Nanofibers can be obtained using various methods such as drawing, self-assembly, phase separation, template synthesis, bicomponent spinning, flash spinning, melt blowing, electrospinning, and some other methods [1, 2, 3]. Electrospinning is a widely applied technology, and it is used to manufacture nanofibers from a wide range of natural and synthetic polymers [4, 5]. The common scheme of the setup, used during conventional electrospinning, is shown in Figure 1. The setup consists of a high power supply, a spinneret (metallic needle mostly), and a grounded collector (rotating drum, covered with aluminum foil, or metallic plate).
Electrospinning is an electrohydrodynamic method, where the discharged polymer solution or melt jet is only stable at the tip of the spinneret (needle). The drop of the polymer solution at the tip of the needle became highly electrified, when a voltage is applied, and the induced charges became evenly distributed over the liquid surface. At this moment, the drop experiences two major types of electrostatic forces: the electrostatic repulsion among surface charges and the Coulomb forces exerted by the external electric field. The liquid drop became distorted into a conical object, under such electrostatic interactions, commonly known as Taylor cone. Once the strength of the electric field has surpassed a threshold value and electrostatic force overcome the surface tension of the polymer solution, the liquid jet is forced to eject from the tip of the needle. During the spinning, the jet undergoes an instable and a rapid whipping elongation process in the space between the capillary (needle) tip and the collector, which allows the jet to become very long and thin [1, 6, 7]. Solutions or melts are pumped through the nozzle at very low flow rates, and the solvent in the jet stream in the way to the counter electrode rapidly evaporates during the fiber formation [7, 8, 9]. Attracted by the opposite charge of the grounded collector, the charged nanofibers are deposited as a randomly oriented, porous nonwoven fiber mat (scaffold). These randomly deposited fibrous scaffolds have potential applications as temporary substrates for skin and in bone tissue engineering because they replace the microstructure of natural tissue [10, 11]. Moreover, the high surface area of electrospun fibrous scaffolds allows oxygen permeability at the wound site, making these scaffolds suitable for wound dressings . The drum collector provides additional stretching of the polymer jet which results in more oriented and thin nanofibers. The speed and form of collector influences the diameter and orientation of the produced nanofibers [13, 14]. The electrospun materials exhibit uniform and ultrafine fibers, high surface-to-volume ratio, porous structures, and controllable compositions, resulted in achieving the desired products with needed properties [4, 8].
The orientation of nanofibers in fibrous scaffolds is considered as one of the important features of a perfect tissue scaffold because the fiber orientation greatly influences the cell growth and related functions. Therefore, engineering scaffolds with a control over fiber orientation is essential and a prerequisite for controlling cell orientation and tissue growth [13, 15].
Gnavi et al. Ref.  demonstrated that aligned fibrous scaffolds provide contact guidance to cultured nerve cells, resulting in the alignment and elongation of cells along the contacted fiber direction. Lee et al. in Ref.  found that aligned fibrous scaffolds could serve as ideal materials for bone tissue engineering, and that the orientations of fibers play an important role in the guidance of new bone formation. Nitti et al. Ref.  investigated chitosan (CS)/polyethylene oxide (PEO) nanofibers orientation in produced scaffolds and they concluded that fibers produced near the center of the collector drum appear randomly aligned, while, gradually moving away from the center, an increase in fiber alignment can be observed, more evident at higher collection speed.
