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No description available.

The purpose of this investigation was to examine the effects of thermal history during cooling from the melt on the degree of crystallinity, morphology, and mechanical properties of (polyphenylenesulfide) PPS/carbon fiber composites. Three thermal treatments were employed in this study: isothermal crystallization from the melt at 140,160,180,200, and 220°C, quenching from 315° C and then annealing at 160 and 200° C, and nonisothennal crystallization from the melt at rates varying from 0040 C/minute to 68° C/second. The effect of varying the thermal history of the sample on the degree of crystallinity developed in the matrix polymer was determined using differential scanning calorimetry. The effect of thermal history on and the resulting matrix morphology was examined by scanning electron microscopy. The subsequent effects of the degree of crystallinity and the morphology on the mechanical behavior of the samples were monitored by transverse tensile tests and flexural tests. In all cases, the transverse tensile and flexural moduli increased as the amount of crystallinity in the samples increased. However, samples with greater amounts of crystallinity did not always yield higher transverse tensile or flexural strengths. Upon examination of the composite samples by electron microscopy, it was observed that trends in the values of the transverse tensile and flexural strengths could be correlated with structural changes in the matrix. This paper is concerned with the simulation of the development of crystallinity and morphology (both amount of crystallinity and the size of spherulites) which arise during the cooling of a slab of a semicrystalline polymer reinforced with long continuous carbon fibers. This situation is commonly found during the processing of semicrystalline thermoplastic composites. Whereas published attempts at simulating this process have treated the composite material as a continuum and thereby used mass averaged physical properties (such as thermal conductivity, density, and specific heat), we use a quasi-continuum approach in which locally we consider the properties of the matrix and fiber separately. Once a temperature distribution is calculated using the continuum approach, the fmite element method is applied locally at various points in the slab to calculate the amount of crystallinity and the size of the developing spherulites. This is done by using the Avrami equation and the Hoffman and Lauritzen radial growth equation. The amount of crystallinity and the spherulite size are predicted as a function of fiber spacing and packing geometry, and the predictions are found to be in good agreement with experimental results obtained on polyphenylenesulfide/carbon fiber composites. The advantages of our approach over the continuum approach is that a relatively accurate prediction of the spherulite size is possible due to constraints imposed by the fiber on the spherulitic growth. / Ph. D.

The adhesion of carbon fibers to several high performance polymers, including a phosphorus-containing bismaleimide, a cyanate ester resin, and a pyridine-containing thermoplastic, was evaluated using the microbond single fiber pull-out test. The objective was to determine the chemical and mechanical properties of the fiber and the polymer which affect the fiber/polymer adhesion in a given composite system. Fiber/matrix adhesion is of interest since the degree of adhesion and the nature of the fiber/matrix interphase has a major influence on the mechanical properties of a composite. The surface chemical composition, topography, tensile strength, and surface energy of untreated AU-4 and commercially surface treated AS-4 carbon fibers were evaluated using x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), single fiber tensile tests, and dynamic contact angle analysis. The commercial surface treatment which converted the AU-4 to the AS-4 fiber oxidized the carbon fiber surface. The surface of the AS-4 carbon fiber was further modified using air, oxygen, ammonia, and ethylene plasmas. The AS-4 fiber tow was also characterized following exposure to the aqueous poly(amic acid) solution used to disperse the matrix powder during aqueous suspension prepregging of thermoplastic matrix composites. The air and oxygen plasma treatments significantly oxidized and roughened the surface of the AS-4 carbon fibers. In addition, the air and oxygen plasma increased the the polar component of the AS-4 fiber surface energy. The ammonia plasma increased the concentration of nitrogen on the fiber surface, without significantly altering the fiber topography (at a nlagnification of 50,000X). The atomic oxygen present in the air and oxygen plasma treatments is capable of reacting with both the edge and basal planes in the carbon fiber structure. As a result, the oxygen-containing plasmas progressively ablated the organic material in the carbon fiber surface. Energetic species in the ammonia plasma cleaned the fiber surface and reacted with the carbon fiber surface, increasing the concentration of amine groups in the fiber surface. The ethylene plasma deposited a layer of plasma polymerized polymer on the carbon fiber surface. The AS-4 carbon fibers were coated with poly(amic acid) when the tow was wet with the aqueous suspension prepregging solution. The carbon fiber adhesion of bis(3-maleimido phenoxy) triphenylphosphine oxide was compared to that of Ciba-Geigy's Matrimid 5292 A/B bismaleimide system. With both bismaleimides, the carbon fiber adhesion increased significantly when the fiber received an oxidative commercial surface treatment or was exposed to an air or ammonia plasma prior to bonding. In contrast, the poly(pyridine-bis A) microbond pull-out test results showed that the carbon fiber adhesion of poly(pyridine-bis A) was not affected by the fiber surface chemical composition, fiber surface energy, or topography. / Ph. D.

