دو ماهنامه علوم و تکنولوژی پلیمر
سال سی و چهارم شماره 1 (پیاپی 171، فروردین و اردیبهشت 1400)

Epoxy resins are widely used as encapsulating materials in the electronic and electrical industries, transportation, coatings, adhesives, and advanced composite matrices owing to their excellent heat, moisture and chemical resistance, high tensile strength, low shrinkage on curing, and excellent dimensional stability. Epoxy resins are very flammable; this nature seriously restricts their widespread applications, especially in the areas requiring high flame retardancy. To address this challenging problem, several approaches, such as use of halogen-based flame retardant, organophosphorus compounds (e.g., ammonium polyphosphate and melamine polyphosphate), inorganic flame retardants (most widely used alumina trihydrate and magnesium hydroxide known as ATH and MDH, respectively), nitrogen-based flame retardants, silicon-based flame retardants, intumescent flame retardants, and nanomaterials have been proposed. Phosphorus-containing compounds are mostly used as substitutes for the halogenated compounds in flame-retardant epoxy resins. Various phosphorus compounds are used as additive or reactive flame retardants. Comparing the additive flame retardants, reactive organophosphorus compounds present excellent flame retardant efficiency in the case of epoxy resins and it can be chemically linked to the backbone of the network either through being part of the curing agent or through the structure of epoxy resins itself and giving intrinsic flame retardancy. In this review, classification of flame retardants (halogen-based flame retardants, organophosphorus compounds, inorganic flame retardants, nitrogen- and silicon-based flame retardants, intumescent flame retardants and nanocomposites), the combustion cycle of polymers and application of flame retardants, especially phosphorus-based flame retardants for epoxy resins are introduced and classified. Experimental test methods for evaluation of flame retardancy like UL-94, limiting oxygen index and cone calorimeter are also discussed.

Hypothesis: 

Despite the wide application of nanofiltration (NF) membranes in forward osmosis (FO) process, one of the most important challenges of this process is the internal concentration polarization (ICP) phenomenon. Different methods have been investigated to reduce the effect of this undesirable phenomenon and it is suggested that one of these methods is loading of hydrophilic nanoparticles in the membrane structure. In this study, SiO2/ZIF-8 nanoparticles were used to improve the structure and performance of polyethersulfone membranes (PES) in FO process.

At first, polyethersulfone membrane was synthesized by phase inversion method. In the next step, ZIF-8 and SiO2/ZIF-8 nanoparticles as filler were synthesized at room temperature. Thin film composite membranes were prepared by the interfacial polymerization (IP) of two reactive organic (TMC) and aqueous (MPD) monomers. Finally, the produced membranes and nanocomposite were characterized by contact angle, FTIR spectroscopy, X-ray diffractometry (XRD), field emission scanning electron microscopy (FESEM), and porosity measurements. Also, the performance of all membranes composed of at least two different components was investigated using reverse and forward osmosisi processes.

The outcomes demonstrated that the presence of a small amount of SiO2/ZIF-8 nanoparticle in the membrane led to an increase in the membrane hydrophilicity and porosity, and also improved the water flux and rejection of the FO. The water flux of TFN FO membrane was reported to increase remarkably from 15.23 to 25.13 L/m2.h when 10 mM NaCl and 2 M NaCl salt were utilized as feed solution (FS) and draw solution (DS), respectively. The improvement in FO water flux was ascribed to the lower S parameter of modified PES sublayer and the reduction of internal concentration polarization (ICP).

Hypothesis: 

Adhesives and sealants based on modified silane polymer (MS polymer) are increasingly gaining market share in adhesive and sealant market place due to their desirable performance in various applications.

In order to find the most optimal adhesive processing parameters, different stages of aluminum surface treatment in three stages were investigated, including the surface degreasing, sanding and priming. In this manner, the performance of four different types of degreasing agents and two kinds of silane-based primers were investigated individually. The optimum bond line thickness was also determined through adhesive joint shear strength. The effect of different degreasing agents and silane-based primers were evaluated through lap-shear strength. The effect of adhesive layer thickness was studied through lap-shear strength and cleavage strength tests.

Optimum bond line thickness was determined at 0.5 mm based on the results of the adhesive shear strength, whereas the results of the fracture toughness indicated 1.25 mm as the optimum adhesive layer thickness. The lower adhesive strength in thicker bond lines may well be due to the possible formation of air traps and lower constrained effect of the substrate. Therefore, the least requirements for aluminum surface treatment was surface degreasing followed by sanding which results in a transition from adhesive fracture mode to cohesive fracture. The use of two kinds of silane-based primers containing the amine group, triethoxy silylpropylamine (APS) and the other with epoxy group, glycidoxypropyl trimethoxysilane (GPS) was more efficient in improving joint durability even after 2500 h cyclic aging with the more pronounced results for the APS.

