In this study, it is aimed to observe electrospinnability of bismuth oxide (Bi2O3) microparticles via polymer solution and effect of loading on electrical insulating property of electrospun composite structure. For that purpose, polyamide 6.6 polymer (PA6.6) with high dielectric constant was loaded with bismuth oxide microparticles and it was used for coating polyester spunbonds by electrospinning technique. Morphological properties, thermal behaviors and electrical resistance of coated spunbonds were investigated. Scanning electron microscopy (SEM) and electron dispersive X-ray spectroscopy (EDX) showed that Bi2O3 was succesfully adapted into fiber structure and nano-scale PA6.6/Bi2O3 composite fibers were obtained. Thermal behaviors of coated samples were developed by increasing Bi2O3 loading according to differential scanning calorimetry (DSC) and thermogravimetic analysis (TGA) results. Addition of Bi2O3 caused remarkable increase on electrical resistance of PA6.6 electrospun surfaces. Bi2O3 /PA6.6 nanocomposite fibers are promising and good candidates for thin electrically insulating polymer-based structures for micro- or nanodimensional devices.
Research Article
Polyamide 6.6/Bismuth (III) Oxide Electrospun Nanocomposites for Potential Applications in Electronics
https://doi.org/10.21203/rs.3.rs-173210/v2
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In composite materials, properties of various components are gathered in a unique construction. With respect to single-phase materials, well-design of a composite material provides the opportunity of tailoring different components with dissimilar physical properties for specific or technical applications. Among matrix elements, polymer matrix have been widely used due to ease of processing, higher productivity, inexpensive manufacturing cost, possibility to light-weight material production and processable with organic/inorganic fillers [1,2]. Especially for electrical applications, polymer matrix composites have been increasingly used in both distribution and transmission of various voltage ranges by considering electrical behavior of polymers. Because of their wide ranges of dielectric constants, they also offer many attractive advantages such as; high elastic surface, low surface energy, good thermal stability, good hydrophobic characteristics and compatibility with organic and inorganic fillers for electrical applications [3].
Bismuth oxide (Bi2O3) is a metal oxide semi-insulator with high dielectric constant and has excellent electrical properties such as wide band-gap and high dielectric permittivity [4,5]. It is generally used in various fields such as gas sen- sors, microwave entegrated circuits, optical devices, photovoltaic applications and solid electrolyte applications [6,7]. In manufacturing of functional materials with Bi2O3, electro-deposition, synthesis with microwave method and chemical vapor deposition method are generally used and various composite structures or semiconductive nanofibrillates can be successfully produced in form of nanopar- ticles, nanofilms or nanofibers [7-10]. There are only few studies available in production of Bi2O3-based nanocomposite insulators by electrospinning tech- nique. Demir et al. (2019) claimed that CNF-Bi2O3 nanocomposite material be- haved as anode material exhibiting excellent rate capability [11]. Mills et al. (2013) studied on developing detection sensitivity of polymeric ions by doping them with metallic nanoparticles [12]. Li et al. (2016) resulted that electrospun Bi2O3-decorated carbon nanofibers were next generation super capacitors [13].
Electrospinning is an alternative method to produce functional and thin composite structures. In this technique, solvent is an actor from which is utilized to electrospin polymer or its blends under electrostatic field and deposition of randomly aligned micro or nanofibers is observed on collector [14]. The mostly known advantage of this manufacturing method is the ability of processing with different types of polymers [15-27]. Many researchers focused on manufacturing of nanofibers due to unique and/or superior characteristics of nanostructures by means of electronic, magnetic, optical, mechanical, physical and chemical properties [28-34]. For changing electrical behaviors of materials, many methods have been studied such as; blending polymers with a conductive polymer or material, addition of co-solvent into polymer solution and using different solvents for dissolving polymer[35-39]. Electrical characteristics of electrospun structures have been studied by few researchers but most of these studies focus on improving electrical conductivity of end product [40-44]. On the other hand, a few current studies are available for producing electrically insulating material with different types of polymers and fillers. Song et al. (2018) studied on manufacturing of multilayer nanofibrillated cellulose hybrid films filled with various amounts of graphene and boron nitride and they reported that composite structure exhibited comprehensive performance with electrically insulating properties [45]. Guo et al. (2019) produced electrically insulating nanofibrillated cellulose films with aluminium oxide (Al2O3) and graphene fillers and they claimed that these films were super insulators in next generation electrical packing field [46]. Chen et al. (2020) fabricated electrically insulating polymer nanocomposites by electrospinning and they reported that films prepared by pressing webs of insulating polymer/boron nitride nanocomposite fibers showed superior electrical resistivity [47]. Lee et al. (2009) claimed that P3HT nano-fibrillated webs were promising intermeditary materials for guaranteed charge carriers between semiconductor and supporting insulating material [48]. Chou et al. (2013) reported that insulating performance of P3HT -based electrospun fibers was improved by blending with PSA (poly(stearylacrylates) because PSA acted as a sheath material for P3HT by the help of electrospinning technique [49]. Canesi et al. (2012) studied on electrospinning of n-type semiconductor P(NDI2OD-T2)(poly[N,N-bis(2- octyldodecyl)-naphthalene-1,4,5,8-bis(dicaroximide) -2,6-diyl]-alt-5,5-(2,2- bithiophene)) with PEO and reported that insulating polymer PEO was a good supporting material for manufacturing of long and uniform P(NDI2OD-T2) nano-fibrillated webs [50].
