Effect of viscosity and electrical conductivity on the morphology and fiber diameter in melt electrospinning of polypropylene

Abstract

The feasibility of fabricating polypropylene (PP) nanofibers has been explored by using different additives, such as sodium oleate (SO), poly(ethylene glycol) (PEG) and poly(dimethyl siloxane) (PDMS), during melt electrospinning. PP of high melt flow index (1000) was used with PEG and PDMS for the reduction of the melt viscosity; and it was used with SO for improving the electrical conductivity during melt electrospinning. It was observed that all the additives used in this study helped to reduce the fiber diameter. The most promising additive, SO, was effective in reducing the fiber diameter to the nanometer scale due to the increase in the electrical conductivity. The fiber diameter was decreased by the addition of PEG and PDMS due to the decrease in the melt viscosity. The effect of die shape on the fiber cross-sectional shape was analyzed and an interesting finding is that the die shapes did not have an effect on the cross-sectional shape of the fibers. That is, irrespective of the die shapes (i.e. trilobal, tetralobal, multilobal and circular) used in this study, the cross-sectional shapes of melt electrospun fibers were circular. The distribution of the additives in the fiber was analyzed by energy-dispersive X-ray analysis and was found to be uniform. Tensile tests were performed on single nanofibers with limited success, due to the problems in preparing fiber samples and successfully holding them in the jaws of the testing machine without slippage.

Keywords

melt electrospinning nanofiber polypropylene viscosity electrical conductivity tensile property

In the last two decades, increasing attention is being paid to the fabrication of nanomaterials, including nanofibers in the fiber industry. Nanofibers, fibers with diameters in nanometers, have a high specific surface-area-to-volume ratio, leading to their unique applications in protective clothing, filtration, biomaterials, electronics and other engineering areas.^1–6 Electrospinning is mainly employed for the fabrication of nanofibers. It involves the stretching of a polymer fluid by the electrostatic attraction in the presence of an external electric field. It can be classified into two groups: solution electrospinning and melt electrospinning.⁷

Melt electrospinning has many advantages over solution electrospinning. It is cheaper, more eco-friendly and safer than solution electrospinning.^8,9 The productivity of melt electrospinning is comparatively higher, since no solvent evaporation is involved, which lead to mass loss of the starting materials.^10,11 Polymers without appropriate solvents at room temperature, such as polyethylene (PE) and polypropylene (PP), can be easily melt electrospun. Melt electrospinning also favors the production of multi-component systems, such as blends and composites, as in many cases no common solvent for all the components may exist.¹² There is limited work reported on melt electrospinning because some limiting constraints are associated with the process, such as (i) coarser fibers are produced due to the high viscosity of the polymer melt compared to polymer solution;¹³ (ii) the equipment used for melt electrospinning is complex;¹³ (iii) the polymer melt is associated with the problem of electric discharge;¹⁴ and (iv) intrinsic difficulties associated with melt electrospinning involving high temperature setup and low conductivity of polymer melt.¹²

Majority of the previous research on melt electrospinning have reported the fabrication of microfibers by using PP of high molecular weights (i.e. low melt flow index (MFI)).^15–18 In this study, an attempt was made to fabricate nanofibers of low molecular weight PP by lowering the melt viscosity and increasing the electrical conductivity (by using different additives). The addition of rheology modifiers to lower the viscosity of the medium in which they are used is well known in melt processing. On the basis of the information provided in the literature, it was decided to use the additives poly(ethylene glycol) (PEG)¹⁹ and poly(dimethyl siloxane) (PDMS)²⁰ to lower the melt viscosity.

The addition of ionic salts in solution electrospinning to improve the electrical conductivity has been investigated.²¹ However, the effect of improving the electrical conductivity of polymer melt has not been studied in melt electrospinning. Therefore, sodium oleate (SO) was used in this study to improve the electrical conductivity.²² The effect of different additives with varying percentages on the fiber diameter has been investigated. In addition, the effects of melt electrospinning die shapes, including trilobal, tetralobal, multilobal and circular, on the cross-sectional shape of resultant fibers was investigated. Furthermore, the mechanical characterization of single nanofibers was performed with limited success.

