Tough Polymer Aerogels Incorporating a Conformal Inorganic Coating

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Tough Polymer Aerogels Incorporating a Conformal Inorganic Coating for Low Flammability and Durable Hydrophobicity Hua Sun, Dayong Chen, Danqi Wang, Miguel Sanchez-Soto, and David A. Schiraldi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02829 • Publication Date (Web): 04 May 2016 Downloaded from http://pubs.acs.org on May 11, 2016

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ACS Applied Materials & Interfaces

Tough Polymer Aerogels Incorporating a Conformal Inorganic Coating for Low Flammability and Durable Hydrophobicity Hua Sun‡, Dayong Chen‡, Danqi Wang, Miguel Sánchez-Soto, and David A. Schiraldi*

H. Sun. Author 1, D. A. Schiraldi. Author 5 Department of Macromolecular Science & Engineering Case Western Reserve University Cleveland, OH 44106, USA

D. Chen. Author 2 Department of Chemical Engineering Massachusetts Institute of Technology Cambridge, MA 02139, USA

D. Wang. Author 3 Swagelok Center for Surface Analysis of Materials Case Western Reserve University Cleveland, OH 44106, USA

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M. Sánchez-Soto. Author 4 Centre Català del Plàstic Universitat Politécnica de Catalunya Colom nº 114, Terrassa 08222, Spain

KEYWORDS: nanocomposite, aerogel, coating, fire-resistance, hydrophobicity

ABSTRACT: Both inorganic and polymeric aerogels are well-known in the materials field. Inorganic aerogels are generally susceptible to brittle fracture, while and polymeric aerogels tend to exhibit low modului and high flammability. To overcome these disadvantages, we introduce a new approach to the design of aerogels. A microporous polyvinyl alcohol (PVA) aerogel/silica nanocomposite was prepared by growing a silica conformal coating onto a PVA aerogel scaffold. Such aerogel/silica nanocomposites show significant improvement in their mechanical properties over either individual component. The nanocomposites show excellent fire resistance since the silica conformal coating serves as a barrier for heat transfer and mass loss of the coated organic materials. After a fluorocarbon silane treatment, the nanocomposites also show durable superhydrophobicity. Introduction


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Aerogels are extremely porous materials, which exhibit low densities, low thermal conductivities, high surface-to-volume ratios, and low dielectric constants.1,2 Aerogels are used in thermal and acoustic insulation,3-6 low dielectric substrates,7 catalyst supports,8,9 electrodes for batteries and supercapacitors,10 superhydrophobic coatings11,12 and fire-resistant materials.13,14 Both inorganic and polymeric aerogels are described in the literature; inorganic aerogels, such as silica aerogels, are usually prepared via a sol-gel process, followed by supercritical carbon dioxide extraction.15-17 Silica aerogels can provide durable superhydrophobic surfaces,11,18,19 but generally exhibit extremely poor mechanical properties. Significant efforts have been devoted to the toughening the silica aerogels; a general approach is to incorporate organic components into the aerogels, either by including alkyl groups in precursors20 or by coating the silica aerogels with a thin layer of cross-linked polymer materials.21-24 Polymer aerogels, prepared by the drying of organogels via supercritical CO2 extraction, can be flexible and even foldable.25,26 Polymer aerogels can also be prepared by freeze drying polymer solutions or hydrogels.27,28Polymer aerogels generally exhibit higher fracture toughnesses than those of pristine silica aerogels;26 polymer aerogels are organic, and therefore tend to be flammable, much as are polymer foams. Hybrid organic/inorganic nanocomposite aerogels have also been prepared using a sol-gel transition followed by a supercritical CO2 drying step29-31or a freeze drying step.32Two synthetic approaches are generally employed to prepare hybrid aerogels.33In the first approach, organic groups are chemically bonded with metal alkoxides which undergo hydrolysis and condensation to form hybrid materials.33,34In the second approach, inorganic components can undergo in situ sol-gel transition in the presence of a polymer matrix,31,32,35occurs when silica gels are immersed in an organic monomer solution.36Such prepared aerogels have found broad applications as insulation materials,35oil sorbents and contamination traps.32 Designing the structure of



