Recent progress in surface engineering methods and advanced applications of flexible polymeric foams.

Polymeric foams, also known as three-dimensional (3D) polymeric sponges, are lightweight, flexible, compressible, and possess a high surface area compared with other bulk polymers. These sponges have traditionally been used for mattresses or seat cushions in homes, offices, aircraft, automobiles, and trains, and to insulate against heat, electricity, and noise. Recently, the demand for modern materials has expanded the application of polymeric foams to various high-value technologies, including in areas that need high flame retardancy, flame sensors, oil/water separation, metal adsorption, solar steam generation, piezoresistivity, electromagnetic interference shielding, thermal energy storage, catalysis, supercapacitors, batteries, and triboelectric energy harvesting. Proper modification of foams is a prerequisite for their use in high-value applications. Several new strategies for the surface coating of 3D porous foams and novel emerging applications have been recently developed. Therefore, in this review, current advances in the field of surface coating and the application of 3D polymeric foams are discussed. A brief background on 3D polymeric foams, including the unique properties and benefits of polymeric sponges and their routes of synthesis, is presented. Different coating strategies for polymeric sponges are discussed, and their advantages and drawbacks are highlighted. Different advanced applications of polymeric sponges, in conjunction with specific and detailed examples of the above-mentioned applications, are also described. Finally, challenges and potential applications related to the coating of polymeric foams are discussed. We envisage that this review will be useful to facilitate further research, promote continued efforts on the advanced applications mentioned above, and provide new stimuli for the design of novel polymeric sponges for future modern applications.

Structure of Metal-Organic Frameworks Eco-Modulated by Acid-Base Balance toward Biobased Flame Retardant in Polyurea Composites.

Biobased-functionalized metal-organic frameworks (Bio-FUN-MOFs) stand out from the crowd of candidates in the flame-retardant field due to their multipathway flame-retardant mechanisms and green synthesis processes. However, exploring and designing Bio-FUN-MOFs tend to counteract the problem of compromising the flame-retardant advantages of MOFs themselves, which inevitably results in a waste of resources. Herein, a strategy in which MOFs are ecologically regulated through acid-base balance is presented for controllable preparation of Bio-FUN-MOFs by two birds with one stone, i.e., higher flame-retardant element loading and retention of more MOF structures. Specifically, the buffer layer is created on the periphery of ZIF-67 by weak etching of biobased alkali arginine to resist the excessive etching of ZIF-67 by phytic acid when loading phosphorus source and to preserve the integrity of internal crystals as much as possible. As a proof of concept, ZIF-67 was almost completely etched out by phytic acid in the absence of arginine. The arginine and phytic acid-functionalized ZIF-67 with yolk@shell structure (ZIF@Arg-Co-PA) obtained by this strategy, as a biobased flame retardant, reduces fire hazards for polyurea composites. At only 5 wt % loading, ZIF@Arg-Co-PA imparted polyurea composites with a limiting oxygen index of 23.2%, and the peaks of heat release rate, total heat release, and total smoke production were reduced by 43.8, 32.3, and 34.3%, respectively, compared to neat polyurea. Additionally, the prepared polyurea composites have acceptable mechanical properties. This work will shed light on the advanced structural design of polymer composites with excellent fire safety, especially environmentally friendly and efficient biobased MOF flame retardants.

Data-driven supervised machine learning to predict the compressive response of porous PVA/Gelatin hydrogels and in-vitro assessments: Employing design of experiments.

The purpose of this study is to design and evaluate a series of porous hydrogels by considering three independent variables using the Box-Behnken method. Accordingly, concentrations of the constituent macromolecules of the hydrogels, Polyvinyl Alcohol and Gelatin, and concentration of the crosslinking agent are varied to fabricate sixteen different porous samples utilizing the lyophilization process. Subsequently, the porous hydrogels are subjected to a battery of tests, including Fourier Transform Infrared spectroscopy, morphology assessment, pore-size study, porosimetry, uniaxial compression, and swelling measurements. Additionally, in-vitro cell assessments are performed by culturing mouse fibroblast cells (L-929) on the hydrogels, where viability, proliferation, adhesion, and morphology of the L-929 cells are monitored over 24, 48, and 72 h to evaluate the biocompatibility of these biomaterials. To better understand the mechanical behavior of the hydrogels under compressive loadings, Deep Neural Networks (DNNs) are implemented to predict and capture their compressive stress-strain responses as a function of the constituent materials' concentrations and duration of the performed mechanical tests. Overall, this study emphasizes the importance of considering multiple variables in the design of porous hydrogels, provides a comprehensive evaluation of their mechanical and biological properties, and, particularly, implements DNNs in the prediction of the hydrogels' stress-strain responses.

