Close Packing of Cellulose and Chitosan in Regenerated Cellulose Fibers Improves Carbon Yield and Structural Properties of Respective Carbon Fibers

A low carbon yield is a major limitation for the use of cellulose-based filaments as carbon fiber precursors. The present study aims to investigate the use of an abundant biopolymer chitosan as a natural charring agent particularly on enhancing the carbon yield of the cellulose-derived carbon fiber. The ionic liquid 1,5-diazabicyclo[4.3.0]non-5-enium acetate ([DBNH]OAc) was used for direct dissolution of cellulose and chitosan and to spin cellulose–chitosan composite fibers through a dry-jet wet spinning process (Ioncell). The homogenous distribution and tight packing of cellulose and chitosan revealed by X-ray scattering experiments enable a synergistic interaction between the two polymers during the pyrolysis reaction, resulting in a substantial increase of the carbon yield and preservation of mechanical properties of cellulose fiber compared to other cobiopolymers such as lignin and xylan.

Method of characterization of precursor fibers using XRD and SAXS X-ray diffraction data of the precursor fiber were collected in the transmission mode setting of a CuKα X-ray instrument, SmartLab (RIGAKU) operated at 45 kV and 200 mA. Precursor fibers were cut into small pieces of powder, tightly packed in a Mylar film (Chemplex) and fixed in the sample holder. Powder diffraction data were collected in a continuous mode from 5° to 60° 2θ by θ/2θ scan setting. Scattering profile of Mylar film without sample was collected under the same experimental condition and was subtracted from the obtained scattering profiles of samples. The subtracted data were smoothed using Savitzky-Golay filter with a window size of 29 and a polynomial order of 1. 1 The smoothed data were then corrected for inelastic scattering.
The amorphous contribution was subtracted from remaining elastic scattering profile using a smoothing procedure applying Savitzky-Golay filter from 8° to 55° 2θ for each diffraction profile. 2, 3 Window size and polynomial order for the Savitzky-Golay filter were set to 201 (corresponding to 4° by 2θ) and 1, respectively. Iteration for the background estimation was repeated until the iteration does not reduce the background area significantly. In this experimental and smoothing condition, the smoothing procedure was repeated 50 times. Then the crystallinity index (CI) was estimated using the ratio of the area of total intensity (Stotal) and of the above estimated background intensity (Sbkg) from 9° to 50° 2θ: The background corrected profiles were fitted with three pseudo-Voigt functions for (11 � 0), (110), and (020) diffraction peaks of cellulose II. 4 The software lmfit was used for the fitting. 5 Scherrer equation was used to estimate the crystal widths (CWhkl) as follows: where K = 0.90 is the shape factor, λ is the X-ray wavelength, βhkl is the full width of half maximum (FWHM) of the diffraction peak in radians and θ is the diffraction angle of the peak.
Due to the significant overlap of diffraction peaks, the crystal widths were reported by the average from three diffraction peaks.
The azimuthal intensity profile was obtained from crystallographic (004) lattice plane (34.6° by 2θ), and was used to estimate the orientation distribution between fiber axis and crystallographic (004) lattice plane ( 004 ): Then Hermans orientation parameter (fWAXD) was estimated between fibril axis and crystallographic c axis: Synchrotron X-ray scattering data were collected at beamline D2AM at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). The fiber samples were mounted on a tailor-made multi-position fiber sample holder. The small-angle scattering patterns were collected in transmission mode using 2D detector "XPAD-D5" (imXPAD, France). The X-ray energy was set to 18 keV (λ = 0.688801 Å) and the sample to detector distance was calibrated using silver behenate.
The SAXS data were processed using pyFAI, a python library for azimuthal integration of diffraction data. 6 The data were processed correcting for the detector distortion, normalizing for the incident beam intensity and subtracting the scattering contribution from air. Equatorial intensity profiles of SAXS data were obtained from the azimuthal integration. The range of integration was determined by fitting equatorial streaks at the scattering vector q = 0.01 Å -1 using a pseudo-Voigt function. The fitted profile was used to estimate the orientation distribution of the equatorial streak (〈cos 2 〉): The orientation distribution of the equatorial streak was then converted to the orientation parameter ( ) in the same fashion as the Hermans orientation parameter is estimated from the equatorial diffraction assuming cylindrical symmetry: Equatorial intensity profiles were obtained from the azimuthal integration in the double range     Figure S3 shows the SAXS profile of wet fibers as well as the fitted curves by WoodSAS model.
Although the two fittings gave similar results, we considered the model without interference function was slightly more favorable for fitting the data of our wet fibers for three reasons.
Firstly, no significant difference was observed when additional parameters were applied for the interference function, which indicates the small contribution from the interference function.
Secondly, the reduced chi-square (χr2) from the model with interference function was smaller than the model without interference function, which implies the overfitting by the fitting model.
Thirdly, the packing distance (a) was fitted too close for the CHA sample. This was probably due to the error which occurred in the fit with a too small intensity contribution by the additional interference function. Therefore, the equatorial profile of our cellulose fibers could mostly be expressed by a cylindrical form factor.