Baki Aksakal

Effect of UV-light on the uniaxial tensile properties and structure of uncoated and TiO2 coated Bombyx mori silk fibers

The effect of UV-light on the uniaxial tensile properties and the structure of uncoated and TiO2 coated silk fibers in the bave form by using sol-gel method was investigated with tensile testing and FT-IR/ATR spectroscopy methods after the silk filaments were exposed to UV-light with high intensity of 760W/m(2) for different times from 0.5h to 1day. It was clearly observed that TiO2 coating considerably increased the Youngs modulus of the uncoated silk single filament by around 17% before the UV-irradiation. The yield point and the post yield region disappeared on the stress-strain curves of both uncoated and TiO2 coated silk filaments after UV-irradiation time higher than 1h. Except for the Youngs modulus, most of the tensile characteristics of both uncoated and TiO2 coated silk filaments decreased remarkably with increasing UV-irradiation time, e.g., after 1h irradiation, although the Youngs modulus slightly changed and ultimate tensile strength decreased by only around 18% and 23%, for the uncoated and TiO2 coated silk filaments, respectively; breaking extension decreased dramatically by 67% and 72%, respectively, for uncoated and TiO2 coated silk filaments. Only the Youngs modulus of TiO2 coated silk filaments which can be considered as a more stable tensile characteristic became significantly higher than that of the uncoated silk filaments with increasing UV-irradiation time. After 1day irradiation, even though the uncoated silk filaments could not be tested and completely lost of their fiber properties, the TiO2 coated silk filaments showed a stress-strain curve in initial elastic region with Youngs modulus of ∼13GPa which indicates considerable protective effect of TiO2 on the silk fiber structure, especially on the β-sheet microcrystals against UV-radiation. The FT-IR/ATR spectral results showed that significant photodegradation took place in not only crystalline but also amorphous regions which were deduced from the decrease in the absorbance ratios of the bands assigned to CH3 rocking, Cα-Cβ, Cα-C stretching vibrations in β-sheet crystalline regions as well as the Amide I, II, and III bands for both crystalline and amorphous regions. Even though the ratio of crystalline to amorphous regions in uncoated silk filaments decreased significantly, the ratio in TiO2 coated silk filaments became almost constant with increasing UV-irradiation time which may indicate more stable β-sheet microcrystals against photodegradation.

Recovery Process in Stretched Wool Fibers

The recovery process in stretched wool fibers at different strain levels ranging from 5% to 40% was investigated at room conditions for a long time, up to one year, and in water. The recovery process in stretched wool fibers is quite slow at room conditions; thus this slow recovery process causes quite high remaining deformation on the wool. The recovery process in the strain (ϵ) and logarithm time (log t) coordinates has a linear dependence in the wide time range that allows estimating the required time for a complete recovery. In contrast to the rather slow recovery process at room conditions, a complete recovery in water at room temperature was observed within approximately 30 s. Structural changes during the recovery processes at room conditions and in water were analyzed by an FTIR/ATR method. The influences of water content and new formations of hydrogen bonds in the recovery processes were examined. Slow recovery at room conditions was associated with the reorganization of the hydrogen bonds between microfibrills and matrix which results in formation of a new and rather stable structure. The absorption of water by the matrix phase causes the disruption of the strong hydrogen bonds holding the stretched form of the fiber and leads to a rapid recovery.

Temperature effect on the recovery process in stretched Bombyx mori silk fibers

The recovery process in stretched Bombyx mori silk fibers at different strain levels from 3% to 17% was investigated at room conditions during long period of time from 5min to 20days and more. How the temperature affects the recovery process in the silk fibers stretched at room conditions was examined at temperatures from 25 to 125°C. The results of the recovery process at 25°C revealed that although the recovery process from strain values higher than 3% strain continued slowly which caused quite high remaining deformation, a complete recovery from 3% strain was observed after 3days. However, better recovery process was observed with increasing temperature which led to lower remaining deformations. For instance, a complete recovery from 6% strain was observed after 144h and 3h for the recovery process at 100°C and 125°C, respectively which indicates an important result that the deformations induced by stretching the silk fibers up to 6% strain are reversible and increasing temperature affects the velocity of this process significantly. The recovery process expressed in the strain (ε) and logarithm time coordinates showed a linear dependence for which a linear equation was proposed. Thus, this linear equation enables to estimate the required time for a complete recovery from different strain levels and remaining deformation at any stage of the recovery at different temperatures. The ATR-FTIR spectra of the stretched silk fibers during the recovery process revealed some changes in the absorbance ratios and shifts in the positions of the bands assigned to Cα-C, N-H stretching vibrations, and the Amide III mode. It was suggested that new formation of the hydrogen bonds between polypeptide chains especially in amorphous regions and the changes in the intra-sheet hydrogen bonds in β-sheet crystalline regions greatly contribute to the recovery process.

