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Abstract

Nanocomposite polyacrylonitrile filaments containing titanium dioxide and silver nanoparticles were produced by wet-spinning method with the aim of developing multifunctional filaments showing antibacterial activity, photocatalytic activity, and electrical conductivity. The nanocomposite filaments were characterized regarding their morphology, composition, nanoparticle dispersion, tensile properties, crystallinity, conductivity, thermal properties, photocatalytic, and antibacterial activity. The nanoparticles were observed to be well dispersed. The composite filaments with 3 wt% silver nitrate showed improved crystallinity. The highest breaking tenacity of 8.72 cN/tex was observed for the filament with 1 wt% TiO₂ and 3 wt% AgNO₃. The conductivity of the nanocomposite filaments were on the order of 10⁻⁴ S/cm, which is in the semiconductive range. The nanocomposite filaments displayed both antibacterial and photocatalytic activity. This study showed the possibility of producing multifunctional filaments with the simultaneous addition of different types of nanoparticles into the filament structure.

Keywords

Antibacterial electrical conductivity filament multifunctional nanocomposite photocatalytic polyacrylonitrile silver nitrate titanium dioxide

Introduction

The use of nanotechnology is creating enormous opportunities in many different fields of science and technology including electronics, biomedical materials, human safety, environmental protection, and textiles, etc [1 –4]. Textile industry, being the first to have successfully implemented the advances in nanotechnology and demonstrated the applications of nanotechnology for consumer usage, is experiencing the benefits of nanotechnology in many products starting from nanocomposite fibers and nanofibers to polymeric nanocoatings, nanocomposite coatings, nanodyeing, and nanofinishing applications [1,5 –10].

Nanocomposite fibers are produced with the incorporation of nanoparticles into polymer melt/solution before spinning process. Addition of nanoparticles into fiber structure is an efficient method, which has the advantages of easy processing, durability, and cost-effectiveness [11 –15]. Nanocomposite fibers are reported to show many superior properties [16 –28]. Wide selection of nanoparticles (layered clays, nanotubes, nano-fibers, metal containing nanoparticles, carbon black, etc.) makes it possible to produce a range of functional nanocomposite fibers that can be successfully used in various applications [1,29]. It has been possible to produce fibers with functional properties such as flame retardancy [16], antibacterial properties [15], water repellency [17], soil-resistance [18], electrical conductivity [19,20], far-infrared radiant property [21], barrier properties [22], and so on. Structural properties are also improved with the addition of nanoparticles. Higher modulus [23,24], increased strength [23,24], improved high-temperature mechanical properties [25], enhanced thermal stability [23,26], enhanced dyeability [27,28] have been reported in literature by the addition of nanoparticles in the fiber/filament structure. Dispersion of nanoparticles and optimization of spinning conditions may be the main problems in nanocomposite filament production [30 –34].

Among different nanoparticles, titanium dioxide (TiO₂) and silver nanoparticles (AgNPs) are two of the nanoparticles which are frequently used in textile applications. TiO₂ nanoparticles are reported to improve UV protection [33], antistatic [35], self-cleaning [36 –38], antibacterial [39], hydrophilic [40], ultra-hydrophobic [41], wrinkle resistance [42], water adsorption, and dyeability [43,44] properties. In particular, anatase TiO₂ is commonly used for the elimination of microbiological and toxic contaminants in filtration applications and for self-cleaning surface applications [45]. On the other hand, AgNPs are reported to be strong and broad-spectrum antibacterial agents against diverse species [46]. Futhermore, they improve the electrical conductivity of textile materials [47,48].

