In my research, I have performed many characterization and fabrication experiments that are based on tools of analytical chemistry, materials chemistry, and surface science. My research projects are as follows. (1) Fabrication of transparent polymer templates for nanostructured amorphous silicon photovoltaics was done using low-cost nanoimprint lithography of polydimethylsiloxane. This approach provides a test bed for absorption studies in nanostructured film geometries and should result in improved light capturing designs in thin-film solar cells. Nanopatterned polymer films were characterized by scanning electron microscopy and optical measurements. (2) A straightforward method for fabricating freely suspended, thin, carbon nanotube (CNT) membranes infiltrated with polymers was developed. This process is a new approach for making thin, reinforced, smooth films or membranes with high concentrations of CNTs, which may lead to higher performance materials. Characterization of the film and membrane was performed via scanning electron microscopy and atomic force microscopy. (3) Laser activation-modification of semiconductor surfaces (LAMSS) was carried out on silicon with a series of 1-alkenes. A key finding from this study is that the degree of surface functionalization in a LAMSS spot appears to decrease radially from the center of the spot. These laser spots were studied by time of flight secondary ion mass spectrometry (ToF-SIMS), and the resulting spectra were analyzed using a series of chemometrics methods. (4) A large ToF-SIMS data set from multiple coal samples spanning a wide range of coal properties was subjected to a chemometrics analysis. This analysis separates the spectra into clusters that correspond to measurements from classical combustion analyses. Thus ToF-SIMS appears to be a promising technique for analysis of this important fuel. (5) Several experiments on carbon nanotube processing were performed in my research, including carbon nanotube sheet formation, carbon nanotube purification, carbon nanotube dispersion, and carbon nanotube functionalization. X-ray photoelectron spectroscopy was a key characterization tool for many of these experiments.

Manufacturing induced shape distortions is a common problem for composite materials. Due to the non-isotropic nature of carbon fibre reinforced polymers (CFRP) unavoidable deformations occur during part production. During fabrication of polymer composites, the material obtains its final shape at elevated temperatures. The curing process involves a transition from the liquid state to the solid, glassy state, allowing bonding between fibres and matrix. As the material cools the mismatch in thermal expansion coefficients and cure shrinkage obtained during the matrix polymerization leads to residual stresses on the mechanical level within composite part. There is a great interest from the aircraft and automotive industries, to increase the ability to understand development of shape distortions and residual stresses during the cure, since these deformations often lead to dissatisfaction of tolerances and it is essential to predict the deformations beforehand in order to compensate time and cost.  In this context, a study of residual stresses during the curing process of thermosetting resin composites is presented. A methodology is proposed for predicting the formation and development of manufacturing- induced residual stresses. The present project reports on a comprehensive experimental study on the dependency of different short curing cycles on the build-up of residual stresses in a carbon fibre/fast-curing epoxy system and evaluate of post-curing methods through induction heating and oven post-curing with unidirectional [904] and unsymmetrical [9020] laminates. It includes characterization in thermo-elastic properties and degree-of-cure of the material by Thermal bending test, thermal expansion test, mechanical tensile test and Differential Scanning Calorimetry (DSC) in non-post-cured and post-cured laminates. The results showed slight variation in the thermal properties and not effect in the mechanical properties at different cure and post-curing conditions. Analytical data by Laminate Analysis program validated the experimental thermo-elastic data with analytical simulations. In addition, it is shown improvements in the temperature distributions in the post-curing by induction heating with different experimental set-ups, however, oven post-curing showed a more systematic system, higher heat efficient a low cure temperature, with more consistent mechanisms of shape distortions and residual stresses compared to induction heating. These findings are relevant for the future development of prediction methods for process induced deformations of Fast Curing Epoxy Resins (FCER).

No description available.