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organic/inorganic hybrid aerogels can lead to improved properties and other applications. Designing the structure of organic/inorganic hybrid aerogels can lead to improved properties and other applications. In this study, we present a new method of producing tough aerogels through conformally coating a layer of silica nanoparticles onto an ice-templated microporous polymer scaffold. Such polymer/silica nanocomposite aerogels are more mechanically robust and exhibit enhanced fire resistance compared to their starting polymer aerogels. It is also demonstrated that such aerogels can be used as durable superhydrophobic materials after coating with a fluorocarbon silane. Results Microporous polymer aerogel scaffolds were prepared via ice-templating.28,37,38 Freezing a polymer solution concentrates polymer molecules at the grain boundaries of the growing ice crystals. Removal of the ice crystals by sublimation gives the aerogel scaffold (Figure 1). The size and morphology of micropores can be adjusted by tuning the factors that regulate the formation and growth of ice crystals, such as freezing direction, freezing rate, polymer chemistry and the presence of other additives.28,39,40 Polyvinyl alcohol (PVA) was used as the microporous polymer scaffold because of its high solubility in water, ready formation of a hydrogel physically cross-linked by PVA crystals, 41

and ample numbers of hydroxyl groups that can serve as nucleation sites critical for the

formation of silica gel.42 As illustrated in Figure 1a, a sealed polystyrene vial containing PVA aqueous solution was immersed in a cold bath at -70 °C for 30 min to induce formation of ice crystals, followed by drying at 25 °C under vacuum. Freeze-drying of 5 wt% PVA aqueous solutions provided 4 ACS Paragon Plus Environment

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PVA aerogels with a compressive modulus of 2.5±0.3 MPa and the specific surface area of 38.5±0.3 m2 g-1. The PVA aerogels maintained the cylindrical shape of the vial; in the middle portion of the cylinder, heat transfers mostly in the radial direction, leading to an axial symmetric structure (Figure 1b). A similar structure was reported by Van Olphan for freeze-dried clay aerogels.43 The radial structure (Figure 1c) indicates that the ice crystals nucleated on the perimeter, and grew into the center following the freezing direction. A ‘fish-bone’ morphology along the freezing direction is shown in Figures 1d and 1e, which is in good agreement with a previous report.28 Figures 1d and 1e also show that the pore size (as characterized by the spacing between neighboring layers) decreased radially from the outside (average pore size of 42±11 µm at R=5 mm) to the center of the monolith (average pore size of 6±1 µm at R=1.5 mm). It has been shown that the pore size is strongly dependent on the freezing rate.28 In this study, it was observed that geometric confinement also restricts the ice crystal growth, and therefore leads to a pore size reduction in the central portion of the crystal. A two-step (acid–base) sol–gel process was used to deposit a conformal silica coating on the microporous PVA scaffold. As illustrated in Figure 2a, PVA aerogel was initially soaked in a hydrochloric acid-treated tetraethyl orthosilicate (TEOS) solution in an ethanol/water mixture for 4 h (pH value of the precursor solution is 6). Ammonia was added to the solution to catalyze the sol-gel transition afterwards (a pH of 8 was observed immediately after addition of ammonia into the precursor). A series of solutions with different compositions are listed in Supplementary Table S1. The open channels in the microporous PVA aerogel allow the TEOS solution to wick into the aerogel by capillary action. It is shown in Figure S1 that hydrochloric acid catalyzes slow hydrolysis and condensation of TEOS forming silanol dimers or oligomers. In the second step, ammonia rapidly catalyzes the condensed silanol dimers and oligomers, leading to a highly