In literature, there are some studies about the incorporation of only TiO₂ or only AgNPs to the filament structure which are performed with poly(ethylene phthalate) (PET), polyamide (PA), and polypropylene (PP) polymers. The focus of these studies is mainly spinnability and mechanical properties of the nanocomposite filaments. Zhui et al. investigated spinnability and mechanical properties of meltspun PA 6 filaments with modified TiO₂ (PA 6/modified TiO₂) and reported improved mechanical properties as compared to PA fibers and composite fibers with unmodifed TiO₂ [30]. Han and Yu prepared PET/nano-TiO₂ nanocomposites using in situ polymerization and melt-spinning, and reported improved ultraviolet (UV) protection factor (UPF) of PET/nano-TiO₂ fabrics with a slight decrease in mechanical properties [31]. Erdem et al. produced PP/TiO₂ nanocomposite fibers for UV protection. Filaments were melt-spun from master batches of PP and 0.3, 1, and 3% TiO₂ nanoparticles. While the mechanical properties were slightly affected by the addition of nanoparticles, the filaments exhibited excellent UV protection [32]. Shayestehfar et al. investigated the physical and morphological properties of PA/TiO₂ nanocomposite filaments. They observed improvement in tensile strength with addition of TiO₂ and a decrease in elongation values [49]. Mikołajczyk et al. produced nanocomposite polyacrylonitrile (PAN) filaments by adding AgNPs directly to the spinning solution using mechanical mixing and investigated the influence of the amount of AgNPs on the supramolecular structure, porosity, thermal properties, and tensile strength of nanocomposite filaments [34]. Yazdanshenas et al. investigated the physical, mechanical, and antimicrobial properties of PA filaments with different AgNP contents. Nanocomposite PA filaments with 0.5–1.0 wt% of AgNP content were reported to show improved physical and mechanical properties, as well as, significant antimicrobial activity [15]. Kizildag and Ucar produced PAN filaments with silver nitrate and applied chemical reduction process by immersing the composite filaments into aqueous solution of hydrazinium hydroxide in order to obtain PAN/AgNP nanocomposite filaments. Composite filaments were semiconductive and antibacterial with improved mechanical and thermal properties [48].

PAN is a conventional fiber-forming polymer. It is widely used in the production of textile products such as socks, sweaters, sportswear, rag dolls, toys, blankets, upholstery fabrics, carpets, and rugs. Some unique properties of PAN such as low density, high thermal stability, high strength make it also advantageous in some technical textile applications [50]. Additionally, it is utilized in the production of filter media for food, automotive, chemical, medical, pharmaceutical, mining, and refining industries [51,52]. TiO₂ and silver-embedded PAN filaments are expected to show multifunctions such as antibacterial properties, photocatalytic activity, and antistatic ability. PAN fibers/filaments with antibacterial and self-cleaning ability can be used in the production of apparel, upholstery fabrics, carpets and rugs, toys, protective clothing, medical textiles, and filter media where hygiene and contamination issues are highly important. Besides antistatic property may contribute to the safety of the dust collection systems as they can remove the undesirable static electrical charges that can lead to explosions, fires, or personal injury from shock. Thus in this study, nanocomposite PAN filaments with TiO₂ and AgNPs were produced by wetspinning method with the aim of producing multifunctional filaments showing antibacterial activity, photocatalytic activity, and antistatic properties. With a different approach from the existing literature, TiO₂ and AgNPs were added to the filament structure simultaneously. As the aim was to produce multifunctional nanocomposite filaments without disturbing the structural properties, the composite filaments were also tested for morphology, crystallinity, tensile properties, and thermal properties besides the functional properties. In addition, the composition of the filaments and the nanoparticle dispersion was analyzed.

Experimental details

Materials

PAN, silver nitrate (AgNO₃) (Alfa Aesar Premion, 10858, 99.9995% metals basis), titanium dioxide (TiO₂) (Sigma Aldrich, 634662, nanopowder <100 nm, 99.5% trace metal basis, rutile and anatase mixture), and N,N-dimethylformamide (DMF) (Merck, 103053), hydrazinium hydroxide (N₂H₅OH) (Merck, 804608), and methylene blue (MB) (Sigma Aldrich, 66720) were used as received. AgNO₃ and TiO₂ were used as the additives. Aqueous solution of hydrazinium hydroxide was used for the reduction of the silver ions. MB was used as an organic pollutant in photocatalytic activity tests.

Methods

Preparation of the spinning solutions. The required amounts of the additives were added to DMF, and the dispersion was homogenized with ultrasonic tip for 15 minutes and ultrasonic bath for 45 minutes. Twenty-one weight percent PAN was added to the solution, which was stirred magnetically at 40℃ until the complete dissolvation of PAN. For the production of pure PAN filaments as reference, 21 wt% PAN was added to DMF and mixed with magnetic stirrer at 40℃ until PAN was dissolved. The compositions of the filaments produced are presented in Table 1.