Polyvinyl Alcohol/EuBa2Ca2Cu3O9−x Composites: Dielectric and Mechanical Properties

In the present work, the influence of EuBa2Ca2Cu3O9−x (coded as Eu-1223) ceramic additive on the mechanical and dielectric properties of poly(vinyl alcohol) (PVA) polymer were investigated for the first time. PVA/Eu-1223 polymer-ceramic thick film composites were prepared by solution casting method with different ceramic contents varying from 1 to 5%. The surface morphology of the samples were studied with scanning electron microscope revealed that adding higher concentration of Eu-1223 increases the roughness of the surface as well as the fragility of the PVA. Thermogravimetric analyses results revealed that the increasing Eu-1223 additive enhances slightly both the thermal and oxidative stability of the PVA. Furthermore, FTIR analysis of the composite displayed new FTIR transmittance peaks that can be attributed to the formation of a reaction between the PVA polymer chain and the ceramic additive. The mechanical properties in the context of stress–strain curves of the samples unveiled that increasing ceramic doping improves the Young’s modulus of PVA films. The impedance and dielectric properties of the samples recorded at room temperature indicated a space charge polarization regardless of the doping concentration. The complex electrical modulus analysis also pointed out a pure conduction process. The Nyquist and phase angle plots defined the grain and grain boundary properties along with the ideal capacitor ability of the composites. Especially, the lower ε′ and ε″ values were achieved for the 5% Eu-1223 additive level concentration. This work brings out that 5% Eu-1223 added PVA thick film may be suggested as a low dielectric loss dielectric material for supercapacitor applications.

Thermomechanical Properties of Anti-Parallel β-Sheets with Bombyx mori Silk Nanostructures [Gly-Ser-Gly-Ala-Gly-Ala]n and [Gly-Ala]n

The present study aims to investigate temperature dependence of anisotropic mechanical properties of anti-parallel beta pleated sheets with Bombyx mori silk nanostructures in sequence of [Gly-Ser-Gly-Ala-Gly-Ala]n and commonly used model in sequence of [Gly-Ala]n. Classical molecular dynamics simulation method was used to carry out calculations, preliminary, at 300K. Ionic interactions were modeled by CHARMM force field method. Substitution of Ala with Ser residue results with a gradual conformational change in the stress-strain.

The Tensile Properties of Uncoated and TiO2 Coated Bombyx Mori Silk Yarns Exposed to Thermal Treatment

Bombyx mori silk yarns were coated with a TiO2 network by sol-gel processing. The tensile properties of TiO2 coated silk yarns heated at various temperatures were examined and compared with those of uncoated silk yarns. The thermal properties of uncoated silk yarns and TiO2 coated silk yarns were also discussed. Prominent changes were seen in the rigidity and Youngs modulus. The Youngs modulus of TiO2 coated silk yarn increased by 13% at room condition and remained higher than that of uncoated silk yarns with increasing temperature. The shape of the stress–strain curve of TiO2 coated silk yarns became the same as uncoated silk yarns and showed a similar tendency of change to uncoated silk yarns with increasing temperature. The rupture values of uncoated silk yarns and TiO2 coated silk yarns decreased significantly, e.g., the breaking extension decreased from 17.28 to 2.44% and from 15.25 to 1.96% for uncoated silk yarns and TiO2 coated silk yarns, respectively, when the temperature was increased from 25 to 225°C. Structural changes and degradation processes due to thermal effects for both uncoated and TiO2 coated silk yarns are discussed.

The Preliminary Stretching Effect on Mechanical Properties of Natural and Synthetic Polyamide fibres and Conformational Transitions

The preliminary stretching effect on mechanical properties of natural (wool, hair, and silk) and synthetic polyamide fibres (nylon, capron) and conformational transitions are investigated by using a tensile tester. It is shown that α‐keratin fibres (wool and hair) show silk‐like properties and properties of synthetic polyamide fibres for preliminary extension of 18–24% and 34–38%, respectively.