Electrospinning is one of the facilitative ways to process CS for the medical application, such as drug delivery, wound dressing, scaffolds, and surgical dressing, for example, hemostatic sponges or nerve conduit [18, 19, 20, 21, 22, 23, 24, 25]. CS has multiple bioactivities, for example, nontoxicity, biodegradability, hemostatic properties, and intrinsic biocompatibility, and it has an amine functional group which is strongly reactive with anions and owns excellent wound healing acceleration and natural antibacterial/antifungal properties, applicable, for example, as bacterial membrane perforator [8, 18, 24, 26, 27]. Due to metal-binding properties, dissolved CS can be used as an additive to bind and remove metals from wastewaters during food processing [28, 29]. In addition, because of its free amino groups, CS can be dissolved into an acidic aqueous solution and form gels, films, sutures, beads, and fibers . The electrospinnability of pure CS is limited because of its polycationic charge, rigid chemical structure, and specific inter/intra-chain hydrogen bonding [26, 27, 30, 31]. Inter/intrachain hydrogen bonds hinder polymeric chain segments, exposed to the electrical field, to move freely and results in jet intermission during the spinning process [27, 30, 32]. Moreover, the repulsive force between ionic groups on the polymer backbone of the CS prohibits the formation of sufficient chain entanglements which are responsible for continuous fiber formation during jet stretching, whipping, and bending, generally resulting in nanobeads instead of nanofibers . In order to improve the inherent poor electrospinnability of CS [5, 31], CS derivatives or fiber-forming additives, such as PEO or polyvinyl alcohol are employed to help the fabrication of CS nanofibers [21, 22, 23]. PEO is a synthetic polymer, with low toxicity and biocompatible with other bioactive substances [34, 35]. Since PEO is semi-crystalline polymer with a flexible chain, pure PEO products have relatively poor mechanical and physical properties. Its ability to absorb water limits its application. The most effective way to improve the properties of PEO products is blending pure PEO with other polymers. This action mostly results in products with improved physical and mechanical properties .
Blending of CS and PEO polymers results in colorless products, where CS is responsible for improved mechanical properties and reduced solubility in water, whereas PEO is responsible for the formation of products with increased flexibility. Recent reports have shown that the resulting composite nanofibrous scaffolds are stable, noncytotoxic to cells, and environmentally and sustainably friendly [25, 27, 36, 37]. Since PEO distinguishes an amphiphilic and unique mechanical properties, CS blends with PEO are always electrospun into nanofibers successfully.
Many of the researchers are investigating the manufacturing process of nanofibers from CS and its blends, especially with high molecular weight PEO [14, 31, 38], but still it is quite difficult to predict the characteristics of those polymers and their behavior during the production of nanofibers. This research consists of the analysis of produced porous scaffolds composition and structure and not targeting to predict the product characteristics by controlling the process of production. The aim of the research was to investigate the spinnability of pure CS and quite low molecular weight PEO, as well as their blends, and to analyze viscosity and conductivity of prepared solutions together with electrical, thermal, and morphological properties, especially the orientation of nanofibers of produced nonwoven fibrous materials adapted in biomedical applications as scaffolds.
CS ((C6H11NO4)n) with a molecular weight of 100,000–300,000 g/mol was purchased from ACROS Organics™. A glacial acetic acid (AA) of 99.8% (CH3CO2H) with a molecular weight of 60.05, d =1.05 kg/l was used as a solvent to dissolve CS. The PEO with a molecular weight of 200 000 g/mol was bought from Ontario Inc. At first, the glacial AA was mixed with deionized water to obtain concentrations of 90% and 50%. Then, 3 %wt% of CS was dissolved in 90% and 50% AA solutions. PEO was dissolved in deionized water to obtain solutions with concentrations of 10 wt%, 15 wt%, and 20 wt%. All solutions were left for mechanical stirring for 24 hours and for degassing for 4 hours at room temperature. In order to prepare CS/PEO blends, the 3 wt% CS dissolved in 90% and 50% AA – preparation of solutions described above – were mixed with PEO solutions of 10 wt%, 15 wt% and 20 wt% concentrations – preparation of solutions described above – for 24 hours as well. The mass ratio of CS to PEO solutions was selected as 50:50. All prepared solutions were used without further purification.
The coding of produced solutions is presented in Table 1.