Table 1.

Compositions of the filaments produced.

Filaments	Sample codes	TiO₂ content (wt%)	AgNO₃ content (wt%)
100% PAN	S00	–	–
PAN/1%TiO₂	S10	1	–
PAN/3%TiO₂	S30	3	–
PAN/1%TiO₂/1%AgNO₃	S11	1	1
PAN/1%TiO₂/3%AgNO₃	S13	1	3
PAN/3%TiO₂/1%AgNO₃	S31	3	1
PAN/3%TiO₂/3%AgNO₃	S33	3	3

PAN: polyacrylonitrile.

Filament formation. The filaments were produced using wet-spinning method. An in-house built laboratory type spinning machine, the schematic of which is given in Figure 1, was used in the production of filaments. The spinning solution was fed through an 8-hole spinneret with a hole diameter of 0.5 mm and drawn into filaments. The solution flow rate was adjusted to 1 ml/h. The coagulation baths contained DMF and distilled water. The first coagulation bath contained 80:20 DMF: distilled water at 40℃, the second coagulation bath contained 50:50 DMF: distilled water at 60℃. The third bath, washing bath, contained 100% distilled water at 80℃. Total drawing was set as four times by adjusting the rotational speed of the successive sets of cylinders. The second set of cylinders was running two times faster than the first set of cylinders, while the third set of cylinders was rotating two times faster than the second set of cylinders. Wet-spinning was conducted under standard atmospheric conditions (Temperature: [20 ± 2]℃, Relative humidity: [65 ± 5]%) and the filaments were dried at room temperature. The count of the filaments was around 20 tex.

Figure 1.

Schematic of the laboratory-type wet-spinning machine used for the production of the nanocomposite filaments.

Chemical reduction process. Chemical reduction was performed using aqueous solution of hydrazinium hydroxide. Pieces of nanocomposite filaments were immersed into the aqueous solution of hydrazinium hydroxide (1:20 hydrazinium hydroxide:distilled water) for 30 min at room temperature, then washed with 100 mL distilled water two times and dried at room temperature. “CRv is used in the names of the filaments, which are chemically treated for silver ion reduction.

Characterization. Scanning electron microscope (SEM), X-ray diffraction spectrometer (XRD), tensile tester, conductivity tester, differential scanning calorimeter (DSC), US–visible (Vis) spectrometer, and energy dispersive X-ray spectrometer (EDS) were utilized for the characterization of the filaments after the chemical reduction process. The composite filaments were also tested to determine their antibacterial activity. For SEM imaging, the filaments were sputter coated with gold using Quorum—SC7620 Sputter Coater before taking SEM images. Philips FEI—Quanta FEG 250 SEM was used to take the surface images of the filaments. Diameter of nanoparticle agglomerates in the filament structure was measured from the SEM image of S33-CR taken with 25 kX magnification using Image J. The average size was based on the measurement of 100 nanoparticle agglomerates.