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cross-linked silica gel.44 As illustrated in Figure S1, hydroxyl groups on the PVA scaffold serve as the nucleation sites for silica particles, leading to a conformal silica coating. Samples then were washed with ample amounts of water and ethanol, in order to remove unreacted precursor. Another freeze-drying step was applied to achieve the polymer/silica nanocomposite aerogels. We note that techniques exist to conformally coat microporous materials, such as atomic layer deposition,45 chemical vapor deposition,12,46 and layer-by-layer assembly.47,48 We achieved conformal coatings through an in situ sol-gel method in the present work. This sol-gel process is cost effective, does not require sophisticated equipment, and can be generalized to conformally coat a variety of inorganic materials.49 As illustrated in Figure 2, the PVA aerogel remains the ‘fish bone’ morphology after the sol-gel process and the additional freeze-drying step. Reduction in the spacing between PVA layers is observed, probably due to slight shrinkage of the bulk sample in the second freezedrying step. The lower image in Figure 2c shows that the PVA scaffold was conformally coated by a layer of silica nanoparticles. The diameter of the silica nanoparticles is 14±2 nm, in good agreement with a previous study.32 The thickness of the silica layer can be controlled by tuning the TEOS concentration until a critical concentration is reached, at which point the silica nanoparticles substantially fill the spaces between PVA layers (Figure S2). The specific surface area of aerogels increases with the TEOS concentration (as indicated by BET measurement in Table S1). Even though polymer-reinforced silica aerogels fabricated by in situ sol-gel process have been investigated,31,32 a major advantage of the present process is that ice-templating provides good control over the microporous structure of the polymer scaffold. One advantage of polymer-inorganic hybrid nanocomposites is that mechanical properties of nanocomposites are superior to either of their components. As shown in Figure 3, 6 ACS Paragon Plus Environment

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the compressive modulus in the axial direction increases from 2.5±0.3 MPa for PVA aerogel to 11±2 MPa for nanocomposite aerogel 1 and 14±4 MPa for nanocomposite aerogel 2. The nanocomposites also exhibit improvements in mechanical toughness over the PVA and silica aerogels.50-52 The nanocomposite aerogel 1 can be compressed to beyond 0.6 strain before failure; throughout the compression, the silica nanoparticles remain adhered to the PVA scaffold. During compression, the propagation of small cracks on the silica layer is arrested by the PVA scaffold. Since the formation of small cracks provides an effective method of energy dissipation, catastrophic failure is not observed. As listed in Table 1, the total energy dissipated (by integrating the area under the stress-strain curve up to 0.6 strain) shows nanocomposite aerogels (1 and 2) can absorb as much as 6 times the energy of the PVA aerogel. Higher modulus and toughness can be achieved by a thicker silica layer on the nanocomposite aerogel 2 at the expense of higher density and earlier failure. The following studies focus on nanocomposite aerogel 1, which exhibits the highest specific modulus. We note that while these nanocomposites show good compression properties in the axial direction, directional freezing usually leads to anisotropy.28,39,53 Detail characterizations on how material properties are influenced by freezing conditions will be a focus of a future study. The nanocomposite aerogels were evaluated by thermal gravimetric analysis (TGA) in air, at a heating rate of 10 °C min-1 up to 800 °C. As shown in Supplementary Figure S3, both nanocomposite aerogels and PVA aerogel show a two-step-degradation mechanism as observed previously for PVA materials.54 The PVA aerogel starts to degrade at ≈ 200 °C and completely decomposes at ≈ 430 °C. The nanocomposite aerogels begin to lose mass at ca. 300 °C and the polymer component fully degrades by 600 °C, leaving only silica residue. The inset image of



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Figure S3 illustrates that the silica residue maintains the shape of the original sample without collapsing. Using aerogels for fire-resistance materials is appealing since they possess extremely low densities and provide excellent thermal insulation. Both polymer aerogels and silica aerogels coated with polymers present organic surfaces, limiting their use as fire-resistant materials. During the burning of polymers, the release of volatile combustion products tends to interfere with the formation of a char-like heat transfer barrier at the gas-condensed phase interface, and therefore increases the burning rate.55 One strategy commonly employed to reduce the flammability of polymers is to incorporate nanoscale fillers, which can form a continuous protective surface layer without any opening or cracks. This significantly reduces the flammability of the polymeric materials by inhibiting vigorous bubbling process in the course of the degradation and by serving as a heat barrier for the polymer beneath.56 Figure 4a and Supplementary Movie 1 show the burning process of the PVA and the nanocomposite aerogels. The PVA aerogel readily ignites and burns vigorously, producing a large flame and black smoke, with a black residue remaining at the end of the process. The nanocomposite aerogel, in contrast, generates a small flame only where the burner flame is in contact the flame self-extinguishes within 10 s and the sample remains intact, except for a small char mark underneath the point of flame contact. Cone calorimetry quantifies the heat release rate (HRR) from the samples by measuring the oxygen consumption in the burning process.57 Figure 4b shows HRR for the PVA aerogel and nanocomposite aerogel samples with dimensions of roughly 8×8×0.8 cm. Table S2 summarizes the detailed results of the cone calorimetry measurements. For PVA aerogel, the time to ignition (TTI) is around 6 s. The flame extinguishes after 55 s from ignition. The HRR value increases quickly and reaches its peak. 8 ACS Paragon Plus Environment