Coding of pure CS, PEO, and CS/PEO blend solutions
|Code of solution||CS concentration in 90% AA||CS concentration in 50% AA||PEO concentration in water|
|CS90PEO10||3 wt%||–||10 wt%|
|CS90PEO15||3 wt%||–||15 wt%|
|CS90PEO20||3 wt%||–||20 wt%|
|CS50PEO10||–||3 wt%||10 wt%|
|CS50PEO15||–||3 wt%||15 wt%|
|CS50PEO20||–||3 wt%||20 wt%|
2.2 Solution testing equipment
A rotating viscometer (Brookfield, DV-II) was used for viscosity measurements of prepared solutions. Results were taken at the same shearing rates. The electric conductivity was tested with a conductivity meter (Mettler Toledo), at 22°C temperature and 35% relative humidity. The mean value of five measurements is presented as a result.
The photograph of conventional electrospinning setup used for production of nanofiber mats is shown in Figure 2. The polymer solution was contained in a syringe with a metal capillary (diameter: 0.6 mm). The capillary was connected with a high power supply, which generated positive DC voltage power for 20 kV. The flow rate of polymer solution of 0.5 μl/h was adjusted and controlled by syringe pump. The distance between the capillary and the collector was 15 cm. The collector drum was covered with grounded aluminum foil. The rotating speed of the collector drum was 320 rpm and it was kept constant through the whole production process. We used approximately the same amount of solution (almost 3 ml) to produce samples of similar thickness. The electrospinning process was carried out at 22°C and 35% relative humidity in a closed laboratory air flow chamber.
The coding of produced porous nanofiber scaffolds is presented in Table 2.
Coding of pure CS, PEO, and CS/PEO porous scaffolds
|Code of fiber mat||Code of solution used in production of samples|
2.4 Characterization of fiber mats
The morphology of fiber mats obtained by an electrospinning process from CS, PEO, and CS/PEO blended solutions was investigated using a scanning electron microscope (SEM; Hitachi TM-1000). The specimens of fiber mats for SEM were attached to the carbon tape. The diameter of produced nanofibers was measured on the SEM images in original magnifications of 5000× by the commercial software installed in SEM. The magnification of 5000× was used because selected images reflect the whole picture of the fiber mats produced, since the images of individual nanofibers, despite the conditions and technical parameters of production, are very similar and it would be difficult to find any significant differences. They would be less informative. The diameter of nanobeads and nanodroplets, formed in the pure CS specimens, was calculated and presented using computer software ImageJ 150i. The orientation anisotropy of fabricated nanofibers was evaluated using ImageJ 150i software function Directionality and OrientationJ plugin [38, 39, 40, 41]. The software estimates nanofibers located only on the surface layer of produced porous scaffolds and evaluates the structure tensor of every Gaussian-shaped window (defined as 1 pixel) by computing the continuous spatial derivatives in the x- and y-dimensions using Fourier interpolation. From the structure tensor of every window, the local predominant orientation (corresponding to the orientation of the largest eigenvector of the structure tensor), the energy (defined as the trace of the structure tensor matrix), and the coherency (defined as the ratio between the difference and the sum of the maximum and minimum tensor eigenvalues) were computed. The computed histogram indicates the amount of structures in a given direction. The east direction was defined as 00, and the orientation is counter clockwise. Angles were reported in their common mathematical sense. Images with completely isotropic content are expected to give a flat histogram, whereas images in which there is a preferred orientation are expected to give a histogram with a peak at that orientation.
The thickness of produced scaffolds was determined using SONY Digital Indicator U30 apparatus. The mean value of five measurements is presented in this work. The standard deviation of thickness values in one sample varied till approximately 10%.
The electrical conductivity of porous scaffolds was determined by two point measurement methods: high resistance and low conductance meter HR 2 equipment. Two stainless steel electrodes, with a distance of 1 cm between them, were employed during measurements. The measurements were taken at 22°C and 35% relative humidity. The average of five measurements is presented as a result.
Fourier-transform infrared-attenuated total reflection (FTIR-ATR) spectra were obtained using a Perkin-Elmer, PE Spectrum-GX FT-IR, FT-NIR ATR Infrared spectrometer. The spectra were recorded in 400–4000 cm−1 spectral range with the 4 cm−1 resolution. The number of scans applied was 32.
