Electrospinning for nano-material organization

Distribution and organization of nano-materials is a challenge as they tend to agglomerate and their small size require non-conventional method of arranging them to the desired position. Electrospinning has emerged as a possible technique for organizing nano-materials in a linear form. As a process, electrospinning to form fibers involves stretching of a fluid in a single direction and any solid dispersion in the solution will like-wise be stretched in the same direction. Due to the confinement of the fluid and the viscoelastic force exerted by the stretching fluid, solids that are larger than the diameter of the stretched solution will be forced to align themselves along the length of the stretched fluid. Solids with a major axis will also be forced to align its major axis in the same direction of the stretching force. Electrospinning can be used to produce oriented fibers and advancement in this technique has seen the development of controlled placement of its fiber. This allows greater control in the positioning of the nano-material load which the fiber is carrying.

Alignment of nano-rods due to stretching of electrospinning jet

Incorporation of carbon nanotubes in electrospun fibers have seen the carbon nanotubes being aligned along the length of the electrospun nanofibers. In cases where functional performance of nanorods or nanotubes is affected by their alignment, electrospinning offers a way to achieve it. Roskov et al (2011) used electrospinning to align gold nanorods of aspect ratio 3.1 along a single axis. Gold nanorods have a tunable longitudinal surface plasmon resonance (LSPR) along its longitudinal axis which makes the ability to align them vital for this application. Polyethylene oxide (PEO) was used as the carrier polymer for the gold nanorods and they were able to achieve excellent gold nanorods alignment along the longitudinal axis of the fibers with diameters ranging from 40 nm to 600 nm. However, when the fiber diameter increases beyond 600 nm, alignment of the nanorods starts to deteriorate. Higher aspect ratio of the nanorod was also found to improve alignment as expected due to greater influence by the solution flow profile. However, the amount of nanorods up to a maximum volume fraction of 0.045 in the solution does not influence their alignment. The resultant composite material showed LSPR bands are polarization dependent with maximum extinction when the polarizer is parallel to the long axis of the rod. Zhang et al (2012) used electrospinning of polyvinyl pyrrolidone (PVP) to align silver nanowires (AgNW). The nanowires have diameter of 30 - 40 nm and length of 10 - 20 µm. Loading of up to 20 mg/ml of AgNW was used this resulted in more than ten AgNWs aligned within a single electrospun nanofiber.

Fibers with very high surface roughness are known to demonstrate higher hypdrophobicity. Beaded fibers have been constructed from linear organization of silica beads using electrospinning. To achieve this, a beads colloidal dispersion was prepared and electrospinning is subsequently carried out. Silica beads in the sub-micron diameter have been arranged into fibers using this method [Lim et al 2007]. Sintering was used to remove the polymer matrix that was used to arrange the beads during electrospinning. Zhou et al (2014) used a suspension of polytetrafluoroethylene (PTFE) fine particles in water for blending with water soluble polyvinyl alcohol (PVA). The PVA with PTFE particles were electrospun and the PVA component removed through sintering up to 380°C for 30 minutes. At this temperature the PTFE particles melted and fused together to form an interconnected nanofibrous network of PTFE. Gonzalez et al (2021) used electrospinning to form nanofibers out of latex beads. For electrospinning, a suspension of polyvinyl alcohol (PVA), surfactant and latex copolymer particles was prepared. PVA is the template polymer to enable electrospinning and the surfactant is used to disperse the latex copolymer particles. Their results showed that particles of larger diameter resulted in electrospun fibers of larger diameter. For particle sizes 107 nm and 192 nm, they were able to produce smooth fibers but with particle size of 317 nm, fibers with pearl necklace morphology were produced probably due to poor packing. The mono-suspension of these three particles with the template polymer exhibited similar viscosities. When the ratio of the particles to PVA was increased to 71/29 wt.%/wt.%, beaded fibers were obtained across all particle sizes. With the same particles to PVA ratio but with a bimodal particles size mixture consisting of the smallest particles (diameter 107 nm) and largest particles (diameter 317 nm) in equal ratio, the resultant electrospun fibers were much smoother. It was found that with this bimodal particle suspension, the viscosity is significantly higher than their monomodal suspension. This probably facilitated the production of uniform fibers.