Silver and titanium dioxide contents of the nanocomposite filaments were measured by EDAX energy dispersive X-ray (EDX) spectrometer attached to Philips FEI—Quanta FEG 250 SEM. Elemental mapping was performed to be able to evaluate the dispersion of the nanoparticles in the nanocomposite filament structure. Tensile properties of the filaments were measured according to ASTM D3822 standard [53]. A conventional tensile tester was used in the measurement. The tester was equipped with a 100 N load cell. The filaments were cut 80 mm in length. The gage length was adjusted to 50 mm and the crosshead speed was adjusted to 30 mm/min. At least 10 specimens were tested for each sample. Microtest 6370 LCR meter was used to measure the resistances of the nanocomposite filaments. Conductivity in S/cm was calculated according to ASTM D257 [54] and ASTM D4496 [55] using the resistance value measured, the length and the thickness of the filaments. The average and standard deviation values for conductivity were calculated based on at least seven measurements. Wide-angle XRD traces were obtained using a Bruker® AXS D8 Advance X-ray diffractometer system using nickel filtered CuK_α radiation (λ, 0.15406 nm) with voltage and current settings of 40 kV and 40 mA, respectively. Counting was carried out at 10 steps per degree. The equatorial X-ray scattering data were collected in reflection mode in the 5–40° 2θ range. Cyrstallinity (%) is based on the ratio of the integrated intensity under the resolved peaks to the integrated intensity of the total scatter under the experimental trace [56] in the 2θ range between 5° and 40°. All the XRD traces obtained for the pure PAN and nanocomposite filaments were fitted with a curve fitting procedure developed by Hindeleh and Johnson [56] to separate the overlapping peaks. DSC (TA Q Series DSC Q10) was used to determine the thermal properties of the filaments in the temperature range from 30℃ to 400℃, at a heating rate of 20℃/min in nitrogen environment. For each sample, the cyclization temperature (T_c) and the cyclization enthalpy (ΔH) were obtained from the thermograms. The photocatalytic activities of the nanocomposite filaments were assessed by photodegradation of MB dye under UV irradiation from a distance of 40 cm. Three hundred Watts Osram Ultra-Vitalux lamp was used for the UV-irradiation. Filaments of 0.2 g were immersed in 50 g MB aqueous solution (10 ppm). After exposure to UV light for 2 h, the concentrations of the MB solutions were measured with PharmaSpec 1700 UV–Vis spectrophotometer in 300–800 nm range. MB removal ratio was calculated as [1 − (A₀ − A_x)/A₀] × 100 where A₀ is the absorbance value of MB solution at 663 nm and A_x is the absorbance value of the solution at 663 nm after exposure to UV light for 2 hours. PAN filaments were considered as the reference. Antibacterial activities of the filaments were assessed quantitatively against Staphylococcus aureus according to ASTM E2149 standard [57]. The antibacterial activities of the filaments were evaluated after the specified incubation time and the percentages of the reduction of the tested bacteria were calculated.

Results and discussion

The nanocomposite filaments were characterized regarding their morphology, composition, nanoparticle dispersion, tensile properties, crystallinity, conductivity, thermal properties, photocatalytic activity, and antibacterial activity. The results obtained regarding crystallinity, tensile properties, electrical conductivity, thermal properties, MB removal ratios, and antibacterial efficiency are presented in Table 2, while the elemental analysis results are given in a separate table within the related section.

Table 2.

Properties of nanocomposite PAN/TiO₂/AgNP filaments in comparison with pure PAN filament.

Samples	Crystallinity PAN (%)	Breaking Tenacity (cN/tex)	Breaking elongation (%)	Electrical conductivity (S/cm)	Cyclization temperature T_c (℃)	Cyclization Enthalpy (ΔH) (J/g)	Methylene blue removal ratios (%)	Antibacterial efficiency (%)
S00	29.7	5.88 ± 0.87	14.92 ± 1.40	–	330.00	406.8	42.7	14.815
S10	27.9	5.03 ± 0.23	14.35 ± 1.17	1.95 × 10⁻⁴± 1.74 × 10⁻⁵	331.83	353.1	56.9	81.852
S30	26.3	4.89 ± 0.29	13.10 ± 1.39	1.68 × 10⁻⁴± 2.33 × 10⁻⁵	329.65	337.4	70.2	86.111
S11-CR	23.0	5.52 ± 0.75	19.83 ± 1.20	1.36 × 10⁻⁴± 2.21 × 10⁻⁵	340.00	315.1	70.9	99.856
S13-CR	28.0	8.72 ± 2.15	19.75 ± 5.65	2.77 × 10⁻⁴± 2.78 × 10⁻⁵	339.08	289.1	66.2	99.998
S31-CR	21.4	5.38 ± 0.42	24.13 ± 2.19	1.28 × 10⁻⁴± 3.65 × 10⁻⁵	338.46	276.4	74.1	99.837
S33-CR	29.1	6.91 ± 1.30	21.19 ± 3.06	2.48 × 10⁻⁴± 5.11 × 10⁻⁵	341.06	260.5	64.2	99.991

PAN: polyacrylonitrile.

Morphology

Photographs of the nanocomposite filaments taken with a camera are presented in Figure 2 in order to show how the composite filaments looked like and how their color changed after adding TiO₂ and AgNO₃ and after chemical reduction process. SEM images taken from the surface of the filaments are presented in Figure 3.

