دو ماهنامه علوم و تکنولوژی پلیمر
سال بیست و هفتم شماره 3 (پیاپی 131، امرداد و شهریور 1393)

The starch-based films have some disadvantages such as weak mechanical and poor water barrier properties that restrict their applications in food packaging. In the present research، to improve the properties of the starch films، a constant level of citric acid and polyvinyl alcohol (PVA) (10%) with different amounts of glycerol (GLY) as a lubricating agent، crystal nanowhiskers (CNW) and titanium dioxide (TiO2) nanoparticles were used together. Finally، the effects of these compounds on permeability properties of the obtained starch-based bionanocomposites were studied and their optimum values were determined by central composite design of response surface methodology (RSM). The results of X-Ray diffraction (XRD) test showed that at low levels of TiO2 and CNW there was no diffractogram peak obtained. However، at high levels of TiO2 and CNW there emerged distinct and sharp peak which was attributed to greater crystalline region and probably non-homogeneity in particle distribution. The Fourier transmission infrared (FTIR) data showed that addition of CNW and TiO2 increased hydrogen binding between the nanofillers and biopolymer matrix. The effects of TiO2 and CNW concentrations as quadratic and glycerol concentration as linear and quadratic were significant on water vapor permeability (WVP). The optimum levels of TiO2، CNW and GLY for obtaining minimum WVP corresponded to 0. 118، 0. 3 g and 1. 06 mL، respectively.

Electromagnetic compatibility (EMC) and electromagnetic interference (EMI) have emerged as key issues with respect to commercial and military purposes in association with electromagnetic waves. The importance of protection against electromagnetic interference in wireless communication and electronic toll collection (ETC) systems has undoubtedly increased over the years. Generally، the electromagnetic absorption properties of material depend on their intrinsic electromagnetic properties such as conductivity، magnetic permeability and dielectric constant and also factors such as thickness and frequency. The effect of each parameter on the absorption performance is yet difficult to comprehend due to the complexity of electromagnetic waves propagation in different media. Addition of pure dielectric or magnetic material to a polymer matrix is a possible way to change electromagnetic properties of the materials. In this study nanocomposites of polystyrene/multi-walled carbon nanotubes were prepared using a solution method with three different homogenizer speeds for the purpose of nanotube dispersion and evaluation of the effect of nanotube dispersion on the electromagnetic wave absorption properties. The morphology of the nanocomposits was investigated by scanning electron microscopy (SEM). The capability and properties of electromagnetic wave absorption of nanocomposites were studied in the frequency range of 5 to 8 GHz using a vector network analyzer and finally the results of their absorption were compared with each other. It was found that by improving the dispersion of nanoparticles، both the amount and bandwidth of absorption increase. Moreover، by increasing the homogenizer speed up to 10000 rpm the maximum reflection loss was reported to occur at 8 GHz.

Due to the gel formation in polybutadiene and some of the problems raised in rubber industry such as groove cracking in products، quantification of the gel content and identification of the microstructure of butadiene elastomer is extremely important. In this paper، the gel content in polybutadiene rubber was identified by differential scanning calorimetry. The thermal decomposition of the polybutadiene rubber was studied by thermal analysis methods in 0-650°C. The thermal decomposition، in the same temperature range، of the polymer with different gel contents under the similar conditions showed that the oxidation and decomposition of the samples occur in three steps. The thermal analysis curves showed that the cis-trans isomerization occurs by increases in the gel content at lower temperatures of a wider thermal range with rising trans isomer and decreases in cis isomer contents، respectively. Also، with the increase in the gel content there is a tendency in lowering double bond density and there is higher energy release in oxidation and decomposition regions due to the breakage in crosslink density of polymer networks. According to the values resulting from enthalpy and heat flow rate، the thermal index was obtained for samples at two maximum temperatures related to two processes of cis-trans isomerization and the cyclization reaction. The gel content (C-C crosslinked) in polybutadiene rubber is determined by the interconnection between the ratio of heat flow index and gel content from the differential scanning calorimetry curves.

The micromechanical models are used to investigate mechanical and thermal properties of a polymer matrix nanocomposite containing multi-walled carbon nanotubes (MWCNT) in their effects to reduce residual stresses in nanocomposites. To do this، first nanotubes with different weights and volume fractions were dispersed in ML-506 epoxy resin. By using different micromechanical models، the effect additional nanotubes on elastic modulus and coefficient of thermal expansion (CTE) of nanotubes/epoxy were studied as critical parameters. Comparing the model and available experimental results، the modified Halpin-Tsai model and the modified Schapery model were chosen to calculate the mechanical and thermal properties of the nanocomposites. Then، using the matrix reinforced with MWCNT and classical micromechanics models the elastic modulus and coefficients of thermal expansion of the nanocomposites were determined for a single orthotropic ply. The results showed that the rule of mixture (ROM) and Hashin-Rosen model to determine the longitudinal and transverse elastic moduli and Van Fo Fy model to calculate the coefficient of thermal expansion were in good agreements with the experimental results of a single-layer nanocomposite. Finally، the classical laminated plate theory (CLPT) was used to calculate the residual stresses of the CNT/carbon fiber/epoxy composites with different weights and volume fractions of MWCNT for angle-ply، cross-ply and quasi-isotropic laminated composite materials. The results showed that residual stresses were reduced using a maximum of 1% wt or 0. 675% volume fraction of the MWCNT in polymer composites. Also، the highest reduction in residual stresses was observed in [02/902] cross-ply laminated composite materials.

