Advanced Materials Letters

Self-sealing polymers possess the virtue to seal small punctures/cracks/damages automatically whenever occurred in the materials/components during their service. This behaviour of self-sealing in polymeric materials is inspired by the biological system in which self-sealing is a usual phenomenon. There are many self-sealing approaches demonstrated by researchers which are being applied in many polymeric components and systems to prevent them from catastrophic failure. The sealant materials in these systems are the main parts which play a crucial role in the sealing phenomenon. This review article describes self-sealing phenomenon, sealant materials and their applications in the various strategic materials and systems namely self-sealing spacecraft structures in the meteoroid environments, self-sealing fuel tank for fighter aircrafts, self-sealing hemostatic syringe to prevent bleeding, self-sealing tires to prevent punctures and concrete structures to prevent water leakages etc. 

Gold nanoparticles (GNPs) are of unique and interesting materials being firstly reported 100 years ago. They are one of the most widely studied nanomaterials potential for disease cure. To improve the colloidal stability, biocompatibility, and hemocompatibility of GNPs, chitosan (CS), a naturally produced polysaccharide with excellent biocompatibility and biodegradation, has been modified to generate water-soluble derivatives and used as the stabilizing agent of GNPs. In the presence of these derivatives, GNPs are stabilized, functionalized, and assembled via electronic static and covalent bond interactions. Based on these works, GNPs with different dimensional, morphology, and crystal lattice are obtained, which can be further apply to a variety of applications in sensing, imaging, therapy, and catalysis. 

Internal microscopic damage is ubiquitous in composites, whether this was caused or introduced during the manufacturing process (i.e. via thermal stresses), from machining (i.e. drilling holes for bolted joints), during component assembly or ultimately from in-service loading. Incorporating an in-situ repair solution that can be activated after each of these individual processes could have a significant impact on reducing composite component scrappage rates, post-manufacture and other repairs and increase the time-period for non-destructive testing (NDT) inspection. By utilizing specific self-healing chemistries (i.e. via epoxy-amine polymers containing Diels-Alder based thermo-reversible bonds and/or epoxy resin healing agents) that can achieve multiple repair/healing cycles, damage generated throughout a components life cycle can be repaired and service life extended as well as complete recycling of fibers and resins can be possible. Materials of optimized composition form densely crosslinked networks at room temperature while repeatedly regaining the ability to flow at elevated temperature. Mechanical testing of bulk epoxy and reinforced polymer composite films demonstrated that the thermo-reversible effect is strong enough to achieve repetitive full self-healing of a severely cracked and delaminated test specimens without significantly affecting the mechanics of the resin. The resin has been integrated in prepreg based test specimen and the self-healing efficiency remained around 40% with 5 subsequent healing cycles. The embedded self-healing agents are thermally activated post-damage to repair the internal structure, akin to the healing functionality in animals and plants. This approach represents a truly positive benefit to industry to aid in the optimization of composite manufacture, by reducing post-manufacture inspection time and material wastage costs, and also to maximize the longevity of composite components in service as well as to introduce a true recycling possibility by recovery of the binder material system and of the reinforcing fibers.

In living systems, there are many autonomous and oscillatory phenomena to sustain life such as heart beating. We developed “self-oscillating” polymer gels that undergo spontaneous cyclic swelling–deswelling changes without any on–off switching of external stimuli, as with heart muscle.The self-oscillating gels were designed by utilizing the Belousov-Zhabotinsky (BZ) reaction, an oscillating reaction, as a chemical model of the TCA cycle. We have systematically studied these self-oscillating polymer gels since they were first reported in 1996.  Potential applications of the self-oscillating polymers and gels include several kinds of functional material systems such as biomimetic actuators, mass transport systems and functional fluids.  In this review, our recent progress on the self-oscillating polymer gels is summarized.

