Download Preparation and Grafting Functionalization of Self

coatings
Review
Preparation and Grafting Functionalization of
Self-Assembled Chitin Nanofiber Film
Jun-ichi Kadokawa
Department of Chemistry, Biotechnology, and Chemical Engineering, Graduate School of Science and
Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan;
[email protected]; Tel.: +81-99-285-7743
Academic Editor: Andriy Voronov
Received: 23 June 2016; Accepted: 8 July 2016; Published: 12 July 2016
Abstract: Chitin is a representative biomass resource comparable to cellulose. Although considerable
efforts have been devoted to extend novel applications to chitin, lack of solubility in water and
common organic solvents causes difficulties in improving its processability and functionality.
Ionic liquids have paid much attention as solvents for polysaccharides. However, little has been
reported regarding the dissolution of chitin with ionic liquids. The author found that an ionic liquid,
1-allyl-3-methylimidazolium bromide (AMIMBr), dissolved chitin in concentrations up to ~4.8 wt %
and the higher contents of chitin with AMIMBr gave ion gels. When the ion gel was soaked in methanol
for the regeneration of chitin, followed by sonication, a chitin nanofiber dispersion was obtained.
Filtration of the dispersion was subsequently carried out to give a chitin nanofiber film. A chitin
nanofiber/poly(vinyl alcohol) composite film was also obtained by co-regeneration approach. Chitin
nanofiber-graft-synthetic polymer composite films were successfully prepared by surface-initiated
graft polymerization technique. For example, the preparation of chitin nanofiber-graft-biodegradable
polyester composite film was achieved by surface-initiated graft polymerization from the chitin
nanofiber film. The similar procedure also gave chitin nanofiber-graft-polypeptide composite
film. The surface-initiated graft atom transfer radical polymerization was conducted from a chitin
macroinitiator film derived from the chitin nanofiber film.
Keywords: chitin; grafting; ion gel; nanofiber film; regeneration; self-assembly
1. Introduction
Polysaccharides are widely distributed in nature and exhibit important functions such as
structural materials and energy providers [1,2]. Cellulose and chitin are the representative structural
polysaccharides as the main components of cell wall in plants and of exoskeletons in crustaceans,
shellfishes, and insects, respectively. Cellulose is composed of β(1Ñ4)-linked D-glucose repeating units
(Figure 1) [3]. Chitin has the similar structure as cellulose, but shows the difference in replacement
of the hydroxy group in D-glucose units to the acetamido group at C-2 position (β(1Ñ4)-linked
N-acetyl-D-glucosamine repeating units) (Figure 1) [4–6]. These polysaccharides are the most abundant
biomass resources on the earth, and accordingly, have been identified as eco-friendly and recyclable
organic substances. Indeed, cellulose has practically been used in applications to furniture, clothes,
foods, medicines, and so on. Compared with cellulose, chitin is still an unutilized biomass resource
primary because of highly crystalline and bulk structure owing to numerous intra- and intermolecular
hydrogen bonds, which cause the limitation of solubility with solvents [7]. Solubility problem of
chitin is more serious compared with cellulose, leading to difficulty in practical application of chitin as
functional materials.
Coatings 2016, 6, 27; doi:10.3390/coatings6030027
www.mdpi.com/journal/coatings
Coatings 2016, 6, 27
Coatings 2016, 6, 27 Coatings 2016, 6, 27 2 of 12
2 of 12 2 of 12 Figure 1. Chemical structures of cellulose and chitin. Figure
1. Chemical structures of cellulose and chitin.
Figure 1. Chemical structures of cellulose and chitin. Ionic liquids (ILs) have been identified as powerful solvents for polysaccharides [8–13]. ILs are Ionic
liquids (ILs) have been identified as powerful solvents for polysaccharides [8–13]. ILs are
Ionic liquids (ILs) have been identified as powerful solvents for polysaccharides [8–13]. ILs are low‐melting‐point molten salts, defined as those form liquids at ambient temperatures, e.g., lower low-melting-point
molten salts, defined as those form liquids at ambient temperatures, e.g., lower than
low‐melting‐point molten salts, defined as those form liquids at ambient temperatures, e.g., lower than the boiling point of water [14,15]. The property is owing to that the liquid state is thethan boiling
of water
The[14,15]. property
is owing
to that
the liquid
thermodynamically
the point
boiling point [14,15].
of water The property is owing to state
that is
the liquid state is thermodynamically favorable because of large size and conformational flexibility of the ions, in favorable
because of large
size and
conformational
of the ions,flexibility in whichof these
thermodynamically favorable because of large size flexibility
and conformational the behaviors
ions, in which these behaviors give rise to small lattice enthalpies and large entropy changes that favor the which these behaviors give rise to small lattice enthalpies and large entropy changes that favor the give
rise
to
small
lattice
enthalpies
and
large
entropy
changes
that
favor
the
liquid
state [16].
liquid state [16]. A wide variety of chemical structures in ILs have been known by various liquid variety
state [16]. A wide structures
variety of inchemical structures in ILs have been known by of
various A wide
of
chemical
ILs
have
been
known
by
various
combinations
organic
combinations of organic cations, such as imidazolium, pyridinium, ammonium, and phosphonium, combinations of organic cations, such as imidazolium, pyridinium, ammonium, and phosphonium, cations,
such as imidazolium, pyridinium, ammonium, and phosphonium, with inorganic and organic
with inorganic and organic anions (Figure 2). Over the past decade, ILs have attracted much attention with inorganic and organic anions (Figure 2). Over the past decade, ILs have attracted much attention anions
(Figure
2). Over
thecharacteristics past decade, ILs
have
attracted
much
attention
because
of their vapor specific
because of their specific such as an excellent thermal stability, a negligible because of their specific characteristics such as an excellent thermal stability, a negligible vapor characteristics
such
as an excellent
thermal
stability, properties a negligible
vapor Beyond pressure,
and traditional controllable
pressure, and controllable physical and chemical [17–19]. these pressure, controllable physical and Beyond
chemical properties [17–19]. Beyond ofthese traditional physical
andand chemical
properties
[17–19].
these
traditional
properties
ILs, interests
and
properties of ILs, interests and applications on ILs have been extended to the researches dealing with properties of ILs, interests and applications on ILs have been extended to the researches dealing with applications
on ILs have been extended to the researches dealing with biological macromolecules
biological macromolecules such as naturally occurring polysaccharides [8–13], because ILs have been biological macromolecules such as naturally occurring polysaccharides [8–13], because ILs have been such
as naturally occurring polysaccharides [8–13], because ILs have been identified to specifically
identified to specifically dissolve them. identified to specifically dissolve them. dissolve them.
Cation structures
Cation structures
R"
R"
R'
R'
R'
R'
R'
Me N
R
R'
N
R N R
R P R
Me N
N R
R N R
R P R
R
R
N
R
R
N
R
Imidazolium
Ammonium Phosphonium
R
Imidazolium
Ammonium Phosphonium
Pyridinium
Pyridinium
Anion structures
Anion structures
O
N
N
O
PF6-- NO3-- CF3 S O
Cl-- Br-- BF4-F3CO2S N SO2CF3
NC N CN
BF4
PF6
Cl
Br
NO3
CF3 S O
F3CO2S
SO2CF3
NC
CN
O
O
Figure 2. Typical cation and anion structures of ionic liquids (ILs). Figure 2. Typical cation and anion structures of ionic liquids (ILs). Figure
2. Typical cation and anion structures of ionic liquids (ILs).
In 1934, a molten N‐ethylpyridinium chloride, in the presence of nitrogen‐containing bases, has In 1934, a molten N‐ethylpyridinium chloride, in the presence of nitrogen‐containing bases, has In 1934, a molten N-ethylpyridinium chloride, in the presence of nitrogen-containing bases, has
already been found to dissolve cellulose [20]. Although this study was probably the first example of already been found to dissolve cellulose [20]. Although this study was probably the first example of the cellulose dissolution with IL‐type solvents, it was considered to be of little practical value at the already
been found to dissolve cellulose [20]. Although this study was probably the first example of
the cellulose dissolution with IL‐type solvents, it was considered to be of little practical value at the time because the concept of ILs solvents,
had not it been put forward. an IL, value
1‐butyl‐3‐
the cellulose
dissolution
with IL-type
was considered
to beIn of 2002, little practical
at the
time because the concept of ILs had not been put forward. In 2002, an IL, 1‐butyl‐3‐
methylimidazolium chloride (BMIMCl, Figure 3), was found to dissolve cellulose in relatively high time
because the concept of ILs had not been put forward. In 2002, an IL, 1-butyl-3-methylimidazolium
methylimidazolium chloride (BMIMCl, Figure 3), was found to dissolve cellulose in relatively high concentrations this 3),research opened up a cellulose
new way the development of a class chloride
(BMIMCl,and Figure
was found
to dissolve
in for relatively
high concentrations
andof this
concentrations and this research opened up a new way for the development of a class of polysaccharide solvent systems [21]. Since this report was published, ILs have been used in the research
opened up
a newsystems way for[21]. the development
of a was classpublished, of polysaccharide
systems
[21].
