of the CPCs towards different vapors resulted from the selective localization of MWCNTs and increased polymer matrix crystallinity were investigated and compared. 4) As discussed for the amorphous polymer blends and crystalline polymer blends and their vapor sensing behavior. The comparison of compact and porous structure of CPCs is going to be studied. In the fourth section, studies to further improve the sensing performance and to find out the exact sensing mechanism of CPCs were performed. Therefore, poly(vinylidene fluoride) (PVDF), a solvent resistive polymer, was chosen to be melt-mixed with PC and MWCNTs. In order to compare the MWCNT dispersion and localization in the blends, three kinds of PCs with different molecular weights were selected; hence, the viscosity ratio of immiscible blends was varied. Rheological, morphological, and electrical properties of CPCs were characterized. After that, the cyclic sensing and long-term immersion tests of CPCs towards different vapors were carried out to evaluate the vapor sensing behavior of compact CPCs with different blend viscosity ratios. Moreover, porous CPC sensors were prepared by extracting the PC component. The same sensing protocols were also applied to these porous sensor materials. The sensing mechanisms between compact CPC sensor and porous CPC sensor were compared and investigated.

The study shows strong influence of the conformation of the main chains of polythiophenes in solutions on their spectroscopic properties. The conformational changes affect strength of interactions between neighboring chains and lead to the formation of aggregates of stacked polythiophene chains. Stacked chains show red shift of the optical absorption and lead to visible vibronic structure of absorption bands. Multilayered systems prepared by periodic adsorption of the cationic and anionic polythiophene polyelectrolytes on various substrates show proportionality of the overall layer thickness to the number of coatings, which allows a control of the layer thickness. Adsorption of the conjugated polyelectrolytes on the anatase form of titanium dioxide show better penetration of the anionic polythiophene to porous anatase. The adsorption of polymer into the mesoporous structure is not complete: maximum adsorption of PTTA (poly(thiophene-3-ylacetic acid)) adsorbed on anatase do not correspond to the inner surface of mesoporous anatase. Alternating adsorption from the solutions of polythiophene with anionic and cationic side groups on mesoporous anatase allows preparation of the Graetzel-like photovoltaic cell provided that the adsorption process starts with the anionic polythiophene and the layers are deposited...

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Fiber sizings are used to improve the performance of fiber-reinforced polymer composites made from low-cost fiber and matrix materials. Evaluation of three sizings, poly(vinylpyrrolidone) (PVP), a carboxyl modified polyhydroxyether (PHE), and a standard industrial sizing (G'), have revealed tremendous improvements in static mechanical and enviro-mechanical properties. The focus of this work is to determine if these improvements in performance can be ascertained from a micromechanical test for interfacial shear strength (IFSS) on as-processed materials. The accomplishment of this goal would create more information with fewer experiments and a need for less experimental materials. In this study, a nanoindenter uniquely outfitted with a blunt tip is effectively used to obtain microindentation results where the debond load is extracted directly from the experimental load-deflection curve. Shear lag and finite element analyses are used to evaluate the mechanics of the system, but both methods show limitations with regard to determining interfacial stresses in an experimental system. In the results obtained, the PHE and Gâ materials outperform the PVP in IFSS, but the bulk properties for PVP and PHE outperform the Gâ material, suggesting the presence of another dominant mechanism. Despite better retention of bulk properties after hygrothermal exposure, PHE experiences degradation in IFSS that PVP does not. The PHE loses 10% of its original IFSS after 576 hours of 65ºC moisture exposure, while PVP improves by 25%. The tensile strengths for PHE and PVP decrease 7% and 10% respectively at 576 hours exposure. Finite element modeling shows that matrix swelling due to moisture absorption increases interfacial shear stresses, a finding supported by a comparison of wet and dry specimens subjected to equivalent aging times. Matrix swelling is not, however, responsible for the increase in IFSS of the PVP material. The relationship between tensile strength and IFSS proves to be small as predicted by a tensile strength model, but processing defects and other failure processes that are not included in the tensile strength model appear to have strong influences over the experimental results. IFSS is important in composite materials, but in the case of the G', PHE and PVP materials, other factors dominate fiber direction tensile performance. Therefore, this one simple micromechanical test provides significant insight into the composite material behavior, but it does not provide the same magnitude of information as from bulk composite experiments. / Master of Science

