دو ماهنامه علوم و تکنولوژی پلیمر
سال بیست و پنجم شماره 4 (پیاپی 120، مهر و آبان 1391)

Most recent studies on polymer nanocomposites containing carbon nanotubes include mechanical، rheological، thermal and electrical properties. Optical properties of polymer nanocomposites containing carbon nanotubes are not very clear. Accordingly، in the present work، the optical properties of these naocomposites are studied in the visible region of electromagnetic spectrum. To do that، PMMA nanocomposites including different percentages of carbon nanotubes (CNTs) and carbon black are prepared by applying melt mixing method. To characterize the optical properties، the spectral reflectance values of the prepared samples are measured by a reflectance spectrophotometer with d/8 geometry in the visible region from 400-700 nm. The color parametric values a*، b* and L* of the samples are calculated in CIELAB color space under a simulator of the day light i. e.، D65 and the observer 1964 (10°). For all forms of samples، i. e. powder and different prepared nanocomposites، the blackness index، proposed by Westland and et al.، is calculated. For the latter، the transmission electronic microscope (TEM) images are obtained، also. As a result، with increasing the nanotube content up to 0. 5 %wt، the spectral reflectance factors decreases down to 7% and the blackness content of the samples increases. In addition، the results show in comparison with carbon black، the samples containing carbon nanotubes represent a better blackness index. The TEM images show that for specific carbon nanotube content، the spreading CNT by melt mixing method is better than solution method. It seems that CNT can replace carbon black by its more efficient distribution by an appropriate method such as melt mixing.

Silver nanoparticles have been extensively applied in various fields such as polymers and textile fibers considering their well known antimicrobial properties. In conventional methods nano silver is synthesized through chemical reduction however، in this paper a novel synthesis method based on aqueous solution of ammonia/silver complex with cationic stabilizer along with UV-C irradiation is introduced. On this basis، silver nitrate was oxidized with sodium hydroxide and then transformed into [Ag (NH3) 2]+ aqueous solution with ammonia followed by adding PVP as a reducing and stabilizing agent and irradiated by UV-C. The formation of silver nanoparticles was confirmed from the appearance of surface plasmon absorption and the X-ray diffraction (XRD) demonstrated that the colloidal nanoparticles were pure silver and Zeta sizer showed particle size distribution. Cotton fabric finishing was accomplished in pad process with various concentrations of nano-sized colloidal silver. Some characteristics of the fabric such as antimicrobial against different microorganisms including gram positive bacteria (Staphylococcous aureus)، one gram negative bacteria (Escherichia coli)، UV–vis spectrophotometry، color space a*، b* and L*، scanning electron microscopy، EDAX were investigated. Very good antibacterial efficacy against S. aureus and E. coli (higher than 97%) appeared even by applying a low nanosilver content (200 ppm) for twenty cycles of home laundering. Polyvinyl pyrrolidone resulted in a remarkable control in the release of silver nanoparticle from the coating and can improve the long-term microbiological activity، especially against home laundering.

Chromic behavior and electrical conductivity of poly (3-methylthiophene) nanoparticles - coated polyester fabric was investigated by chemical polymerization method. The electrochromic behavior of poly (3-methylthiophene) coated polyester fabrics were measured by digital voltmeter on the samples surface and the reflectance spectra were recorded. The effects of reaction time، temperature، and oxidant concentration on conductivity and other properties of the coated fabric were studied. Scanning electron microscopy confirmed that the surface of fabric has entirely been coated with poly (3-methylthiophene) particles at range of nanometer. Further characterizations were investigated using Fourier transform infrared spectroscopy to provide evidence of forming nanoparticles onto the fabric، UV-Vis absorption spectroscopy، electrical surface resistivity and pressure dependence visible reflectance spectrophotometer measurements. It can be found that the oxidant polymerization has been done physically without any structural changes. Results showed that the increasing of reaction time and temperature (less than 40°C) and oxidant to monomer molar ratio up to 3:1 led to a decrease in the electrical resistance of polymer-coated fabric. Also، the strength of samples was decreased during the polymerization (raw polyester fabric''s strength، 23. 36 kgf; coated polyester fabric''s strength، 19. 42 kgf). The blue shift in the reflectance spectra under high pressure represented the piezochromism in poly (3-methylthiophene) nanoparticlescoated polyester fabric.