Experimental details

Materials

PP of 1000 MFI (at 230°C, 21.6 N) was used for melt electrospinning. The 1000 MFI PP was synthesized by the chain scission of the base polymer (Moplen 241R from Lyondellbasell with a MFI of 30) using the radical initiators in an extruder.²³ Before melt electrospinning, the PP pellets were converted to fine powder by cryogenic (liquid nitrogen) grinding²⁴ in a grinder (Glenmills, Model 500). The additives or rheology modifiers used for the current study include SO (British Drug House), PEG (PEG-300, Ega Chemie) and PDMS (United Chemical Technologies Inc.). The chemical structures of the additives are shown in Scheme 1. The additives were mixed thoroughly with the PP powder on the basis of percentage weight (i.e. 4, 7 and 12 wt%) prior to electrospinning, as per the information provided in the literature.^20–22
Scheme 1.
Chemical structure of the additives (a) sodium oleate, (b) poly(ethylene glycol) and (c) poly(dimethyl siloxane).

Melt electrospinning equipment

The basic equipment used for melt electrospinning is shown in Figure 1. It consisted of five major components, namely the temperature controller, high-voltage power supply, heating assembly, syringe pump and collector. The temperature controller (BTC 909C) was able to control the temperature with an accuracy of ± 2°C. The temperature was measured using a K-type thermocouple. Negative voltage was applied to the collector using a high-voltage power supply (Spellman SL 150) with a voltage range of 0–50 kV.
Figure 1.
Schematic of the equipment used for melt electrospinning.

The heating assembly consisted of a metallic barrel of 10 mm inner diameter and 125 mm length. Heating was carried out by electrical heating elements housed inside the barrel. The equipment consisted of a digitally controlled syringe pump (PHD 2000, Harvard Apparatus). The polymer melt was pumped with a flow rate of 0.0013 ml/min through the die. Although different dies (such as 0.3 mm circular (C), 0.2 mm trilobal (T), 0.3 mm circular protruding (CP) and 0.2 mm CP) were successfully used for fiber production in preliminary experiments, the 0.2 mm CP die was mainly employed for the final investigations. The collector was a flat aluminum plate (150 mm (H) × 80 mm (W)) overlaid with aluminum foil on which fibers were collected. The collector was placed at varied distances of 120, 140 and 150 mm. The barrel was preheated up to 200°C. The piston was removed and about 4 g of the powdered polymer (mixed with the additives) was loaded into the barrel and kept for about 10 min so that a homogenous melt was prepared and steady state was achieved. The piston was reloaded and the delivery of the polymer melt was initiated.

Characterization of fibers

The surface morphology of the electrospun fibers was analyzed using a Field Emission Scanning Electron Microscope (Philips XL30 FE-SEM) with an accelerating voltage of 30 kV. The fibers were placed on stubs and coated with iridium by using a high-resolution ion beam sputtering system. Fiber diameters were measured by an image processing software (Image J, NIST) using the SEM images. The average fiber diameter was determined from at least 150 measurements from at least 10 SEM images.

The shear viscosity was measured using a rheometer (Ares) equipped with parallel plates in the dynamic mode. The shear viscosity of pure polymer and polymer with the additives were measured over a wide range of shear rates (0.1–100 s⁻¹). Disc samples of 1 mm thickness were prepared by using the heat press for the tests. Frequency scans were performed with 20% strain at a temperature of 200°C. The electrical conductivity of pure polymer and polymer with SO was measured at 200°C by an electrometer (Keithley 2612). The configuration used for the measurement of the electrical conductivity is shown in Figure 2. A circular polymer disc, prepared by the heat press, was used. The polymer disc was melted by band heaters to the specific temperature. Two electrodes (separated by 5 mm) were dipped in the melt and connected to the electrometer. The electric current flowing between the electrodes was measured by applying a variable voltage (0–50 V).
Figure 2.
Configuration used for the measurement of electrical conductivity.

