Search

We report the fabrication of hierarchical poly(vinylidene fluoride) (PVDF) fibrous structures using a unique fabrication technique based on electrospinning. Electrospinning was used to fabricate aligned PVDF fibrous membranes. These membranes were then brought in contact with anodized aluminum oxide (AAO) template and then heat-treated above the glass transition temperature (T g) of the polymer to assist the flow of polymer within the cylindrical pores of AAO template. Scanning electron microscopy images confirmed that this approach lead to the growth of nanopillars on the surface of PVDF fibers. Nanoindenter was used to measure the pull-off force that was required to completely detach the indenter from the samples. To investigate the effect of hierarchy, pull-off force required to detach the indenter from neat PVDF fibers was determined and compared with the pull-off force recorded for hierarchical fibers. The effect of indentation depth was also investigated on both PVDF fibers and PVDF fiber with nanopillars. Significant pull-off force recorded indicates that these PVDF hierarchical fibrous structures can be potentially used for dry-adhesive applications.

Poly(N-isopropylacrylamide) (PNIPAM) has been used extensively for numerous biomedical applications. However, there is not enough information in the literature on the wettability and hygroscopic properties of electrospun PNIPAM fibers, relevant for water harvesting applications. This study focuses on investigating the wettability and hygroscopic properties of electrospun PNIPAM fibers at room temperature and elevated temperature. The wettability properties of electrospun PNIPAM fibers were compared to spin-coated PNIPAM thin films. The wettability properties of the electrospun fibers were enhanced by 56% compared to spin-coated films. Water contact angle (WCA) measured on electrospun fibers was determined to be 137° at elevated temperatures while WCA on spin cast PNIPAM film was determined to be 81° at elevated temperatures. Furthermore, hygroscopic properties of the electrospun PNIPAM fibers were studied using thermogravimetric analysis (TGA). The PNIPAM fibers are seen to exhibit moisture absorption capacity of about 16.6 wt. % under humid conditions.

In this study, we used electrospinning combined with template wetting method to fabricate hierarchical poly(methyl methacrylate) (PMMA) fibrous structures. The adhesion performance of these samples was investigated using a nanoindenter. A flat circular diamond indenter of 10 μm diameter was brought in contact with the sample and then retracted back. The force required to detach the indenter from the sample was determined to be the pull-off force. The effect of indentation depth on the pull-off force was also investigated. Following this, an empirical relationship to predict the pull-off strength (σ) was established for a given fibril radius (r), fibril height (l), preload (p), and effective Young's modulus (E*). The pull-off force values recorded for hierarchical PMMA fibrous structure were also used to validate the empirical relationship. The empirical relationship demonstrated good correlation between the recorded pull-off strength and system parameters. We believe that this empirical relationship will be helpful in designing high strength synthetic dry-adhesives as the relationship can be used to predict the pull-off strength a priori.

Moth's eye inspired multiscale ommatidial arrays offer multifunctional properties of great significance in optoelectronic devices. However, a major challenge remains in fabricating these arrays on large-area substrates using a simple and scalable technique. Here we present the fabrication of these multiscale ommatidial arrays over large areas by a distinct approach called sacrificial layer mediated nanoimprinting, which involves nanoimprinting aided by a sacrificial layer. The fabricated arrays exhibited excellent pattern uniformity over the entire patterned area. Optimum dimensions of the multiscale ommatidial arrays determined by the finite-difference time domain simulations served as the design parameters for replicating the arrays on glass. A broadband suppression of reflectance to a minimum of ∼1.4% and omnidirectional antireflection for highly oblique angles of incidence up to 70° were achieved. In addition, superhydrophobicity and superior antifogging characteristics enabled the retention of optical properties even in wet and humid conditions, suggesting reliable optical performance in practical outdoor conditions. We anticipate that these properties could potentially enhance the performance of optoelectronic devices and minimize the influence of in-service conditions. Additionally, as our technique is solely nanoimprinting-based, it may enable scalable and high-throughput fabrication of multiscale ommatidial arrays.

Smart functional polymers have gained a huge amount of interest in recent times due to their innumerable applications in areas including sensors, actuators, switchable wettability, bio-medical and environmental applications. Numerous intensive research studies have been carried out to develop smart functional polymers using stimuli responsive polymeric moieties. This review article encapsulates recent developments in the area of smart functional polymers with a focus on various (physical, chemical and biological) stimuli responsive systems and their applications. Furthermore, this review also provides useful insights and in-depth analysis on the feasibility of utilizing stimuli responsive polymeric materials/composites in anti-fouling and water harvesting applications which hold tremendous potential to create a sustainable environment.

