Superhydrophobic and Slippery Lubricant-Infused Flexible

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Superhydrophobic and Slippery Lubricant-Infused Flexible Transparent Nanocellulose Films by Photo-Induced Thiol-Ene Functionalization Jiaqi Guo, Wenwen Fang, Alexander Welle, Wenqian Feng, Ilari Filpponen, Orlando J. Rojas, and Pavel A. Levkin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11741 • Publication Date (Web): 16 Nov 2016 Downloaded from http://pubs.acs.org on November 17, 2016

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ACS Applied Materials & Interfaces

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Superhydrophobic and Slippery Lubricant-

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Infused Flexible Transparent Nanocellulose

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Films by Photo-Induced Thiol-Ene

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Functionalization

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Jiaqi Guo 1, Wenwen Fang 2, Alexander Welle 3, Wenqian Feng 4, Ilari Filpponen 1, Orlando J.

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Rojas 1,5, Pavel Levkin 4,6*

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University, FI-00076, Espoo, Finland

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Department of Materials Science and Engineering, Aalto University, FI-00076, Espoo, Finland

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Institute of Functional Interfaces, Karlsruhe Institute of Technology and Karlsruhe Nano Micro

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Facility, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany

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4

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Eggenstein-Leopoldshafen, Germany

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5

Department of Applied Physics, Aalto University, FI-00076, Espoo, Finland

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Institute of Organic Chemistry, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany

Bio-based Colloids and Materials (BiCMat), Department of Bioproducts and Biosystems, Aalto

Institute of Toxicology and Genetics, Karlsruhe Institute of Technology (KIT), 76344

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1 ACS Paragon Plus Environment


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ABSTRACT

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Films comprising nanofibrillated cellulose (NFC) are suitable substrates for flexible devices

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in analytical, sensor, diagnostic and display technologies. However, some major challenges in such

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developments include the high moisture sensitivity of NFC and the complexity of current methods

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available for functionalization and patterning. In this work, we present a facile process for tailoring

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the surface wettability and functionality of NFC films by a fast and versatile approach. Firstly, the

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NFC films were coated with a layer of reactive nanoporous silicone nanofilament by

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polycondensation of trichlorovinylsilane (TCVS). The TCVS afforded reactive vinyl groups,

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thereby enabling simple UV-induced functionalization of NFC films with various thiol-containing

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molecules via the photo “click” thiol-ene reaction. Modification with perfluoroalkyl thiols resulted

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in robust superhydrophobic surfaces, which could then be further transformed into transparent

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slippery lubricant-infused NFC films that displayed repellency against both aqueous and organic

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liquids with surface tensions as low as 18 mN·m-1. Finally, transparent and flexible NFC films

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incorporated hydrophilic micropatterns by modification with OH, NH2 or COOH surface groups,

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enabling space-resolved superhydrophobic-hydrophilic domains. Flexibility, transparency,

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patternability and perfect superhydrophobicity of the produced nanocellulose substrates warrants

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their application in biosensing, display protection, biomedical and diagnostics devices.

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KEYWORDS: photochemistry; thiol-ene reaction; nanocellulose; superhydrophobicity; slippery

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lubricant-infused porous surface; SLIPS; superhydrophobicity; surface patterning.

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INTRODUCTION

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Numerous efforts have been devoted to replace glass and plastic films with sustainable

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materials with competitive strength, coefficient of thermal expansion and heat-resistance.1-3 Films

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and nanopapers prepared from nanofibrillated cellulose (NFC, also named as CNF) are ideal

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alternatives owing to the fact that they are mechanically strong (up to 350 MPa strength to rupture),

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light-weighted (density of ~1.6 kg·m-3) and transparent (above 90% optical transmission); also,

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they exhibit a low coefficient of thermal expansion (CTE < 8.5 ppm·K-1), are non-toxic and

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biocompatible.4-8 Taken the above advantages into account, NFC films have been considered in

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recent years for utilization in electronic devices including displays, transistors, organic light-

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emitting diodes (OLEDs), touchscreens, lithium ion batteries, solar cells, transparent conductive

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electrodes and antennas.9-14 More practical applications have taken advantage of the inherent

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properties of NFC, which can be developed upon modification for uses as gas barriers, fire

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retardant, shape-memory materials, optical systems, oil/water stabilizers, sensors, etc.15-20

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However, owing to the high density of hydroxyl and oxygen-containing groups, NFC are highly

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sensitive to moisture.21 To a large extent, this inherent characteristic related to water sorption

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affects the properties of NFC-based materials (e.g. mechanical and barrier properties)22,23 and

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further limits their applications.24,25 Thus, there is a strong need of methods for tailoring chemical

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properties and enabling patterning of NFC films to generate additional functionality and improve

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their applicability.

