Sustainable Chitin Nanofibrils Provide Outstanding Flame-Retardant

Subscriber access provided by United Arab Emirates University | Libraries Deanship

Article

Sustainable Chitin Nanofibrils Provide Outstanding Flame-Retardant Nanopapers Felix Riehle, Daniel Hoenders, Jiaqi Guo, Alexander Eckert, Shinsuke Ifuku, and Andreas Walther Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b01766 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 8, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21 1 2 3 4 5 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

Sustainable Chitin Nanofibrils Provide Outstanding Flame-Retardant Nanopapers Felix Riehle,†, ‡, § Daniel Hoenders,†, ‡, § Jiaqi Guo,†, ‡, § Alexander Eckert,// Shinsuke Ifuku,∔ and Andreas Walther†, ‡, §, ^* †

Institute for Macromolecular Chemistry, Stefan-Meier-Strasse 31, University of Freiburg, 79104 Freiburg, Germany ‡

Freiburg Materials Research Center, Stefan-Meier-Strasse 21, University of Freiburg, 79104 Freiburg, Germany §

Freiburg Center for Interactive Materials and Bioinspired Technologies, Georges-Köhler-Allee 105, University of Freiburg, 79110 Freiburg, Germany //

DWI – Leibniz-Institute for Interactive Materials, Forckenbeckstr. 50, 52056 Aachen, Germany

∔

Graduate School of Engineering, Tottori University, 101-4 Koyama-cho Minami, Tottori, 6808502, Japan ^

Freiburg Institute for Advanced Studies, University of Freiburg, 79104 Freiburg, Germany

* Correspondence to [email protected]

ACS Paragon Plus Environment

1

Biomacromolecules 1 2 3 4 5 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 21

Abstract Sustainable polysaccharides nanofibrils formed from chitin or cellulose are emerging biobased nanomaterials for advanced materials requiring high mechanical performance, barrier properties, for bioactive materials or other functionalities. Here, we demonstrate a single-step, waterborne approach to prepare additive-free flame-retardant and self-extinguishing, mechanical high-performance nanopapers based purely on surface-deacetylated chitin nanofibrils (ChNFs). We show that the flammability can be critically reduced by exchanging the counterions, e.g. to the phosphate type, using the respective acid providing electrostatic stabilization in the preparation of the ChNFs. This exchange renders beneficial elemental combinations of high contents of N/P in the final nanopapers, known to provide outstanding performance in halogen- and heavy metal-free flame-retardant materials. Full fire barrier nanopapers can even be obtained by hybridizing the ChNF with nanoclay. Comprehensive fire retardancy tests, including vertical and horizontal flame tests and microscale cone combustion calorimetry, as well as fire breakthrough tests elucidate excellent flame-retardant properties and high structural integrity when being burned. The intrinsic elemental composition of chitin, containing nitrogen, and the simple modification of the counterions to include phosphorus provides key advantages over related, but flammable nanocellulose materials, that often require significant chemical modifications and additives to become fire retardant. By activating a global food waste, this study presents a critical advance for bioinspired, green and mechanical highperformance materials with extraordinary flame-retardant and fire barrier properties based on sustainable feedstock, using benign water-based room temperature processing, and by avoiding heavy metals and halogen atoms in their composition.


2

Page 3 of 21 1 2 3 4 5 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

Introduction The transformation towards future biobased and sustainable economies requires to identify pathways towards bio-based, high-performance and multifunctional materials using sustainable feedstocks. Chitin is next to cellulose the most relevant and abundant biopolymer on earth with a largely untapped potential.1, 2 The fishery industry produces several million tons of chitincontaining crustacean cuticles and shells as waste products worldwide every year. 3Beside its natural abundancy, chitin is also known to be nontoxic, antibacterial and biodegradable, which attracts attention in wide range of applications.4-6 Its current applications are mostly based on the chitin-derived water-soluble chitosan polymers (deacetylated chitin) as used in cosmetics, water treatment or biomedical devices.7-10 Similar to cellulose, chitin forms extended fibrillar crystal structures, which lead to high mechanical strength and Young’s modulus (E ≈ 4160 GPa), resulting in highly attractive mechanical properties.11-14 Nature has optimized the nanocrystalline dimensions and morphologies for excellent combinations of stiffness, strength and toughness in natural nanocomposite materials.15, 16 In contrast, classical recrystallization approaches starting from molecular dissolution of chitin or cellulose, do typically not allow to recover similar morphologies and mechanical properties, and often require the use of harsh, non-green solvents to intermediately break the interfibrillar hydrogen bonds.17 This has motivated to isolate nanofibrillar chitin or nanofibrillar cellulose directly from natural organisms without excessive degradation of the crystalline structure and morphology. For both material classes, chitin nanofibrils (ChNFs) as well as cellulose nanofibrils (CNFs), the last decade has seen large progress in their isolation, chemical modification and integration into multifunctional and mechanical high performance materials.9, 11, 12, 18-36 Overall, most attention in functional materials has so far been devoted to CNFs, whereas ChNF may have unexplored advantages due to their different chemical makeup. For instance, CNFs have been researched rather intensely already for flame-retardant and fire barrier materials, but their inherent flammability due to being a CHO material requires tailored modifications to become suitable.19, 37-45 On the contrary, very little attention has been devoted to the potential for fire barrier and flame retardant, and yet mechanically most robust materials, based on ChNF. In contrast to CNFs, surface-deacetylated chitin nanofibrils (ChNFs) are rich in nitrogen (internal amides and surface amines, CHNO material), which have a critical functional benefit when it comes to inherent fire retardancy due to the formation of ammonia during combustion, which increases the thermo-oxidative stability.46 In addition, it is known that phosphoruscontaining species, and in particular the combination of phosphorus and amines can greatly improve flame retardancy.47-50 Phosphorus acts in both the gas and condensed phase.51, 52 PO radicals can slow down the oxidation of hydrocarbons through recombination with highly reactive radicals in the gas phase, which renders them less harmful degradation products. 53, 54 Moreover, phosphorus-containing flame retardants lead to endothermic dehydration and char formation due to the formation of strong phosphoric acid.55, 56 Advantageously, from a chemical and materials perspective, surface-deacetylated ChNFs provide a high density of surface amine groups, that are easily modified in their counterion composition using appropriate acids in late stages of their preparation.18, 57 This motivates strongly to investigate and understand the potential of ChNFs for fire barrier materials, as it could provide a possibly very powerful and thus far unexplored sustainable materials platform for intensely needed halogen- and heavy metal-free flame retardant materials. Replacing heavy metals and halogens, which are of high ACS Paragon Plus Environment

3


Page 4 of 21

toxicity to living organisms and tedious to recycle, will be a key aspect for promoting green future flame retardant materials, and because they are likely to being banned in the near future.58-61 To this end, we here show a facile, waterborne approach to mechanically robust and translucent, flame-retardant and fire barrier ChNF nanopapers that can be tuned and improved in their performance by simple exchange of the counterions of the surface amine groups in surfacedeacetylated ChNFs. We study the fire barrier and fire retardancy properties as a function of the counterion by thermogravimetric analysis (TGA), fire break-through test (FBT), vertical flame test (VFT), comparable to the UL94 standard, and micro-scale cone combustion calorimetry (MCC). In addition, we prepare ChNF/nanoclay nanocomposites to increase the thermal stability even further while maintaining the high-performance materials character in terms of mechanical properties. These nanocomposites enable the formation of nanopapers with a layered arrangement of nanoclay surrounded by a ChNF matrix and an improved stiffness, as well as high structural integrity when being burned. The mechanical properties of the nanocomposite can be tuned by the ratio of ChNF to nanoclay and we show that even small ratios of the inorganic component further reduce the flammability significantly. Hence, this study establishes a platform for bioinspired, green, and mechanical high-performance materials with extraordinary flame-retardant and fire barrier properties based on sustainable feedstock and using benign water-based room temperature processing.