Formation of polymer/CNT nanocomposites enhances the physical and mechanical properties of polymer matrices. Basically، the mechanical properties are investigated by means of polymer/filler interactions. The other properties such as the rheological and electrical properties are examined by particle distribution and dispersion in the polymer matrices. Due to the variety of industrial applications of polyurethane and the need to improve their properties، in the current work، the improvements of PTMEG with carbon nanotubes have been studied. The particles were acid treated in a mixture of sulfuric/nitric acid، providing uniform dispersion of the nanotubes in the polymer matrix. The rheological measurements have been used to obtain the optimum degree of MWCNT dispersion، and the formation of nanoparticles superstructure were quantified by two percolation thresholds (Ф*،Ф**) at low deformation rates. The critical percolation for PTMEG/MWCNT was estimated at 0. 25-0. 5 wt%. A significant improvement in the rheological characteristics was observed due to the agglomeration of particles at Ф* and continuous network of clusters at Ф**. The interfacial interaction of composites was also measured by means of relaxation time spectra and showed high improvement at Ф** (0. 5 wt%). Considering the FE-SEM images of specimens، it was found that there is a nanotube network or cluster agglomeration at the concentration 0. 75 wt% of MWCNT in the polymeric matrix.

The thermal residual stresses generated during curing process of laminated polymer composites can negatively affect their mechanical strength and performance. Premature failure، delamination، out-of-plane deformation and matrix cracking are negative effects of residual stresses. In order to predict the residual stresses effects on the performance of composite components، it is important to develop techniques to measure those stresses. The slitting method is one of the most conventional methods of residual stress measurement. In this method، a slit is created incrementally through the thickness of the stressed part and the released strains in each incremental depth are then measured by a strain gauge. A major assumption of this method is that the measured strains during the slitting experiment are only caused by the residual stress component perpendicular to the slit face. However، slitting process will also release two shear stress components in the slit plane، which may influence the values of the measured strains. This paper investigates the effect of the released in-plane and out-of-plane residual shear stresses in the slitting process on the measured strains for a carbon/epoxy laminate as well as a steel specimen. The results of the finite element simulations show that both components of the residual shear stress may significantly affect the measured strains during the slitting experiment. For simultaneous determination of residual normal and shear stresses، a new method using two strain gauges attached in both sides of the slit is presented. Finally، the validity of the simulation results was demonstrated by carrying out the slitting experiments on two carbon/epoxy composite specimens.

The nanoscale fracture behavior of epoxy-based nanocomposites reinforced with double-walled carbon nanotube (DWNT) was investigated by molecular dynamics (MD) simulations technique. In order to prepare a nanocomposite model including polymer and DWNT، the exact atomic structure of epoxy was adopted as in previous experimental studies made by authors. Tersoff and Amber potential، which are well known potentials، were used for simulation of polymer and DWNT، respectively. Among different available methods to simulate the cross-linking process، a technique was adopted with closer similarity to what happens in real conditions. Therefore، when some especial atoms of monomer and hardener molecules were closer than a specific potential distance، the chemical bonds were created between them. To verify the prepared model، a pull-out simulation was carried out and the results were compared with those of previous studies. It was found that although a rather wide range for interface strength has been presented by different researchers and different techniques، the strength obtained in this study is in the middle of this range. In addition، the fracture energy obtained from the simulations for pure epoxy was compared with that of experimental results and good agreement was obtained. To evaluate the effect of nanocomposite structure at nanometer scale، DWNT was modeled in three different angles relative to the loading direction، including 0°، 45°and 90°. It was found that when DWNT is parallel with the loading direction (i. e. 90°) it has the least impact on the fracture energy. The maximum fracture energy was obtained when MWNT was at 45° relative to loading direction. These results were compared with the theories provided for conventional composites.

The effect of microwave thermal treatment on mechanical properties of composites made from MDF dust-polypropylene was investigated. In this regard، nanoclay (Cloisite 15A) was added in three different (2، 4 and 6 %wt) levels into the composites. Furthermore، polypropylene (PP) was used as a matrix material and maleic anhydride-grafted polypropylene (MAPP) was used as coupling agent at 4%wt. The medium density fibreboard (MDF) dust was used as lignocellulosic material in the present study. Test samples were made، using extrusion process (twin screw)، as granules. Then، samples with dimensions 30×20×1 cm were prepared with 1 g/cm3 nominal density for the polymer composites using hot pressing method. The bending strength and impact strength were measured according to the technical specifications CEN/TS15534:2007 and D-4495 regulation of ASTM standard، respectively. Field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) techniques were employed to evaluate the extent of intercalation and exfoliation of silicate layers in the nanocomposites. Finally، the mechanical tests of flexural strength، flexural modulus and impact strength were performed on the samples. The results showed that the highest flexural strength، flexural modulus and impact strength were obtained at level 2 wt% nanoclay particles. The mechanical strength of the samples that were treated by microwave radiation was the highest. Also، X-ray diffraction studies showed increases in the interlayer spacing of silicate layers and intercalation of polymer chains between the clay layers. Scanning electron microscopy results showed that in the nanocomposites treated by microwave radiation، MDF dust was desirably compounded with thermoplastic materials.