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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/).