• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 3807
  • 581
  • 440
  • 415
  • 152
  • 135
  • 133
  • 121
  • 121
  • 121
  • 121
  • 121
  • 119
  • 74
  • 74
  • Tagged with
  • 7490
  • 2153
  • 1100
  • 1054
  • 1012
  • 933
  • 612
  • 449
  • 431
  • 410
  • 387
  • 350
  • 350
  • 339
  • 329
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
681

Thermorheology and processing of polyethylene blends : macromolecular structure effects

Velazquez, Omar Delgadillo 11 1900 (has links)
Rheological and processing behavior of a number of linear low-density polyethylene(LLDPE)/low-density polyethylene (LDPE) blends was studied with emphasis on the effects of long chain branching. First, a linear low-density polyethylene (LL3001.32) was blended with four LDPE's having distinctly different molecular weights. At high LDPE weight fractions, DSC melting thermograms have shown three different polymer phases; two for the pure components and a third melting peak of co-crystals. Different rheological techniques were used to check the thermo rheological behavior of all blends in the melt state and the effect of long chain branching. It was found that all blends are miscible in the melt state at small LDPE concentrations. The elongational behavior of the blends was studied using a uniaxial extensional rheometer, SER. The blends exhibit strain hardening behavior at high rates of deformation even at LDPE concentrations as low as 1%, which suggests the strong effect of branching added by the LDPE component. On the other hand, shear rheology was found to be insensitive to detect addition of small levels of LDPE up to lwt%. The second set of blends prepared and studied consisted of two Ziegler-Natta LLDPE's (LL3001.32 and Dowlex2045G) and two metallocene LLDPE's(AffinityPL1840 and Exact 3128) blended with a single LDPE. In DSC melting thermograms, it was observed that blends with metallocence LLDPE's exhibit a single melting peak at all compositions; whereas the Ziegler-Natta blends exhibit three melting peaks at certain compositions. It was found also that the metallocene LLDPE's are miscible with the LDPE at all concentrations. On the other hand, the Ziegler-Natta LLDPE's were found to be miscible with LDPE only at small LDPE concentrations. The processing behavior of all blends with emphasis on the effects of long chain branches was also studied in capillary extrusion. The critical shear stresses for the onset of sharkskin and gross melt fracture are slightly delayed with the addition of LDPE into LLDPE. Furthermore, the amplitude of the oscillations in the stick-slip flow regime, known as oscillating melt fracture, were found to scale with the weight fraction of LDPE. Amounts as low as 1 wt% LDPE have a significant effect on the amplitude of pressure oscillations. These effects are clearly due to the presence of LCB. It is suggested that the magnitude of oscillations in the oscillating melt fracture flow regime can be used as a method capable to detect low levels of LCB. Finally, the sharkskin and stick-slip polymer extrusion instabilities of a linear low-density polyethylene were studied as a function of the type of die geometry. The critical wall shear stress for the onset of flow instabilities, the pressure and flow rate oscillations, and the effects of geometry and operating conditions on the instabilities are presented for a LLDPE. It was found that sharkskin and stick-slip instabilities were present in the capillary and slit extrusion. However, stick-slip and sharkskin in annular extrusion are absent at high ratios of the inside to outside diameter of the annular die. This observation also explains the absence of these instabilities in polymer processing operations such as film blowing. These phenomena are explained in terms of the surface to volume ratio of the extrudates. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate
682

