دو ماهنامه علوم و تکنولوژی پلیمر
سال سی و دوم شماره 3 (پیاپی 161، امرداد و شهرویور 1398)

Designing a biocompatible scaffold that mimics the mechanical, electrical, chemical, and topographical properties of extracellular matrix of the target tissue is one of the main challenges in regulating cellular behaviors. For this reason, conductive scaffolds are very much considered in the engineering of electroactive tissues such as nerve, bone, and heart, and are the ideal tools for transmitting the electrical signals to these tissues and regulating their cells behaviors. On the other hand, nanostructures of conductive polymers have become a topic of interest to many researchers, because by the combination of conductivity and nanostructures, new functional materials are obtained with unique physicochemical properties, which can simultaneously simulate physical and electrical properties of the extracellular matrix. In this regard, several researchers have been working on the design of conductive scaffolds with the consideration of topographical properties. Conductive polymeric nanofibers are prepared using various conductive materials and different methods. Intrinsically conductive polymers, carbon materials such as graphene and carbon nanotubes, and metallic nanoparticles such as gold are the most common materials used for the production of conductive polymer-based nanofibers. This review covers the spinning of conductive polymer or the blend of carrier polymer and conductive agents by electrospinning and wet spinning, conductive agent deposition onto template nanofibers (in situ chemical polymerization, electrochemical polymerization, admicellar polymerization, vapor-phase polymerization, carbon and metal coating on nanofibers through immersion, coating of metal vapor on nanofibers), and template-free synthesis (interface polymerization, electrochemical polymerization) among methods of fabrication of conductive nanofibrous scaffolds and finally their advantages and disadvantages are compared together.

Epoxy resins are thermoset polymers with extensive industrial applications. Their superior properties have attracted great attention in different fields. Having the potential to provide enhanced strength-to-weight and stiffness-to-weight ratios, reinforced polymers are superior to unreinforced ones. Using nanoparticles as reinforcement in a polymer can improve toughness, aging resistance, strength and fracture of the composites.

Molecular dynamics method was used to study the effects of silicon carbide (SiC) nanoparticles on the mechanical and thermal properties of the Araldite LY 5052/Aradur HY 5052 epoxy resin. Different simulation phases, including the minimization, equilibration, curing, and calculation of mechanical properties were carried out by NPT and NVT ensembles, based on the COMPASS II force field.

The simulation results indicated that the mechanical properties of the epoxy resin system at 300 K were not only in good agreement with other experimental and theoretical properties, but also they produced greater accuracy than the previous work by COMPASS force field. The results also indicated that the addition of SiC reinforcement to the epoxy resin system improved the mechanical properties such as strength and hardness as well as the thermal properties of the system while its density increased slightly. They showed that the optimum mechanical properties were related to low concentration of SiC nanoparticles. In epoxy resin with a higher nanoparticles percentage, by increasing the weight percentage, an agglomeration phenomenon occurred, porosity increased, and consequently the mechanical and thermal properties decreased. The effect of particle size on the mechanical and thermal properties of the epoxy resin system also showed that by increasing the particle size, the mechanical and thermal properties of the system were reduced. SiC nanoparticles with 3 different nanoparticle geometries were added to the epoxy resin system. The results showed that, due to a higher surface-to-volume ratio (0.6 1/cm), the epoxy resin system with the spherical reinforcement presented higher mechanical properties than the cylindrical (0.5 1/cm) and planar (0.26 1/cm) reinforcements.

Methyl styrene monomer is a styrene derivative, created by the methyl group(s) substituted on the benzene ring. Polymethyl styrene has similar properties as with polystyrene (PS) but a lower density and higher glass transition than PS. The knowledge of polymerization kinetics allows us to construct complex polymeric architectures with different polymerization techniques and controlling their polymerization conditions. The aim of this study is to study the kinetics of free radical polymerization of methyl styrene (64% meta- and 36% para-methyl styrene isomers) in the temperature range of 80-140°C, using second- and third-order thermal initiation models and moment equations.

All polymerization runs were carried out through the ampoule method. Bulk thermal polymerization of methyl styrene was established in the range of 80-140°C and conversions were measured gravimetrically. Modeling of methyl styrene polymerization was simulated by MATLAB and moment equations. Moment equations were obtained by using the polymerization reaction. The assumed models for the initiation step of polymerization included second- and third-order in the monomer.

The theoretical results obtained from both initiation methods showed good agreement with the experimental results acquired from the gravimetric method. Also, it was demonstrated that the third-order model has better adaptation with experimental results of conversion than the second-order model. The similar results were obtained for modeling of average molecular weights. It was revealed that the gel effect had stronger effect on the average molecular weight than monomer transfer. This effect was more significant for weight average molecular weight than number average molecular weight. On the other hand, an ideal polymerization model, with no gel effect assumptions and having the same rate constants throughout, does not make good agreement with the experimental results.