Diabetes mellitus is a serious life-time health issue which has been increasing among the greater population, approximately 285 million people carrying this disease worldwide. In this study, we have functionalised the gold nanoparticles (AuNPs) with biocatalytic enabled optical property, it is the subject of the study for detecting glucose towards the development of photometric nano-transducer. The citrate capped AuNPs were used to warrant the electrostatic self-assembly of glucose oxidase (GOx) in the colloidal state. Glucose biocatalysis was studied through the nano-optical function of glucose on the surface of AuNPs. Using surface plasmonic resonance as analytical technique, we have determined the molecular binding interaction between glucose molecule and AuNPs surface. Based on the visible spectrum, successful immobilization of GOx onto AuNPs was demonstrated. The GOx functionalized AuNP exhibits catalytic activities for the oxidation of glucose and resulting change in the absorption peak of colloidal bio-assembly. It was observed that the absorbance at 520 nm was proportional to the concentration of glucose in the test samples. The Lambert-Beer law expresses the linear relationship between the absorbance and glucose concentration at a fixed wavelength, i.e., λmax at 520 nm. The precise detection of glucose is essential to monitor the biological level of glucose in the body. It can be concluded that the nano-(bio) gold surface exhibits a rapid photometric response with changes of glucose concentration in the test samples.

It is now commonly acknowledged that G-rich polynucleotide sites can fold into G-quadruplex (G4) structures in vivo. In terms of molecular programming, the G4-folding propensity can be regarded as a build-in nucleic acid function with multiple implications for genomic regulation. Here we review several important advances in the studies of G4 self-assemblies in genomic context. We discuss prerequisites and consequences of G4 formation upon transcription or replication and analyze recent data on G4-dependent genomic rearrangements, including translocation and recombination. Hypothetical mechanisms of those G4-dependent rearrangements suggest self-association of G-rich sites. We outline the general molecular basis for possible self-association pathways, i.e., formation of intermolecular G4 assemblies or interquadruplex stacking. Intermolecular G4s and multimeric G4 stacks attract widespread interest as scaffolds for the development of complex junctions in DNA nanotechnology and have prospects in aptamer design, but in this review we focus on fundamental aspects of such higher-order G4 assemblies.

Bio-fouling is a major issue in all membrane-based water treatment systems and there are several cleaning methods available to address this problem. Current membrane modification methods are focused on improving the hydrophilicity of membranes by blending hydrophilic additives or blending antibacterial compounds with the dope solution. In this study, we demonstrate a new method to impart bio-fouling resistance to a membrane surface by developing a water-insoluble unique copolymer additive, namely poly (acrylonitrile co maleic acid co di-amino maleio-nitrile) (PANCMACDAMN) with highly hydrophilic carboxylic and amine functional groups. Hydrophilic polyethylene glycol functionalized with silver (Ag) was grafted to the copolymer backbone to further improve the hydrophilicity. The final additive PEG-Ag attached PANCMACDAMN was used to modify polyethersulfone (PES) ultrafiltration (UF) membrane. Characterization tests indicate that the innovative surface chemistry increases the hydrophilicity of the membrane by reducing the water contact angle (CAw) by 78.1% and increases its permeability by 120% compared to the control membrane. More importantly, the innovative surface chemistry prevents protein attachment and exhibits inhibition to microbes even after 720 min of continuous protein solution filtration.

In this study, it is possible to disperse effectively cracked using synthetic fiber, an examination of the most suitable self-healing conditions was performed on the above crack width 0.1mm. As a result, effective crack dispersion using polyvinyl alcohol (PVA) fibers with polar OH - groups, as well as improved self-healing for cracks that are larger than 0.1 mm in width, posing concerns of CO2 gas and Cl - penetration, were observed. Also, CO3 2- reacts with Ca 2+ in the concrete crack, resulting in the precipitation of a carbonate compound, CaCO3. Based on this, it is deemed possible for the recovery of effective water tightness and strength recovery through effective freezing-thawing resistance to be made from cracks that are larger than 0.1 mm in width. In addition, it was determined that, as for the most suitable self-healing conditions in the inside and surface of the cracks, calcium hydroxide (Ca(OH)2) solution with CO2 micro-bubble was more effective in promoting the self-healing capability than water with CO2 micro-bubble. 