Development of carbon prepreg/epoxy self-healing composites by inserting bio lumen carriers: A novel approach.

The current study focused on abaca fiber lumens with a thermoset healing resin mechanism integrated into high-performance carbon prepreg composites. Self-healing composites with a fiber orientation of [0°/90°]4s and similar fiber volume fractions were manufactured and tested using a compression after impact (CAI) test to assess the post-impact behavior. The experimental results showed that the healed composites had an improved restoration strength of 19.25% and were supported by micro analysis with no degradation effects owing to the presence of the healing carriers. The effect of reinforcing healing carriers (HC) improved the tensile and flexural strengths of carbon prepreg composites by 5.14 and 61.11%, respectively, and the alkali treatment enhanced the tensile/flexural modulus to 23.61 and 21.17%, respectively. Overall, the healing carriers effectively healed the damage to the carbon prepreg/epoxy composite after residual compression characteristics. The fracture toughness values of the self-healing composites were significantly higher than those of the pure composites.

Study on the Effect of Core-Shell Abaca Vascular Carriers on the Self-Healing and Mechanical Properties of Thermoset Panels.

Self-healing panels were prepared using vinyl ester (VE) and vascular abaca fibers (unidirectional) through the hand lay-up process. Initially, two sets of abaca fibers (AF) were prepared by filling the healing resin VE and hardener and stacking both core-filled unidirectional fibers in a 90° direction to obtain sufficient healing. The experimental results demonstrated that the healing efficiency increased by approximately 3%. SEM-EDX analysis further confirmed the healing process by exhibiting spill-out resin and the respective fibers' major chemical elements at the damaged site after self-healing. The tensile, flexural, and Izod impact strengths of self-healing panels indicated improved strengths of 7.85%, 49.43%, and 53.84%, respectively, compared with fibers with empty lumen-reinforced VE panels due to the presence of a core and interfacial bonding between the reinforcement and matrix. Overall, the study proved that abaca lumens could effectively serve as healing carriers for thermoset resin panels.

Development of Self-Healing Carbon/Epoxy Composites with Optimized PAN/PVDF Core-Shell Nanofibers as Healing Carriers.

Two-component self-healing carbon/epoxy composites were fabricated by incorporating healing agents between to carbon fiber laminates via the vacuum bagging method. Vinyl ester (VE), cobalt naphthalene (CN), and methyl ethyl ketone peroxide (MEKP) were encapsulated in a polyacrylonitrile (PAN)/Poly(vinylidene fluoride) (PVDF) shell via co-axial electrospinning. Varying nanofiber compositions were fabricated, namely, 10, 20, 30, and 40% PAN in PVDF nanofibers. The 20% PAN fibers were finalized as the shell material owing to their superior tensile properties and surface morphology. The behavior of the PAN/PVDF nanofibers encapsulating the healing agents was studied via Fourier-transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), and thermogravimetric analysis (TGA) to affirm the presence of the healing agents. Mechanical analysis in the presence of core-shell nanofibers indicated an enhancement of 7 and 5% in flexural strength and Izod impact strength, respectively. Three-point bending tests confirmed the autonomous healing characteristics of these nanofibers, which retained 62% of their initial strength after 24 h. FESEM and energy dispersive X-ray (EDX) analyses of the fracture surface confirmed that the resin was released from the nanofibers, restoring the initial properties of the composites.

Harvesting of flow current through implanted hydrophobic PTFE surface within silicone-pipe as liquid nanogenerator.

Harvesting of flow current through implanted hydrophobic surface within silicone pipe as liquid nanogenerators where Tap water (TW), and DI water (DIw) as liquid reservoirs to successfully convert induced mechanical energy into electrical energy. Here, we used a commercial PTFE film for the generation of a hydrophobic surface as a source of mechanical energy. The surface roughness of the hydrophobic surface is confirmed using atomic force microscopy, and contact angle analyses. The generation of power through the interaction of TW and DI with inbuilt PTFE in silicone tube is described. The higher output voltage (Voc), and short circuit currents (Isc) were attained through an interaction of TW and DIw with N-PTFE. The lower Voc, and Isc's were produced when DI water interacts with N-PTFE electrode, whereas TW produced higher Voc and Isc's, respectively, due to a lack of free mobile ions in DIw than TW. The TW-Sh-TENG and DIw-Sh-TENG are produced the maximum peak-to-peak Voc, and Isc of 29.5 V and 17.4 V and 3.7 µA, and 2.9 µA, respectively. Significant power output enhancement of ~ 300% from TW-Sh-TENG from DIw-N-TENG due to the formation of higher surface roughness and lead to the slipping of water droplets by super-hydrophobicity.

