This work presents a simple and reliable method for directly generating polyvinylidene fluoride (PVDF) nanofibers with secondary surface morphology (e.g., porous surfaces, rough surfaces, grooved surfaces, and interior porosity) by using single/binary solvent systems and relative humidity. We clarified the mechanisms responsible for the formation of these morphologies by systematically exploring the molecular interactions among the polymer, solvent(s), and water vapor. Our results proved that the formation of secondary surface morphology needed the presence of water vapor, a non-solvent of the polymer, at an appropriate level of relative humidity. The formation of secondary surface morphology was dependent on the speed of evaporation of the solvent(s) (ACE, DMF, and their mixtures), as well as the inter-diffusion and penetration of the non-solvent (water) and solvent(s). The results of N2 physical adsorption-desorption isotherms showed that the macro-porous fibers (> 300 nm) exhibited the highest specific surface area of 23.31 ± 4.30 m2/g and pore volume of 0.0695 ± 0.007 cm3/g, enabling the high oil absorption capacities of 50.58 ± 5.47 g/g, 37.74 ± 4.33 g/g, and 23.96 ± 2.68 g/g for silicone oil, motor oil, and olive oil, respectively. We believe this work may serve as guidelines for the formation of different structures of macro-porous, rough, and grooved nanofibers with interior porosity through electrospinning.Electronic supplementary materialThe online version of this article (10.1186/s11671-018-2705-0) contains supplementary material, which is available to authorized users.

Harvesting energy has been attracting the attention of researchers in recent years. This work comprehensively reports the fabrication and characterization of poly(vinylidene fluoride) fiber webs with comparable diameters and diversified surface morphologies (wrinkled, smooth, and porous) based on randomly oriented and aligned fiber webs which are used as active layers to directly make a piezoelectric nanogenerator (PENG). The results show that wrinkled fibers are preferable fiber webs for the PENG owing to their outstanding properties represented by high fibers friction, supreme β phase content (F[β]), and interior pores. Furthermore, we found that the electrical outputs of the PENG based on aligned fiber webs are higher than those based on randomly oriented fiber webs due to the increase in the friction area. The electrical outputs of the PENG based on the aligned wrinkled fiber webs are able to run microelectronic devices when it subjected to a mechanical impact. We believe that our study may inspire the research area for future energy harvesting applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47049.

The surface evolution of fibers has been attracting great attention in many areas. In this work, we demonstrated the feasibility of directly electrospinning grooved and porous polyvinylidene fluoride fibers by varying polymer concentration and using both single/binary solvent systems. We discovered that grooved fibers can be generated from a variety of binary solvent systems, whereas porous fibers can be achieved using low boiling point solvent (LBPS) systems. The results indicated that the key factors for the formation of grooved fibers are the polymer concentration, the difference of evaporation rate (the difference of boiling points between each of the two studied solvents) between the two solvents, and the solvent ratio. While the polymer concentration, the presence of LBPS, and the solvent ratio are the decisive factors for the formation of porous structures. The formation mechanism of grooved fibers was attributed to void-based elongation, wrinkle-based elongation, and collapsed jet-based elongation. The mechanism for the formation of porous fibers was ascribed to the coexistence of both thermally induced phase separation and vapor induced phase separation. These structures can be used in different applications, such as harvesting energy and filtration. Importantly, we believe our work can serve as guidelines for the preparation of grooved and porous nanofibers through electrospinning.

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Recent advances in self-powered electronic devices have urged the development of energy-harvesting technology. Batteries are gradually unable to satisfy the practical requirements for powering the different types of microelectronic devices owing to their drawbacks such as occupying a significant percentage and weight of portable products, the need to replace or recharge them, constructing an important environmental impact, and the probable seepage of electrolyte solutions. Various technologies for converting renewable energies into electricity have been reported. Particularly, energy harvesters based on piezoelectricity to convert mechanical energy into usable electricity have received considerable attention. Electrospun fibers from piezoelectric polymers and inorganic nanowires as emerging piezoelectric materials have shown great potential for energy-harvesting applications. This review paper summarizes energy-harvesting technology based on piezoelectric polymeric fibers, inorganic piezoelectric fibers, and inorganic nanowires. A comprehensive overview of fundamentals of piezoelectric effect, types of piezoelectric materials, energy harvesting from fibers, energy harvesting from inorganic nanowires, and energy harvesting from polymeric/inorganic fibers and nanowires composites are discussed.

Engineering the secondary surface morphology of fibers (fibers without smooth surface and solid interior) has been attracting significant awareness in various areas and applications. Among different methods of forming nanofibers, electrospinning is the most widely adopted technique due to the ease of forming fibers with a broad range of properties and its unique advantages such as the flexibility to spin into a variety of shapes and sizes as well as adjustable porosity of electrospun structures. In this review paper, more emphasis is put on the secondary surface morphology. A comprehensive overview of the basic principles about the electrospinning process, types of electrospinning, materials, formation mechanisms, characterizations, and applications of electrospun fibers with the secondary surface morphology (e. g. the porous structure, grooved structure, wrinkled structure, rough structure, cactus structure, and hollow structure) are discussed in detail. Importantly, we believe our work can serve as guidelines for the preparations, characterizations, and applications of electrospun fibers with the secondary surface morphology.

A novel PVDF cactus-like nanofiber was directly electrospun. The mechanism of formation, properties, and possible applications were demonstrated.

Structures modification of fibers has been attracting significant attention in various fields and applications. Among different techniques of fabricating ultrathin fibers, electrospinning is the most commonly adopted method because of the ease of forming fibers with a wide range of properties and its exceptional advantages, such as the ability to spin into different shapes and sizes, as well as the adaptable porosity of electrospun fiber webs. The crimped structure has been attracting the attention of scientific researchers owing to its unique properties (eg, spring-like behavior, supreme strain, remarkable specific surface area, good piezoelectric properties, excellent biological properties, and so on). Therefore, this study summarizes a review of the strategies and methods, reported so far, of generating electrospun crimped ultrathin fibers of various polymers. The review focuses on the polymer types, formation methods, characterizations, and applications of the electrospun crimped ultrathin fibers. We believe this work can serve as an important reference for the materials, strategies, and applications of crimped fibers.