In the present study, it is examined whether electrospinning technique can be used for production of a thin composite insulator and how Bi2O3, a semi-insulator, affects electrical characteristics of electrospun surface. For that purpose, PA6.6 was doped with Bi2O3 micro particles and nanocomposite fibers were deposited on aluminium oil or PES spunbond in order to obtain adequate thickness for testing and transport easily without being damaged. The reason of using PA6.6 polymer as matrix is its insulating character and easily electrospinnability. Besides, components in binary solvent system evaporate fastly and PA6.6 polymer facilitates manufacturing of ultrafine electrospun fiber.
The objective of current study was to produce lightweight polymer-based insulator using electrospinning technique with dispersion of Bi2O3 in polymer fiber webs and examine the effect of Bi2O3 doping on electrical resistivity, morphological properties and thermal characteristics on PA6.6 electrospun surfaces.
2.1 Materials
Bismuth oxide (Bi2O3) powder (purity ≥ 99%) was purchased from Across Organics. PA6.6 (polyamide 6.6 with molar mass of 262.35 g mol-1) polymer and polyester (PES) spunbond (50 gm-2) was supplied from Sigma Aldrich and Mogul Nonwovens, respectively. Formic acid (FA) and dichloromethane (DCM) were acquired from Merck Chemicals. All reagents were used as received without fur- ther purification.
2.2 Method
Solution Preparation: PA6.6 pellets were dissolved in FA/DCM (3:1) binary solution [15, 51] under magnetic stirring for 5 h at 35 0C. Bi2O3 microparticles were added to 10 wt % PA6.6 polymer solutions within concentration ranges of 0-10 wt %, homogenized in ultrasonic bath for 6 h at 350C and then stirred vig- orously for 8 h at room temperature.
Fabrication of Samples: Bi2O3 doped polymer solutions were transferred to a 10 ml syringe with 11.99 mm inside diameter. The applied voltage and flow rate was ∼37-38 kV and 1 ml h-1, respectively. Distance between needle tip and target was 8 cm. The resulting randomly-oriented PA6.6/Bi2O3 fibers were de- posited on flat collector covered with foil or PES spunbond for 45 minutes.
Characterization: The average fiber diameter of as-electrospun PA6.6/Bi2O3 fibers and morphology of Bi2O3 powder and PA6.6/ Bi2O3 composite fibers were determined by Zeiss EVO/LS10 scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDX). On behalf of possible bond occurrence, the samples were examined by Agilent Technologies Cary 630 fourier transform infrared spectroscopy (FT-IR). Thermal decomposition behaviors of electrospun fibers were characterized by increasing heat flow from 30 0C up to 300 0C at a heating rate of 20 0C per minute via Shimadzu DSC-60 differential scanning calorimetry. Mass loss of samples were determined within a temperature range of 30 0C up to 800 0C at a heating rate of 10 0C per minute via Shimadzu TGA-50 thermogravimetric analysis device. Thickness of PA6.6/ Bi2O3 composite fibers coated PES spunbonds were measured by LYK 5318 digital fabric thickness measurement device and average thickness values of samples were calculated after three readings for each sample. Finally, DC conductivity measurements were performed at ambient temperature with standard temperature-controlled four point probe technique via Entek Electronics FPP 510 electrical resistance and conductivity measurement system. The average value was obtained after 50 readings for each sample by considering sample thickness. In four point probe technique, if sample thickness is less than probe spacing, the resistivity can be calculated with the following equation:
Herein; V is the potential across the probes; I is the current, t is the material thickness [52]
3.1 Scanning Electron Microscopy & Electron Dispersive X-Ray Spectroscopy
Table 1 shows presence of bismuth element in PA6.6-based composite fi- bers doped with different amount of Bi2O3. Results showed that amount of Bi2O3 in electrospun webs increased as increasing powder concentration in polymer solutions. Besides, Bi2O3 powder was adapted into nanofiber struc- tures by considering SEM analysis of PA6.6/ Bi2O3 nanocomposite fibers.
