دو ماهنامه علوم و تکنولوژی پلیمر
سال سی و چهارم شماره 6 (پیاپی 176، بهمن و اسفند 1400)

 Epoxy resin is used in various industries such as adhesives, paints and coatings, aerospace and electronics due to its unique attributes. Epoxy curing agents can be generally classified in two groups of normal (room or high temperature) and latent curing agents. Latent curing agents are mixed with epoxy resins to obtain stable compounds at normal conditions. These compounds can cure epoxy resins rapidly when exposed to external stimulation, such as heat. Capsulation of curing agent as a cost-effective method has attracted an extensive attention to prepare non-reactive or latent curing agents. The concentration of microencapsulated latent curing agent significantly affects the final mechanical properties of cured epoxy resins.

The effect of concentration of microcapsules containing curing agent of 1-methyl imidazole by solid epoxy shell on the mechanical properties of epoxy resin was investigated using dynamic mechanical thermal analysis.

The effect of 20, 25, 30 and 35 phr (per hundred resin) microcapsules concentration in liquid epoxy on storage modulus (E′) and phase angle tangent (Tanδ) was investigated. The results showed that increasing the concentration of microcapsules in cured samples causes to advance storage modulus due to increasing the amount of curing agent and consequently increasing the density of crosslinks. On the other hand, it was found that liquid epoxy resin cured with 30 phr microcapsules has the highest glass transition temperature (48°C). The hardness test results also confirmed the results of thermal-mechanical dynamic test at the optimum microcapsule concentration. The results also indicated that at 30°C the storage module decreased by increasing microcapsule concentration from 20 to 25 phr.  The storage modulus of cured epoxy resins increased to higher values by increases in microcapsule concentration. Therefore, the epoxy resin cured by 35 phr microcapsule showed the highest storage module (723 MPa).

 Poly(m-phenylenediamine) and barium titanate nanoparticles have promising physical and chemical properties in the electrical and optical fields. One of the attractive properties of these materials is their nonlinear optical behavior. This property allows these materials to be used in high-tech systems, the manufacture of various optical parts as well as lasers.

Poly(m-phenylenediamine/barium titanate) nanoparticles (PmPDA/BaTiO3) nanocomposite was prepared by in situ polymerizations. The prepared materials were characterized by various methods. Nonlinear optical studies of materials were investigated by the Z-scan technique with open aperture and closed aperture to obtain the absorption coefficient and nonlinear refractive index at different concentrations 0.3, 0.5, and 0.7 mg/L with four different intensities at a wavelength of 532 nm.

X-ray diffraction and field emission electron microscopy results showed a semi-crystalline pattern and an irregular aggregate structure for PmPDA/BaTiO3 nanocomposite, respectively. The thermal stability of the nanocomposite increased due to the presence of BaTiO3 nanoparticles relative to PmPDA. The presence of BaTiO3 nanoparticles in the nanocomposite shifted the absorption peak of PmPDA to a shorter wavelength (325 nm). The optical results show that at concentrations of 0.3, 0.5, and 0.7 mg/L with varying intensity of moderate light on PmPDA, BaTiO3, and PmPDA/BaTiO3 samples, the values of nonlinear refractive index (n2) and the nonlinear absorption coefficient (β) are obtained differently. In addition, the results show that by changing the intensity, the samples have a nonlinear refractive index with a negative sign (n2 <0). This result shows that the samples are self-focal in nature and can play an important role in the correction of laser pulses. The samples also have a saturation absorption (SA) nature. This feature plays an important role in the fabrication of optical switches and optical limiters in lasers.

 Chitosan is a biodegradable biopolymer used today as an effective and practical material in various industries. To produce chitosan hydrogels, different crosslinkers are utilized, most of which are toxic and, dangerous. Due to the many applications of chitosan in medicine, pharmaceuticals, tissue engineering and food industries, it is better to use safe and non-toxic crosslinkers to produce biopolymers and hydrogels. Vanillin is a natural and non-toxic aldehyde that can be used as a crosslinker to produce chitosan hydrogels. The cross-links that vanillin make with chitosan are, on the one hand, the Schiff base type bond, and on the other hand, the hydrogen bond, which makes the chitosan-vanillin (CV) hydrogel more mechanically weaker than other chitosan-dialdehyde hydrogels. Chitosan-vanillin hydrogels have been studied in micro- and nanoscale and biofilm shape. In this study, montmorillonite (MMT) fillers have been used to fabricate chitosan-vanillin-montmorillonite (CMV) macrohydrogels, and their effect on improving mechanical properties has been investigated.

In this study, chitosan hydrogels were fabricated with vanillin crosslinker and montmorillonite as fillers. Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were performed to identify the bonds formed and to examine the morphology of the hydrogels, respectively. Then, the gel content, swelling, porosity, and mechanical properties of hydrogels were investigated.

