High Performance Polymers

Bis-{[(4-hydroxybutoxyphenyl)methylidene]aminophenoxyhexyl}benzene-1,3-dicarboxylate – a novel A₂ diol monomer – and a series of new blocked isocyanates (B₃ monomers) were synthesized in high yield and their structures were confirmed. Using these monomers, different hyperbranched polyurethanes (HBPUs) containing azomethine and aryl-ether connectivities were prepared via an A₂ + B₃ approach. All possible parameters were varied to study their effect on molecular weight build-up. The effects of concentrations of A₂ and B₃ and the A₂: B₃ monomer ratio were well reflected in the molecular weight of the polymer. The structure of the B₃ monomer was also found to have a significant effect on the molecular weight. The polymers were characterized by Fourier transform infrared, proton nuclear magnetic resonance, gel permeation chromatography, differential scanning calorimetry, thermogravimetric analysis, wide-angle X-ray diffraction and impedance spectroscopy. All the polymers registered unusual sub-T _g; and they invariably underwent decomposition at three distinct stages with significantly improved thermal stability at initial stage. The ionic conductivity of HBPU increased remarkably upon doping with KI and I₂. The dye-sensitized solar cell fabricated using this polymer as electrolyte yielded an overall conversion efficiency of 3.5% upon illumination with visible light.

Thermosetting blends of 4-aminophenoxyphthalonitrile (APN)/epoxy resin (ER) were investigated by differential scanning calorimetry (DSC), dynamic rheological analysis and thermogravimetric analysis (TGA). The self-promoted curing behaviors and processing properties of the APN/ER blends and prepolymers were studied by DSC and rheological analysis under dynamic and isothermal conditions. The APN/ER blends and prepolymers exhibited two-step polymerization reactions in the absence of any other curing additives. The APN/ER blends and prepolymers had large processing windows with low melt viscosity, and the size of the processing windows was dependent on the APN monomer concentration. The thermal behaviors of the APN/ER copolymers were studied by TGA. These cured copolymers exhibited decomposition temperatures over 367 ^oC in either flowing nitrogen or air, and high char yield at 800 ^oC of 68% under nitrogen atmosphere. Consequently, these studies revealed that the APN/ER system exhibits attractive self-promoted curing reaction, desirable processing feature, excellent thermal and thermal-oxidative stabilities, and high char yields. The APN/ER copolymer will be a good candidate as a matrix for high-performance polymeric materials.

A novel heat-resistant epoxy resin based on N, N’-bis(5-hydroxy-1-naphthyl) pyromellitic diimide was prepared and its cure kinetics with 4,4-diaminodiphenysulfone (DDS) was investigated by differential scanning calorimetry (DSC) under non-isothermal and isothermal conditions. Under non-isothermal condition, the effective activation energy calculated from the advanced isoconversional method (AICM) varies with curing conversion, which is different from that obtained by Kissinger’s model. This should be attributed to the formation of highly crosslinked network later in the curing stage. The isothermal curing process at different temperatures could be well fitted by Kamal’s model in the initial curing stage but deviates subsequently. The deviation is corrected by introducing a diffusion factor in Kamal’s model. In addition, the naphthyl-imide epoxy resin is used to prepare carbon fiber reinforced composites and their applications were explored. The composites exhibit a high glass transition temperature, low moisture absorption, sufficient flame retardance and especially very low tensile strength loss at high temperature.

Nanocomposites of poly(butylene terephthalate) (PBT)/modified montmorillonite organoclay were prepared via in-situ polymerization of terephthalic acid and butanediol in the presence of different clay content. Cloisite 30B, a commercially available amino modified montmorillonite, was modified with 3-aminopropytriethoxysilane (APS) through a silylation reaction. Morphology and properties of all samples were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared (FTIR) spectroscopy, proton nuclear magnetic resonance (¹HNMR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The extent of clay layers in samples was confirmed by XRD. Results of XRD and TEM showed that an exfoliation structure was produced in nanocomposites. Studies of crystallization showed that the presence of nanoclay leads to an increase in the crystallization rate and enhances the thermal stability of nanocomposites. Crystallization kinetics were described by the Avrami equation. Crystal growth was spherulitic. According to dynamic mechanical analysis (DMA), storage modulus of nanocomposites was remarkably improved compared with homo PBT. Moreover, a shrinkage test was carried out. Results showed a reduction in shrinkages along and across the flow direction that means a decrease in free volume. Flammability based on the UL-94 test was applied to study the flame retardancy effect of nanoclay on the samples.