Table 1. Elemental constituents of samples (wt %)
| |
PA6.6/0 |
PA6.6/5 |
PA6.6/10 |
| C |
73.38 |
62.36 |
54.15 |
| N |
14.29 |
17.69 |
15.79 |
| O |
12.32 |
14.16 |
14.61 |
| Bi |
- |
5.79 |
15.44 |
SEM micrographs of Bi2O3 powder showed that Bi2O3 was layerless material having irregular spherical shapes with uncertain surface uniformity. Particle sizes of Bi2O3 powder changed from 650.4 nm to 1.724 µm and average particle size was found as 1.246 µm. According to SEM images of PA6.6-based webs, nanocomposite fibers were manufactured with/without microparticles. Average fiber diameter of neat PA6.6 nanofibers (PA6.6-0) was 68.25 ± 20 nm. PA6.6/ Bi2O3 nanocomposite fibers with average diameter of 81.5 ± 10 nm and 84.10 ± 12 nm were obtained for PA6.6-5 and PA6.6-10, respectively. Despite of gradual increase in Bi2O3 loading, there is no meaningful or direct relationship between fiber diameter and Bi2O3 weight percentage. Due to continuous and partially beaded nanofiber manufacturing, it was concluded that FA/DCM binary solvent mixture did not satisfactorily contribute to homogeneity of PA6.6 polymer solution containing Bi2O3 but studying on suitable concentration range with these solvents supported nanofiber occurrence [53,54].
Figure 1 shows the SEM images of Bi2O3 powder and PA6.6/ Bi2O3 nano-composite webs.
3.2 Fourier Transform Infrared Spectroscopy (FT-IR)
FT-IR spectra of electrospun webs were examined by considering EDX test results to detect possible bond formation in nanocomposite fibers. In Figure 2, bands at 3296.655 cm−1, 2919.519 cm−1 and 1634.330 cm−1 wavelengths represented stretching vibrations of N-H, CH2 and C=O bonds, respectively. Besides, N-H bending vibration was shown at wavelength of 1634.330 cm−1. Characteristic peaks of PA6.6 nanofibers were consistent with previous studies [55]. Peak intensity of PA6.6/Bi2O3 nanocomposite fibers increased without any shift in characteristic band ranges and these increases were results of an increase in Bi2O3 concentration.
3.3 Differential Scanning Calorimetry & Thermogravimetric Analysis
In Figure 3, DSC results showed that PA6.6/Bi2O3 electrospun webs had similar pattern with little shifts due to increase in Bi2O3 loading. Glass transition temperature (Tg) values of Bi2O3-free PA6.6 sample was consistent with webs electrospun with polymer concentration of 10 % (w) in previous studies and it was reported as 56.44 0C [56-58]. Bi2O3 loaded nanofibers showed higher Tg values with comparison of unloaded ones. DSC thermograms of Bi2O3-free electrospun fiber were relevant with literature but these peaks were shifted and observed at higher temperature values for electrospun fibers containing Bi2O3 micro particles. Electrospinning was performed at room temperature and further process was not applied to surfaces, such as drying in an oven for a while. Therefore, no crystallization would occur during electrospinning. Besides, randomly aligned electrospun fibers were manufactured and perfect alignment of nanofibers were not observed which was a factor triggering crystallization [58]. Tm and Tg values of samples are given in Table 2.
To investigate the effects of Bi2O3 on the thermal stability of nanocomposite web, TGA analysis was performed and weight loss due to thermal degradation was determined. For neat PA6.6 (PA6.6-0), small weight loss was observable above 200 0C due to evaluation of traces of moisture or unreacted monomers and decomposition was available above 350 0C due to decomposition of base polymer to leave residual carbon from polymer backbone [59]. For PA6.6/Bi2O3 nano-composites, onset decomposition temperature increased with corresponding to neat PA6.6 by 5 % and 10 % Bi2O3 loading. Weight loss of loaded samples were seen above 450 0C. This case explained that Bi2O3 doping developed thermal behaviors of samples. % weight loss of samples are given in Figure 4.
3.4 Thickness of Samples
By the help of a digital fabric thickness measurement device, the thickness measurements were taken from five different points of electrospun mats. PA6.6-0 and PA6.6-10 mats were measured as 0.066 mm and 0.108 mm, respectively. In terms of fiber diameter, fineness of PA6.6-based nanofibers was measured between the values of 68.25 nm and 84.10 nm. By taking average fiber diameter into consideration, there was a consistent relationship between Bi2O3 loading and fabric thickness as well as fiber diameter. Bi2O3 loading caused to increase in fiber dia and thereby web thickness of PA6.6-10 was found as higher than that of others. Relationship between thickness and aver-age fiber diameter of mats are illustrated in Figure 5. The average thickness of PES spunbond was also determined by taking five measurements from different points of sample and recorded as 0.192 mm.
