2.3. Physicochemical properties of lignin/polycaprolactone nanofibers
To examine molecular structures, presence of ordered domains, and chemical features of KL, HIL, and HOL, their powders were investigated using 1H nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. Figure 3a shows the 1H NMR spectra observed from KL, HIL, and HOL powders. The characteristic peaks of KL observed at δ = 2.57, 3.41, and 3.87 ppm are attributed the protons of methylene and methoxy groups,23-27 and the broad peak between δ = 6.79 and 6.97 ppm represents the proton of the phenyl ring in KL.26, 28, 29 With the chemical treatment of KL to HIL, the new peak was detected at δ = 3.14 ppm, which corresponds to the proton of methanimine group with N-CH3bond.30, 31 In the spectra of HIL, the characteristics peaks associated with KL were found at δ = 3.87 and 6.79 to 6.97 ppm, indicating that the lignin backbone structure retained even after chemical modification. In addition, the prominent peak observed atδ = 4.70 ppm is attributed to the absorption of H2O, thereby indicating that the hydrophilic surface exhibits the capability to adsorb water molecules in air through the formation of hydrogen bonds with hydroxyl groups in HIL.32-34
Upon subjecting KL to hydrophobic chemical treatment, the characteristic peaks of KL were found at δ = 2.57, 3.84, and 6.79 to 6.97 ppm, confirming the preservation of the macromolecular lignin backbone. Furthermore, the resulting NMR spectrum of HOL revealed characteristic peaks associated with palmitic groups. The peaks corresponding to different functional groups within the palmitic structure, namely CH3 (methyl), aliphatic CH2 (methylene), −CH2−CH2−C(=O)−, and −CH2−(C=O)−, were observed at δ = 0.91, 1.27, 1.63–1.77, and 7.26 ppm, respectively.35, 36 The emergence of these new peaks, specifically those associated with the methyl (CH3) and methylene (CH2) groups, can be attributed to the increased hydrophobicity of HOL.37, 38
Chemical distinctions between KL, HIL, and HOL were also identified in the FTIR spectra (Figure S3a ). The FTIR band assignment obtained from KL were presented in Table S1 .11, 39-42 Chemical modification of KL induced a change in the chemical structures of both HIL and HOL. In the FTIR spectrum of HIL, the weak signal of band at 1708 cm‒1, which is observed in the KL spectrum, indicated the presence of unconjugated carbonyl groups after KL modification.43On the other hand, in the FTIR spectrum of HOL, the absence of broad band from 3600 to 3000 cm‒1 was attributed to substitute palmitic groups (C15H31) for O–H group. The prominent emergence of band at 2915 cm‒1 and strong signal of band at 2848 cm‒1 in the HOL spectrum, corresponding to the asymmetric and symmetric C–H stretching vibrations of aliphatic methylene (–CH2–),44-46 signify the transformation of kraft lignin into hydrophobic lignin. Furthermore, the strong signal of band at 1708 cm‒1 was ascribed to the presence of C=O bond as substituting palmitic groups (C15H31) for O–H group.
The XRD patterns revealed significant variations in the phase characteristics of lignin due to hydrophilic and hydrophobic treatments. As presented in Figure S3b , two broad peaks corresponding to the ordered domain and amorphous region of biomass were observed in the XRD spectra of KL at 2θ = 22.2° and 40°, respectively.44 With the functionalization of KL to HIL, the position of the peak associated with the ordered domain of lignin shifted from its original position at 2θ = 22.2° to a lower angle of 2θ = 19.1°. This shift was attributed to the presence of a uniformly tensioned ordered domain within the lignin structure, achieved through the attachment of trimethylamine groups to the lignin framework.47, 48 However, the XRD pattern of HOL exhibited significant changes compared to those of KL and HIL. These changes were attributed to the incorporation of the palmitic group in the KL molecule. The peaks observed in the XRD pattern of HOL at 2θ = 21.3° and 23.9° were identified as the presence of fatty acid crystals corresponding to the palmitic groups.49, 50 In addition, the HOL peak associated with the ordered domain of lignin shifted toward 2θ = 20.3° owing to the uniform tensile stress exerted on the ordered domain. This resulted in the attachment of the palmitic groups onto the lignin framework.
The M w of KL, HIL, and HOL were determined using gel permeation chromatography (GPC). An alternative approach combining elemental analysis and degree of substitution was employed to analyze the M w of HIL, which was insoluble in the GPC mobile phase. The results presented in Table 1 reveal that KL, HIL, and HOL had M w values of 4.826, 6.654, and 13.430 g mol-1, respectively. In comparison toM w of KL, the mass percentages of HIL and HOL significantly increased by 37.8% and 178.3%, respectively, which was attributed to the presence of functional groups in HIL and HOL.
