دو ماهنامه علوم و تکنولوژی پلیمر
سال سی و پنجم شماره 4 (پیاپی 180، مهر و آبان 1401)

Polyurethanes, mainly synthesized by addition reactions of polyols with diisocyanates, have been considered as the most widely used polymers in various industries due to the variety of raw materials, synthetic methods and properties. The high demand, the significant share of the world market and the abundant use of this category of polymers have involved the major consumption of petroleum-based polyols and diisocyanates. Nonetheless, environmental challenges and the crude oil crisis have led to greater interest in renewable resources and systems based on green chemistry. Today, with the synthesis of waterborne polyurethanes) WPUs) and the use of water as a safe solvent, the release of volatile organic compounds and the production of solvent-based systems have been prevented to a considerable extent. The non-toxicity, non-flammability, environmentally friendly, and the wide applications of WPUs in plastics, paints, adhesives, printing ink and biomaterials are among the reasons for further development of these water-based systems. Because there are only a few commercially available diisocyanates in the synthesis of WPUs, it is the choice of polyol which may determine the WPUs properties. The most convenient renewable raw materials are natural oils, polysaccharides, wood and proteins, and vegetable oils which are the most beneficial options and have been widely studied. Today bio-polyols obtained from different vegetable oils have become vital in the synthesis of WPUs. However, castor oil has attracted special attention, as an excellent substitute for petroleum-based polyols and has been used in polyurethane compounds due to its availability, biodegradability and inherent hydroxyl groups. Considering the importance of bio-polyols and their role in the development of vegetable oil-based WPUs, in this article, while briefly introducing WPUs and their synthetic methods, the properties of WPUs based on castor oil, as the only natural polyol, are reviewed. Some green approaches to acquire a clear picture of the current and potential future applications of WPUs based bio-polyols in various fields are introduced.

Hypothesis: 

Bismaleimide resin, due to its favorable mechanical and thermal properties, can improve the properties of epoxy resin. 4,4'- Bis(maleimido)diphenylmethane resin (BMI) is one of bismaleimide resins that can be cured simultaneously with epoxy resin by amine curing agents and both resins can be cured with the same curing cycle. This improves the physical-mechanical properties of epoxy-based composites.

BMI resin was synthesized using the reaction between maleic anhydride and 4,4'-diaminodiphenyl methane in acetone to form an intermediate of amic acid. The dehydration of the amic acid was carried out to form an imide using acetic anhydride, triethylamine and sodium acetate. The product was characterized by FTIR and 1H NMR spectroscopy techniques. The synthesized resin was blended with DGEBA epoxy resins in different amounts of 10, 20, 30 and 40 phr. Next, the blends were cured by 4,4'-diaminodiphenyl methane as a curing agent, and the composites were prepared from the blends and glass fibers. The interlaminar shear strength (ILSS) of the prepared composites was measured as a key parameter of composites. The curing behavior of epoxy/bismaleimide blend was investigated using isothermal differential scanning calorimetry (DSC) and FTIR spectroscopy.

4,4'-Bis(maleimido)diphenylmethane resin was used to improve ILSS properties of the composite prepared based on DGEBA epoxy resin and glass fibers. The value of 52.50 MPa was obtained as the optimum value of ILSS at 78°C for the mixture with 30 phr of bismaleimide. DGEBA and BMI resins have the ability to simultaneously cure by 73% and 78% using MDA curing agent at 160°C. Full curing of the epoxy and bismaleimide mixture requires a temperature higher than 160°C, such as 220°C.

Hypothesis:

 The modification of aromatic polymers, such as poly(ether ether ketone) (PEEK), by sulfonation modification, can result in fabricating polyelectrolyte membranes (PEMs) as the alternatives to Nafion for direct methanol fuel cell (DMFC) applications. Due to the effective role of nanomaterials in reducing the permeability in nanocomposites, the addition of natural or organically modified montmorillonite (OMMT) nanofillers to the sulfonated matrix, with the optimum degree of sulfonation, can reduce the methanol permeability and increase the efficiency of the fuel cell.

PEEK was sulfonated at various degrees in solution state. Based on the selectivity parameter, the optimal degree of sulfonation (DS) was introduced. In order to prepare the nanocomposite membranes, using an ultrasonic agitator, different amounts of MMT and OMMT (Cloisite 15A or chitosan-modified MMT (CMMT)) nanofillers were added to the sulfonated polymer with optimal DS, and the resulting mixtures were cast. In this study, the ion exchange capacities (IEC) of the membranes were measured. The selectivity parameter (as ratio of proton conductivity to methanol permeability) at 25°C, as well as DMFC performance at 25°C and 1M feed of methanol for different membranes were determined and the results were compared with those of Nafion 117.

The optimum DS for sulfonated poly(ether ether ketone) (SPEEK) was 62%. X-ray diffraction (XRD) patterns proved that nanoclays were exfoliated in the structure of nanocomposites at small loading weight of 1% (by wt). The proton conductivity and methanol permeability, as well as the performance test, showed that SPEEK/CMMT-based nanocomposite membranes have the highest maximum power generation density compared to other nanocomposite membranes or Nafion 117. Accordingly, SPEEK/CMMT polymer electrolyte membranes are promising candidates for direct methanol fuel cell (DMFC) applications.

