Chapter 6 Fabrication of graphene fibres
6.5. Preliminary electrospinning trial of PVA and graphene
a b
Figure 6-7 SEM images of deposit of electrospun PVA [2%]: graphene (1:1) at low magnification (a) and high magnification (b).
Observations
During the SEM analysis of the samples, it was observed that the electron beam appeared to damage the samples since they were not gold coated. It was decided to coat the samples in gold in the future to prevent damage of the samples. Following analysis of the SEM images obtained from these experiments, it was observed that the fibres produced using a ratio of 1:2 (PVA: GO) did not produce fibres. The reason for this was that the solution was not viscous enough [192, 195, 205]. The GO that was added had been suspended in distilled water therefore upon addition to the PVA solution, unintended dilution of the polymer solution occurred resulting in a less viscous solution. A solution having a viscosity that is too low results in discontinuous fibre production [169]. This observation suggested that a 1:1 or 2:1 ratio (PVA: GO or G) should be used. Furthermore, the absence of fibres could indicate that the PVA concentration of the solution itself was too low thereby resulting in a solution of very low viscosity.
When the graphene suspended in distilled water (from the previous experiment) was added to the 4 % and 6 % PVA aqueous solutions, the solution became a gel almost immediately after the introduction of the graphene to the solution. The gel-like solution was heated to no avail as this only served to turn the gel into a slime. The cause of this gelation was at first unclear;
however, after further testing the cause was identified as the presence of residual NaBH4 in the graphene sample that was added to the PVA solution. This conclusion was reached since when the NaBH4 was added in isolation to the PVA solution, the liquid congealed after rapid bubbling was observed. This bubbling was similar to that observed during the reduction reactions involving GO and NaBH4. This compound reacts with water and since the solvent in the PVA solution was in fact distilled water, the NaBH4 reacted unfavourably with it thus resulting in the gel formation.
Electrospinning of PVA and graphene – Horizontal configuration
The electrospinning apparatus for the electrospinning of the PVA and graphene solutions was assembled in the horizontal configuration. The experiments conducted in this section are summarised in the table below which shows the parameters used for each experiment.
Table 6-4 Parameters used for the preliminary electrospinning trial of PVA and graphene.
Experiment
no. Material Needle
(G)
Flow rate (µl/min)
Syringe size (ml)
Voltage (kV)
Tip- to- collector
distance (cm)
Time (mins)
I PVA [4 %] : G (2:1) 18 30 5 22 13 3
II PVA [4 %] : G (1:1) 18 30 5 22 13 3
III PVA [6 %] : G (2:1) 18 30 5 22 13 3
IV PVA [6 %] : G (1:1) 18 30 5 22 13 3
V PVA [10 %] : G (2:1) 18 30 5 22 13 3
VI PVA [10 %] : G (1:1) 18 30 5 22 13 3
Experiments I – V
The SEM images of the samples produced from each of these five experiments revealed that no fibres had been produced. The results from Experiment I and III were largely similar with the Experiment III revealing more fibre-like deposits however actual fibres were still absent. All the other experiments displayed similar findings of fairly consistent deposits yet no actual fibres.
Experiment VI
All the previous experiments from this particular set of experiments failed to produce fibres except for the last experiment involving a solution of 10 % (wt. /vol.) PVA: G (1:1). The SEM images of this sample revealed chain-like structures of fibres. There was beading present on these fibres, however, the primary feature of interest was the presence of the fibrous material.
a b
Figure 6-8 SEM images of fibres produced from the electrospinning of PVA [10 %]: G (1:1) at low magnification (a) and higher magnification of the same area (b).
Observations
The concentration of the graphene present in the solutions was determined on a volumetric basis; i.e.: 1 or 2 ml of graphene suspended in distilled water was added to 2 ml of each of the PVA solutions to obtain the desired concentration. This method was unreliable as it introduced unwanted distilled water to the solution. An improvement upon this method was to determine the concentration of the graphene present in the solutions by mass instead. The graphene in the proceeding experiments was dried prior to addition to the PVA solution thereby avoiding dilution of the PVA solution.
In addition, when the samples were initially coated in gold to avoid sample damage, the deposits were not visible under the SEM therefore gold coating of the samples proved to be counterproductive. Thus, an electron beam of lower voltage was instead used when viewing samples from the following experiments to prevent damage to the sample.
The results from this set of experiments were largely inconclusive as fibres were only present in the sample from Experiment VI. This could be caused by the low viscosity of the solutions as mentioned above [206, 207]. The only conclusion which could be reached at this point was that PVA solutions of high concentrations, 10 % (wt. /vol.) and possibly higher, appear to produce fibres. Below this point, no fibre formation was observed, therefore this appeared to be close to the critical solution concentration. However, as mentioned above, this concentration was not the true reflection of the concentration of the solution since distilled water was introduced to the solution with the addition of the graphene which would have diluted the solution somewhat.
It was thus concluded that the logical approach was to optimise the electrospinning processing and solution properties for the PVA solutions in isolation prior to the introduction of the graphene to the solutions.