3D Printed Parahydrophobic Surfaces as Multireaction Platforms.

Parahydrophobic surfaces (PHSs) composed of arrays of cubic µ-pillars with a double scale of roughness and variable wettability were systematically obtained in one step and a widely accessible stereolithographic Formlabs 3D printer. The wettability control was achieved by combining the geometrical parameters (H = height and P = pitch) and the surface modification with fluoroalkyl silane compounds. Homogeneous distribution of F and Si atoms onto the pillars was observed by XPS and SEM-EDAX. A nano-roughness on the heads of the pillars was achieved without any post-treatment. The smallest P values lead to surfaces with static contact angles (CAs) >150° regardless of the H utilized. Interestingly, the relationship 0.6 ≤ H/P ≤ 2.6 obtained here was in good agreement with the H/P values reported for nano- and submicron pillars. Furthermore, experimental CAs, advancing and receding CAs, were consistent with the theoretical prediction from the Cassie-Baxter model. Structures covered with perfluorodecyltriethoxysilane with high H and short P lead to PHSs. Conversely, structures covered with perfluorodecyltrimethoxysilane exhibited a superhydrophobic behavior. Finally, several aqueous reactions, such as precipitation, coordination complex, and nanoparticle synthesis, were carried out by placing the reactive agents as microdroplets on the parahydrophobic pillars, demonstrating the potential application as chemical multi-reaction array platforms for a large variety of relevant fields in microdroplet manipulation, microfluidics systems, and health monitoring, among others.

Superhydrophobic µ-pillars via simple and scalable SLA 3D-printing: the stair-case effect and their wetting models.

In nature, superhydrophobic surfaces (SHSs) exhibit microstructures with several roughness scales. Scalable fabrication and build-up along the X-Y plane represent the promise of 3D printing technology. Herein we report 3D printed microstructures with a dual roughness scale that achieves SHS using a readily available Formlabs stereolithography (SLA) printer. Pillar-like structure (PLS) arrangements with a wide range of geometrical shapes were 3D printed at three resolutions and two printing orientations. We discovered that a tilted printing direction enables a stair-case pattern on the µ-PLS surfaces, conferring them a µ-roughness that reduces the solid-liquid contact area. The programmed resolution governs the number of polymerized layers that give rise to the stepped pattern on the µ-PLS surfaces. However, this is reduced as the printing resolution increases. Also, all samples' experimental contact angles were consistent with theoretical predictions from Cassie-Baxter, Wenzel, and Nagayama wettability models. The underlying mechanisms and governing parameters were also discussed. It is believed that this work will enable scalable and high throughput roughness design in augmenting future 3D printing object applications.

The Role of α, γ, and Metastable Polymorphs on Electrospun Polyamide 6/Functionalized Graphene Oxide.

The present paper describes the addition of nitroxide-functionalized graphene oxide (GOFT) into polyamide 6 (PA6) micro- and nanofibers, which are obtained through electrospinning. Scanning electron microscopy micrographs demonstrate the presence of fibers. Tensile testing presents an unexpected and non-obvious behavior, in which the Young's modulus, tensile strength, and elongation simultaneously and remarkably increase compared to the pristine polymer nanofibers. GOFT induces the hydrogen bonding between the NH group from PA6 with the functional groups, thus promoting higher crystallinity of the polymer matrix. Nonetheless, deconvoluted curves by differential scanning calorimetry reveal the presence of two quasi-steady polymorphs (ß and Î´ phases) contributing to 46% of the total crystallinity. This evidence suggests that their presence and high ratios are responsible for the unexpected and simultaneous enhancement of tensile properties.

EELS Analysis of Nylon 6 Nanofibers Reinforced with Nitroxide-Functionalized Graphene Oxide.

A detailed analysis by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) of nitroxide-functionalized graphene oxide layers (GOFT) dispersed in Nylon 6 nanofibers is reported herein. The functionalization and exfoliation process of graphite oxide to GOFT was confirmed by TEM using electron diffraction patterns (EDP), wherein 1 to 4 graphene layers of GOFT were observed. The distribution and alignment of GOFT layers within a sample of Nylon 6 nanofiber reveals that GOFT platelets are mainly within the fiber, but some were partially protruding from it. Furthermore, Nylon 6 nanofibers exhibit an average diameter of 225 nm with several microns in length. GOFT platelets embedded into the fiber, the pristine fiber, and amorphous carbon were analyzed by EELS where each spectra [corresponding to the carbon edge (C-K)] exhibited changes in the fine structure, allowing a clear distinction between: i) GOFT single-layers, ii) Nylon-6 nanofibers, and iii) the carbon substrate. EELS analysis is presented here for the first time as a powerful tool to identify functionalized graphene single-layers (< 4 layers of GOFT) into a Nylon 6 nanofiber composite.