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Fitting the peak hints the two step degradation mechanism of PVA,54 as shown in Figure S4. In remarked contrast, the TTI of the conformally-coated sample increases to 40 s, and the flame extinguishes at 138 s. Only whitish residue was observed and its size is only slightly smaller than that of the original sample. The HRR of the conformally-coated nanocomposite sample never reaches a peak value but plateaus through the entire burning process at 39% of the peak HRR of the PVA aerogel. Even though the total heat release from the two samples is similar, the conformally-coated sample is much more difficult to ignite and also a significantly lower burning rate.56 Aerogels are used in the form of surface coatings in many applications.58-60 Superhydrophobic surfaces are attractive in coating applications due to the self-cleaning mechanism.61,62 In order to achieve robust superhydrophobicity, a combination of a low surface energy material and a hierarchical micro-nanoscale surface structure is generally required.63 Increasing the surface roughness of a hydrophobic surface with micro-nanoscale structures could create a Cassie-Baxter state in which the grooves of the surface are not wetted by water, yielding a large apparent contact angle.64 Such micro-nanoscale surface structures are extremely vulnerable to mechanical wear, which leads to the loss of superhydrophobicity. There are different strategies of designing robust superhydrophobic surfaces;

65-67

one attractive method is

to utilize porous substrates, for which new surfaces with similar structures will be exposed as mechanical wear removes surface materials.11,66 The nanocomposite aerogels in this study have porous micro-nanoscale structures, which make them ideal candidates for durable superhydrophobic surfaces, except that they are made of hydrophilic materials. In order to increase their hydrophobicity, the aerogels were soaked in a solution of perfluorooctyl trichlorosilane in hexane at a concentration of 4 µL/mL for 12 h, followed by annealing at 60° C



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for 3 h to ensure that the perfluorooctyl trichlorosilane molecules are chemically bonded to the aerogels. The advancing and receding contact angles for water on the silanized PVA aerogels were 126±1° and 115±2°, respectively, whereas these values for the nanocomposite aerogel were 144±2° and 136±2°, respectively. The difference in contact angles is likely due to the introduction of nanoscale roughness by the silica particles. As shown in Figure 5, the water contact angles on nanocomposite aerogel remain the same after linear abrasion test by sandpaper (3M 1200 grade) for a prolonged period of time.68,69 The change in the pore size of the exposed surface does not appear to alter the apparent contact angle within the range of measurement certainty. The rolling-off behavior of water droplets on such surfaces is also maintained after sandpaper treatment (Supplementary Movie 2). Such durable superhydrophobicity of the nanocomposite aerogel implies that the PVA scaffold is conformally coated by a layer of silica particles. The micro-nanoscale roughness exists throughout the entire sample. Surfaces of similar roughness will be exposed when the sample surface layer is removed by abrasion, leading to a robust Cassie-Baxter state, indicated by the silvery air film (plastron) observed in Supplementary Movie 3. A sand paper-treated nanocomposite aerogel immersed in water remains completely dry after withdrawing from the water bath. Conclusions Tough polymer/silica nanocomposite aerogels were produced by an in situ sol-gel method to grow a conformal continuous silica-nanoparticle coating on the microporous PVA aerogel scaffolds prepared via ice-templating. As-fabricated polymer/silica nanocomposite aerogels possess higher compressive moduli and higher fracture toughness compared to those of either individual component. The nanocomposite aerogels show good fire resistance, and durable superhydrophobicity after fluorocarbon silanization. Such ice–templating and in situ sol-gel 10 ACS Paragon Plus Environment