Experimental Section Preparation of ChNF Based on our previous procedure a 1.3 wt% suspension of ChNF with deacetylated surface was prepared by first putting chitin powder into a NaOH aqueous solution bath and refluxing for 6 h.18, 62 The resulting product was washed several times by centrifugation and subjected to homogenization in a stone grinder MKCA6-3 (Masuko Sangyo Co., Ltd.) at a speed rotation of 1500 rpm using -1.5 as gap height. Acetic acid (HAc) was added to facilitate disintegration (pH = 4). Counterion exchange The counterion exchange consists of five main steps: (i) 0.5 wt% ChNF-Ac dispersions are prepared by addition of DI water, (ii) adjustment of the pH to pH = 10 with NaOH to form colloidally unstable ChNF-NH2, (iii) centrifugation for 20 min at 6000 rpm to collect a pellet of ChNF-NH2 and remove excess NaOAc, (iv) redispersion of ChNF-NH2 at pH = 5.5 with either HCl or H3PO4 to yield ChNF-Cl or ChNF-P, respectively. Steps i-iv are repeated twice to ensure maximum exchange of counterions. Finally, (v) the dispersions are dialyzed (MWCO: 6-8 kDa) against the diluted acidic solutions (pH = 5.5) water to remove excess acid. Preparation of ChNF-nanopapers 40 mL of a 0.5 wt% suspension were added to polystyrene petri dishes and dried at room temperature to yield films of ca. 25 µm thickness.


4

Page 5 of 21 1 2 3 4 5 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

Preparation of CNF-nanopapers The CNFs are prepared by TEMPO-mediated oxidation at neutral conditions and subsequent microfluidization according to literature.26 The content of carboxyl groups is 0.44 mmol g-1, the degree of crystallinity is 77%. The nanopapers from CNFs are prepared by film casting of a 0.19 wt% suspension of CNF at pH = 9 at room temperature to give films of ca. 25 µm thickness. Exfoliation of montmorillonite A 1.8 wt% aqueous dispersion of montmorillonite (BYK) was centrifuged at 12.000 rpm for 90 minutes. After the centrifugation, three different phases were obtained. A solid brown bottom phase, which consists mainly of aggregated clay, a yellow gel phase in the middle, which contains the desired delaminated nanoclay with 8 wt% solid content and a clear aqueous top phase. The yellow gel phase was collected and diluted with DI water to 0.5 wt%. Preparation of ChNF/MTM nanopapers ChNF/MTM nanopapers were prepared by first slowly adding a well delaminated montmorillonite dispersion (0.5 wt%) to a ChNF-P dispersion (0.5 wt%). Then, the mixtures were stirred for 18 h and cast in polystyrene petri dishes at room temperature to yield films of ca. 25 µm thickness. Characterization techniques Fourier-transform infrared spectroscopy (FTIR) was recorded on a CARY-630 FTIR (Agilent Technologies, USA) using attenuated total reflectance (ATR). UV-Vis spectra were measured on a UV-1800 spectrometer (Shimadzu, Japan). Elemental analysis was conducted on an Elementar Vario EL (Elementar, Germany) or via ion chromatography on a IC 100 (Metrohm, Switzerland). Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was recorded on an iCap 6500 (Thermo Fisher Scientific, USA). Zeta potentials were measured using a Zetasizer Zen 3600 (Malvern Instruments, USA). Measurements were performed three times in disposable capillary cells. X-ray diffraction (XRD) was measured either on a XRD 3003 TT (Seifert Analytical X-ray, Germany), equipped with a NaI scintillation counter, using a copper anode utilizing Cu Kα1 radiation (λ = 1.54 Å), or on an Empyrean setup (Malvern Panalytical, USA) using Cu-Kα1 radiation (λ = 1.54 Å) and a Bragg-Brentano parallel-beam geometry. Deconvolution of the x-ray diffractogram (XRD) and calculation of the areas from the crystalline and amorphous parts was used to determine the crystallinity index. Field emission-scanning electron microscopy (SEM) was performed on a FEI Scios 2 field emission microscope (Thermo Fisher Scientific) using 5 kV acceleration voltage. Samples were sputtered with a thin Au/Pd layer. Conductometric titrations were recorded on a Titrando 907 (Metrohm, Switzerland) at 25 °C. Atomic force microscopy (AFM) was performed on a MultiMode V (Digital Instruments, USA) under ambient conditions in tapping mode. The samples were obtained from deposition of a diluted suspension in water (ca. 0.005 wt%) onto freshly cleaved mica. The heights of the ChNFs, which are not subject to tip broadening artifacts, were used to determine the diameter. A sample size of 200 nanofibrils was used for the calculations. Mechanical tensile tests were carried out on a MT200 (Deben, UK) equipped with a 20 N load cell at room temperature. The specimen size used was ACS Paragon Plus Environment

5


Page 6 of 21

10 mm × 2 mm × 25 µm. At least five specimens were tested for each material. A nominal strain rate of 1.0 mm/min was used. The samples were conditioned at 50% relative humidity (% RH) for 40 h prior to measuring. The slope of the linear region of the stress/strain curves was used to determine the Young’s modulus. Thermogravimetric analysis (TGA) was conducted using a STA409C instrument (Netzsch, Germany) in N2 as well as O2 atmosphere from 50 to 550 °C at a heating rate of 10 °C/min. Microcone calorimetry (MCC) was measured on a MCC-2 (Govmark, UK) according to ASTM D 7309B. Three measurements were conducted for each sample. Fire break through tests (FBT) were performed using a Soudogaz (CR) X 2000 PZ gas torch with a flame temperature of ca. 1750 °C. The samples were fixed in a circular metal holder with a diameter of 7 cm and exposed to the flame at a distance of 13 cm and an angle close to 90°, and monitored using a video camera. Bench-scale vertical flame tests (VFT), were performed on the films with dimensions of 6 × 1 cm (L×W) in analogy to EN ISO 11925-2 and the UL94 standards. A gas flame was kept at the bottom of the specimens for a duration of 3 sec and the height of the burned area was monitored using a video camera and subsequently quantified by averaging three samples.

Results and Discussion Material Design and Structural and Mechanical Characterization The basic component for our polysaccharide nanofibril-derived flame-retardant and fire barrier materials are surface-deacetylated ChNFs. Those are prepared by mild fibrillation of crustacean wastes to preserve the highly crystalline character and high aspect ratio of the nanofibrils.18 During the process, parts of the chitin amide groups on the fibrils are deacetylated, leading to surface-deacetylated ChNFs, which are stabilized by electrostatic repulsion of the ammonium groups typically using acetic acid. Atomic force microscopy (AFM) depicts entangled micrometer-long nanofibrils with an average diameter of 5.4 ± 1.1 nm, as deduced by statistical height analysis (Figure 1a). Most importantly, the crystalline character of the nanofibrils is largely preserved during the extraction with a crystallinity index of 44% according to x-ray diffraction. Conductometric titration of the ChNFs yields a degree of deacetylation of 33.7 ± 0.3%, corresponding to 1.78 mmol g-1 free amine groups, which guarantees a sufficiently high fraction of ionizable amine groups to induce a noticeable effect after counterion exchange (Figure 1b).

Figure 1. Characterization of the surface-deacetylated ChNFs. (a) AFM height image of ChNF and their diameter distribution as inset (z-scale = 10 nm). (b) Conductometric titration of ChNF-Cl with NaOH. I = ACS Paragon Plus Environment

6

Page 7 of 21 1 2 3 4 5 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

neutralization of free H+-ions, II = deprotonation of surface ammonium groups, III = increasing excess of OH- ions in solution.