Thermal energy storage (TES) consists in storing heat for a later use, thereby reducing the gap between energy availability and demand. The most diffused materials for TES are the organic solid-liquid phase change materials (PCMs), such as paraffin waxes, which accumulate and release a high amount of latent heat through a solid-liquid phase change, at a nearly constant temperature. To avoid leakage and loss of material, PCMs are either encapsulated in inert shells or shape-stabilized with porous materials or a nanofiller network. Generally, TES systems are only a supplementary component added to the main structure of a device, but this could unacceptably rise weight and volume of the device itself. In the applications where weight saving and thermal management are both important (e.g. automotive, portable electronics), it would be beneficial to embed the heat storage/management in the structural components. The aim of this thesis is to develop polymer composites that combine a polymer matrix, a PCM and a reinforcing agent, to reach a good balance of mechanical and TES properties. Since this research topic lacks a systematic investigation in the scientific literature, a wide range of polymer/PCM/reinforcement combinations were studied in this thesis, to highlight the effect of PCM introduction in a broad range of matrix/reinforcement combinations and to identify the best candidates and the key properties and parameters, in order to set guidelines for the design of these materials. The thesis in divided in eight Chapters. Chapter I and II provide the introduction and the theoretical background, while Chapter III details the experimental techniques applied on the prepared composites. The results and discussion are then described in Chapters IV-VII. Chapter IV presents the results of PCM-containing composites having a thermoplastic matrix. First, polyamide 12 (PA12) was melt-compounded with either a microencapsulated paraffin (MC) or a paraffin powder shape-stabilized with carbon nanotubes (ParCNT), and these mixtures were used as matrices to produce thermoplastic laminates with a glass fiber fabric via hot-pressing. MC was proven more suitable to be combined with PA12 than ParCNT, due to the higher thermal resistance. However, also the MC were considerably damaged by melt compounding and the two hot-pressing steps, which caused paraffin leakage and degradation, as demonstrated by the relative enthalpy lower than 100 %. Additionally, the PCM introduction decreased the mechanical properties of PA12 and the tensile strength of the laminates, but for the laminates containing MC the elastic modulus and the strain at break were not negatively affected by the PCM. Higher TES properties were achieved with the production of a semi-structural composite that combined PA12, MC and discontinuous carbon fibers. For example, the composite with 50 wt% of MC and 20 wt% of milled carbon fibers exhibited a total melting enthalpy of 60.4 J/g and an increase in elastic modulus of 42 % compared to the neat PA. However, the high melt viscosity and shear stresses developed during processing were still responsible for a not negligible PCM degradation, as also evidenced by dynamic rheological tests. Further increases in the mechanical and TES properties were achieved by using a reactive thermoplastic matrix, which could be processed as a thermosetting polymer and required considerably milder processing conditions that did not cause PCM degradation. MC was combined with an acrylic thermoplastic resin and the mixtures were used as matrices to produce laminates with a bidirectional carbon fabric, and for these laminates the melting enthalpy increased with the PCM weight fraction and reached 66.8 J/g. On the other hand, the increased PCM fraction caused a rise in the matrix viscosity and so a decrease in the fiber volume fraction in the final composite, thereby reducing the elastic modulus and flexural strength. Dynamic-mechanical investigation evidenced the PCM melting as a decreasing step in ’; its amplitude showed a linear trend with the melting enthalpy, and it was almost completely recovered during cooling, as evidenced by cyclic DMA tests. Chapter V presents the results of PCM-containing thermosetting composites. A further comparison between MC and ParCNT was performed in a thermosetting epoxy matrix. First, ParCNT was mixed with epoxy and the mixtures were used as matrices to produce laminates with a bidirectional carbon fiber fabric. ParCNT kept its thermal properties also in the laminates, and the melting enthalpy was 80-90 % of the expected enthalpy. Therefore, ParCNT performed better in thermosetting than in thermoplastic matrices due to the milder processing conditions, but the surrounding matrix still partially hindered the melting-crystallization process. Therefore, epoxy was combined with MC, but the not optimal adhesion between the matrix and the MC shell caused a considerable decrease in mechanical strength, as also demonstrated by the fitting with the Nicolais-Narkis and Pukanszky models, both of which evidenced scarce adhesion and considerable interphase weakness. However, the Halpin-Tsai and Lewis-Nielsen models of the elastic modulus evidenced that at low deformations the