polysaccharide solvent Since this report ILs have solvent
been used in the processing of cellulose and other polysaccharides, which mainly concern the dissolution, Since
this
report
was
published,
ILs
have
been
used
in
the
processing
of
cellulose
and
other
processing of cellulose and other polysaccharides, which mainly concern the dissolution, homogeneous derivatization and modification, and regeneration [8–13]. In contrast, only limited polysaccharides,
mainly concern
the dissolution,
homogeneous
derivatization
and
modification,
homogeneous which
derivatization and modification, and regeneration [8–13]. In contrast, only limited investigations have been reported regarding the dissolution of chitin with ILs [22–24]. So far, two and
regeneration [8–13]. In contrast, only limited investigations have been reported regarding the
investigations have been reported regarding the dissolution of chitin with ILs [22–24]. So far, two types ILs, i.e., 1‐allyl‐3‐methylimidazolium bromide (AMIMBr, Figure 3) and some imidazolium dissolution
chitin
with ILs [22–24]. So far, two
types (AMIMBr, ILs, i.e., 1-allyl-3-methylimidazolium
bromide
types ILs, ofi.e., 1‐allyl‐3‐methylimidazolium bromide Figure 3) and some imidazolium acetates with different substituents (1‐allyl, 1‐butyl, and 1‐ethyl‐3‐methylimidazolium acetates, (AMIMBr,
3) and some
imidazolium
acetates
with
different
substituents (1-allyl, 1-butyl,
and
acetates Figure
with different substituents (1‐allyl, 1‐butyl, and 1‐ethyl‐3‐methylimidazolium acetates, Figure 3) have been reported to dissolve chitin in several wt % concentrations [23,25–27]. The former Figure 3) have been reported to dissolve chitin in several wt % concentrations [23,25–27]. The former 1-ethyl-3-methylimidazolium
acetates, Figure 3) have been reported to dissolve chitin in several wt %
IL, which was found to dissolve chitin by the author, is more stable than the latter ILs. Furthermore, IL, which was found to dissolve chitin by the author, is more stable than the latter ILs. Furthermore, concentrations
The
IL,quaternarization which was found
to dissolve
chitin by the author,
more
AMIMBr can [23,25–27].
be prepared by former
a simple reaction of 1‐methylimidazole with isallyl AMIMBr can be prepared by a simple quaternarization reaction 1‐methylimidazole with reaction
allyl stable
than the
latter
ILs. Furthermore,
AMIMBr
can be prepared
by of a simple
quaternarization
Coatings 2016, 6, 27 Coatings
2016, 6, 27
Coatings 2016, 6, 27 3 3 of 12 of 12
3 of 12 bromide in high yield [28]. The dissolution of chitin with deep eutectic solvents (DESs), that is, ionic bromide in high yield [28]. The dissolution of chitin with deep eutectic solvents (DESs), that is, ionic of
1-methylimidazole
with allyl
yield
[28]. The chlorocholine dissolution of chloride‐urea, chitin with deep
liquid analogues, composed of bromide
mixtures in
of high
choline halide‐urea, and liquid analogues, composed of ionic
mixtures choline halide‐urea, chlorocholine chloride‐urea, and eutectic
solvents (DESs),
that is,
liquidof analogues,
composed of
mixtures of choline
halide-urea,
choline chloride‐thiourea were also investigated [29]. Consequently, maximum dissolution of chitin choline chloride‐thiourea were also investigated [29]. Consequently, maximum dissolution of chitin (9% w/w) was obtained in the choline chloride‐thiourea system. chlorocholine
chloride-urea, and choline chloride-thiourea were also investigated [29]. Consequently,
(9% w/w) was obtained in the choline chloride‐thiourea system. maximum
dissolution of chitin (9% w/w) was obtained in the choline chloride-thiourea system.
Figure 3. Representative ILs, which dissolve cellulose and chitin. Figure
3. Representative ILs, which dissolve cellulose and chitin.
Figure 3. Representative ILs, which dissolve cellulose and chitin. The author reported that AMIMBr dissolved a chitin powder from crab shells in concentrations The
author reported that AMIMBr dissolved a chitin powder from crab shells in concentrations
The author reported that AMIMBr dissolved a chitin powder from crab shells in concentrations up to ~4.8 wt %, and further formed ion gels with 6.5–10.7 wt % contents of chitin [26]. From the ion up
to ~4.8 wt %, and further formed ion gels with 6.5–10.7 wt % contents of chitin [26]. From the ion
up to ~4.8 wt %, and further formed ion gels with 6.5–10.7 wt % contents of chitin [26]. From the ion gel, chitin self‐assembled into nanofibers by regeneration, which further formed a nanofiber film by gel,
chitin self-assembled into nanofibers by regeneration, which further formed a nanofiber film by
gel, chitin self‐assembled into nanofibers by regeneration, which further formed a nanofiber film by entanglement [30–32]. Moreover, surface‐initiated graft polymerization from the nanofiber film has entanglement
[30–32]. Moreover, surface-initiated graft polymerization from the nanofiber film has
entanglement [30–32]. Moreover, surface‐initiated graft polymerization from the nanofiber film has been conducted to produce chitin nanofiber‐graft‐synthetic polymer composite materials [33]. In this been conducted to produce chitin nanofiber‐graft‐synthetic polymer composite materials [33]. In this been
conducted
produce
chitin nanofiber-graft-synthetic
polymer
materials [33]. Inof this
review article, to
the comprehensive results on the preparation and composite
grafting functionalization the review article, the
the comprehensive
comprehensive results
results on
on the
the preparation
preparation and grafting
grafting functionalization
functionalization of
of the
the review
article,
and
self‐assembled chitin nanofiber film are presented. self‐assembled chitin nanofiber film are presented. self-assembled
chitin nanofiber film are presented. 2. Preparation of Self‐Assembled Chitin Nanofiber Film from Ion Gel 2.
Preparation of Self-Assembled Chitin Nanofiber Film from Ion Gel
2. Preparation of Self‐Assembled Chitin Nanofiber Film from Ion Gel The author reported that a clear solution of chitin (4.8 wt %) with AMIMBr was obtained by The
author reported that a clear solution of chitin (4.8 wt %) with AMIMBr was obtained by
The author reported that a clear solution of chitin (4.8 wt %) with AMIMBr was obtained by heating a mixture of a commercial chitin powder from crab shells with AMIMBr at 100 °C for 48 h heating
mixture
of a commercial
powder
crab
shells
with AMIMBr
100 ˝ C for
48 h
heating a mixture of a commercial chitin powder from crab shells with AMIMBr at 100 °C for 48 h (Figure a4) [26]. When mixtures of chitin
6.5–10.7 wt % from
chitin with AMIMBr were left atstanding at room (Figure
4)
[26].
When
mixtures
of
6.5–10.7
wt
%
chitin
with
AMIMBr
were
left
standing
at
room
(Figure 4) [26]. When mixtures of 6.5–10.7 wt % chitin with AMIMBr were left standing at room temperature for 24 h, heated at 100 °C for 48 h, and then cooled to room temperature, furthermore, temperature
for 24 h, heated at 100 ˝ C for 48 h, and then cooled to room temperature, furthermore,
temperature for 24 h, heated at 100 °C for 48 h, and then cooled to room temperature, furthermore, they totally turned into the gel‐like form (Figure 4). The dynamic rheological measurements showed they
totally turned into the gel-like form (Figure 4). The dynamic rheological measurements showed
they totally turned into the gel‐like form (Figure 4). The dynamic rheological measurements showed that both 4.8 wt % and 6.5 wt % chitins with AMIMBr behaved as weak gels. However, the 6.5 wt % that
both
4.8 wt
% and did 6.5 wt
%flow, chitins
withthe AMIMBr
weak
gels. However,
theflow 6.5 wt
%
that both 4.8 wt % and 6.5 wt % chitins with AMIMBr behaved as weak gels. However, the 6.5 wt % chitin with AMIMBr not while 4.8 wt behaved
% chitin as
with AMIMBr started to upon chitin
with
did
not
flow,
while
the 4.8
chitin
with AMIMBr
startedstarted to flow to upon
leaning.
chitin with AMIMBr
AMIMBr did not flow, while the wt
4.8 %wt % chitin with AMIMBr flow upon leaning. leaning. Figure 4. Dissolution (~4.8 wt %) and gelation (6.5–10.7 wt %) of chitin with 1‐allyl‐3‐
Figure 4. Dissolution (~4.8 wt %) and gelation (6.5–10.7 wt %) of chitin with 1-allyl-3-methylimidazolium
Figure 4. Dissolution (~4.8 wt %) and gelation (6.5–10.7 wt %) of chitin with 1‐allyl‐3‐
methylimidazolium bromide (AMIMBr). bromide
(AMIMBr).
methylimidazolium bromide (AMIMBr). Coatings 2016, 6, 27
Coatings 2016, 6, 27 4 of 12
4 of 12 Coatings 2016, 6, 27 The regeneration of chitin from the 9.1 wt %–10.7 wt % chitin ion gels with AMIMBr was4 of 12 carried
The regeneration of chitin from the 9.1 wt %–10.7 wt % chitin ion gels with AMIMBr was carried out by soaking in methanol at room temperature for 24 h. The subsequent sonication gave dispersions
out by soaking in methanol at room temperature for 24 h. The subsequent sonication gave dispersions The regeneration of chitin from the 9.1 wt %–10.7 wt % chitin ion gels with AMIMBr was carried of the regenerated
chitin (Figure 5) [30]. The SEM image of the sample, which was prepared by dilution
of the regenerated chitin (Figure 5) [30]. The SEM image of the sample, which was prepared by out by soaking in methanol at room temperature for 24 h. The subsequent sonication gave dispersions of the resulting dispersion, showed the nanofiber morphology with ca. 20–60 nm in width and several
dilution of the resulting dispersion, showed the nanofiber morphology with ca. 20–60 nm in width of the regenerated chitin (Figure 5) [30]. The SEM image of the sample, which was prepared by hundred nm in length (Figure 6a), indicating the self-assembling formation of the chitin nanofibers
and several hundred nm in length (Figure 6a), indicating the self‐assembling formation of the chitin dilution of the resulting dispersion, showed the nanofiber morphology with ca. 20–60 nm in width bynanofibers the regeneration
fromapproach the ion gels.
