Ultralight, Thermally Insulating, Compressible Polyimide Fiber

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Ultralight, thermally insulating, compressible polyimide fiber assembled sponges Shaohua Jiang, Bianca Uch, Seema Agarwal, and Andreas Greiner ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11045 • Publication Date (Web): 25 Aug 2017 Downloaded from http://pubs.acs.org on August 25, 2017

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ACS Applied Materials & Interfaces

Ultralight, Thermally Insulating, Compressible Polyimide Fiber Assembled Sponges Shaohua Jiang, Bianca Uch, Seema Agarwal*, Andreas Greiner* Macromolecular Chemistry, Bavarian Polymer Institute, University of Bayreuth, Universitätsstraße 30, 95440 Bayreuth, Germany KEYWORDS Polyimide; electrospinning; sponge; thermal resistance; self-reinforced

ABSTRACT

Tunable density, thermally and mechanically stable, elastic and thermally insulating sponges are required for demanding applications. Hierarchically structured sponges with bimodal interconnected pores, porosity more than 99% and tunable densities (between 7.6-10.1 mg/cm3) are reported using polyimide (PI) as high temperature stable polymer. The sponges are made by freeze-drying a dispersion of short PI fibers and precursor polymer, poly (amic acid) (PAA). The concept of “self-gluing” the fibrous network skeleton of PI during sponge formation was applied to achieve mechanical stability without sacrificing the thermal properties. The sponges showed initial degradation above 400 and 500 °C in air and nitrogen, respectively. They have low thermal conductivity of 0.026 W/mK, and thermal diffusivity of 1.009 mm2/s for a density of

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10.1 mg/cm3. The sponges are compressible for at least 10000 cycles and good thermal insulators even at high compressions. These fibrous PI sponges are promising candidates for potential applications in thermal insulation, light weight construction, high temperature filtration, sensors, and catalyst carrier for high temperature reactions.

INTRODUCTION Lot of interesting work is reported by several research groups regarding polyimide (PI) aerogels with an intention to combine high thermal stability and glass transition temperature with light weight, flexibility and thermal insulation for aerospace and other applications.1-8 The aerogels had densities, in general, in the range 200 mg/cm3 - 600 mg/cm3 (higher and lower (as low as 50 mg/cm3) are also reported) and porosities till about 90%. Large shrinkage (up to 40-50%) during PI aerogel formation is a problem.1,2 Therefore, different chemistries and starting monomers were used for the formation and cross-linking of PIs to reduce the shrinkage during aerogel formation and enhance the mechanical stability.1,4, 9-10 Recently, fibrous sponges derived from polymeric and inorganic electrospun fibers have attracted a lot of attention due to their three dimensional (3D) interconnected network, high porosity and pore volume, hierarchical pore structures, low densities (< 10 mg/cm3), and high flexibility and elasticity.11-23 These sponges are highly promising for applications in tissue engineering, oil-catalysis, energy absorption, electrodes, super capacitor, and oil/water separation. In spite of few literature efforts for the formation of polymer sponges from fiber dispersions, it is still a great challenge to achieve stable sponges with good mechanical properties. There is no universal method and each new sponge requires an appropriate new technique for stabilization.


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The incorporation of organic or inorganic materials is an effective approach to enhance the mechanical stability of fibrous sponges only when the additives do not sacrifice other desired properties of the sponge.15, 18-21, 23 The sponges have a bimodal open pore size structure in which bigger pores (100 µm or more) are interconnected through small pores (< 10 µm) formed by entanglement of fibers. This special architecture allows penetration and retainment of large volumes of a liquids and gases. This is of interest for catalysis and thermal insulation. The inherent characteristics of polymeric fibrous sponges such as, high porosity and pore volume, thermal insulation and easy modulation of densities in combination with material properties (for example, high temperature stability of PI) could provide novel sponges for high temperature applications. PI nanofibers could be produced by electrospinning their precursor solutions in organic solvent or in water followed by a thermal imidization and have been reported for many kinds of applications, such as filtration, battery separator, reinforcements, fuel cell proton exchange membranes, sensors, microelectronics, and precursor for carbon nanofibrous materials.24-25 In this work, we report high temperature stable and thermally insulating PI sponges with tunable low densities, up scalable, with very high porosity and pore volume. The concept of “self-gluing” the fibrous network skeleton of PI during sponge formation was applied to achieve mechanical stability without sacrificing the thermal properties. The obtained PI sponges showed enhanced and excellent mechanical properties, superior thermal resistance and ultralow thermal conductivity. These fibrous PI sponges are promising candidates for potential applications in thermal insulation, light weight constructions, high temperature filtration, sensors, and catalyst carrier for high temperature reactions.