A 185/65R15 steel belted radial tire was analyzed for the prediction of its rolling resistance using finite element method. The Abaqus code was used for this purpose. A two-dimensional axisymmetric model was first designed to form the tire layout in the mold. After analyzing for rim mounting، an internal pressure was applied to the tire. Having rotated the tire cross-section about rolling axis، a three-dimensional model was then created and used for the analyses under static vertical load and steady state rolling conditions. Owing to the use of arbitrary Lagrangian/Eulerian framework، a constant linear velocity was assumed and the analysis was performed for a range of angular velocity of the tire. An in-house developed user subroutine was employed and linked to the Abaqus for the accurate determination of the free rolling rotational speed (angular velocity) of the tire based on zero force/torque. Two sets of analyses were performed. In the first set، it was assumed that the mechanical behaviors of the rubbery parts could be described by the well-known Ogden hyperelastic model. In the second set، hyper-viscoelastic behaviors were assumed in which the Ogden model was combined with the Prony series to take the material history and time effect into consideration. The difference between the calculated longitudinal forces in rolling state using the mentioned models was attributed to the rolling resistance of the tire. In order to check the accuracy of the proposed method، the predicted rolling resistance force was compared with that of experimentally obtained data which confirmed the applicability and robustness of the model. The contact pressure distributions have been presented and discussed in relation to different types of material model.

Phenolic resin composites reinforced with short carbon fiber are one of the most usable materials in ultra-high-temperature applications such as thermal protective in aerospace industries. In this work، novolac type of phenolic resin matrix was modified with graphite nanoparticles to prepare multi-layered nanocomposites. The effect of graphite nanoparticles was studied on the thermal stability، ablation and mechanical properties of novolac/short carbon fiber composites to achieve nanocomposite with optimum properties for ultra-high-temperature applications. In order to evaluate thermal stability and ablation properties of composite and nanocomposites، a sample containing 40 wt% short carbon fiber was prepared as a reference and the structure of its polymeric matrix was modified with nanographite particles. The amounts of nanographite powders in nanocomposite samples were chosen as 6، 9 and 12 wt%. XRD Spectroscopy was used to study and investigate the dispersion of the graphite nanoparticles and morphology in the polymeric matrix. The compression molding under hot press method was used to fabricate the composite and nanocomposite specimens. Thermal properties of the nanocomposites were studied by TGA and oxy-acetylene flame test. Three-point bending and wear tests were performed to measure the mechanical and wear properties of the nanocomposites. The obtained results showed that the addition of nanographite improved the thermal stability، decreased the rate of degradation and at the same time decreased the weight loss and ablation rate of the nanocomposites. Addition of 12 wt% nanographite particles increased thermal stability by about 12% compared to the reference sample. Moreover in nanocomposite with 12 wt% graphite، the rate of ablation decreased by more than 19% compared to the reference composite.

The rheology and swelling behavior of a silicone rubber was studied at various cure states using bis (2،4-dichlorobenzoyl) peroxide as the curing agent. To this end، first an optimization was carried out on the percentage of crosslinking agent، the time and temperature of curing process. Then، samples، at various cured states، were prepared at zero (uncured sample)، gel point، 5%، 15%، 30%، 60% and 100% gel content using a moving die rheometer RPA2000. The moving die rheometer RPA2000 offered obvious priorities over the old oscillating disc rheometers and it was able to measure the behavior of rubber compounds prior، during and after cure in a single test. The results of frequency sweep tests indicated that the modulus of semi-cured samples increased with increasing the frequency and cure state. The increase in frequency was ineffective on the storage modulus of samples of higher cure states، exhibiting a plateau throughout the whole frequency range. The samples with lower percentage of the crosslinking exhibited a shoulder at low frequencies resembling the behavior commonly observed for immiscible polymer blends at frequency sweep tests. The viscosity behavior versus frequency for all semi-cured compounds obeyed the power law model with a power law index being quite far from Newtonian behavior. The crosslink density was determined using the Flory-Rhener equation. The swelling magnitude، the weight decreasing rate as a result of the extraction of the sol part and the swelling rate decreased with increasing the cure state.