As outlined in the introduction, it is known that halogens and phosphorus, as well as the combination of phosphorus and amine, can greatly reduce the flammability of organic materials.47, 51, 54 Therefore, we hypothesized that a simple counterion exchange of the acetate counterions (Ac), typically present due to acidification with acetic acid at the final step of the preparation of ChNFs, against phosphate-type (pH-dependent character; H2PO4-; HPO42-; PO43) or chloride would have a substantial impact on the flammability of ChNF-based materials. Using HCl or H3PO4, chloride and phosphate-type counterions can be introduced easily via ionic interactions at the ammonium group of the surface-deacetylated ChNFs. To this end, we applied a stringent and highly standardized protocol to ensure the counterion exchange (Scheme 1). First, we purposely induced aggregation of the ChNFs by setting 0.5 wt% ChNF dispersions to pH = 11 with NaOH. This deprotonates the ammonium groups on the ChNF surface, and the ChNF-NH2 dispersion coagulates and can be pelletized by centrifugation. This pellet can be redispersed using the appropriately diluted acids at pH = 5.5. Repeating the precipitation and redispersion step and additional dialysis against the diluted acidic solutions (pH = 5.5) ensure efficient counterion exchange.

Scheme 1: Counterion exchange towards ChNF-P and ChNF-Cl and preparation of the corresponding nanopapers.

The corresponding samples are called ChNF-P and ChNF-Cl, respectively. The success of the counterion exchange can be traced using energy-dispersive X-ray spectroscopy (EDX) in scanning electron microscopy (SEM) of solid samples (Figure 2a). A distinct signal for phosphorus (Kα = 2.013 keV) is clearly visible in the ChNF-P film, while it is absent in ChNF-Ac and ChNF-Cl. Correspondingly, the chlorine signal (Kα = 2.621 keV) is only visible in ChNF-Cl, which indicates proper substitution. Moreover, the counterion substitution can be quantified by elemental analysis with inductively coupled plasma optical emission spectroscopy (ICP-OES). The measurements yield 2.42 wt% chlorine and 2.14 wt% phosphorus in ChNF-Cl and ChNF-P, respectively. Based on the degree of deacetylation of 33%, this demonstrates the decoration of ca. 38% of all deacetylated amine groups with chloride counterions in ChNF-Cl and ca. 40% with phosphate-type counterions in ChNF-P, respectively. Additionally, X-ray diffraction (XRD) demonstrates that the anion exchange does not affect the crystallinity of the ChNFs, as expected (Figure 2b). Overall, the strategy shows a convenient way to exchange the counterions of ChNF from acetate to either chloride or phosphate.


7


Page 8 of 21

Figure 2: Characterization of the counterion exchange and of the properties of the corresponding nanopapers. (a) EDX spectra to monitor counterion exchange, (b) X-ray diffractogramms displaying similar crystallinity independent of the counterion, (c) UV-Vis spectra demonstrating good transparency, (d) Tensile curves of the nanopapers indicating similar mechanical properties.

Nanopapers can be formed from these ChNFs by film casting the aqueous dispersions for instance in petri dishes to yield transparent and strong films, prepared with a typical thickness of 25 µm. These nanopapers show a slight decrease of transparency after counterion exchange (Figure 2c). The ChNF-Ac films exhibit a transmission up to ca. 75% at 600 nm, while ChNFP and ChNF-Cl show a transparency of ca. 60%. This slight decrease may arise from the formation of some aggregates during the precipitation/centrifugation step, which do not fully redisperse during acidification. The corresponding tensile tests however show very similar mechanical properties independent of the counterions (Figure 2d, Table 1). The Young’s moduli of the various ChNF nanopapers range between 8.5 and 9.2 GPa and the tensile strength reaches up to almost 200 MPa at an elongation to break of ca. 5%. These values are at the top end of ChNF-based nanopapers and partly surpass them.31, 63 We assume that better fibrillation during preparation and the degree of deacetylation may be source of this beneficial behavior. Overall, these mechanical properties are in range with the mechanical properties frequently reported for CNF-based nanopapers.24 Table 1: Mechanical properties of nanopapers with different counterions. Specimen

E (GPa)a

σUTS (MPa)b

εb (%)c

Ut (MJ/m3)d

ChNF-P

8.5 ± 0.2

196 ± 10

5.2 ± 0.9

7.1 ± 1.6

ChNF-Cl

8.8 ± 1.0

182 ± 13

5.3 ± 1.0

7.2 ± 1.8

ChNF-Ac

9.2 ± 0.5

199 ± 13

5.0 ± 0.4

6.8 ± 0.9

a

Young’s modulus obtained from the slope in the region of linear elastic deformation. b Tensile strength and c strain at break obtained from the last point before fracture. d Work of fracture calculated from the area under the σ−ε curve.

Thermal Characterization and Fire Retardancy We used vertical flame tests (VFT), in analogy to the UL94 standard, thermogravimetric analysis (TGA) and microscale combustion cone calorimetry (MCC) to quantify the influence of the different counterions on the flame retardancy and the thermo-oxidative behavior. A nanopaper formed from tempo-oxidized CNFs was tested using exactly the same methods, so as to provide relevant data for the most appropriate material comparison exhibiting a similar multifunctional property profile (polysaccharide nanofibrils, high crystallinity, high mechanical strength, transparency).


8

Page 9 of 21 1 2 3 4 5 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

For the bench-scale VFT, the nanopapers were exposed to a gas flame at the bottom of the specimens (size 6 x 1 cm) for a duration of ca. 3 s, and the height of the burned area was constantly monitored via a video camera (Figure 3a-c). Nanopapers consisting of CNF readily ignite with a bright flame and burn off completely after 2 s without residue. ChNF-Ac nanopapers are more difficult to ignite especially at initial stages and even resist a direct flame for a short time. Compared to CNFs, they burn more slowly, but show a bright flame as well. ChNF-Ac reaches a plateau at 84% relative burned height after 2 s, and displays char formation. ChNF-Cl and ChNF-P nanopapers exhibit a distinctly decreased flammability. The maximum burned height is reduced and the initial ignition is delayed when acetate is swapped for chloride or phosphate. ChNF-Cl burns slowly and reaches a relative burn height of ca. 61% after 3 seconds flame exposition, while ChNF-P behaves even better with a maximum burn height of only ca. 49%. Therefore, ChNF-P shows the greatest flame-retardant effect. A complete monitoring of the relative burned height (Figure 3c) clearly quantifies a delayed initial ignition and reduced relative burned height for ChNF-P compared to all other samples. The black residue of ChNF-P also shows much higher structural integrity with a high char content compared to ChNF-Ac. In summary, the VFTs show a clear positive effect of nitrogen in the polysaccharide structure (ChNF-Ac vs. CNF), as well as a pronounced beneficial effect of introducing chloride, or even better phosphate-type counterions on the flame-retardant properties and the char formation.

Figure 3: Impact of different counterions on the fire retardant and thermo-oxidative stability of ChNF nanopapers. (a) Photographs of various nanopapers at times indicated during the VFTs showing the delayed ignition when ChNF-Ac is swapped for ChNF-Cl or ChNF-P. (b) ChNF-P after being exposed to a flame for 3 sec with indicated burning height. (c) Time-dependent evaluation of the VFTs with shaded confidence areas of triplicated VFTs indicating the standard deviation. (d) TGA and dTGA under air demonstrate the better charforming ability of ChNF-P over the other materials by higher residual mass. (e) Heat release rates (HRR) obtained from MCC. (f) Photographs of 45° flame tests of ChNF-Ac, ChNF-P and CNF showing the self-extinguishing behavior of ChNF-P over ChNF-Ac and CNF. CNF is shown for comparison in all measurements.


