Materials

PALF were received from a local Malaysia company in fibers form. Polyethylene terephthalate (PET) resin was obtained from Sigma-Aldrich and received in granular form with an average molecular weight (M_v) of 18000. The chemicals were supplied by Aldrich Merck Company. Purity of Trifluoroacetic Acid (TFA) is 99%, dichloromethane (DCM) is 99,8 %, cyclohexane is 99 % and ethanol is 99,8 %.

Dewaxing by soxhlet method

PALF were dewaxed using soxhlet apparatus (Buchi, E-812, Flavil, Switzerland) to remove wax and terpenes. This procedure was done to improve fibers' surface for better interface interaction. PALF were rinsed with 200 ml of ethanol:cyclohexane at 1:1 ratio for 6 hours. The samples were rinsed using distilled water and oven dried (Memmert, BM 500, Schwabach, Germany) at 40 °C for 12 hours.

Dissolution of PALF and PET

Dried PALF and PET were weighed accordingly as tabulated in Table 1. Five different concentrations were prepared for the electrospinning process. All samples were then diluted in the mixture of 5 mL and 6,5 mL of trifluoroacetic acid (TFA) and dichloromethane (DCM) solution respectively. The mixture was stirred for 6 hours until homogenous solution was obtained.

Table 1: Weight of PALF and PET for different ratio. 

Electrospinning

Figure 1 illustrates the electrospinning machine set up in this study. As sufficient high voltage was supplied from the power supply, the electric charge will move into the solution through a metallic needle. High charge in solution causes solution instability thus forces the solution to move to different charge emitted from the metallic collector. Electrospinning parameters were set up at 12 kV~15 kV of voltage 0,2 ml/min flow rate and 10 cm distance (Santos et al. 2019).

Figure 1: Electrospinning machine set up.  

Characterization

Wettability testing

The wettability of the samples was tested using contact angle analyzer (goniometer AST/VCA -3000 s). The samples were cut into 2 cm x 2 cm and placed on the test surface and a camera was used to capture the images. 3 microliters of purified water were gently dropped on the sample surface. Three replicates of contact angle were taken at different positions of the sample. The image of the water droplet was captured within each 5 s of delivery until 40 s (^{Goetz et al. 2016}).

In the case of complete wetting, the contact angle is 0?. Between 0? and 90?, the solid is wettable and above 90? is not wettable. In the case of superhydrophobic materials with the so-called lotus effect, the contact angle approaches the theoretical limit of 180?.

Water absorption

Water absorption (WA) testing was used to determine the moisture absorption of nanofibers. Increasing in water absorption without disrupting the physical bond will produce fibers with great absorbing ability.

WA test was conducted according to ^{ASTM D570-98 (2018)}. Two replicates of each ratio were soaked into distilled water for 24 hours. Final weight of the samples was recorded. The percentages were calculated using Equation 1.

(1)

Wm: weight of sample after soak

Wo: initial weight

X-Ray diffraction (XRD)

Nanofiber electrospun mats were cut into several mats of 3 cm x 3 cm of rectangular shape and carefully placed in the sample slot. The XRD patterns for the samples were recorded at room temperature from 2θ = 50 to 800 (degrees) by X-ray diffractometer (PANalytical X’Pert PRO) using CuKa radiation (λ=1,5418Å). The XRD measurements were performed at a scanning speed of 2,50 degrees per minute. The results were drafted using ^{Origin Pro 8.1 (2018)} Origin software and the crystallite size and the percentage of crystallinity were determined using Equation 2 and Equation 3 respectively.

(2)

Crystallite size,

Where:

d = Crystallite size

K = Scherrer constant (0,94)

λ = X-ray wavelength (0,1542nm)

β= width of the peak (FWHM)

θ = Bragg’s angle

Crystallinity:

(3)

Thermo gravimetric analysis (TGA)

TGA analysis were carried out using thermogravimetric analyser (Netzch Tg 200 F3 Tarsus). The samples were then heated from room temperature to 800 °C at a heating rate of 10 °C/min under nitrogen gas. The crucible used was an aluminium pan. The results were analysed using NATZCH software to obtain degradation temperature, dTG graph, residual mass and percentage mass loss.

Differential scanning calorimetry (DSC)

DSC analysis was carried out using Differential Scanning Calorimetry analyser (Netzch Dsc 200 F3 Tarsus). The samples about 5 mg were heated from room temperature to 800 °C at a heating rate of 10 °C/min under nitrogen gas. The crucible used was a closed lid aluminium pan. The obtained results were analysed using NATZCH software to determine the glass transition temperature, melting temperature and crystalline temperature.