The thermal behavior of the PEO/CS blend nanofiber mats was estimated by measuring melting (Tm), crystallization (Tc) temperatures, and specific heat capacities, based on the endothermic and exothermic peaks. The measurements were carried out in a differential scanning calorimetry (DSC) model (Q10, TA Instruments) equipped with a refrigerated cooling system, under nitrogen atmosphere at a flow rate 50 ml/min. During DSC analyses, specimens were heated and cooled within a certain temperature interval ranging from −50°C to 180°C at 100C/min−1 and held at 180°C for 3 min, and then quenched to −50°C. Then investigated samples were reheated again to 180°C. Accurately weighed samples (±0.1 mg) were placed into a covered aluminum sample holder with a central pin hole. Indium metal (99.99%) has been used to calibrate the DSC modulus in relation to temperature and enthalpy.
Only a two-stage heating process was carried out during investigation. The first stage heating was used to obtain evidence of any effect of processing on the development of the fiber structure (to eliminate the thermal history and to ensure the thermal connection between the fiber mat (solid material) and the bottom of the DSC crucible, and the second one was used for results obtained .
The value of fusion enthalpy (ΔHf0) used while calculating the crystallinity of pure PEO and CS/PEO blend samples was 213.7 J/g .
3 Results and discussion
The conductivity and viscosity parameters measured for solutions, later used for electrospinning of nanofiber mats, are listed in Table 3. The thickness and electrical conductivity of fabricated nanofiber scaffolds are presented in Table 4.
Properties of pure CS, PEO, and CS/PEO blend solutions
|Code of solution||Conductivity (μS/cm)||Viscosity (cP)|
Properties of pure CS, PEO, and CS/PEO fiber mats
|Code of fiber mat||Conductivity (pS)||Thickness (mm)|
The data presented in Tables 3 and 4 show that viscosity of pure CS solutions influences the values of conductivity, that is, higher viscosity results in lower solution conductivity. As the same quantity of CS polymer was added in the solvent, we can conclude that the viscosity and conductivity are influenced by the concentration of AA added, that is, quantity of water. Contrary to pure PEO and CS/PEO blend solutions – higher values of viscosity give lower conductivities of solution. Lubentsov et al. Ref.  also concluded that water influences the crystal nature of the polymer and change its conductivity. As in preparation of all CS, PEO and CS/PEO solutions we used distilled water, whose conductivity is quite small, another factor may influence the electrical conductivity of solutions. The increase in conductivity of solutions might be due to free volume of polymer which affected this conductivity behavior . We could observe the same dependence for CS/PEO fiber mats as well, while for pure PEO scaffolds conductivity increases with the decrease in viscosity and increase in electrical conductivity in solutions. Later, in SEM images of pure PEO scaffolds, we can see that the solvent was not evaporated completely while spinning, which is the reason for such electrical conductivity values.
We have not investigated more rheological properties of solutions because time is very dominant for CS and CS/PEO solutions. Longer relaxation time for such solutions results in more viscous solutions. That is why all solutions after mechanical stirring (for 24 hours) were left only for 4 hours at room temperature (for degassing). If we would prolong the preparation time for solutions, we would be able to investigate the rheology of prepared solutions, but, we would not be able to electrospin nanofibers with less or no defects because the viscosity of solutions would be too high. More viscous solutions result in fibers with bigger diameter (even microfibers instead of nanofibers). The investigation of viscosity dependence from time we can find in Refs. [47, 48].
Also it is seen from the presented data that conductivity of fiber mats is linearly dependent on conductivity of solution. Table 3 shows that the concentration of AA, in which the CS was dissolved, highly influences the viscosity values of CS/PEO solutions. Higher AA concentration results in more viscous CS/PEO solutions at the same overall polymer concentration. The same effect was observed by Kriegel et al.  while investigating CS/PEO solutions.