The physicochemical difference depending on lignins was also evident in the lignin/PCL NF mats. Figures 3b and 3c present the FTIR spectra and XRD patterns of the pristine PCL, KLP, HILP, and HOPL NF mats. The FTIR spectra of all NF mats exhibited the characteristic bands of the PCL NF mat at 2947, 2866, and 1720 cm‒1, corresponding to asymmetric elongation of the methylene-oxygen (CH2‒O), symmetric methylene groups (CH2‒), and vibration of –C=O bonds, respectively.51, 52 This indicates the presence of PCL within all NF mats. The FTIR spectra of the KLP and HILP NF mats revealed the same bands observed from KL and HIL powders; O–H stretching (3600~3000 cm‒1) and aromatic C=C (1600 and 1515 cm‒1). However, discernible changes in the FTIR spectrum of the HOLP NF mats were observed, i.e. , the elimination and the reduction of the characteristic bands of O–H and aromatic C=C. Meanwhile, new bands of C–H bonds (2915 and 2848 cm‒1) and C=O ester (1708 cm‒1), which are related to the hydrophobic feature and observed from HOL powder, were also observed in the FTIR spectrum of HOLP NF mat (Figure 3b ).
Contrary to the analyses above, no discernible difference between the NF mats was observed in the XRD result (Figure 3c ). For the pristine PCL NF mat, the XRD pattern exhibited two intense peaks at 2θ = 21.4° and 23.6°, indicative of the (110) and (200) PCL lattice phases, respectively.53 A broad peak in the range of 10–20° attributed to the semicrystalline phase was also observed. The dominant (110) and (200) lattice planes in the PCL NF mat were also observed in both the KLP and HILP NF mats. In case of the HOLP NF mat, a peak at 2θ = 23.9°, which corresponds to the presence of fatty acid crystal, was also found.
The thermal stability and degradation behavior of the HILP and HOLP NF mats, compared to the KLP NF mat, were evaluated through thermogravimetric analysis (TGA). Figure 3d shows the TG curves of the specimens. The thermal degradation process of the HILP NF mat was observed to occur in four stages, whereas that of the KLP and HOLP NF mats occurred in two stages. The two thermal degradations observed in the KLP and HOLP NF mats correspond to the decomposition of phenolic groups in the lignin (150–320 °C) and the decomposition of PCL and lignin (320–500 °C), respectively.54, 55 A significant decomposition occurred around 250 °C of the HOLP NF mat is attributed to the thermal degradation of palmitic groups. The first thermal degradation of the HILP NF mat between 30 and 100 °C, presenting a weight loss of 2%, resulted from the loss of water initially absorbed from ambient moisture because of the hydrophilicity of HILP. This result agrees well with the 1H NMR and FTIR analyses. The second degradation occurred between 130 and 300 °C, corresponding to a weight loss of 12%, is attributed to the decomposition of the cationic group substituted from the hydroxyl group in KL. The third degradation observed between 300 and 350 °C ascribed to the cleavage of inter-unit linkages of lignin.56, 57 The fourth degradation between 350 and 500 °C corresponds to the comprehensive decomposition of PCL and lignin.
The modified functional groups in HIL and HOL resulted in changes in the wettability of the corresponding NF mats (Figure 3e ). The water contact angle (WCA) of the PCL NF mat was 134°, predominantly because of the hydrophobic properties of the C–H bonds in the methylene group. The introduction of KL into PCL led to a reduction in WCA to 108°. The hydrophilic O–H groups from KL is responsible for this wettability change. The HILP NF mat, where the C–H bonds of the methylene and methoxy groups were eliminated through the chemical treatment, exhibited a significant further decrease in WCA to 28°. The HOLP NF mat showed the highest WCA of 138°. The additional C–H bonds present in HOLP contributed to its enhancement in hydrophobic nature.
The modified functional groups resulting from the chemical treatment of KL to HIL and HOL conversely influenced the electrostatic potential states of the resulting NF mats. This was evaluated through surface potential (φ ) measurements (Figure 3f ). For HILP NF mat,φ was measured to be 248 mV, indicating the presence of a positive charge on its surface. The positive charge is attributed to the cationic site of the quaternary ammonium substituents (–N+(CH3)3) introduced during the modification process to transform KL into HIL.58, 59 On the other hand, the HOLP NF mat exhibited φ of 82 mV, implying the degree of negative surface charge is higher than the HILP NF mat. This is due to the protonation of the carboxyl group in HOLP.59, 60 The presence of positive or negative surface charges can affect the charge transfer and accumulation during the triboelectric process. Indeed, the chemical treatments of KL to HIL and HOL significantly impacted the energy-harvesting performance of the resulting NF mats, which is further discussed in the forthcoming section.