When
the gels. resulting
self-assembled
were
by the approach
regeneration from the ion When the resulting nanofibers
self‐assembled and several hundred nm in length (Figure 6a), indicating the self‐assembling formation of the chitin isolated
by
filtration
of
the
dispersion,
a
film
was
obtained
(Figure
5).
The
SEM
image
of
the
resulting
nanofibers were isolated by filtration of the dispersion, a film was obtained (Figure 5). The SEM image nanofibers by the regeneration approach from the ion gels. When the resulting self‐assembled film
exhibited
the morphology
ofthe highly
entangledof nanofibers
(Figure nanofibers 6b). Such entangled
structure
of the resulting film exhibited morphology highly entangled (Figure 6b). Such nanofibers were isolated by filtration of the dispersion, a film was obtained (Figure 5). The SEM image from
the
nanofibers
probably
contributed
to
formation
of
the
films.
entangled structure from the nanofibers probably contributed to formation of the films. of the resulting film exhibited the morphology of highly entangled nanofibers (Figure 6b). Such entangled structure from the nanofibers probably contributed to formation of the films. Figure 5. Preparation of self‐assembled chitin nanofiber dispersion/film and preparation of chitin Figure 5. Preparation of self-assembled chitin nanofiber dispersion/film and preparation of chitin
Figure 5. Preparation of self‐assembled chitin nanofiber dispersion/film graft and preparation of radical chitin nanofiber macroinitiator dispersion and subsequent surface‐initiated atom transfer nanofiber macroinitiator dispersion and subsequent surface-initiated graft atom transfer radical
nanofiber macroinitiator dispersion and subsequent surface‐initiated graft atom transfer radical polymerization (ATPR) of methyl methacrylate (MMA). polymerization
(ATPR) of methyl methacrylate (MMA).
polymerization (ATPR) of methyl methacrylate (MMA). Figure 6. SEM images of chitin nanofiber dispersion and film obtained using methanol ((a) and (b), Figure
6. SEM images of chitin nanofiber dispersion and film obtained using
methanol ((a) and (b),
Figure 6. SEM images of chitin nanofiber dispersion and film obtained using methanol ((a) and (b), 2･2H2O/methanol solution respectively), chitin nanofiber dispersion obtained using 0.765 mol/L CaBr
･
22H
2H
2O/methanol solution respectively), chitin nanofiber dispersion obtained using 0.765 mol/L CaBr
respectively),
chitin
nanofiber
dispersion
obtained
using
0.765
mol/L
CaBr
¨
O/methanol
solution (c),
2
2
(c), and chitin nanofiber/PVA composite film (d). (c), and chitin nanofiber/PVA composite film (d). and
chitin nanofiber/PVA composite film (d).
Morphologies of self‐assembled chitin nanofibers were changed when the regeneration from the Morphologies of self‐assembled chitin nanofibers were changed when the regeneration from the ion gels were conducted using the several kinds of calcium halide･2H
2O/methanol solutions instead Morphologies of self-assembled chitin nanofibers were changed when
the regeneration from the
ion gels were conducted using the several kinds of calcium halide･2H
2O/methanol solutions instead of a pure methanol [34]. The nanofiber assembly was not induced by the regeneration from the ion ion gels were conducted using the several kinds of calcium halide¨ 2H2 O/methanol solutions instead
of a pure methanol [34]. The nanofiber assembly was not induced by the regeneration from the ion 2･2H
2O and CaBr
2･2H2O/methanol solutions in high concentrations (3.06 mol/L and of gel using CaCl
a pure methanol
[34].
The nanofiber
assembly was not induced by the regeneration from the ion
gel using CaCl2･2H2O and CaBr2･2H2O/methanol solutions in high concentrations (3.06 mol/L and 1.53 mol/L, respectively), whereas the regeneration using CaBr
2･2H2O/methanol solution in lower gel1.53 using
CaCl2 ¨ 2H2 O and CaBr2 ¨ 2Hthe solutions
in high
concentrations (3.06 mol/L and
2 O/methanol
mol/L, respectively), whereas regeneration using CaBr2･2H2O/methanol solution in lower concentration (0.765 mol/L) gave self‐assembled nanofibers with higher aspect ratio as confirmed by 1.53
mol/L,
respectively),
whereas
the
regeneration
using
CaBr
¨
2H
O/methanol solution in lower
2
2
concentration (0.765 mol/L) gave self‐assembled nanofibers with higher aspect ratio as confirmed by the SEM image (Figure 6c). The tensile strength value of the film, which was obtained by filtration of concentration
(0.765
mol/L)
gave
self-assembled
nanofibers
with
higher
aspect ratio as confirmed by
the SEM image (Figure 6c). The tensile strength value of the film, which was obtained by filtration of the dispersion from such 0.765 mol/L CaBr
2･2H2O/methanol solution, was larger than that of the films thethe dispersion from such 0.765 mol/L CaBr
SEM image (Figure 6c). The tensile strength
value of the film, which was obtained by filtration of
2･2H2O/methanol solution, was larger than that of the films the dispersion from such 0.765 mol/L CaBr2 ¨ 2H2 O/methanol solution, was larger than that of the
Coatings 2016, 6, 27
Coatings 2016, 6, 27 5 of 12
5 of 12 films produced using the other methanol-based solutions. These results indicated that the mechanical
produced using the other methanol‐based solutions. These results indicated properties
were
affected
by morphologies
of the
chitin assemblies
in the
films. that the mechanical properties were affected by morphologies of the chitin assemblies in the films. Co-regeneration approach has been conducted to fabricate a self-assembled chitin
Co‐regeneration approach has been conducted to 7) fabricate a self‐assembled chitin nanofiber/poly(vinyl
alcohol)
(PVA)
composite
film (Figure
[30]. For the
composite preparation,
nanofiber/poly(vinyl alcohol) (PVA) composite film (Figure 7) [30]. For the composite preparation, a a solution of PVA (DP = ca. 4300) with a small amount of hot water was first mixed with the 9.1 wt %
solution of PVA (DP = ca. 4300) with a small amount of hot water was first mixed with the 9.1 wt % chitin
ion gel with AMIMBr (feed weight ratio of chitin to PVA = 1:0.3). The co-regeneration of the
chitin ion gel with AMIMBr (feed weight ratio of chitin to PVA = 1:0.3). The co‐regeneration of the two polymers was then conducted by soaking the mixture in methanol, because methanol is a poor
two polymers was then conducted by soaking the mixture in methanol, because methanol is a poor solvent not only for chitin, but also for PVA. The resulting dispersion was subjected to filtration to
solvent not only for chitin, but also for PVA. The resulting dispersion was subjected to filtration to isolate the regenerated polymeric materials, which were purified by Soxhlet extraction with methanol,
isolate the regenerated polymeric materials, which were purified by Soxhlet extraction with methanol, to obtain the self-assembled chitin nanofiber/PVA composite film. The SEM image of the resulting
to obtain the self‐assembled chitin nanofiber/PVA composite film. The SEM image of the resulting composite
film showed the nanofiber morphology (Figure 6d). This result suggested immiscibility of
composite film showed the nanofiber morphology (Figure 6d). This result suggested immiscibility of thethe two polymers in the composite. The PVA component probably filled in spaces among the fibers. two polymers in the composite. The PVA component probably filled in spaces among the fibers.
TheThe DSC result of the composite film suggested that PVA might partially be miscible at the interfacial DSC result of the composite film suggested that PVA might partially be miscible at the interfacial
area
on the
nanofibers
by hydrogen
bonding
between
the two
polymers.
Both the
tensile
strength
area on chitin
the chitin nanofibers by hydrogen bonding between the two polymers. Both the tensile value
and
the
elongation
value
at
break
of
the
composite
film
under
tensile
mode
were
larger
than
strength value and the elongation value at break of the composite film under tensile mode were larger those
of the self-assembled chitin nanofiber film. This result indicated reinforcing effect of PVA present
than those of the self‐assembled chitin nanofiber film. This result indicated reinforcing effect of PVA in the
chitin nanofiber film.
present in the chitin nanofiber film. Figure 7. Preparation of chitin nanofiber/poly(vinyl alcohol) (PVA) composite film. Figure 7. Preparation of chitin nanofiber/poly(vinyl alcohol) (PVA) composite film.
The self‐assembled chitin nanofibers were also formed by gelation of chitin with the choline The
self-assembled chitin nanofibers were also formed by gelation of chitin with the choline
chloride‐thiourea DES system (10% w/w) and subsequent dilution with water [35]. The resulting self‐
chloride-thiourea
DES system (10% w/w) and subsequent dilution with water [35]. The resulting
assembled chitin nanofibers were employed to prepare calcium alginate bio‐nanocomposite gel beads self-assembled
chitin nanofibers were employed to prepare calcium alginate bio-nanocomposite gel
[35]. beads [35].