interfacial interaction is good, and this also agrees with the data of thermal conductivity, which resulted in excellent agreement with the Pal model calculated considering no gaps at the interface. These epoxy/MC mixtures were then reinforced with either continuous or discontinuous carbon fibers, and their characterization confirmed that the processing conditions of an epoxy composite are mild enough to preserve the integrity of the microcapsules and their TES capability. For continuous fiber composites, the increase in the MC fraction impaired the mechanical properties mostly because of the decrease in the final fiber volume fraction and because the MC phase tends to concentrate in the interlaminar region, thereby lowering the interlaminar shear strength. On the other hand, a small amount of MC enhanced the mode I interlaminar fracture toughness (Gic increases of up to 48 % compared to the neat epoxy/carbon laminate), as the MC introduced other energy dissipation mechanisms such as the debonding, crack deflection, crack pinning and micro-cracking, which added up to the fiber bridging. Chapter VI introduces a fully biodegradable TES composite with a thermoplastic starch matrix, reinforced with thin wood laminae and containing poly(ethylene glycol) as the PCM. The wood laminae successfully acted as a multifunctional reinforcement as they also stabilized PEG in their inner pores (up to 11 wt% of the whole laminate) and prevent its leakage. Moreover PEG was proven to increase the stiffness and strength of the laminate, thereby making the mechanical and TES properties synergistic and not parasitic. Finally, Chapter VII focused on PCM microcapsules. The synthesis of micro- and nano-capsules with an organosilica shell via a sol-gel approach clarified that the confinement in small domains and the interaction with the shell wall modified the crystallization behavior of the encapsulated PCM, as also evidenced by NMR and XRD studies and confirmed by DSC results. In the second part of Chapter VII, a coating of polydpamine (PDA) deposited onto the commercial microcapsules MC. The resulting PDA coating was proven effective to enhance the interfacial adhesion with an epoxy matrix, as evidenced by SEM micrographs. XPS demonstrated that the PDA layer was able to react with oxirane groups, thereby evidencing the possibility of forming covalent bond with the epoxy matrix during the curing step.

<p>Cellulose is the most abundant biopolymer on earth, derived from a variety of living species. An attractive source to obtain cellulose is from agriculture wastes, for instance, potato peel. Potato is one of the most important crops for human consumption, but in recent years its consumption in raw form has decreased, especially in developed countries. Many potatoes are processed into value-added products to meet the demand of fast food industries. So far the main use of the potato peel is sold for animal feed at very low prices. In addition, there are significant quantities of rotten potatoes generated during the years of heavy rain fall, which represent a substantial financial loss to the farmers unless an alternative industrial use can be found for the biomass. Therefore, extracting cellulose from potato peel and processing them into a higher valuable product is not only an environment-friendly solution to the disposal issues but also creates a non-food based economy for potatoes.</p> <p>Cellulose nanocrystals (CN) are a promising material and have been widely studied over the past two decades. This material is interesting as nanofiller due to its nanoscale dimensions, high specific area, and highly rigid crystalline structure. In comparison to mineral or metal nanofillers that are industrially available, cellulose nanocrystals are prepared from renewable feedstocks, feature low density, are relatively low cost, and remain biodegradable.</p> <p>This study investigated the effectiveness of cellulose nanocrystal derived from potato peel waste to improve the mechanical and barrier properties of a polymer. The nanocrystals were chemically derived from the cellulosic material in potato peel waste by alkali treatment and subsequently acid hydrolysis with sulfuric acid. Infrared spectroscopy indicated sufficient removal of lignin and hemicellulose from the raw potato peel biomass whereas X-ray diffraction confirmed that the prepared nanocrystals maintained their original crystalline lattice structure as the extracted cellulose, with a crystallinity of 85%. TEM images showed that the average fiber length of the nanocrystals was 410 nm with a diameter of 10 nm (aspect ratio of 41). Cellulose nanocrystal-filled polyvinyl alcohol (PVA) and thermoplastic starch (TPS) were prepared by solution casting method to maintain uniform dispersion of the 1-2% (w/w) fibers. An increase of 19% and 38% (starch composite) and 32% and 54% (PVA composite) in Young’s modulus was observed for the 1% and 2% CN-reinforced composites, respectively. Water vapor transmission rate measurements showed a reduction of water permeability for the PVA nanocomposite, whereas no effect was observed for starch nanocomposite.</p> / Master of Applied Science (MASc)