Nitroxide-Functionalized Graphene Oxide from Graphite Oxide.

A facile method for preparing functionalized graphene oxide single layers with nitroxide groups is reported herein. Highly oxidized graphite oxide (GO=90.6%) was obtained, slightly modifying an improved Hummer's method. Oxoammonium salts (OS) were investigated to introduce nitroxide groups to GO, resulting in a one-step functionalization and exfoliation. The mechanisms of functionalization/exfoliation are proposed, where the oxidation of aromatic alcohols to ketone groups, and the formation of alkoxyamine species are suggested. Two kinds of functionalized graphene oxide layers (GOFT1 and GOFT2) were obtained by controlling the amount of OS added. GOFT1 and GOFT2 exhibited a high interlayer spacing (d0001 = 1.12nm), which was determined by X-ray diffraction. The presence of new chemical bonds C-N (~9.5 %) and O-O (~4.3 %) from nitroxide attached onto graphene layers were observed by X-ray photoelectron spectroscopy. Single-layers of GOFT1 were observed by HRTEM, exhibiting amorphous and crystalline zones at a 50:50 ratio; in contrast, layers of GOFT2 exhibited a fully amorphous surface. Fingerprint of GOFT1 single layers was obtained by electron diffraction at several tilts. Finally, the potential use of these materials within Nylon 6 matrices was investigated, where an unusual simultaneous increase in tensile stress, tensile strain and Young's modulus was observed.

Enhancing Visible Light Photocatalytic Degradation of Bisphenol A Using BiOI/Bi2MoO6 Heterostructures.

With the growing population, access to clean water is one of the 21st-century world's challenges. For this reason, different strategies to reduce pollutants in water using renewable energy sources should be exploited. Photocatalysts with extended visible light harvesting are an interesting route to degrade harmful molecules utilized in plastics, as is the case of Bisphenol A (BPA). This work uses a microwave-assisted route for the synthesis of two photocatalysts (BiOI and Bi2MoO6). Then, BiOI/Bi2MoO6 heterostructures of varied ratios were produced using the same synthetic routes. The BiOI/Bi2MoO6 with a flower-like shape exhibited high photocatalytic activity for BPA degradation compared to the individual BiOI and Bi2MoO6. The high photocatalytic activity was attributed to the matching electronic band structures and the interfacial contact between BiOI and Bi2MoO6, which could enhance the separation of photo-generated charges. Electrochemical, optical, structural, and chemical characterization demonstrated that it forms a BiOI/Bi2MoO6 p-n heterojunction. The free radical scavenging studies showed that superoxide radicals (O2•-) and holes (h+) were the main reactive species, while hydroxyl radical (•OH) generation was negligible during the photocatalytic degradation of BPA. The results can potentiate the application of the microwave synthesis of photocatalytic materials.

Strong and Lightweight Stereolithographically 3D-Printed Polymer Nanocomposites with Low Friction and High Toughness.

Strong and lightweight polymer nanocomposites with low friction, high toughness, and complex shapes were obtained for the first time through an affordable stereolithographic 3D printer, using low amounts of TiO2 nanoparticles. Tridimensional solid structures (i.e., tensile bars, compressive test specimens, gyroid-type structures, and dense lattices) were obtained. Herein, we found that the compressive stress, compressive strain, yield strength, and toughness corresponding to 3D-printed polymer nanocomposites were simultaneously increased-which is uncommon-using low amounts (0.4 wt.%) of TiO2 nanoparticles. Furthermore, we obtained lightweight cylindrical structures exhibiting high resistance to compression with a low friction coefficient (µ~0.2), and the printability of complex and hollow structures was demonstrated.

Sustainable xanthophylls-containing poly(ε-caprolactone)s: synthesis, characterization, and use in green lubricants.

Three xanthophylls [(3R,3'R,6'R)-lutein (1), (3R,3'S)-zeaxanthin (2), and (3R,3'S)-astaxanthin (3)] were used for the first time as initiators in the ring-opening polymerization (ROP) of ε-caprolactone (CL) catalyzed by tin(ii) 2-ethylhexanoate [Sn(Oct)2] for the synthesis of novel sustainable xanthophyll-containing poly(ε-caprolactone)s (xanthophylls-PCL). The obtained polyesters were characterized by 1H and 13C NMR, FT-IR, DSC, SEC, and MALDI-TOF MS, and their use as additives in green lubricants was evaluated using a sliding friction test under boundary conditions. Xanthophylls-PCL were obtained with good conversions and with molecular weights determined by SEC to be between 2500 and 10 500 Da. The thermal properties of xanthophyll-polyesters showed a crystalline domain, detected by DSC. Lastly, the green lubricant activity of these polymers was evaluated and the results showed that xanthophylls-PCL could be employed as additives for biodegradable lubricant applications since they have better tribological behavior than current additives, which demonstrates their potential as future commercial materials with interesting eco-friendly properties for diverse applications.