Physical phenomena of thin surface layers

Thomas, Katherine Ruth January 2010 (has links)
This thesis explores different physical phenomena observed in, or involving thin surface films. Thin surface layers are ubiquitous. Found in nature and used in almost every aspect of daily life, thin surface films are invaluable. While the applications and roles may be varied, to be used effectively, the physical properties of these films and the factors influencing their stability need to be well understood. Surfaces can have a strong effect on the stability of thin films. In thin films of polymer blends, wetting layers rich in one component often form at the film interface prior to phase separation. Here the formation of these wetting layers are seen to result in destabilisation of the film, even when the blend is far from phase coexistence. A spinodal like instability with a characteristic wavelength is shown to form. A theoretical model is developed, which describes the observed behaviour in terms of coupled height and composition fluctuations in the wetting layer. Spin coating is a common technique for the formation of thin polymer films. Films formed in this way however, are often seen to exhibit anomalous properties, which strongly differ from that of the bulk behaviour of the material. Here the rheological properties and stored stresses in spin cast films are explored, with focus on the role that the casting solvent plays in the properties of the film. The results suggest that the observed deviation comes from a lowered density of chain entanglements. The effective viscosity and residual stresses in the as-spun film are seen to strongly depend on the casting solvent properties and the solvent-polymer interactions. The use of organometallic polymers as precursors for the formation of magnetic ceramics is investigated. Emphasis is placed on doping the polymers with metallic compounds prior to pyrolysis, allowing for the formation of technologically interesting metallic alloys, without the need for new polymers to be synthesised. The formation of iron-palladium alloys is demonstrated using this method. These are highly desirable due to their potential use in hard-disk drive technologies. Thin films can be used to influence the optical signature of a material and are widely used in nature to produce vibrant, pure, iridescent colours. Here the optical properties of the tropical plant Selaginella willdenowii are explored. The bright blue colouration is seen to arise from a multilayer lamella structure on the upper surface of the leaves. Light is important to plants, who use it both as an energy source and an environmental signal. Blue iridescence occurs in a wide range of plant species, suggesting that it has some adaptive benefit. These are considered and discussed.
683

Emulsion polymerization of vinyl chloride

Laudie, Harry January 1977 (has links)
Abstract not available.
684

Sequencing synthetic copolymers using electrospray ionization mass spectrometry

Giguere, Marie-Soleil January 2003 (has links)
The dissociation of gas-phase synthetic homopolymers poly(butyl acrylate) (PBA), poly(methyl methacrylate) (PMMA) and poly(vinyl acetate) (PVAc) ionized by Li+ or Na+ was investigated by electrospray-ionization tandem mass spectrometry. Based on these results it was possible to use tandem mass spectrometry to sequence three copolymers produced by free radical polymerization (PBA/PVAc, PBA/PMMA and PMMA/PVAc). In the case of PVAc containing copolymers, tandem mass spectrometry yielded partial sequence information including the number of VAc monomers and consecutive BA or MMA monomers while the complete sequence (and mixture of sequences) could be deduced for PBA/PMMA. The poly(vinyl acetate) ions lose acetic acid molecules from the polymer side chain, and ultimately metal acetate, to form a polyene with a positive charge on the carbon backbone while PBA and PMMA fragment along the polymer backbone. An extensive study was made of the dissociation characteristic of PVAc. Collision-induced dissociation (CID) was performed at different center-of-mass collision energies to study the sequential appearance of the fragment ions. Molecular mechanics/molecular dynamics (MM/MD), Hartree-Fock, semi-empirical and density functional calculations were employed to model the lowest energy dissociation processes.
685

Development of and gas permeation study of homo and copolymers from the family of polyphenylene oxides

Tran, Anh January 2004 (has links)
Membrane gas separation emerged as a commercial process on a large scale during the 1980s. When compared to other gas separation processes such as cryogenics, adsorption and absorption, membrane separation competes primarily on the basis of overall economics and convenience, but not on the basis of product purity. For example, membranes have been used for offshore natural gas processing because they reduce the size and the weight of the processing units, require less maintenance and less operator supervision. In order to improve product purity, development of new membrane materials, and modification of existing membrane material for better gas separation properties, is required. This research focused on preparation and characterization of new copolymers from the family of polyphenylene oxides. This family of polymers was chosen because of the properties of its best-known representative, poly(2,6-dimethyl-1,4-phenylene oxide), which shows one of the highest rates of permeability for gases among glassy polymers, exhibits excellent film-forming properties, and is resistant to a number of chemical agents. The study was divided into two parts: (1) homopolymers and both block and random copolymers of different composition were synthesized by oxidative coupling of 2,6-diphenylphenol and 2,6-dimethylphenol; (2) the physical and gas transport properties of the polymers were determined. (Abstract shortened by UMI.)
686