3. Surface‐initiated Graft Polymerization from Self‐Assembled Chitin Nanofiber Film 3. Surface-Initiated Graft Polymerization from Self-Assembled Chitin Nanofiber Film
Surface‐initiated graft polymerization of cyclic and vinyl monomers from the self‐assembled chitin nanofiber film has been conducted to fabricate composite materials [33]. Such composites can Surface-initiated graft polymerization of cyclic and vinyl monomers from the self-assembled
be expected exhibit functions to
such as influence of interfacial adhesion on mechanical chitin
nanofiberto film
has specific been conducted
fabricate
composite
materials
[33]. Such
composites
properties [36–38]. For example, surface‐initiated ring‐opening graft copolymerization of L‐lactide can be expected to exhibit specific functions such as influence of interfacial adhesion on mechanical
(LA)/ε‐caprolactone monomers initiated from hydroxy groups on the self‐assembled chitin properties
[36–38]. For(CL) example,
surface-initiated
ring-opening
graft copolymerization
of L-lactide
nanofiber was attempted in the presence of tin(II) 2‐ethylhexanoate as a Lewis acid catalyst to (LA)/ε-caprolactone
(CL) monomers
initiated
from hydroxy
groups on
the
self-assembled
chitin
produce chitin nanofiber‐graft‐poly(LA‐co‐CL) film (Figure 8) [39]; poly(LA‐co‐CL) is a well‐known nanofiber was attempted in the presence of tin(II) 2-ethylhexanoate as a Lewis acid catalyst to
biodegradable polyester [40,41] and can be synthesized by ring‐opening copolymerization of LA/CL produce
chitin nanofiber-graft-poly(LA-co-CL) film (Figure 8) [39]; poly(LA-co-CL) is a well-known
monomers initiated by alcohols [42,43]. To efficiently take place the ring‐opening graft biodegradable polyester [40,41] and can be synthesized by ring-opening copolymerization of
copolymerization of LA/CL on surface of the nanofibers, spaces among the fibers were made by LA/CL monomers initiated by alcohols [42,43]. To efficiently take place the ring-opening graft
immersing the film in water for 10 s. Then, the surface‐initiated ring‐opening graft copolymerization copolymerization of LA/CL on surface of the nanofibers, spaces among the fibers were made by
of LA/CL (feed molar ratio = 10:90) from the pre‐treated film was conducted as follows. After the film immersing the film in water for 10 s. Then, the surface-initiated ring-opening graft copolymerization
was immersed in a solution of LA/CL in toluene, tin(II) 2‐ethylhexanoate (ca. 5 mol% for the of LA/CL (feed molar ratio = 10:90) from the pre-treated film was conducted as follows. After the
monomers), was added and the mixture was heated at 80 °C for 48 h to occur the ring‐opening film
was immersed in a solution of LA/CL in toluene, tin(II) 2-ethylhexanoate (ca. 5 mol% for−−1
the
copolymerization. The IR spectrum of the resulting film exhibited carbonyl absorption at 1739 cm
Coatings 2016, 6, 27
6 of 12
monomers),
was added and the mixture was heated at 80 ˝ C for 48 h to occur the ring-opening
Coatings 2016, 6, 27 6 of 12 copolymerization. The IR spectrum of the resulting film exhibited carbonyl absorption at 1739 cm´1
due to the ester linkage of poly(LA‐co‐CL), suggesting the presence of poly(LA‐co‐CL) in the product, due
to the ester linkage of poly(LA-co-CL), suggesting the presence of poly(LA-co-CL) in the product,
which was explanatorily bound to the nanofibers by covalent linkage. The LA/CL unit ratio (30/70) which
was explanatorily bound to the nanofibers by covalent linkage. The LA/CL unit ratio (30/70)
in the polyester, which was determined by H NMR analysis, was higher than that in feed (10/90) in
the polyester, which was determined by 11H
NMR analysis, was higher than that in feed (10/90)
because of the higher reactivity of LA than CL in the copolymerization [44]. The SEM image of the because
of the higher reactivity of LA than CL in the copolymerization [44]. The SEM image of the
resulting film indicated increase of the fiber widths (60–100 nm) compared with those of the original resulting
film indicated increase of the fiber widths (60–100 nm) compared with those of the original
pre‐treated film and some fibers were merged at the interfacial areas, that was probably caused by pre-treated
film and some fibers were merged at the interfacial areas, that was probably caused by
the grafted polyesters present on the nanofibers (Figure 9a). The stress‐strain curve of the resulting the
grafted polyesters present on the nanofibers (Figure 9a). The stress-strain curve of the resulting
film under tensile mode exhibited the larger elongation value (7.2%) at break than that of the original film
under tensile mode exhibited the larger elongation value (7.2%) at break than that of the original
pre‐treated chitin
chitin nanofiber
nanofiber film
film (1.1%)
(1.1%) (Figure
(Figure 10).
10). When
When the
the surface-initiated
surface‐initiated ring-opening
ring‐opening graft
graft pre-treated
copolymerization of LA/CL was conducted in various monomer feed ratios, the grafting amounts of copolymerization
of LA/CL was conducted in various monomer feed ratios, the grafting amounts of
poly(LA‐co‐CL)s on the nanofibers increased with increasing the LA/CL feed molar ratios. poly(LA-co-CL)s
on the nanofibers increased with increasing the LA/CL feed molar ratios. ‐lactide (LA)/ε‐caprolactone (CL), ‐benzyl L‐
Figure 8. Surface‐initiated graft (co)polymerization of Figure
8. Surface-initiated graft (co)polymerization of LL-lactide
(LA)/ε-caprolactone (CL), γ-benzyl
glutamate‐N‐carboxyanhydride (BLG‐NCA), and 2‐hydroxyethyl acrylate (HEA) from chitin L -glutamate-N-carboxyanhydride (BLG-NCA), and 2-hydroxyethyl acrylate (HEA) from chitin
nanofiber films with appropriate initiating sites. nanofiber films with appropriate initiating sites.
Coatings 2016, 6, 27
Coatings 2016, 6, 27 Coatings 2016, 6, 27 7 of 12
7 of 12 7 of 12 Figure Figure 9. 9.
SEMimages images of of chitin chitin nanofiber‐graft‐poly(LA‐co‐CL) nanofiber-graft-poly(LA-co-CL)film film(a) (a)and and chitin chitin nanofiber‐graft‐
nanofiber-graftFigure 9. SEM SEM images of chitin nanofiber‐graft‐poly(LA‐co‐CL) film (a) and chitin nanofiber‐graft‐
polyBLG film (b). polyBLG
film
(b).
polyBLG film (b). Figure Figure 10. 10.
Stress-straincurves curvesof ofpre‐treated pre-treatedchitin chitinnanofiber nanofiberfilm film (a) (a) and and chitin chitin nanofiber‐graft‐
nanofiber-graftFigure 10. Stress‐strain Stress‐strain curves of pre‐treated chitin nanofiber film (a) and chitin nanofiber‐graft‐
poly(LA‐co‐CL) film (b). poly(LA-co-CL)
film (b).
poly(LA‐co‐CL) film (b). Chitin nanofiber‐graft‐synthetic film was also Chitin nanofiber‐graft‐synthetic polypeptide polypeptide also fabricated fabricated by by surface‐initiated surface‐initiated Chitin nanofiber-graft-synthetic
polypeptide film film was was also
fabricated
by
surface-initiated
polymerization approach. Because synthetic polypeptides with well‐defined structure are polymerization approach.
approach. Because synthetic polypeptides with well‐defined structure are polymerization
Because
synthetic
polypeptides
with well-defined
structure are
synthesized
synthesized by ring‐opening polymerization of ‐amino acid N‐carboxyanhydrides (NCAs) synthesized by ring‐opening polymerization of ‐amino acid N‐carboxyanhydrides (NCAs) by ring-opening polymerization of α-amino acid N-carboxyanhydrides (NCAs) accompanied with
accompanied decarboxylation initiated by groups [45,46], surface‐initiated graft accompanied with with decarboxylation by amino amino [45,46], the the surface‐initiated graft decarboxylation
initiated
by amino initiated groups [45,46],
the groups surface-initiated
graft
polymerization
of
polymerization of a NCA monomer from the self‐assembled chitin nanofiber film having amino polymerization of a NCA monomer from the self‐assembled chitin nanofiber film having amino a NCA monomer from the self-assembled chitin nanofiber film having amino initiating groups was
initiating groups was performed [47]. As the NCA monomer, ‐benzyl L‐glutamate‐NCA (BLG‐NCA) initiating groups was performed [47]. As the NCA monomer, ‐benzyl ‐glutamate‐NCA (BLG‐NCA) performed
[47]. As the NCA monomer, γ-benzyl L-glutamate-NCAL(BLG-NCA)
was particularly
was particularly selected because its ring‐opening polymerization and subsequent hydrolysis of was particularly selected because its ring‐opening polymerization and subsequent hydrolysis of selected because its ring-opening polymerization and subsequent hydrolysis
of benzyl
ester gives
benzyl ester gives poly(
L‐glutamic acid) having carboxylic acid functional groups (Figure 8). Partial benzyl ester gives poly(
L‐glutamic acid) having carboxylic acid functional groups (Figure 8). Partial poly(
L -glutamic acid) having
carboxylic acid functional groups (Figure 8). Partial deacetylation
deacetylation of acetamido groups in the chitin nanofiber film was conducted by the treatment with deacetylation of acetamido groups in the chitin nanofiber film was conducted by the treatment with of acetamido groups in the chitin nanofiber film was conducted by the treatment with 40% (w/v)
40% (w/v) NaOH aq. at 80 °C for 7 h [48] to generate amino initiating groups on the nanofibers (degree 40% (w/v) NaOH aq. at 80 °C for 7 h [48] to generate amino initiating groups on the nanofibers (degree NaOH
aq. at 80 ˝ C for 7 h [48] to generate amino initiating groups on the nanofibers (degree of
of deacetylation = 24%). The resulting partially deacetylated chitin nanofiber film was immersed in a of deacetylation = 24%). The resulting partially deacetylated chitin nanofiber film was immersed in a deacetylation = 24%). The resulting partially deacetylated chitin nanofiber film was immersed in
solution of BLG‐NCA (20 equiv. with an amino group) in ethyl acetate at 0 °C for 24 h to take place asolution of BLG‐NCA (20 equiv. with an amino group) in ethyl acetate at 0 °C for 24 h to take place solution of BLG-NCA (20 equiv. with an amino group) in ethyl acetate at 0 ˝ C for 24 h to take place
the surface‐initiated graft polymerization from amino groups. The IR of the surface‐initiated graft
graft polymerization from amino groups. The IR spectrum spectrum of the the product product the surface-initiated
polymerization
from
amino
groups.