A Simple and Expeditious Route to Phosphate-Functionalized, Water-Processable Graphene for Capacitive Energy Storage.

Phosphate-functionalized carbon-based nanomaterials have attracted significant attention in recent years owing to their outstanding behavior in electrochemical energy-storage devices. In this work, we report a simple approach to obtain phosphate-functionalized graphene (PFG) via anodic exfoliation of graphite at room temperature with a high yield. The graphene nanosheets were obtained via anodic exfoliation of graphite foil using aqueous solutions of H3PO4 or Na3PO4 in the dual role of phosphate sources and electrolytes, and the underlying exfoliation/functionalization mechanisms are proposed. The effect of electrolyte concentration was studied, as low concentrations do not lead to a favorable graphite exfoliation and high concentrations produce fast graphite expansion but poor layer-by-layer delamination. The optimal concentrations are 0.25 M H3PO4 and 0.05 M Na3PO4, which also exhibited the highest phosphorus contents of 2.2 and 1.4 at. %, respectively. Furthermore, when PFG-acid at 0.25 M and PFG-salt at 0.05 M were tested as an electrode material for capacitive energy storage in a three-electrode cell, they achieved a competitive performance of ∼375 F/g (540 F/cm3) and 356 F/g (500 F/cm3), respectively. Finally, devices made up of symmetric electrode cells obtained using PFG-acid at 0.25 M possess energy and power densities up to 17.6 Wh·kg-1 (25.3 Wh·L-1) and 10,200 W/kg; meanwhile, PFG-salt at 0.05 M achieved values of 14.9 Wh·kg-1 (21.3 Wh·L-1) and 9400 W/kg, with 98 and 99% of capacitance retention after 10,000 cycles, respectively. The methodology proposed here also promotes a circular-synthesis process to successfully achieve a more sustainable and greener energy-storage device.

Super-Anticorrosive Materials Based on Bifunctionalized Reduced Graphene Oxide.

The design of new materials with two or more functional groups must be strongly considered to achieve multifunctional coatings with outstanding properties such as active-passive protection against corrosion, low-friction, antifouling, and sensing, among others. In this sense, nanocomposites based on solvent-free epoxy resin/bifunctionalized reduced graphene oxide layers with NH2 and NH3+ groups (ER/BFRGO) with super-anticorrosive properties are for the first time reported here. The amine groups (-NH2) act as cross-linker agents, which react with epoxy terminal groups from resin, thus closing the gap between the BFRGO layers and the polymeric matrix. Meanwhile, the ammonium ions (-NH3+) are effective trapping agents of negatively charged atoms or molecules (e.g., Cl-). This novel combination enables us to obtain nanocomposite coatings with passive-active protection against corrosion. ER/BFRGO deposited onto A36 mild steel exhibited a remarkably enhanced barrier against corrosion into a saline medium (1 M NaCl; 58.4 g/L), wherein the corrosion current density (icorr) was diminished 6 orders of magnitude (icorr = 5.12 × 10-12 A/cm2), with respect to A36 mild steel coated only with ER (icorr = 2.34 × 10-6 A/cm2). Also, the highest polarization resistance Rp = 6.04 × 107 Ω/cm2 was obtained, which represents the lowest corrosion rate and corresponds to 3 orders of magnitude higher than A36 mild steel coated with ER (Rp = 1.43 × 104 Ω/cm2). The strategy of bifunctionalization proposed herein to obtain bifunctionalized reduced GO with NH2 and NH3+ groups has not been disclosed in the literature before; in consequence, this work opens a new pathway toward the design of smart materials based on multifunctional nanomaterials.

On the Effect of Ultralow Loading of Microwave-Assisted Bifunctionalized Graphene Oxide in Stereolithographic 3D-Printed Nanocomposites.