Figure 2.

Photographs of (a) S00 pure PAN; (b) S11; (c) S11-CR nanocomposite filaments.

Figure 3.

SEM images of (a) S00; (b) S10; (c) S30; (d) S11-CR; (e) S13-CR; (f) S31-CR; (g) S33-CR taken with 10 kX magnification.

While pure PAN filament and nanocomposite PAN filaments with TiO₂ nanoparticles were white in colour, the nanocomposite filaments became brown with the addition of AgNO₃ (Figure 2(b)) due to reduction effect of DMF on silver ions [58,59]. After chemical reduction process, the brown color of the nanocomposite filaments darkened showing the further reduction of the silver ions due to reduction of AgNO₃ by hydrazinium hydroxide (Figure 2(c)). The darkening with the chemical reduction process was less for the nanocomposite filaments with TiO₂.

The nanocomposite filaments were uniform in structure (Figure 3). The nanoparticles on the surface of filaments could be observed in SEM micrographs. While the surface of the pure PAN filaments were smooth, the nanocomposite filaments had rough surfaces due to the presence of nanoparticles in the filament structure. The roughnesses of the nanocomposite filaments were observed to increase as the additive content increased. Grooves were observed on filament surfaces along the drawing direction. They were likely due to voids forming in the nanocomposite filament structure by the effect of nanoparticles and taking the form of a groove after drawing. The average diameter of the nanoparticle agglomerates in the S33-CR nanocomposite filament structure was measured as 92.28 ± 26.64 nm (Figure 4).

Figure 4.

SEM image of S33-CR taken with 25 kX magnification.

Elemental analysis

The presence of Ag and TiO₂ in the filament structure was confirmed by elemental analysis, the results of which are presented in Table 3. The amount of Ti detected in nanocomposite filament structure increased from 0.65 to 1.70 as the amount of TiO₂ added to filament structure increased from 1 wt% to 3 wt%. The amount of silver on the surface of the nanocomposite filaments decreased as the content of TiO₂ in the filament structure increased. The addition of TiO₂ might have reduced the Ag nanoparticles on the surface of the nanocomposite filaments.

Table 3.

Elemental analysis of S13-CR and S33-CR nanocomposite filaments.

Element	wt% in S13-CR	wt% in S33-CR
C	73.96	73.69
N	15.91	14.90
O	8.01	8.48
Ag	1.46	1.23
Ti	0.65	1.70

The dispersions of Ag and TiO₂ nanoparticles were assessed using elemental mapping. The images, presented in Figure 5, pointed out that the nanoparticles were dispersed homogeneously in the nanocomposite filament structure although some agglomerates of nanoparticles were apparent on the SEM images.

Figure 5.

Images of elemental mapping (a) Ag mapping in S13-CR nanocomposite filament; (b) TiO₂ mapping in S13-CR nanocomposite filament; (c) Ag mapping in S33-CR nanocomposite filament; (d) TiO₂ mapping in S33-CR nanocomposite filament.

X-ray diffraction

XRD measurements were carried out in order to investigate the crystalline nature of wet-spun PAN/TiO₂/AgNP nanocomposite filaments. Figure 6 shows the resolved equatorial XRD traces of pure PAN and nanocomposite filaments incorporating varying proportions of TiO₂ and AgNPs, while the crystallinity values of the filaments are presented in Table 2.

Figure 6.

XRD spectra and curve fittings of (a) S00 pure PAN; (b) S30; (c) S33-CR composite filaments.

A typical equatorial XRD trace of wet-spun PAN filament shows two well-defined and sharp and one diffuse equatorial reflections with d-spacings of 0.52 nm, 0.3 nm, and 0.34 nm, which have the scattering angles (2θ) of 16.7°, 26.7° and 29.2°, respectively. These characteristic reflections of PAN are observed in the qualitative examination of equatorial XRD traces. Two prominent reflections with d-spacings of 0.52 and 0.3 nm are indexed as (100) and (110) reflections of a hexagonal unit cell [60] with basal plane dimensions of a = b = 0.6 nm. The diffuse equatorial reflection, which is observed at 0.34 nm, can be assigned to the disordered material.