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approach for nanocomposite aerogels can be adopted to produce a variety of polymer/inorganic nanocomposite aerogels, which can potentially be used in demanding practical applications such as fire-resistance, filtering, and substrate-assisted catalysis. Experimental Section Preparation of PVA aerogels 5 % PVA aqueous solution was prepared by stirring 5 grams of PVA (Mw=130,000, 99+% hydrolyzed, Sigma-Aldrich) in 100 ml deionized water at 80° C for 4 h. 15 ml PVA solution was then placed in each polystyrene vial, and immersed in a mixture of solid carbon dioxide and ethanol (-70° C) for 30 min to freeze. The vials were then uncapped and placed in a freeze drier (VirTis AdVantage EL-85 lyophilizer, 25 oC under vacuum < 50 µbar) for 4 days to remove the ice. Preparation of PVA/silica nanocomposite aerogels A series of silica gel precursor solutions were prepared by mixing tetraethyl orthosilicate (TEOS, 98%, Sigma-Aldrich) with hydrochloric acid at different volume ratios, (listed in Table S1) in a mixture of water and ethanol. The pH value of the precursor solution was 6. One piece of the PVA aerogel was placed in a polystyrene vial together with 14 ml of the precursor solution. After the PVA aerogel was soaked in solution for 4 h to reach the saturated weight, 0.85 ml ammonia (0.1 mol/l) was added in each vial to catalyze the sol-gel transition, and the reaction was allowed to proceed for 24 h. The vial was then placed in the oven at 60° C for 3 h to complete the sol-gel reaction. After the sol-gel process, the sample was an opaque hydrogel. Since there is a small space between the PVA scaffold and the polystyrene vial, the fragile silica gel growing in the gap outside the PVA scaffold was peeled off from the PVA/silica nanocomposite gel. The 11 ACS Paragon Plus Environment


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PVA/silica nanocomposite gels were then soaked and washed in ethanol, an ethanol/water mixture, and water for 12 h in a beaker. One piece of nanocomposite gel was then placed in a polystyrene vial with water, followed by freezing in a mixture of solid carbon dioxide and ethanol. The vials were then uncapped and dried in a freeze drier for 3 days to remove the ice and form the PVA/silica nanocomposite aerogels. For the cone calorimeter test, rectangular PVA aerogel sheets are prepared. For each PVA aerogel sample, a 10×10×1 cm3 stainless steel mold (no cover) fully filled with PVA solution is immersed into a glass tray filled with a mixture of solid carbon dioxide and ethanol (70° C) to freeze for 30 min, followed by vacuum drying. PVA/silica nanocomposite aerogels are made following exactly the same procedure as described above. For each sample, 140 ml precursor and 8.5 ml ammonia are added into the reaction. Hydrophobic modification PVA/silica nanocomposite aerogels were placed in a clean, dry glass jar containing a solution of perfluorooctyltrichlorosilane (Sigma-Aldrich) in hexane at concentration of 4 µl/ml. The reaction was allowed to proceed for at least 5 h before removing the aerogel from the solution. The nanocomposite aerogel was then washed twice with hexane and dried in air. To ensure complete silanization, these aerogel samples were baked at 60° C for 2 h. Linear abrasion test Sandpaper (3M, 1200 grade) was chosen as the abrading material for the test. The top surface of a nanocomposite aerogel cylinder was placed in contact with the sandpaper. A pressure of around 20 kPa was applied to the sample. The sample was moved back and forth for 5 cm on the sandpaper with a speed of about 2 cm/s until 1 mm thickness of materials was 12 ACS Paragon Plus Environment

