2.2. Mechanical properties of lignin/polycaprolactone nanofibers
Photos and scanning electron microscopy (SEM) images of the KLP, HILP, and HOLP NF mats are shown inFigure 2. Lignins, the main component of LP-TNEGs, retained their inherent coloration, and these colors revealed their identities when the lignins were combined with PCL (Figure S1b). The introduction of different lignins into the PCL resulted in a noticeable reduction in the brown color of the resulting NF mats (Figures 2a, 2b, and 2c). The degree of color change in the lignin/PCL NF mats was found to be correlated with the specific lignin powder employed, with KL, HIL, and HOL powders resulting in a lighter shade in order. In addition, discernible changes were observed in the morphology and mechanical properties of the lignin/PCL NF mats. The average diameter (Davg) of the lignin/PCL NF varied depending on the wettability of lignin, despite using a constant lignin/PCL ratio and the same electrospinning conditions. Compared to the pristine PCL NFs with Davg =103 nm (Figures S2a and Sb), the lignin/PCL NFs showed an increase in Davg for all lignin cases. Among the different lignin/PCL NF cases, the HOLP NFs exhibited a smallerDavg of 136 nm than those of KLP and HIL NFs, which had Davg of 218 and 273 nm, respectively (Figure 2d).
The wettability manipulation of lignin also had a significant influence on the mechanical properties of the resulting lignin/PCL NF mats.Figure 2e presents the results of the tensile tests conducted on the KLP, HILP, and HOLP NF mats. The values of Young’s modulus (E ) and yield strength (Y ) were obtained by using the elastic–plastic Green equation [σ = Ytanh( /Y ), where σ and ε are the tensile stress and strain measured experimentally].20 The values of E and Y for the pristine PCL NF mat were 6.16 ± 0.02 and 0.05 ± 0.02 MPa, respectively (Figure S2c ). The incorporation of KL, HIL, and HOL into PCL led to improvement in mechanical properties. The HILP NF mat exhibited the highest Evalue of 15.95 ± 4.32 MPa among all the NF mats, which is seemingly attributed to its highest value in D . On the other hand, theY values of the KLP, HILP, and HOLP NF mats were 2.66 ± 0.69, 2.93 ± 0.57, and 7.31 ± 0.12 MPa, respectively, which shows that blending higher M w lignins into PCL resulted in higher Y values (Table 1 and Figure 2e ). The higher M w lignins possess longer polymeric chains, which can enhance intermolecular interactions and entanglement inside of individual NFs.21, 22 Thus, these enhanced intermolecular behaviors improved the mechanical strength of the NF mats.