Energy-dispersive X-ray (EDX) microanalysis was used to investigate the presence of the additives by mapping the relative distribution of the elements, for example sodium (Na) for SO and Silicon (Si) for PDMS. The Philips XL30 FE-SEM used for analyzing the surface morphology was also used for the EDX analysis. An accelerating voltage of 30 kV was used for all the specimens. To determine the SO and PDMS concentration on the surface of the fiber, X-ray net counts were obtained with a collection time of 500 s. All the fiber samples were carbon coated before the EDX analysis.

The mechanical property of single nanofibers fabricated by melt electrospinning was tested in the Chatillon tensile tester (Chatillon TCD 200) with a load cell of 30 g. The gauge length was 10 mm and the traverse speed was 2.5 mm/min. A single nanofiber was carefully removed with the help of tweezers under the stage of a microscope from the aluminum foil and mounted onto a square paper using double-face tape and polyvinyl acetate (PVA) wood glue/Aquadhere, as described in Figure 3. During the tensile testing, the paper was cut on both sides after it was mounted into the jaws and only the fiber was held by the jaws.
Figure 3.
Preparation of single nanofiber for tensile testing in the Chatillon tensile tester. PVA: polyvinyl acetate.

Results and discussion

Effect of die size and shape

Initially, the experiments were performed with different dies (at 48 kV, 200°C and a feed rate of 0.0013 ml/min), as listed in Table 1, with pure PP. The cross-sectional area of the dies was determined by using a microscope (Olympus). As a smaller nozzle diameter¹² and low molecular weight of the polymer (i.e. high MFI)¹⁷ favor the formation of fibers with smaller diameter, preliminary experiments were performed with the smallest die size (0.05 mm) before moving to the larger sizes. It was found that the die sizes of 0.2 mm or lower did not produce any fiber, because the high viscosity of the polymer melt prevented the flow through the small orifice.
Table 1.
Specification of the dies used in the melt electrospinning

Die size (mm) Die shape C/T/CP Cross-sectional area of die orifice (µm²) × 10⁴ Fiber formation

0.05 C ^a No

0.1 C 0.64 No

0.15 C 1.62 No

0.2 C 2.84 No

0.3 C 6.64 Yes

0.2 T 34.03 Yes

0.3 CP 6.17 Yes

0.2 CP 2.55 Yes

C: circular, T: trilobal and CP: circular protruding.

a
Area could not be measured.

Figure 4 shows the SEM images and fiber diameter distribution plots of the electrospun PP fibers using different dies. It can be observed from the diameter distribution plots that the 0.2 T die with the highest cross-sectional area produced the coarsest fiber, whereas the 0.2 CP die with the lowest cross-sectional area produced the finest fiber. The presence of nanofibers can be observed only in the case of the CP dies (i.e. 0.3 and 0.2 CP in Figure 4 (d) & (h)). The circular die (0.3 C) with almost the same area compared to the CP die (0.3 CP) produced only microfibers. The nanofiber fabrication in the case of CP dies could be due to the fact that the applied electric field was concentrated at the end of protrusion, whereas in circular dies (without protrusion) it is distributed in a wider area (Figure 5).
Figure 4.
Scanning electron micrographs showing the effect of die size on the diameter of melt electrospun PP fibers fabricated by: (a) 0.2 trilobal (T) die; (b) 0.3 circular protruding (CP) die; (e) 0.3 circular (C) die; and (f) 0.2 CP die; diameter distribution plots of fibers fabricated by: (c) 0.2 T die; (d) 0.3 CP die; (g) 0.3 C die; and (h) 0.2 CP die.

Figure 5.
Electric field distribution patterns: (a) circular (C) die and (b) circular protruding (CP) die.

The effect of die shape on the fiber cross-section using dies other than the circular, such as trilobal, tetralobal and multilobal (not listed in Table 1), was also analyzed. To the best of our knowledge, for the first time it was observed that there is no effect of the die shape on the fiber cross-section. The cross-sections of fibers fabricated by different dies are shown in Figure 6. It was noticed that all the dies (i.e. trilobal, tetralobal and multilobal) produced circular fibers similar to the circular dies. Normally in melt spinning, the cross-section of the fiber follows the die shape used (i.e. a trilobal die produces trilobal fibers). The shape of fibers not following the die cross-section is because of the presence of a Taylor cone, which is essential for electrospinning. When the high voltage is applied, the polymer melt is distorted into a conical object commonly known as the Taylor cone.²⁵ As the electrospinning process starts from the Taylor cone formed at the apex of the die, the die cross-section plays no role in shaping the fiber cross-section.
Figure 6.
Effect of die shape on the cross-section of the melt electrospun polypropylene fibers fabricated by (48 kV, 200°C and feed rate of 0.0013 ml/min): (a) circular die, (b) trilobal die, (c) tetralobal die and (d) multilobal die.