The IR
spectrum
of the product
showed
−1, strongly suggesting the presence of showed C=O absorption due to ester linkage at 1735 cm
−1
´
1
showed C=O absorption due to ester linkage at 1735 cm
, strongly suggesting the presence of C=O absorption due to ester linkage at 1735 cm , strongly suggesting the presence of polyBLG in the
polyBLG in the product, which was explanatorily bound to the nanofibers by covalent linkage. The polyBLG in the product, which was explanatorily bound to the nanofibers by covalent linkage. The product, which was explanatorily bound to the nanofibers by covalent linkage. The grafting amount
grafting amount of the polyBLG chains was evaluated by the weight difference of the films before grafting amount of the polyBLG chains was evaluated by the weight difference of the films before of
the polyBLG chains was evaluated by the weight difference of the films before and after the graft
and polymerization to 18 %. The SEM of film observed and after after the the graft graft to be be 18 wt wt SEM image image of the the produced produced observed polymerization
to bepolymerization 18 wt %. The SEM
image
of%. theThe produced
film observed
nanofiberfilm morphology,
nanofiber morphology, but some fibers were merged at the interfacial areas, that was probably nanofiber morphology, but at
some fibers were merged at the interfacial areas, that was probably but
some fibers
were merged
the interfacial
areas,
that was
probably
caused
by the
grafted
polyBLG
caused by the grafted polyBLG chains present on the nanofibers (Figure 9b). caused by the grafted polyBLG chains present on the nanofibers (Figure 9b). chains present on the nanofibers (Figure 9b).
A highly flexible chitin nanofiber‐graft‐poly(
L‐glutamic acid sodium salt) (chitin nanofiber‐graft‐
A highly flexible chitin nanofiber‐graft‐poly(
A highly flexible chitin nanofiber-graft-poly(LL‐glutamic acid sodium salt) (chitin nanofiber‐graft‐
-glutamic acid sodium salt) (chitin nanofiber-graftpolyLGA) network film was obtained by alkaline hydrolysis of ester linkages in the polyBLG chains polyLGA) network film was obtained by alkaline hydrolysis of ester linkages in the polyBLG chains polyLGA) network film was obtained by alkaline hydrolysis of ester linkages in the polyBLG chains to
to the polyLGA chains, followed by of the sodium carboxylate to generate generate polyLGA chains, followed by condensation condensation the produced produced carboxylate generate
the the polyLGA
chains,
followed
by condensation
of the of produced
sodiumsodium carboxylate
groups
groups with amino groups present in the film (Figure 11). The chitin nanofiber‐graft‐polyBLG film groups with amino groups present in the film (Figure 11). The chitin nanofiber‐graft‐polyBLG film with amino groups present in the film (Figure 11). The chitin nanofiber-graft-polyBLG film was first
was first immersed in 1.0 mol/L NaOH aq. at 60 °C for 5 h for the alkaline hydrolysis of benzyl esters was first immersed in 1.0 mol/L NaOH aq. at 60 °C for 5 h for the alkaline hydrolysis of benzyl esters Coatings 2016, 6, 27
8 of 12
immersed in 1.0 mol/L NaOH aq. at 60 ˝ C for 5 h for the alkaline hydrolysis of benzyl esters to obtain
Coatings 2016, 6, 27 8 of 12 the chitin nanofiber-graft-polyLGA film having sodium carboxylate groups. The IR spectrum of the
film after
the alkaline
treatment
supported that benzyl
esters were
completely
hydrolyzed
to generate
to obtain the chitin nanofiber‐graft‐polyLGA film having sodium carboxylate groups. The IR spectrum of the groups.
film after the alkaline treatment benzyl esters were sodium
carboxylate
The
condensation
of the supported generated that carboxylate
groups
withcompletely amino groups
hydrolyzed to generate sodium carboxylate groups. The condensation of the generated carboxylate present
at the terminal end of the polyLGA chains or the remaining amino groups present on the
groups with amino groups present at the terminal end of the polyLGA chains or the remaining amino nanofibers, which did not induce the initiation of the graft polymerization, was then conducted using
groups present on the nanofibers, which did not induce the initiation of the graft polymerization, was the N-hydroxysuccinimide/
1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (NHS/EDC) condensing
then conducted using the N‐hydroxysuccinimide/ 1‐ethyl‐3‐(3‐dimethylaminopropyl)carbodiimide agent (10 equiv. with a carboxylate group) in water at room temperature for 12 h to construct polyLGA
(NHS/EDC) condensing agent (10 equiv. with a carboxylate group) in water at room temperature for or polyLGA/chitin
networks in the film. The stress-strain curve of the chitin nanofiber-graft-polyBLG
12 h to construct polyLGA or polyLGA/chitin networks in the film. The stress‐strain curve of the film under
tensile
mode
showed the lager elongation value at break than that of the chitin nanofiber
chitin nanofiber‐graft‐polyBLG film under tensile mode showed the lager elongation value at break film with
the comparable
strength
values.
This resulttensile suggested
the values. more elastic
nature of
than that of the chitin tensile
nanofiber film with the comparable strength This result the chitin
nanofiber-graft-polyBLG film. The stress-strain curve of the chitin nanofiber-graft-polyLGA
suggested the more elastic nature of the chitin nanofiber‐graft‐polyBLG film. The stress‐strain curve of the chitin nanofiber‐graft‐polyLGA network film exhibited the larger values both in tensile strength network
film exhibited the larger values both in tensile strength and elongation at break than those
and elongation at break than those of the chitin nanofiber and chitin nanofiber‐graft‐polyBLG films. of the chitin nanofiber and chitin nanofiber-graft-polyBLG films. These data indicated the superior
These data indicated the superior mechanical property of the chitin mechanical
property
of the
chitin
nanofiber-graft-polyLGA
network
filmnanofiber‐graft‐polyLGA as this showed the highly
network film as this showed the highly flexible nature and was bended without breaking. flexible nature and was bended without breaking.
Figure 11. Alkaline treatment of chitin nanofiber‐graft‐polyBLG film and subsequent condensation to Figure
11. Alkaline treatment of chitin nanofiber-graft-polyBLG film and subsequent condensation to
produce chitin nanofiber‐graft‐polyLGA network film. produce chitin nanofiber-graft-polyLGA network film.
Surface‐initiated graft atom transfer radical polymerization (ATRP) from a chitin nanofiber Surface-initiated
atom
transfer radical
(ATRP)
from aliving chitinradical nanofiber
macroinitiator film graft
was also investigated. ATRP polymerization
is one of the facile and versatile macroinitiator
film
was
also
investigated.
ATRP
is
one
of
the
facile
and
versatile
living
radical
polymerization techniques and has been employed to synthesize a wide range of polymeric materials with well‐defined structure [49,50]. Because of the fact that ATRP can be initiated by ‐haloalkylacyl polymerization
techniques and has been employed to synthesize a wide range of polymeric
groups, the well-defined
chitin nanofiber macroinitiator with the initiating groups was by by
materials
with
structure
[49,50]. film Because
of the
fact that
ATRP
canprovided be initiated
esterification of the hydroxy groups on the self‐assembled chitin nanofibers with ‐bromoisobutyryl α-haloalkylacyl groups, the chitin nanofiber macroinitiator film with the initiating groups was provided
bromide (Figure 12) [51,52]. The degree of substitution of ‐bromoisobutyrate groups on the chitin by esterification
of the hydroxy groups on the self-assembled chitin nanofibers with α-bromoisobutyryl
nanofiber film surface was calculated by SEM‐EDX measurement to be 0.61. The surface‐initiated bromide (Figure 12) [51,52]. The degree of substitution of α-bromoisobutyrate groups on the chitin
ATRP of 2‐hydroxyethyl acrylate (HEA) (20 equiv. with an initiating site) from the macroinitiator nanofiber
film surface was calculated by SEM-EDX measurement to be 0.61. The surface-initiated ATRP
film was then examined in the presence of CuBr (catalyst)/2,2’‐bipyridine (Bpy, ligand) in 3 wt % of 2-hydroxyethyl
acrylate (HEA) (20 equiv. with an initiating site) from the macroinitiator film was
LiCl/DMAc at 60 °C to produce chitin nanofiber‐graft‐polyHEA films (Figure 12) [52]. The intensity then ratio examined
in
the
of CuBrto (catalyst)/2,2’-bipyridine
(Bpy,
ligand)
in 3 wt
% LiCl/DMAc
of the ester presence
C=O absorption the amido C=O absorption in the IR spectra of the products at
˝
60 Cincreased to produce
chitin nanofiber-graft-polyHEA
films (Figure
12) [52]. The
ratio
the ester
compared with those of the macroinitiator film, indicating the intensity
progress of the ofgraft conversions of HEA inincreased with ofincreasing reaction times. For gel with
C=O polymerization. absorption to theThe amido
C=O absorption
the IR spectra
the products
increased
compared
thosepermeation chromatography (GPC) analysis of the resulting polyHEAs, the graft chains were cleaved of the macroinitiator film, indicating the progress of the graft polymerization. The conversions
of HEA increased with increasing reaction times. For gel permeation chromatography (GPC) analysis
of the resulting polyHEAs, the graft chains were cleaved from the composite films by alkaline
Coatings 2016, 6, 27
Coatings 2016, 6, 27 9 of 12
9 of 12 from the to
composite films by alkaline hydrolysis yield poly(acrylic which acrylate)s
were then by
hydrolysis
yield poly(acrylic
acid)s, which
wereto then
converted
into acid)s, poly(methyl
converted into poly(methyl acrylate)s by methyl poly(methyl
esterification. acrylate)s
The GPC peaks toof the resulting methyl
esterification.
