• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 1221
  • 392
  • 263
  • 190
  • 118
  • 116
  • 78
  • 75
  • 61
  • 15
  • 15
  • 14
  • 13
  • 12
  • 12
  • Tagged with
  • 2911
  • 692
  • 419
  • 301
  • 262
  • 250
  • 225
  • 214
  • 184
  • 179
  • 175
  • 168
  • 165
  • 160
  • 158
  • 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.

Preparation of ionic cellulose for wrinkle resistant fabrics

Vargantwar, Pruthesh Hariharrao 03 May 2007 (has links)
Conventional treatment of cellulosic fabrics by formaldehyde-based cross-linkers provides improved wrinkle recovery angles (WRA) and durable press (DP) performance. But these treatments suffer from strength loss and later release of formaldehyde, a known carcinogen. Ionic crosslinking offers a potential solution to these problems, and has shown improved wrinkle recovery performance in previous studies. In the current novel method of ionic crosslinking for wrinkle resistant fabrics, the cellulosic fabric is treated with salt of mono chloroacetic acid and 3-chloro-2-hydroxypropyl trimethyl ammonium chloride (CHTAC) sequentially or in mixture to form covalently bonded anionic and cationic sites on cellulose, which are durable to washing, and which form inter/intra molecular ionic cross-links. There is no later release of hazardous chemicals involved with this treatment and improved wet WRA are obtained. Fabric treated by this method gained tensile strength and breaking strain compared to the untreated fabric. Different routes for chloroacetate treatment are presented. Pad-dry-pad-cure is the most efficient route and a functional relationship between the anionic content and the process parameters is established. Analytical techniques like confocal microscopy and scanning electron microscopy are used to confirm the morphological changes and occurrence of carboxymethylation reaction in the fiber interior.

Novel Reactive Dyes Based on Pyrimidine and Quinoxaline Systems

Horton, Aaron Michael 13 August 2009 (has links)
No description available.


Dondero, William Edward 12 July 2005 (has links)
The mechanical properties and morphology of multi-wall carbon nanotube (MWNT)/polypropylene (PP) nanocomposites were studied as a function of nanotube orientation and concentration. Through melt mixing followed by melt drawing, using a twin screw mini-extruder with a specially designed winding apparatus, the dispersion and orientation of multi-wall carbon nanotubes was optimized in polypropylene. Tensile tests showed a 32% increase in toughness for a 0.25 wt % MWNT in PP (over pure PP). Moreover, modulus increased by 138% with 0.25 wt % MWNTs. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) all demonstrated qualitative nanotube orientation. Wide angle X-ray diffraction was used to calculate the Herman?s Orientation Factor for the composites as function of nanotube loading and orientation. No significant changes in PP crystal orientation were found indicating that the alignment of the nanotubes did not significantly affect the orientation of the PP crystals. In addition, differential scanning caloriometry (DSC) qualitatively revealed little change in overall crystallinity. In conclusion this work has shown that melt mixing coupled with melt drawing has yielded MWNT/PP composites with a unique combination of strength and toughness suitable for advanced fiber applications, such as smart fibers and high performance fabrics.

Automated Method For Fiber Length Measurement

Tompkins, Michael 04 August 2006 (has links)
The price of cotton is dictated by quality and the most significant factor of contributing to the fiber quality is the length distribution of the fibers contained within the population. Therefore it is of importance to accurately and repeatably measure the length of fibers within a population so that it is graded properly. Current methods are inadequate and thus prior work focused on designing a machine to directly measure individual cotton fibers using digital imaging. The current work begins with the evaluation of the effectiveness of the digital imaging machine. The machine was evaluated and sources of error identified. Modifications were implemented in an attempt to improve the error. After multiple modifications with little success an entirely new design was conceptualized. The new design aimed to eliminate all major sources of error with the existing machine while not creating new sources of error. The new design is discussed and the results are compared to those obtained by the original imaging machine. The new machine was better able to accurately measure the length of cut length fibers. The variation between fibers within a sample and entire samples of cut length fiber was significantly decreased when compared to the variation of the previous system.