Polyacrylonitrile (PAN) and its copolymers are used in a wide variety of applications ranging from textiles to purification membranes, packaging material and carbon fiber precursors. High performance polyacrylonitrile copolymer fiber is the most dominant precursor for carbon fibers. Synthesis of very high molecular weight poly(acrylonitrile-co-methyl acrylate) copolymers with weight average molecular weights of at least 1.7 million g/mole were synthesized on a laboratory scale using low temperature, emulsion copolymerization in a closed pressure reactor. Single filaments were spun via hybrid dry-jet gel solution spinning. These very high molecular weight copolymers produced precursor fibers with tensile strengths averaging 954 MPa with an elastic modulus of 15.9 GPa (N = 296). The small filament diameters were approximately 5 'm. Results indicated that the low filament diameter that was achieved with a high draw ratio, combined with the hybrid dry-jet gel spinning process lead to an exponential enhancement of the tensile properties of these fibers. Carbon fibers for polymer matrix composites are currently derived from polyacrylonitrile copolymer fiber precursors where solution spinning accounts for ~40 % of the total fiber production cost. To expand carbon fiber applications into the automotive industry, the cost of the carbon fiber needs to be reduced from $8 to ~$3-5. In order to develop an alternative melt processing route several benign plasticizers have been investigated. A low temperature, persulfate-metabisulfite initiated emulsion copolymerization was developed to synthesize poly(acrylonitrile-co-methyl acrylate) copolymers with acrylonitrile contents between 91-96 wt% with a molecular weight range of 100-200 kg/mol. This method was designed for a potential industrial scale up. Furthermore, water was investigated as a potential melting point depressant for these copolymers. Twenty-five wt% water lead to a decrease in the Tm of a 93/7 wt/wt % poly(acrylonitrile-co-methyl acrylate) of Mw = 200 kg/mol to 160 0C as measured via DSC. Glycerin, ethylene glycol and glycerin/water combinations were investigated as potential plasticizers for high molecular weight (~200,000 g/mol), high acrylonitrile (93-96 mole:mole %) content poly(acrylonitrile–co-methyl acrylate) statistical copolymers. Pure glycerin (25 wt %) induced crystallization followed by a reduced "Tm" of about 213 °C via DSC. However this composition did not melt process well. A lower MW (~35 kg/mol) copolymer did extrude with no apparent degradation. Our hypothesis is that the hydroxyl groups in glycerin (or water) disrupt the strong dipole-dipole interactions between the chains enabling the copolymer endothermic transition (Tm) to be reduced and enable melting before the onset of degradation. Additionally high molecular weight (Mw = 200-230 kg/mol) poly(acrylonitrile–co-methyl acrylate) copolymers with lower acrylonitrile content (82-85 wt %) were synthesized via emulsion copolymerization and successfully melt pressed. These materials will be further investigated for their utility in packaging applications. / Ph. D.

