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Formation of microcracks is a critical problem in polymers and polymer composites during their service in structural applications. Development and coalescence of microcracks would bring about catastrophic failure of the materials and then reduce their lifetimes. Therefore, early sensing, diagnosis and repair of microcracks become necessary for removing the latent perils. In this context, the materials possessing self-healing function are ideal for long-term operation. Self-repairing polymers and polymer composites have attracted increasing research interests. Attempts have been made to develop solutions in this field. The present article reviews state-of-art of the achievements on the topic. According to the ways of healing, the smart materials are classified into two categories: (i) intrinsic self-healing ones that are able to heal cracks by the polymers themselves, and (ii) extrinsic in which healing agent has to be pre-embedded. The advances in this field show that selection and optimization of proper repair mechanisms are prerequisites for high healing efficiency. It is a challenging job to either invent new polymers with inherent crack repair capability or integrate existing materials with novel healing system.
InIn this work, it was investigated to use of poly(N-ethyl-4-vinylpyridinium) bromide stabilized palladium nanoparticles in the Suzuki reaction between phenylboronic acid and aryl halides in aqueous solution. The nanoparticles were isolated and re-used several times with low loss of activity.
Nanocomposites were prepared using an ethylene vinyl acetate copolymer (EVA) and organically modified Cloisite® 93A clay in the absence and presence of dicumyl peroxide (DCP) and dibenzyl peroxide (DBP) as cross-linking agents. The results clearly show differences in the EVA-clay morphology of nanocomposites prepared in the absence of organic peroxides, and of those prepared in the presence of respectively DCP and DBP. It seems as if DCP may initiate grafting between the polymer and the clay, which results in an exfoliated morphology. The presence of clay seems to inhibit the initiation of EVA crosslinking by the DBP free radicals. These free radicals probably initiate hydroxylated edge-edge interaction between the clay layers, which gives rise to a flocculated morphology and reduced polymer-clay interaction. There is a good correlation between these morphologies and the thermal stabilities and total crystallinities of the nanocomposites. Clay incorporation and peroxide treatment did not significantly change the tensile properties.
A variety of opaque white to light yellow polymeric material have been prepared by two methods, one copolymerization of styrene (ST), divinylbenzene (DVB), and grafting of linseed oil (LIN), and the second involves the copolymerization of the same comonomers with pre-reacted (with initiator) linseed oil. All of the reactant mixtures in different concentrations start to solidify at 100°C and give rise to a solid crosslinked polymer at 130°C. These polymeric materials contain approximately 30 to 74% of crosslinked materials. Their 1H NMR spectra indicate that the polymeric samples contain both soft oily and hard aromatic segments. The insoluble material left after soxhlet extraction contains finely distributed micropores. The heat deflection temperatures (HDT) of the polymer samples range from 26 to 44°C. The glass transition temperature for different linseed oil polymer samples ranges from 66 to 147°C (from dynamic mechanical analysis) and 158 to 182°C (from differential scanning calorimetry). The crosslinking density of samples ranges from 35.0 to 6.01•104 mol/m3. Irrespective of methods, the storage modulus decreases with increasing oil content in the copolymers. The polymers prepared by the first method show minimum swelling in saline water and maximum swelling in tetrahydrofuran. On the other hand, the polymers from the second method show maximum swelling in alkaline solution and a minimum in acidic solution.
Acrylic pressure-sensitive adhesives (PSA) based on two monomers: 2-ethylhexyl acrylate and acrylic acid were synthesized in organic solvent ethyl acetate using AIBN (2, 2'-azo-diisobutyronitrile) and new synthesized azo-peresters as radical initiators. After polymerization the viscosity, molecular weight and polydispersity of synthesized acrylic PSA were evaluated. The novel synthesized radical azo-perester initiators were synthesized, isolated and compared with industrial predominant usable azo-initiator AIBN.
Low-temperature reactive mixing of controlled electron beam modified Polytetrafluoroethylene (PTFE) nanopowder with Ethylene-Propylene-Diene-Monomer (EPDM) rubber produced PTFE coupled EPDM rubber compounds with desired physical properties. The radiation-induced chemical alterations in PTFE nanopowder, determined by electron spin resonance (ESR) and Fourier transform infrared (FTIR) spectroscopy, showed increasing concentration of radicals and carboxylic groups (–COOH) with increasing irradiation dose. The morphological variations of the PTFE nanopowder including its decreasing mean agglomerate size with the absorbed dose was investigated by particle size and scanning electron microscopy (SEM) analysis. With increasing absorbed dose the wettability of the modified PTFE nanopowder determined by contact angle method increased in accordance with the (–COOH) concentration. Transmission electron microscopy (TEM) showed that modified PTFE nanopowder is obviously enwrapped by EPDM. This leads to a characteristic compatible interphase around the modified PTFE. Crystallization studies by differential scanning calorimetry (DSC) also revealed the existence of a compatible interphase in the modified PTFE coupled EPDM.
The present work aims at the study of molecular relaxations in PVDF/BaTiO3 nanocomposites using broadband dielectric spectroscopy. The nanocomposites of PVDF with BaTiO3 (10–30% by wt%) are prepared using simple melt mixing method. In dielectric permittivity study, two relaxation processes are identified corresponding to the crystalline, glass transition in the PVDF/BaTiO3 nanocomposites. The peaks shift to higher frequencies as the temperature is increased. Electric modulus formalism is used to analyze the dielectric relaxations to overcome the conductivity effects at low frequencies. In M" spectra two peaks are observed only at high temperature and low frequency whereas a single relaxation peak appears at low temperatures. The single relaxation peak appearing at low temperatures is the αc relaxation attributed to crystalline chain relaxation in PVDF and the second relaxation peak which appears only at high temperatures and at a frequency lower than αc relaxation is identified as MWS relaxation. The temperature dependence of αc relaxation and MWS relaxation follows Arrhenius type behavior.
In this work, diglycidyl ether of bisphenol A (DEGBA) type epoxy resin has been modified with maleated depolymerised natural rubber (MDPR). MDPR was prepared by grafting maleic anhydride onto depolymerised natural rubber. MDPR has been characterized by Fourier transform infrared (FT-IR) spectroscopy and nuclear magnetic resonance spectroscopy. MDPR was blended with epoxy resin at three different ratios (97/3, 98/2 and 99/1), by keeping the epoxy resin component as the major phase and maleated depolymerised natural rubber component as the minor phase. The reaction between the two blend components took place between the acid/anhydride group in the MDPR and the epoxide group of the epoxy resin. The proposed reaction schemes were supported by the FT-IR spectrum of the uncured Epoxy/MDPR blends. The neat epoxy resin and Epoxy/MDPR blends were cured by methylene dianiline (DDM) at 100°C for three hours. Thermal, morphological and mechanical properties of the neat epoxy and the blends were investigated. Free volume studies of the cured, neat epoxy and Epoxy/MDPR blends were correlated with the morphological and mechanical properties of the same systems using Positron Annihilation Lifetime Studies.