Nanocellulose, as a biobased versatile
nanomaterial that can be
derived with tailorable surface functionalities, dimensions, and morphologies,
has considerable implications for modifying the rheology, mechanical
reinforcement, and influencing the carbonization efficiency in the
production of polyacrylonitrile (PAN)-based carbon fibers. Herein,
we report the influence of three different nanocellulose types, varying
in the derivatization method, source, and aspect ratio, on the mechanical
properties and thermal transformations of solution-spun PAN/nanocellulose
nanocomposite fibers into carbon fibers. The incorporation of 0.1
wt % nanocellulose into solution-spun PAN fibers led to a 7–19%
increase in tensile modulus and 0–27% increase in tensile strength in the solution-spun
fibers, compared to a control PAN fiber. These improvements varied
depending on the nanocellulose type. After low-temperature carbonization
at 1200 °C, improvements in the mechanical properties of the
nanocellulose-reinforced carbon fibers, compared with a PAN fiber,
were also observed. In contrast to the precursor fibers, the improvement
% in the carbonized fibers was found to be dependent on the nanocellulose
morphology and was linearly correlated with increasing aspect ratio
of nanocellulose. For example, in carbon fibers with a cotton-derived
low-aspect-ratio cellulose nanocrystal and spinifex-derived high-aspect-ratio
CNC and nanofiber, up to 4, 87, and 172% improvements in tensile moduli
were observed, respectively. Due to the processing methods used, the
nanocellulose aspect ratio and crystallinity are inversely related,
and as such, the increase in the carbon fiber mechanical properties
was also related to a decrease in crystallinity of the nanocellulose
reinforcers. Raman spectra and electron microscopy analysis suggest
that mechanical improvement after carbonization is due to internal
reinforcement by highly ordered regions surrounding the carbonized
nanocellulose, within the turbostratic carbon fibers.