Wastewater from paper industries is one the most polluting effluents. Due to presence of polymeric compounds such as lignin, this effluent is harmful for environment and public health. In this study, ordinary and electrospun (ES) polyacrylonitrile (PAN) ultrafiltration membranes were used to separate the lignin from paper mill effluent.

To improve the ultrafiltration process, response surface methodology (RSM) was employed to model and optimize the effective factors including effluent concentration, pressure and PVP concentration against rejection and flux. In addition, ‎the electrospun membranes were used to separate lignin, moreover the electrospun membranes ‎were modified by DMF vapor. Finally, the fouling effect was evaluated on all 3 types of membranes (ordinary, ES, and modied ES).

The results showed that, the performance of PAN ultrafiltration membrane was acceptable to separate the lignin from the wastewater. In fact, by increasing the waste concentration the flux decreased, but the rejection first increased and then decreased gradually. Pressure increment increased the flux and decreased the rejection linearly, however, this behavior at high pressures was taken place gradually. To improve the hydrophilic effect of PAN membranes, PVP was added to the ultrafiltration membranes. So, the flux increased significantly but PVP had a negative effect on the rejection. At optimum condition, the flux and rejection of ultrafiltration membrane reached 14.76 L/m2.h and 93.91%, respectively. In electrospun membranes the flux increased at least by twice in comparison with the optimized ultrafiltration membrane, though the rejection was lower. To increase the rejection, the electrospun membrane was modified by DMF vapor. For this study, exposing of electrospun membrane to DMF vapor for 20 min gave the best results. Finally, the fouling test was accomplished on all 3 types of membrane. The electrospun membrane displayed the longest fouling time of about 210 min, however, the ultrafiltration and modified electrospun membranes were blocked after 190 and 110 min, respectively.

In recent years, there have been several studies on characterization and preparation of polymeric organogels and alcohol-absorbent organogels which are the subgroup of polymeric organogels. Considering the importance of sulfonic monomers to improve the absorption of alcoholic acrylic acid-based adsorbent gels, due to their ability to separate ionic groups in a low-dielectric solvent such as ethanol, and the use of carbopol in medical and pharmaceutical industries, in the present study, the alcohol-absorbent gels were prepared by grafting sulfonated monomer on the surface of carbopol particles.

Alcohol-absorbent gels were prepared by grafting sulfonated monomer through the ultrasonic method. The effect of monomer content on alcohol and saline solution absorbency was investigated through swelling measurement.

In samples with similar particle size, an increase in ionic monomer content increased the absorption of alcohol due to dissociation ability of sulfonic acid groups. For example, in a sample containing 75% ionic monomer, 13% increase in ethylene glycol adsorption and 27% increase in salt absorption were observed compared to that containing 25% ionic monomer. The polymer chains resulting from the grafting of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) on the carbopol particle surface helped to increase absorption under load because of retaining the consistency of the swollen samples. Glass transition temperature (Tg) of the prepared sample decreased by 48% compared to carbopol, because the side chains caused steric hindrance and negative charge on the grafted poly(AMPS) chains resulted in electrostatic repulsion. The study on the effect of physical parameters such as particle size on the alcohol absorbency showed that by increasing the particle size of the prepared samples, the absorption of alcohol dropped significantly because of reduction in the contact area of the particles with the solvent.

Epoxy resins are one of the most prominent resins in the world. Many factors affect the mechanical properties of this family of polymers, but the effects of parameters such as molecular packing on mechanical properties are less investigated. The most important factor in molecular packing and mechanical properties is the chemical structure of the curing agent. Herein, we report the effect of aromatic structure of the curing agent on the molecular packing and mechanical properties of the cured epoxy resin.

The curing of the epoxy resin was performed by melting the curing agent and addition of the epoxy resin. To study the effect of curing agent structure, three different curing agents including meta-phenylenediamine (m-PDA), ortho-phenylenediamine (o-PDA) and 2,4-diamino toluene (2,4-DAT) were used. The molecular packing was studied by X-ray diffraction (XRD) technique. Also, the density measurements of cured epoxy resins were carried out using the Archimedes method and finally, the effect of molecular packing on the mechanical properties was investigated by tensile test.

The results obtained from XRD and tensile test measurements showed that there is a direct relationship between the molecular packing and mechanical strength which by increases in the molecular packing the tensile strength increased as well. By changing the curing agent from m-PDA to o-PDA, the molecular packing was increased and consequently, led to an increase in the tensile strength of the epoxies. In using 2,4-DAT as a curing agent, the molecular packing and hence the mechanical strength were decreased due to the steric hindrance of the methyl group.