The GPC peaks
of the
resulting
shifted
higher
molecular
Coatings 2016, 6, 27 9 of 12 poly(methyl acrylate)s shifted to higher molecular weight regions with increasing reaction weight regions with increasing reaction times. These results strongly supported that thetimes. longer
These results strongly supported that the longer polyHEA generated on the chitin from the composite films by on
alkaline hydrolysis to yield acid)s, which were then polyHEA
chains
were generated
the chitin
nanofibers
by poly(acrylic a chains gradualwere ATRP
process
with
prolonged
nanofibers a gradual ATRP process with prolonged reaction times. The SEM image of chitin converted into poly(methyl by methyl esterification. The GPC peaks of the
the resulting reaction
times.by The
SEM
image
ofacrylate)s chitin
nanofiber-graft-polyHEA
film
yielded
from
lower
monomer
nanofiber‐graft‐polyHEA film yielded from the lower monomer conversion (6%, polymerization time poly(methyl acrylate)s shifted to 3
higher molecular weight morphology
regions with increasing reaction times. conversion
(6%, polymerization
time
h) showed
nanofiber
with the increased
fiber width
results strongly morphology supported that the the longer polyHEA chains were compared generated on the that chitin 3 h) These showed nanofiber with increased fiber width with of the compared with that of the macroinitiator film (20 and 40 nm, (Figure 13a,b) respectively). The SEM
nanofibers by a gradual ATRP process with prolonged reaction times. The SEM image of chitin macroinitiator film (20 and 40 nm, (Figure 13a,b) respectively). The SEM image of chitin nanofiber‐
image of
chitin nanofiber-graft-polyHEA film with a higher monomer conversion (71%, polymerization
nanofiber‐graft‐polyHEA film yielded from the lower monomer conversion (6%, polymerization time graft‐polyHEA film with a higher monomer conversion (71%, polymerization time 24 h), on the other time 243 h),
on the other
hand,
did not with observe
the nanofiber
morphology
13c),
probably
h) showed nanofiber morphology the increased fiber width compared (Figure
with that of the hand, did not observe the nanofiber morphology (Figure 13c), probably because that the nanofibers macroinitiator film (20 and 40 nm, (Figure 13a,b) respectively). The SEM image of chitin nanofiber‐
because
that
the
nanofibers
were
totally
covered
by
the
longer
graft
polyHEA
chains.
The
stress-strain
were totally covered by the longer graft polyHEA chains. The stress‐strain curves of the chitin graft‐polyHEA film with a higher monomer conversion (71%, polymerization time 24 h), on the other curves
of
the chitin nanofiber-graft-polyHEA
under
tensile
the values larger elongation
nanofiber‐graft‐polyHEA films under tensile films
mode showed the mode
larger showed
elongation at break hand, did not observe the nanofiber morphology (Figure 13c), probably because that the nanofibers values
at
break
compared
with
the
original
chitin
nanofiber
film.
Moreover,
the
values
increased
with
compared with the original chitin nanofiber film. Moreover, the values increased with increasing were totally covered by the longer graft polyHEA chains. The stress‐strain curves of the chitin increasing
monomer
conversions,
while
the
tensile
strength
values
decreased.
These
data
indicated
monomer conversions, while the tensile strength values the decreased. These values data indicated nanofiber‐graft‐polyHEA films under tensile mode showed larger elongation at break the theenhancement of flexibility by grafting the longer polyHEA chains on the chitin nanofiber film. enhancement
of flexibility
bychitin grafting
the longer
polyHEA
chains
the chitin
compared with the original nanofiber film. Moreover, the values on
increased with nanofiber
increasing film.
monomer conversions, while the tensile strength values decreased. These data indicated the CH3
enhancement of flexibility by grafting the longer polyHEA chains on the chitin nanofiber film. OH
O
HO
Br C
O
OH
NHAcO n
O
ChitinHO
nanofiber
NHAc n
O
C
Br
CH3CH3
Br C
O
C
Br
CH3
OR1
O
R1O
O
R1O
R1 = H,
Chitin nanofiber
R1 = H,
OR1
O
NHAc
O
CH3
C
O
C
OR2
HEA/BPy/CuBr
NHAc
C
n
HEA/BPy/CuBr
n
Br
CH3
CH3
C
O
OR2
R2O
Br
Chitin nanofiberOmacroinitiator
CH3
Chitin nanofiber macroinitiator
O
R2O
R2 = H,
R2 = H,
O
CH3
C
CH3
O
C
C
O
CH3
O
n
NHAc
n
NHAc CH
C
2
CH
CH32
CH
C=O
CH
C=O
n
n
Br
Br
OCH2CH2OH
Chitin nanofiber-graft-polyHEA
OCH2CH2OH
Figure 12. Preparation of chitin nanofiber macroinitiator film and subsequent surface‐initiated graft Figure 12. Preparation of chitin nanofiber macroinitiator film and subsequent surface-initiated graft
Figure 12. Preparation of chitin nanofiber macroinitiator film and subsequent surface‐initiated graft atom transfer radical polymerization (ATRP) of HEA. Chitin nanofiber-graft-polyHEA
atom transfer
radical polymerization (ATRP) of HEA.
atom transfer radical polymerization (ATRP) of HEA. Figure 13. SEM images of chitin nanofiber macroinitiator film (a) and chitin nanofiber‐graft‐polyHEA Figure 13.
SEM images of chitin nanofiber macroinitiator film (a) and chitin nanofiber-graft-polyHEA
Figure 13. SEM images of chitin nanofiber macroinitiator film (a) and chitin nanofiber‐graft‐polyHEA films obtained by the reaction times of 3 and 24 h ((b) and (c), respectively). films obtained
by the reaction times of 3 and 24 h ((b) and (c), respectively).
films obtained by the reaction times of 3 and 24 h ((b) and (c), respectively). Surface‐initiated graft ATRP of methyl methacrylate (MMA) on the chitin nanofibers under Surface-initiated
graft
(MMA)on onthe thechitin chitinnanofibers nanofibers
under
Surface‐initiated graft ATRP
ATRP of
of methyl
methyl methacrylate
methacrylate (MMA) under dispersion conditions was investigated, followed by entanglement of the products to fabricate chitin dispersion
conditions was investigated, followed by entanglement of the products to fabricate chitin
nanofiber‐graft‐polyMMA films with nanofiber morphology (Figure 5) [53]. For graft ATRP on the dispersion conditions was investigated, followed by entanglement of the products to fabricate chitin self‐assembled chitin nanofibers under dispersion conditions, (Figure
the reaction of ‐bromoisobutyryl nanofiber-graft-polyMMA
films with
nanofiber
morphology
5) [53].
For graft ATRP on the
nanofiber‐graft‐polyMMA films with nanofiber morphology (Figure 5) [53]. For graft ATRP on the bromide with hydroxy groups on the chitin nanofibers was conducted in the dispersion to produce self‐assembled chitin nanofibers
nanofibers under
under dispersion
dispersion conditions, self-assembled
chitin
conditions, the thereaction reactionof of‐bromoisobutyryl α-bromoisobutyryl
the macroinitiator in‐situ. Because the large amounts of methanol present in the dispersion bromide with hydroxy groups on the chitin nanofibers was conducted in the dispersion to produce bromide
with hydroxy groups on the chitin nanofibers was conducted in the dispersion to produce the
potentially react with ‐bromoisobutyryl bromide, exchange of dispersion medium from methanol the macroinitiator Because the amounts
large amounts of methanol present in the dispersion macroinitiator
in-situ.in‐situ. Because
the large
of methanol
present in
the dispersion
potentially
to other liquids that did not react with ‐bromoisobutyryl bromide was attempted. Consequently, potentially react with ‐bromoisobutyryl bromide, exchange of dispersion medium from methanol react with
α-bromoisobutyryl
bromide, exchange
of dispersion
medium state fromwas methanol
the exchange to N,N‐dimethylformamide (DMF) with retaining dispersion achieved. to other
to other liquids that did not react with ‐bromoisobutyryl bromide was attempted. Consequently, Accordingly, the reaction of ‐bromoisobutyryl bromide (20 equiv. with a repeating unit of chitin) liquids
that did not react with α-bromoisobutyryl bromide was attempted. Consequently, the exchange
exchange to N,N‐dimethylformamide (DMF) dispersion
with retaining state Accordingly,
was achieved. with hydroxy groups on the chitin nanofibers in the DMF dispersion was carried out in the presence to the N,N-dimethylformamide
(DMF) with retaining
statedispersion was achieved.
the
of pyridine to give the chitin nanofiber macroinitiators. The SEM image of the resulting dispersion Accordingly, the reaction of ‐bromoisobutyryl bromide (20 equiv. with a repeating unit of chitin) reaction of α-bromoisobutyryl bromide (20 equiv. with a repeating unit of chitin) with hydroxy groups
showed the nanofiber morphology. From the intensity ratio of the ester C=O absorption to the amido onwith hydroxy groups on the chitin nanofibers in the DMF dispersion was carried out in the presence the chitin nanofibers in the DMF dispersion was carried out in the presence of pyridine to give the
of pyridine to give the chitin nanofiber macroinitiators. The SEM image of the resulting dispersion chitin nanofiber macroinitiators. The SEM image of the resulting dispersion showed the nanofiber
showed the nanofiber morphology. From the intensity ratio of the ester C=O absorption to the amido morphology. From the intensity ratio of the ester C=O absorption to the amido C=O absorption
in the IR spectrum of the product, the functionality of the initiating sites on chitin nanofibers was
estimated to be 0.37 with a repeating unit of chitin. Surface-initiated graft ATRP of MMA (5 or 10 equiv.