The results showed that the presence of vanillin increased the porosity and caused regular porosity in the chitosan hydrogel. Chitosan and vanillin macrohydrogels have good mechanical properties with a porosity greater than 90%, gel content > 86%, swelling, and mechanical strength. The addition of filler to chitosan-vanillin hydrogels also reduces the porosity and swelling and increases the mechanical properties of this system

 The effect of carbon black nanoparticles and thermoplastic polyurethane on the tensile strength and impact properties of polyacetal (POM), which is widely used in the application of automotive parts such as bumper brackets, has been investigated. Improving the impact resistance of polyacetal is one of the challenges of automotive industry, which would diminish the car damage in accidents. The incorporation of thermoplastic polyurethane into the polyacetal matrix can create good compatibility and increase its impact resistance. In addition, the presence of carbon block in the polyacetal matrix can simultaneously elevate the tensile strength and impact resistance and increase the UV resistance of polyacetal.

Standard mechanical testing specimens of the POM/CB/TPU nanocomposites, containing 0.42% (by wt) carbon black and different fractions of 2.5, 5 and 7.5 % (by wt) of thermoplastic polyurethane (TPU) were produced through a twin-screw extruder and injection molding. Standard tensile and impact tests were performed to evaluate the mechanical performance of nanocomposites. The morphology of fractured surfaces of impact specimens and the toughening mechanisms were investigated using scanning electron microscopy (SEM). 

The results of tensile test showed that the presence of carbon black nanoparticles increases the Young's modulus and the tensile strength of polyacetal. However, the inclusion of thermoplastic polyurethane into the POM/CB reduced the tensile behavior. The incorporation of a phase with soft segments to the polymeric matrix with hard segments reduces the tensile strength. In addition, the carbon black and the thermoplastic polyurethane increase the elongation-at-break of this three-phase nanocomposite. The results of impact test showed that the presence of carbon black nanoparticles and thermoplastic polyurethane in the polyacetal matrix leads to enhanced impact resistance. Plastic deformation, crazing, fibrillated structure and microvoid were the dominant toughening mechanisms in nanocomposites.

 Acrylic adhesives based on methyl methacrylate are thermoplastic polymers due to their linear polymerization. By adding two or more functional acrylate or methacrylate monomers it is expected that cross-linking may occur partially in leading to their chemical and thermal resistance improvements. It is also possible to increase their adhesion strength by adding acidic acrylate or acidic methacrylate additives.

Compounds containing different percentages of methyl methacrylate monomer and polymethyl methacrylate were prepared. By measuring viscosity and contact angle, a suitable formulation based on surface wetting property was selected. Gel time measurement was used to find optimum amount of the second component of these two-component adhesives. For improving the properties, some modifying monomers including acidic monomer for increasing the adhesion property and a two-functional monomer for crosslinking were added. Crosslink density was assessed by dynamic mechanical analysis. Thermal gravimetry analysis, chemical resistance and lap shear test were used to evaluate other properties.  

Results show that the lap shear strength of acrylic adhesive on aluminum substrate increased from 1.1 MPa to 2.7 MPa by adding acidic monomer as adhesion promoter. The strength increased to 3.5 MPa by using 4% of ethylene glycol dimethacrylate as a two-functional crosslinking agent. The decomposition temperature at 5% weight loss increased from 194 to 248 ᵒC. The chemical resistance of cured adhesives in water, ethanol, acetone and toluene was also studied. The results show that the specimen without crosslinking was dissolved in acetone while the crosslinked specimen showed 14.9% weight loss after 24 h immersion. Both the thermoplastic and thermoset adhesives showed good water resistance.

 Accurate temperature measurement is of particular importance in various medical and industrial fields. Researchers have recently developed heat-sensitive sensors with the development of nanotechnology. The goal of the present research is the fabrication of an ultra-sensitive thermal nanosensor that can be applied to monitor human body temperature and industrial tasks. 

For this purpose, polypyrrole and graphene nanocomposites were synthesized with different percentages. The structural characteristics of the obtained nanocomposites were assessed by electron scanning microscopy and X-ray diffraction spectroscopy (XRD). 

The results showed that synthetic graphene and polypyrrole are in the shape of sheets and fiber with a thickness less than 100 nm and diameter of 150 nm, respectively. The XRD spectrum of the 0.5% (by wt) nanocomposite also indicated a suitable combination of graphene and polypyrrole. The thermal biosensor evaluations of samples disclosed that pure polypyrrole allocated the first rank compared to other samples in the temperature range of 25-80°C, with a sensitivity of 218 kΩ/°C, but its nonlinear behavior limited its applicability. In this temperature range, 0.5% (by wt) nanocomposite sensor showed the highest optimal performance with the sensitivity, temperature coefficient resistance (TCR), response and recovery time of 197 kΩ/°C, -1.17 %°C-1, 78 and 170 s, respectively. In the temperature range of 35-40°C, to control the human body temperature, the nanocomposite sensor with the concentration of 0.5% (by wt) has the best linear performance with a sensitivity of 20.5 kΩ/˚C, TCR of -2.26% per°C and response and recovery times of 21 and 34 s. In  comparison to similar samples, this nanocomposite has improved by 23.9 and 1.8 times, with respective to the above recovery time. In the final conclusion, the nanocomposite sensor with a concentration of 0.5% (by wt) was designated as the most ideal nanosensor that can be utilized in industrial as well as medical fields.