The transition to a Circular Economy scheme that enables a more efficient usage of the resources is one of the most pressing needs in our society. From the industrial perspective this has been translated into new design philosophies and the search for more efficient systems. Polymeric composites have played a key role in the development of lighter components with good mechanical and tribological properties. Specifically, the demand of Carbon Fiber Reinforced Polymers (CFRP) has had an increasing trend since 1970s-1980s, becoming one of the kind of composites with the highest demand in the market to supply industries such as aerospace, automotive, construction, renewable energies, among others. With the increasing demand of CFRP materials some of the main challenges that arise are their disposal, environmental impact and cost of production to maintain the required supply. The use of Carbon Fibers as a reinforcement for polymeric matrices has been widely documented over the last decades, however the characterization of recycled Carbon Fibers for tribological applications is still scarce. Therefore, this investigation is focused on the mechanical and tribological characterization under water lubricated conditions of Ultra High Molecular Weight Polyethylene (UHMWPE) composites reinforced with virgin and recycled Carbon Fibers and Graphene Oxide. The findings of this work provide an important panorama regarding the performance of recycled Carbon Fibers, showing that they can have a comparable performance in mechanical properties and tribological behavior. This enables the use of recycled Carbon Fibers without compromising performance while reducing the environmental impact and cost.

Le caractère entièrement sans contact de la technique d'Ultrasons-Laser en fait un outil de premier choix pour étudier la propagation des ondes acoustiques dans diverses géométries. Les problèmes rencontrés lors du couplage de transducteurs piézoélectriques sur une surface courbe sont ainsi évités et l'étude de structures cylindriques de diverses tailles, notamment à l'échelle du micron, peut être envisagée. Le développement de l'acoustique picoseconde dans les années 80 a permis, en effet, de générer des ultrasons de très haute fréquences (>GHz) présentant des longueurs d'onde de l'ordre de quelques nanomètres. Il devient alors possible de sonder des couches minces de tailles microniques à sub-microniques. Une extension de ces travaux au cas de fibres de diamètres micrométriques est ici proposée. Le mécanisme de génération acoustique par laser nécessite quel'échantillon soit optiquement absorbant. Plusieurs régimes de génération sont abordés suivant la fluence laser et la longueur de pénétration optique. Des modèles semi-analytiques de la génération acoustique dans des structures cylindriques optiquement absorbantes sont ainsi proposés. Une première approche, s'intéresse à la modélisation de la propagation dans un multi-couche cylindrique pour lequel la source acoustique est distribuée selon un profil d'intensité imposée par l'absorption de l'impulsion optique. Une seconde approche consiste à rechercher la fonction de Green d'une source enfouie. Une convolution par la distribution de source permet ensuite de calculer la réponse du cylindre. Une validation expérimentale est menée sur un barreau de verre de 5 mm de diamètre à l'aide d'un laser Nd:Yag délivrant des impulsions de 5ns. Une confrontation expérience/théorie est enfin proposée pour une fibre de tungstène de 5 microns de diamètre étudiée par un dispositif d'acoustique picoseconde. Enfin, des expériences similaires ont été réalisées sur des fibres de carbone d'un diamètre de 5 et 10 microns et ont permis d'évaluer localement leur indice optique complexe ainsi que leur élasticité transverse. / Having emerged in the 80s, the laser ultrasonics technique with its non-contact generation and detection process overpasses the difficulties of coupling piezoelectric transducers with curved surfaces. With the picosecond ultrasonics technique it is now possible to generate acoustic waves with very high frequency (>GHz) and very short wavelengths of few nanometers giving access to micrometric and sub-micrometric samples probing. To date, the acoustic generation for cylinders opaque at a given laser wavelength where the acoustic source is located at the cylinder surface was investigated. In this PhD thesis, an extension of this work is proposed in the case of micro-fibers where taking into account the optical penetration depth becomes essential. Semi-analytical models for acoustic waves generation and propagation in a optically absorptive cylinder are derived. In a first way, we adapt modelling techniques used to calculate the response of elastically heterogeneous materials to the case of an homogeneous material with a radially distributed source imposed by the optical absorption. In a second model, the radial displacement for an inner point source is derived, in a 2D Fourier domain. The response to a volume-source distribution along a radius is obtained as a convolution of the above Green function with the corresponding source distribution caused by optical absorption. Two inverse transforms are then applied. Picosecond ultrasonics experiments are performed for different micrometric fibers and compared with calculated waveforms for different optical absorptive properties. Experiments made on carbon micro-fibers have permitted to locally evaluate the transverse elasticity of the fibers as well as their complex optical index.

