دو ماهنامه علوم و تکنولوژی پلیمر
سال سی و پنجم شماره 3 (پیاپی 179، امرداد و شهریور 1401)

Gas-liquid membrane contactors are promising alternatives to conventional absorption technologies. However, despite their significant advantages, such systems face the major problem of gradual wetting of the porous polymeric membranes with liquid absorbents. In fact, wetting of porous polymeric membranes by liquid absorbents increases the mass transfer resistance of the membrane phase, which in turn, reduces the gas absorption efficiency. Accordingly, in this paper, the effect of membrane wetting on mass transfer resistance and absorption efficiency has been studied. The influence of effective parameters on membrane wetting phenomenon, such as absorbent and membrane properties, is investigated. In addition, different prevention methods of membrane wetting is discussed in detail. The results show the increase in the rate of wetting at high velocity and pressure of the absorbent. In the case of absorbents-containing organic compounds, their surface tension is decreased rapidly with the increasing concentration of organic compounds, which increased the wetting rate of the membrane. In addition, studies on various polymeric membranes have shown that the structural and chemical changes of the membrane surface, which occur due to long-term contact of the membranes with amine absorbents, strongly increase the wetting of the membranes, and thus, its functionality. Moreover, studies have shown that modifying the surface of membranes is one of the most effective methods to prevent the problem of wetting. By modifying the surface, the free energy of the membrane surface can be reduced and its roughness can be increased, thus increasing the hydrophobicity of the membrane surface. Among these, nanotechnology has been introduced as one of the most important technologies in the production of superhydrophobic membranes to prevent the problem of wetting the porous polymeric membranes.

Hypothesis: 

Nowadays, nanotechnology has an important role in the medical industry. Electrospun nanofibers with interconnected structure, high surface-to-volume ratio and good porosity are used in various medical fields. Nanofibers morphology is very effective in their mechanical, thermal, physical and biological properties. The aim of this study is to produce structures with smooth surface and minimized diameter. 

Production assessments are made on hybrid nanofibrous structures using combinations of poly(vinyl alcohol)/chitosan (PVA/CS), and gelatin/polycaprolactone (Gel/PCL). The process is done through the electrospinning process of two nozzles facing each other. For this purpose, Gel/PCL has been injected from a nozzle and CS/PVA from another nozzle and the produced nanofibers are collected on a rotating collector. By a Taguchi test design the effect of variable parameters (PVA/CS ratios of 90:10, 80:20, 70:30; gel/PCL ratios of 80:20, 50:50, 20:80; and CS/PVA feed rates of 1, 1.5, 2 mL/h) on morphological properties of the produced scaffold is evaluated.

According to the results, nanofibers with the blend ratios of 75:25 and 80:20 for CS/PVA and gel/PCL, respectively, and flow rate of 1 mL/h for PVA/CS showed an average diameter of about 130 nm and a suitable morphology. A hybrid scaffold made of gel/PCL-PVA/CS can be considered as a very suitable and practical biomaterial in the field of diabetic wound healing, because the presence of gel and CS as natural polymers with excellent biological properties and PCL with high elastic property provide good conditions for imitating natural skin behavior.

Hypothesis: 

Due to the presence of intra- and inter molecular hydrogen bonding in alginate chemical structure, its electrospinning capability is weak. However, this weakness can be improved through substitution of hydroxyl groups by sulfate groups. This article focuses on the role of degree of substitution of sulfate groups on the physicochemical properties of electrospinning solutions, such as viscosity, electrical conductivity and electrospinning conditions.

Sodium sulfated alginate (SSA) was synthesized through the reaction of sodium alginate and chlorosulfonic acid in formamide as the solvent. The amount of chlorosulfonic acid was varied in order to obtain the SSA samples with different degrees of substitution. The chemical structures of neat alginate and SSA were studied by FTIR and 1H NMR spectroscopy. Degree of sulfation of samples was measured using CHNS elemental analysis, and the electrical conductivity and viscosity of SSA solutions were measured. The SSA nanofibers were fabricated using electrospinning and further crosslinked by a solution of calcium chloride to improve its hydrolytic stability. Finally, the fiber diameter and mechanical properties of the nanofibrous mat were studied by SEM and a tensile mechanical machine.

Both FTIR and 1H NMR analyses have confirmed the formation of sulfate groups in SSA structure. Based on elemental analysis, the degree of substitution (DS) of SSA samples has been measured as 0.9 and 0.5 for SSA1 and SSA0.5, respectively. The electrical conductivity and viscosity of the SSA solutions also increased and decreased by increasing DS, respectively.  The SSA1 sample showed better electrospinning capability and higher SSA content in dry electrospun mat compared to those in SSA0.5 sample. Finally, the crosslinked SSA1 mat revealed a lower mechanical strength compared to SSA0.5 mat due to lower crosslink density and higher chain scission of polymeric chains resulted from sulfation reaction.

