Self-Healing Superhydrophobic Materials Showing Quick Damage

Aug 24, 2017 - Because of the silicone micro/nanograss structures on the PDMS surfaces and the effective preserve/protection system of a large quantit...
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Self-healing Superhydrophobic Materials Showing Quick Damage Recovery and Long-term Durability Liming Wang, Chihiro Urata, Tomoya Sato, Matt W. England, and Atsushi Hozumi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02343 • Publication Date (Web): 24 Aug 2017 Downloaded from http://pubs.acs.org on August 26, 2017

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Self-healing Superhydrophobic Materials Showing Quick Damage Recovery and Longterm Durability Liming Wang, Chihiro Urata, Tomoya Sato, Matt W. England, Atsushi Hozumi*

National Institute of Advanced Industrial Science and Technology (AIST), 2266-98, Anagahora, Shimoshidami, Moriyama, Nagoya 463-8560, Japan *Email: [email protected]

KEYWORDS:

superhydrophobicity ・ self-healing ・ self-assembly ・ silicone

micro/nanograss・ poly(dimethylsiloxane) (PDMS)・ plasma exposure

ABSTRACT: Superhydrophobic coatings/materials are important for a wide variety of applications, but the majority of these man-made coatings/materials still suffer from poor durability because of their lack of a self-healing ability. Here we report novel superhydrophobic materials which can quickly self-heal from various severe types of damage. In this study we used poly(dimethylsiloxane) (PDMS) infused with two liquids: trichloropropylsilane, which reacts with ambient moisture to self-assemble into grass-like microfibers (named silicone micro/nanograss) on the surfaces, and low-viscosity silicone oil

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(SO), which remains within the PDMS matrices and acts as a self-healing agent. Due to the silicone

micro/nanograss

structures

on

the

PDMS

surfaces

and

the

effective

preserve/protection system of a large quantity of SO within the PDMS matrices, our superhydrophobic materials showed quick superhydrophobic recovery under ambient conditions (within 1-2 hours) even after exposure to plasma (24 hours), boiling water, chemicals, and outside environments. Such an ability is superior to the best self-healing superhydrophobic coatings/materials reported so far.

INTRODUCTION Superhydrophobic coatings/materials have attracted much attention due to their applicability in a wide variety of applications.1-5 To achieve superhydrophobicity, simultaneous control of hierarchical surface topography and surface chemistry are required.6-7 However, these man-made coatings/materials easily and permanently lose their liquid repellency once they are chemically or mechanically damaged. Their poor stability and lack of self-healing properties seriously limits their practical applications.8-11 To overcome these shortcomings, several approaches for preparing specific surface topologies that sustain their liquid repellency have been proposed, for example, introducing hierarchical rough structures using hydrophobic nanoparticles of two different size ranges, or covering mechanically stable hydrophobic microstructures with hydrophobic nanoparticles or nanofilaments, to reduce mechanical damage to the rough surface features.11-14 Chemical degradation of low-surfaceenergy materials on surfaces by sunlight irradiation, oxidation, acid/base-catalyzed decomposition, heat degradation, and hydrolysis in humid environments is also a serious and inevitable problem encountered during long-term usage.15 In contrast, lotus leaves can keep their surfaces clean, and self-heal damage by the continuous secretion of epicuticular wax.16

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To mimic these smart self-healing systems, appropriate preservation of self-healing agents is required to achieve surface recovery and extend durability.17 Based on the status of the preserved self-healing agents, current strategies for self-healing can be classified into two categories: chemically preserved agents9,18-28 and physically preserved liquid-state agents.29-32 In the former case, self-healing agents (mainly perfluorinated compounds with reactive functionalities) infused into the porous materials through a vapor or liquid phase are chemically embedded on the surfaces, while in the later case, self-healing agents (mainly unreactive low-surface-tension fluids) infused into the porous structures form continuous liquid films through the good affinity between them. Although these strategies are promising, they are inadequate for long-term usage due to their inherent limitations. Their disadvantages are summarized as follows: first, there is a limited loading capacity for chemically preserved self-healing agents. Second, their mobility is relatively low, and thus some appropriate triggers like heat,18-20 high humidity,9, 21-24 or UV light25 are required to accelerate the recovery kinetics. Third, while physically infused selfhealing agents have higher mobility and larger loading capabilities, such as slippery liquidinfused porous surfaces (SLIPSTM),29-32 current strategies do not consider the protection for the healing agents, which results in their degradation, wasteful draining and evaporation, and eventually the loss of long-term durability.29-32 In contrast to textured superhydrophobic surfaces focusing on the increase in water contact angles (CAs), liquid-infused surfaces like SLIPSTM are flat and exhibit excellent dynamic dewettability, with small volume of various probe liquids sliding off the surface at low tilt angles, regardless of the magnitude of CAs. We have recently developed poly(dimethylsiloxane) (PDMS)-based, self-healing superhydrophobic materials, which can regenerate superhydrophobicity from mechanical damages.33 In this case, octadecyltrichlorosilane (ODS, hydrophobic self-healing agent) was infused with isocetane within the PDMS matrices. Due to the syneresis of ODS to the outer

