Nanofibre fabrication in a temperature and humidity controlled environment for improved fibre consistency

The process was carried out in a ClimateZone climate control cabinet (a1-safetech Luton, UK) which allows the process to be performed under controlled atmospheric conditions. The temperature and RH were selected and kept constant throughout each electrospinning event from a temperature range: 17–35 °C (resolution of 0.1 °C), and a humidity range: 20–80% RH (resolution of 1% RH). A 5-mL polymer solution was loaded into a sterile syringe and attach to a Harvard PHD 4400 syringe pump (Harvard Apparatus Ltd. Kent, UK), with a programmable flow rate range from 0.0001 up to 13.25 L/h, to deliver the polymer solution to a 0.5-mm ID stainless steel micro needle. The pump is set at a flowrate of 800 μL/h. The tip of the needle was placed 30 cm above the grounded collector plate. The collector plate used was a rectangular (20 × 26 cm) aluminium foil covered earth steel plate. The process was run for 1 h. These conditions were selected based on preliminary experiments and are known to yield solid dry nanofibres with diameters from 0.1 to 1 μm. Electrospinning samples for the defined parameters were repeated on three different days allowing for the comparison of fibre consistency.

Thermal properties of the fibres were evaluated by a Netzsch DSC 200 F3 Maia (NETZSCH-Gerätebau GmbH, Selb, Germany) at a rate of 10 °C/min, heating at 25–260 °C in a nitrogen atmosphere. Ten samples of nanofibres were measured consisting of one sample from each of the nine possible combinations of temperature and humidity conditions and one sample of CA powder. The melt enthalpy values were calculated by taking the integral of the melt temperature curve using the DSC software.

Complete drying of the fibres was allowed before characterisation by scanning electron microscopy. Nonwoven fibre samples were analysed from three SEM images each with 20 individual measurements of nanofibre diameters. The 60 measurement points per fibre sample were selected randomly and gave a good coverage of the SEM images. Data was collected using imaging software by selecting ‘x’ and ‘y’ coordinate points along the nanofibre edges using magnified images. This does provided opportunity for human error in conjunction with the error given by pixelated images at high magnification. This was repeated for three different cuttings from a single electrospun fibre mat fabricated under a single set of constant conditions to calculate the average nanofibre diameter and standard deviation. The total number of samples imaged by the SEM was 81 (27 samples with 3 cuttings each) which generated a data total of 1620 fibre diameter measurements. Fibres diameters measured above 1 μm were excluded from the average fibre diameter determination as they appeared to originate from bead formation. The SEM used was a Hitachi TM-1000 Tabletop microscope (Hitachi High-Technologies Europe Gmbh).

DSC was used to measure the energy required to heat each of the nine nanofibre samples from 25 to 260°C. Melt enthalpy values were determined by taking the integral of the melting temperature curve from the thermograms. Enthalpy data is particularly relevant to the study of nanofibres as it expresses information about the morphology of the fibres. Previous studies have described pre and post electrospinning treatments to alter the fibre’s structure conferring different glass transition temperature (T _g) and melt temperature (T _m) enthalpy values [21‐25]. Any changes in melt enthalpy values should be caused and interpreted by the changes in the degree of crystallinity and/or macromolecular orientation within the electrospun fibres.

Figure 3 shows that the melt enthalpy decreases with increasing temperature. At 20% RH, the melt enthalpy decreased from 8.96 to 4.47 J/g from a process temperature of 17.5–32.5 °C. This decrease of 50% is significant and suggests that with increasing temperature comes a less ordered molecular structure since the energy required to make the phase change is greatly reduced. In this study where polymer solution parameters and process conditions have been kept constant, the observed reduction in melt enthalpy values are likely to be caused by faster solvent evaporation at a higher temperature which leaves less time for fibre solidification. This means that polymer macromolecules have less time to arrange, resulting in a lower degree of crystallization and less molecular orientation. This effect is concurrent with a reduction in fibre diameter since faster solvent evaporation will result in fewer interconnected chain entanglements making the energy required to separate these polymer molecules less than that for a thicker fibre. Lee et al. [26] demonstrated a similar effect using DSC to show that nanofibres fabricated from higher molecular weight polyvinyl acetate solutions shifted the T _m from 224.7 to 232.7 °C due to the effectively increased crystalline structure. Figure 3 also shows a clear distinction between the melt enthalpies of nanofibres fabricated at different controlled RHs, with the largest value being 12.07 J/g for the nanofibres produced under the atmospheric conditions 17.5 °C and 70% RH. Using 17.5 °C as a constant variable, the melt enthalpy decreased from 12.07 to 8.96 J/g for process humidities of 70 and 20%, respectively.