Integrated electronic skin (e-skin) for harvesting of TENG energy through push-pull ionic electrets and ion-ion hopping mechanism.

The development of highly durable, stretchable, and steady triboelectric nanogenerators (TENGs) is highly desirable to satisfy the tight requirement of energy demand. Here, we presented a novel integrated polymeric membrane that is designed by PEDOT: PSSa-naphthalene sulfonated polyimide (PPNSP)-EMI.BF4 Electronic skin (e-skin) for potential TENG applications. The proposed TENG e-skin is fabricated by an interconnected architecture with push-pull ionic electrets that can threshold the transfer of charges through an ion-hopping mechanism for the generation of a higher output voltage (Voc) and currents (Jsc) against an electronegative PTFE film. PPNSP was synthesized from the condensation of naphthalene-tetracarboxylic dianhydride, 2,2'-benzidine sulfonic acid, and 4,4'diaminodiphenyl ether through an addition copolymerization protocol, and PEDOT: PSSa was subsequently deposited using the dip-coating method. Porous networked PPNSP e-skin with continuous ion transport nano-channels is synthesized by introducing simple and strong molecular push-pull interactions via intrinsic ions. In addition, EMI.BF4 ionic liquid (IL) is doped inside the PPNSP skin to interexchange ions to enhance the potential window for higher output Voc and Iscs. In this article, we investigated the push-pull dynamic interactions between PPNSP-EMI.BF4 e-skin and PTFE and tolerable output performance. The novel PPNSP- EMI.BF4 e-skin TENG produced upto 49.1 V and 1.03 µA at 1 Hz, 74 V and 1.45 µA at 2 Hz, 122.3 V and 2.21 µA at 3 Hz and 171 V and 3.6 µA at 4 Hz, and 195 V and 4.43 µA at 5 Hz, respectively. The proposed novel TENG device was shown to be highly flexible, highly durable, commercially viable, and a prospective candidate to produce higher electrical charge outputs at various applied frequencies.

Synthesis of non-phosphorylated epoxidised corn oil as a novel green flame retardant thermoset resin.

This study aimed to produce a new potential flame retardant thermoset resin from epoxidised corn oil through a one-pot method using liquid inorganic catalysed with hydrogen peroxide. Using a gas chromatography-mass selective detector, attenuated total reflectance-fourier transform infrared spectroscopy, proton nuclear magnetic resonance imaging, optical microscopy, and scanning emission microscopy, we synthesised a bio-based resin based on newly designed parameters. The flame retardant capacity was fully established using thermogravimetric analysis and a micro calorimeter. The produced epoxidised corn oil had a relative percentage conversion of oxirane of approximately 91.70%, wherein the amount of double bonds converted into epoxides was calculated. A significant reduction from 17 to 40% in peak heat rate release (pHRR) and 26-30% in total heat release was observed, confirming its flame retardant property. Thus, the potential of epoxidised corn oil was demonstrated.

Mechanically Sustainable Starch-Based Flame-Retardant Coatings on Polyurethane Foams.

The use of halogen-based materials has been regulated since toxic substances are released during combustion. In this study, polyurethane foam was coated with cationic starch (CS) and montmorillonite (MMT) nano-clay using a spray-assisted layer-by-layer (LbL) assembly to develop an eco-friendly, high-performance flame-retardant coating agent. The thickness of the CS/MMT coating layer was confirmed to have increased uniformly as the layers were stacked. Likewise, a cone calorimetry test confirmed that the heat release rate and total heat release of the coated foam decreased by about 1/2, and a flame test showed improved fire retardancy based on the analysis of combustion speed, flame size, and residues of the LbL-coated foam. More importantly, an additional cone calorimeter test was performed after conducting more than 1000 compressions to assess the durability of the flame-retardant coating layer when applied in real life, confirming the durability of the LbL coating by the lasting flame retardancy.

Excellent Fire Retardant Properties of CNF/VMT Based LBL Coatings Deposited on Polypropylene and Wood-Ply.

