Highly porous polymeric hollow nanofibers were developed using a method based on coaxial electrospinning with inner silicon oil and outer polymer solutions. This method was verified by the fabrication of polymeric hollow fibers, whose diameter and wall thickness could be varied by controlling the coelectrospinning parameters, such as the dielectric constant of the solvents, concentration of the polymer solution, molecular weights of the polymers and viscosity of the inner silicon oil phase. The entire diameter and wall thickness of the hollow fibers could be varied from 5 to 15 µm and 180 to 900 nm, respectively. Highly porous polymeric hollow nanofibers were fabricated by coaxial electrospinning with a highly volatile solvent. The interior surface was quite smooth without pores. Therefore, pore formation occurred at the outer surface of the hollow fibers due to rapid solvent evaporation because the jet only occurred between the surface of the polymer solution and air. The smooth interior and highly porous outer surface, circular crosssection and uniform size of the hollow polymer nanofibers are expected to have attractive applications in areas, such as catalysis, optoelectronics, nanofluidics, drug delivery or biosensorics.
ABSTRACT:Carbon black (CB)-filled high density polyethylene (HDPE)/syndiotactic polystyrene (sPS) composites were prepared by the conventional melt-mixing procedure. The effect of the HDPE/sPS weight ratio, CB content and processing speed on the thermoelectric behavior such as positive temperature coefficient (PTC) and negative temperature coefficient (NTC) was investigated in detail. The non-crosslinked HDPE/sPS composites containing 10-phr CB, a typical PTC behavior was observed when the temperature increased toward the melting point of HDPE. At high processing speed, the composites exhibited high room temperature resistivity and high PTC intensity. In a high sPS content composite, the dispersion of sPS particles are homogeneous and the distance of sPS particles are very close. The CB-coated sPS particles could be formed a continuous conductive network; therefore the PTC effect became weaker and weaker in these systems. The elimination of the NTC effect in CB-filled composites can be achieved by using a very high melting semicrystalline polymer as one of its components. An interesting phenomenon exhibited by the some conductive polymer is a positive temperature coefficient (PTC) effect. The main feature of PTC materials is that heating causes the conductive system to show a sharp resistivity increase near the melting region of a semicrystalline polymer matrix. 1,2A well-known technique that brings conducting properties into an intrinsically insulating polymer incorporates it with conducting fillers such as carbon black, 3 metal powder and fibers 4 or graphite fibers. 5Although metal powders are intrinsically more conductive than carbon black, metal has a tendency to oxidation to form an insulating layer on its surface and is not used as frequently as carbon back (CB). One of the most common additives used in conducting polymers is CB, which initially forms isolated structures within the matrix; then isolated clusters form, and finally through-going paths arise.The critical amount of CB necessary to form a conductive network is referred to as the percolation threshold. It is desirable for the conducting filler content to be as low as possible to achieve good processability, good mechanical properties and low cost.It is well known that when the temperature is above the melting point of semicrystalline polymers, noncrosslinked CB-filled semicrystalline polymer composites exhibit a very sharp decrease in resistivity. This phenomenon is referred to as the negative temperature coefficient (NTC) effect. 6,7 In general, non-crosslinked CB-filled semicrystalline polymer composites cannot be used as thermistors in over-temperature and over-current protections due to their NTC effects and poor reproducibility in thermal recycling. To overcome these disadvantages, researchers have proposed and developed many methods to eliminate the NTC effect of CB-filled semicrystalline polymer composites. Among these methods, the approach used to crosslink the semicrystalline polymer matrix by a crosslinking agent, such as a peroxide 8...
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