Langston, Thomas Brice 05 June 2003 (has links)
The purpose of this study was to investigate the properties of air-textured aramid yarn (ATAY), in a single yarn composite (SYC), in a 3D woven fabric preform, and in a 3D preform composite. Yarn tensile tests demonstrated textured yarn was 70-77% lower in tensile strength, 82-85% lower in tensile modulus, and 60-190% higher in breaking strain than those of the control yarn. The results of SYC testing illustrated that the control yarn composite had only a 5% higher tensile strength, a 27% higher modulus, and 11% lower energy to break than the textured single yarn composite. Fabric tensile tests demonstrated a low initial modulus and a much larger secondary modulus for all 3D woven preforms. The ATAY fabric had a similar initial modulus and a much lower secondary modulus in the weft direction compared to the control fabric. The ATAY fabric had a significantly higher yield shear stress and strain, primary and secondary shear moduli, energy to yield point, and total energy absorbed to 4° than those of the control. With the same fiber volume fraction, the ATAY composite had a slightly lower tensile strength and modulus, but a 120% higher shear modulus, than the regular aramid yarn (RAY) composite. Unlike the RAY composite brittle failure behavior, the ATAY composite failed in a ductile manner with multiple diverting cracks propagating during failure. The ATAY composite had a much higher yield point in the 45° direction tensile test, a much higher softening point in the warp direction tensile test, and increased the interlaminar shear strength of a laminated composite by 37% as compared with the control.

Use of Modified Cellulose for the Improvement of Water Repellency

Goli, Kiran Kumar 16 June 2008 (has links)
A novel method is developed for imparting durable water repellency to cotton cellulosic fabrics based on ionic interactions. Most of the traditional water repellent finishing chemicals such as paraffin waxes, pyridinium compounds, formaldehyde based N-methylol crosslinkers, siloxanes and fluoro-carbon polymers are either non-durable to washing or environmentally unsafe or expensive. Our method includes cationization of cotton fabric with 3-chloro-2-hydroxypropyl trimethyl ammonium chloride (CHTAC) followed by subsequent treatment with a salt of stearic acid to form ionic attractions between cationic groups of cationized cotton fabric and anionic groups of stearate anion. These ionic interactions hold the stearate or hydrophobic molecules on the surface of cotton fabric outwards giving durable water repellency without releasing any hazardous chemicals present in almost all other durable water repellent treatments for textiles.

Fabrication and characterization of novel single and bicomponent electrospun nanofibrous mats

Ojha, Satyajeet Sooryakant 08 August 2007 (has links)
Nanofibers were produced using relatively new electrospinning technique. Single layer nanofibers were fabricated using nylon-6. Several parameters such as polymer molecular weight, concentration, surface tension, applied electric voltage, distance between tip to grounded electrode and feed rate were investigated to optimize fiber consistency and diameter. Scanning electron microscopy was employed to study fiber morphology and diameter. Understanding the effects of various parameters mentioned above, electrospinning strategy was further utilized to produce nanofibers with novel core-sheath structure using chitosan, a biopolymer and polyethylene oxide (PEO). Chitosan is very difficult to electrospin, to alleviate this problem PEO was used as sheath to support chitosan core. For this purpose, rheology of polymer solutions was evaluated for successful fabrication of core-sheath nanofibers. Only 3 wt % chitosan was found to produce coaxial structure with 4 wt % PEO, due to their proximity in rheological behavior. Coaxial morphology of nanofibers was verified by transmission electron microscopy having 250 nm and 100 nm as sheath and core diameters respectively. Fourier transform infrared spectroscopy was employed to investigate the effect of de-ionized water treatment of core-sheath mats where in PEO layer was removed off in order to get pure chitosan nanofibers. Coaxial nanofibers with one component were also fabricated using pure PEO as core and PEO doped with Multi-walled carbon nanotubes as sheath material. Results showed that as carbon nanotubes were subjected to relatively smaller volumes, predominantly on the surface culminated in appreciable increase in conductivity as well as mechanical properties. Coaxial nanofibers produced from electrospinning are of particular interest in tissue engineering and wound healing scaffolds.

Molecular Dynamics Simulations of Single-Walled Carbon Nanotubes Wrapped by Various Polymers