Three crystalline structures of TiO₂ are known as anatase (tetragonal, a = b = 0.3785 nm, c = 0.9514 nm), rutile (tetragonal, a = b = 0.4593 nm, c =0.2959 nm), and brookite (orthorhombic, a = 0.9182 nm, b = 0.5456 nm, c = 0.5143 nm) [61,62]. Within the scanned range of 5–40° 2θ, an anatase peak indexed as 101/011 diffraction at 2θ = 25.32°, and a rutile peak indexed as 110 diffraction at 2θ = 27.45° are expected since TiO₂ used is reported to be a mixture of anatase and rutile crystal forms by the supplier. However, only anatase crystal form (2θ = 25.3475°) is observed for all of the samples containing TiO₂ (Figure 6(b), (c)). It is possible that rutile form may be in very small amount to be detected by the XRD.

On the other hand, AgNPs seems to be easily detected by the XRD system. The diffraction peaks at 2θ = 31.01° and 2θ = 38.08° respectively indicated the presence of Ag (110) and Ag (111) planes in the composite filament structure [63].

The crystallinity values presented in Table 2 suggest that the incorporation of TiO₂ depresses the values of crystallinity whereas the incorporation of AgNPs improves crystallinity depending on the loading amount. For example, the crystallinity value of pure PAN (29.7%) is reduced to 27.9% upon incorporation of 1 wt% TiO₂, whereas the crystallinity is further reduced to 26.3% upon incorporation of 3 wt% TiO₂. In literature, decrease in crystallinity is reported with the addition of TiO₂ into PET filaments. The decrease was assigned to the formation of more imperfect crystals as a result of TiO₂ acting as an nucleating agent [31]. When TiO₂ and AgNPs are incorporated together, the values of crystallinity seem to be influenced by the amount of TiO₂ and AgNPs added to the filament structure. Generally, the simultaneous incorporation of AgNO₃ and TiO₂ seems to disturb the crystallinity of the filaments as all the composite samples produced show lower crystallinity than pure PAN. It is possible to expect higher amorphization in filament structure with the increase in additive amount. However, AgNO₃ is reported to be an additive, which is very well dispersed in PAN due to its interaction with cyano nitrogen groups. It is reported in literature that Ag ions and cyano nitrogen of PAN form coordination bonds. The coordination between cyano nitrogen and silver ion encourages the dispersion of AgNPs in PAN more homogenously [64]. AgNO₃ is considered to have an improving effect on the degree of order of the PAN macromolecules due to its homogeneous dispersion and interactions with PAN. One weight percent AgNO₃ was not sufficient to maintain some order in the filament structure while 3 wt% AgNO₃ could provide some order although the crystallinity of the composite filament with 3 wt% AgNO₃ was still below the crystallinity of pure PAN filament.

Tensile properties

The mechanical properties of nanocomposite polymers are reported to be dependent on several factors such as filler type, filler shape, size and dispersion of filler in the matrix, filler orientation, interaction of the host polymer with filler, and interactions between fillers [65 –68].

While the tensile properties of the pure PAN and nanocomposite filaments are presented in Table 2, representative tenacity–elongation curves are presented in Figure 7.

Figure 7.

Tenacity–elongation curves of pure PAN and nanocomposite filaments.

In parallel with the literature [31,65], decrease was observed in breaking tenacity and breaking elongation of the filaments with TiO₂ addition. The decrease in tenacity was most likely due to the formation of agglomerates and voids in the filament structure behaving as defects during tensile testing [15]. Besides TiO₂ nanoparticles might have reduced the interactions between the polymer chains [31] which was confirmed by XRD analysis showing a decreasing crystallinity with increased amount of TiO₂. The decrease in breaking elongation might have been due to TiO₂ restricting the chain movement in filament structure [49].