Nafion is a currently used as a proton exchange membrane in vanadium redox flow batteries (VRBs). It is an excellent proton-conducting and fully hydrated membrane, but it is costly. In order to reduce the cost of the membrane used in VRBs and to reduce vanadium permeability across the membrane, sulfonated poly(ether ether ketone) (S-PEEK) and sulfonated poly(1,4-phenylene ether ether sulfone) (S-PPEES) membranes were fabricated and studied as a function of the sulfonation reaction time. The increase in the degree of sulfonation from 46 to 86% induced increases in the water uptake, ion exchange capacity, proton conductivity, and vanadium permeability. The vanadium permeability coefficients of S-PEEK and S-PPEES membranes are in the range of 0–24.95 × 10^–7 cm² min^–1, which are significantly lower than that of Nafion 117, which is 30.84 × 10^–7 cm² min^–1. The proton conductivity of S-PEEK in our study is nine times higher than sulfonated poly(arylene ether ketone) in a previous work and more suitable for using in VRBs.

Si₃N₄ fillers were added to polyethersulfone (PES) using the solution mixing and ball-mill mixing methods to increase the thermal conductivity. 3-Aminpropyltriethoxysilane was used as a coupling agent for the surface modification of the Si₃N₄ particles. The morphology, thermal stability, mechanical properties, and thermal conductivity of the Si₃N₄/PES composites were then studied in detail. The dispersion of the Si₃N₄ particles into the matrix resin was higher in ball-mill mixing than in solution mixing. As the Si₃N₄ content increased, the coefficient of thermal expansion of the composites gradually decreased, whereas the Vickers hardness and thermal conductivity increased. At 19 vol.% Si₃N₄, the thermal conductivities of the composites via ball-mill mixing and solution mixing reached 0.677 and 0.7 W/m·k, respectively.

The objective of this research article is to evaluate the physico-mechanical and tribological properties of nylon 66 and polytetrafluoroethylene (PTFE) powder composites in the presence of graphite (Gr) particulate filler. Nylon 66 containing 2 wt% Gr powder was extruded with 5, 10, 15 and 20 wt% PTFE powder in a co-rotating twin-screw extruder. The fabricated nylon 66/Gr/PTFE composites have been evaluated for physico-mechanical properties, such as tensile behaviors, impact strength, water absorption, density and void content. The thermal characteristics of the composites have been studied by using dynamic mechanical analysis and thermogravimetric analysis. The effect of PTFE content, loads, sliding velocities and sliding distances on wear characteristics of the composites were evaluated using pin-on-disc equipment. The nylon 66/Gr/PTFE composites showed enhanced impact strength, thermal stability, wear resistance and reduced coefficient of friction. It was found that wear performance was greatly influenced by selected experimental parameters. Worn surfaces were examined with scanning electron microscopy to have better insight into the wear mechanism. It was observed that the PTFE played a main role in the tensile resistant and wear resistant properties of nylon 66 composites.

Poly o-anisidine (POA) was prepared by chemical oxidation polymerization, and the nanocomposite of this polymer was mixed with different nanopowder metal oxides. The size and morphology of these synthesized POA/nanocomposites were shown using a transmission electron microscope (TEM). Fourier-transform infrared (FTIR) spectroscopy has been used to investigate the microstructure of the synthesized POA/nanocomposites. Thermal stability thermogravimetric analysis (TGA) and electrical conductivity of POA/nanocomposites were studied in the temperature range 40–120°C. It was found that the nanopowder metal oxides improve the thermal stability of POAs, except TiO_2, while the POA/CuO nanocomposite has the most thermal stability. Also the thermal stability is increases when the temperature is increased. The POA/nanocomposites reveal enhanced conductivity compared to conventional bulk POA. The highest electrical conductivity is obtained when the polymer mixes with CuO.

