Polyvinylidene fluoride PVDF is a thermoplastic material used commonly as membrane substrate in filtration and as separator in batteries. PVDF by nature is characterized by high electrical resistance property. That’s why, PVDF is not well explored as piezoelectric material for electronic applications such as nonlinear optics, microwave transducers despite its robustness and weather-proof and high temperature stability.
The team of experts Ali Sarvi, Uttandaraman Sundararaj, Vinicius Chimello, Aline Bruna da Silva, and Rosario Elida Suman Bretas from Canada — University of Calgary and Brasil – Federal University of São Carlos – used advanced electrospinning to transform PVDF from insulative material into conductive one and made it electronic-friendly material. The team touched up PVDF with conductive fillers (multiwalled carbon nanotubes, MWCNT) in a novel way using electrospinning approach to form semiconductive PVDF mats that showed promise to be highly piezoelectric.
In regular electrospinning process, a polymeric solution is spun from a high-voltage single nozzle spinneret, and the nanofibers are deposited on a grounded collector where they form a nonwoven mat.
The team adopted advanced electrospinning, called coaxial electrospinning, because of its potential in transforming conventional materials into extraordinary materials, imparting unusual feature into a regular material. In coaxial electrospinning process, coaxial nozzle spinneret is used, which allows two different polymeric solutions to spin as two-concentric layered fiber at high voltage which are collected as composite bi-layered nanofibrous mat.
Bi-layered nanofibrous mat
Utilizing this concept, the team took PVDF as core (inner) solution, but for the shell (outer) layer, they mixed PVDF with the conductive materials, namely MWCNT and polyaniline (PANi) at optimal ratio, 82wt% PVDF : 8wt% PANi : 10wt% MWCNTs. Upon electrospinning, the team was able to produce core-shell nanofibers; PVDF in the core and the composite PVDF-PANi-MWCNTs in the shell. Electrospinning process firstly achieved the high orientation of PVDF molecules as nanofibers which led to the formation of crystalline β phase of PVDF dominantly, which imparted piezoelectricity characterisitc. Electrospinning process was proved better in enhancing β phase in PVDF than the compression-molded process for PVDF. The resultant nanofiber PVDF mats showed higher piezoelectricity than compression-molded PVDF. This was evidenced by the greater piezoelectric strain constant (d33~−57.6pm/V) for PVDF core-shell nanofibers compared with commercially available PVDF thin films (d33~−15pm/V).
The conductivity property was induced in PVDF material due to the result of conductive shell layer. The conductivity of the PVDF nanofiber mat enabled use of the electricity that is generated via the piezoelectric effect. It is necessary to make conductive connections between nanofibers besides increasing the conductivity of the nanofibers to obtain a conductive nanofiber mat. In coaxial electrospinning process, the percolation threshold was attained because the incorporation of PANi in the PVDF nanofibers promoted the formation of conductive bridges between nanofibers, which eliminated air resistance between the fibers. MWCNTs in the shell layer enhanced the probability that a network forms at lower concentrations and increased conductivity. The combined MWCNT effect and the PANi bridges resulted in a significant increase in conductivity of the nanofiber mat, achieved the conductivity value of 2.75 x 10-7 S/m.
Thus this study proposes the coaxial electrospinning a feasible and industry viable approach to create novel conductive materials from the cheaper cost robust materials that are lack of conductive/electronic features.
This work is published by Society of Plastics Engineers (SPE), which can be accessed here below; http://www.4spepro.org/view.php?article=005274-2014-04-18&category=Composites
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