Effect of additives on fiber morphology and diameter

Experiments to investigate the effect of additives on the fiber diameter were performed by using the 0.2 CP die, as it produced the finest fibers out of all the dies investigated. The additives (SO, PEG and PDMS) were used at different weight percentages (i.e. wt% of 4, 7 and 12). The surface morphology and diameter distribution plots of the melt electrospun PP fibers fabricated with different additives (at 7 wt%, 48 kV, 200°C and a feed rate of 0.0013 ml/min) are shown in Figure 7.
Figure 7.
Scanning electron micrographs of melt electrospun polypropylene (PP) fibers with different additives (at 7 wt%, 48 kV, 200°C and a feed rate of 0.0013 ml/min): (a) pure PP; (b) PP with 7% sodium oleate (SO); (e) PP with 7% poly(ethylene glycol) (PEG); (f) PP with 7% poly(dimethyl siloxane) (PDMS); diameter distribution plots of the fibers: (c) pure PP; (d) PP with 7% SO; (g) PP with 7% PEG; (h) PP with 7% PDMS.

It can be observed that the fibers fabricated by melt electrospinning of pure PP and PP with different additives are circular. Melt electrospinning of pure PP resulted in fibers of micrometer diameter. The inability to fabricate nanofibers in melt electrospinning can be attributed to the high viscosity of polymer melts, which is normally one order of magnitude higher than the viscosity of polymer solutions.¹² Therefore, the shear viscosity of the polymer melt was analyzed; the shear viscosity curves of PP with additives (7 wt% at 200°C) are shown in Figure 8. It was found that the shear viscosity of PP decreased in the presence of PEG and PDMS, while it increased with SO. Similar behavior was observed with adding the additives at 4 and 12 wt%. The decrease in the viscosity with the addition of PEG and PDMS resulted in the fabrication of finer fibers with varying diameter (also refer to Figure 11). Although all the additives helped to reduce the fiber diameter, uniform nanofibers were fabricated only with the addition of SO.
Figure 8.
Shear viscosity of polypropylene (PP) with different additives (7 wt%, 200°C). SO: sodium oleate; PEG: poly(ethylene glycol); PDMS: poly(dimethyl siloxane).

The results obtained from the rheological investigation showed that the viscosity of the polymer melt with the additive SO was increased. Hence, the other possibility for the fabrication of nanofibers is the increase in the electrical conductivity by the addition of SO. Therefore, the electrical conductivity of the polymer with SO was measured by an electrometer (Keithley 2612). Figure 9 shows the change in the electrical conductivity of PP with the addition of SO at 200°C.
Figure 9.
Change in the electrical conductivity with the addition of sodium oleate at 200°C. PP: polypropylene.

It can be observed from the figure that pure PP polymer showed electrical conductivity at the level of 10⁻¹² S/cm at 200°C. It can be observed that, as the amount of additives increased from 4 to 12%, the electrical conductivity increased. The electrical conductivity increased from the level of 10⁻⁸ to 10⁻⁶ S/cm when the amount of SO increased from 4 to 12 wt%. Electrical conductivity in the polymer material is governed by the generation and mobility of the charge carriers or ions. The increase in the electrical conductivity with the addition of SO can be attributed to the effect of the ions from the dissociation of SO. During melt electrospinning, the ions increased the charge-carrying capacity of the polymer melt-jet. The melt-jet underwent whipping instability during its travel to the collector. The presence of more charges when SO was added increased the charge repulsion in the jet. Therefore, the whipping instability of the jet was increased and the jet was subjected to stronger stretching forces, resulting in the formation of finer fibers.²¹ In addition, the jet was repeatedly split into smaller jets by the increased charge density, resulting in smaller fiber diameter.²