The T _g was not obvious but the thermograms showed a small feature before the main melting peak at around 194 °C which is likely to correspond to bond breaking and irreversible plastic deformation occurrences at this point although this observation could have also been cause by the melting of less stable crystalline structure.

The powder form of CA was also investigated using DSC and yielded smaller peaks at 230 and 242 °C. The melt enthalpy was not measurable here suggesting that the nanofibre structures confer a higher degree of order than the powder form. Zong et al. [27] used DSC to show that polylactic acid polymer had a crystallinity degree of 36% whereas polylactic acid nanofibres exhibited a much lower value. XRD results supported that although the polymer chains were non-crystalline in nanofibre form, they are highly orientated. This suggests that nanofibres do confer a high degree of order of polymer chains though their crystallinity is not high.

The distribution of the data displayed in Fig. 4 is typical throughout. The data does not fit a Gaussian function; however, normal means were calculated to determine average fibre diameters which could then be plotted. The non-normal distribution observed could be an artefact from various factors such as the beading effect in this polymer system, the human aspect of collecting the data or the image resolution as described in the SEM section. The standard deviation error calculated is based on a normal distribution and is used is subsequent graph analyses.

The average fibre diameter for each sample was determined by taking an average of 60 measurements from nine SEM images for each of the controlled conditions. This was performed three times on three separate days. The change in average fibre diameter for the variable ranges was larger than the values reported by Mit-Uppatham et al. [28], which is not surprising because they employed a different polymer solvent system of polyamide-6 in formic acid. Figure 5 shows that the fibre diameter decreases with increasing temperature. Using the 20% RH as a constant variable the fibre diameter decreased from 360 to 284 nm for process temperatures 17.5 and 32.5 °C, respectively. The rate of change in fibre diameter caused by the temperature change was not independent of humidity as shown by the percentage increase in fibre diameter of 31.2 and 11.0% for humidity conditions 20 and 70% RH, respectively. The average of the entire data showed an increase in fibre diameter of 1.29% per 1 °C.

At an increased temperature the viscosity of the polymer–solvent solution is reduced. The lower viscosity allows the columbic forces to increase stretching, giving finer fibres. Mit-Uppatham et al. [28] demonstrated that an increase in temperature caused the decrease of solution viscosity, surface tension conductivity and the resulting polyamide-6 fibre diameter. Increasing temperature also increases evaporation rate, this in conjunction with greater solubility allows for more even stretching and the deposition of more uniform fibres. This was observed by Demir et al. [29] in their study of parameters affecting the electrospinning of polyurethane fibres.

Figure 6 indicates that the fibre diameter increases with increasing humidity. Using the 25 °C samples as a constant variable, the average fibre diameter increased from 300 to 352 nm for process humidities 20 and 70%, respectively. Here the average change in fibre diameter was 0.30% per 1% RH. As mentioned previously, the rate of change in fibre diameter caused by temperature change was not independent of process humidity. This is clearly visible in Fig. 6 and is due to the wider range of fibres observed at lower humidities caused by the beading effect.

An increase in relative atmospheric humidity results in a decrease in evaporation rate favouring finer fibre formation. The increased water in the atmosphere would also suggest a slowing of the solidification process resulting in a longer flight time and therefore finer fibre formation; however, the effects observed here suggest the opposite occurrences. This case is perhaps specific to CA which upon addition of water results in fast polymer precipitation; therefore, as the humidity increases, it can be speculated that the increased water absorption causes the jet to precipitate out of solvent more quickly thereby reducing the flight time and elongation of the polymer fibre, resulting in thicker fibre. Most likely the more dominant effect is the increase in charge dissipation at higher RH due to the additional water present in the atmosphere resulting in lower charge repulsion by the polymer during the whipping stage, favouring the formation of thicker fibres. If the relative humidity is too high the deposition of wet fibres can occur causing them to fuse together before drying. These occurrences, explained by Baumgarten [30], affect the fibre diameter but perhaps the more interesting effect from changes in humidity is that to the fibre surface roughness or porosity. The correlation observed could also be due to a less dense fibre being formed at a higher humidity. At sufficient atmospheric humidity, water condenses on the surface of the fibre during electrospinning. If a volatile solvent is being used, pores form when both the water and solvent evaporate. Pore size increases with increasing humidity until they coalesce to form large non-uniform pores. Casper et al. [31] showed that electrospinning polystyrene at a RH of 31–38% was enough to see the formation of fibre surface pores. They also showed that surface pores of size increased with RH, seeing average pore sizes of 85 nm at 31–38% RH and 135 nm at 66–72% RH. Bognitzki et al. [32] reported that using a solvent with a lower vapour pressure reduced the formation of pores on polylactic acid porous nanofibres. Megelski et al. [33] observed the same effect of reduced pore formation with decreasing vapour pressure using the example of polystyrene and varying ratio THF/DMF solvent mixtures. As the ratio of the less volatile DMF increases, therefore reducing the vapour pressure, surface roughness or microtexture decreased, at 100% DMF, smooth fibres were formed. Many of the lower RH samples contained a beaded fibre system and thus fibre diameters over 1 μm were excluded from the data. It also yielded a large error due to some partially formed beads represented as thicker sections of fibre.

