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.