Blend electrospinning of citronella or thyme oil-loaded polyurethane nanofibers and evaluating their release behaviors

2.1 Materials

In this study, thermoplastic TPU (Mw: 107.010 g·mol⁻¹, Elastollan C59D, BASF) was used as the polymeric component for nanofiber production. Steam distilled CEO (Monoville) and cold pressed TEO (Tijda) were used as the essential oils. N,N-dimethylformamide (DMF) (98%, Sigma-Aldrich Co.) was used as the solvent for TPU solution preparation.

2.2 Methods

This study consists of three steps as presented in Figure 1.

Figure 1

Experimental procedure followed in the study.

3.1 Solution properties

Table 2 shows the properties of the pure CEO, TEO, and TPU solutions with different essential oil concentrations. Viscosity is a critical parameter in the electrospinning process for the bead-free production of uniform and continuous fibers. With the addition of 5% CEO into the TPU solution, the viscosity increased from ⁓518 to ⁓806 cP. Increasing the CEO concentration to 10% decreased the viscosity to ⁓585 cP, while 15% CEO addition resulted in a viscosity of ⁓848 cP. For TEO-loaded solutions, 5% TEO addition decreased the solution viscosity to ⁓377 cP, while the viscosities of 10% and 15% TEO-loaded solutions increased to ⁓389 and ⁓524 cP, respectively. These viscosity fluctuations can be related to various effects including the increasing amount of substance in the unit area, molecular interactions between the oil and the polymer, or plasticization effects of oils (37). The basic nature of the essential oils decreased the pH values, while essential oil addition generally increased the surface tension compared to pure TPU solution.

3.2 SEM analyses

Initial step of the present study is to optimize blend electrospinning parameters to produce decent TPU, TPU/CEO, and TPU/TEO nanofibers.

Electrospinning is a complex process that is affected by many parameters including viscosity of the polymer solution and process parameters such as voltage, tip-to-collector distance, flow rate, etc. In order to produce thin and bead-free nanofibers, all these parameters should be optimum. For instance, although increasing voltage is known to have a thinning effect on nanofiber diameter, above a certain value, it may reversely affect the fiber morphology (38). Therefore, the resultant nanofiber morphology and diameter should be investigated by taking all the parameters into account.

In this study, the effects of essential oil concentration, tip-to-collector distance, and voltage on the nanofiber morphology were investigated. The resultant fiber morphologies were examined by SEM analyses, as presented in Figures 2–4.

Figure 2

SEM images of the pure TPU nanofibers produced at different electrospinning parameters.

Figure 3

SEM images of the TPU/CEO nanofibers produced at different CEO concentrations and electrospinning parameters.

Figure 4

SEM images of the TPU/TEO nanofibers produced at different CEO concentrations and electrospinning parameters.

Figure 2 shows SEM images of nanofiber mats produced at different tip-to-collector distance and voltage values from pure TPU solutions. As can be seen, beaded nanofibers – resulting from its low viscosity – were obtained with diameters ranging from 175.80 to 284.73 nm depending on the distance and voltage.

One can see that the beaded structure was prominent particularly at lower voltage values. The increase in the voltage resulted in less beaded nanofibers with more uniform diameters. However, the increase in tip-to-collector distance and voltage led to increase in fiber diameters. This unexpected behavior may be explained by the low viscous nature of the polymer solution, leading to poor jet stability and insufficient jet stretching during electrospinning (39).

For CEO-loaded nanofibers, although the fiber diameters decreased with the increase in CEO concentration, all samples showed thicker and more uniform nanofibers compared to pure TPU (Figure 3). With the addition of 5% CEO, the viscosity increased and fibers with higher diameters ranging from ⁓335 to 423 nm were produced (Table 2 and Figure 3). With the increase in CEO concentration (10% and 15%), the nanofiber diameters decreased. Although 15% CEO-loaded nanofibers were electrospun from solutions with the highest viscosity (Table 2), they exhibited the lowest nanofiber diameters (down to ⁓195 nm) with the smoothest morphology depending on the process parameters.

It can be seen that the increase in distance led to decrease in nanofiber diameters for 5% and 10% CEO-loaded nanofibers, as expected. On the other hand, nanofiber diameters were increased for 15% CEO-loaded nanofibers with the increase in distance. The high solution viscosity might limit the polymer jet stretching in the electrical field during electrospinning, resulting in nanofibers with thicker diameters (40). The increase in voltage led to different effects depending on the tip-to-collector distance. At 15 cm distance, increasing the voltage from 15 to 20 kV resulted in increased nanofiber diameters for 5% and 10% CEO-loaded samples. This might occur due to the limited time given to the polymer jet during electrospinning at 15 cm. When tip-to-collector distance was increased to 20 cm, all CEO-loaded samples showed lower diameters since optimum stretching and enough evaporation time was given to the polymer jet, regardless of the viscosity (41).

