Nanofibre fabrication in a temperature and humidity controlled environment for improved fibre consistency
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- Hardick, O., Stevens, B. & Bracewell, D.G. J Mater Sci (2011) 46: 3890. doi:10.1007/s10853-011-5310-5
Fabricating nanofibres with reproducible characteristics is an important demand in the membrane industry in order to establish commercial viability. In this study, the effect of controlled atmospheric conditions on electrospun cellulose acetate (CA) nanofibres was evaluated for temperatures ranging 17.5–32.5 °C and relative humidity ranging 20–70%. CA solution (0.2 g/mL) in a solvent mixture of acetone/dimethylformamide/ethanol (2:2:1) was electrospun into nonwoven fibre mesh with the fibre diameter ranging from 150 nm to 1 μm. The resulting nanofibres were analysed by differential scanning calorimetry, showing a correlation of reducing melt enthalpy with increasing atmospheric temperature. The opposite was seen with increasing atmospheric humidity, which conferred increasing melt enthalpy. Analysis of scanning electron microscopy images provided a correlation of reducing average fibre diameter with increasing atmospheric temperature and increasing fibre diameter with increasing atmospheric humidity. These results correlate with the melt enthalpy results, suggesting that finer CA nanofibres infer a lower melt enthalpy. Together these studies provide strong evidence that the controlled atmospheric conditions affect the fibre diameter of the resulting electrospun nanofibres. A salient observation in this study was that increased humidity reduced the effect of fibre beading yielding a more consistent and therefore better quality of fibre. This has apparent implications for the reproducibility of nanofibre production and offers a new method of controlling fibre morphology. This study has highlighted the requirement to control atmospheric conditions during the electrospinning process to fabricate reproducible fibre mats.
Scanning electron microscopy
Differential scanning calorimetry
Glass transition temperature
The first significant report of electrospinning to produce polymer fibres came in 1934 when a patent by Formhals  was issued which described electrospinning as a process for forming textile fibres. To date, applications of nanofibres have included textiles, medical materials, filtration devices, bioengineering materials and even energy cells [2, 3, 4, 5]. In the medical sector, nanofibres have been used to produce artificial organ components, implant material, tissue replacement and wound dressing and are the subject of much recent attention [6, 7, 8].
The application of nanofibres continues to grow through a multitude of industries where product reproducibility is expected or required by validation and regulation [12, 13, 14, 15]. Indeed all products should be sold with relevant documentation stating the product specification. This requires rigorous quality assurance testing of which reproducibility will be a key part. Therefore, to assure nanofibre use is accessible and applicable for many markets, an ability to control the production must be established. This study is particularly relevant to membrane operations which are commonly limited by poor membrane pore size uniformity and axial and radial diffusion which results in poor system dispersion, yielding low utilisation of membrane capacity [16, 17]. Previous studies have highlighted the importance of designing a membrane with regards to the proposed system and operating conditions, optimising the membrane pore size by balancing mass transfer against and fouling issues [18, 19, 20].
Polymer solution parameters which involve rheological and chemical properties of solutions.
Processing conditions which include applied voltage, flowrate and spinneret and collector properties.
Ambient parameters where atmospheric conditions interact with the system to affect fibre morphology.
Varying any of these parameters even by small amounts can have a large effect on the structure of fibres produced; this enables the formation of fibres with defined features such as fibre diameter, flat ribbon or cylindrical fibres, level of fibre surface porosity and bead formation. Depending on the intended application, these properties have the potential to be selected and specifically expressed. For example, thinner fibres may be preferred due to the larger surface area that they convey but often a small diameter presents a reduction in fibre strength.
In the current literature which covers nanofibre production, it has typically been the polymer solution parameters and processing conditions which have been investigated. However, to ensure a reproducible and optimised product, the effect of the electrospinning environment by controlling air temperature and humidity was investigated. The system chosen here is the electrospinning of cellulose acetate (CA) nonwoven nanofibre mats using three different temperatures and relative humidities (RHs) in a controlled environment cabinet to evaluate the effect on the resulting average nanofibre diameter by scanning electron microscopy (SEM) and the corresponding thermal properties of the nanofibres by differential scanning calorimetry (DSC). The polymer solution parameters and processing conditions remain fixed for the entire investigation.
A solution of CA (Mr = 29,000; 40% acetyl groups; 0.20 g/mL) in acetone/dimethylformamide/ethanol (2:2:1) was electrospun to obtain CA nanofibre nonwoven membranes. All materials were bought from Sigma–Aldrich (Sigma–Aldrich Company Ltd. Dorset, UK) and used without further purification.
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).
Results and discussion
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 (Tg) and melt temperature (Tm) enthalpy values [21, 22, 23, 24, 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.
The Tg 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.  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.
Fibre diameter and surface characteristics
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.  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.  in their study of parameters affecting the electrospinning of polyurethane fibres.
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 , 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.  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.  reported that using a solvent with a lower vapour pressure reduced the formation of pores on polylactic acid porous nanofibres. Megelski et al.  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.
Consistency and reproducibility
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 . 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 .
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.  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 . Koski et al.  observed that higher molecular weight polymer–solvent systems of poly(vinyl alcohol) in water produced ribbon-like fibres. Luo et al.  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.
Cellulose acetate nanofibres were electrospun under nine different ambient conditions over three separate days and the effects of process temperature and process humidity on the resulting fibre morphology were investigated. The SEM results indicated the expected correlation of decreasing average fibre diameter with increasing process temperature. A correlation was also observed whereby the fibre diameter increased with increasing humidity. The fibre diameters were also found to be consistent over the 3 days, suggesting a high reproducibility. In addition, the RH levels during the electrospinning process appeared to be the dominant factor in determining the level of bead formation. This is most likely due to the change in evaporation rate which determines the stability of the chain entanglements due to flight time. This notable observation has given new possibility for the controlling of beading nanofibre systems and has highlighted the importance of controlling atmospheric conditions during the electrospinning process.
The atmospheric conditions most suitable for nanofibre production in this study are 25.0 °C and 50% RH, which gives the highest level of fibre diameter uniformity, the lowest level of beading and maintains a low fibre diameter for increased surface area and increased pore size homogeneity. The DSC results support the SEM study and show the critical parameter to be atmospheric humidity because of the effect to homogeneity caused by fibre beading. The effect of the atmospheric humidity on the melt enthalpy was comparable with changes in the polymer solution parameters or other processing conditions. Therefore, the ambient parameters require an equal amount of consideration in the reproducible production of nanofibres by electrospinning. This study is particularly relevant to the membrane industry where beading would result in a poor quality membrane structure.
Support for O.H. as part of the IMRC for Bioprocessing in the Advanced Centre for Biochemical Engineering by the Engineering and Physical Sciences Research Council (EPSRC) under the Innovative Manufacturing Research initiative is gratefully acknowledged. Support from Dr. Rob McKean at the Micro and Nanotechnology Centre, Science and Technology Facilities Council was greatly appreciated. Also, the authors thank Stewart R Dods and Dr. Anke Lohmann for their assistance during manuscript preparation.