Hypothesis:

 In recent years, gel polymer electrolytes (quasi-solid state electrolytes) have attracted great attention as a suitable substitute for liquid electrolytes. On the other hand, ionic liquids could dramatically enhance the ionic conductivity of electrolytes. In this work, gel polymer electrolytes based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)/poly(ethylene oxide) (PEO) blends (for application in dye-sensitized solar cells (DSSCs)) and imidazolium-based ionic liquids were prepared. It is supposed that blending these two polymers could reduce the degree of crystallization and increase the porosity of the electrolyte blend to yield a higher electrolyte uptake and ionic conductivity.   

Polymer blend electrolytes were prepared in different blend ratios and in the presence of either one of the ionic liquids including BMII or BMIMBF4 through phase inversion method and their properties were investigated by differential scanning calorimetry (DSC), mercury porosimetry, electrolyte uptake, and morphology to optimize the blend ratio.  

It was found that the blend ratio of 60/40 (w/w) PVDF-HFP/PEO has the highest porosity and electrolyte uptake. Crystallization investigations by DSC showed that there is a direct relationship between the decrease of crystallinity of two polymers and the increment of electrolyte ionic conductivity. Electrolyte uptake gradually increased with increasing PEO component concentration up to 40 wt%, and reached a maximum of 98.49% and 89.48% for BMIMBF4 and BMII, respectively. Beyond this concentration, a decrease in electrolyte uptake was seen, which is an undesirable feature for the produced samples. In this blend ratio ionic conductivity was measured as 2.07 mS/cm and 1.78 mS/cm for PVDF-HFP/PEO/BMIMBF4 and PVDF-HFP/PEO/BMII electrolytes, respectively.

Hypothesis: 

Loading of herbal drug within the polymer hydrogel matrix significantly improves its performance as wound dressing. In this way, the drug amount loaded into the hydrogel plays a key role and can influence the physicomechanical properties, drug release behavior, and cell proliferation of hydrogel.

Hydrogels based on poly(vinyl alcohol) (PVA)/agar/poly(ethylene glycol) (PEG) containing various amounts (0, 0.2, 0.5, 1 and 2% by wt) of aloe vera were prepared by electron beam irradiation method at a dose of 25 kGy and keeping their water solution under the ambient atmosphere. The effect of aloe vera amount on the gel content, equilibrium swelling degree, dehydration and tensile mechanical properties of hydrogel along with aloe vera release behavior from the hydrogel and fibroblast cell proliferation was investigated.

The results obtained from the measurements indicated that with increasing aloe vera content from 0.2 wt% to 1 wt%, the gel content decreased from 55 to 47%, and the swelling degree increased from 195 to 294%. Unexpectedly, the incorporation of aloe vera to 1 wt% into the hydrogel enhanced its tensile strength by 30%, and its elongation-at-break by 119%. The dehydration phenomenon of the hydrogel was notably accelerated in the presence of aloe vera. The release kinetics of aloe vera from the hydrogel showed linear behavior and the highest release rate and amount (about 70%) was obtained for the hydrogel containing 1 wt% aloe vera. The proliferation of fibroblast cells was also the most for the hydrogel with 1 wt% aloe vera. Finally, based on the results of studies, the optimal amount of aloe vera in hydrogel was determined to be 1 wt%.

Hypothesis:

 Palierne emulsion model is usually used to investigate the rheology-morphology correlation for immiscible polymer blends by droplet-matrix morphology. The Palierne model describes the linear viscoelastic behavior of emulsions and suspensions properly only when the dispersed phase volume fraction is not high. In the case of nanocomposites, the Palierne model cannot predict the experimental data due to the interactions between the nanoparticles. Therefore, researchers modified Palierne emulsion model by considering the shear amplification rate effect and stress amplification rate effect for nanocomposites. However, modified Palierne models could not predict the complex modulus of binary polymeric blends containing nanoparticles due to the complexity of these systems.

A new model is proposed using a suitable combinative method to predict the complex modulus of binary polymer blends with nanoparticles localized in the droplets phase. The proposed model considers particle-particle interaction, polymer-particle interaction and droplets crowding effect. On the other hand, for the validation of the proposed model, a poly(styrene)/poly(methyl methacrylate) polymer blend was prepared in the presence of 1 wt% CNT by melt mixing method.

The morphology was characterized as dispersed PS droplets within the PMMA matrix, and the transmission electron microscopy results indicated that the CNT was localized in the droplet phase. The validation results of the proposed model were more consistent with the experimental data compared to other modified Palierne models, and the parameters obtained from the proposed model provided more details on viscoelastic properties such as droplet phase-nanoparticles interfacial tension, disperse droplets crowding effect, nanoparticles amplification rate, and nanocomposite interfacial tension.