Subscriber access provided by UNIV LAVAL
Article
Highly transparent and self-extinguishing nanofibrillated cellulose-monolayer clay nanoplatelet hybrid films Siyi Ming, Gang Chen, Jiahao He, Yudi Kuang, Yu Liu, Ruiqiang Tao, Honglong Ning, Penghui Zhu, Yingyao Liu, and Zhiqiang Fang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01665 • Publication Date (Web): 03 Aug 2017 Downloaded from http://pubs.acs.org on August 6, 2017
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 free 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 accessible to all readers and 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.
Langmuir 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 23
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
Langmuir
Highly transparent and self-extinguishing nanofibrillated cellulose-monolayer clay nanoplatelet hybrid films Siyi Ming,1 Gang Chen,1 Jiahao He,1 Yudi Kuang,1 Yu Liu,1 Ruiqiang Tao,2 Honglong Ning,2* Penghui Zhu,1 Yingyao Liu1, Zhiqiang Fang1,2,3* 1. State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640 Guangdong, China Email:
[email protected] 2. Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent Materials & Devices, South China University of Technology, Guangzhou 510640 Guangdong China Email:
[email protected] 3. Key Laboratory of Pulp and Paper Science & Technology of Ministry of Education/Shandong Province, Qilu University of Technology, Jinan 250353 Guangdong, China *Corresponding authors
Abstract A viable solution towards “green” optoelectronics is root in our ability to fabricate optoelectronics on transparent nanofibrillated cellulose (NFC) film substrates. However, the flammability of transparent NFC film poses a severe fire hazard in optoelectronic devices. Despite many efforts towards enhancing the fire-retardant features of transparent NFC film, making NFC film fire-retardant while maintaining its high transparency (≥ 90 %) remains an ambitious objective. Herein, we combine NFC with NFC-dispersed monolayer clay nanoplatelets as a fire retardant to prepare highly transparent NFC-monolayer clay nanoplatelet hybrid films with a superb self-extinguishing behavior. Homogeneous and stable monolayer clay nanoplatelet dispersion was initially obtained by using NFC as a green dispersing agent with an assistance of ultra-sonication and then used to blend with NFC to prepare highly transparent and selfextinguishing hybrid films by a water evaporation-induced self-assembly process. As the content of monolayer clay nanoplatelets increased from 5 wt% to 50 wt%, the obtained hybrid films presented enhanced self-extinguishing behavior (limiting oxygen index sharply increased from
ACS Paragon Plus Environment
Langmuir
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
21% to 96.5%) while retaining a ~90 % transparency at 600 nm. More significantly, the underlying mechanisms for the high transparency and excellent self-extinguishing behavior of these hybrid films with a clay nanoplatelet content of over 30 wt% were unveiled by a series of characterizations such as SEM, XRD, TGA, and limiting oxygen index tester. This work offers an alternative environmentally friendly, self-extinguishing, and highly transparent substrate to next-generation optoelectronics, and is aimed at providing a viable solution to environmental concerns that are caused by ever-increasing electronic waste.
Introduction Enormous quantities of electronic waste (E-waste), that result from the widespread use of consumer electronics in our daily routine and the considerable decrease of their lifetime, pose serious environmental concerns because most electronics contain petroleum-derived nonbiodegradable materials.1-3 In the past decade, the integration of bio-based and renewable materials into devices has been a trend in the field of electronics to reduce environmental contaminations caused by electronic waste.4, 5 Recently, nanofibrillated cellulose (NFC) film that is derived from the most abundant and renewable wood fibers globally has emerged as an promising substrate for optoelectronics because of its unsurpassed quintessential physical and mechanical properties such as its light weight, mechanical ductility, high transparency, strong mechanical strength, nano-scale surface roughness, and good barrier properties.6-9 A number of proof-of-concept optoelectronics with satisfying device performance, including touchscreens,10, 11
displays,12, 13 photovoltaics,14-16 and organic light emitting diodes,4, 17 have been demonstrated
on various NFC films. The increasing number of incidents that has been reported related to the ignition of electronic devices has raised awareness regarding the flame-retardant feature of electronics. NFC
ACS Paragon Plus Environment
Page 2 of 23
Page 3 of 23
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
Langmuir