Coatings 2016, 6, 27 10 of 12 C=O absorption in the IR spectrum of the product, the functionality of the initiating sites on chitin Coatings
2016, 6, 27
10 of 12
nanofibers was estimated to be 0.37 with a repeating unit of chitin. Surface‐initiated graft ATRP of MMA (5 or 10 equiv. with an initiating site) on the resulting chitin nanofiber macroinitiators was then with
an initiating site) on the resulting chitin nanofiber macroinitiators was then performed using
performed using CuBr and Bpy in the dispersion with DMF at 60 °C for 24 h. The resulting materials CuBr
and Bpy in the dispersion with DMF at 60 ˝ C for 24 h. The resulting materials were isolated
were isolated from the dispersion by filtration, which formed films. The intensities of the carbonyl from
the dispersion by filtration, which formed films. The intensities of the carbonyl absorptions
absorptions due to ester linkage in the IR spectra of the products was higher than those of the chitin due
to
estermacroinitiators, linkage in the IR
spectra ofthe the progress products of wasthe higher
those of the chitin
nanofiber
nanofiber indicating graft than
polymerization. Moreover, the macroinitiators,
indicating
the
progress
of
the
graft
polymerization.
Moreover,
the
intensity
ratios
intensity ratios in IR spectra also suggested the generation of the longer graft chains using 10 equiv. in
spectra
also
suggested
the generation
of the longer
graft
chainsfilms usingobserved 10 equiv. of
than
of IR
MMA than using 5 equiv. The SEM images of the resulting the MMA
nanofiber using
5
equiv.
The
SEM
images
of
the
resulting
films
observed
the
nanofiber
morphologies,
which
morphologies, which were further merged at interfacial areas (Figure 14). The SEM image of the were
further merged
at nanofiber‐graft‐polyHEA interfacial areas (Figure 14).
The
SEM
image
of the
aforementioned
aforementioned chitin film with the longer graft chains, which chitin
was nanofiber-graft-polyHEA
film
with
the
longer
graft
chains,
which
was
produced
by
surface-initiated
produced by surface‐initiated graft ATRP on the film, did not show nanofiber morphology (Figure graft
onsurface‐initiated the film, did not graft show nanofiber
morphology
(Figure
13c). In the
13c). ATRP
In the ATRP under dispersion conditions, on surface-initiated
the other hand, graft
the ATRP
under dispersion
conditions,
on the other
hand, the polymerization
occurred
the surfaces
of
polymerization occurred on the surfaces of individual nanofibers. Then, the on
nanofibers were individual
nanofibers.
Then,
the
nanofibers
were
entangled
during
filtration
and
merged
among
the
entangled during filtration and merged among the non‐crystalline polyMMA graft chains on the non-crystalline
nanofibers. polyMMA graft chains on the nanofibers.
Figure 14. SEM images of chitin nanofiber‐graft‐polyMMA film ((a) MMA 5 equiv. and (b) MMA 10 Figure
14. SEM images of chitin nanofiber-graft-polyMMA film ((a) MMA 5 equiv. and (b) MMA 10 equiv.).
equiv.). 4. Conclusions
4. Conclusions Based on the fact that AMIMBr formed a chitin ion gel, the self-assembled chitin nanofiber film
Based on the fact that AMIMBr formed a chitin ion gel, the self‐assembled chitin nanofiber film was easily fabricated by the regeneration from the gel using methanol, followed by sonication and
was easily fabricated by the regeneration from the gel using methanol, followed by sonication and filtration. The highly entanglement from the nanofibers during the process contributed to formation
filtration. The highly entanglement from the nanofibers during the process contributed to formation the film. The co-regeneration approach from the ion gel achieved to produce the chitin nanofiber/PVA
the film. The co‐regeneration approach from the ion gel achieved to produce the chitin nanofiber/PVA composite film. The surface-initiated graft polymerization technique from the chitin nanofiber film
composite film. The surface‐initiated graft polymerization technique from the chitin nanofiber film was conducted to obtain the composite films with polyester, polypeptide, and poly(meth)acrylates.
was conducted to obtain the composite films with polyester, polypeptide, and poly(meth)acrylates. The present approaches provide the efficient use of the representative biomass, chitin to fabricate the
The present approaches provide the efficient use of the representative biomass, chitin to fabricate the nanofiber film, which further convert into composite films with synthetic polymers. These chitin-based
nanofiber film, which further convert into composite films with synthetic polymers. These chitin‐
nanofiber films have a potential to be employed as practical materials in biomedical, tissue engineering,
based nanofiber films have a potential to be employed as practical materials in biomedical, tissue and environmentally benign fields in the future.
engineering, and environmentally benign fields in the future. Acknowledgments:
The author is indebted to the co-workers, whose names are found in references from his
Acknowledgments: The author is indebted to the co‐workers, whose names are found in references from his papers, for their enthusiastic collaborations.
papers, for their enthusiastic collaborations. Conflicts of Interest: The author declares no conflict of interest.
Conflicts of Interest: The author declares no conflict of interest. References
References 1.
1.
2.
2.
3.
3.
4.
4.
5.
Schuerch, C. Polysaccharides. In Encyclopedia of Polymer Science and Engineering, 2nd ed.; Mark, H.F.,
Schuerch, C. Polysaccharides. In Encyclopedia of Polymer Science and Engineering, 2nd ed.; Mark, H.F., Bilkales, N., Overberger, C.G., Eds.; John Wiley & Sons: New York, NY, USA, 1986; Volume 13, pp. 87–162.
Bilkales, N., Overberger, C.G., Eds.; John Wiley & Sons: New York, NY, USA, 1986; Volume 13, pp. 87–162. Berg, J.M.; Tymoczko, J.L.; Stryer, L. Biochemistry, 7th ed.; W.H. Freeman: New York, NY, USA, 2012.
Berg, J.M.; Tymoczko, J.L.; Stryer, L. Biochemistry, 7th ed.; W.H. Freeman: New York, NY, USA, 2012. Klemm, D.; Heublein, B.; Fink, H.P.; Bohn, A. Cellulose: Fascinating biopolymer and sustainable raw
Klemm, D.; Heublein, B.; Fink, H.P.; Bohn, A. Cellulose: Fascinating biopolymer and sustainable raw material. Angew. Chem. Int. Ed. 2005, 44, 3358–3393. [CrossRef] [PubMed]
material. Angew. Chem. Int. Ed. 2005, 44, 3358–3393. Kurita, K. Chitin and chitosan: Functional biopolymers from marine crustaceans. Mar. Biotechnol. 2006, 8,
Kurita, K. Chitin and chitosan: Functional biopolymers from marine crustaceans. Marine Biotechnol. 2006, 203–226. [CrossRef] [PubMed]
8, 203–226. Rinaudo,
M. Chitin and chitosan: Properties and applications. Prog. Polym. Sci. 2006, 31, 603–632. [CrossRef]
Coatings 2016, 6, 27
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
11 of 12
Pillai, C.K.S.; Paul, W.; Sharma, C.P. Chitin and chitosan polymers: Chemistry, solubility and fiber formation.
Prog. Polym. Sci. 2009, 34, 641–678. [CrossRef]
Muzzarelli, R.A.A. Biomedical exploitation of chitin and chitosan via mechano-chemical disassembly,
electrospinning, dissolution in imidazolium ionic liquids, and supercritical drying. Mar. Drugs 2011, 9,
1510–1533. [CrossRef] [PubMed]
El Seoud, O.A.; Koschella, A.; Fidale, L.C.; Dorn, S.; Heinze, T. Applications of ionic liquids in carbohydrate
chemistry: A window of opportunities. Biomacromolecules 2007, 8, 2629–2647. [CrossRef] [PubMed]
Liebert, T.; Heinze, T. Interaction of ionic liquids with polysaccharides 5. Solvents and reaction media for the
modification of cellulose. Bioresources 2008, 3, 576–601.
Feng, L.; Chen, Z.I. Research progress on dissolution and functional modification of cellulose in ionic liquids.
J. Mol. Liq. 2008, 142, 1–5. [CrossRef]
Pinkert, A.; Marsh, K.N.; Pang, S.S.; Staiger, M.P. Ionic liquids and their interaction with cellulose. Chem. Rev.