9


Page 10 of 21

TGA supports the trend visible in the VFTs (Figure 3d). The degradation onset under air is around 220 °C for all specimens. CNF with the earliest degradation and ChNF-Cl with the most delayed thermal degradation frame the most interesting ChNF samples with regard to environmentally more acceptable counterions (P, Ac). The subsequent weight loss is highly dependent on the material and counterion. ChNF-P exhibits an onset of mass loss comparable to ChNF-Ac, but the degradation is considerably slowed down at temperatures above 300 °C. At 550 °C, ChNF-P yields a residue of 47 wt% while ChNF-Ac, ChNF-Cl and CNF are mostly decomposed with residual masses as low as 10 wt%. This demonstrates the increased char formation at elevated temperatures induced by phosphorus as well as the beneficial influence of nitrogen in the polysaccharide structure. Importantly, even the halogen-modified ChNF is clearly inferior in terms of char-forming ability compared to ChNF-P. To further understand the influence of the different counterions, we investigated the thermooxidative behavior of the various nanopapers using MCC following ASTM D 7309B (Figure 3e, Table 2). MCC provides similar quantification of the flame-retardant properties as classical cone calorimetry, but can be applied to much smaller material quantities, allowing for a facile access to the important combustion characteristics without requiring excessive material. The reference material, CNF, which serve as a benchmarking material presently exploited in multifunctional polysaccharide-nanofibril materials, degrades faster than any sample containing ChNF. Its early onset of degradation of ca. 220 °C with a peak heat release rate (PHRR) of 124 W/g and a total heat release (THR) of 5700 J/g indicates a high flammability. ChNF-Ac reduces the THR and PHRR by 15 and 35%, compared to CNF, which is due to additional nitrogen being present in the chitin structure, which contributes to splitting off ammonia during combustion, and increases the thermo-oxidative stability.47 While ChNF-Cl shows relatively fast degradation in TGA, it possesses a lower THR of 2800 J/g, which is due to the radical scavenging effect of chloride. Critically, ChNF-P shows the lowest THR of 1900 J/g and a PHRR of 49 W/g, which are reductions of 60% and 40% compared to ChNFAc, respectively. This is a clear proof of the positive effect of phosphate-type counterions on the thermo-oxidative stability, as they catalyze, when converted to strong acids during fire, the dehydration of the oxygen-containing biopolymers. This leads to a fast and short initial degradation, while then producing thermally stable residues. The MCC data thus corroborate the TGA. The resulting char-like structures act as thermal shield during combustion as well as hinder O2 diffusion, thus slowing down pyrolysis at elevated temperatures. Table 2: Characterization of different nanopapers by MCC and TGA. Specimen

THRa (J/g)

PHRRb (W/g)

Tonsetc (°C)

TGAd (%)

ChNF-P 1900 ± 70 49 ± 3 255 ± 4 47 ChNF-Cl 2800 ± 130 34 ± 2 263 ± 1 16 ChNF-Ac 5000 ± 300 84 ± 7 275 ± 5 11 CNF 5700 ± 140 124 ± 24 221 + 6 9 a Total heat release. b Peak heat release rate. c Onset temperature of degradation. d Residual mass at 550 °C under air.

Of critical importance for practical applications is an understanding of the self-extinguishing behavior. VFT, in which the sample is lit by a flame underneath the specimen, are not very suitable to judge on this behavior for flammable samples. Therefore, we turned to tests of specimens mounted at 45° regarding to the flame and lit the samples for only 1 s (Figure 3f). ACS Paragon Plus Environment

10

Page 11 of 21 1 2 3 4 5 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

CNF and ChNF-Ac nanopapers completely burn once ignited, while ChNF-Cl and ChNF-P are self-extinguishing and do not burn once the flame is removed. This simple test provides another clear evidence of the improved behavior after simple counterion exchange in ChNF with its own comparisons and moreover to CNF-based nanopapers. Fire Barrier Materials including Nanoclay While all of these experiments have shed light on the flame-retardant behavior, another important aspect is to find ways to stabilize materials against the breakthrough of flames for extended periods of time with a barrier function. To this end, we prepared layered nanocomposites consisting of ChNF-P and montmorillonite (MTM, delaminating nanoclay, aspect ratio ca. 260, thickness of nanosheets ca. 1 nm)64, for which we and others have shown interesting fire barrier effects, while another relevant effect for the addition of nanoplatelets may be the increase of mechanical or gas barrier properties.19, 21, 64-77 The nanocomposites were obtained by slowly adding a well exfoliated MTM dispersion (0.5 wt%) to a ChNF-P dispersion (0.5 wt%) and subsequent film casting. This procedure yielded several hybrid nanopapers with different weight ratios of ChNF-P/MTM (98/2, 95/5, 90/10, 70/30, 30/70). To understand structure formation in these nanocomposites, it is important to consider the charges of both entities. The partly surface-deacetylated ChNFs are positively charged and can interact with the negatively charged surfaces of the nanoclay. Depending on the mixing ratios, charge densities and pH between (here 5.5 – 6), the mixtures are homogeneous to the naked eye at an excess of one component, whereas a one-to-one mixture (50/50) of MTM and ChNF-P flocculates at pH 5.7 (Figure 4b).

Figure 4: Colloidal dispersion properties of ChNF-P/MTM dispersions. (a) ζ-potential of ChNF/MTM dispersions at different mixing ratios. (b) Photograph of various ChNF/MTM dispersions with ChNF-P50-MTM50 precipitating.

To assess the colloidal interactions, which are also important for mechanical stress transfer in bulk, ζ-potential measurements of the respective particle mixtures were conducted at pH = 5.7 (Figure 4a). Pure MTM shows a ζ-potential of -44 mV, while pure ChNF-P shows one of 45 mV. Mixtures between both components display a continuous transition of the ζ-potential, which indicates a strong polyelectrolyte-type complexation of both components. A 1:1 mixture by mass exhibits a ζ-potential of roughly 0 mV, which leads to coagulation. Since we showed earlier that excess precipitation leads to inferior mechanical properties in nanopapers (and for processing challenges) we omitted this combination for the following studies, and prepared ChNF-P/MTM nanopapers at various composition (98/2, 95/5, 90/10, 70/30, 30/70) with a ACS Paragon Plus Environment

11


Page 12 of 21

focus towards so-called nacre-mimetic nanocomposite compositions at high fractions of platelet-shaped reinforcements.20, 66, 68, 69, 78 XRD as well as SEM allow to probe details of the nanocomposite structure. SEM consistently shows a layered arrangement of the nanoclay in the materials, which is beneficial for barrier properties and also for unidirectional reinforcement (Figure 5a). XRDs of the ChNF-P/MTM nanocomposites (Figure 5b) exhibit primary diffraction peaks (q*) that are significantly shifted to smaller angles compared to pure MTM, and correspond to d-spacings of 2.9 to 1.8 nm. Pure MTM with its peak at 5.1 nm-1 equals a gallery spacing of 1.2 nm. Hence, the expanded gallery spacing indicates a successful intercalation upon increasing the ChNF-P content (Figure 5c). Nonetheless, the determined values are below the average height of the ChNF nanofibrils (ca. 5.4 nm). This allows to conclude that the MTM galleries are only partially expanded by either having a sub monolayer intercalation of ChNFs, or potentially include some cleaved chitosan chains. The overall structure corresponds to a partly exfoliated tactoid structure in a ChNF matrix, as also similarly reported for CNF/nanoclay material.19, 21 The comparably small peak at 6.4 nm-1, which is present in all samples, originates from the 020 plane of ChNF.

Figure 5: Mesostructural characterization of nacre-inspired ChNF-P/MTM nanocomposites. (a) Cross sectional SEM image of ChNF-P70-MTM30 hybrid nanopaper. Layered arrangement is observed at all ratios of ChNF-P/MTM. (b) X-ray diffractogramms of MTM/ChNF-nanocomposites. (c) Decrease of the gallery spacing with increasing MTM content.