The strength of nanocomposite filaments increased to 5.52 cN/tex with the addition of 1 wt% AgNO₃ and further increased to 8.72 cN/tex with the addition of 3 wt% AgNO₃ to the filament structure at the TiO₂ content of 1 wt%. The similar trend was also observed with the addition of AgNO₃ to the filament structure at the TiO₂ content of 3 wt%. The increase in breaking tenacity with AgNO₃ addition is attributed to the formation of coordination bonds between Ag ions and nitrile groups of PAN [64] and consequent increase in the crystallinity (Table 3). The strengthening effect of AgNO₃ was pointed out when it was added to the PAN filament structure alone [48]. While the breaking elongation values decreased with the addition of TiO₂, they increased with AgNO₃ addition, which might have been due to the increased amount of voids in the filament structure leading to the increased ductility of the nanocomposite filaments.

Electrical conductivity

The electrical conductivity values can be seen in Table 2.

While the conductivity of pure PAN filament was not measurable by the used conductivity tester, the conductivities of the nanocomposite filaments were measured at around 10⁻⁴ S/cm, which is in the semiconductive range [69,70] and meet the electrical conductivity requirements of static dissipative applications [71]. Although the change in the color of the filaments, SEM images, and EDS analysis confirmed the higher amount of nanoparticles in the filament structure with the increase in the additive content, the conductivity was not significantly affected by the increase in the additive amount.

DSC analysis

The cyclization temperatures (T_c) and cyclization enthalpies (ΔH) of pure PAN and nanocomposite filaments with different contents of nanoparticles are presented in Table 2.

Pure PAN and nanocomposite filaments exhibited a relatively large exothermic peak at around 330℃ which is attributed mainly to the cyclization reactions [72 –74]. While the cyclization temperature was not very much affected by the addition of TiO₂, it shifted to higher temperatures with the addition of AgNO₃ which means increased thermal stability. The shift to higher temperatures may be the result of the higher thermal conductivity of the AgNPs resulting in the distribution of the applied heat initially between the nanoparticles.

The cyclization enthalpy, showing the total heat released during the cyclization reactions, decreased with the addition of the additives. The decrease in total heat of cyclization reactions seems to result from a decrease in the activity between polar cyanogen groups of PAN macromolecules caused by the presence of nanoparticles between them [34]. Besides, PAN might have undergone cyclization reactions at room temperature with presence of nanoparticles acting as catalysts which might have formed another reason for the lower heat of cyclization reactions during DSC testing.

Photocatalytic activity

When TiO₂ nanoparticles are exposed to UV light with an energy that matches or exceeds their band gap energy, an excitation of electrons from the valence band to the conduction band occurs, leaving holes in the valence band. While the holes react with water to produce hydroxyl radicals, the electrons react with molecular oxygen molecules producing superoxide radical anions, both of which can decompose organic materials [75 –77]. In this study, MB was used as the organic material. The changes in the absorbance values of the MB solutions with the immersion of the nanocomposite filaments were measured and the MB removal ratios were determined.

In Table 2, the MB removal ratios calculated for pure PAN and nanocomposite filaments are presented. When the MB solution was exposed to UV light, the absorbance value at 663 nm reduced from 1.41 to 1.34 which meant 5 % of the MB was decomposed by photolysis.

Pure PAN filaments displayed a MB removal ratio of 42.7% after 2 h UV irradiation which was considered to be the result of both dye absorption by the fibers and photolysis. As the TiO₂ content increased, MB removal ratios increased.

When the nanoparticles were used together, an increase was observed with addition of 1 wt% AgNO₃ at the TiO₂ content of 1 wt% and a further increase at the TiO₂ content of 3 wt%. Although silver has no photocatalytic activity, its combined use with TiO₂ resulted in an improvement in the photocatalytic activity most probably due to the Ag acting as electron traps aiding electron-hole separation and also facilitating electron excitation by creating a local electric field [61]. This synergistic effect disappeared with the increase in the additive amounts. The increase in AgNO₃ content resulted in a decrease in MB removal ratio, which was most likely due to the poor dispersion of the nanoparticles.