The fabrication of lubricants-infused textured surfaces has opened up a new route towards omniphobicity. However, achieving a homogeneous thin film of lubricating material on a flat/smooth surface still remains a challenge. This work shows the successful fabrication of a thin, transparent, and homogeneous coating of perfluoropolyether (PFPE, a lubricating material) on a smooth glass surface by the electrospraying technique. The sol–gel solution for electrospraying was prepared by adding a small amount of (tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane (FTS) with PFPE which was subsequently electrosprayed on a glass substrate. After curing the coated samples at 80 °C, a transparent, homogeneous, and slippery coating with a low surface energy (12.5 mN m−1) was obtained. It was observed that the presence of FTS with PFPE, assisted significantly in the stacking of PFPE on the substrate resulting in the formation of smooth, uniform blended (PFPE + FTS) films. The surface nature of the blended films was characterized by spectroscopy and microscopy. The blended surface exhibited omniphobic properties. The surface contact angles and slipping angles made by water and acetone droplets were measured to be (116°, 40.8°) and (6°, 10°), respectively. Furthermore, the coating showed good optical (transmittance – 91%) and mechanical properties with strong adherence to glass surfaces, thus revealing the potential for applications in windows and solar modules.

This review article exemplifies the importance of self-cleaning materials and coatings. Self-cleaning coatings are becoming an integral part of our daily life because of their utility in various applications such as windows, solar panels, cements, and paints. In this review, various categories of materials for the fabrication of hydrophilic, hydrophobic, oleophobic, amphiphobic and multifunctional coatings and their synthesis routes have been discussed. Furthermore, different natural organisms exhibiting superhydrophobic behaviour have been analysed. This review also covers the fundamentals of self-cleaning attributes such as water contact angle, surface energy, and contact angle hysteresis.

Fabrication of large-area anti-reflective coatings (ARCs) on glass substrates by using a cost-efficient and simple approach, especially for applications in photovoltaics, remains a challenge. This study proposes electrospinning as a technique to fabricate porous SiO 2 ARCs on large-area glass substrates (20×20 cm 2 ). The existing electrospinning setup is modified to enable large-area glass coatings and electrospinning process parameters are optimized to achieve sub-wavelength ARCs. The post-sintered SiO 2 ARC is thoroughly characterized for the film morphology and optical properties. The transmittance for one-side and both-sides coated glass is found to be 94.3% and 96% respectively. The anti-reflective glass is incorporated in solar modules to determine the increase in short circuit current. The increase in short-circuit current is found to be 3%. Electrospinning as a fabrication technique has potential in offering a cost effective solution for synthesizing ARCs on large-area substrates for photovoltaic applications.

Rice-shaped TiO2 nanostructures are fabricated by electrospinning for creating a robust superamphiphobic coating on glass substrates. The as-fabricated TiO2 nanostructures (sintered at 500 °C) are superhydrophilic in nature which upon silanization turn into superamphiphobic surface with surface contact angle (SCA) values achieved using water (surface tension, γ = 72.1 mN/m) and hexadecane (surface tension, γ = 27.5 mN/m) being 166° and 138.5°, respectively. The contact angle hysteresis for the droplet of water and hexadecane are measured to be 2 and 12°, respectively. Thus, we have successfully fabricated superior self-cleaning coatings that possess exceptional superamphiphobic property by employing a simple, cost-effective, and scalable technique called electrospinning. Furthermore, the coating showed good mechanical and thermal stability with strong adherence to glass surface, thus revealing the potential for real applications.

Herein, the fabrication of poly(vinylidene fluoride) (PVDF) fibrous membrane using electrospinning is reported and its use for dry-adhesive applications is demonstrated. The shear and normal adhesion performance of the samples was investigated using an Instron tensile tester and an atomic force microscope (AFM) respectively. For shear adhesion measurements, the electrospun membrane was finger pressed on to a glass slide and pulled in shear mode using a tensile tester. The thickness of the electrospun membrane was varied and the effect of thickness on shear adhesion was investigated. The shear adhesion strength increased when the thickness of the samples was reduced. Shear adhesion strength of a 200 µm thick sample was determined to be approximately 0.165 N/cm. For normal adhesion measurements, a flat tipless cantilever was used to indent the sample and then retract back to measure the pull-off force. High shear adhesion strength and normal pull-off force recorded are attributed to the fine size of the fibers that conform to the asperities present on the surfaces of the glass slide and the AFM cantilever. The durability of the adhesive was also verified by repeating the AFM adhesion measurements over 1000 consecutive attachment–detachment cycles. The pull-off force was seen to be constant over 1000 attachment–detachment cycles. Our results indicate that these electrospun fibrous membranes can potentially be used as reusable dry-adhesives. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44393.

Temperature-triggered switchable nanofibrous membranes are successfully fabricated from a mixture of cellulose acetate (CA) and poly(N-isopropylacrylamide) (PNIPAM) by employing a single-step direct electrospinning process. These hybrid CA-PNIPAM membranes demonstrate the ability to switch between two wetting states viz. superhydrophilic to highly hydrophobic states upon increasing the temperature. At room temperature (23 °C) CA-PNIPAM nanofibrous membranes exhibit superhydrophilicity, while at elevated temperature (40 °C) the membranes demonstrate hydrophobicity with a static water contact angle greater than 130°. Furthermore, the results here demonstrate that the degree of hydrophobicity of the membranes can be controlled by adjusting the ratio of PNIPAM in the CA-PNIPAM mixture.