In this report, layer by layer (LBL) fire retardant coatings were produced on wood ply and Polypropylene Homopolymer/Flax fiber composites. FE-SEM and EDAX analysis was carried out to analyze the surface morphology, thickness, growth rate and elemental composition of the samples. Coatings with a high degree of uniformity were formed on Polypropylene composite (PP/flax), while coatings with highest thickness were obtained on wood ply (wood). FTIR and Raman spectroscopy were further used for the molecular identifications of the coatings, which confirmed the maximum deposition of the solution components on the wood substrate. A physiochemical analysis and model was proposed to explain the forces of adhesion between the substrate and solution molecules. Fire protection and thermal properties were studied using TGA and UL-94 tests. It was explored, that the degradation of the coated substrates was highly protected by the coatings as follows: wood > PP/flax > PP. From the UL-94 test, it was further discovered that more than 83% of the coated wood substrate was protected from burning, compared to the 0% of the uncoated substrate. The flammability resistance of the samples was ranked as wood > PP/flax > PP.

Synthesis of vinyl ester resin-carrying PVDF green nanofibers for self-healing applications.

Self-healing on the engineering applications is smart, decisive research for prolonging the life span of the materials and the innovations have been mounting still smarter. Connecting to advancements in self-healing carriers, in altering the chemical structure by optimizing the brittleness for self-healing performance and introducing the bio-degradability, for the first time TPS was blended to PVDF for the synthesis of nanofibers, as carriers of a vinyl ester (VE) resin (medication), by the coaxial electrospinning technique. TPS was mechanically mixed with PVDF base polymer and optimized the TPS content (10 wt%) based on mechanical performance. The novel nanofibers were characterized via field emission scanning electron microscopy (FESEM), Fourier-transform infrared spectroscopy, X-ray diffraction, thermal, moisture analysis, and a mechanical line with FESEM and energy-dispersive X-ray analysis studied the self-healing. The TPS/PVDF fibers having hydrogen bonding and increased the crystallinity (40.57 → 44.12%) and the diameter (115 → 184 nm) along with the surface roughness of the fibers with increasing the TPS content. Microanalysis presented the flow-out of the VE resin at the scratched parts in the pierced fibers; interestingly, after some time, the etched part was cured automatically by the curing of the spread resin. Mechanical stretching of the nanofibers in the tensile tests up in the plastic region showed a decrement in the elasticity (TPS/PVDF fibers) and an increment in the brittle nature (cured VE resin) with the increase in Young's modulus at each stretching, clearly elucidating the healing performance.

An aerobic oxidation of alcohols into carbonyl synthons using bipyridyl-cinchona based palladium catalyst.

We have reported an aerobic oxidation of primary and secondary alcohols to respective aldehydes and ketones using a bipyridyl-cinchona alkaloid based palladium catalytic system (PdAc-5) using oxygen at moderate pressure. The PdAc-5 catalyst was analysed using SEM, EDAX, and XPS analysis. The above catalytic system is used in experiments for different oxidation systems which include different solvents, additives, and bases which are cheap, robust, non-toxic, and commercially available on the industrial bench. The obtained products are quite appreciable in both yield and selectivity (70-85%). In addition, numerous important studies, such as comparisons with various commercial catalysts, solvent systems, mixture of solvents, and catalyst mole%, were conducted using PdAc-5. The synthetic strategy of oxidation of alcohol into carbonyl compounds was well established and all the products were analysed using 1H NMR, 13CNMR and GC-mass analyses.

In situ generated hydrophobic micro ripples via π-π stacked pop-up reduced graphene oxide nanoflakes for extended critical heat flux and thermal conductivities.

We report the synthesis of thermally heated pop-up reduced graphene oxide (Pop-rGO) and its nanofluid (Pop-rGO-Nf) in DI water for extended critical heat flux (CHF) in a nucleate pool boiling experiment. When Pop-rGO-Nf is boiled over a nichrome (NiCr) wire heater the CHF values were increased up to 132%, 156%, and 175% with increasing concentrations of 0.0005 vol%, 0.001 vol%, and 0.005 vol% at heat fluxes of q'' = 264 333 kW m-2, 339 202 kW m-2, and 327 895 kW m-2, respectively, because of the higher surface area of 430 m2 g-1. We also found a decrease in the CHF value from 0.05 vol% (175%) to 0.01 vol% (153%) for Pop-rGO-Nf due to the nanofluid concentration reaching the saturation point. After nucleate pool boiling, the developed Pop-rGO-Nf built-up layer on the NiCr wire surface showed regular π-π stacking with novel micro-rippled structures having uniform nanocavities and nanochannels. The nanocavities strongly helped vapor bubbles to escape from the NiCr wire surface. In addition, the nanochannels were formed by hydrogen bonding of adjacent carboxyl groups of each Pop-rGO nanosheet. The surface hydrophobicity of the built-up layers increased with the increase of the concentration of the Pop-rGO-Nfs, and the surface morphology, roughness average (R a) and hydrophobicity were determined using FE-SEM, AFM and contact angle (CA) analysis. In our present investigation, during and after the nucleate CHF experiments with Pop-rGO-Nfs, for the first time, we obtained a higher CHF value of 175% at 0.01 vol% and a higher CA of 118° obtained at 0.05 vol%, due to the increase in surface hydrophobicity and the novel micro-rippled structures. We anticipate that the present results suggest that pool boiling employing Pop-rGO-Nf can dissipate the critical heat flux of electronic chips to a greater extent, allowing the enhancement of the cooling performance in existing two-phase heat transfer devices.

