Nanofibres yield stronger composites

Nanofibres enhance binding in carbon fibre and other fibre-reinforced polymer composites.

A group at Oak Ridge National Laboratory, USA, says the approach yields a 50% improvement in tensile strength and a nearly two-fold increase in toughness, through carefully tailored nanofibres.

'The challenge of improving adhesion between carbon fibres and the polymer matrix that surrounds them has been a concern in industry for some time,' asserts Sumit Gupta, the researcher who led the project.

The scientists say industry has tried texturing the fibres’ exterior or injecting chemicals with limited success.

The team’s hybrid technique uses carbon nanofibres for improved chemical and mechanical bonding.

They use electrospinning, whereby the carbon fibre precursor polyacrylonitrile (PAN) is extruded through a strong electric field to produce strands ~200nm wide, which land on a spinning metal drum overwrapped with carbon fibre fabric.

By varying the strength of the electric field, the drum speed and other parameters, the researchers have created nanofibres that chemically bond to the matrix and mechanically bond to other carbon fibres, creating ‘bridges’ between the two dissimilar materials.

'Poly(acrylonitrilebutadiene-styrene) (ABS), PAN, and carbon fibre were selected as the polymer matrix, the nanofibre scaffold and the core fibre, respectively,' shares researcher, Chris Bowland. 

The carbon fibre diameter is 10 times larger than the electrospun nanofibres, which creates a hierarchical network at the core fibre-matrix interphase. 

'PAN and ABS have compatible nitrile groups that can aid the desired hierarchical nanostructures-matrix molecular coupling when subjected to suitable thermal stimuli,' explains Bowland.

He says they chose electrospinning as it is a scaleable and low-cost method to integrate the high aspect ratio PAN nanofibres at the fibre-matrix interphase. They can control the chemical bonding and fibre orientation by tweaking the electrospinning conditions.

The rotation speed of the drum can be precisely adjusted by controlling the motor’s revolutions per minute, enabling control over fibre alignment. Similarly, the electrospinning voltage can be easily tuned by adjusting the gain on the high-voltage amplifier, allowing for fine control over the electric field during fibre formation.

Gupta adds, 'Specifically, by reducing the electric field from 1.4kV/cm to 0.5kV/cm, the average fibre diameter decreased from approximately 0.4μm to 0.2μm.'

Explaining how the fibres chemically bond to the matrix, Bowland says, 'The number of available functional groups on nanofibres for covalent bonding with the matrix was controlled by altering the nanofibres’ surface area. This, along with nanofibre geometric and chain alignment, was utilised to enhance the reinforcing effect at the fibre-matrix interface'.

They have studied PAN nanofibres at ultra-low concentrations of 0.01-0.05wt.% of the total composite mass, but they are pursuing experiments to identify the optimal PAN loading 
to achieve a balance of desirable properties.

Another strategy is to vary the PAN-dope concentration during electrospinning, which can tailor the fibre architecture.

Gupta notes, 'One potentially cost-effective route involves deriving PAN from lignocellulosic biomass, offering an alternative to conventional petroleum-based sources.' 

The research team is seeking out industrial partners to license the approach to mitigate costs and improve commercial competitiveness.