To investigate the pollution of produced scaffolds, that is, to assess whether extraneous compounds or side effects were formed during spinning, FTIR-ATR analysis was carried out. The spectra of pure PEO samples of various concentrations are presented in Figure 3. It is seen from the spectra of 10PEOinH2O, 15PEOinH2O, and 20PEOinH2O, that a large broad band at 3600 cm−1 which is due to PEO hydration. The PEO is highly hydroscopic and absorbs water and gets hydrated . Many peaks in this band can also be attributed to C–H stretching mode . The typical absorption band at 2980 cm−1 corresponds to CH2 asymmetric vibration. The absorbance intensity of CH2 stretching vibration increases when PEO content is increased. However, there is no strong evidence that the shift of an absorption band establishes a hypothesis for the intermolecular interaction with PEO weight ratio change . The same stretching mode appears at 2620 cm−1. Broad band at 1960 cm−1 corresponds to C–O mode. CH2 wagging and some C–C stretching mode are seen at 1420 and 1320 cm−1, respectively. The CH2 twisting mode at 1260 cm−1 and also stretching vibration of ether group (C–O–C) can be seen at 1160 cm−1. CH2 symmetrical rocking at 880–600 cm−1 and C–O–C vibration mode at 980 and 480 cm−1 are observed .
The FTIR investigations of CS/PEO blends showed a broad band between 3490 and 3670 cm−1 attributed to intermolecular hydrogen bonding (OH peaks) between CS molecules and NH stretching of the primary amino groups. The sharp peaks at 2920 and 2540 cm−1 prove PEO presence in the samples and corresponds to aliphatic CH2 stretching. These peaks indicate chemical interactions and complex formation between CS and PEO . The absorption bands at 1960 and 1340 cm−1 can be attributed to –NH2 stretching and N–H bending of the primary amino groups, and the carboxyl stretching of the amide bands . The band was also produced from the crosslinking reaction between COOH groups in acid with CS. Peaks were produced due to the formation of ester bonding (C=O) resulting from the reaction between COOH of acid and OH groups of CS. The peak at 1340 cm−1 also can be attributed to the overlapping peak of C–H stretching and C–N stretching of amide bonding, resulting from interaction between COOH groups of acid with amino groups (NH2) of CS . Also the FTIR spectrum showed peaks in bands at 1160–600 cm−1 identifying saccharide groups of CS (C–O stretching vibration). In these parts, the overlapping of alcohol C–O stretching and ether linkage, as well as C–O–C stretching band are observed (see Figure 3).
The FTIR analyses have shown that the variations of polymer concentrations have not altered the chemical nature of materials used for investigations and that the chemical structure of CS was stable during electrospinning process. The same conclusions were made by Homayoni et al. Ref.  and Feng et al. Ref. .
Thermal analysis by DSC was carried out for fabricated porous scaffolds to explore the influence of CS on the crystallization kinetics of PEO. The heat flow was measured during the DSC test, when fiber mats were heated from −50°C to 180°C. Generally, polysaccharides do not melt but degrade upon heating above a certain temperature due to their links through hydrogen bonding [35, 53, 54]. On the other hand, glass transition temperature is still a complicated matter for CS. Below the temperatures of degradation, their thermograms show a very broad endotherm that is associated with the water evaporation. Therefore, mainly thermal characteristics of PEO in electrospun fibers were to be focused on in these experiments.
The DSC results and the calculated parameters are presented in Figure 3 and Table 5. ΔHm is the amount of energy (joules/gram) a sample absorbs while melting. ΔHc is the amount of energy (joules/gram) a sample releases while crystallizing. Results were obtained from the second run of DSC scans. The melting enthalpies were determined using constant integration limits. The melting temperature, Tm, here refers to the peak temperature of the main endothermic peak, and it varies from 62.6 to 64.1°C for pure PEO nanofibers, that is, endothermic events were noticed. It is seen from DSC thermograms, presented in Figure 4, that the peaks of PEO were shifted toward a higher temperature when the PEO content increased.
