دو ماهنامه علوم و تکنولوژی پلیمر
سال بیست و هشتم شماره 3 (پیاپی 137، امرداد و شهریور 1394)

An Experimental and numerical study on the flexural behavior of new types of sandwich structures with glass-epoxy skins and a combinatorial core consisting of PVC foam and a corrugated composite was performed. The purpose of inserting a corrugated composite into the core was the reinforcement of the core and so of the sandwich structure without substantial increment in its weight. Samples were prepared by vacuum assisted resin transform molding industrial technique and tested using three-point bending load test according to ASTM C393 and then the load-deflection curves were obtained. The finite element analysis was performed using Abaqus software to determine the maximum deflection of the samples. In order to increase the precision of numerical results، the tensile test was carried out according to ASTM D3039 to obtain the mechanical properties of the skins and corrugated composite. In addition to a reference sample consisting of a simple foam core، three series of samples were prepared، which consisted of corrugated composites، with square، trapezoidal and triangular geometries، inserted in a PVC foam core. For each experiment، three samples were prepared and tested and the data were used as mean values. It was revealed that the highest and lowest increases in flexural stiffness and flexural stiffness-to-weight ratio were obtained for the samples with trapezoidal and triangular geometries، respectively. Finally، the experimental and numerical results were compared and a good agreement was observed in all samples.

Alightweight polymer composite with a broad bandwidth، tunable absorption frequency and multi-functionality is an ideal material for making a radar absorber. In general، composite microcellular foams have many potential applications due to their lightweight، high mechanical properties and monotonous cell structure. In this research، the effect of foaming method on the radar absorbing properties of composite radar absorbers was investigated. In the first step، a controllable، repeatable and high pressure/temperature operation foaming system by supercritical CO2 gas as foaming agent was designed and built. The composites based on poly (methyl methacrylate) (PMMA) and multiwall carbon nanotube (MWCNT) at different weight percentages were prepared by solvent-anti solvent coagulation method. The sample sheets with 3 mm thickness were molded using hot compression molding method. Then، the foaming process was performed and the cell morphology of the prepared foams was studied using scanning electron microscopy. Monotonous cell structure of the composite foams revealed a good dispersion of nanoparticles in the polymer matrix. The data of the reflected radar waves (before and after foaming) showed that the foaming reduced the reflection of the radar waves to less than 10 percent in all the samples. It is important to note that the absorption of radar waveswas increased with the foaming of neat PMMA. It was observed that the foaming of composites increased the threshold of absorption of radar waves from less than 1 wt% nanotube for the unfoamed samples to 1-3 wt% nanotube for the foamed samples.

Effect of different formulation ingredients on the abrasion behavior، crack growth and modulus of tire tread formulation was studied using two different case studies. In the first case study، the effect of the partial substitution of natural rubber by cis-butadiene and the content variation of oil and sulfur in the presence of modified clay was studied on the basis of central composite design experiment in a NR/SBR-based truck tire tread formulation. In the second case study، the effect of oil، sulfur and highly dispersible silica level was investigated via Box-Benken design experiment in a SBR/BR-based passenger tire tread formulation. In each study a suitable response surface model was developed on the basis of the data obtained using the experimental design. Artificial neural network models with forwarding multi-layers were also developed to investigate the potential of the current approach in modeling of fracture behavior of rubber materials. It was observed that the complex dependency of the fracture/abrasion behavior of rubbery materials on formulation variations could be modeled with high accuracy through response surface and artificial neural models. The response surface profiles were developed to explain the abrasion behavior better. The observed behaviors for the abrasion of rubber formulations were also investigated with the aid of the modulus statistical analysis، deMattia crack growth model and also the Fukahori and mechano-chemical abrasion theories. In the presence of high levels of cis-butadiene، the abrasion with the mechano-chemical mechanism is dominant. However، according to the Fukahori model، the mean amplitude strain has a key effect on the abrasion of rubbery materials.

Nanocomposite membranes of ethylene-propylene-diene monomer/multiwalled carbon nanotubes (EPDM/MWCNT) were prepared by solution casting، solvent evaporation and cross-link technique to be applied in CO2/N2 gas separation. Both simple and functionalized MWCNTs have been used. The effect of incorporated different amounts multiwalled carbon nanotubes (0-4 wt%)، of both simple and functionalized types، on the performance of nanocomposite membranes was studied. Fourier transform infrared (FTIR) spectroscopy and field emission scanning electron microscopy (FESEM) were used to evaluate the structural/morphological observations of nanocomposite membranes. Comparing the FTIR results of pure and functionalized nanotubes confirmed the presence of carboxylic groups on the functional carbon nanotubes. The FESEM images indicated that at low concentrations، carbon nanotube particles were dispersed well in the EPDM matrix، but they formed agglomerates at concentrations beyond 1 wt%. By incorporation of MWCNTs، the mechanicalproperties of nanocomposite membranes including tensile strength، Young''s modulus and elongation-at-break considerably were improved. By increasing carbon nanotube loading up to 0. 75 wt%، the permeability of both CO2 and N2 and the CO2/N2 selectivity increased. Further loading led to higher permeability of CO2/N2، while the selectivity ofthe system decreased that could be attributed to further agglomeration of carbon nanotube particles. Furthermore، functionalization of carbon nanotubes improved their dispersion and the mechanical properties and gas separation performance of nanocomposite membranes. Through functionalizing of MWCNTs، both the CO2 permeability and CO2/N2 selectivity of the optimum membrane (0. 75 wt% MWCNTs) increased from 37. 95 and 18. 03 Barrer to 57. 57 and 23. 43 Barrer، respectively. At ambient temperature، by the increase in feed pressure a slight increase in the permeability of both CO2 and N2 gases was observed، while the CO2/N2 selectivity was not highly affected.