Wetting properties of PALF/PET electrospun nanofibers

Wetting properties of nanofibers is one of the most important properties for surface interaction. Contact angle (CA) measurement was carried out to study the wetting properties of electrospun PALF/PET mat. The CA values of electrospun nanofibers at different PALF:PET ratios are tabulated in Table 2. From the table, pure PET displayed the highest CA which is 148,17°. The CA values decreased with increasing ratio of PALF. At the ratio of 1/10 (PALF/PET), the CA value of the mat was recorded at 117,25° and drastically dropped to the lowest CA of 27,76° at the ratio of 1,3/1. The reduction of surface CA was due to the existence of polar chemical groups on the PET surface after the addition of PALF. As a result, the surface energy of the samples was enhanced and led to lower surface CA.

Table 2: Average Water Contact Angle (WCA) values after 1 second water drop. 

Figure 2 displays the change of CA over time (s) on pure PET and PALF/PET electrospun nanofibers at different PALF/PET ratios. As can be seen from the figure, the CA of the pure PET and mats at 1/10 remained static even after several seconds, indicating that the surfaces of both materials are not absorbing any water molecule or hydrophobic. Meanwhile, samples with ratio 1/7,5; 1/5; 1/1 and 1,3/1 showed decreases in CA values at different rates as the time taken for the CA values to achieve 0° differed between samples. As the ratio of PALF increased, the time needed for the water droplet to reach 0° is shortened. At a PALF/PET ratio of 1,3/1, it only required 10 s for the water droplet to be fully absorbed by the nanofibers.

By increasing the PALF content, the CA values show a decreasing trend indicating that PALF addition changed the properties of the nanofibers from hydrophobic to be more hydrophilic. Water drop becomes more easily absorbed through the decreasing in fiber size and due to the high O-H bonding that can react with water (^{Hekmati et al. 2014}). Synthetic polymers with hydrophobic nature of the electrospun materials have limited its application and efficiency (^{Goetz et al. 2016}). To overcome this problem, a strategy of blending together both natural and synthetic polymers was adopted. Hydrophobic properties of PET were modified to become more water absorbent due to the presence of PALF cellulosic materials. This result was supported by Goetz et al. (2016) that studied on the potential of chitosan as natural fibers to become super hydrophilic materials for water filtration.

Figure 2: Graph of Contact angle and Time adsorb.  

Water absorption (WA)

From the application point of view, it is important to know the behaviour of the PALF/PET electrospun in water. For this purpose, PALF/PET samples were soaked in water for 24 hours and its water absorption were observed. Figure 3 shows the results of the water uptake percentage for PALF/PET sample at different ratios. Pure PET shows the lowest percentage of water uptake which is 23,4 %. It is probably due to its fiber size and hydrophobicity property. PALF has been known as a highly hygroscopic material due to its high cellulose content (^{Aji et al. 2013}). Therefore, addition of PALF undoubtedly increased the water uptake of the electrospun nanofibers. From Figure 3, it can be seen that the water absorption increased along with increasing PALF ratio. It is interestingly to note that a significant increment in water absorption was observed in sample with ratio of 1/5 compared to that of sample with ratio of 1/7,5. The increment in water absorption was more than two-folds (72 % in sample with ratio of 1/7,5 and 162 % in samples with ratio of 1/5). This is probably due to the decreases in the nanofiber diameter which is from 120 nm to 46 nm as shown in Table 3. Table 3 shows the average diameter of nanofibers. The SEM micrographs in Figure 4 shows the image of nanofibers mat at 20 k magnification for ratios of 1/7,5 and 1/5. Small diameter of fiber also led to an increase in pore size of nanofibers thus allowing a higher amount of water to be adsorbed (^{Hekmati et al. 2014}). Sample ratio 1/10 and sample ratio 1/7,5 shows bigger diameter, due to the different solubility and the dielectric constant of the PET and PALF (^{Jung et al. 2007}). Increasing in PALF ratio had narrowed the fiber diameter distribution.

As expected, the percentage of water uptake increases steadily as the ratio of PALF increases until it reaches the saturated level. Further increasing the PALF ratio did not resulted in much difference on the water absorption percentage. This is possibly due to it having reached the maximum absorption capacity. These findings proved that degradation of PALF, which can lead to dissolution, had not occurred during this time period and under these conditions.

Figure 3: Percentage of water uptake of electrospun nanofibers mat made from different PALF/PET ratios. 

Figure 4: FESEM micrographs of PALF/PET nanofibers (a) ratio 1/5 (b) ratio 1/7,5.  

Table 3: Diameter size of electrospun nanofibers. 