A one-dimensional morphology of electrospun nanofibers has been used as a template to fabricate a robust and transparent superamphiphobic coating. The template is created by the deposition of a thick layer of SiO2 nanofibers on glass. The developed template (SiO2 nanofibers) is coated with an ultrathin (25 nm) porous silica membrane by the vapor deposition technique. After heat treatment (600 °C), a transparent, superhydrophilic coating consisting of a hybrid silica network (SiO2 nanofibers enclosed by the silica membrane) is obtained. It is observed that during the heat treatment process, the coated silica membrane reinforces the SiO2 nanofibers and prevents the fibers from disintegrating into nanoparticles, resulting in the formation of a hybrid silica network. The fiber morphology assisted the hybrid silica network to keep its roughness and surface texture. After silanization, the coating exhibited superamphiphobic property with surface contact angles achieved using water and hexadecane are 161° and 146.5°, respectively.

Polymer-based anti-reflective coatings (ARCs) on glass pose major challenges for outdoor photovoltaic applications due to their incompatible mechanical and thermal properties. Here we demonstrate durable, chemically and thermally stable polyhedral oligomeric silsesquioxane-based (POSS) anti-reflective moth's eye nanostructures on glass fabricated by double-side nanoimprint lithography. These anti-reflective nanostructures exhibited excellent broadband and quasi-omnidirectional anti-reflective properties. An optimum resist composition for nanoimprinting was obtained by mixing a methacryl POSS cage mixture with 1,6-hexanediol diacrylate in a 1 : 12 molar ratio. Thermal free radical co-polymerization during nanoimprint lithography produced a uniform array of moth's eye nanostructures on both sides of a glass substrate with yields ∼90 to 100%. The transmittance of the resulting glass was enhanced to 98.2% (reflectance 1.26%) with excellent quasi-omnidirectional transmittance observed from −50° to +50° of angles of incidence. Furthermore, a series of ASTM-based tests on the imprinted ARC structures showed strong adherence to glass, better hardness and mechanical strength with superior chemical and thermal stability, thus suggesting their strong potential for commercial applications.

A photocatalytic, superhydrophilic, transparent, porous TiO2 film consisting of rice-shaped nano/mesostructures deposited on glass substrates by electrospinning has been developed. Optical properties such as transmittance and absorbance, as well as the superhydrophilicity and self-cleaning properties (photocatalysis) of the deposited TiO2 film are studied. The water contact angle decreases with increase in the thickness of the TiO2 resulting in superhydrophilic transparent coatings which may be used for applications in window coatings and photovoltaic modules. The self-cleaning property of the TiO2 film consisting of rice-shaped nano/mesostructures is compared with that of commercially available Degussa P-25 in the photodegradation of Alizarin red dye and found to be more effective.

Anti-reflective coatings (ARCs) have evolved into highly effective reflectance and glare reducing components for various optical and opto-electrical equipments. Extensive research in optical and biological reflectance minimization as well as the emergence of nanotechnology over the years has contributed to the enhancement of ARCs in a major way. In this study the prime objective is to give a comprehensive idea of the ARCs right from their inception, as they were originally conceptualized by the pioneers and lay down the basic concepts and strategies adopted to minimize reflectance. The different types of ARCs are also described in greater detail and the state-of-the-art fabrication techniques have been fully illustrated. The inspiration that ARCs derive from nature (‘biomimetics’) has been an area of major research and is discussed at length. The various materials that have been reportedly used in fabricating the ARCs have also been brought into sharp focus. An account of application of ARCs on solar cells and modules, contemporary research and associated challenges are presented in the end to facilitate a universal understanding of the ARCs and encourage future research.

This review article summarizes the key areas of self-cleaning coatings, primarily focusing on various materials that are widely used in recent research and also in commercial applications. The scope of this article orbits around hydrophobic and hydrophilic coatings, their working mechanism, fabrication techniques that enable the development of such coatings, various functions like Anti-icing, Electro-wetting, Surface switchability and the areas where selfcleaning technology can be implemented. Moreover, different characterization techniques and material testing feasibilities are also analyzed and discussed. Though several companies have commercialized a few products based on self-cleaning coating technology, much potential still remains in this field.

This study investigated the switchable wettability behavior of poly-N-(isopropyl acrylamide) (PNIPAM) and poly-(vinylidene fluoride) (PVDF) blend membranes that were fabricated using electrospinning. The wettability of the membranes was determined by water contact angle measurements. The wettability of the membranes was controlled by varying the concentration of PNIPAM in the blend. The results demonstrated that the addition of PVDF to the PNIPAM did not change the thermoresponsive behavior of PNIPAM. All PNIPAM/PVDF blend samples switched from hydrophilic to hydrophobic state when the temperature was increased from room temperature to 40 °C. However, blend samples with 25 wt% PNIPAM were demonstrated to be more hydrophilic and hydrophobic compared to other samples at room temperature and elevated temperature respectively. This was attributed to the phase separation of the polymers that resulted in domains with drastically different surface energies on the surface of the fibers.