Figure 7 shows that as the fibre diameter decreased so did the melt enthalpy, which was expressed by a broad low endothermic peak. The largest fibres (17.5 °C, 70% RH: 385 nm) yielded the largest melt enthalpy of 12.07 J/g compared with the 4.47 J/g yielded by the finest fibres (32.5 °C, 20% RH: 276 nm). Thus, a decreased crystallinity and a less ordered molecular structure occur with decreasing fibre diameter. This clearly suggests that controlled atmospheric conditions affect fibre diameter.

Reproducible fabrication of nanofibres is essential for their mass production for commercial purposes. The nine controlled condition parameters repeated on three separate days were assessed. Although there are bead formation issues that occur when electrospinning CA, the consistency of average fibre diameter from the samples electrospun on different days can be seen in Fig. 8. The greatest difference in average fibre diameter among samples electrospun under the same conditions but on different days was 27.3 nm which corresponded to a difference of 7.5%. The average difference, however, was 4.3% which is perfectly respectable considering the variety of fibre characteristics. The distribution of data over the three separate days was consistent, supporting that this polymer–solvent system has sufficient reproducibility.

Figure 9 shows an SEM image matrix, suggesting that the polymer–solvent system was in a state near to or actually producing a beaded fibre structure. Figure 10 summarises the extent of beading as determined by analysing 27 SEM images from the nine controlled conditions over the 3 days. The most uniform fibres, and the ones with the least amount of beading, were those electrospun at high humidity. High RH appeared to be the dominant process variable in the goal for homogeneity though atmospheric temperature did have an effect (Fig. 9).

The effect of increased humidity, and to a lesser extent temperature, on the formation of bead could be due to the reduced evaporation rate, which allows greater stability of the chain entanglements due to increased flight time. The reduced beading effect improves homogeneity of the fibres, which can be seen as an advantage in the effort of fabricating reproducible fibres. The change in beading effect with change in temperature could be caused by the reduced viscosity of the polymer solution allowing for a more even stretching due to the more dominant effect of the columbic forces [29]. However, this effect is not as noticeable due to the converse effect of the increased surface tension that occurs at increased temperatures. Surface tension can be a common cause of bead formation in electrospinning. Where there is a high concentration of free solvent molecules there is a tendency for them to congregate, adopting a spherical shape and giving rise to the bead formation. The use of low surface tension solvents such as ethanol encourages smooth fibre formation as does adding surfactants to the solution to reduce surface tension [34].

The increased beading effect displayed with decreasing humidity could also be due to a polymer–solvent solution droplet drying at the tip of the micro-needle. With reduced humidity, the evaporation of the volatile solvent mixture occurs at such a rate that the polymer begins to dry before it is spun out into fibre jets. This causes pulses of fibre jets leading to the formation of polymer beads along the fibres formed. Zong et al. [27] noted a similar effect due to using a polymer solution at a very high concentration. Another effect formed by a mechanism relating to fast solvent evaporation is the formation of ribbon-like or flattened fibres. Should the polymer–solvent system be at such a state where a thin polymer skin forms on the liquid jet surface as a result of hyper solvent volatility the liquid core can succumb to the atmospheric pressure allowing the fibres to collapse in on them at the same time as complete solvent evaporation resulting in flattened fibre formation [35]. Koski et al. [36] observed that higher molecular weight polymer–solvent systems of poly(vinyl alcohol) in water produced ribbon-like fibres. Luo et al. [37] demonstrated the importance of the solvent choice and developed a spinnability–solubility map to enable the systematic selection of solvents creating electrospinnable binary solvent systems for polymethylsilsesquioxane.