Mechanical Reinforcement of Free-Standing Polymeric

Jan 11, 2019 - Owing to their nanometer thickness, large lateral dimensions, and self-supporting properties, free-standing nanomembranes (FS-NMs) exhi...
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Letter Cite This: ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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Mechanical Reinforcement of Free-Standing Polymeric Nanomembranes via Aluminosilicate Nanotube Scaffolding Anteneh Mersha†,‡ and Shigenori Fujikawa*,†,‡,§,∥ Graduate School of Engineering, ‡WPI International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), and §Center for Molecular Systems (CMS), Kyushu University, Fukuoka 819-0395, Japan ∥ Laboratory for Chemistry and Life Science, Tokyo Institute of Technology, 4259 Nagatsutacho, Midori-ku, Yokohama 226-8503, Japan Downloaded via SWINBURNE UNIV OF TECHNOLOGY on February 3, 2019 at 19:38:23 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: Owing to their nanometer thickness, large lateral dimensions, and self-supporting properties, free-standing nanomembranes (FS-NMs) exhibit outstanding advantages. However, FS-NMs suffer from macroscopic instability, making mechanical reinforcement still a detrimental challenge for their full exploitation. In this paper, we reported a simple strategy for the mechanical enhancement of nanomembranes based on free-standing polymers with thicknesses of less than 100 nm via the incorporation of aluminosilicate nanotube (ASNT) bed scaffolds. The composite nanomembranes of ASNT/ polydimethylsiloxane (PDMS) demonstrated a 4-fold increase in tensile strength and biaxial modulus over 43 times higher compared to the PDMS film. This approach could be extended to other polymers, even polymers with a less-film-forming nature. KEYWORDS: mechanical strength, flexible, free-standing, ASNTs, polymer, nanomembrane

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mechanical reinforcement in organic/inorganic composite materials. However, control of polymer−filler compatibility (such as interfacial interactions) is a research challenge for the development of chemically stable and defect-free membranes.12 In addition, molecular-level designs, such as organic/inorganic interpenetrating networks,15 double-network hydrogels16 and layer-by-layer assembly,10,17 could be used to prepare mechanically enhanced nano- and microthickness membranes. However, such procedures often require materials to satisfy both the properties of membrane formation ability and functional moieties in a membrane form, resulting in the limitation of material selection. Herein, we report a different strategy employing aluminosilicate nanotubes (ASNTs) as bed scaffolds to deposit polymeric nanofilms. Polydimethylsiloxane (PDMS) is chosen as a starting polymer because it is commonly used as a gasseparation membrane material. Acting as a strong backbone, ASNT networks strengthen the PDMS nanomembrane. ASNTs are synthetic analogues of natural imogolite with internal and external diameters of about 1 and 2 nm, respectively.18 The choice of ASNTs is based on the utilization of its reactive hydroxyl functionalized surfaces to make crosslinked nanofiber backbones for high mechanical reinforcement. For example, the characteristic OH-functionalized surfaces allow sol−gel reaction with metal oxides.19 Such surfaces also

ree-standing nanomembranes (FS-NMs) have received rapidly growing interest for a wide range of applications including energy conversion and storage,1,2 sensing,3,4 and biomedical applications4 and rarely in molecular separations,5,6 owing to their nanometer thickness and large lateral dimensions. FS-NMs comprise thicknesses of less than a few hundred nanometers1 with dual surfaces that can physically separate two spaces,7,8 by sustaining their size and shape without support.9,10 FS-NMs offer superlative advantages over their bulk counterparts and other material forms because of the following features: (i) FS-NMs can be transferred onto any substrate of diverse configurations such as planar, curvilinear, or wavy structures;1,4 (ii) they have high lateral size to thickness aspect ratios of greater than 106;7 and (iii) FS-NMs have unique interfacial and mechanical properties, including non-covalent adhesiveness and flexibility.7,11 Nevertheless, to fully exploit such unique features, FS-NMs need to have sufficient mechanical and chemical stabilities in the macroscopic scale. For example, FS-NMs are expected to enhance mass transport with less energy in separation applications. However, membrane thinning is often encountered by mechanical weakening. Also, the excellent mechanical stability of FS-NMs is required in practical applications, such as wound dressing and antibacterial therapeutics,4 in which FSNMs are installed onto uneven surfaces. Therefore, to fully utilize their potential, improving the macroscopic stability of FS-NMs is still a critical challenge. The incorporation of inorganic fillers into a polymer matrix12−14 have been a common practice for synergetic © XXXX American Chemical Society