The application of glass fiber reinforced composites has grown rapidly due to their high strength-to-weight ratio, low cost, and chemical resistance. However, the increasing demand for fiber reinforced composites results in the generation of more composite wastes. Mechanical recycling is a cost-effective and environmentally-friendly recycling method, but the loss in the quality of recycled glass or carbon fiber composite hinders the wide-spread use of this recycling method. It is important to develop novel composite materials with higher recyclability. Thermotropic liquid crystalline polymers (TLCPs) are high-performance engineering thermoplastics, which have comparable mechanical performance to that of glass fiber. The TLCP reinforced composites, called in situ composites, can form the reinforcing TLCP fibrils during processing avoiding the fiber breakage problem. The first part of this dissertation is to study the influence of mechanical recycling on the properties of injection molded TLCP and glass fiber (GF) reinforced polypropylene (PP). The processing temperature of the injection molding process was optimized using a differential scanning calorimeter (DSC) and a rheometer to minimize the thermal degradation of PP. The TLCP and GF reinforced PP materials were mechanically recycled up to three times by repeated injection molding and grinding. The mechanical recycling had almost no influence on the mechanical, thermal, and thermo-mechanical properties of TLCP/PP because of the regeneration of TLCP fibrils during the mold filling process. On the other hand, glass fiber/PP composites decreased 30% in tensile strength and 5% in tensile modulus after three reprocessing cycles. The micro-mechanical modeling demonstrated the deterioration in mechanical properties of GF/PP was mainly attributed to the fiber breakage that occurred during compounding and grinding. The second part of this dissertation is concerned with the development of recyclable and light weight hybrid composites through the use of TLCP and glass fiber. Rheological tests were used to determine the optimal processing temperature of the injection molding process. At this processing temperature, the thermal degradation of matrix material was mitigated and the processability of the hybrid composite was improved. The best formulation of TLCP and glass fiber in the composite was determined giving rise to the generation of a recyclable hybrid composite with low melt viscosity, low mechanical anisotropy, and improved mechanical properties. Finally, TLCP reinforced polyamide composites were utilized in an additive manufacturing application. The method of selecting the processing temperature to blend TLCP and polyamide in the dual extrusion process was devised using rheological analyses to take advantage of the supercooling behavior of TLCP and minimize the thermal degradation of the matrix polymer. The composite filament prepared by dual extrusion was printed and the printing temperature of the composite filament that led to the highest mechanical properties was determined. Although the tensile strength of the TLCP composite was lower than the glass fiber or carbon fiber composites, the tensile modulus of 3D printed 60 wt% TLCP reinforced polyamide was comparable to traditional glass or carbon fiber reinforced composites in 3D printing. / Doctor of Philosophy / The large demand for high performance and light weight composite materials in various industries (e.g., automotive, aerospace, and construction) has resulted in accumulation of composite wastes in the environment. Reuse and recycling of fiber reinforced composites are beneficial from the environmental and economical point of view. However, mechanical recycling deteriorates the quality of traditional fiber reinforced composite (e.g., glass fiber and carbon fiber). There is a need to develop novel composites with greater recyclability and high-performance. Thermotropic liquid crystalline polymers (TLCP) are attractive high performance materials because of their excellent mechanical properties and light weight. The goal of this work is to generate recyclable thermotropic liquid crystalline polymer (TLCP) reinforced composites for use in injection molding and 3D printing. In the first part of this work, a novel recyclable TLCP reinforced composite was generated using the grinding and injection molding. Recycled TLCP composites were as strong as the virgin TLCP composites, and the mechanical properties of TLCP composites were found to be competitive with the glass fiber reinforced counterparts. In the second part, a hybrid TLCP and glass fiber reinforced composite with great recyclability and excellent processability was developed. The processing conditions of injection molding were optimized by rheological tests to mitigate fiber breakage and improve the processability. Finally, a high performance and light weight TLCP reinforced composite filament was generated using the dual extrusion process which allowed the processing of two polymers with different processing temperatures. This composite filament could be directly 3D printed using a benchtop 3D printer. The mechanical properties of 3D printed TLCP composites could rival 3D printed traditional fiber composites but with the potential to have a wider range of processing shapes.