Thermal properties (melting points and transition enthalpies) of nanofibers tested
|Code of fiber mat||Tm (°C)||ΔHm (J/g)||ΔHf (J/g)||Tc (°C)||ΔHc (J/g)||Xc (%)|
Addition of CS into fiber blend showed no significant influence on the melting temperature; on the other hand, it lowered the degree of crystallinity of blend. The same tendencies were found by Chen et al. . Authors also stated that most of the polysaccharides do not melt, but degrade upon heating above a certain temperature because of their associations with hydrogen bonding. Below the temperatures of polymer degradation, their thermograms show a very broad endotherm that is associated with the water evaporation. Therefore, mainly thermal characteristics of PEO in electrospun fibers were to be focused on. It was found earlier in Refs. [36, 56] that PEO is characterized by a high tendency for crystallization and its crystals are outlined by monoclinic unit cells of crystals with four radially oriented PEO chains. These crystals became stretched, but strongly hold each other, when PEO concentration is increased. Nevertheless, addition of CS in the polymer solution would destabilize PEO crystallines probably by interrupting PEO–PEO interactions and consequently monitored peaks were moved to lower temperatures.
Another observation from Figure 4 is that the melting temperature of the pure samples is higher than that of the blended samples, which is due to the presence of acid in the nanofibers.
From the thermal analyses, the crystallinity (Xc) percentages of the nanofiber samples were calculated using Equation (1). The ΔHf and Xc values of the nanofibers are also presented in Table 5. As it could be seen from Table 5, all the nanofibers are semi-crystalline, and the crystallinity of the nanofibers, like the melting point, decreases with the increase in the acid proportion.
The morphology of electrospinned products, produced from pure 3 wt% CS dissolved in different concentrations of AA, is shown in Figure 5. From Figure 5a and 5e, it is seen that only nanobeads and droplets in the average of approximately 2–3 μm (micrometres) were obtained. The high crystalline structure of CH and the formation of hydrogen bonds during production of high viscosity spinning solutions (see Table 3) complicate the spinability of pure CS nanofibers [31, 57, 58, 59]. The distribution histograms composed of nanobeads and droplets in the samples from pure CS, dissolved in 90% and 50% AA, are shown accordingly in Figure 5d and 5h. It is seen from SEM images and histograms that standard deviations of diameter measurements of randomly located nanodroplets are less than 3 and indicates that the data points tend to be close to the mean value. It is visible from binary and outline images (see Figure 5d and 5c) that when CS was dissolved in 90% AA, more nanobeads and droplets of various forms were formed during spinning. While spinning from CS50 solution (AA concentration almost twice smaller), we obtained fewer droplets, but they were almost circular in shape (Figure 5f and 5g). We have not noticed any footprint or hint of nanofibers formation. Spinnability may be also explained by viscosity and electrical values of produced solutions (see Table 1). The viscosity of CS90 solution is almost twice higher and conductivity is approximately 4.5 times lower than that of CS50 solution. So, as we noticed earlier, more various beads were formed on the collector from more viscous and less conductive solution, that is, the drops from solution CS90 reached the collector much easily than from solution CS50.
Some other authors in Refs. [9, 26, 30, 35, 47, 55, 56, 57, 60] also concluded that it is impossible to successfully spin CS, unless another spinnable polymer was added. A droplet was formed at the tip of the needle while spinning pure CS. It elongated very slowly and the jet formed was not continuous. We observed spraying, an explosion-like behavior, instead of continuous jet flowing. This was a result of the electromagnetic field vibrations, generated between the needle and the collector. Such impulsive solution spraying produced nanobeads and very small droplets which reached drum collector by forming coating. The same behavior of pure CS during electrospinning process was observed by Pakravan et al. .
As we have not succeeded to electrospun nanofibers from pure CS, we continued electrospinning of pure PEO nanofibers. Later, we mixed pure CS and PEO solutions, and fabricated fine nanofibers from CS/PEO blends. The distribution of diameters of nanofibers produced along with confidential intervals is shown in Figure 6. The diameters of fibers produced from PEO solutions of different concentrations vary in nanoand micro-scales. The diameter of pure PEO depends on the polymer concentration of PEO – smaller concentration results in bigger diameter and wider distribution of values. The same dependence in nanofiber diameters can be found for CS/PEO blends, where 90% AA was used to dissolve CS. Quite opposite is with nanofibers from CS/PEO blends with 50% AA in the solution. The wider nanofibers were received while spinning CS/PEO solution with the highest amount of PEO. We should remember that viscosity depends on concentration, resulting from the intermolecular and intramolecular interactions (see Table 1) . Fibers cannot be form, when solutions are in gel-like state, because the polymer is not fully dissolved and the solvent cannot evaporate properly. And in the case of blend solutions, such gels prevent PEO from forming stable nanofibers. For pure PEO products, viscosity resulted in thinner fibers, but for CS/PEO blends contrary – more viscous solutions resulted in finer nanofibers. The same dependences were obtained: CS/PEO blends by Garcia et al  and pure PEO by Lemma .