X-Ray Diffraction (XRD)

Figure 5 shows the XRD result obtained for each sample. The diffraction peaks for Pure PET were observed at 2θ = 16° and 2θ = 23° in which the peak at 23° is moderately sharp and the other peak is broader. The peaks observed were in agreement with the previous study by ^{Mahar et al. (2017}) who suggested that the observed patterns were similar to that of semicrystalline structure of PET. Sharp peak is usually attributed to crystalline structure while broader peak is attributed to amorphous structure (Mahar et al. 2017). According to ^{Mahardika et al. (2018}), PALF also exists with semi crystalline structure with broad peak at 2θ = 18° and sharp peak at 2θ = 22,6°.

An Increase in PALF ratio did not change the peak position angle, indicating that the sample still exhibits the same structure as pure PET and pure PALF. There are no new peaks observed for any sample. However, the peak at 23° for samples with ratio 1/1 and ratio 1,3/1 has higher intensity compared to that of pure PET peak. According to ^{Karuppuchamy and Jeong (2005}), hydrophilic materials with high crystallinity produced materials that come with long durability and good absorbing abilities. These properties are said to be beneficial to applications such as biomedical and water treatment (^{Alharbi et al. 2016}).

Figure 5: XRD diffactograms for PET and PALF/PET electrospun nanofibers. 

Thermogravimetric analysis (TGA)

The TGA or thermal profile testing is aimed to uncover the thermal stability of the materials. Figure 6 shows TGA graph for pure PET electrospun and PALF/PET electrospun samples with different ratios. Pure PET sample shows single step degradation which occurred at temperature of 369,7 °C. This was in agreement with ^{Mahar et al. (2017}) who mentioned that degradation of PET electrospun usually occurred at temperature between 350 °C to 400 °C.

In addition, one step degradation proved that water is absent in PET which usually caused slope degradation at temperature below 100 °C. However, the degradation peak increased along with PALF addition. As can be seen from Figure 6, nanofibers with addition of PALF exhibit additional stages of decomposition due to the presence of PALF. Due to the hygroscopicity of the PALF, an initial weight loss could be observed in the samples with different PALF ratios. The weight loss was corresponding to the evaporation of water and volatile organic compounds in PALF (^{Lee et al. 2018}).

Figure 6: TGA of electrospun nanofibers mat made from different PALF/PET ratios.  

Increasing in PALF ratio showed new peak degradation with increasing mass change at temperature 260 °C - 320 °C (cellulose degradation) and at temperature 360 °C - 400 °C (lignin degradation). The data obtained from the graph were tabulated in Table 4. Sample with PALF shows initial degradation temperature (IDT) which is approximately around 200 ºC - 280°C compared to pure PET at 369,7 °C due to the presence of hemicellulose and cellulose that has lower thermal resistance (^{Sridhar et. al. 2015}). This is supported by ^{Neto et al. (2013}) who indicated that cellulose degradation temperature occurred at around ~260 °C. According to ^{Adam et al. (2018}), cellulose content together with hemicellulose will play a profound effect on fiber strength and self resinification of fiber during exposure to high temperature and pressure.

Addition of PALF in PET does not really affect much on PET degradation temperature which is around 370 ºC - 390 °C. A peak which is at high temperature at 480 ºC - 510°C represents the lignin degradation where lignin with strong hydrogen bond is well known for its high thermal stability up to 600 °C (^{Dorez et al. 2013}). This peak was absent for the pure PET sample and sample ratio 1/10 due to lower amount of PALF in the sample.

Table 4: Data Summarize of TGA for PET and PALF/PET electrospun nanofibers.  

Differential scanning calorimetry (DSC)

Differential scanning calorimetry, DSC analyses were carried out. Through this analysis, the glass transition temperature, melting temperature and crystalline temperature were determined. The thermal profiles of the samples were shown in Figure 7. Data obtained from the DSC were summarized in Table 5. According to ^{Nadirah et al. (2011)}, PALF exhibits T_m = 290 °C but does not produce any peak for T_g and T_c , while PET shows T_g at around 58°C and two melting point at 221 °C and 253 °C respectively.

Glass transition temperature, T_g occurred at temperature range 55 °C - 57 °C for all the samples except for ratio 1/1 and ratio 1,3/1 where T_g value shifted to higher temperature at 63 °C and 59 °C respectively. Increasing T_g values indicate that the sample becomes more crystalline in its structure. This result was supported by XRD results that show sample ratio 1/1 and 1,3/1 exhibit high crystallinity compared to pure PET.

Figure 7: DSC Thermogram of electrospun nanofibers mat made from different PALF/PET ratios.  

Two melting temperatures were shown for pure PET which is at 221 °C and 253 °C respectively. Two different melting point is associated with different melting of the lamellae layer (^{Kong et al. 2003}). From the graph, the melting temperature for PET was undetected for sample ratio 1,3/1 due to the insufficient quantity of PET. This further highlights the presence of PALF as represented by its melting point, T_m at 196,2 °C.

Table 5: Summarized data of DSC for PET and PALF/PET electrospun nanofibers.