Received: November 6, 2018 Accepted: January 11, 2019 Published: January 11, 2019 A

DOI: 10.1021/acsapm.8b00104 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Polymer Materials

Figure 1. (a) Schematic representation of nanomembrane preparation procedure. (b−e) Surface and cross-sectional morphologies under SEM observation. Scale bar: 200 nm. (b) Cross-linked ASNTs network structure after 10 cycles of alternate ASNT and TiO2 coating steps, (A-T)10. (c) The film surface became smooth following PDMS coating on panel a, i.e., (A-T)10/PDMS. (d, e) Cross-sectional view of panels b and c, respectively. (f, g) Digital images of detached nanomembranes. Scale bar: 1 cm. (f) Nanomembrane lifted off by a tweezer and dried in air (color is due to reflection of light). (g) Nanomembrane transferred onto a printed text.

Information. Breifly, ASNTs were first synthesized according to a modified reported procedure.22 Then, a randomly interlaced ASNTs network (Figure 1b) was deposited on the precoated sacrificial layer from an aqueous dispersion of ASNTs. Here, spin-coating conditions were essential for uniform ASNTs deposition. Spinning speed was ramped up from 500 rpm (held for 10 s) to 1500 rpm (held for 30 s) and finally to 3000 rpm (held for 30 s). Afterward, a 1 mM chloroform solution of titanium nbutoxide was spin-coated at 3000 rpm for 60 s. The titanium nbutoxide reacts with the surface hydroxyl groups via the sol− gel reaction,19 wrapping ASNTs with thin layer of TiO2. The alternate spin-coating of ASNTs and TiO2 was repeated as required. A distinct layer of TiO2 film was not formed, as confirmed by scanning electron microscopy observation (Figure 1b). However, the presence of TiO2 demonstrated an apparent change in the stability of ASNT networks. This phenomenon was observed during film detachment process. In the absence of TiO2, the repeatedly coated ASNTs were unstable and fragmented into small pieces that are not visible to the naked eye. However, the same cycles of ASNT/TiO2 remained stable in the detachment solvent, even though the film was broken into large flakes. It was also observed that the

facilitate excellent solution processability and good adhesion with polymers.20 In addition to the above merits, the onedimensional structure of ASNTs20 provides an opportunity for multiple hydrogen bonding that enhances the mechanical property of the ASNT scaffold. These properties are advantageous than the inert chemical nature of other nanotubular materials, such as carbon nanotubes (CNTs). Although CNTs are the most widely used nanotubular materials in membrane-based separations 12 and other applications,21 harsh surface modifications are often required to improve their solution dispersibility and interaction with polymers.12 Aggressive treatments alter CNT integrity and properties.21 In this report, FS-NMs of the general designation (A-T)n/ PDMS (where A is ASNTs, T is TiO2, and n is the number of cycles of alternate ASNTs and TiO2 coating) were fabricated by spin-coating method. A similar procedure was followed to prepare An/PDMS and Tn/PDMS, except that TiO2 was absent in the case of An/PDMS and ASNT was absent in the case of Tn/PDMS. The schematic illustration of nanomembrane preparation procedure is shown in Figure 1a. The detailed experimental condition and procedure are described in the Supporting B