2009, 109, 6712–6728. [CrossRef] [PubMed]
Gericke, M.; Fardim, P.; Heinze, T. Ionic liquids—Promising but challenging solvents for homogeneous
derivatization of cellulose. Molecules 2012, 17, 7458–7502. [CrossRef] [PubMed]
Isik, M.; Sardon, H.; Mecerreyes, D. Ionic liquids and cellulose: Dissolution, chemical modification and
preparation of new cellulosic materials. Int. J. Mol. Sci. 2014, 15, 11922–11940. [CrossRef] [PubMed]
Welton, T. Room-temperature ionic liquids. Solvents for synthesis and catalysis. Chem. Rev. 1999, 99,
2071–2083. [CrossRef] [PubMed]
Hallett, J.P.; Welton, T. Room-temperature ionic liquids: Solvents for synthesis and catalysis. 2. Chem. Rev.
2011, 111, 3508–3576. [CrossRef] [PubMed]
Wasserscheid, P.; Keim, W. Ionic liquids—New “solutions” for transition metal catalysis. Angew. Chem.
Int. Ed. 2000, 39, 3772–3789.
Davis, J.H. Task-specific ionic liquids. Chem. Lett. 2004, 33, 1072–1077. [CrossRef]
Lee, S.G. Functionalized imidazolium salts for task-specific ionic liquids and their applications.
Chem. Commun. 2006, 1049–1063. [CrossRef] [PubMed]
Giernoth, R. Task-specific ionic liquids. Angew. Chem. Int. Ed. 2010, 49, 2834–2839. [CrossRef] [PubMed]
Graenacher, C. Cellulose solution. US Patent 1943176, 1934.
Swatloski, R.P.; Spear, S.K.; Holbrey, J.D.; Rogers, R.D. Dissolution of cellose with ionic liquids. J. Am.
Chem. Soc. 2002, 124, 4974–4975. [CrossRef] [PubMed]
Zakrzewska, M.E.; Bogel-Łukasik, E.; Bogel-Łukasik, R. Solubility of carbohydrates in ionic liquids.
Energy Fuels 2010, 24, 737–745. [CrossRef]
Wang, W.T.; Zhu, J.; Wang, X.L.; Huang, Y.; Wang, Y.Z. Dissolution behavior of chitin in ionic liquids.
J. Macromol. Sci. Part B Phys. 2010, 49, 528–541. [CrossRef]
Jaworska, M.M.; Kozlecki, T.; Gorak, A. Review of the application of ionic liquids as solvents for chitin.
J. Polym. Eng. 2012, 32, 67–69. [CrossRef]
Wu, Y.; Sasaki, T.; Irie, S.; Sakurai, K. A novel biomass-ionic liquid platform for the utilization of native chitin.
Polymer 2008, 49, 2321–2327. [CrossRef]
Prasad, K.; Murakami, M.; Kaneko, Y.; Takada, A.; Nakamura, Y.; Kadokawa, J. Weak gel of chitin with ionic
liquid, 1-allyl-3-methylimidazolium bromide. Int. J. Biol. Macromol. 2009, 45, 221–225. [CrossRef] [PubMed]
Qin, Y.; Lu, X.M.; Sun, N.; Rogers, R.D. Dissolution or extraction of crustacean shells using ionic liquids to
obtain high molecular weight purified chitin and direct production of chitin films and fibers. Green Chem.
2010, 12, 968–971. [CrossRef]
Zhao, D.B.; Fei, Z.F.; Geldbach, T.J.; Scopelliti, R.; Laurenczy, G.; Dyson, P.J. Allyl-functionalised ionic liquids:
Synthesis, characterisation, and reactivity. Helv. Chim. Acta 2005, 88, 665–675. [CrossRef]
Sharma, M.; Mukesh, C.; Mondal, D.; Prasad, K. Dissolution of a-chitin in deep eutectic solvents. RSC Adv.
2013, 3, 18149–18155. [CrossRef]
Kadokawa, J.; Takegawa, A.; Mine, S.; Prasad, K. Preparation of chitin nanowhiskers using an ionic liquid
and their composite materials with poly(vinyl alcohol). Carbohydr. Polym. 2011, 84, 1408–1412. [CrossRef]
Kadokawa, J. Ionic liquid as useful media for dissolution, derivatization, and nanomaterial processing of
chitin. Green Sustain. Chem. 2013, 3, 19–25. [CrossRef]
Coatings 2016, 6, 27
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
12 of 12
Takada, A.; Kadokawa, J. Fabrication and characterization of polysaccharide ion gels with ionic liquids and
their further conversion into value-added sustainable materials. Biomolecules 2015, 5, 244–262. [CrossRef]
[PubMed]
Kadokawa, J. Fabrication of nanostructured and microstructured chitin materials through gelation with
suitable dispersion media. RSC Adv. 2015, 5, 12736–12746. [CrossRef]
Tajiri, R.; Setoguchi, T.; Wakizono, S.; Yamamoto, K.; Kadokawa, J. Preparation of self-assembled chitin
nanofibers by regeneration from ion gels using calcium halide¨ dihydrate/methanol solutions. J. Biobased
Mater. Bioenergy 2013, 7, 655–659. [CrossRef]
Mukesh, C.; Mondal, D.; Sharma, M.; Prasad, K. Choline chloride-thiourea, a deep eutectic solvent for the
production of chitin nanofibers. Carbohydr. Polym. 2014, 103, 466–471. [CrossRef] [PubMed]
Felix, J.M.; Gatenholm, P. The nature of adhesion in composites of modified cellulose fibers and
polypropylene. J. Appl. Polym. Sci. 1991, 42, 609–620. [CrossRef]
Valadez-Gonzalez, A.; Cervantes-Uc, J.M.; Olayo, R.; Herrera-Franco, P.J. Effect of fiber surface treatment on
the fiber–matrix bond strength of natural fiber reinforced composites. Compos. Part B Eng. 1999, 30, 309–320.
[CrossRef]
Vautard, F.; Fioux, P.; Vidal, L.; Schultz, J.; Nardin, M.; Defoort, B. Influence of the carbon fiber surface
properties on interfacial adhesion in carbon fiber–acrylate composites cured by electron beam. Compos. Part A
Appl. Sci. Manuf. 2011, 42, 859–867. [CrossRef]
Setoguchi, T.; Yamamoto, K.; Kadokawa, J. Preparation of chitin nanofiber-graft-poly(L-lactide-co-e-caprolactone)
films by surface-initiated ring-opening graft copolymerization. Polymer 2012, 53, 4977–4982. [CrossRef]
Albertsson, A.C.; Varma, I.K. Aliphatic polyesters: Synthesis, properties and applications. Adv. Polym. Sci.
2002, 157, 1–40.
Seyednejad, H.; Ghassemi, A.H.; van Nostrum, C.F.; Vermonden, T.; Hennink, W.E. Functional aliphatic
polyesters for biomedical and pharmaceutical applications. J. Control. Release 2011, 152, 168–176. [CrossRef]
[PubMed]
Lou, X.D.; Detrembleur, C.; Jérôme, R. Novel aliphatic polyesters based on functional cyclic (di)esters.
Macromol. Rapid Commun. 2003, 24, 161–172. [CrossRef]
Jérôme, C.; Lecomte, P. Recent advances in the synthesis of aliphatic polyesters by ring-opening
polymerization. Adv. Drug Deliv. Rev. 2008, 60, 1056–1076. [CrossRef] [PubMed]
Grijpma, D.W.; Pennings, A.J. Polymerization temperature effects on the properties of L-lactide and
e-caprolactone copolymers. Polym. Bull. 1991, 25, 335–341. [CrossRef]
Kricheldorf, H.R. Polypeptides and 100 years of chemistry of alpha-amino acid N-carboxyanhydrides.
Angew. Chem. Int. Ed. 2006, 45, 5752–5784. [CrossRef] [PubMed]
Deming, T.J. Synthetic polypeptides for biomedical applications. Prog. Polym. Sci. 2007, 32, 858–875. [CrossRef]
Kadokawa, J.; Setoguchi, T.; Yamamoto, K. Preparation of highly flexible chitin nanofiber-graft-poly(g-L-glutamic
acid) network film. Polym. Bull. 2013, 70, 3279–3289. [CrossRef]
Phongying, S.; Aiba, S.; Chirachanchai, S. Direct chitosan nanoscaffold formation via chitin whiskers. Polymer
2007, 48, 393–400. [CrossRef]
Kamigaito, M.; Ando, T.; Sawamoto, M. Metal-catalyzed living radical polymerization. Chem. Rev. 2001, 101,
3689–3745. [CrossRef] [PubMed]
Matyjaszewski, K. Atom transfer radical polymerization (ATRP): Current status and future perspectives.
Macromolecules 2012, 45, 4015–4039. [CrossRef]
Yamamoto, K.; Yoshida, S.; Mine, S.; Kadokawa, J. Synthesis of chitin-graft-polystyrene via atom transfer
radical polymerization initiated from a chitin macroinitiator. Polym. Chem. 2013, 4, 3384–3389. [CrossRef]
Yamamoto, K.; Yoshida, S.; Kadokawa, J. Surface-initiated atom transfer radical polymerization from chitin
nanofiber macroinitiator film. Carbohydr. Polym. 2014, 112, 119–124. [CrossRef] [PubMed]
Endo, R.; Yamamoto, K.; Kadokawa, J. Surface-initiated graft atom transfer radical polymerization of methyl
methacrylate from chitin nanofiber macroinitiator under dispersion conditions. Fibers 2015, 3, 338–347.
[CrossRef]
© 2016 by the author; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).