Surface functionalization of graphene oxide (GO) is one of the best ways to achieve homogeneous dispersions of GO within polymeric matrices and composites. Nonetheless, studies regarding how the level of GO functionalization affects the macroscopic properties of three-dimensional (3D) printed nanocomposites are still few. Furthermore, the bifunctionalization of GO with the NH2/NH3+ groups to obtain improved thermomechanical macroscopic properties at ultralow loads has not been reported. In this paper, fast and straightforward surface bifunctionalization of GO with a controlled ratio of NH2/NH3+ groups at low, medium, and high functionalization levels (AGOL, AGOM, and AGOH) in a one-step microwave-assisted synthesis is reported for the first time. The functionalization mechanism was disclosed, wherein three graft densities (Gφ) were obtained. A plateau of maximum functionalization (Gφ = 4.9 µmol/m2 = 2.9 molecules/nm2) was reached, suggesting that full coverage of the GO surface is achievable. Also, an increase in the exfoliation of functionalized layers was obtained, ranging from d002 = 8.6 Å up to d002 = 15.8 Å. X-ray photoelectron spectroscopy (XPS) reveals the successful functionalization of GO, as well as an atomic relationship NH2/NH3+ of about 50/50% in all functionalized samples. Stereolithographic (SLA) 3D-printed nanocomposites (AGOL/R, AGOM/R, and AGOH/R) were obtained using ultralow loads (0.01 wt %) of each bifunctionalized material. This ultralow amount was sufficient to enhance thermal stability (up to 4 °C) and a significant increase in the glass transition temperature (93 °C ≤ Tg ≤ 120 °C). Interestingly, we found that low and medium grafting density promotes a ductile material (ε > 5%); meanwhile, a high graft density produces brittle materials. Also, we observe that the toughness can be tuned as a function of the graft density (AGOH: 24 MPa, AGOM: 342 MPa, AGOL: 562 MPa) at ultralow loadings. The 3D-printed nanocomposites using GO with low graft density (AGOL) increase their tensile strain by 90% in comparison with the control sample (without filler). Finally, the underlying mechanisms were discussed to explain the findings.

Optimizing the Coverage Density of Functional Groups over SiO2 Nanoparticles: Toward High-Resistant and Low-Friction Hybrid Powder Coatings.

Hybrid powder coatings (HPC) with low friction and high hardness enhance the sliding speed and allow interlocking or meshing products to slide effortlessly within each other, saving energy. In automobiles, they decrease fuel consumption and greenhouse gas emission. In the present work, a new insight of the key role played by the coverage density of triethoxyphenylsilane (TPS) grafted to SiO2 nanoparticles over the friction coefficient, hardness, elastic modulus, and roughness of HPC is presented for the first time. In all cases, a very low amount (0.1 wt %) of functionalized or unfunctionalized SiO2 nanoparticles were added to a powder-coating formulation based on polyester resin. HPC formulated with functionalized nanoparticles at a suitable coverage density (HPC-TPS3) exhibited significantly low friction coefficient (µ = 0.12), strong wear resistance (under dry sliding conditions at 1 and 5 N of load), low roughness (R q = 3.5 nm), and high hardness and elastic modulus on the surface. We demonstrated that it is possible to tune the macroscopic properties by varying only the coverage density of TPS that is chemically attached to SiO2 nanoparticles. Also, a physicochemical explanation was disclosed, wherein a hydrophilic-hydrophobic balance between -OH and phenyl groups was proposed. In all cases, the phenyl group allows the migration of functionalized nanoparticles through the polyester matrix, enhancing the hardness and elastic modulus on the surface. Thus, the functional nanomaterial design with tunable coverage density is a powerful tool to improve the physical and superficial properties of powder coatings using low amounts of nanomaterial.

Chemical Modification of Butyl Rubber with Maleic Anhydride via Nitroxide Chemistry and Its Application in Polymer Blends.

Butyl rubber (isobutylene⁻isoprene⁻rubber, IIR) was functionalized in solution with a nitroxide moiety taking advantage of the unsaturations present in the isoprene units of IIR, and was further grafted with maleic anhydride (MA) or styrene⁻MA (SMA) to produce IIR-g-MA and IIR-g-SMA. In one of the functionalization techniques used, the molecular structure of the IIR was preserved as the chain-breaking reactions are prevented from occurring. The resulting graft copolymers were tested as compatiblizers/impact modifiers blended with Nylon-6, and one of them was preliminarily tested as a coupling agent in the preparation of nanocomposites of IIR and an organo-clay. Blends of PA-6/IIR-g-MA exhibited a significant increase in impact resistance at increasing loads of the modified IIR, as well as a good rubber particle dispersion in the polyamide matrix. On the other hand, the performance of IIR-g-SMA as an impact modifier of PA, or as a coupling agent in the preparation of rubber-organoclay nanocomposites, is marginal.

Theoretical study of absorption of 2,2,6,6-tetramethylpiperidine-1-oxoammonium cation (TEMPO) on TiO2(110) rutile surface.

We present a theoretical study of the adsorption of 2,2,6,6-tetramethylpiperidine-1-oxoammonium cation (TEMPO) onto the TiO2(110) surface rutile, investigating its bonding nature, electron properties and structural stability. Based on the results obtained with the PBE0/def2-SVP method, natural bond orbital (NBO) analysis suggests a bond order for the O--O bond in complexes 5 and 6, of 0.25 and 0.88, respectively. We also described NBOs for the main interactions of the TiO2-TEMPO complexes.