DOI: 10.1021/acsapm.8b00104 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials

referred to as glass tube) and affixed by a Kapton tape (Figures 2b and S3a). Distilled water was continuously loaded dropwise onto the nanomembrane surface inside the tube; the corresponding membrane deflection was recorded by video camera (Movie S2), which was set perpendicular to the deflection direction. The nanomembrane deflection and curve lengths (Figure 2c) were determined by processing the captured images in ImageJ software. To avoid error in the measurement of deflection height, the stiffness of the tape was confirmed by a control test, in which no sign of deflection was observed (see the Supporting Information). Nanomembranes responded differently to the applied equal hydraulic pressure of 312.1 Pa, which is equivalent to 2.5 g of load (Figure 3a). We prepared a series of nanomembranes of comparable thickness (within 85−95 nm) to study the contribution of each membrane component for the mechanical property change in the overall composite nanomembrane; Pristine PDMS, T10/PDMS, A10/PDMS, (A-T)5/PDMS, and (A-T)10/PDMS. It is assumed that a small difference in thickness has an insignificant effect on the mechanical property of the composite nanomembrane. The pristine PDMS membrane demonstrated the highest deflection height, followed by titania-scaffolded PDMS with only a small decrease in height (Figure 3a1 and a2). An apparently lower deflection was observed in ASNT-scaffolded PDMS (Figure 3a3) compared with that of TiO2-supported and pristine PDMS nanomembranes. This is due to the presence of nanofiber networks in A10/PDMS. Even if they are not cross-linked by TiO2, the hydrogen bonding-reinforced ASNT interlaced structure in A10/PDMS improved its stiffness. When the ASNTs were cross-linked by TiO2 via sol−gel reaction, as in (A-T)5/PDMS and (A-T)10/PDMS, the mechanical property of the nanofiber scaffold layer was significantly enhanced, which ultimately reinforced the composite nanomembrane. This phenomenon was reflected by the corresponding minimal deflection of the nanomembranes (Figure 3a4,a5,b). The stress−strain curves showed linear relationship (Figure 3c). The maximum percent strain was 83.6% for PDMS, while the lowest was 18.2% for (A-T)10/PDMS, signifying the contribution of cross-linked ASNTs on the dramatic increase in membrane stiffness. However, an 18.2% elongation in the case of (A-T)10/PDMS indicated that the composite nanomembrane was flexible. The maximum membrane deflections before membrane breaking (Figure 3b) were also consistent with strain values. To further characterize the mechanical robustness of the ASNT-scaffolded PDMS nanomembrane, the ultimate tensile strength and biaxial modulus were calculated, evaluated, and presented graphically in Figure 4a,b. The ultimate tensile strength indicates the highest stress at the rupture point. When comparing the tensile strength of nanoscale and bulk PDMS, the 90 nm thick pristine PDMS (calculated as 8.9 MPa) was slightly higher than that of bulk PDMS (7.1 MPa) reported elsewhere.24 The difference in mechanical strength could be due to higher rearrangement of the polymer chains during stretching in the nanoscale PDMS film, which is less probable in the bulk PDMS (thickness of >200 μm). However, the ASNT-scaffolded nanomembrane, (A-T)10/ PDMS, experienced a high tensile strength of 34.5 MPa, which is approximately 4 times higher than that of pristine PDMS nanomembrane of comparable thickness. Similarly, the biaxial modulus of (A-T)10/PDMS was 172 MPa, much larger than