film as an important component of “green” optoelectronics requires efficient flame-retardant performance. However, the highly flammable character of NFC films is a primary obstacle for their use in optoelectronics. Combing the NFC films with flam-retardant additives is an effective approach to suppress their flammability. Halogenated compounds have been considered the most effective fire retardant for polymer composites, but tremendous studies have reported their toxicity for human and animals because of the risk of bioaccumulation.18 Nontoxic inorganic compounds are of scientific and industrial interests to improve the fireretardant feature of NFC films. Clay nanoplatelets have garnered increasing interests for ecofriendly fire-retardant applications owing to their intriguing characters such as a natural abundance, nontoxicity, high surface interaction, outstanding mechanical properties, high intrinsic thermal stability, and layered structure with tunable aspect ratios.19-26 Employing clay nanoplatelets as a fire retardant for NFC films to reduce their flammability has been widely studied over the past decade. Liu et.al mixed NFC with clay with an average size of 110 nm to prepare fire-retardant NFC-clay composite film by a large-scale paper-making procedure.27 When the clay nanoplatelet content reached 50 wt%, the hybrid film presented prominent selfextinguishing behavior with flame retraction. Considerable research efforts have since been devoted to exploring the mechanism for superior fire retardancy of the NFC-nanoclay composite films and their thermal shielding effect. In spite of the obviously improved fire-retardant feature of NFC films by clay nanoplatelet addition,28-30 the opaque appearance or low transparency of the hybrid films in previous literature hindered their optoelectronic applications where a high optical transparency (≥ 90 %) is compulsory. In this study, we intend to prepare highly transparent and superior self-extinguishing NFCclay nanoplatelets hybrid films by using NFC-dispersed monolayer clay nanoplatelets as a fire
ACS Paragon Plus Environment
Langmuir
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
retardant. Figure 1 shows the procedure to prepare NFC-monolayer clay nanoplatelet hybrid films with a high light transmittance and superior self-extinguishing behavior. Softwood pulp was pretreated with a 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidation system to weaken the interfibrillar hydrogen bonding in the cell walls.. NFC was then disintegrated from pretreated softwood pulp through a homogenization process and served as an effective and green dispersant to disperse the clay nanoplatelets in water uniformly by using ultrasonication. The addition of NFC is only 10 wt% based on the dry mass of the clay nanoplatelets. The NFCdispersed monolayer nanoclay suspension, which acts as a fire-retardant additive, was blended with NFC (0.5 wt%) to form a homogeneous and stable NFC-monolayer clay nanoplatelet suspension by a combination of mechanical stirring and ultrasonication. Consequently, highly transparent and self-extinguishing NFC-monolayer clay nanoplatelet hybrid films were prepared by water evaporation-triggered self-assembly in a constant temperature and humidity chamber. Results and Discussion Dispersion of Clay Nanoplatelets Using Carboxylated NFC as a Dispersing Agent A well-dispersed and stable clay nanoplatelet suspension is necessary to achieve highly transparent NFC-clay nanoplatelet hybrid films with self-extinguishing behavior. Previous publications have confirmed that NFC is an amphiphilic material that can be applied to disperse nanomaterials in an aqueous solution, for instance, carbon nanotubes, graphene, molybdenum disulfide, and boron nitride, which was ascribed to the existence of hydrophilic hydrogen groups and hydrophobic C-H moieties in cellulose molecular chain.31-33 Therefore, we used NFC to disperse clay nanoplatelets in water, assisted by ultrasonic treatment. Figure 2a shows the homogenous and stable clay nanoplatelet dispersion (3.2 mg/L) and the morphology of the dispersed nanoplatelets was characterized by atomic force microscopy (AFM) analysis. As
ACS Paragon Plus Environment
Page 4 of 23
Page 5 of 23
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
Langmuir
shown in Figure 2b, clay nanoplatelets with a lateral size of 100-300 nm and NFC of several hundreds of nanometer in length are visible. An AFM topological line scan reveals that the height of dispersed clay flakes is ~ 1.5 nm (Figure 2c), which indicates the monolayer structure of NFC-dispersed clay nanoplatelets. In addition, because of electrostatic repulsion and steric hindrance of the carboxylated NFC, the NFC-dispersed monolayer clay nanoplatelet suspension is stable for at least 1 week without any sedimentation (Figure S1).