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EXPERIMENTAL SECTION Preparation of poly(amic acid) (PAA) The PAA was prepared according to our previous report.25 In brief, equal molar ratio of pyromellitic dianhydride and 4, 4’-oxidianiline were reacted in DMF at 0-4 °C for 24 h. Then the obtained PAA solution (7.1 wt.%) was precipitated by deionized water. The obtained PAA solid was filtered and dried at 50 °C in vacuum for 12 h. The intrinsic viscosity of the PAA powder was 1.22 dL/g, which was measured by Ubbelohde viscometer at 25 °C in DMF. Preparation of short PI fibers Electrospun PI fibrous membranes made by polycondensation of pyromellitic dianhydride and 4, 4’-oxidianiline followed by thermal imidization are kindly provided by Jiangxi Xiancai Nanofibers Technology Co., Ltd, China. The PI membranes (20 g) were cut into small pieces and mixed with 2 L of mixture of solvents (dioxane/water, 50/50, v/v). The mixture was cooled with liquid nitrogen until it is like a paste. Then the cooled mixture was cut by a mixer (Robot Coupe Blixer 4, Rudolf Lange GmbH & Co. KG) at 3500 rpm for 2 min into short fibers. After that, the powder of short fibers was obtained by freeze-drying for 24 h. Preparation of fibrous PI sponges 120 mg of PI short fibers were mixed with 29 mL of dioxane and 1 mL of PAA solutions in DMSO containing 5, 10, 20, and 50 mg PAA, respectively. The obtained dispersions were slowly frozen at -20 °C and then dried by a freeze-dryer for 48 h under 0.03 mbar. Then the obtained sponges were dried in a vacuum oven at 60 and 100 °C for 1 and 2 h, respectively followed by thermal imidization using the protocol: 1) heating to 150 °C (3 °C/min) and


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annealing for 1 h; 2) heating to 350 °C (1.5 °C/min) and annealing for 1 h; 3) cooling down to room temperature. The obtained PI sponges depending on the addition amount of PAA were denoted as PISG-5, PISG-10, PISG-20 and PISG-50, with density of 7.6, 8.0, 8.4 and 10.1 mg/cm3, respectively. The density (mass of sponge / volume) of the sponges was determined by weighing them on a precision balance and volume determination using radius and height of the cylindrical samples ( ℎ). Characterization The morphologies of the fibers and sponges were observed by scanning electron microscope (SEM, Zeiss Leo 1530). A 3 nm layer of platinum was coated on the sponges before SEM measurement. ImageJ software was used to determine the fiber diameter and the fiber length. Thermogravimetric analysis (TGA) was performed on a Netzsch TG 209 F1Libra under air and N2 with a heating rate of 10 °C / min from 100 to 800 °C. The compression test and cyclic compression test were performed on a Zwick Z2.5 machine equipped with a 20 N sensor at a compression rate of 50 mm/min. 10000 compression cycles were performed and the curves for first and 100n (n = 1, 2, …… , 100) cycles were recorded. The sample for cyclic compression tests is PISG-50 with a diameter of 2.5 cm and height of 1.4 cm. The thermal conductivity was measured by a Hot Disk Thermal Constants Analyser (TPS 2500s) with a kapton sensor (Hot Disk 7577 2.001 mm). The applied measurement time and heating powder were 10 s and 5 mW, respectively. The porosity (P) and specific pore volume (SPV, cm3/g) of the sponges can be calculated from equation (1) and (2): = 1 −