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removed. Then the cylinder was turned by 90o, the side cylindrical surface was in contact with the sandpaper. The same parameters of the abrasion test are applied until the radius of the cylinder was reduced by 1 mm. ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publication website at DOI: 10.1021/acsami.******. Figure S1-S6 representing in situ sol-gel reaction details, more SEM images for different TEOS input ratios, TGA traces and fitting for HRR of PVA aerogel. Table S1 showing reactant inputs and specific surface for different aerogels. Table S2 showing detailed information from cone calorimetry. Movie 1 showing the burning process of PVA aerogel and composite aerogel. Movie 2 showing water droplets bouncing off the composite aerogels before and after sand paper treatment. Movie 3 showing a silvery air film between the aerogel and water observed when sandpaper treated nanocomposite aerogel immersing and withdrawing in and out of a water bath.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

Author Contributions ‡ H. Sun and Dr. D. Chen contributed equally to this work. Notes The authors declare no competing financial interest. 13 ACS Paragon Plus Environment


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ACKNOWLEDGMENTS This research was supported by the Case School of Engineering.

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Figure 1. Freeze-drying of PVA aqueous solution in a cylindrical vial yielding an aerogel with axial symmetry. a) A schematic illustration of the freezing setup showing the PVA aqueous solution in a sealed polystyrene vial, immersed in a mixture of solid carbon dioxide and ethanol. b) A cross sectional image showing the axially symmetric structure of the cylindrical PVA aerogel. c, d, e) SEM images showing portions of the cross section. The pore spacing decreases from outside to inside as observed in c from right to left, and at two different distances from the center (42±11 µm at R=5 mm in e, 6±1 µm at R=1.5 mm in d).


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Figure 2. In situ sol-gel transition is employed to conformally grow a layer of silica nanoparticles onto microporous PVA scaffold. a) A schematic illustration showing the process of conformal coating by soaking PVA aerogel in the TEOS solution, followed by an in situ sol-gel reaction and an additional freeze-drying process to yield PVA/silica nanocomposite aerogels. Light blue color represents the TEOS precursor and dark blue represents the silica coating b, c) SEM images (acquired at half the radius to the center) show the conformal coating and additional freeze-drying leads to microporous PVA scaffold conformally coated with a layer of silica nanoparticles. b: unmodified, and c: modified. The spacing between PVA main frames is reduced in size from 42±11 µm to 29±7 µm after the modification.

Figure 3. Stress-strain curves of the PVA aerogel and PVA/silica nanocomposite aerogels under compression. 23 ACS Paragon Plus Environment


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Figure 4. Conformal coating leads to low flammability. a) Temporal images showing the burning process of PVA aerogel and PVA/silica nanocomposite aerogel ignited by a Bunsen burner. While PVA aerogel burned vigorously (upper row), nanocomposite aerogel selfextinguished in 10 s after removing from the burner flame (lower row). b) HRR curves for both PVA and nanocomposite aerogels by the cone calorimeter test with a heat flux of 50 kW m-2. Insert photos show the residues after the cone calorimeter test.


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Figure 5. Durable superhydrophobicity. a) A schematic illustration of mechanical abrasion removing the surface layer, exposing the underlying surface with similar surface texture. b) Sessile drop measurements showing water contact angle remains the same after sand paper treatment due to the sacrificial mechanism illustrated in a.

Table 1. Mechanical properties of PVA and PVA/silica nanocomposite aerogels. Density [g cm-3]

Modulus [MPa]

Specific Modulus [MPa cm3 g-1]

PVA

Energy per volume dissipated up to 0.6 compression [MPa] 0.25

0.066 ± 2.5 ± 0.3 39 ± 5 0.001 Nanocomposite 1a) 0.184 ± 11.4 ± 1.7 62 ± 9 1.09 0.010 Nanocomposite 2b) 0.258 ± 14.1 ± 3.8 55 ± 14 1.42 0.022 a) Nanocomposite 1: PVA/silica nanocomposite aerogel with 3 mL TEOS in precursor. b) Nanocomposite 2: PVA/silica nanocomposite aerogel with 6 mL TEOS in precursor. (Detailed information on aerogel preparation is tabulated in Table S1).



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Table of contents


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Silica-modified polymer aerogel 254x190mm (96 x 96 DPI)

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Tough Polymer Aerogels Incorporating a Conformal Inorganic Coating

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