In this work, TiO2 and Ag/TiO2 thin films were synthesized on glass by combination of sol-gel method and dip-coating deposition technique. Thermal treatment in temperatures ranging from 100 °C to 500 °C was used to evaluate changes in structure, morphology and texture of these materials. Adherent and microcrack-free films were obtained. The structural and morphological evolution with temperature was studied by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Average particle size and roughness were determined by atomic force microscopy (AFM). The films were tested for wettability by measuring the contact angle between a drop of distilled water and the material surface. Results of hydrophobic/hydrophilic tests using UV-C irradiation showed that the films change their hydrophobic character to hydrophilic reaching even the superhydrophilic character which indicates their potential application as self-cleaning coatings. 

Novel self-repairing Urea-Phenol-Formaldehyde (UPF) microcapsules containing linseed oil were prepared via in-situ polymerization in an oil-in-water emulsion. The main purpose of encapsulation is to control the release of linseed oil, when external conditions such as mechanical stress or energy cause microcapsules to break. These controlled release mechanisms of linseed oil make them suitable for application in self-healing coatings. Chemical structure analyses of microcapsules were studied by Fourier transform infrared spectroscopy (FTIR), optical microscopy and scanning electron microscopy for their structural & morphological illustrations. Controllable particle sizes were determined under optical microscope and as well using particle size analyzer. To determine the healing efficiency, the microcapsules, were incorporated in the epoxy coatings in varying proportions. The effects of the same on anti-corrosion performance was carried out in 5% NaCl aqueous solution (ASTM B117) and Decreasing trend of pencil hardness, scratch hardness, Impact resistance with the increase in concentration of microcapsules was observed. Chemical resistance could also be attributed to the presence of aromatic structures in epoxy which impart chemical stability. Secondary hydroxyl moiety in epoxy chain forms hydrogen bonding with the metal substrate that would contribute to good adhesive forces. Epoxy coatings incorporated with microcapsules showed better corrosion resistance than neat epoxy coating, where neat epoxy coating showed rust and spreading of rust observed on tested panel. Mechanical properties decreased on incorporating microcapsules into epoxy matrix, hence development of mechanical properties without effecting the corrosion properties shall be studied further.

To deliver epoxy composites with enhanced self-healing ability, this study investigates healing efficiency of dual component epoxy system consisting of microcapsules containing epoxy (DGEBA) and different variants of hardener (TETA) microcapsules. Morphological investigation under FESEM confirms formation of spherical shaped intact TETA microcapsules at high agitation speed with average size of the ~65.32 µm and reduced wall thickness of ~1.823 µm. Reaction temperature is found to play significant role to tune the roughness of the microcapsule surfaces. The single edge notched bending (SENB) test was performed to evaluate the healing ability. It was found that with incorporation of microcapsules, the fracture toughness decreases but the healing efficiency increases with increase in content of microcapsules. The maximum healing efficiency observed was 65.61%. High concentration of TETA microcapsule (prepared at high agitation speed) in epoxy network gives the essence for their applicability as a potential ingredient to elevate the healing efficiency. To enhance the healing ability further of the composites as well as fibre reinforced composites with unaltered mechanical properties we believe synthesis nanocapsules and their incorporation could have significant impact.

Zinc oxide (ZnO) nanoparticles, self-assembled in the form of one dimensional ZnO nanofibers were synthesized using electrospinning technique from solution of polyvinyl alcohol (PVA) and zinc acetate followed by calcination at 600°C in oxidizing environment. Scanning Electron Microscope (SEM) analysis demonstrates that morphology of ZnO nanofibers having rough surface and corresponding Energy Dispersive Spectrometry (EDAX) confirmed the Zn: O atomic ratio approximately in 50:50. Transmission electron microscopy (TEM) images clearly demonstrate the rough morphology is due to the self-assembling of ZnO nanoparticles having diameter approximately 50nm. X-ray Diffraction (XRD) reveals the polycrystalline structure and Raman spectra show some shifts in phonon modes. The PL graph show exceptional emission at 342nm due to band-band transition. Under solar radiations as produced ZnO nanofibers degrades the 99% of 25ppm acid fuchsine which proven through UV spectra when compared to blank dye solution. This shows that natural solar radiations are sufficient to excite theses self-assembled high surface area ZnO nanofibers to show its photocatalytic activity.