In this study, two new fluorinated curing agents bearing imidazole-bisphenol and imidazole-diamine groups in their structures, namely: 4,4′-(2-(4-(trifluoromethyl)phenyl)-1H-imidazole-4,5-diyl)diphenol (TFIDO) and 4,4′-(4,4′-(2-(4-(trifluoromethyl)phenyl)-1H-imidazole-4,5-diyl)bis(4,1-phenylene)bis(oxy)bis (3-(trifluoromethyl)aniline) (TFIA) were synthesized, fully characterized and used to cure diglycidylether of bisphenol-A (DGEBA)-based epoxy resin and blend of DGEBA with nano ZnO (NZ) particles. Two mechanisms were anticipated to play roles in the curing reactions based on adducts formation and ionic complexes. The DGEBA/TFIDO system was more reactive than the DGEBA/TFIA system, whereas the thermal stability of DGEBA/TFIA was higher than that of DGEBA/TFIDO. These results can be explained by the increase in cross-linking density and more CF₃ groups in TFIA. The results indicated that cured materials showed red shift in absorption and fluorescence emission spectra and significant improvements in the water repellency and thermal stability. The NZ particles as catalyst increased the rate of cure reaction by decreasing the activation energy (E _a) and increasing the rate constant values.

The sound absorption properties of open-celled polyimide foams at room temperature and high temperatures were tested by an AWA6122A standing wave tube and a temperature controllable impedance tube, respectively. The sound absorption characteristics of polyimide foams at room temperature were analyzed by applying the rigid end model and Helmholtz resonator model. The effects of material structures (thickness, density and cell size) and material service environments (frequency of sound wave, ambient temperature and mechanical crushing) on the sound absorption coefficients (α) were investigated. The results showed that the models can predict the numerical relationship between the structure parameters and α. At frequencies below 1600 Hz the value of α increased with the increase of foam thickness and foam density and decreased with the increase of cell size. In contrast to untreated polyimide foams, the first resonant frequencies of mechanically crushed polyimide foams moved to higher frequencies and αincreased at higher frequencies. With an increase in temperature, at the same frequencies below 2000 Hz, the αof polyimide foams decreased and the first resonant frequencies trended to move towards higher frequencies. The experimental data indicated that the sound absorption properties of polyimide foams were significantly influenced by foam structures and service environments.

Nitrile butadiene rubber (NBR)-nanoclay composites have been prepared. Maleic acid anhydride-treated organoclay (MOC) was synthesized and utilized as a compatibilizer. The prepared clay structures were analyzed by Fourier transform infrared (FTIR) spectroscopy. Furthermore, the FTIR investigation confirmed the incorporation of (MOC) in the NBR matrix, X-ray diffraction analysis indicated that an exfoliated structure was obtained, and scanning electron microscope images displayed the good interaction between the MOC and the rubber matrix. The influence of the MOC content (i.e. 1, 3, 5, 10, 20 phr) relative to the micrometer clay (i.e. 10, 20 phr) on the NBR compounds was analyzed through rheometric and swelling characteristics. The rheometric study demonstrated an increase in minimum and maximum torques, delayed scorch time and optimum cure time for the MOC (3 phr)-incorporated NBR compound compared to pure NBR. The cure rate index followed the same trend. In addition, these particular compounds showed decreased equilibrium swelling in toluene solvent, increased crosslinking density, a greater volume fraction and bound rubber content compared to the corresponding rubber filled with micrometer clay.

The influence of maleic acid anhydride-treated organoclay (MOC) content (i.e. 1, 3, 5, 10, 20 phr) relative to the micrometer clay (i.e. 10, 20 phr) on the nitrile butadiene rubber (NBR) compounds was analyzed through physico-mechanical properties (i.e. tensile strength, elongation at break, Young’s modulus, hardness Shore A and crosslink density). The effect of (MOC) content on aging resistance of NBR nanocomposites at 90 ± 1°C for different time intervals (namely 2, 4, 6, 7 days) was also investigated through changes in aging properties, compared to unmodified micrometer clay composites (10, 20 phr) and unfilled NBR. The constants of the relative change in the physico-mechanical properties of NBR vulcanizate compounds were calculated. Physico-mechanical properties demonstrated an increase in the crosslink density for the MOC (3 phr)-incorporated NBR compound compared to conventional composites and pure NBR. Thermal aging data of the aged samples revealed that under aerobic hot aging conditions, NBR compounds undergo crosslink reactions that lead to embrittlement and ultimately failure. Incorporation of MOC filler, however, resulted in significant improvement of the degradation profile of the nanocomposites at 90 ± 1°C. Loss of tensile strength and flexibility during aging of the NBR nanocomposites with 3 phr MOC was milder, relative to unfilled polymer, indicating a restricted degradation by the MOC-filled rubber, thus prolonging the modulus. However, beyond 3 phr MOC, a deterioration in the aging properties of the nanocomposites takes place.