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Получены композиции на основе альгината натрия и фторкаучука СКФ-26, наполненные частицами магнитных материалов: Ni, Fe, Fe3O4, SrFe12O19, MQP-Системы изучены методами изотермической микрокалориметрии, механического динамического анализа и магнитометрии. Методом микрокалориметрии измерены энтальпии смешения компонентов композитных пленок и рассчитаны значения предельной энтальпии адгезии полимерных матриц к поверхности магнитных порошков. Показано, что для систем на основе СКФ-26 энтальпия адгезии является отрицательной величиной, абсолютные значения которой увеличиваются в ряду Ni-Fe-Fe3O4. В системах альгинат натрия/Ni и альгинат натрия/Fe реализуется атермическое смешение, а в системах альгинат натрия/ФС и альгинат натрия/ Fe3O4 – экзотермическое. С помощью метода ДМА получены концентрационные зависимости динамических модулей потерь и упругости, а также угла сдвига для систем на основе раствора альгината натрия и магнитных порошков Fe, Fe3O4 и SrFe12O19 при частоте 1 Гц и напряжении 1 Па. Обнаружено, что с увеличением концентрации частиц неорганического вещества в суспензиях наблюдается увеличение динамических модулей и уменьшение угла сдвига, причем для суспензии, содержащей магнетит, характер течения не меняется. При течении суспензий, содержащих порошки Fe или SrFe12O19 начинают преобладать упругие свойства. Методом магнитометрии для систем СКФ-26/Ni и CКФ-26/Fe были получены петли магнитного гистерезиса в диапазоне напряженности магнитного поля от -15 кЭ до 15 кЭ. Показано, что с ростом содержания порошка в композите остаточная намагниченность и намагниченность насыщения не меняются, а коэрцитивная сила уменьшается, т. е. на размагничивание магнитного материала требуется меньше энергии. Методом магнитометрии получены данные для систем на основе альгината натрия, содержащих 30% Fe или Fe3O4. Обнаружено, что для системы альгинат натрия/Fe значения намагниченности насыщения для композита больше, чем для индивидуального порошка Fe, что может быть обусловлено упорядочением спинов на поверхности магнитных частиц при формировании композита. / Compositions based on sodium alginate and SKF-26 fluoro-rubber filled with particles of magnetic materials: Ni, Fe, Fe3O4, SrFe12O19, MQP-systems were studied by isothermal microcalorimetry, mechanical dynamic analysis and magnetometry. The enthalpy of mixing of composite film components was measured by microcalorimetry and the values of the limiting enthalpy of adhesion of polymer matrices to the surface of magnetic powders were calculated. It is shown that for systems based on SKF-26, the enthalpy of adhesion is a negative value, the absolute values of which increase in the series Ni-Fe-Fe3O4. In the sodium alginate/Ni and sodium alginate/Fe systems, athermic mixing is realized, and in the sodium alginate/FS and sodium alginate/ Fe3O4 systems, exothermic mixing is realized. The DMA method is used to obtain the concentration dependences of the dynamic loss and elasticity modulus, as well as the shear angle, for systems based on a solution of sodium alginate and magnetic powders Fe, Fe3O4, and SrFe12O19 at a frequency of 1 Hz and a voltage of 1 Pa. It is found that with an increase in the concentration of inorganic matter particles in the suspensions, an increase in the dynamic modules and a decrease in the shear angle are observed, and for a suspension containing magnetite, the flow pattern does not change. During the course of suspensions containing Fe or SrFe12O19 powders, elastic properties begin to prevail. Magnetic hysteresis loops in the magnetic field strength range from -15 kE to 15 kE were obtained by magnetometry for the SKF-26/Ni and SKF-26/Fe systems. It is shown that with an increase in the powder content in the composite, the residual magnetization and saturation magnetization do not change, and the coercive force decreases, i.e., less energy is required to demagnetize the magnetic material. Data for systems based on sodium alginate containing 30% Fe or Fe3O4 were obtained by magnetometry. It is found that for the sodium alginate/Fe system, the saturation magnetization values for the composite are greater than for the individual Fe powder, which may be due to the ordering of spins on the surface of magnetic particles during the formation of the composite.