Tallury, Srinivasa Syamal Sanmath 07 August 2009 (has links)
Carbon nanotubes (CNTs) possess superior electrical and mechanical properties and thus are excellent candidates for nanostructured materials. Due to the very high length-to-diameter ratio of CNTs, they are ideal reinforcements for polymer nanocomposites. Engineering of the polymer-CNT interface through noncovalent modifications is necessary to achieve the desired mechanical properties and yet preserve the inherent properties of the CNTs. However, the effects of chemical composition and backbone stiffness on the adsorption characteristics of polymers are not well understood. Molecular dynamics simulations in vacuum were used to study the interaction between a (10,0) zig-zag type single-walled carbon nanotube (SWCNT) and a series of polymers. These simulations investigate whether the polymers prefer to wrap the SWCNT, what the molecular details of that interface are, and how the interfacial interaction is affected by the chemical composition and structure of the polymer. The simulations indicate that polymers with both flexible and stiff backbones tend to wrap around the SWCNT, although in different conformations. Flexible backbones like nylon6 (N6) and poly(lactide) (PLA) wrap in a random conformation along both the longitudinal axis and the diameter of the SWCNT. One flexible polymer, poly(acrylnitrile) (PAN), preferred to extend along the longitudinal axis rather than wrap the diameter of the SWCNT as a means of optimizing pi-pi overlap between the cyano side chain and the SWCNT; PAN was the only flexible backbone polymer that exhibited preferential orientation of chemical groups along the SWCNT surface. Flexible polymers with bulky and aromatic side groups such as poly(methylmethacrylate) (PMMA) and poly(styrene) (PS) prefer intra-chain coiling rather than wrapping the SWCNT. Poly(ethylene terephthalate) (PET), the only polymer with a semi-flexible backbone in this study, exhibited a partial wrap in an S-conformation along the side of the SWCNT. Polymers with stiff backbones such as poly(acetylene) (PA), poly(p-phenylene vinylene) (PPV), poly(pyrrole) (PPy), and poly(arylene ethynylene) (PPE) exhibit distinct conformations upon adsorption. Helical-like wrapping conformations were only obtained for PPV and PPE. Aromatic groups along the backbone tend to dictate the adsorption conformation due to pi-pi interactions with the SWCNT, although the presence of bulky aliphatic side chains can have a slight impact on this interaction. Plots of the rotational moment of inertia of each polymer about the SWCNT longitudinal axis as a function of time quantify the interplay between intra-chain coiling and adsorption to the SWCNT surface. These plots indicate that the adsorption of polymers with stiff backbones tends to be a two-step process, whereas flexible backbones tend to exhibit a multi-step wrapping mechanism, especially those that have a preference for intra-chain coiling. To quantify the correlation between the chemical composition of the repeat unit and the conformational limitations of long polymer chains, MD simulations were also performed with small molecules that correspond to the repeat units of a subset of the polymers. These simulations indicate that the individual molecules have more conformational freedom, yet still exhibit some orientation characteristics similar to the polymers, such as adsorption of both aromatic rings and aliphatic hydrocarbons along the SWCNT surface.

Mechanical and physical properties of electrospun nanofibers

Zhang, Shu 13 August 2009 (has links)
The process of electrospinning was utilized to fabricate randomly aigned nylon6 nanofibers and aligned nylon6 nanofibers. Polymer concentration affecting electrospinning was investigated. This parameter was evaluated using degree of crystallinity by differential scanning calorimetry (DSC) as well as visual images produced by scanning electron microscopy (SEM). DSC data demonstrated that more crystals were formed with lower polymer concentrations; SEM images revealed that slimmer fibers were produced by lower concentrations. The mechanical properties of unoriented fibers and aligned fibers were tested on Instron 5544. The result of tensile tests indicated higher Youngâs modulus and tensile strength of aligned nanofibers than that of unaligned fibers. The SEM images at broken edges of fibers illustrated different broken mechanisms of these two forms of nanofibers. The broken mechanism of aligned nanofibers was further confirmed by crystallinity parameters obtained from DSC and fiber diameter shown from SEM images.

Novel Supramolecular Polyamides

Saunders, Joshua Daniel 25 July 2005 (has links)
The objective of this research is to use low DP poly(p-benzamide) (PBA) segments, terminated by units forming supramolecular bonds, able to extend the overall DP of the aromatic polyamide. PBA fibers, and the related industrially produced PPTA (Kevlar), exhibit their most interesting ultra-high strength properties only when a considerably large DP (>100) is attained. Use of cumbersome and expensive syntheses and solvents are required to attain DP in the range (~200-300) of industrial interest. Moreover, the fully covalent polymers thus far produced are highly insoluble in common organic solvents. On the other hand, easier processing becomes feasible if the DP of conventional PBA (prepared by the Yamazaki reaction) is increased by supramolecular bonding through ionic or hydrogen bond interactions. The effects of three different binding methods were first investigated on short rigid monomers with promising results the same binding was then used on rigid segments of PBA. The binding methods used two diamine binders triethylenediame (TED) and bipiperidine (Bipip) to form ionic bonds with the monomer, and polymer segments. The last method utilized a 2(6-iso cyanato hexylamino carbonyl amino)-6-methyl-4[1H]pyrimidinone (Upy) end group covalently bonded to the PBA polymer. This end group has the ability to form 4 hydrogen bonds with itself and thus could be used to increase the overall DP of the polymer starting material. This is believed to be the first recorded hydrogen bonded supramolecular interaction in amide type solvents. The novel and revolutionary idea of using low DP segments of PBA to increase the overall DP of polymer could be an industrially viable way to produce the highly sought after industrial polyamides.

Page generated in 0.0257 seconds