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surface of the sample, it reacted with the moisture in the air to form superhydrophobic surfaces (rough surfaces without any specific surface features). Although, thanks to the continuous syneresis of ODS, regeneration of superhydrophobicity through the reconstruction of surface microstructures could be achieved even after mechanical damage, the self-healing ability of our sample gradually worsened, and was lost within a few days because ODS reacted very quickly with the moisture. Thus, the effective protection of self-healing agents from evaporation and other exogenous factors is important to realizing long-term durability in man-made superhydrophobic coatings/materials. It is well known that trichlorosilanes terminated with short functional groups, such as trichloromethylsilane (TCMS)34-36 and trichlorovinylsilane,37 are easily hydrolyzed in the presence of water and self-assemble into a large number of silicone nanofilaments, through a vapor phase or solution phase reaction. The resulting nanofilament surfaces exhibited excellent superhydrophobicity or superamphiphobicity when coated with perfluoroalkylsilane, which are featured with small difference between liquid advancing CA (θA) and receding CA (θR), i.e., CA hysteresis (CAH, ∆θ = θA - θR), on the surfaces. Droplets move very easily without pinning on these surfaces, and roll off of at low tilt angles.34-38 However, these nanofilaments possess poor mechanical properties (easily collapsed by touching) and weak adhesion to the substrates despite their positive attributes. In our preliminary study, we noticed that unreactive low-surface tension liquids, such as low-viscosity silicone oil (referred to as SO), are ideal self-healing agents, because they can be massively pre-loaded in the PDMS precursors and rapidly migrate within the matrices, particularly to damaged areas. In addition, trichloropropylsilane (TCPS), which is also soluble in the PDMS precursors, was found to self-assemble into unique micro/nanofibers. The simple combination of these two chemicals, which effectively work to control surface chemistry and geometry, respectively, was found to be effective in creating novel PDMS-

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based superhydrophobic materials that exhibit both quick recovery kinetics and exceptionally durable self-healing abilities against prolonged plasma irradiation, as well as possessing reasonable mechanical strength compared to the conventional trichlorosilane-based nanofilaments reported so far. To date, the durability and self-healing abilities of reported coatings/materials have generally been tested by briefly exposing them to plasma, from tens of seconds23, 28 to a maximum of several minutes.9,18-22,26 In spite of such short exposure times, the surfaces were seriously damaged and required of recovery times ranging from hours9,19-22 to days.18,23,26,28 To the best of our knowledge, there have been no reports of self-healing superhydrophobic coatings/materials that are able to survive and quickly recover under ambient conditions from severe damage caused by prolonged plasma irradiation (e.g., 24 hours).

RESULTS AND DISCUSSION In this study, we infused two types of liquid, i.e., TCPS and low-viscosity SO in PDMS matrices. A new type of micro/nanometer-scale polysilsesquioxane fibers, with conical grasslike features (named “silicone micro/nanograss”), was formed by self-assembly from SO/TCPS-infused PDMS (referred to as PDMS-SO/TCPS, Figure 1A). Grass-like fibers have been previously prepared on silicon wafers, i.e., silicon grass, through complex cyclic deep reactive ion etching (c-DRIE) (a top-down approach),39,40 but by contrast, we have used a bottom-up (i.e., self-assembly) approach here to fabricate silicone micro/nanograss. In this present case, TCPS works as a “nutrient” for silicone micro/nanograss growth. TCPS gradually leaches out from the “ground” (i.e., PDMS-SO/TCPS) by syneresis and reacts with moisture in the air, through a combination of hydrolysis and polycondensation reactions, while the SO works as a self-healing agent which remains inside PDMS matrices. As shown in Figure 1B, the silicone micro/nanograss has the average diameter and length of ~300 nm‒

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1.0 µm and ~10‒30 µm, respectively. Like real grass, the silicone micro/nanograss branches near its root, and grows separately in different directions. Moreover, judging from TEM image of Figure 1C, the surfaces of the micro/nanograss are not smooth, but rough with many spherical protrusions.

Figure 1. (A) Schematic illustration of the formation of “silicone micro/nanograss” on a PDMS-SO/TCPS surface. (B) SEM image showing the silicone micro/nanograss, grown on the PDMS-SO/TCPS surface. Silicone micro/nanograss has conical features which taper from the bottom to the top. (C) TEM image of the silicone micro/nanograss. Scale bar in the inset, 200 nm.