It can be concluded from Figure 6 that fiber diameter is very sensitive to the AA concentration used for CS dissolution. Higher concentration of the AA resulted in thicker nanofibers, higher values of solutions viscosity, and smaller conductivities of solutions and fiber mats (see Tables 1 and 2). The same discovery was made by Kriegel et al. .
As the concentration of PEO polymer increased in the CS/PEO blend solution, where 90% AA was used, the bead shapes changed from spherical to spindle-like, unlike in the CS/PEO blend samples, where 50% AA was used – here increase in PEO concentration resulted in change of beads from spindle-like to the spherical ones. The bead formation is a result of solution viscosity, which is closely related with surface tension [61, 64, 65].
One would not have expected that nanofibers from pure PEO will be about 1.5 times bigger in diameter (and with wider spread of radius results) than nanofibers from CS/PEO blends.
It is seen from SEM images (see Figure 7a, 7d, and 7g) that the solvent is not clearly evaporated in the pure PEO specimens. The fibers are not highly oriented and they are stick to each other. The best orientation is obtained in fiber mat 10PEOinH2O: almost flat histogram was received for this sample, only a peak at approximately −800, representing a dominant direction of nanofiber (see Figure 7c). The goodness of the fit is quite good, 0.63. According to Refs. [40, 41], the flat histogram represents absolutely isotropic composition of sample structure. The distribution of orientation is also low (see Figure 7b). As it can be seen in Figure 7f and 7k, the directionality in porous scaffolds 15PEOinH2O and 20PEOinH2O is similar: approximately −900 and goodness is 0.2. But, distributions of orientation are different (see Figure 7e and 7h). The mesh-like structures are mostly formed in the PEO specimens. When concentration of PEO was increased till 20 wt%, some of the broken fibers and some of very fine fibers with different sizes of beads are seen (Figure 7g). Experiments have shown that it is impossible to spin beadless ultra-fine PEO nanofibers using conditions, described in this paper.
As the experiments have shown that pure 3 wt% CS is unspinnable and it is problematic to produce highly oriented ultra-fine PEO nanofibers without beads. For further investigations, pure CS was mixed with PEO solutions of different concentrations (see Table 2). The aim of this was to produce finely oriented nanoscale fibers with no beads.