Magnetically tunable colloidal nano clusters (CNC’s) have been fabricated using superparamagnetic cobalt doped iron oxide CNCs for the first time. This has the regular interparticle spacing and strongly diffracts light, which is being controlled by an external field. The size of the nanoclusters varies from 10nm–200nm. It reveals that it can be used for wide magnetic tunability. The response to the magnetic field was studied using reflection spectra by varying field sample distance. This shows good response in the UV and visible region. We obtained eight prominent peaks in the UV region which enhances the prosperity of our CNC’s sensing the UV predominately. Hysteresis behavior shows the presence of superparamagnetic nature, which is promising candidate for drug delivery, bioseperation and magnetic resonance imaging.

We discuss experimental results demonstrating the inhibition and enhancement of spontaneous emission of dye molecules embedded in a nanophotonic structure. This is achieved in our all-solid self-assembled photonic crystals consisting of dye-doped polystyrene spheres. Our samples exhibit well-resolved photonic stop gap with high reflectivity and photonic strength. Laser induced emission experiments reveal an inhibition of ~ 70% of emission intensity in the photonic stop gap wavelength range. Also, we discuss the enhancement of spontaneous emission intensity near the blue side of the stop gap. Our results have implications in photonic devices, such as, low-threshold lasers and efficient lighting devices.

We present here our results on surface micro-structuring via nanosecond pulsed laser irradiation of Titanium and Stainless Steel cathode samples. Laser based surface micro-structuring leading to formation of self-assembled micro-tips can potentially enhance field emission efficiency of such surface treated cathodes. Microstructure of the laser treated surfaces has been observed under a Scanning Electron Microscope (SEM) and SEM images were further analyzed using software attached with Optical Microscope. To study the effect of laser fluence on developed surface microstructure, the target surface was irradiated in different regions with laser beams at varying laser fluence levels ranging from 2-10 J/cm 2 for a period of 1 to 15 minutes corresponding to 600 to 9000 laser pulses. Mean height of the generated micro-cones was observed to increase from 17 to 30µm on increasing number of irradiating laser pulses from 3000 to 9000 in case of Stainless Steel samples. In case of laser treated Titanium average periodicity of generated self-assembled micro-cones decreased from 10.8μm to 6.5μm when laser fluence was increased from 5 to 10J/cm 2 with a total of 600 laser pulses used for irradiating the sample.

Copolymerization of self doping monomer aniline and oxalic acid (OA)/ acetic acid (AA) in different molar ratio via the self-assembly process were conducted to prepare self-doping polyanilines (SD-PANIs). In this polymerization process, AA or OA plays the roles of surfactant and dopant for the self-doping PANIs. The morphology, UV–Vis absorption behaviour, crystalline density and electrical conductivity of self-doped PANIs are investigated. Depending on molar ratio of aniline to OA/AA, nanotubes structure of polyaniline (PANI) can be formed. Higher concentration of OA leads to increase in the diameter of the tubes in which micelles act as the template in the self-assembly of PANI to form nanotube structures, whereas increase in concentration of AA leads to change the structure of polyaniline from microspheres to nanotubes. The nanotubular structure aggregates to form a bundle structure as the concentration of AA increases. More uniform structure is observed in case of OA than that of AA, which may be due to the bulky structure of OA than AA. Higher absorption intensity in UV-Vis spectra of self-doping PANIs was observed for lower concentration of OA/AA. The crystal structure for the synthesized self-doped PANIs is orthorhombic and the C-N-C angle is larger. High electrical conductivity of the self-doped PANIs was observed as a function of degree of doping.