Carbon nanotubes have been of great interest given their unique electronic and mechanical properties. Scholars have focused on the addition of carbon nanotubes to various matrices in order to develop novel materials. These new hybrid materials would combine the properties of both the nanotubes and the matrix of choice, which can both enhance the mechanical and electronic properties of the matrix material, and allow for the matrix to be used for other applications. In order to take advantage of the properties of the nanotubes, it is vital for them to be well dispersed in a solution or matrix as individual tubes, rather than as bundles. Additionally, it is cost effective to have individually dispersed tubes in a matrix. In order to individually disperse the tubes in the matrix, they can be pre-treated or functionalized via both covalent and non covalent processes. Subsequent to functionalization, the nanotubes can be coated with the matrix material or other metal compounds. This can help with the dispersion and interface interaction with the matrix material, or create materials with novel properties. This thesis focuses on conditions of growing various metal compounds or metal oxides on nanotubes using chemical bath deposition (CBD) and liquid phase deposition (LPD) methods. CBD and LPD use aqueous mediums for growth and deposition of compounds, which makes it both environmentally friendly and cost effective. Different pre-treatments are first employed on the nanotubes in order for them to be both well dispersed in solution and provide nucleation sites for the deposition and growth of various metal and metal oxides on the surface of the nanotubes. Once an ideal deposition is achieved, applications of the coated tubes are studied, tested and discussed.

Electrochemical copolymerizations of thiophene (Th) and 3,4-ethylenedioxythiophene (EDOT) was performed in this study. Incorporation of Th with EDOT units have accelerated deposition rate in relative to the simple polymerization behavior of EDOT. The electrochemical properties of poly(thiophene-co-3,4-ethylenedioxythiophene) (PTh-EDOT) are different from the homopolymers of polythiophene (PTh) and poly(3,4-ethylenedioxythiophene) (PEDOT). PTh-EDOT were then served as cathode materials of lithium-ion (Li-ion) batteries to test their capability to transfer lithium ion in 1.0 M LiPF6/ethylene carbonate/dimethyl carbonate solution. PTh-EDOT copolymer prepared from the monomer ratio of 1/1 (Th/EDOT) shows better stability than PEDOT and PTh homopolymers, polymer property enhancement by copolymerization is thus demonstrated. A composite electrode material PEDOT/LiCoO2 was prepared from the electrochemical polymerization of EDOT on LiCoO2 electrode was primarily prepared to inspect the influence of PEDOT on the electrochemical features of LiCoO2. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) show the successful deposition of PEDOT over LiCoO2 particles. Compared to the simple LiCoO2 electrode, PEDOT/LiCoO2 composite cathode shows enhanced properties including rate capability and cycle stability for potential Li-ion battery application. Nevertheless, differential scanning calorimetry (DSC) scans on the fully charged cathodes imply that PEDOT may reduce the thermal stability of LiCoO2. Two carbon materials, vapor grown carbon fibers (VGCF) and nano-scaled Ketjen black EC (KB), were implemented into LiCoO2 electrode. The influence of different carbon additive and their content on the performance of LiCoO2 such as rate capability and cycle ability has been evaluated. KB shows more positive effects than VGCF even in the case of a low 1 wt% content. Furthermore, incorporation of PEDOT was made by electrochemical deposition of EDOT on the preformed LiCoO2-VGCF and LiCoO2-KB composite electrodes. The influence of the carbon additives and the conductive PEDOT polymer on LiCoO2 was then investigated. Compared to the electrodes without PEDOT coating, PEDOT-incorporated composite electrodes show larger capacity, better transfer rate of lithium ions in electrolytes, and enhanced cycle ability. The electrochemical deposition of PEDOT on the LiCoO2/nano-carbon cathodes provides a new approach to implement the conducting polymers in Li-ion batteries.