The surface structures were largely dependent on the ratios of TCPS, SO, and PDMS. When the ratio of TCPS/PDMS was maintained at 20 vol%, and the ratio of SO/PDMS changed from 50 to 100 vol%, all surfaces showed superhydrophobicity. The surface with negligible CAH (θA/θR = 163.0°/162.2°, ∆θ = 0.8°) was obtained from a mixture with TCPS:SO:PDMS at 20:70:100 vol% (referred to as Sample A, See Figure S1A in the Supporting Information). Typical SEM images of Sample A are shown in Figure 2A, B. We found brain-like topographical features with densely grown silicone micro/nanograss, as well as irregular microvoids (tens of micrometers in width and hundreds of micrometers in length), which were loosely dispersed on the surface. When the ratio of TCPS/PDMS was increased to 30 vol%, and the ratios of SO/PDMS were changed from 80 to 200 vol%, the surface with

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the minimum CAH (∆θ 99.0%), toluene (>99.0%), HCl (1.0 M), sulfuric acid (96.0 wt%), and NaOH (1.0 M) were purchased from Wako Pure Chemical Industries Ltd. Deionized water with a resistivity of 18.2 MΩ·cm (Milli-Q) was used for all rinsing processes and water contact angle (CA) measurements, unless otherwise stated.

Preparation of PDMS matrices PDMS base and curing agent were mixed in a weight ratio of 10:1. The mixtures were degassed for 1 hour in vacuum, and then cured at 100 °C for 2 hours in a Teflon® container (~3.5 cm in diameter).

Preparation of PDMS organogels infused with SO/TCPS PDMS precursors (PDMS base to curing agent ratio was fixed at 10:1 by weight) were mixed with SO at various ratios in Teflon® containers (the ratio between SO to PDMS (precursors) is denoted as SO/PDMS), and degassed for 1 hour in vacuum. The containers were then moved to a glove box filled with a dry N2 atmosphere (RH < 10 %). TCPS was added into PDMS-SO mixtures at different amounts in the glove box. The containers were then sealed, and the multicomponent liquids were thoroughly mixed on a Vortex mixer for 5 minutes each, then heated at 100 °C for different curing time (Sample A for 1 hour and 50 minutes, Sample B for 2 hours, Samples C & D for 3.5 hours, respectively). Preparation of silicone micro/nanograss on the samples

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After thermal curing of the PDMS-SO/TCPS, cured samples were removed from the containers, and then placed in a chamber with a K2CO3 saturated aqueous solution and a small stream of N2 gas for 8 hours (RH was in the range of 30 and 50 %). The transparent PDMS-SO/TCPS gradually turned opaque during this period. The samples were further aged in a vessel at ambient conditions for 2 days to remove evaporative TCPS residues before use.

Characterizations Water CAs were obtained using a CA goniometer (Drop Master DM-501, Kyowa Interface Science, Japan) under ambient conditions (temperature between 20 and 25 °C, and relative humidity (RH) between 35 and 70%). Water advancing (θA) and receding (θR) CAs were obtained while a water droplet was added to and withdrawn from the drop, respectively. Each reported value is the average from at least 5 independent measurements. Mechanical durability was characterized by using a home-made scratch tester with a 1500-mesh sandpapers (Trusco).9 Plasma irradaition was carried out in a Harrick oxygen plasma cleaner PDC-32G using air as a gas source at ~102 Pa and the maximum power (18 W was applied to the radio frequency coil). UV irradiation was also carried out in a vacuum chamber using a 172 nm vacuum UV (VUV) lamp (Ushio, UER-20-172VA). The light intensity of this VUV lamp is about 10 mW/cm2. Some of the samples were irradiated with VUV light for 1 to 24 hours at ~102 Pa. The dose was about 36-864 J/cm2. The optical images were taken with a Canon Powershot SD 4500 IS. Optical properties were characterized by UV-Vis-NIR spectrometer (Cary 5000 Spectrophotometer, Agilent Technologies Inc., USA). Confocal microscopy images of surface topography were obtained on a confocal laser microscope (OPTELICS® HYBRID, Lasertec Corporation, Japan). TEM images were obtained using a JEOL JEM-2010 microscope operating at 200 kV. SEM and EDX measurements were

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carried out by a Phenom Pro scanning electron microscope without a conductive coating on the samples.

SUPPORTING INFORAMATION The Supporting Information is available free of charge on the ACS Publications website at DOI: Figures S1-S11 (PDF) Self-healing ability of our sample after 172 nm VUV irradiation for 24 hours (movie S1, mp4), lost of supehydrophobicity of living lotus leave after 172 nm VUV irradiation for 30 seconds (movie S2, mp4), and durable superhydrophobicity of our sample (Sample A) after an outdoor exposure test for over one year (movie S3, mov)

ACKNOWLEDGMENT This work was partially supported by Advanced Research Program for Energy and Environmental Technologies (No. P14004) from New Energy and Industrial Technology Development Organization (NEDO), Japan, and by JSPS KAKENHI Grant Number JP24120005 in Scientific Research on Innovative Areas “Innovative Materials Engineering Based on Biological Diversity”.

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