Tethered polymers in a shear flow: A molecular dynamics study of the good and bad solvent cases

Gratton, Yannick January 2005 (has links)
Nanotechnology is now a reality although it is still very much in its infancy. This being said much work still needs to be done. During the last past few decades, new experimental tools such as videomicroscopy and optical tweezers, which allow us to both visualize and manipulate in real time the dynamics of macromolecules, have emerged. These two tools alone provide endless possibilities. For example, it is now possible to tether a polymer at one end with optical tweezers while a constant flow of solvent extends it. If the chain is fluorescently labeled, dynamical properties can be extracted via videomicroscopy. In this thesis, we study a similar problem. With the help of Molecular Dynamics (MD) simulations, we examine the physics of a Freely Jointed Chain (FJC) tethered by one end to a hard surface while being submitted to a Poiseuille flow. A good solvent is used during the first part of this thesis, while for the second part, we look at the dynamics of the chain in poor solvents. In both cases, we compare actual stretching theories to our simulation data and extract the effects of Hydrodynamic Interactions (HI). For good solvents, we propose a new empirical equation which relates the unstretched fraction of the chain to the full range of shear rates. We also observe a peculiar cyclic motion of the chain which was first reported experimentally. For poor solvents, we study the collapsed chain properties while changing the strength of the shear flow linearly in time. A novel hysteretic effect for the chain extension as a function of shear rate is observed in our simulations.
687

Mass spectrometry of polymers: From synthesis to sequence

Alhazmi, Abdulrahman M January 2008 (has links)
Electrospray ionization (ESI) and matrix assisted laser desorption-ionization (MALDI) mass spectrometry were used to determine the composition (monomer ratios) and structure (end group analysis), relative to 1H NMR spectroscopy and theoretical predictions, for three different copolymers: poly(butyl acrylate/vinyl acetate) (PBA/PVAc), poly(methyl methacrylate/vinyl acetate) (PMMA/PVAc) and poly(butyl acrylate/methyl methacrylate) (PBA/PMMA). The ESI results were found to be in excellent agreement with 1H NMR spectroscopy for PBA/PVAc and PBA/PMMA copolymers whereas there was more divergence in the case of PMMA/PVAc. In the case of PBA/PMMA copolymers similar distributions of products were observed by ESI-MS and MALDI-MS; two major product classes were observed differing by their end-groups. One class has hydrogen and dodecylthio end groups while in the other the dodecylthio has been replaced by alpha-cyanoisopropyl from the initiator. The relative abundance of these distributions as a function of copolymer conversion for a series of reaction conditions was investigated by both ESI and MALDI. The collision-induced dissociation (CID) mass spectra for a variety of chain lengths of four ionized polymer samples have been quantified according to their observed total relative fragment ion abundances. The CID mass spectra of oligomers of ionized PMMA with three different types of end groups and polystyrene, PS, were obtained at fixed centre-of-mass collision energies and collision numbers. For the PMMA polymers, the total fragment ion abundance increases with increasing chain length, consistent with an increase in internal energy deposition with size of the ion. A discontinuity in the increase in total fragment ion abundance appears to correspond to a change in conformation of the polymer ions from linear (at short chain lengths) to cyclic (at long chain lengths). Ionized PS does not exhibit this change in conformation as all chain lengths show compact structures and accordingly the total fragment ion abundance does not change with increasing chain length. Protonation of polymers in the gas-phase was achieved by the dissociation of proton-bound complexes of the oligomers with small peptides and amino acids. Protonated PMMA and PBA oligomers were shown to fragment in unique pathways that involved losses of neutral molecules from their side chains, until all that is left is a hydrocarbon backbone. For PMMA the neutrals were primarily CO and methanol, while for PBA butyl ether is lost. We also explored the threshold fragmentation of the proton-bound complexes as a function of the amino acid used (and its proton affinity) which allowed a kinetic bracketing of the oligomer PA as a function of chain length. The results were consistent with a change from bidentate to tridentate binding of the proton with increasing length of the PMMA oligomers.
688