Hypothesis: 

Modifications in chemical formulations of existing commercial polymerization catalysts may deteriorate other catalyst properties, especially stereo- and region-selectivity. Therefore, an absolute necessity for petrochemical polymerization facilities is to find feasible non-chemical routes for tailoring essential parameters, including molecular weight distribution (MWD) width, activity and fragmentation mechanism in order to modify existing catalytic systems. 

To this goal, use is made of a recently developed single-particle multipore model (MPM), which describes the reaction-diffusion processes involved in the heterogeneous olefin polymerization to investigate the impacts of initial catalyst porosity, initial catalyst particle size, bulk monomer concentration and pore size arrangement on the above-mentioned parameters. 

Modeling a supported Ziegler-Natta catalyst system showed that increasing the initial catalyst porosity or initial particle size or decreasing the bulk monomer concentration decreased the local reaction rate distribution width, resulting in narrower MWDs. Although, the polydispersity index generally changed oppositely due to its dependence on the location of the MWD in addition to its width. The model has elucidated and rationalized two unexplained experimental observations, i.e., increasing initial porosity reduces the catalyst activity in some studies and that polydispersity index generally changes irregularly and unpredictably with bulk monomer concentration. For the physical quantities studied in this work, the reaction rate is directly related to the MWD width, revealing that a trade-off between MWD width and yield should be sought for applications that require higher resistance to melt fracture phenomena, edge waviness and draw resonance. While, the reaction rate, MWD width and polydispersity index did not show any relationship with the participation ratio of the two fragmentation mechanisms. Increasing the initial catalyst porosity or the initial particle radius intensified the more preferred continuous bisection mechanism, thereby dropping the probability in fouling.

Hypothesis: 

Poly(dimethyl siloxane) block copolymers membranes are a good choice for gas separation application. The mechanical strength and gas separation properties of these polymers can be improved by the preparation of layered membranes. In layered membranes, the coating layer generally controls the flow and selectivity of gas, while the porous substrate plays the role of mechanical strength supplier. The dense layer, at the interface of the substrate and selective layer, can also affect the efficiency of the layered membranes gas separation, and its performance depends on the morphology of the substrate and the gas permeation properties of the substrate and the coating layer. The goal of this research is to prepare and investigate the effect of substrate morphology on the performance of poly(dimethyl siloxane) block copolymers/polyurethane layered membranes.

Polyurethane substrates were prepared through the non-solvent-induced phase separation (NIPS) method. Different substrates with sponge-like and finger-like morphology structures were prepared with the help of changing the concentration of dope solution concentration (10, 20, 25, 30% by weight of polymer) and coagulation bath (molar ratio of water/methanol=100/0, 80/20, 70/30 and 50/50), and used for the preparation of poly(dimethyl siloxane) block copolymer/polyurethane layered membranes.

The polyurethane substrates revealed a finger-like morphology structure when a water coagulation bath was used due to the high exchange rate of solvent/non-solvent. Meanwhile, by adding methanol to the coagulation bath, the morphology of the substrate changed to a sponge-like one. In an overview, the gas separation properties of poly(dimethyl siloxane) block copolymers were improved by using a polyurethane substrate. The results revealed that by changing the morphology of the substrate from finger-like to sponge-like, the CO2 permeability of the membranes improved from 1.58 GPU to 4.53 GPU. While the permselectivity of the layered membranes (CO2/N2) decreased from 21.94 to 12.57.

Hypothesis: 

The surface modification of a heterogeneous cation exchange membrane was carried out through a chitosan nanocomposite layer containing copper oxide nanoparticles in an electrodialysis process. The effect of the formed surface layer on the structure and transfer, separation and antibacterial properties of the membranes was investigated.   

Double layer membranes were produced by dip-coating method. Scanning electron microscopy (SEM), X-ray diffractometry (XRD) and Fourier transform infrared spectroscopy (FTIR-ATR), electrical resistance, ionic flux, ability to remove heavy metal ions, water content, water contact angle and antibacterial experiments were employed to examine the membranes.

The EDX and FTIR results confirmed the formation of chitosan-copper oxide nanocomposite layer on the surface of pristine membrane. The SEM images also showed the formation of a uniform layer on the modified membranes. The amount of water content for double-layer membranes showed an increasing trend compared to pristine membranes, although the contact angle results proved an increase in surface roughness for double-layer membranes. The results of ionic properties also showed that the electrical resistances of double-layer membranes decreased to 46% initially by utilizing CuO nanoparticles in the surface layer, whereas the monovalent ionic flux and bivalent flux for heavy metals were enhanced by 50% and >300%, respectively. At high ratios of CuO nanoparticles in the surface layer, the electrical resistance of membranes increased again and the flux showed a decreasing trend. Double-layer nanocomposite membranes showed a high ability to remove copper heavy metal ions and their antibacterial performance was suitable against Escherichia coli. Among the prepared membranes, the double-layer membrane containing 0.001% (by weight) copper oxide nanoparticles showed better performance compared to a pristine and other modified membranes.