For 5% and 10% TEO-loaded nanofibers, generally beaded nanofibers were obtained (Figure 4). This is expected, since TEO addition decreased the solution viscosities (Table 2). For 15% TEO-loaded nanofibers, generally thicker nanofibers were obtained, compatible with the solution’s higher viscosity (Table 2). Moreover, the overall surface characteristics were improved with 15% TEO addition and the least bead formation was observed for this sample.

The alterations in tip-to-collector distance and voltage also affected the morphology of TEO-loaded samples. For instance, the increase in the distance resulted in decreased diameters for 5% and 10% TEO-loaded samples. However, when the voltage was kept constant at 20 kV, the increase in the distance from 15 to 20 cm led to thickened but more uniform fibers.

Same trend was observed with the increase in voltage. When the distance was kept constant at 20 cm, increasing the voltage from 15 to 20 kV resulted in more uniform nanofibers with thicker diameters, except for 5% TEO. These results could be also explained by the complex relationship among viscosity, distance, and voltage, as explained above.

Considering all these results, SEM analyses revealed that nanofibers (TPU10/CEO15:D20_V20 and TPU10/TEO15:D20_V20) electrospun from TPU solutions containing 15% essential oil with a distance of 20 cm and voltage of 20 kV were optimum parameters for an acceptable nanofiber production.

3.3 Wettability and porosity

Wettability of a material is the interaction between the water and the material surface. It plays crucial role depending on the application area. Contact angle test is one of the most widely used method to quantify the wettability of the material.

The key factors affecting contact angle measurements are the sample’s chemical composition, structural properties such as surface roughness, surface porosity, and crystallinity, as well as heterogeneities and other minor influences (42–47). In general, the contact angles <90° are considered as hydrophilic, whereas the contact angles >90° indicate hydrophobicity (48).

In this study, we examined the contact angles and bulk porosities of the three selected samples as presented in Table 3. One can see that pure TPU sample had a contact angle of 102°, indicating its hydrophobic structure. CEO and TEO addition into nanofiber structure resulted in higher contact angle values pointing out the increased hydrophobicity. The increased hydrophobicity can be attributed to the hydrophobic nature of both CEO and TEO, which results from the presence of terpenes and terpenoids in CEO and thymol, carvacrol, and other phenolic compounds in TEO.

Table 3

Structural properties and contact angle values of the samples

Sample	ρ p (g·cm−3)	Porosity (%)	Contact angle (°)
TPU10/D20_V20	1.19	76.07 ± 1.73	102.00 ± 0.02
TPU10/CEO15:D20_V20	0.98	64.08 ± 4.19	125.00 ± 1.74
TPU10/TEO15:D20_V20	1.02	58.02 ± 3.91	121.00 ± 0.60

It should be kept in mind that porosity is one of the most essential parameters of an electrospun mat affecting the contact angles (49,50). Pure TPU showed a porosity around 76.07%. The addition of essential oils into the nanofiber structure resulted in lower porosities and CEO and TEO containing samples exhibited porosities of 64.08% and 58.02%, respectively (Table 3). In literature it is stated that an increase in fiber diameter results in a decrease the porosity (51–53). For, TEO containing sample, the decrease in the porosity can be explained by the thickened nanofibers compatible with the literature. On the other hand, the lower porosity of CEO containing sample despite its decreased diameter can be explained by its large diameter distribution.

The higher contact angle values can be also associated with the lower porosities of these samples along with the hydrophobic character of the essential oils. The lower porosities of essential oil-containing samples can prevent water from spreading into the material, further contributing to the increased contact angle values.

3.4 FTIR analyses

To investigate the presence of essential oil, and the interaction between the polymer matrix and the essential oil, FTIR analyses were performed. Figure 5 shows the FTIR spectra of pure TPU nanofiber mat, pure CEO, and CEO containing TPU nanofiber mat, respectively.

Figure 5

FTIR spectra of TPU, CEO, and TPU/CEO nanofiber mat.

For pure TPU, peaks were observed at 3,327, 2,953, 2,872, 1,701, 1,527, 1,064, and 815 cm⁻¹. In the spectrum, the peak at 3,327 cm⁻¹ corresponds to N‒H stretching vibration within the characteristic urethane group of the TPU polymer (54). The peaks at 2,953, 2,872, 1,527, and 815 cm⁻¹ are attributed to the asymmetric stretching of CH₂, symmetric stretching of CH ₃ , urethane amide II band, and bending vibration in benzene ring, respectively (55). Another characteristic peak was observed at 1,701 cm⁻¹ which corresponds to C═O stretching in the urethane carbonyl group (hard segment of TPU), while the peak at 1,060 cm⁻¹ is assigned to C‒O stretching in the alcohol group (56).