<p>Polymer based multifunctional material systems (MFMS) have gained increasing attention in the past two decades. The addition of nanofillers and nanoparticles allows for modification of physical properties as well as the discovery of new features. Multifunctionalization of composites allows us to “do more with less”. For example, electrically conductive additives can eliminate the need for sensors through self-sensing principles, shape morphing matrices can reduce the need for actuators, and the inclusion of fire-resistant constituents can reduce flammability in stringent fire protection measures. With added capabilities, the applications of multifunctional composites extends beyond the aerospace and automotive industries to healthcare, infrastructure, electronics, among others, and optics.</p> <p><br></p> <p>The current state of the art is largely focused on single-filler composites or multifiller composites with complementary attributes. For example, carbon nanotubes (CNTs) when mixed with graphene produces higher conductivity than can be achieved via modification with either CNTs or graphene alone. The majority of investigations conducted in this domain have fillers selected with the aim of imparting a singular property. Much less has been done in the area of multifiller and multifunctional polymer matrix composites (PMCs) which can exhibit multiple properties. Consequently, this work seeks to contribute towards the field of synergistic functional composites. That is, a multifiller composite material system comprised of differently functional fillers. This approach has potential to yield smart material systems that outperform single-filler or single-functionality materials through the discovery of novel synergistic coupling between the differently functional phases.</p> <p><br></p> <p>In light of the preceding motivation, this work presents the results on the experimental electromagnetic and mechanical characterization of multi-walled carbon nanotubes (MWCNTs) + carbon-coated iron nanoparticle (CCFeNP)-modified polymers. Carbon nanotubes with their electrical properties and iron nanoparticles with their magnetic attributes present potential for synergistic electromagnetic interactions in a well-percolated network. We report on the electro-magnetic properties of MWCNT + CCFeNP/epoxy composites including DC and AC conductivity, dielectric permittivity, magnetic permeability, and piezoresistance as a function of varying relative MWCNT and CCFeNP concentrations. The results are in a large part linked to the manufacturing process described herein. This work seeks to establish the foundations of synergistic functional filler combinations that could lead to new multifunctional capabilities in the future.</p>

The research field of composite materials is particularly fascinating due to the design freedom they offer and the infinite number of constituent combinations, including those that are already explored, and many more that are yet to be tried. One composite material that holds great potential contains carbon in its fiber shape. Carbon fibers possess unique properties that excel in mechanical aspects, as well as interesting electrical and thermal properties that are yet to be fully explored. These fibers are readily available on the market and can be introduced as reinforcement in various lengths and orientations, yielding diverse results depending on the intended effect. Although carbon fiber reinforced polymer composites (CFRP) are present on the market for quite some time, specifically in high-performance applications, they are predominantly used when their performance outweighs their cost. Meanwhile, carbon fiber composite waste is starting to cumulate in noticeable amounts. This waste originates from both, production scrap and end-of-life scenarios, as components introduced in service life in the past 30 years are being decommissioned and discarded. Unfortunately, the prevalent solution for handling this waste is landfilling, due to its ease, affordability, and accessibility. Consequently, substantial amounts of composite waste are accumulating worldwide. Furthermore, it has finally come to our attention that our planet's resources are finite. Our exploitation of these resources has been largely devoid of consideration for the needs of future generations. As a result, recently, sustainability has emerged as a key enabler for a circular economy, driven by increasing environmental concerns and demands from customers and users for market transformation. The implementation of sustainable practices is now underway, albeit at a gradual pace.   In summary, we find ourselves facing a trifold predicament: a splendid material being underutilized due to production costs, the cumulative generation of CFRP waste resulting from a lack of foresight and suitable alternatives, and the urgent need to transition towards a circular economy due to resource depletion. This research work aims to address all three challenges by developing an integrated solution.   The current work demonstrates that it is possible to recycle carbon fiber model composites through a two-step pyrolysis treatment, a fully mature recycling technology. The study has been done in two stages which are presented in two journal papers included in the thesis. The primary objective of the first paper is to identify and optimize process parameters that maximize the retention of mechanical properties in the recovered fibers. The overall results achieved show good retention value; with over 90% retention on stiffness and 90% on strength. Encouraging results from initial experimental work, have spurred the research focus towards further investigation. Thus, the second paper reports on repetitive manufacturing and recycling cycles of two sets of identical model composites by using the two most effective recycling treatments identified through the parameter optimization. The mechanical performance and structural changes of the recycled fibers are characterized and analyzed. Although further analysis is required, current mechanical behavior shows recovered fibers suitable for secondary applications after two recycling cycles, with an abrupt decay in fiber properties after the third cycle.   With the waste challenge under control, through successful recycling of composite waste, it is time to find concrete applications for this research. Having recycled carbon fibers (rCF) with comparable performance to virgin carbon fibers (vCF) opens up opportunities for rCF mats and other intermediate products to compete in previously inaccessible markets.