Adrug delivery process at a particular organ or site and at a specific time requires drug dose adjustment to reduce side effects and accelerate faster healing. The three parameters of time، site and release rate can be modulated by controlled drug delivery systems. Hydrogels are hydrophilic polymers and copolymers with three-dimensional network structures that nowadays are used in new controlled drug delivery systems. These macromolecules can respond to external stimuli such as temperature، pH and ionic strength. In this study، biocompatible acrylic hydrogels، synthesized by ultrasound، were studied to examine controlled drug release of Fluvoxamine. A pulsed power ultrasound was applied to the reaction mixture from the tip of a probe unit. It was found that hydrogel formation was faster using ultrasound. The results showed that ultrasonic irradiation significantly reduced the reaction time and increased efficiency. Additionally، increasing glycerol in the solution improved the viscosity of the reaction towards higher reaction rate. Also، we studied the stimuli sensitivity and swelling of hydrogels، and the rate of drug release at different temperatures and pH media. The results showed that the ultrasound irradiated acrylic hydrogels were sensitized towards pH and temperature variations. These hydrogels، due to their highly porous structure، were capable to load and release the drug rapidly and their performances were affected by pH and temperature. Also، the results showed that in a simulated body environment، the hydrogels were suitable options as controlled drug delivery systems in intestinal media. The ultrasonic polymerization method described here has a wide range of applications in biomaterial synthesis where initiators are not desired. The method adopted in this study can be developed for other gels and drugs.

Nitrile rubber/clay nanocomposites were prepared via in-situ emulsifier-free emulsion polymerization technique in the presence of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) and pristine sodium montmorillonite (Na-MMT). The morphology of the nanocomposites was studied using X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM). The XRD results showed exfoliated morphology for the nanocomposites containing up to 3 wt% nanoclay and exfoliated-intercalated morphology for the nanocomposite containing 5 wt% nanoclay. The basal spacing of the nanocomposite، containing 5 wt% nanoclay، was increased to 1. 8 nm، which was 0. 61 nm wider than that obtained using pristine Na-MMT. Compared to neat rubber، the thermogravimetric analysis (TGA) revealed improvements in the thermal stability of all nanocomposite samples، in which the thermal degradation temperature was increased by increasing the clay content. The maximum increase in the thermal stability of the nanocomposites was obtained for the nanocomposite containing 5 wt% nanoclay with exfoliated-intercalated morphology. The tensile testing results showed remarkable improvements in the mechanical strength of the nanocomposites. In comparison with the neat nitrile rubber، the exfoliated nanocomposite، containing 3 wt% nanoclay، showed 302% and 219% increases in tensile modulus and tensile strength، respectively. The reason for improvementin mechanical properties of a nanocomposite، containing 3 wt% of nanoclay with respect to the nanocomposite containing 5 wt% of nanoclay، was related to the better dispersion of nanoclay platelets in the matrix and formation of exfoliated morphology. In addition، elongation-at-break and hardness were increased in the nanocomposites، when compared with the neat nitrile rubber.

Polystyrene/MCM–41 nanocomposites were synthesized by atom transfer radical polymerization (ATRP) at 110°C. Activators generated by electron transfer (AGET) and activators regenerated by electron transfer (ARGET)، as two novel initiation techniques، for ATRP were used. Specific structure، surface area، particles size and their distribution and spongy and porous structure of the synthesized MCM–41 nanoparticles were evaluated using X–ray diffraction، nitrogen adsorption/desorption isotherm analysis، scanning and transmission electron microscopy images، respectively. The final monomer conversion was determined using gas chromatography. Number and weight average molecular weights (Mn and Mw) and polydispersity index (PDI) were also evaluated by gel permeationchromatography. According to the results، addition of 3 wt% MCM–41 nanoparticles into the polymerization media resulted in lowering conversion from 81 to 58% in the AGET ATRP system. Moreover، a reduction in the molecular weight of the products from 17116 to 12798 g/mol was also occurred، although، the polydispersity index increased from 1. 24 to 1. 58. The similar results were also obtained by ARGET ATRP system; lowering conversion from 69 to 43% and molecular weight from 14892 to 9297 g/mol، and an increase of PDI from 1. 14 to 1. 41. The improvement in thermal stability of the nanocomposites، as a result of higher MCM–41 nanoparticles loading، was confirmed by thermogravimetric analysis. In addition، according to the analytical results of differential scanning calorimetry، a decrease in glass transition temperature، due to the addition of 3 wt% of MCM–41 nanoparticles (from 100. 1 to 91. 5°C in AGET ATRP system and from 100. 3 to 85. 8°C in ARGET ATRP)، was achieved.