Hypothesis: 

Among the various pollutants found in natural water, the heavy metal arsenic is more important due to its high toxicity. One of the most efficient methods to remove this pollution from water streams is the surface adsorption method. Zeolite nanocomposites can be considered powerful among arsenic adsorbers. Powder adsorbents are not very effective in industrial systems due to the problems such as clogging of filters, high pressure drop and also the problem of separation from water.

To solve this problem, zeolite nanocomposite powder was transformed into beads using the chitosan gel method in three different types of cross-linking solutions including sodium hydroxide, sodium tripolyphosphate and joint sodium hydroxide/sodium tripolyphosphate. The effect of various parameters such as the type and initial ratio of the materials on the formation of beads was investigated.

The results showed that the beads formed in the sodium hydroxide+sodium tripolyphosphate cross-linking solution and the optimal initial ratio of 1:3 from chitosan to the nanocomposite have a more suitable appearance and strength and better performance in arsenic absorption. In order to confirm and justify the mentioned Findings, SEM, BET and AAS analyses were performed. Operational parameters of initial arsenic concentration and adsorbent dose which are effective on the beads’ adsorption efficiency were investigated and the optimal amount of adsorbent dose was determined as 1 g/L with an efficiency of 92.9%. In order to obtain more information about the method of adsorption and determining the maximum capacity of adsorbents, Langmuir and Freundlich isotherms for granular adsorbents were investigated. The highest adsorption capacity of 7450.7 mg/g was obtained and Freundlich isotherm was in better agreement with the results.

Hypothesis: 

The use of plasma is widely used as a method to change polymer surfaces. The use of atmospheric cold plasma has more advantages than other plasma, laser and X-ray methods. This method is simple and it uses inexpensive equipment. Considering the many uses of polyethylene in industry, it can be effective to investigate its changes against cold plasma.

A dielectric barrier discharge (DBD) plasma under atmospheric pressure was used to increase the hydrophobicity of low-density polyethylene (LDPE). After studying the optical emission spectrum (OES) of the produced plasma, its effects on surface and depth changes including surface morphology, chemical composition and polymer crystal structure were studied through scanning electron microscopy (SEM), attenuated total reflectance-Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry (XRD) and positron lifetime spectroscopy (PALS). Moreover, the contact angle analysis (CA) was used to examine the changes in the hydrophobicity of the polymer. 

Based on the data from FTIR and XRD analyses, it was found that plasma irradiation for 180 s affects the depth of a few nanometers of the polyethylene surface and does not cause significant changes in the chemical bonds and crystal structure of the polymer. In other words, plasma radiation can be used for nanometer-scale modification of the surface. On the other hand, the SEM images indicate that the plasma radiation changes the primary flat surface of the polymer into a porous surface. The results of CA analysis, while confirming this issue, show an increase in the hydrophobicity of the polymer after plasma irradiation. The results of PALS spectroscopy also reveal that at micron depth due to the sudden rise in temperature during plasma irradiation, the free volume of the material increases as a result of pore merging.

Hypothesis: 

In the synthesis of vinyl polymers, one of the important parameters that play a significant role in the physical-mechanical properties is the tacticity of monomers within the polymer chain. Polymethyl methacrylate is considered one of the important industrial polymers. The method of its synthesis can have a major effect on this parameter and finally on the final properties. One of the parameters that has a significant effect on the stereoregularity of this polymer is the reaction temperature.

The poly methyl methacrylate was synthesized at three different temperatures of 50, 150, and 250 °C via bulk thermal polymerization method. The most important instrument that can be used to study tacticity order is nuclear magnetic resonance spectroscopy. The tacticity of the polymethyl methacrylate (PMMA) was investigated and studied through alpha-methyl protons splitting and alpha-methyl and carbonyl carbons splitting, respectively, by proton (1HNMR) and carbon (13CNMR) nuclear magnetic resonance spectroscopy in deuterated chloroform (CDCl3) and deuterated tetrahydrofuran (THF-d8). The assignment of all stereosequences at triad level for alpha-methyl proton and pentad level for alpha - methyl carbon and carbonyl carbon were carried out by liquid nuclear magnetic resonance spectroscopy in deuterated chloroform. Bernoullian and first-order Markov statistics models were calculated for the synthesized sample and compared with the experimental results.

The results indicated that probability of meso (Pm) was increased by increasing the methyl methacrylate polymerization temperature. The corresponding probability of meso values determined for synthesized polymethyl methacrylate at 50, 150, and 250°C were 0.203, 0.274, and 0.356, respectively. Finally, the effect of tacticity on glass transition temperature using differential scanning calorimetry (DSC) is discussed. The temperature glassy (Tg) values by DSC results were shown that the synthesized polymethyl methacrylate at 50, 150, and 250°C had 126.0, 125.1 and 102.9 oC, respectively.