Vacuum assisted resin transfer molding (VARTM) is a low cost resin infusion process being developed for the manufacture of composite structures. VARTM is being evaluated for the manufacture of primary aircraft structures, including foam sandwich composite materials. One of the benefits of VARTM is the ability to resin infiltrate large or complex shaped components. However, trial and error process development of these types of composite structures can prove costly and ineffective. Therefore, process modeling of the associated flow details and infiltration times can aide in manufacturing design and optimization. The purpose of this research was to develop a process using VARTM to resin infiltrate stitched and unstitched dry carbon fiber preforms with polymethacrylimide foam cores to produce composite sandwich structures. The infiltration process was then used to experimentally verify a three-dimensional finite element model for VARTM injection of stitched sandwich structures. Using the processes developed for the resin infiltration of stitched foam core preforms, visualization experiments were performed to verify the finite element model. The flow front progression as a function of time and the total infiltration time were recorded and compared with model predictions. Four preform configurations were examined in which foam thickness and stitch row spacing were varied. For the preform with 12.7 mm thick foam core and 12.7 mm stitch row spacing, model prediction and experimental data agreed within 5%. The 12.7 mm thick foam core preform with 6.35 mm row spacing experimental and model predicted data agreed within 8%. However, for the 12.7 mm thick foam core preform with 25.4 mm row spacing, the model overpredicted infiltration times by more 20%. The final case was the 25.4 mm thick foam core preform with 12.7 mm row spacing. In this case, the model overpredicted infiltration times by more than 50%. This indicates that the model did not accurately describe flow through the needle perforations in the foam core and could be addressed by changing the mesh elements connecting the two face sheets. / Master of Science

Ziel der Untersuchung war es, eine Charakterisierung des Dauerlaufverhaltens dynamisch und tribologisch belasteter Maschinenteile aus Wood Polymer Composite (WPC) in tragenden Anwendungen durchzuführen. Zu diesem Zweck wurde ein Hybridprofil aus Aluminium und WPC (70% PP, 30 % Weichholz) zu dem bestehenden Profil aus Aluminium in ein Hängefördersystem eingebaut und die dynamische Belastung (Vertikalbeschleunigung) auf das Fördergut während des Anlagenbetriebes gemessen. Im Testbetrieb wurde das System mit Ersatzlasten beladen und die Amplituden der Vertikalbeschleunigung zu Versuchsbeginn und deren Veränderung nach 1000h Laufzeit hinsichtlich beider Profilarten bewertet und das Hybridprofil auf sichtbare Schäden überprüft. Zu Beginn und nach 1000h Versuchszeit waren keine relevanten Veränderungen des charakteristischen Beschleunigungsbildes bezüglich beider Profilaufbauten feststellbar. Trotz Verschleiß am Antrieb des Fördersystems blieb das Hybridprofil voll funktionsfähig (kein sichtbarer Verschleiß, keine merkliche Schädigung). Darauf aufbauend wird der Versuch mit höherer Belastung fortgesetzt und die Entwicklung eines Profils, welches nur aus WPC besteht, vorangetrieben. / Aim of the study was to analyze the characteristics of dynamic and tribological stressed wood polymer machine elements. For this purpose an endurance test with an overhead conveyor was executed. A sectional beam in hybrid design (Aluminum & WPC [70 % PP / 30 % softwood]) was implemented in this aluminum beam overhead conveyor. During the conveying process the vertical acceleration of the transported material (dummy loads) was measured. At the beginning of the endurance test the acceleration patterns of the aluminum and the hybrid beams did not differ. Despite of mechanical wear of the drive system after 1000 operating hours, alterations of the acceleration patterns still could not be detected in the different beams. The hybrid beam remained fully functional with no visible wear or damage after 1000 operating hours. Based on these encouraging results the endurance test is continued with enhanced dummy loads and the development of a beam completely composed of WPC is aspired.