Antibacterial activity

While pure PAN does not show antibacterial activity [78], AgNPs either alone or in polymer matrices are reported to display antibacterial activity [58,79 –81]. Although the exact mechanism of antimicrobial effect of silver nanoparticles is not clearly known, there are various theories on their action on microorganisms. According to one of them, AgNPs anchor to the bacterial cell wall and subsequently penetrate it, thereby causing structural changes in the cell membrane leading to the death of the cell [82,83]. According to another mechanism suggested, free radicals, which have the ability to damage the cell membranes, form when AgNPs get in contact with the bacteria [84,85]. Others have suggested the possible reaction of silver ions released by the silver nanoparticles with the sulfur groups of both the cell membrane and the cell DNA [86,87]. TiO₂ nanoparticles are also reported to inhibit growth of bacteria [88 –91]. The antibacterial effect of TiO₂ is frequently ascribed to OHċ radicals and other reactive oxygen species (ROS) that form upon photoactivation [88]. Some researchers have shown that initial oxidative attack on the outer/inner cell membrane of the microorganism, alterations of coenzyme A-dependent enzyme activities, and damage to DNA via hydroxyl radicals are the main parts of the TiO₂-based antibacterial mechanism which results in cell death [92 –96]. In this study, AgNPs and TiO₂ were incorporated to the structure of PAN filaments and the nanocomposite filaments were expected to display antibacterial activity. While the pictures of agar plates with pure PAN and nanocomposite filaments after 24-hour incubation are presented in Figure 8, the antibacterial efficiency test results of nanocomposite filaments after 24-hour incubation are presented in Table 2.

Figure 8.

Agar plates with (a) control solution; (b) S00 pure PAN; (c) S10; and (d) S11-CR nanocomposite filaments after 24-hour incubation.

The composite filaments containing TiO₂ displayed antibacterial activity which was observed to be dependent on TiO₂ concentration. The antibacterial activity of the filaments was significantly improved with the addition of 1% TiO₂ and further increased with increase in the TiO₂ content from 1% to 3%. The antibacterial activity of nanoparticles are reported to be dependent on chemical composition [97], concentration [85], size [98], shape of the nanoparticles [99], and type of the target microorganism [100]. In particular, the antibacterial activity of TiO₂ is reported to be proportional to the concentration of TiO₂ before achieving a plateau [101].

The highest percentages of antibacterial activity were obtained when TiO₂ and AgNPs were simultaneously incorporated to the filament structure. All of the nanocomposite filaments, which were produced with the combined addition of TiO₂ and AgNPs, showed over 99% antibacterial activity and thus could be called antibacterial materials according to FTTS-FA-002 standard [102].

Conclusions

Nanocomposite PAN filaments with different contents of TiO₂ and AgNPs were produced by wet-spinning method and characterized by SEM, EDS, XRD, tensile tester, conductivity tester, DSC, UV–Visible spectroscopy, and antibacterial activity testing. The addition of TiO₂ and AgNO₃ to the filament structure had considerable effects on both structural and functional properties of the filaments. SEM, EDS, and XRD results confirmed the presence of nanoparticles in the filament structure. Elemental mapping confirmed the homogeneous dispersion of the nanoparticles in the filament structure. While decrease was observed in both breaking tenacity and breaking elongation of the filaments with TiO₂ addition, some improvement was observed with the addition of AgNO₃ especially at the content of 3 wt%. The conductivities of the nanocomposite filaments were around 10⁻⁴ S/cm, which is in the semiconductive range. The nanocomposite filaments displayed MB removal efficiency of up to 74.1 in 2-hour period and excellent antibacterial activity. This study showed the possibility of producing multifunctional filaments with the simultaneous addition of different types of nanoparticles into filament structure. The nanocomposite filaments with titanium dioxide and silver nanoparticles were semiconductive and displayed both photocatalytic and antibacterial activity.

Footnotes

Acknowledgements

We would like to thank Prof. Dr. Ismail Karacan for his help in XRD analysis and MSc. Olcay Eren for her help in filament production.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by The Scientific and Technological Research Council of Turkey (TUBITAK) (Project 112M877) and Istanbul Technical University-Scientific Research Projects Department (BAP) (Project 37676).

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Nanocomposite polyacrylonitrile filaments with titanium dioxide and silver nanoparticles for multifunctionality

Abstract

Keywords

Introduction

Experimental details

Materials

Methods

Results and discussion

Morphology

Elemental analysis

X-ray diffraction

Tensile properties

Electrical conductivity

DSC analysis

Photocatalytic activity

Antibacterial activity

Conclusions

Footnotes

Acknowledgements

Declaration of Conflicting Interests

Funding

References