Lignin without purification was hard to spin into fibers due to
the existence of impurities and lignin-carbohydrate complexes.
However, continuously spooled lignin fibers can be easily prepared
after purification. Fig. 4(a) depicts the melt processed lignin fiber
spools and Lignin/PLA fiber spools. Despite their good spinnability
and continuous spinning, the obtained lignin fibers were brittle and
present poor mechanical properties. The addition of PLA can
improve the toughness of lignin fibers effectively [17], while the
thermal stability of Lignin/PLA blends was almost unchanged.
Detailed analysis of thermal stability of Lignin/PLA blends and the
tensile strength of Lignin/PLA fibers can be found in the
Supplementary information.
SEM was utilized to further study the effect of PLA on the
morphology of Lignin/PLA based carbon fibers. Fig. 4(b) and (c)
show the overall structures and amplified images of Lignin/PLA
based carbon fibers, respectively. All of the carbon fibers with the
diameter of 30e60 mm exhibited uniform cylindrical structures
independently of the composition of the blends. The surface of
lignin based carbon fibers was smooth, and some microvoids
caused by the volatilization of PLA could be seen on the surface of
Lignin/PLA based carbon fibers when 10 wt.% of PLA was incorporated.
With the increasing content of PLA, more microvoids
appeared on the surface of carbonized fibers. When PLA content
was up to 20 wt.%, the surface of Lignin/PLA based carbon fibers
became rough and the whole surface was filled up by microvoids.
Fig. 4(d) shows the cross-section images of Lignin/PLA based carbon
fibers. Due to the presence of a small amount of carbohydrates in
lignin, volatile gas was produced during the stabilization and
carbonization processes, leading to the formation of some voids in
lignin based carbon fibers. Besides, the amount and the size of voids
in Lignin/PLA based carbon fibers increased with the increasing
content of PLA, due to the coalescence of voids formed by the
volatilization of PLA.