It is seen from Figures 8 and 9 that combination of CS and PEO solutions resulted in manufacturing of straight and fine fibers with cylindrical morphology. However, it is clearly seen from SEM micrographs that solvent has not fully evaporated during spinning, that is, water has not evaporated during manufacturing process, and some of the nanofibers were bonded to each other at their contact sites. Beads in fiber mats are seen only when PEO concentration of 10 wt% and 15 wt% was used in blend solutions. When polymer concentration in the solution is low, usually beads are formed in fibers, due to the surface tension, or dripping of solution may occur, when electric field is created while spinning . When PEO concentration increased till 20 wt%, beadless fibers were produced, but their diameter is large when compared to fibers with lower concentration of PEO (see Figure 7). The directionality histograms of samples 3CSin90AA+10PEO and 3CSin90AA+20PEO show that fiber directionality is almost isotropic (see Figure 8c and 8k) with low distribution of orientation (see Figure 8b and 8h). The goodness of scaffold 3CSin90AA+20PEO is close to 1, which represents good fit. While a directionality histogram of nanofibers in fiber mat 3SCin90AA+15PEO has a peak at approximately −300, but the goodness is high and the distribution of orientation is quite low (see Figure 8e and 8f), but the coherency values close to zero denote no preferential orientation of the fibers. The histogram of nanofiber directionality of scaffold 3CSin50AA+10PEO has a peak at approximately 700, which do not allow to state that fibers have completely isotropic content, but the goodness value is close to unity, which shows good fit (see Figure 9b and 9c). The distribution of orientation is quite high with low value of coherency coefficient. Results obtained for 3Chin50AA+15PEO show that the distribution of orientation is low and directionality histogram represents more isotropic behavior of nanofibers, but with low goodness and coherency coefficient values (see Figure 9e and 9f). The straight and beads free nanofibers of 3Shin50AA+20PEO SEM photo (in Figure 9g) obtained quite high goodness value (approximately 0.6). But directionality histogram shows a dominant peak at approximately −20 and high distribution in orientation (see Figure 9h and 9k). But, overall SEM images of the composite fiber mats surface shown in Figures 8g and 9g prove that fibers have straight and quite uniform structures without any “bead-on-a-string” morphology. They have smooth surfaces, with no particles separating out from the fiber matrix. Nanofibers without defects mostly compose, when concentrations of solutions are high, because high viscosity solutions have cohesive nature .
The aim of this research was to investigate the electrospun CS, PEO, and CS/PEO solutions and fiber mats. The estimations of viscosity and electrical conductivity have shown that the concentration of polymers used in solutions influences conductivity values. In the case of pure CS, increase in solution viscosity resulted in decrease in conductivity, which was influenced by concentration of AA. Opposite results were obtained for pure PEO and CS/PEO solutions – higher viscosities gave lower electrical conductivity values. The same influence was observed with CS/PEO blend fiber mats. Conductivity of fiber mats is linearly dependent on conductivity of solution. The conductivity of pure PEO scaffolds was influenced by the solvent that is not evaporated. The DSC – thermal – tests showed that melting temperature of PEO was shifted toward a higher temperature when the PEO content was increased. The addition of CS into the fiber blend lowered the degree of crystallinity of blends. The presence of acid in the samples influenced the melting temperature and crystallinity as well. The composition analysis with FTIR has not shown any formation of extraneous compounds or side effects. The morphological (SEM images) analysis proved that it was impossible to spin pure 3 wt% CS using neither 90% AA nor 50% AA. Only nanobeads and droplets in the range of 2–3 μm (micrometres) were obtained while spinning pure CS. The processed pure PEO nanofibers were not highly oriented and stick to each other. The best orientation of nanofibers was obtained for 10PEOinH2O, where the values in directionality histogram showed almost isotropic character and the goodness was 0.63, representing good fit. Increase in PEO concentration resulted in broken fibers and some of the very fine fibers with different sizes of beads.
The introduction of PEO into CS solutions accelerated the quality of the nanofibers and decreased the amount of defects in them. The combination of CS/PEO blend solutions resulted in the manufacturing of straight and fine fibers with cylindrical morphology. Beads in fibers were seen only when PEO concentration of 10 wt% and 15 wt% was conducted in the blends. Fiber mats with 20 wt% of PEO had quite uniform and smooth structures without any “bead-on-a-string” morphology. This was also influenced by the viscosity and surface tension of solutions.
The directionality of nanofibers in all produced CS/PEO blend scaffolds has not shown completely isotropic character, but we obtained almost isotropic one for samples 3CHin90AA+20PEO, 3CHin90AA+10PEO, 3Chin50AA+10PEO, and 3Chin50AA+20PEO. The last one was revealed with high goodness value, 0.74. The smallest distributions of orientation were obtained in samples 3Chin90AA+10PEO and 3Chin50AA+15PEO.
The obtained porous scaffolds, especially CS/PEO samples with highest concentration of PEO, may be used as matrices for medical applications, that is, tissue engineering, wound dressing, or homeostatic material.
The authors would like to thank for the Support of the National Agency of the Republic of Estonia (Archimedes AS), agreement No. AM-EE-2014-LT-1309.
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