As the addition of SO resulted in uniform nanofibers, an attempt was made to find out the optimum amount of SO for the fabrication of the lowest fiber diameter. The average diameters of the melt electrospun fibers with different amounts of SO were measured, the results of which are graphically shown in Figure 10. The figure shows that the smallest fiber diameter was achieved with 7 wt% SO. The average fiber diameter decreased up to 7 wt% of SO and then increased. When the amount of additives increased beyond this value, the fiber diameter started increasing. The increase in the conductivity after a certain level increased the instability of the melt electrospinning process and the formation of a larger Taylor cone. This led to the increase in the fiber diameter after a certain percentage of SO.
Figure 10.
Optimum concentration of sodium oleate (melt electrospinning was performed at 48 kV, 200°C and a feed rate of 0.0013 ml/min; the absence of bars in the graphs indicates no fibers being fabricated). PP: polypropylene.

The effects of different levels of additives on fiber diameter are shown graphically in Figure 11. It can be observed that in the case of pure PP and PP with PEG and PDMS, the average fiber diameter was in the micrometer range. Nanofibers were fabricated only in the case of SO. The effect of increasing the amount of additives from 4 to 12 wt% on fiber diameter shows a decrease followed by an increase trend in the case of SO. The fiber diameter increased when the PEG concentration was increased from 4 to 7 wt% and then decreased at 12 wt%. In case of PDMS, an increasing trend was observed when the PDMS concentration was increased from 4 to 12 wt%.
Figure 11.
Effect of the amount of additives (sodium oleate (SO), poly(ethylene glycol) (PEG) and poly(dimethyl siloxane) (PDMS)) on the fiber diameter (melt electrospinning was performed at 48 kV, 200°C and a feed rate of 0.0013 ml/min): (a) 4%; (b) 7%; (c) 12% (the absence of bars in the graphs indicates no fibers being fabricated). PP: polypropylene.

It can also be observed from the figures that as the collector distance increased, the fiber diameter decreased. At a collector distance of 150 mm, in the case of pure PP and PP with PDMS, no fiber was fabricated. This can be attributed to the insufficient electrostatic drag at very high collector distance.

Elemental composition analysis

The presence and distribution of SO and PDMS in the electrospun PP fiber were analyzed by EDX. EDX analysis cannot be used to uniquely identify the elements of PEG, as it is a long-chain polymer of carbon (C), hydrogen (H) and oxygen (O). In the EDX spectra (Figure 12), the elements observed in the electrospun fibers with SO were C, O, aluminum (Al) and Na. The presence of Na indicates the presence of SO in the fiber. The presence of Al can be attributed to the underlying aluminum foil used to collect the fibers, which was present during the EDX analysis. Similarly, the EDX spectra of electrospun PP fibers with PDMS (Figure 13) showed the presence of Si, which was due to the PDMS. Figures 12 and 13 also indicate the distribution of the additives in the electrospun PP fiber. The net counts of Na or Si indicate the amount of the respective additives (i.e. SO and PDMS) in the fiber. It can be observed that as the percentage of SO or PDMS increased, the amount of Na or Si increased, respectively. The EDX mappings also showed the uniform distribution of the additives in the electrospun fiber.
Figure 12.
Energy-dispersive X-ray (EDX) spectra of electrospun polypropylene (PP) fibers (48 kV, 200°C and a feed rate of 0.0013 ml/min) with (a) 0%, (b) 4%, (c) 7% and (d) 12% of sodium oleate (SO). EDX mapping of electrospun PP fibers with (e) 0%, (f) 4%, (g) 7% and (h) 12% of SO.

Figure 13.
Energy-dispersive X-ray (EDX) spectra of electrospun polypropylene (PP) fibers (48 kV, 200°C and a feed rate of 0.0013 ml/min) with (a) 0%, (b) 4%, (c) 7% and (d) 12% of poly(dimethyl siloxane) (PDMS). EDX mapping of electrospun PP fibers with (e) 0%, (f) 4%, (g) 7% and (h) 12% of PDMS.