Fabrication and characterisation of starch/chitosan/flax fabric green flame-retardant composites.

The study reveals the fabrication of eco-friendly bio-composites by employing natural, widely available biopolymers such as starch, chitosan (CS) and flax fabric (FF). In a typical process, starch was used in the form of thermoplastic starch prepared via mechano ball milling and subsequently, composites were fabricated via compression with CS and FF. The nature of the composites was analysed using FTIR. Good compatibility and homogeneous dispersion of reinforcements was corroborated using FESEM (EDX). The influence of CS (3, 6, & 9 wt%) on the mechanical (UTM) and thermal (TGA) properties, biodegradability (soil burial test), and flammability (horizontal burning test (UL94), limited oxygen index (LOI)) of the composites was investigated. An improvement in tensile strength from 16.45 to 20.78 MPa, thermal stability 10 wt% @ 800 °C (N2 atmosphere) and flame retardancy showed remarkable withstandability (UL94 = Vo & LOI = 40) of the composites with flame and flame self-annihilate were speculated to arise from the dense char formed by the carbonaceous agent CS. A delay in biodegradation was observed for CS composites, indicating longer durability of the composites.

Improved flame-retardant and tensile properties of thermoplastic starch/flax fabric green composites.

This article highlights the development of biodegradable flame-retardant composites using a compression technique on low-cost starch, flax fabric (FF) and ammonium polyphosphate (APP) raw materials. The starch was plasticized into thermoplastic starch through a mechano-ball milling process and composites were developed by reinforcing the FF and incorporating varying amounts of APP. The effects of APP on the flammability and thermal properties of the composites were studied. Limited oxygen index and horizontal-burning tests exhibited significant sustainability of the composites toward flame and direct flame self-extinguishment. It was observed that at higher temperatures, APP leads to formation of thermally stable char. The flame retardant properties of the composites were speculated to be due to the protective compact crosslinked network (POP and POC) of the char. The reported effects of APP include improvement in mechanical and biodegradation properties. This investigation provides the design of novel flame-retardant green composites with excellent properties.

Surface nanocrystallization of pure Cu induced by ultrasonic shot peening.

Peening is mainly used as a method of surface treatment for microstructural modification in order to improve surface mechanical properties. The ultrasonic shot peening (USP) technique can cause severe plastic deformation with its high strain rate on the surface of metallic parts. However, systematic studies of microstructural refinement mechanism upon plastic deformation with consideration of alloy systems are rare. In this study, USP-treated Cu samples of 99.96% purity was examined using analytical techniques, Vickers microhardness test, electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). Results of EBSD and microhardness analyses indicated grain refinement with deformation structures and hardness increase down to 400 µm in depth upon treatment. Depth specific TEM analysis of the samples revealed the steps of the grain refinement process to the formation of randomly oriented fine grains.

Growth of nanolaminate structure of tetragonal zirconia by pulsed laser deposition.

Alumina/zirconia (Al2O3/ZrO2) multilayer thin films were deposited on Si (100) substrates at an optimized oxygen partial pressure of 3 Pa at room temperature by pulsed laser deposition. The Al2O3/ZrO2 multilayers of 10:10, 5:10, 5:5, and 4:4 nm with 40 bilayers were deposited alternately in order to stabilize a high-temperature phase of zirconia at room temperature. All these films were characterized by X-ray diffraction (XRD), cross-sectional transmission electron microscopy (XTEM), and atomic force microscopy. The XRD studies of all the multilayer films showed only a tetragonal structure of zirconia and amorphous alumina. The high-temperature XRD studies of a typical 5:5-nm film indicated the formation of tetragonal zirconia at room temperature and high thermal stability. It was found that the critical layer thickness of zirconia is ≤10 nm, below which tetragonal zirconia is formed at room temperature. The XTEM studies on the as-deposited (Al2O3/ZrO2) 5:10-nm multilayer film showed distinct formation of multilayers with sharp interface and consists of mainly tetragonal phase and amorphous alumina, whereas the annealed film (5:10 nm) showed the inter-diffusion of layers at the interface.