CEO contains a diverse range of terpenes mainly citronellal, citronellol, and geraniol. The observed peaks in its spectrum correspond to the chemical functional groups of these terpenes (57). The broad peak at 3,600–3,300 cm⁻¹ is associated with O–H stretching vibration, primarily due to the presence of primary alcohols such as citronellol and geraniol. These primary alcohols can engage in intermolecular hydrogen bonding, leading to an increase in O–H bond length (58). The absorption band observed in the 3,000–2,800 cm⁻¹ range corresponds to C–H stretching (59), while the peaks at 2,727 and 1,726 are associated with H−C terminal aldehydic stretching and C═O stretching of aldehyde, respectively (58). Other characteristic peaks of CEO appearing at 1,645, 1,377, and 1,014 cm⁻¹, correspond to O–H bending, C–O–H group deformation, and C–O stretching, respectively (57).

For the TPU/CEO nanofiber mat, distinctive peaks were observed at 3,327, 2,956, 1,701, and 1,074 cm⁻¹. In the spectrum, the characteristic peaks of TPU appeared more dominantly, likely due to peak overlapping and the relatively small amount of CEO in the structure. However, shifts in peak intensities and locations indicate the presence of CEO within the matrix. The broad O–H stretching peak of CEO disappeared, while the N–H stretching peak of TPU at 3,327 cm⁻¹ intensified. Additionally, the C–H stretching peak of TPU at 2,953 cm⁻¹ shifted to 2,956 cm⁻¹, while the C═O stretching peak at 1,701 cm⁻¹ remained unchanged. The C–O stretching peak at 1,064 cm⁻¹ shifted to 1,074 cm⁻¹. Furthermore, the intensities of these peaks increased, supporting the successful incorporation of CEO into the TPU matrix.

Figure 6 shows the FTIR spectra and functional groups of pure TPU nanofiber mat, pure TEO, and TEO containing TPU nanofiber mat, respectively. TEO is rich in monoterpenes and phenolic compounds, mainly thymol, cymene, and carvacrol (60–62). In its spectrum, the broad peak appearing at ∼3,600–3,200 cm⁻¹ is associated with the hydroxyl (–OH) stretching vibrations from thymol and carvacrol. The peak at 2,958 cm⁻¹ corresponds to the stretching vibration due to the aliphatic bonds of C–H₂. The peaks of C═C bonds of thymol and carvacrol and the presence of an aromatic ring in the cymene were observed at 1,589 and 1,458 cm⁻¹, respectively. The peaks at 1,381 and 1,361 cm⁻¹ can be attributed to the asymmetric and symmetric bending vibrations of isopropyl and methyl groups. Finally, the peak at 810 cm⁻¹ can be associated with the overlapping of thymol and cymene bands which corresponds to the out-of-plane CH wagging vibrations (60–62).

Figure 6

FTIR spectra of TPU, TEO, and TPU/TEO nanofiber mat.

For TPU/TEO nanofiber mat, the characteristic peaks of both TPU and TEO were observed. Moreover, there have been slight shifts in some peak locations and peak intensities which might be resulted from the interactions between TPU and TEO. In the spectrum, a broad absorption band in the range of 3,600–3,200 cm⁻¹ and a peak at 3,323 cm⁻¹ were seen due to the TEO and TPU presence. It can be seen that TPU peak at 1,701 cm⁻¹ remained, while the peak at 1,589 cm⁻¹ from TEO shifted to 1,595 cm⁻¹. These results suggest the successful integration of TEO into TPU matrix.

3.5 Release behavior

To investigate the CEO and TEO release from TPU nanofiber mats, TGA analyses were performed, and the cumulative release values are presented in Figure 7. An initial burst release was observed within the first hour for both samples, followed by a decrease in the release rate. For TPU/CEO, approximately 1% of the total release occurred within the first hour, and around 5% of CEO was released by the end of 18 h. In the case of TPU/TEO nanofiber mats, TEO release exceeded 1% within the first hour, and the total release of TEO reached approximately 10% at the end of 18 h.

Figure 7

Release behavior of TPU/CEO and TPU/TEO nanofiber mats.

The total release at the end of 18 h for both samples was relatively low, which may be attributed to the interactions between the essential oils and TPU, as shown in FTIR. Additionally, the lower release of CEO might be due to stronger interactions between CEO and TPU compared to TEO.

The release behavior is also influenced by the physical properties of the nanofiber mats, such as fiber diameter, porosity, and pore size. As shown in Figure 4, the TPU/CEO sample had an average fiber diameter of 274 nm and a porosity of 70%, whereas TPU/TEO had a larger diameter (∼353 nm) and lower porosity (∼64%). These structural differences may have influenced the release behavior. As a result, the higher nanofiber diameter and lower porosity of TPU/TEO may have contributed to TEO’s faster diffusion.