Optical transparency of NFC-Monolayer Clay Nanoplatelet Hybrid Films Previous studies have shown that the optical transparency of NFC-clay nanoplatelet hybrid films would deteriorate seriously with an increasing content of clay nanoplatelets,34-36 which is due to the occurrence of clay nanoplatelet aggregation during drying process. By blending the NFC-dispersed monolayer clay nanoplatelet suspension mechanically with a NFC dispersion, followed by ultra-sonication and centrifugation, homogeneous and stable NFC-monolayer clay nanoplatelet suspensions with different mass ratios were obtained and enabled us to prepare highly transparent hybrid films with a densely oriented lamellar microstructure by water evaporation-induced self-assembly. Figure 3a shows the transparent appearance of NFCmonolayer clay nanoplatelet hybrid films with 5 wt%, 10 wt%, 30 wt%, and 50 wt% single-layer clay nanoplatelets, respectively. The word “university” beneath various NFC-monolayer clay nanoplatelet hybrid films is visible to the naked human eye. An ultraviolet-visible spectrometer was applied to measure quantitatively the total light transmittances of the hybrid films over the visible range. The total optical transmittance spectra of each transparent hybrid film with different monolayer clay nanoplatelet contents are shown in Figure 3b, which shows that all hybrid films exhibit a total transmittance of approximately 90%
ACS Paragon Plus Environment
Langmuir
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
at 600 nm and that their transmission curves almost overlap at 600 nm to 800 nm. This phenomenon is different from previous results in literature in which the transparency of hybrid films decreases sharply as the clay nanoplatelet content exceeds 30wt %.34-36 When the wavelength is lower than 600 nm, the transparency of all hybrid films begins to decrease, and they exhibit a much larger decrease for hybrid films with a content that exceeds 30 wt%. As shown in Table 1, in comparison to other hybrid films with maximum clay contents, our hybrid film with a 50 wt% monolayer clay nanoplatelets exhibits a highest light transmittance of ~90 % at 600 nm.
1 2 3 4 5 6 7
Table 1Comparison of the optical transmittance of various hybrid films Thickness Transmittance Component 1 Component 2 References (µm) (% at 600 nm) 50 wt % NFC 50% Montmorillonite 5-7.7 ~46 34 — 80 wt % NFC 20 % Vermiculite ~70 35 60 wt % CMC 40 % Montmorillonite 40-50 ~69 36 20 wt % CMC 80 % Montmorillonite 70 — 37 30 wt% Polyacrylic 70 % Synthetic clay 10 ~83 38 acid sodium 50 % Synthetic 50 wt % NFC 5.0–11.7 ~83 39 saponite 50 % Monolayer clay 50 wt % NFC 50 ~90 Our work nanoplatelets
Mechanism for the High Transparency of Hybrid Films To explore the mechanism for the high light transmittance of hybrid films at different mass ratios of monolayer clay nanoplatelets, cross-sectional scanning electron microscopy (SEM) images of highly transparent hybrid films that contain 5 wt%, 30 wt%, and 50 wt% monolayer clay nanoplatelets are displayed in Figure 2a-c, respectively. The NFC-monolayer clay nanoplatelet hybrid building blocks were stacked together by water evaporation-induced selfassembly to form a well-aligned layered arrangement over large areas, which is similar to the brick-and-mortar structure of nacre, regardless of the monolayer clay nanoplatelet content. No
ACS Paragon Plus Environment
Page 6 of 23
Page 7 of 23
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
Langmuir
obvious micro-scale pores or cavities are visible within hybrid films with different monolayer clay nanoplatelet contents in the through-thickness direction, which favors the high light transmission of hybrid films. Energy-dispersive X-ray mapping of elemental silicon was also carried out to evaluate the distribution of monolayer clay nanoplatelets in the hybrid films. The squares in Figure 4a-c show that red dots that correspond to elemental Si were distributed uniformly in the thickness direction, which indicates a homogeneous distribution of monolayer clay nanoplatelets. The increasing density of red dots in the square is ascribed to an increase of monolayer clay nanoplatelets in hybrid films. Figure 4d shows the X-ray diffraction (XRD) spectra of a hybrid film with 50 wt% monolayer clay nanoplatelets and pure clay nanoplatelets. For the pure clay nanoplatelets, a sharp characteristic peak occurs at 7.0° that corresponds to a d001 spacing of 1 nm. However, the XRD peak of the hybrid film with 50 wt% monolayer clay nanoplatelets almost disappears, which suggests that the monolayer structure of the clay nanoplatelet was well maintained during the water evaporation process. These XRD data also prove indirectly that a complete exfoliation of original stacked clay nanoplatelets into single-layer nanosheets occurs, with NFC assisting as a natural dispersant. In summary, the high transparency of various hybrid films are mainly due to the monolayer clay nanoplatelets with a relatively low aspect ratio and the oriented lamellar nanostructure of NFC-monolayer clay nanoplatelet building blocks resulting from the homogeneous and stable NFC-dispersed monolayer clay nanoplatelet suspension. Thermogravimetric Analysis and Flammability Test The fire-retardant properties of transparent NFC films is of significance for optoelectronic applications. Therefore, the thermal stability and flammability of transparent hybrid films with
ACS Paragon Plus Environment
Langmuir
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
different monolayer clay nanoplatelet contents were investigated by thermogravimetric analysis (TGA), vertical flame test, and limiting oxygen index (LOI) testing, aiming to unravel the underlying mechanism for the excellent self-extinguishing behavior of hybrid films with a monolayer clay nanoplatelet content that exceeds 30 wt%. TGA was used to evaluate the thermal degradation behavior of dissimilar transparent hybrid films in nitrogen and the results are presented in Figure 5a. The onset of decomposition for various transparent hybrid films is ~ 200 °C, which is almost 100 °C lower than that of natural cellulose because of the introduction of carboxyl groups into cellulose chains.40-42 As the monolayer clay nanoplatelet content increases, the decomposition rate of NFC in the hybrid film decreases gradually whereas the total residue mass shows an increasing trend compared with pure NFC film. Vertical flame test and limiting oxygen index (LOI) analysis were employed to investigate the flammability of hybrid films with varying monolayer clay nanoplatelet concentrations. A vertical flame test was applied to evaluate the tendency of these transparent hybrid films to initiate a flame, and enabled the assessment of their flame-resistant performance. Rectangle transparent hybrid films (50-µm thick, 10 cm × 1.5 cm) were subjected to vertical flammability testing. Figure 5b presents digital images of burnt hybrid films and Table 2 illustrates the observations during the test. Pure NFC film exhibits the highest flammability. The sample burns with vigorous flames when exposure to a direct flame (Figure 5c). After burning vigorously for an average of 8 s, the fire is extinguished and a smoldering phenomenon that lasts 32 s is observed to consume the remaining NFC (residue < 1 wt%). However, transparent hybrid films, with only a 5 wt% single-layer clay nanoplatelet addition, demonstrate improved fire-retardant properties compared with pure NFC film. All clay-
ACS Paragon Plus Environment
Page 8 of 23
Page 9 of 23
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
Langmuir
containing samples display a self-extinguishing behavior yet a different flammability upon removal from the flame. With increasing monolayer clay nanoplatelet contents, the char mass increases after burning. A 10 wt% monolayer clay nanoplatelet sample showed a similar flammability as compared to the hybrid film with 5 wt% monolayer clay nanoplatelets, but its shape was retained better and the afterglow time was reduced from 33 s to 22 s. As the monolayer clay nanoplatelet content increased to 30% or more, a radically different phenomenon resulted (Figure 5b). The flame was extinguished immediately after its removal, which shows a threshold in flammability similar to previous publication,27, 29 and the flame was less aggressive during vertical flammability testing. More significantly, no flameless combustion resulted for samples with a 30 wt% and 50 wt% monolayer clay nanoplatelet content (afterglow time of 0 s), respectively, which indicates that the formed char is sufficiently durable to hinder extra NFC pyrolysis and starve the fire of fuel, and thereby the integrity of the two samples is well preserved. As a consequence, residues after combustion of the two samples are high (as shown in Table 2) at 71 wt% and 83 wt% based on total mass, respectively. The transparent hybrid film with monolayer clay nanoplatelets of > 30 wt% demonstrates a sharp expansion in the through-thickness direction (intumescent behavior) when it is exposed to flames. For example, the burned region of the hybrid film with 50 wt% monolayer clay nanoplatelets is almost five-fold thicker than unburned area (Figure S2). LOI testing is widely used to determine the flammability of polymer materials. The higher the value of LOI, the greater the flame retardancy. In this study, the LOI of pure NFC film and hybrid films with different mass ratios of monolayer clay nanoplatelets was measured and the results are listed in Table 2. Pure NFC film has a LOI value of 18.3%. As the addition of NFCdispersed monolayer clay nanoplatelets, the LOI of hybrid films improves from 21.0% to 96.5%.
ACS Paragon Plus Environment
Langmuir
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 10 of 23
When the content of clay nanoplatelets is ≤ 10 wt%, the hybrid films show a slightly increase in LOI value. However, the LOI improves sharply while the content of clay nanoplatelets increases to 30 wt%. For a hybrid film with 50 wt% clay nanoplatelets, it presents a LOI value of 96.5%. Table 2 Flammability data of hybrid films at dissimilar contents of monolayer clay nanoplatelets Samples
Total burning time (s)
Afterglow time (s)
Residue (%)
Limiting oxygen index (%)
0%
8
32
