The
ester cross-linking of cellulose nanofibril (CNF) aerogel was
carried out by a facile and highly effective gas–solid fluorinating
reaction without the assistance of organic solvent. The fluorination
achieved the coexistence of covalent cross-linking and more hydrophilic
groups, simultaneously endowing the fluorine-treated CNFs (F-CNFs)
aerogel with enduring water-durable and enhanced hydrophilic properties.
Hydrogen bond interactions between cellulose nanofibers are replaced
by covalent cross-linking of ester bonds, which makes the CNFs aggregate
more tightly and thus facilitates the enhancement of mechanical properties
of F-CNF aerogel. Meanwhile, the F-CNF aerogel exhibited ultralight
weight, excellent mechanical properties (123.4 KPa at 85% strain),
and water absorption (123.65 g/g), while having good shape memory
performance. What’s more, there was not any exotic element
introduced into F-CNFs, which can thus maximize the possibility for
the final materials to maintain the excellent biological properties
of the original CNFs.
Heteroatom-doping
reactions are essential to achieve advanced graphene-based
materials for energy and biological areas. Unfortunately, considerably
less is known regarding the detailed reaction pathways up to now.
Here, we focus on investigating the nitrogen (N) doping process of
fluorinated graphene (FG) under the assistance of defluorination based
on modified in situ Fourier transform infrared spectroscopy. It was
demonstrated FG possesses a higher and more effective reactivity with
ammonia in comparison with other graphene derivatives, which enable
N-doping to proceed efficiently with assistance of defluorination
even at a lower temperature (16.8 at. % of N at 300 °C and 19.9
at. % of N at 400 °C). Combining with Density functional theory,
it was proved that, at the initial reaction step of N-doping, ammonia
molecule attacked and substituted the C–F of FG by the new
C-NH2. Sequentially, amino group was cyclized to the three-membered
ring of ethylenimine. More importantly, the dissociation and migration
of C–F bonds facilitates the dissociating of C–C bonds
and the recombining of C–N bonds, thus significantly promoting
the N atom in ethylenimine ring to transform to the pyridinic-N or
graphitic-N in graphene skeleton.
Miscible
thermosetting/thermoplastic aromatic polyimide foam is
prepared by in situ simultaneous orthogonal polymerizations, where
the linear polyimide (PI) is formed by condensation polymerization
from polyester ammonium salt (PEAS) and the cross-linked bismaleimide
(BMI) is synthesized through addition polymerization from 4,4′-bismaleimide
diphenylmethane (BDM). A unique pleated cellular structure is formed
after the polyblend foam is cured at high temperature; here, 2D FT-IR
correlation analysis is employed to detect the detailed chemical reactions
during the thermal foaming progress. The simultaneous orthogonal polymerization
is confirmed, and the cross-linking reaction of BDM is found to be
divided into two stages. The pleated structure formed in the second
stage significantly improves the thermal insulation property of the
polyblend foamthe effective thermal conductivity decreases
from 427.5 to 77.5 mW·m–1·K–1 at 300 °C with 15 wt % BMI. Meanwhile, the polyblend is miscible
and foams only have one single glass transition temperature (Tg),
which increases with the content of BMI. This polyblend polyimide
foam will be a promising porous material for thermal insulation applications
in extreme environments, and the method to prepare the pleated cellular
structure in this work should provide a strategy to develop advanced
polymeric foam materials.
This
paper presents a feasible and universal approach to fabricate
regionally controllable nitrogen-heteroatom-doped multiwalled carbon
nanotubes with a skin-core heterostructure (N-o-FMWCNT) with the assistance
of defluorination in outer-layered fluorinated MWCNTs (o-FMWNCT).
First, o-FMWCNT with outer fluorinated tubes encapsulating intact
inner tubes was prepared through well-designed direct heating fluorination
with temperatures from 25 to 180 °C. Subsequently, nitrogen-doping
was implemented at a mild temperature under NH3 atmosphere
(400 °C). According to the results of X-ray photoelectron spectroscopy,
high-resolution transmission electron microscopy, and elemental analysis,
about 4.97 atom % nitrogen-heteroatom with primary pyridine-nitrogen
configuration was successfully doped into the carbon skeleton at the
outer tube of o-FMWCNT with the recombination of C–N bonds.
Because of the integrity of its inner tubes, the N-o-FMWCNT was endowed
with a relatively high-level conductivity (5.46 × 10–2 S·m–1) in contrast to other reported highly
N-doped carbon materials. Correspondingly, the mass-specific capacitance
of N-o-FMWCNT as a supercapacitor electrode material was spectacularly
improved by 437% at the current density of 0.5 A/g in comparison to
that of pristine MWCNTs (p-MWCNT). Meanwhile, nitrogen-doped MWCNTs
with a homogeneous doping structure (N-i-FMWCNT) were also prepared
through direct isothermal fluorination for comparison. Our work proposes
a nondestructive postdoping strategy for preparing functionalized
CNTs with a regionally controllable heteroatom-doping structure.
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