presence of TiO2 suppressed bundling of the ASNTs during spin-coating. The thickness of (A-T)10 was observed to be only about 14 nm on the sacrificial layer (Figure 1d). Finally, a 1.3 wt % hexane solution of PDMS was spincoated at a speed of 4000 rpm for 180 s. It was then crosslinked at 80 °C for 12 h. Upon PDMS coating, the film surface appeared smooth (Figure 1c). As can be seen in Figure 1e, the boundary of the PDMS and (A-T)10 layers was hardly distinguished. PDMS was embedded into the nanofiber layer and filled the space between the interlaced ASNTs, resulting in a fused structure. The penetration of PDMS into the ASNT network structure was further confirmed by the detection of siloxane form of Si 2p peak in the vicinity of the ASNT network side under X-ray photoelectron spectroscopy (see the Supporting Information). Mechanical robustness of the prepared FS-NMs was, in part, demonstrated by simple lift off process from the membrane detachment solvent. Once detached by dissolving the sacrificial layer in ethanol, the composite nanomembranes with Kapton frame could be lifted off by simply holding them with tweezers (Figure 1f and Movie S1) with an almost-zero failure rate. A large-size (as high as 40 mm diameter) and ultrathin (as low as 65 nm) nanomembrane could be picked up and manipulated in air without any damage, revealing excellent macroscopic mechanical stability. In contrast, the lifting off of a pristine PDMS nanomembrane with comparable thickness was difficult and has a high failure rate (∼50%). A similar observation was made by Khang et al.,23 claiming less than 50% success rate in the transfer of PDMS nanomembranes thinner than 100 nm. The enhanced mechanical stability in the manipulation of the (A-T)10/PDMS nanomembrane is ascribed to the presence of cross-linked ASNT scaffolding. The composite nanomembranes were also transparent (Figure 1g) and smooth. In addition to physical manipulation, quantitative evaluation of mechanical properties was conducted by hydraulic bulging test depicted by the apparatus in Figure 2a. FS-NMs were suspended onto the bottom of the cylindrical funnel (hereafter

Figure 2. (a) Schematic diagram of the hydraulic bulging test apparatus. (b) Digital image of the nanomembrane assembled to the base of a cylindrical funnel by Kapton tape for bulging test (light gray area between the dashed lines indicates membrane exposed for loading, the orange circle indicates the membrane covered by Kapton tape, and the rest of the area refers membrane sandwiched between the glass and the tape). (c) Schematic description of the bulging process upon pressure exertion. C

DOI: 10.1021/acsapm.8b00104 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials

Figure 3. (a) Digital image showing deflection of different nanomembranes against an equal hydraulic pressure of 312.1 Pa. The blue semicircles under the images represent a clearer view of the bulges. (b) Maximum deflection at the corresponding rupture pressure. (c) Tensile stress−strain plots of nanomembranes and their linear fits.

Figure 4. (a) Ultimate tensile strength (UTS) of nanomembranes determined from the stress−strain curve. (b) The biaxial modulus of nanomembranes. (c) Schematic representation of vertical structure of (A-T)10/PDMS nanomembrane depicting the TiO2-cross-linking of ASNTs and the PDMS top layer.

that the composite nanomembrane was quite flexible. In addition, when looking closely the interfacial structure of the nanofiber scaffold and PDMS, the polymer was embedded into the rough nanotube network structure (Figure 1b,e), as illustrated schematically in Figure 4c. The partial penetration prevents the delamination of PDMS and nanotube network and improves the polymer-scaffold interfacial binding,25 contributing to the mechanical reinforcement of the whole composite nanomembrane. Compared with the well-known epoxy−resin-based robust polymer nanomembranes,26 ASNT-

that of pristine PDMS (3.9 MPa). This dramatic improvement in tensile strength and biaxial modulus is indeed the result of the cross-linked nanofiber scaffolds (Figure 4c). The high aspect ratio of ASNTs and OH-functionalized surfaces20 create high interfacial area for TiO2-cross-linking and H-bonding interactions among ASNTs themselves. These interactions strengthen the network of ASNTs. Consequently, wellentangled and cross-linked network of ASNTs reinforced the overall nanomembrane. Again, the nanofiber scaffold was not too rigid due to the inherently flexible nature of the ASNTs so D