The stress/strain curves for the respective nanocomposites show an increase of the Young’s modulus for higher amounts of MTM (Figure 6, Table 3). An increase in Young’s modulus of ca. 18% is achieved with very small loadings of 2 wt% of MTM. The Young’s modulus rises in total from 8.5 GPa in ChNF-P100 to 12 GPa in ChNF-P70-MTM30. However, higher MTM contents, such as in ChNF-P30-MTM70, lead to a decrease in stiffness and ultimate tensile strength, because such nanocomposites with very high nanoclay content cannot form intercalated structures with thick nanofibrils, which diminishes the ability to transfer stress from the matrix ChNF phase. This also induces brittleness with smaller elongation at break. Very small amounts of MTM in ChNF-P98-MTM2 have no effect on the elongation, while a pronounced decrease of 45% is visible in ChNF-P90-MTM10.


12

Page 13 of 21 1 2 3 4 5 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

Figure 6: Mechanical properties of ChNF/MTM hybrid nanopapers. Stress/strain curves of (a) ChNF-P-MTM nanocomposites, (b) Young’s modulus and (c) ultimate tensile strength of ChNF-P-MTM nanopapers.

Table 3: Mechanical properties of several ChNF-P-MTM nanocomposites. Specimen

E (GPa)a

σUTS (MPa)b

εb (%)c

Ut (MJ/m3)d

ChNF-P100

8.5 ± 0.2

196 ± 10

5.3 ± 0.9

7.1 ± 1.6

ChNF-P98-MTM2

10.1 ± 0.5

207 ± 26

5.3 ± 1.4

7.7 ± 2.9

ChNF-P95-MTM5

9.5 ± 0.3

199 ± 8

4.8 ± 0.6

6.4 ± 1.0

ChNF-P90-MTM10

10.9 ± 0.5

168 ± 10

2.8 ± 0.3

3.1 ± 0.5

ChNF-P70-MTM30

12.1 ± 1.2

148 ± 6

2.5 ± 0.3

2.5 ± 0.4

ChNF-P30-MTM70

10.4 ± 0.9

98 ± 14

1.7 ± 0.2

a

1.1 ± 0.2 b

Young’s modulus obtained from the slope in the region of linear elastic deformation. Tensile strength and c strain at break obtained from the last point before fracture. d Work of fracture calculated from the area under the σ−ε curve.

Next, we analyzed the thermal and thermo-oxidative stability of the nanocomposites by MCC, VFT, fire break-through tests (FBT) and TGA in air (Figure 7a-e). As mentioned above, nanoclay may positively influence the fire-barrier properties through increased oxygen barrier, as well as acting as a thermal shield. Additionally, MTM catalyzes char formation, which results in less formation of ignitable gas.79 The TGA shows that increased nanoclay content leads to higher residual mass (Figure 7c, Table 4), which is however expected, because MTM does not decompose. To understand whether there might be a positive synergetic effect on the char formation, we subtracted the MTM mass from the total nanopaper mass to quantify a possible influence of the clay on the degradation of the organic content: 𝑚445℃ − 𝑚898 Normalized organic residual mass = (1) 𝑚: − 𝑚898 m550C, mMTM and mi are the mass at 550 °C, the mass of MTM in the respective nanocomposite and the initial mass of the nanocomposite, respectively. The normalized values are written in brackets in Table 4. After normalization to the organic content, it becomes evident that there is no significant change in residual mass of the organic component at low clay concentrations and even slightly less organic residual mass at high concentrations. Therefore, incorporating MTM does not have a beneficial influence on the char formation in this particular material.


13


Page 14 of 21

Figure 7: Analysis of the impact of different ratios of ChNF-P/MTM in nanocomposites on the fire retardant and thermo-oxidative stability. (a,e) Photographs and corresponding shaded confidence areas of triplicated VFTs of various nanopapers and nanocomposites at times indicated during the VFT showing the improved ignition time when MTM is added. (b) Fire break-through tests of corresponding films in (a) show that nanocomposites containing more than 30 wt% MTM withstand a direct high intensity flame. (c) TGA under air show the increase in residual mass when MTM is incorporated. (d) MCC shows increased thermo-oxidative stability of ChNF/MTM nanocomposites with increasing the content of MTM.

MCC shows a correlated decrease of THR and PHRR with increasing nanoclay content (Figure 7d). Especially at low MTM loadings, the PHRR is reduced by 30% in ChNF-P90-MTM10 compared to ChNF-P100. Furthermore, higher MTM contents move the onset temperature of degradation to higher temperatures, e.g. from 256 °C to 289 °C when changing from pure ChNF to ChNF-P30-MTM70. Thus, the addition of nanoclay increases the thermo-oxidative stability of the nanocomposite, as a nanoclay-rich barrier decreases the oxygen permeability and shields subjacent ChNF from heat exposition. This results in a lower THR and PHRR. This trend is also reflected in VFTs, which show a small decrease on the relative height of the film burned when adding MTM (Figure 7a,e). All samples exhibit a plateau in the range of 30-40% of relative burned height.


14

Page 15 of 21 1 2 3 4 5 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

Table 4: MCC and TGA data of various nanoclay nanocomposites. Specimen

THRa (J/g)

PHRRb (W/g)

Tonsetc (°C)

TGAd (%)

ChNF-P100

1900 ± 70

49 ± 3

256 ± 4

47

ChNF-P90-MTM10

1640 ± 50

36 ± 2

260 ± 1

46 (40)

ChNF-P70-MTM30

1350 ± 60

28 ± 1

270 ± 4

55 (36)

ChNF-P30-MTM70

630 ± 65

10 ± 2

289 ± 10

78 (27)

a

b

c

d

Total heat release. Peak heat release rate. Onset temperature of degradation. Residual mass at 550 °C under air and residual mass calculated according to equation 1 in brackets.

Finally, we turn to fire break-through tests (FBTs) to examine the macroscopic effect of MTM on the fire barrier properties, i.e. the ability to withstand direct high intensity flames from a short distance (130 mm). Therefore a 25 µm thick nanopaper is fixed in a circular holder and exposed to a high intensity torch close to 90° from a distance of 130 mm (Figure 7b). ChNFAc and CNF nanopapers instantly catch fire and are mostly burned after 1 second. ChNF-P burns as well at longer flame exposition, but yields a much higher char content, leaving behind a lot of black residue. In contrast, ChNF-P nanopapers with MTM show much higher structural integrity. ChNF-P90-MTM10 nanopapers remain mostly intact, but form cracks one second after ignition and show enlarged fracture areas after 30s. However, at 30 wt% MTM content, the hybrid nanopapers char into a shape-persistent solid barrier, which can withstand prolonged exposure to the high intensity gas torch. Therefore, the structural integrity rises with increasing nanoclay content and is sufficient to prevent the crack formation starting from 30 wt% MTM – despite being only 25 µm thick. This demonstrates that the incorporation of nanoclay within ChNF-P nanocomposites has a tremendously positive effect on the thermal and thermooxidative stability and provides some key advantages for further real-life fire barrier applications.

Conclusion We established ChNFs as a highly relevant and sustainable, halogen-free and heavy metal-free class of flame-retardant and fire barrier polysaccharide nanofibril materials. The substitution of the counterion on the surface-deacetylated ChNFs is simple, yet crucial, to achieve beneficial combinations of N/P inside the final nanopapers and to critically enhance the fire-retardant properties. The phosphate-coordinated ChNFs are highly flame-retardant, instantly selfextinguishing and substantially better than normally prepared ChNF-Ac (acetate counterions), or presumably pure ChNFs without ionic groups. It is important to emphasize that an exchange to phosphate counterions can in future be easily done in the last step of the preparation of surface-deacetylated ChNF, which is typically an acidification to impart electrostatic stabilization to the nanofibrils. Furthermore, the incorporation of nanoclay platelets yields wellordered nanocomposites with an increased stiffness of 40% in Young’s modulus compared to pure ChNF nanopapers, and provides exceptional, shape-persistent fire barrier properties against direct exposure to high intensity flames. Moreover, the resulting materials have attractive multifunctional properties, being one the hand translucent, and, on the other hand of ACS Paragon Plus Environment

15


Page 16 of 21

high mechanical stiffness and strength, typically one order of magnitude above classical polymer materials and in range with nanopapers prepared from CNFs. As a practical benefit to the materials community, ChNF-P nanopapers and composites can be prepared simply from single-step waterborne processing routes. In comparison to their sibling material, nanocellulose, the elemental compositions of chitin (N) and the simple coordination of counterions (P) provide relevant and critical advantages in elemental composition (N/P) over presently researched flame-retardant nanocellulose materials. This motivates to apply recent sophisticated improvements from flame-retardancy research in nanocellulose materials to ChNF-based materials to identify how much further the performance of such sustainable nanofibrillar high performance biomaterials can be improved. Overall, this multifunctional material concept activates a global food waste, and presents a critical advance for biobased, green and mechanical high-performance materials with extraordinary flame-retardant and fire barrier properties. We believe that such materials can be applied in bulk, as surface coating, or can in future be transformed into other multifunctional material classes such as thermally insulating foams or mechanically robust fibers for textile applications.

Acknowledgements. We thank the BMBF for funding the Aquamat Research Group.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

Barikani, M.; Oliaei, E.; Seddiqi, H.; Honarkar, H., Preparation and Application of Chitin and its Derivatives: A Review, Iran. Polym. J. 2014, 23, 307-326. Tharanathan, R. N.; Kittur, F. S., Chitin — The Undisputed Biomolecule of Great Potential, Crit. Rev. Food Sci. Nutr. 2003, 43, 61-87. Yan, N.; Chen, X., Sustainability: Don’t Waste Seafood Waste, Nature 2015, 524, 155. Beier, S.; Bertilsson, S., Bacterial Chitin Degradation—Mechanisms and Ecophysiological Strategies, Front. Microbiol. 2013, 4, 149. Benhabiles, M. S.; Salah, R.; Lounici, H.; Drouiche, N.; Goosen, M. F. A.; Mameri, N., Antibacterial Activity of Chitin, Chitosan and Its Oligomers Prepared from Shrimp Shell Waste, Food Hydrocoll. 2012, 29, 48-56. Jayakumar, R.; Menon, D.; Manzoor, K.; Nair, S. V.; Tamura, H., Biomedical Applications of Chitin and Chitosan Based Nanomaterials—A Short Review, Carbohydr. Polym. 2010, 82, 227-232. Azuma, K.; Ifuku, S.; Osaki, T.; Okamoto, Y.; Minami, S., Preparation and Biomedical Applications of Chitin and Chitosan Nanofibers, J. Biomed. Nanotechnol. 2014, 10, 28912920. Liu, D.; Zhu, Y.; Li, Z.; Tian, D.; Chen, L.; Chen, P., Chitin Nanofibrils for Rapid and Efficient Removal of Metal Ions from Water System, Carbohydr. Polym. 2013, 98, 483489. Torres-Rendon, J. G.; Femmer, T.; Laporte, L. D.; Tigges, T.; Rahimi, K.; Gremse, F.; Zafarnia, S.; Lederle, W.; Ifuku, S.; Wessling, M.; Hardy, J. G.; Walther, A., Bioactive Gyroid Scaffolds Formed by Sacrificial Templating of Nanocellulose and Nanochitin ACS Paragon Plus Environment

16

Page 17 of 21 1 2 3 4 5 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

22. 23. 24. 25. 26. 27.

Hydrogels as Instructive Platforms for Biomimetic Tissue Engineering, Adv. Mater. 2015, 27, 2989-2995. Morganti, P.; Morganti, G., Chitin Nanofibrils for Advanced Cosmeceuticals, Clin. Dermatol. 2008, 26, 334-340. Torres-Rendon, J. G.; Schacher, F. H.; Ifuku, S.; Walther, A., Mechanical Performance of Macrofibers of Cellulose and Chitin Nanofibrils Aligned by Wet-Stretching: A Critical Comparison, Biomacromolecules 2014, 15, 2709-2717. Das, P.; Heuser, T.; Wolf, A.; Zhu, B.; Demco, D. E.; Ifuku, S.; Walther, A., Tough and Catalytically Active Hybrid Biofibers Wet-Spun From Nanochitin Hydrogels, Biomacromolecules 2012, 13, 4205-4212. Nishino, T.; Matsui, R.; Nakamae, K., Elastic Modulus of the Crystalline Regions of Chitin and Chitosan, J. Polym. Sci. B 1999, 37, 1191-1196. Ogawa, Y.; Hori, R.; Kim, U.-J.; Wada, M., Elastic Modulus in the Crystalline Region and the Thermal Expansion Coefficients of α-Chitin Determined Using Synchrotron Radiated X-Ray Diffraction, Carbohydr. Polym. 2011, 83, 1213-1217. Wegst, U. G. K.; Bai, H.; Saiz, E.; Tomsia, A. P.; Ritchie, R. O., Bioinspired Structural Materials, Nat. Mater. 2015, 14, 23-36. Meyers, M. A.; McKittrick, J.; Chen, P.-Y., Structural Biological Materials: Critical Mechanics-Materials Connections, Science 2013, 339, 773-779. Zhong, C.; Cooper, A.; Kapetanovic, A.; Fang, Z.; Zhang, M.; Rolandi, M., A Facile Bottom-up Route to Self-Assembled Biogenic Chitin Nanofibers, Soft Matter 2010, 6, 5298-5301. Ifuku, S.; Nogi, M.; Abe, K.; Yoshioka, M.; Morimoto, M.; Saimoto, H.; Yano, H., Preparation of Chitin Nanofibers with a Uniform Width as α-Chitin from Crab Shells, Biomacromolecules 2009, 10, 1584-1588. Liu, A.; Walther, A.; Ikkala, O.; Belova, L.; Berglund, L. A., Clay Nanopaper with Tough Cellulose Nanofiber Matrix for Fire Retardancy and Gas Barrier Functions, Biomacromolecules 2011, 12, 633-641. Benítez, A. J.; Torres-Rendon, J.; Poutanen, M.; Walther, A., Humidity and Multiscale Structure Govern Mechanical Properties and Deformation Modes in Films of Native Cellulose Nanofibrils, Biomacromolecules 2013, 14, 4497-4506. Jin, H.; Cao, A.; Shi, E.; Seitsonen, J.; Zhang, L.; Ras, R. H. A.; Berglund, L. A.; Ankerfors, M.; Walther, A.; Ikkala, O., Ionically Interacting Nanoclay and Nanofibrillated Cellulose Lead to Tough Bulk Nanocomposites in Compression by Forced Self-Assembly, J. Mater. Chem. B 2013, 1, 835-840. Wang, B.; Benitez, A. J.; Lossada, F.; Merindol, R.; Walther, A., Bioinspired Mechanical Gradients in Cellulose Nanofibril/Polymer Nanopapers, Angew. Chem. Int. Ed. 2016, 55, 5966-5970. Hoenders, D.; Guo, J.; Goldmann, A. S.; Barner-Kowollik, C.; Walther, A., Photochemical Ligation Meets Nanocellulose: A Versatile Platform for Self-Reporting Functional Materials, Mater. Horiz. 2018, 5, 560-568. J. Benítez, A.; Walther, A., Cellulose Nanofibril Nanopapers and Bioinspired Nanocomposites: A Review to Understand the Mechanical Property Space, J. Mater. Chem. A 2017, 5, 16003-16024. Benítez, A. J.; Walther, A., Counterion Size and Nature Control Structural and Mechanical Response in Cellulose Nanofibril Nanopapers, Biomacromolecules 2017, 18, 1642-1653. Ling, S.; Kaplan, D. L.; Buehler, M. J., Nanofibrils in Nature and Materials Engineering, Nat. Rev. Mater. 2018, 3, 18016. Azuma, K.; Nishihara, M.; Shimizu, H.; Itoh, Y.; Takashima, O.; Osaki, T.; Itoh, N.; Imagawa, T.; Murahata, Y.; Tsuka, T.; Izawa, H.; Ifuku, S.; Minami, S.; Saimoto, H.; ACS Paragon Plus Environment

17


28.

29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44.

Page 18 of 21

Okamoto, Y.; Morimoto, M., Biological Adhesive Based on Carboxymethyl Chitin Derivatives and Chitin Nanofibers, Biomaterials 2015, 42, 20-29. Tabuchi, R.; Anraku, M.; Iohara, D.; Ishiguro, T.; Ifuku, S.; Nagae, T.; Uekama, K.; Okazaki, S.; Takeshita, K.; Otagiri, M.; Hirayama, F., Surface-Deacetylated Chitin Nanofibers Reinforced with a Sulfobutyl Ether β-Cyclodextrin Gel Loaded with Prednisolone as Potential Therapy for Inflammatory Bowel Disease, Carbohydr. Polym. 2017, 174, 1087-1094. Bamba, Y.; Ogawa, Y.; Saito, T.; Berglund, L. A.; Isogai, A., Estimating the Strength of Single Chitin Nanofibrils via Sonication-Induced Fragmentation, Biomacromolecules 2017, 18, 4405-4410. Fan, Y.; Fukuzumi, H.; Saito, T.; Isogai, A., Comparative Characterization of Aqueous Dispersions and Cast Films of Different Chitin Nanowhiskers/Nanofibers, Int. J. Biol. Macromol. 2012, 50, 69-76. Mushi, N. E.; Utsel, S.; Berglund, L. A., Nanostructured Biocomposite Films of High Toughness Based on Native Chitin Nanofibers and Chitosan, Front. Chem. 2014, 2, 99. Kontturi, E.; Laaksonen, P.; Linder, M. B.; Nonappa; Gröschel, A. H.; Rojas, O. J.; Ikkala, O., Advanced Materials through Assembly of Nanocelluloses, Adv. Mater. 2018, 30, 1703779. Fan, Y.; Saito, T.; Isogai, A., Preparation of Chitin Nanofibers from Squid Pen β-Chitin by Simple Mechanical Treatment under Acid Conditions, Biomacromolecules 2008, 9, 1919-1923. Yokoi, M.; Tanaka, R.; Saito, T.; Isogai, A., Dynamic Viscoelastic Functions of LiquidCrystalline Chitin Nanofibril Dispersions, Biomacromolecules 2017, 18, 2564-2570. Yang, Q.; Saito, T.; Berglund, L. A.; Isogai, A., Cellulose Nanofibrils Improve the Properties of All-Cellulose Composites by the Nano-Reinforcement Mechanism and Nanofibril-Induced Crystallization, Nanoscale 2015, 7, 17957-17963. Lundahl, M. J.; Klar, V.; Ajdary, R.; Norberg, N.; Ago, M.; Cunha, A. G.; Rojas, O. J., Absorbent Filaments from Cellulose Nanofibril Hydrogels through Continuous Coaxial Wet Spinning, ACS Appl. Mater. Interfaces 2018, 10, 27287-27296. Horrocks, A. R.; Kandola, B. K.; Davies, P. J.; Zhang, S.; Padbury, S. A., Developments in Flame Retardant Textiles – A Review, Polym. Degrad. Stab. 2005, 88, 3-12. Gaan, S.; Sun, G., Effect of Phosphorus and Nitrogen on Flame Retardant Cellulose: A Study of Phosphorus Compounds, J. Anal. Appl. Pyrolysis 2007, 78, 371-377. Wicklein, B.; Kocjan, A.; Salazar-Alvarez, G.; Carosio, F.; Camino, G.; Antonietti, M.; Bergström, L., Thermally Insulating and Fire-Retardant Lightweight Anisotropic Foams Based on Nanocellulose and Graphene Oxide, Nat. Nanotech. 2015, 10, 277-283. Wicklein, B.; Kocjan, D.; Carosio, F.; Camino, G.; Bergström, L., Tuning the Nanocellulose–Borate Interaction To Achieve Highly Flame Retardant Hybrid Materials, Chem. Mater. 2016, 28, 1985-1989. Ghanadpour, M.; Carosio, F.; Larsson, P. T.; Wågberg, L., Phosphorylated Cellulose Nanofibrils: A Renewable Nanomaterial for the Preparation of Intrinsically FlameRetardant Materials, Biomacromolecules 2015, 16, 3399-3410. Nechyporchuk, O.; Bordes, R.; Köhnke, T., Wet Spinning of Flame-Retardant Cellulosic Fibers Supported by Interfacial Complexation of Cellulose Nanofibrils with Silica Nanoparticles, ACS Appl. Mater. Interfaces 2017, 9, 39069-39077. Uddin, K. M. A.; Ago, M.; Rojas, O. J., Hybrid Films of Chitosan, Cellulose Nanofibrils and Boric Acid: Flame Retardancy, Optical and Thermo-Mechanical Properties, Carbohydr. Polym. 2017, 177, 13-21. Köklükaya, O.; Carosio, F.; Grunlan, J. C.; Wågberg, L., Flame-Retardant Paper from Wood Fibers Functionalized via Layer-by-Layer Assembly, ACS Appl. Mater. Interfaces 2015, 7, 23750-23759. ACS Paragon Plus Environment

18

Page 19 of 21 1 2 3 4 5 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

45. 46. 47. 48. 49. 50.

51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64.

Ghanadpour, M.; Wicklein, B.; Carosio, F.; Wågberg, L., All-Natural and Highly FlameResistant Freeze-Cast Foams Based on Phosphorylated Cellulose Nanofibrils, Nanoscale 2018, 10, 4085-4095. Horacek, H.; Grabner, R., Advantages of Flame Retardants Based on Nitrogen Compounds, Polym. Degrad. Stab. 1996, 54, 205-215. Gaan, S.; Engelhard, M. H.; Sun, G.; Hutches, K., Effect of Nitrogen Additives on Flame Retardant Action of Tributyl Phosphate: Phosphorus–Nitrogen Synergism, Polym. Degrad. Stab. 2008, 93, 99-108. Leu, T.-S.; Wang, C.-S., Synergistic Effect of a Phosphorus–Nitrogen Flame Retardant on Engineering Plastics, J. Appl. Polym. Sci. 2004, 92, 410-417. Pan, H.; Wang, W.; Pan, Y.; Song, L.; Hu, Y.; Liew, L. M., Formation of SelfExtinguishing Flame Retardant Biobased Coating on Cotton Fabrics via Layer-by-Layer Assembly of Chitin Derivatives, Carbohydr. Polym. 2015, 115, 516-524. Ghanadpour, M.; Carosio, F.; Ruda, M. C.; Wågberg, L., Tuning the Nanoscale Properties of Phosphorylated Cellulose Nanofibril-Based Thin Films To Achieve Highly FireProtecting Coatings for Flammable Solid Materials, ACS Appl. Mater. Interfaces 2018, 10, 32543-32555. Schartel, B., Phosphorus-Based Flame Retardancy Mechanisms—Old Hat or a Starting Point for Future Development?, Materials 2010, 3, 4710-4745. Perret, B.; Pawlowski, K. H.; Schartel, B., Fire Retardancy Mechanisms of Arylphosphates in Polycarbonate (PC) and PC/Acrylonitrile-Butadiene-Styrene, J. Therm. Anal. Calorim. 2009, 97, 949. Hastie, J. W., Molecular Basis of Flame Inhibition, J. Res. Natl. Stand. A 1973, 77A, 733. Green, J., Mechanisms for Flame Retardancy and Smoke suppression - A Review, J. Fire Sci. 1996, 14, 426-442. Kishore, K.; Mohandas, K., Action of Phosphorus Compounds on Fire-Retardancy of Cellulosic Materials: A Review, Fire Mater. 1982, 6, 54-58. Gaan, S.; Sun, G., Effect of Phosphorus Flame Retardants on Thermo-Oxidative Decomposition of Cotton, Polym. Degrad. Stab. 2007, 92, 968-974. Ifuku, S.; Saimoto, H., Chitin Nanofibers: Preparations, Modifications, and Applications, Nanoscale 2012, 4, 3308-3318. Aziz, H.; Ahmad, F.; Zia-ul-Mustafa, M., Effect of Titanium Oxide on Fire Performance of Intumescent Fire Retardant Coating, Adv. Mat. Res. 2014, 224-228. Cusack, P. A.; Monk, A. W.; Pearce, J. A.; Reynolds, S. J., An Investigation of Inorganic Tin Flame Retardants Which Suppress Smoke and Carbon Monoxide Emission from Burning Brominated Polyester Resins, Fire Mater. 1989, 14, 23-29. Kaspersma, J.; Doumen, C.; Munro, S.; Prins, A.-M., Fire Retardant Mechanism of Aliphatic Bromine Compounds in Polystyrene and Polypropylene, Polym. Degrad. Stab. 2002, 77, 325-331. de Wit, C. A., An Overview of Brominated Flame Retardants in the Environment, Chemosphere 2002, 46, 583-624. Ifuku, S.; Nogi, M.; Yoshioka, M.; Morimoto, M.; Yano, H.; Saimoto, H., Fibrillation of Dried Chitin into 10–20nm Nanofibers by a Simple Grinding Method under Acidic Conditions, Carbohydr. Polym. 2010, 81, 134-139. Mushi, N. E.; Butchosa, N.; Salajkova, M.; Zhou, Q.; Berglund, L. A., Nanostructured Membranes Based on Native Chitin Nanofibers Prepared by Mild Process, Carbohydr. Polym. 2014, 112, 255-263. Das, P.; Malho, J.-M.; Rahimi, K.; Schacher, F. H.; Wang, B.; Demco, D. E.; Walther, A., Nacre-mimetics with Synthetic Nanoclays up to Ultrahigh Aspect Ratios, Nat. Commun. 2015, 6, 5967.


19


65. 66. 67.

68.

69.

70. 71. 72. 73. 74. 75. 76. 77. 78. 79.

Page 20 of 21

Mäkiniemi, R. O.; Das, P.; Hönders, D.; Grygiel, K.; Cordella, D.; Detrembleur, C.; Yuan, J.; Walther, A., Conducting, Self-Assembled, Nacre-Mimetic Polymer/Clay Nanocomposites, ACS Appl. Mater. Interfaces 2015, 7, 15681-15685. Eckert, A.; Rudolph, T.; Guo, J.; Mang, T.; Walther, A., Exceptionally Ductile and Tough Biomimetic Artificial Nacre with Gas Barrier Function, Adv. Mater. 2018, 30, 1802477. Shahzadi, K.; Zhang, X.; Mohsin, I.; Ge, X.; Jiang, Y.; Peng, H.; Liu, H.; Li, H.; Mu, X., Reduced Graphene Oxide/Alumina, A Good Accelerant for Cellulose-Based Artificial Nacre with Excellent Mechanical, Barrier, and Conductive Properties, ACS Nano 2017, 11, 5717-5725. Walther, A.; Bjurhager, I.; Malho, J.-M.; Ruokolainen, J.; Berglund, L.; Ikkala, O., Supramolecular Control of Stiffness and Strength in Lightweight High-Performance Nacre-Mimetic Paper with Fire-Shielding Properties, Angew. Chem. Int. Ed. 2010, 49, 6448-6453. Zhu, B.; Jasinski, N.; Benitez, A.; Noack, M.; Park, D.; Goldmann, A. S.; Barner‐ Kowollik, C.; Walther, A., Hierarchical Nacre Mimetics with Synergistic Mechanical Properties by Control of Molecular Interactions in Self-Healing Polymers, Angew. Chem. Int. Ed. 2015, 54, 8653-8657. Das, P.; Thomas, H.; Moeller, M.; Walther, A., Large-scale, thick, self-assembled, nacremimetic brick-walls as fire barrier coatings on textiles, Sci. Rep. 2017, 7, 39910. Das, P.; Schipmann, S.; Malho, J.-M.; Zhu, B.; Klemradt, U.; Walther, A., Facile Access to Large-Scale, Self-Assembled, Nacre-Inspired, High-Performance Materials with Tunable Nanoscale Periodicities, ACS Appl. Mater. Interfaces 2013, 5, 3738-3747. Lazar, S.; Carosio, F.; Davesne, A.-L.; Jimenez, M.; Bourbigot, S.; Grunlan, J. C., Extreme Heat Shielding of Clay/Chitosan Nanobrick Wall on Flexible Foam, ACS Appl. Mater. Interfaces 2018, 10, 31686-31696. Li, Y.-C.; Schulz, J.; Mannen, S.; Delhom, C.; Condon, B.; Chang, S.; Zammarano, M.; Grunlan, J. C., Flame Retardant Behavior of Polyelectrolyte−Clay Thin Film Assemblies on Cotton Fabric, ACS Nano 2010, 4, 3325-3337. Priolo, M. A.; Gamboa, D.; Holder, K. M.; Grunlan, J. C., Super Gas Barrier of Transparent Polymer−Clay Multilayer Ultrathin Films, Nano Lett. 2010, 10, 4970-4974. Laufer, G.; Kirkland, C.; Cain, A. A.; Grunlan, J. C., Clay–Chitosan Nanobrick Walls: Completely Renewable Gas Barrier and Flame-Retardant Nanocoatings, ACS Appl. Mater. Interfaces 2012, 4, 1643-1649. Guin, T.; Krecker, M.; Milhorn, A.; Hagen, D. A.; Stevens, B.; Grunlan, J. C., Exceptional Flame Resistance and Gas Barrier with Thick Multilayer Nanobrick Wall Thin Films, Adv. Mater. Interf. 2015, 2, 1500214. Carosio, F.; Kochumalayil, J.; Fina, A.; Berglund, L. A., Extreme Thermal Shielding Effects in Nanopaper Based on Multilayers of Aligned Clay Nanoplatelets in Cellulose Nanofiber Matrix, Adv. Mater. Interfaces 2016, 3, 1600551. Zhu, B.; Noack, M.; Merindol, R.; Barner-Kowollik, C.; Walther, A., Light-Adaptive Supramolecular Nacre-Mimetic Nanocomposites, Nano Lett. 2016, 16, 5176-5182. Carosio, F.; Kochumalayil, J.; Cuttica, F.; Camino, G.; Berglund, L., Oriented Clay Nanopaper from Biobased Components—Mechanisms for Superior Fire Protection Properties, ACS Appl. Mater. Interfaces 2015, 7, 5847-5856.


20

Page 21 of 21 1 2 3 4 5 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Biomacromolecules

Table of Contents Use:


21

Sustainable Chitin Nanofibrils Provide Outstanding Flame-Retardant

Recommend Documents