Photochemistry applied in optical lithography Use of spectroscopic and imaging techniques for the characterization of chemical processes in thin polymer films

Ivan, Marius Gabriel January 2007 (has links)
Most of the research presented in this thesis deals with photochemistry applied to optical lithography, a technology used to manufacture computer chips, memories, and integrated circuits in the micro- and nano-regime. Photochemical reactions induced in thin polymer films by laser radiation at 248, 193 and 157 nm were studied extensively, employing spectroscopy and microscopy techniques. Fluorescent dyes that would change their properties upon interaction with a chemical species were used to gain insights about the photoproducts formed during irradiation. Photolysis of photoacid generators (PAGs) leads to formation of acid, which will then catalyze deprotection of polymer pendant groups. The amount of acid formed as a function of incident dose was determined for different PAGs at various wavelengths and in different resist formulations using Coumarin 6, a fluorescent dye that changes its spectroscopic properties upon interaction with the proton. New and more transparent PAGs, obtained in the group, were characterized at different wavelengths. The amount of acid determined by in polymer titration was used in calculation of the catalytic chain length (CCL). This is a novel method which employs two fluorescent sensors embedded in a polymer film. Hydrofluoric acid (HF) was detected in films of fluorinated polymers exposed to laser radiation at 157 nm, using a fluorescent sensor especially designed and synthesized in the group. The influence of laser radiation on the polymer film was studied using pyrene as fluorescent probes sensitive to the environment changes. Formation of excimers was observed in exposed areas; a very interesting finding is that the molecules forming the excimers do not separate after they emit light, remaining in close proximity for days. The influence of temperature on the excimers in thin polymer films was studied, using polymers with different Tg.
689

Probing Chromophore Assemblies In Solution : Study Of Polymer Folding And Micellization

Ghosh, Suhrit 04 1900 (has links) (PDF)
No description available.
690

Drying Behavior of Oil Sand Mature Fine Tailings Pre-dewatered with Superabsorbent Polymer

Roshani, Anis January 2017 (has links)
Oil sand processing to extract bitumen generates large volumes of slurry comprising water, silt, sand, clay, unrecovered bitumen, and residual chemical aides and solvents added during the extraction process. The by-product stream of the bitumen extraction is pumped into constructed tailings ponds. Managing these tailings is one of the most difficult environmental challenges for the oil sand industry. This study aims to develop a novel technique to assist in the assessment of the technologies for managing mature fine tailings (MFT) in oil sands. Innovative application of a superabsorbent polymer in the oil sands industry may provide a new method for tailings management. However, thus far, no study has addressed this research gap. In fact, fundamental knowledge of the behavior of MFTs pre-dewatered with the superabsorbent polymer could provide an important way to positively affect the speed of reclamation. To this end, comprehensive instrumentation, geo-environmental, and geotechnical analyses are carried out to obtain essential knowledge on the behavior of MFTs pre-dewatered with the polymer. The results of this study reveal that the mechanical, hydraulic, and thermal properties of the MFTs are related. Evaporation and drying shrinkage can affect the hydro-mechanical properties of the tailings and have a significant influence on the developed shear strength of the MFTs. In addition, the process-affected water includes a high concentration of the dissolved ions and organic chemicals that stem from ore extraction chemicals and tailings treatments, or that may be released from oil sands ores. Through the application of a superabsorbent polymer in the dewatering of oil sand MFTs, the chemical components are entrapped in the polymer chains, thus limiting the mobility of the major ions and trace metals in water bodies beneath the oil sand tailings pond. Results show that the application of the superabsorbent polymer considerably reduces the rate of drainage from the oil sand MFTs into water bodies, which can help mitigate the risk of seepage. The author believes that the superabsorbent polymer dewatering technique can be considered as an environmentally friendly promising approach for management of oil sands MFTs. This new technique can accelerate the pace of reclamation and thus minimize the footprint of the oil industry in Canada.

Page generated in 0.0428 seconds