3.5 Electrical Characteristics
The electrical resistivity of coated PES spunbonds are illustrated in Figure 6 as a function of fiber diameter. The resistivity of uncoated PES spunbond was measured as 170.535 kΩcm. Coating PES spunbond with undoped PA6.6 nano-fibers (PA6.6-0) increased electrical resistivity with three-and-a-half times and the value was measured as 592.287 kΩcm. Besides, 5 wt% Bi2O3 loading contributed electrical resistance and electrical resistivity of PA6.6-5 coated PES was 5 times higher than that of uncoated PES. The values were determined as 855.917 kΩcm to 949.398 kΩcm for PA6.6-5 and PA6.6-10 coated samples, respectively. Coated PES spunbonds showed more insulating property than uncoated ones. This can be correlated with tortuosity effect of nanofibers collected on PES spunbond and slow discharge of electrical energy in nanofiber layers. It is reported that increase in electrical resistivity was related with polymer type and nanofiber production utilized both Bi2O3 propagation and decisive characteristics’ exhibition in composite structures [60-62]. The resistivity of samples increased with the fiber diameter. Increase in Bi2O3 loading caused thicker fiber manufacturing and this lead to decrease in contact probability of electrospun fibers due tolarge fiber diameter. On the other hand, insulating character of polymer matrix triggered the decrease in resistivity. As a consequence, these two effects contribute to an increase in electrical resistivity as fiber diameter increases [63,64].
Figure 7 shows the electrical conductivity values of samples with different thickness. The thickness of uncoated PES was 0.192 mm whereas PA6.6-10 coated was 0.300 mm. Increase in fiber diameter lead to thicker coating and thereby PES spunbond coated with PA6.6-10 mat with average fiber diameter of 84.10 nm was found as one-and-half higher than uncoated one. Due to insulating character of both polymer matrix (PA6.6 polymer) and filler (Bi2O3) and less contact among nanofibers, a remarkable increase was observed in electrical resistivity. But negligible decrease was also available in electrical conductivity. Coating of PES spunbond with doped or undoped PA6.6 nanofibers caused to a slight decrease in electrical conductivity. The obtained values were 5,864 E-06 S/cm for uncoated PES, 1,688E-06 S/cm for PA6.6-0 and 1,168E-06 S/cm for PA6.6-5 coated PES. PA6.6-0 coating decreased conductivity at a ratio of 71.22 % whereas PA6.6-5 coating of 80.09 %. This case can be explained by absence of perfect contact among electrospun fibers, presence of less fill factor and lack of accurate thickness measurement for electrospun webs. The electrical conductivity of coated samples were slightly higher than that of each other due to high porosity. Besides, in four point probe method, conductivity measurement is based on totalvolume by disregarding spaces among fibers [64,65]. It is expected to obtain considerable decrease in electrical conductivity of an individual electrospun nanofiber.
Web morphology is depend on both deposition technique and materials/additives from which it is manufactured. Electrospinning technique is a simple method for forming film or web from polymers, polymer blends and addivites, controlling its structural and physical parameters and gaining improved characteristics to end product by size effect. Nanoor micro-fibers are available in electrospun webs and desired characteristic is adapted by selection of proper materials on whole structure due to fiber entanglement. In this study, we developed thin nanocomposite surfaces with electrically insulating properties by electrospinning technique. In particular, Bi2O3 microparticles were adapted to PA6.6 nanocomposite fibers and fiber fineness was decreased as increasing Bi2O3 content. FT-IR spectra showed that structural formations were preserved and negligible shifts were observed due to concentration changes of Bi2O3 loadings in PA6.6 nanocomposite fibers. According to DSC and TGA thermograms, thermal characteristics of samples developed and Bi2O3 loading decreased mass loss percentages for studied samples. Electrical conductivity of PA6.6/ Bi2O3 coated PES spunbonds was in similar ranges but electrical resistivity was developed. Despite of semi-insulating character of Bi2O3, studied webs showed more insulating property even in low doping rate than that of undoped PA6.6 coated or uncoated PES spunbonds. This result was correlated with size effect of electrospinning technique. PA6.6-5 nanocomposite structures are promising candidates for being used as insulating materials in miniaturized electronic devices with advantages of their lightness in weight and fineness in thickness. Further studies should be performed on different concentration values of various polymers by focusing on developing of bulkiness of electrospun webs for obtaining highest electrical resistivity and the least conductivity.
Acknowledgements
The authors would like to send special thanks to Prof. Dr. Suat Çetiner for his kind help and technical supports. This work was supported by Scientific Research Unit of Kahramanmaras Sutcu Imam University under grant number of BAP2015/1-48D.
Competing interests:
The authors declare no competing interests.
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