Factors affecting carbon nanotubes-reinforced nanofibers

Alignment of nano-rods due to stretching of electrospinning jet.

Carbon nanotubes are known for its high strength and modulus and this makes it an attractive filler material in the fabrication of composites. Numerous studies have shown that carbon nanotubes showed good alignment in electrospun nanofibers. 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.

TEM image of nylon-6 nanofiber with multi-walled carbon nanotube [Saeed et al. Nanoscale Research Letters 2008 4:39-46 doi:10.1007/s11671-008-9199-0. See Copyright]

There are several factors that influence the mechanical reinforcement effect of carbon nanotubes in nanofibers. These factors include dispersion, interfacial adhesion between the carbon nanotube and its matrix and aspect ratio of the carbon nanotube.

Dispersion of reinforcement fillers is vital to obtain maximum mechanical reinforcement. Poor dispersion and agglomeration of fillers material often leads to a deterioration of the mechanical strength. To ensure uniform distribution of the carbon nanotubes within the solution mixture, it is often sonicated to disperse the carbon nanotubes prior to electrospinning. Other additives may also be added to aid the dispersion of the carbon nanotubes. Even so, there is a finite amount of carbon nanotubes that can be added before agglomeration sets in. Jeong et al (2007) found that the dispersion of multi-walled carbon nanotubes (MWCNT) in polyvinyl alcohol (PVA) electrospun nanofibers were good only up to a concentration of about 2.5 wt%. Beyond this, notches, bumps and beads start to appear on the surface of the nanofibers. The peak tensile strength was at 1 wt% MWCNT and modulus was at 2.5 wt% MWCNT. Tensile strength starts to drop significantly with increasing MWCNT to levels below neat PVA nanofiber.

To improve interfacial adhesion between carbon nanotube and its matrix, the surface of the carbon nanotube may be functionalized. Sen et al (2004) compared the mechanical property of electrospun polystyrene (PS) and polyurethane (PU) membrane loaded with as-prepared single-walled carbon nanotubes (AS-SWCNT) and ester functionalized SWCNT (EST-SWCNT). PU-EST-SWCNT had the highest tensile strength and modulus compared to AS-SWCNT and pure polyurethane membranes which showed the benefits of ester functionalization of SWCNT for improving interfacial strength. The type of functionalization is dependent on the polymer matrix which the carbon nanotube is going to be used in. For polyacrylonitrile (PAN), Eren et al (2014) used amine-functionalized multi-walled carbon nanotubes (MWCNT). Strong π- π interaction between amine group and nitrile group on PAN should lead to better bonding strength. This was demonstrated at 1% MWCNT loading, the functionalized MWCNT in PAN nanofiber showed a tensile strength of 2.41 N/mm² which was greater than non-functionalized MWCNT in PAN nanofiber (2.18 N/mm²). At higher MWCNT loading of 3%, the tensile strength and modulus were reduced.

Mechanical property of carbon-nanotube-reinforced nanofiber nonwoven membrane.

Polymer	Filler	Diameter	Mechanical Property	Reference
Polyacrylonitrile, Mw 100,000 g/mol	None	210 nm	Ultimate tensile strength: 1.9 MPa; Tensile strain: 33%	Yousefzadeh et al 2010
Polyacrylonitrile, Mw 100,000 g/mol	Multi-walled carbon nanotube, 1 wt%.	520 nm	Ultimate tensile strength: 3.1 MPa; Tensile strain: 68%	Yousefzadeh et al 2010
Nylon-6	None	100 to 400 nm	Ultimate tensile strength: 207 kgf cm/g	Saeed et al 2008
Nylon-6	Multi-walled carbon Nanotube, 1 wt%	100 to 400 nm	Ultimate tensile strength: 359 kgf cm/g	Saeed et al 2008
Nylon-6 grafted with multi-walled nanotube	Multi-walled carbon Nanotube, functionalized with amine groups, 1 wt%	100 to 400 nm	Ultimate tensile strength: 389 kgf cm/g	Saeed et al 2008
Polyvinyl alcohol	None	684 nm	Ultimate tensile strength: 3.11 MPa; Tensile strain: 142%	Naebe et al 2007
Polyvinyl alcohol	Multi-walled carbon nanotube, 4.5 wt%	295 nm	Ultimate tensile strength: 4.24 MPa; Tensile strain: 143%	Naebe et al 2007
Polyvinyl alcohol, MW 30 000-70 000 g mol-1	poly(m-aminobenzenesulfonic acid)-functionalized Single-walled carbon nanotube, 0.43 vol%	150 nm	Ultimate tensile strength: 10 MPa	Blond et al 2008
Silk fibroin	None	290 nm	Ultimate tensile strength: 6.7 MPa	Kang et al 2009
Silk fibroin	Multi-walled carbon nanotube, 0.2 wt%	420 nm	Ultimate tensile strength: 10 MPa	Kang et al 2009

Studies on cylindrical reinforcements have shown that aspect ratio of the fillers have a significant impact on the stress transfer from the matrix to the fillers. While this has been widely studied in larger composite materials, few studies have been conducted on carbon nanotube in electrospun nanofibers. A study by Wong et al (2009) showed that there is a significant improvement in the elastic modulus of polyvinyl alcohol/multi-walled carbon nanotube/lignosulfonic acid sodium salt composite nanofiber membranes when the average value of the aspect ratio of the carbon nanotube reached above 36.

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