Mechanical properties

The mechanical properties of the single nanofibers were evaluated using the Chatillon tensile tester. Ten specimens were tested for the nanofibers fabricated from 1000 MFI PP with the additive SO (7%). Out of the 10 specimens, only 2 were able to break properly at the center (results are given in bold in Table 2); 2 samples were broken during specimen mounting and the remaining 6 samples slipped at the jaws instead of breaking.
Table 2.
Tensile properties of single nanofibers fabricated by melt electrospinning

Sample no. Elongation at break (%) Breaking load (cN)

1 1.19 0.029

2 1.4 0.148

3 3.03 1.369

4 ^a ^a

5 3.52 1.945

6 0.9 0.034

7 1.41 0.08

8 1.69 0.085

9 1.51 0.083

10 ^a ^a

a
Broken during specimen loading.

Figure 14 shows the load-elongation curve of a single nanofiber. The general shape of the load-elongation curve is similar to most of the textile fibers. Comparison of the breaking load of the fibers fabricated in this study with other results is not possible, as the fiber diameters are different and so are the test methods. For a general idea, the breaking strength of a commercial PP fiber is 16.31 cN,²⁶ whereas in our study it is about 1.6 cN. The preparation of a single nanofiber for tensile testing was a tedious process. In addition, trying with different adhesives to prevent the jaw slippage was not successful.
Figure 14.
Load-elongation curve of a single nanofiber fabricated by melt electrospinning.

Conclusions

Melt electrospinning of high MFI (1000 MFI) PP with different additives (SO, PEG and PDMS) has been successfully achieved. The addition of additives helped in lowering the fiber diameter. The fiber diameter was decreased by the addition of PEG and PDMS, due to the decrease in the melt viscosity. The most promising additive, SO, was effective in reducing the fiber diameter to the nanometer scale due to the increase in the electrical conductivity. Nanofibers with average diameter of 880 nm were obtained by adding SO. In addition, it was observed that there is no significant effect of die shape on the cross-sectional shape of the melt electrospun fibers. All the die shapes (i.e. trilobal, tetralobal, multilobal and circular) produced fibers with circular cross-sections. EDX microanalysis showed the presence of the additives in the fiber structure. EDX microanalysis also provided evidence of the uniform distribution of the additives in the fiber structure. Tensile tests were performed on single nanofibers with limited success, due to the problems in preparing fiber samples and successfully holding them in the jaws of the testing machine without slippage.

Die size (mm)	Die shape C/T/CP	Cross-sectional area of die orifice (µm²) × 10⁴	Fiber formation
0.05	C	^a	No
0.1	C	0.64	No
0.15	C	1.62	No
0.2	C	2.84	No
0.3	C	6.64	Yes
0.2	T	34.03	Yes
0.3	CP	6.17	Yes
0.2	CP	2.55	Yes

Sample no.	Elongation at break (%)	Breaking load (cN)
1	1.19	0.029
2	1.4	0.148
3	3.03	1.369
4	^a	^a
5	3.52	1.945
6	0.9	0.034
7	1.41	0.08
8	1.69	0.085
9	1.51	0.083
10	^a	^a

Footnotes

Acknowledgments

The technical support from Muthu Pannirselvam and Mike Allan (Rheology and Materials Processing Centre, School of Civil, Environmental & Chemical Engineering, RMIT University); Phil Francis, Peter Rummel, Matthew Field and Frank Antolasic (School of Applied Sciences, RMIT University); Geraldine Van Lint (School of fashion & Textiles, RMIT University); Gary Peeters and Lance Nichols (MSE, CSIRO, Clayton); Mark Greaves, John Ward and (Scanning Electron Microscopy, Digital Imaging & Surface Analysis Facility of MSE, CSIRO, Clayton) Wendy Tian (MSE, CSIRO, Clayton); Liz Goodall and Winston Liew (Materials Characterisation Services of MSE, CSIRO, Clayton); Youssof Shekibi (ET, CSIRO, Clayton); and David Sutton and Peter Kouwenoord (Lyondellbasell) is gratefully acknowledged.

Funding

The authors gratefully acknowledge the financial support provided by RMIT University Higher Degree by Research Publications Award (HDRPA-2012).

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