DOI: 10.1021/acsapm.8b00104 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials scaffolded PDMS nanomembranes demonstrated higher mechanical robustness with more than double tensile strength and flexibility about 2 orders of magnitude higher than the epoxy nanomembranes. Because one of the challenges in the development of FSNMs is difficulty in avoiding pinhole formation, we have conducted gas separation experiments to confirm the absence of defects in our nanomembranes. The detailed gas permeation measurement procedure was described elsewhere.10 A 50:50 mixture of CO2 and N2 was used as a feed gas. The (A-T)10/ PDMS nanomembrane could hold a feed pressure of up to 0.2 MPa. The CO2/N2 selectivity was about 7, and the permeance values of CO2 and N2 were 109 and 16 GPU, respectively (1 GPU: 7.5 × 10−12 m3/m2·s·Pa at standard temperature and pressure). The pressure holding capacity and selective gas separation of CO2 over N2 implied that the ASNT-scaffolded composite nanomembrane does not have serious defects that leads to simple gas leaks. The unexpected low permeance compared to pure PDMS may be due to the presence of amorphous TiO2 around ASNT networks. The key factor behind our tough, flexible, and free-standing nanomembrane is the incorporation of a very thin (∼15 nm) ASNTs backbone structure (Figure 4c). The thickness contribution of the ASNT scaffold layer is only about 15% of the whole composite nanomembrane. However, this ultrathin scaffold layer brought about a dramatic effect on the mechanical property of the PDMS nanofilm. In our membrane design, the structural function of the aluminosilicate nanotube scaffold resembles that of the cytoskeleton in biological membranes.27 The cytoskeleton is the “frame” of the cell, keeping structures in place, providing support, and giving the cell a definite shape. Like the role of TiO2 to crosslink ASNTs, the cytoskeletal microtubules and filaments have cross-linkers to control the structural organization of their networks. When shear stresses are applied to the cell, such cytoskeletal networks stiffen and resist additional deformation. They are also elastic enough to undergo softening after compressive stress. Similarly, the ASNT networks maintain stiffness during stress while experiencing stretching upon a progressive increase in mechanical load. The elongation of the ASNT scaffolds together with the polymer networks could be due to the disassembling of the ASNT network structure, which allows the rearranging and alignment of the fiber structures in the direction of tensile stretching. In summary, mechanically strong, flexible, free-standing and sub-100 nm thick nanofiber-scaffolded polymer nanomembrane has been developed. The incorporation of cytoskeletoninspired cross-linked ASNT networks presented remarkable enhancement in the tensile strength and biaxial modulus of the polymer nanomembrane while maintaining flexibility in the composite nanomembrane. Such a biomimetic strategy can be extended to the development of other mechanically strong FSNMs of diverse polymer materials, including polymers of lessfilm-forming nature, for a broad spectrum of applications such as sensing devices, energy conversion and storage, wound dressing, and cellular organization.

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Additional experimental details; figures showing TEM micrographs, the ratio of Al to Si in the ASNT, XPS analysis, a schematic illustration of the membrane assembly, the cylindrical funnel, and a schematic description of the mechanical property determination parameters; and a table showing a comparison of Al-toSi ratios (PDF) A movie showing the lift-off process of the free-standing (A-T)10/PDMS nanomembrane from the detachment solvent (AVI) A movie showing the deflection of the (A-T)10/PDMS nanomembrane against applied pressure (MPG)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Anteneh Mersha: 0000-0002-3042-5646 Shigenori Fujikawa: 0000-0001-7902-0191 Author Contributions

Both authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Japanese Government, Ministry of Education, Culture, Sports, Science, and Technology (MEXT) Scholarship Program and the World Premier International Research Center Initiative (WPI), MEXT, Japan. We gratefully acknowledge the financial support from a Grant-in-Aid for Scientific Research on Innovative Area “Coordination Asymmetry” (grant no. 16H06513).We also thank Prof. Atsushi Takahara and Prof. Ken Kojio for guidance and discussion on composite mechanical models as well as for suggestion to XPS measurement.

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ASSOCIATED CONTENT

S Supporting Information *

Movie S1, Movie S2, The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsapm.8b00104. E

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DOI: 10.1021/acsapm.8b00104 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX