Nanofibrous Kevlar Aerogel Threads for Thermal Insulation in Harsh

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Nanofibrous Kevlar Aerogel Threads for Thermal Insulation in Harsh Environments Zengwei Liu, Jing Lyu, Dan Fang, and Xuetong Zhang ACS Nano, Just Accepted Manuscript • Publication Date (Web): 01 May 2019 Downloaded from http://pubs.acs.org on May 1, 2019

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Nanofibrous Kevlar Aerogel Threads for Thermal Insulation in Harsh Environments Zengwei Liua,b†, Jing Lyub†, Dan Fangb, and Xuetong Zhangb,c*

aSchool

of Nano Technology and Nano Bionics, University of Science and Technology

of China, Hefei 230026, P. R. China

bSuzhou

Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences,

Suzhou 215123, P. R. China

cDepartment

of Surgical Biotechnology, Division of Surgery & Interventional Science,

University College London, London, NW3 2PF, UK

*E-mail:

[email protected] or [email protected]

ABSTRACT: Aerogel with low density, high porosity and large surface area is a promising structure for next generation of high-performance thermal insulation fibers and

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textiles. However, aerogel fibers are suffering from weak mechanical properties or complex fabricating processes. Herein, a facile wet-spinning approach for fabricating nanofibrous Kevlar (KNF) aerogel threads (i.e. aerogel fibers) with high thermal insulation under extreme environments is demonstrated. The aerogel fibers made form nanofibrous Kevlar render a high specific surface area (240 m2/g) and wide-temperature thermal stability. The flexible and strong KNF aerogel fibers are woven into textiles to illustrate the excellent thermal insulation property under extreme temperature (-196 °C or 300 °C) and at room temperature. COMSOL simulation is applied to calculate the thermal conductivity of a single aerogel fiber and find an effective way to improve the thermal insulation property of the aerogel fiber. Furthermore, a series of functionalized fibers or textiles based on KNF aerogel fibers, such as phase-change fibers, conductive fibers and hydrophobic textiles have been prepared. Such KNF aerogel fiber represents a promising direction for the next generation of high-performance fibrous thermal-insulation materials.

KEYWORDS: aerogel threads, nanofibrous Kevlar, thermal insulation, extreme environments, simulation analysis


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For millenniums, nature-based fibers, ranging from plant cellulose (e.g. flax and cotton) to animal protein fibers (e.g. wool and silk), have served mankind as abundant and important thermal insulation materials.1–3 After the industrial revolution, with the scientific development and technological progress, chemists have been capable of not only modifying natural materials but also synthesizing new ones. The first man-made fiber (rayon) was manufactured from nitrocellulose solution in 1855, while the first synthetic fiber, nylon 66 was introduced to the world in 1939.4 Since then, the family of synthetic fiber ushered in rapid development, which has grown to include polyester fibers,5 polyolefin fibers,6 and polyamide fibers.7,8 To date, synthetic fibers with novel structures and functionalities such as hollow fibers and ultrafine fibers can exceed the performance limits of the best natural fibers that grown in fields, or spun from the fleece of animals (Figure 1a).9 Hollow fibers have higher thermal resistance in comparison with solid ones, while thermal resistance of ultrafine fibers is better than those of macro-fibers in both hollow and solid fibers. It is believed that the high thermal insulation performance of hollow fibers or ultrafine fibers is ascribed to the high surface area and abundant air trapping between/inside the fibers. Previous reports also revealed that the main effects of thermal


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insulation on thermal performance depend on material density, porosity, specific surface area and the number and size of void in the material.10 Based on that, aerogel structures containing large porous volume for effective thermal insulation could be an alternative for next-generation synthetic fiber with outstanding thermal regulation. Aerogels are highly porous materials with low density, large surface area, showing promising for various applications, including environmental treatment,11,12 catalytic carriers,13 energy storage devices,14,15 and thermal insulation materials.16,17 Currently, the most attractive and widespread application of aerogels is used as thermal insulation due to the significantly low thermal conductivity which can be as low as 0.025 W/m∙K (similar to that of air at ambient conditions, 0.026 W/m∙K).18,19 This ultralow value can be attributed to (1) the high porosity and tortuosity of the solid nanostructure, which minimizes thermal conduction, (2) the effective suppression of thermal radiation, and most importantly, (3) the pore sizes below the mean free path of the gas phase (ca. 70 nm for ambient air), which effectively reduces thermal convection contributions.19,20 It is proposed that shaping aerogels into one-dimensional fibers, which may give some


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fascinating properties such as favorable flexibility, facilitating the design and manufacturing of thermal insulation textiles. In the past decade, many efforts have been dedicated to developing aerogel fibers, e.g., graphene aerogel fibers,21,22 titanium oxide aerogel fibers,23 silica aerogel fibers24 and cellulose aerogel fibers25 with many different functionalities. For example, graphene aerogel composite fibers with tunable functions of thermal conversion and storage under multi-stimuli was reported providing broad applications in the next-generation of wearable systems. Cellulose aerogel fibers with higher mechanical strength and low thermal conductivity at room temperature was fabricated.26 However, all these aerogel fibers either suffer from poor mechanical properties or are fabricated from complex processes including chemical, enzymatic, and/or mechanical treatment. Besides, it is difficult to directly measure the thermal conductivity of a single aerogel fiber. In this work, we reported an aerogel thread (or fiber) which was fabricated through a simple wet-spinning process from nanofibrous Kevlar (KNF) dispersion, where the KNF is in essence the nanoscale version of poly (paraphenylene terephthalamide), a paraaramid polymer better known by its trade name, Kevlar®. The obtained aerogel fibers with


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excellent mechanical performance can be tied into a knot or weaved into a textile. Moreover, the aerogel structure with three-dimensional interconnected porous networks and a high still air layer content provides the aerogel fibers with an extraordinary thermal insulation, even under harsh environments. COMSOL was employed to provide an example of the effect of aerogel structure on thermal insulation performance. In addition to thermal insulation, the aerogel fibers could also function as phase-change fibers, conductive fibers and hydrophobic textile, when incorporated with phase-change materials, nickel and fluorocarbon (FC) resin, respectively. These results demonstrated the aerogel fibers hold great potential for next-generation synthetic fibers in the field of thermal insulation and thermal management. RESULTS AND DISCUSSION Fabrication and Characterization of KNF aerogel fibers. The schematic diagram for fabricating the aerogel fibers is shown in Figure 1b, which involves wet-spinning, sol-gel process and freeze-drying. The nanofiber solution with different concentration was prepared through dissolving Kevlar® in dimethyl sulfoxide (DMSO) according to previous reports,27–31 and the concentration of nanofibrous Kevlar dispersion was adjusted from


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0.1 wt% to 2.0 wt%. Compared to monomer-to-nanofiber synthesis strategy,32 this facial top-down method is simpler and the resulting nanofiber is thinner. Wet-spinning was applied to extrude the well-dispersed nanofiber solution into a coagulation bath from a syringe at a constant speed. The main factor that determined the diameter of aerogel fibers was the inner diameter of the needles, and in this work, the inner diameter of spinning needle was from 200 µm to 600 µm. Accordingly, a series of aerogel fibers with controllable diameters were obtained. The gelation rate can be controlled by regulating the volume fraction of DMSO to DI water in the coagulation bath. After removing DMSO through solvent replacement, the KNF hydrogel fibers were obtained. Subsequently, freeze-drying was employed to acquire the corresponding aerogel fibers. Although supercritical fluid drying is a traditional method to prepare aerogels, due to the efficiency in maintaining the original porous structure of aerogels, freeze-drying is simpler and more energy conservation. The as-prepared aerogel fibers exhibit excellent mechanical and thermal insulation properties, endowing them with good weaveability and hence applications as outstanding thermal insulation textiles (figure 1c). In order to improve the performance of the aerogel fibers and broaden their applications, a variety of functional


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modification was utilized. For example, after chemical modification or simply coating a barrier layer of fluorocarbon resin, the hydrophobic thermal insulation textile can be fabricated. A colorful textile also can be prepared from KNF aerogel fibers which were colored by simply immersing hydrogel fibers into a dye solution before freeze-drying. Besides, the strong capillary force throughout the aerogel fiber attributed to high specific surface area was applied to absorb phase change materials to fabricate form-stable phase-change composite fibers, and electroless plating as an important metallization method was utilized to transfer electrical isolation aerogel fibers to conductive ones.


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Figure 1. (a) The history of thermal insulation fibers. (b) Schematic description of the fabrication of KNF aerogel fibers and (c) its various applications.

Figure 2a shows the photograph of the aerogel fibers prepared from 2 wt% nanofibrous Kevlar dispersion with approximate 300 µm in diameter and light yellow in color. This aerogel fiber exhibits unusually high mechanical strength where a single fiber can withstand a tensile load of 20 g (Figure 2b and Movie S1). The surface and cross-section morphologies of aerogel fibers were revealed by scanning electron microscopy (SEM), which are shown in Figure 2c and Figure S1. All the fibers were fabricated via freezedrying from 2 wt% dispersion. The as-prepared aerogel fibers present good integrity and wrinkled surface (Figure 2c1 and c2). The cross-sectional morphology shows interconnected three-dimensional nanofibrillar networks (Figure 2c3 and c4), facilitated by flexibility and branching of the individual nanofibers.28 With the increase of DMSO concentration in coagulation bath, the depth of wrinkles increased and hence the roughness increased (Figure S1), but it has no significant effect in controlling the specific surface area and the pore size. With the pH of coagulation bath increased from 1 to 13,


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the specific surface area increased from 180 m2/g to 250 m2/g, but the pore size didn’t display the same trend, it fluctuated from 9.0 nm to 12.8 nm (Figure S2). Supercritical CO2 drying was applied to prepare the aerogel fibers for comparison with freeze-dried one, and its SEM images demonstrate smooth surface and homogeneous interconnected nanofibrillar networks both on surface and cross-section (Figure S3). Notably, for freezedrying process, the ice crystal growth could break the network structure and generate large pores, resulting in severe shrinkage, as show in Figure S4. To protect the network of Kevlar hydrogel fiber, tert-butanol (TBA)/water solution was applied to replace water in Kevlar hydrogel fiber, which would suppress the ice growth by forming tert-butanol-water eutectic. 25% TBA solution was chosen as a typical solvent system for the preparation of Kevlar aerogel fibers, except for special instruction. To study the effects of the dissolution and protonation procedures on the chemical structure, FT-IR spectrum of KNF aerogel fibers was measured. As shown in Figure S5, the upshifts of C=O stretching vibrations was observed when compared with that of Kevlar threads. The higher energy of these vibrations is related to the higher average strength of hydrogen bonds in the material, which can be associated with the optimization of chain


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conformations in nanofibers.28 The X-ray diffraction (XRD) patterns of Kevlar and its aerogel are shown in Figure S6, indicating that the dissolution process did not change the structure of the nanofibers. To investigate the thermal stability of the KNF aerogel fibers, thermogravimetric analysis (TGA) was conducted and the result shows the extraordinary high temperature resistance (Figure S7), which is similar to the thermal decomposition behavior of bulk Kevlar.27,30 This is very important because high thermal stability enables the aerogel fiber as thermal insulating material to work under extreme environments. Uniaxial tensile test was carried out for the aerogel fibers fabricated from different conditions. Typical stress-strain curves are shown in Figure S8, which illustrates that the initial concentration of KNF dispersion has a significant effect on the mechanical properties. With the increasement of initial concentration, the ultimate tensile stress increased gradually from 0.6 MPa for 0.4 wt% to 3.3 MPa for 2.0 wt%, which was attributed to the enhanced proportion of skeleton structure, especially manifested on the surface. The tensile stress reached 350 MPa for neat fiber fabricated from KNF dispersion

via vacuum oven drying (Figure S8 inset). Figure S9 summarizes the variation of tensile strength and diameters of the aerogel fibers prepared form TBA/water solution with


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different TBA concentration. Associating with SEM images in Figure S4, 25% TBA aqueous solution was an appropriate solvent system for the preparation of aerogel fibers. Therefore, the aerogel fiber prepared from 2 wt% KNF dispersion, and solvent exchange with 25% TBA aqueous solution, showed the optimal mechanical properties with a tensile strength of 3.3 MPa and a strain of 35.2%, exceeding those of cellulose aerogel fibers fabricated from the 2 wt% initial cellulose solution (0.99 MPa in strength, 14% in strain) or some silica aerogel-based composites.33,34 The Figure 2d is the SEM image of a knot which shows the flexibility of the aerogel fiber. Based on the excellent flexibility and strength, the KNF aerogel fiber could easily weave into textiles which proves a practical application possibility. The photograph, polarizing microscope image, and SEM image of the aerogel textiles are shown in Figure 2e-g, respectively. The textile is weaved by densely arranged aerogel fibers and the weaving process would not cause any damage to the aerogel fiber (Figure 2g).


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Figure 2. Images of KNF aerogel fibers and its textiles. (a) Photograph of the aerogel fiber with a diameter of 300 µm in a roll. (b) Photograph of a single aerogel fiber with a tensile load of 20g. (c) Surface (c1 and c2) and cross-section (c3 and c4) SEM images of freezedried aerogel fibers with different magnifications. (d) SEM image of aerogel fiber knot. (e) Photograph of a textile woven from the aerogel fiber. (f) Polarizing microscope image of the aerogel textile. (g) SEM image of the aerogel textile.

The porosity of the aerogel fiber was investigated by nitrogen sorption characterization. The type IV nitrogen adsorption/desorption isotherm of aerogel fiber shows a hysteresis loop at high relative pressure, indicating the existence of plentiful mesopores in the


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aerogel (Figure S10). The BET surface area of aerogel fibers was approximate 240 m2/g, which was attributed to their interconnected nanofibers.21 As far as we know, the specific surface area of this aerogel fiber is comparable and even higher than previous reported other aerogel fibers, including cellulose aerogel fibers.35 In addition, there was no remarkable distinction in BET surface area between aerogel fibers prepared from different original nanofibrous Kevlar dispersion or drying methods (Figure S11). This aerogel fiber with ultralow density (23 kg/m3 prepared from 2 wt% dispersion) and up to 98% porosity (calculated from Equation S1) hold great potential in thermal insulation. It was made into a mat to measure the thermal conductivity and compare it with that of commercial microfiber mats and hollow fiber mats. The thermal conductivity at different temperatures is shown in Figure S12. The room temperature thermal conductivity of aerogel fibers mat is ∼0.037 W/m·K, which is lower than that of hollow fiber mat and comparable to that of microfiber mat (Figure S12). When the temperature in the range of -20 to 25 °C, the aerogel fiber mat exhibits the lowest thermal conductivity, revealing the most superior thermal insulation performance. This behavior was attributed to the low density and high porosity of the aerogel, as well as the size confinement


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enhanced boundary scattering and phono-defect scattering.16,18,34,36 The thermal conductivity of the KNF aerogel fiber remains competitive among all kinds of aerogelbased thermal insulating materials, including organic aerogels (e.g., cellulose and polyimide-based aerogels) and inorganic aerogels (e.g., SiO2-based aerogels) (Figure S13). Although some bulk aerogels, such as SiO2 aerogels and SiO2-PAN nanofiberassembled aerogels exhibited lower thermal conductivity, they suffered from ultralow mechanical strength (

Nanofibrous Kevlar Aerogel Threads for Thermal Insulation in Harsh

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