× 100% 1)


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=

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2) × 10

Where ρSG is the density of the sponge and ρbulk = 1.4 g/cm3 is the density of polymer in bulk state. RESULTS AND DISCUSSION Ultralight PI sponges were prepared from dispersion of different concentration of short PI nanofibers, which were obtained from annealed electrospun PAA nanofibers followed by mechanical cutting in dispersion. The short fibers had an average length of 41 ± 22 µm (Figure 1a). The PI electrospun fiber nonwovens were processed into short PI fiber dispersions of different concentrations and freeze-dried for making sponges. However, these short PI fibers could not self-assemble into a stable sponge (Figure 1b), which is probably due to the lack of junction points between the fibers (Figure 1c, d). This assembly showed very poor mechanical performance and could be re-dispersed in solvents (Figure 1e).


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Figure 1. The length distribution of short electrospun PI fibers (a), photo of self-assembled short electrospun PI fibers (b), the corresponding SEM images (c, d), and the re-dispersed PI short fiber dispersion (e). Conformal coating of polymers such as epoxy, cyanoacrylates and styrene for increasing the stability of silica aerogel which otherwise is highly brittle is known.26-27 The polymers used for coating have limited thermal stability in the range 150-200 °C. High temperature and chemically stable poly(p-xylylene) (PPX) was used as coating material by gas-phase polymerization on polymer sponges for improving their mechanical stability in the past by us.21 but this method is very slow for large scale samples. In the present study, we show the concept of self-gluing of fibers during sponge formation during freeze-drying by using precursor PAA in the PI short fiber dispersion followed by its imidization. The self-glued fibers are expected to have homogenous chemical composition and therefore not compromising the thermal stability. The PI is infusible and non-swellable polymer in organic solvents. Therefore, for self gluing of the fibers, the precursor polymer PAA was added in the PI fiber dispersion. The addition of dissolved PAA to the dispersions and additional imidization by annealing was extremely important for the stability of PI sponges. After freeze-drying and imidization, the PAA was converted into PI and coated around and in-between the PI fibers and formed very stable PI sponges (Figure 2). The PI sponge showed less than 20 % shrinkage in length and flat surfaces on slow cooling during preparation. A fast cooling in liquid nitrogen during preparation procedure should be avoided as it leads to the collapse of PI sponges in the centre (Figure 2).


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Figure 2. Size changes during freeze-drying by two freezing approaches. (a) A slowly immersing freeze process and (b) fast freeze process. Table 1. Density, porosity and specific pore volume (SPV) and compressive strength at 50% compressive strain of the PI sponges. Sample

PISG-5

PISG-10

PISG-20

PISG-50

Density (mg/cm3)

7.6

8.0

8.4

10.1

Porosity (%)

99.46

99.43

99.40

99.28

SPV (cm3/g)

130.9

124.3

118.3

98.3

1.45

1.76

2.20

108 ± 56

90 ± 32

73 ± 38

Compressive (MPa) Big pore size (µm)

strength 1.15 121 ± 72

As the amount of PAA increased, the density of the sponges also slightly increased so that PISG-5, PISG-10, PISG-20 and PISG-50 showed density 7.6, 8.0, 8.4 and 10.1 mg/cm3 (still one of the lightest polymer sponges), respectively (Figure 3a), but their porosities were always larger than 99% and specific pore volumes were larger than 98 cm3/g (98 – 131 cm3/g depending upon the sponge density) (Table 1). All the sponges presented hierarchical microporous structures as shown in Figure 3b-e. The big pores from the evaporation of dioxane crystals during freeze-drying process possessed pore sizes from


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tens of micrometers, while the small pores between the fibers showed pores sizes

Ultralight, Thermally Insulating, Compressible Polyimide Fiber

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