The sectional beam is the essential detail of an overhead conveyor. The construction element is stressed by time-varying mechanical and tribological loads. Here, we discuss the influence of the manufacturing process on the mechanical properties and the serviceability of the extruded profile in a selected application. The development of existing formulas and processing parameters are shown with the objective to expand the material application from WPC-decking to use in mechanical engineering. / Das Tragprofil ist das zentrale Element in einem Hängefördersystem. Das Bauteil wird durch zeitlich veränderliche mechanische und tribologische Lasten beansprucht. Nachfolgend wird der Einfluss des Herstellungsprozesses auf die mechanischen Eigenschaften und die Gebrauchsfähigkeit eines extrudierten Trag- und Gleitprofils aus WPC im gewählten Anwendungsfall vorgestellt. Die notwendige Weiterentwicklung bestehender Rezepturen und Verarbeitungsverfahren wird aufgezeigt, um den Anwendungsbereich des Werkstoffes WPC vom Bereich Terassendielen auf den Maschinenbau zu erweitern.

A exploração não racional dos recursos florestais nativos, no Brasil, tem provocado a redução da oferta de espécies de uso consagrado em diversos segmentos, notadamente na construção civil e na indústria de móveis. A alternativa mais imediata tem sido o emprego da madeira de reflorestamento, em particular, dos gêneros Eucalyptus e Pinus, freqüentes nas regiões sul e sudeste do país. Porém, muitas das espécies disponíveis comercialmente não apresentam propriedades fisico-mecânicas que as tornem capazes de promover a substituição das espécies tradicionalmente empregadas. Neste contexto, o presente trabalho tem como objetivo demonstrar a viabilidade de se obterem compósitos polímero-madeira (CPMs) que podem apresentar comportamento equivalente ou superior ao da madeira sem tratamento, proveniente das regiões de reflorestamento dos mencionados gêneros. Para tal, foi feita a impregnação da madeira das espécies Eucalyptus grandis e Pinus caribaea var. hondurensis por apresentarem maior disponibilidade e densidade compatível para viabilizar o processo. Empregaram-se os monômeros poliméricos de estireno e metacrilato de metila e, como iniciador no processo de polimerização, o peróxido de benzoíla. Foi utilizado o método de vácuo-pressão para a impregnação da solução monômero-iniciador. Os resultados mostraram, para os CPMs de Pinus, um significativo aumento nos valores de todas as propriedades estudadas. Nos CPMs de Eucalyptus, em decorrência de sua baixa permeabilidade, somente os valores das durezas paralela e normal às fibras apresentaram aumento. / Large exploration of native forestry resources in Brazil has led to a decrease in supply of the most widely used species in several sectors, notably in civil construction and furniture industries. The most immediate remedy has been the use of reforestation timber, obtained from the common Eucalyptus and Pinus species available in the south and southwest regions of the country. However, most of these species do not present adequate physical and mechanical properties to viably the mentioned uses. In this context, the main objective of the present research is to demonstrate the viability of obtaining wood-polymer composites (WPCs), which may exhibit similar or superior physical and mechanical properties than untreated aforementioned species from reforestation regions of Brazil. In order to achieve this goal, the impregnation of Eucalyptus grandis and Pinus caribaea var. hondurensis, which have compatible density and are available in large amounts to permit such processing, was carried out. In the process, polymeric monomers of styrene and methyl methacrylate were employed with benzyl peroxide, whose functions as an initiator in the polymerization process. Vacuum-pressure method was used in the impregnation of the monomer-initiator solution. Properties of the WPCs – Pinus were significantly improved in all tests, however, because of its low permeability, only hardness parallel and perpendicular to grain showed increase for the WPCs – Eucaliptus.