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Introducing hydrophobic moieties to tailor cellulose wettability has been reported in the

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literature via non-covalent adsorption through physical/chemical processing or covalent

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attachment of hydrophobic moieties.26,27 Coating with cationic surfactants, mixing with lignin and

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other procedures have been attempted.28,29 Chemical modification by esterification, amidation and



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polymer grafting have been proposed.30-33 Sehaqui et al. reported on the grafting of alkyl chains

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on NFC film. 30 Shimizu et al. published a procedure to attach quaternary alkylammoniums (QAs)

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that resulted in the nanocellulose with hydrophobic properties.28 Given their rather smooth surface,

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the maximum water contact angle (WCA) of NFC films has been limited to 130° while displaying

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water droplet adhesion and high WCA hysteresis.28,30 Moreover, to the best of our knowledge,

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there are no simple methods available for the fabrication of functional (hydrophobic-hydrophilic)

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patterns on nanocellulose films.

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Superhydrophobic surfaces (WCA > 150° and WCA hysteresis < 10°) possess extreme water

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repellency, self-cleaning and various other unique properties that are useful in many applications.34

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Numerous techniques have been reported to create superhydrophobic surfaces.35 Among them, a

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promising route is the growth of polydimethylsiloxane nanofilaments, as was first reported in 2004

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by Jung et al.36 and Zimmermann et al.,37,38 the nature of which was later described in detail by

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Korhonen, Ras and coworkers.39 This simple approach has shown excellent superoleophobic

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properties as well as resilience toward chemical and mechanical abrasion.40 More recently, we

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expanded this approach by developing UV-reactive silicone nanofilaments on glass substrate,

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thereby enabling formation of functional superhydrophobic patterns.41

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Here we report a novel and facile process to functionalize, pattern and tailor the wetting

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properties of NFC films. First, transparent and reactive porous TCVS silicone nanofilaments

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(TCVS-SNFs) were installed onto the surface of the NFC films by in-situ polycondensation of

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trichlorovinylsilane, which introduced both nanoscale roughness and reactive vinyl groups that

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can be functionalized via photo-click thiol-ene reaction.42,43 Thereafter, superhydrophobicity was

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developed by further modifying the NFC film with either alkyl or perfluoroalkyl thiols. In addition,


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slippery lubricant-infused NFC films (termed here as SLI-NFC), displaying perfect repellency

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against both aqueous and organic liquids with low surface tension, was demonstrated (Figure 1).

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Figure 1. Schematic illustration of the functionalization of films of nanofibrillated cellulose (NFC). The

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NFC film surface is used as a substrate for growing silicone nanofilaments (SNFs) carrying reactive vinyl

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groups (TCVS-SNFs) (i). The vinyl groups can be reacted with thiolated compounds via photo-induced

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thiol-ene click reaction (ii and iii). For instance, they can carry hydrophilic or hydrophobic groups to endow

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superhydrophobicity (ii) or superhydrophilicity (iii), respectively. The fabrication of a slippery, liquid-

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infused porous NFC film surfaces (SLI-NFC) can be achieved by impregnation of porous NFC films that

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have been fluorinated by the thiol-ene click reaction with a low-surface-energy, chemically inert lubricant

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(iv).

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EXPERIMENTAL SECTION

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Materials. NFC films (A4 size) were provided by VTT Technical Research Center of

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Finland.

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hydrochloride and 1H, 1H, 2H, 2H-perfluorodecanethiol were purchased from Sigma-Aldrich,

Trichlorovinylsilane

(TCVS),

1-butanthiol,

2-mercaptoethanol,

cysteamine



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Germany. Dichloromethane, n-hexane (HEX), ethyl acetate, dimethyl sulfoxide (DMSO), and

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glycerol (GLY) were obtained from Merck, Germany. 4 Å molecular sieves and tetrahydrofuran

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were purchased from Alfa Aesar, Germany. Toluene (99.5%, extra dry over 4 Å molecular sieves)

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and dimethylformamide (DMF) were provided by VWR, Germany. Krytox 103 was purchased

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from DuPond, Germany. Anhydrous ethanol and methylene blue hydrate were purchased from

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Fluka, Germany. Food dye was obtained from August Teomasen Corp. (USA).

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Growth of functional TCVS silicone nanofilaments (TCVS-SNFs) on NFC film. The

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presence of traces of water in toluene was found critical for the success of the reaction. In order to

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study the effect of water concentration, TCVS content and reaction time, certain volume (3.6 µL,

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5.0 µL, 7.2 µL) of Millipore water (18.2 MΩ·cm) was added to 40 mL of dry toluene in a Falcon

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tube. In order to fully disperse water in toluene, the mixture was sonicated at 50 °C for 40 min.

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One piece of dried NFC film (1.5*1.5 cm) was immediately immersed into the toluene-water

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mixture. Certain volume of TCVS (25 µL, 50 µL) was then added. After certain reaction time (6

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or 24 h), the NFC film was washed thoroughly with ethanol and then dried under nitrogen flow.

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Prior to immersion into toluene-water solution, the NFC film was dried under vacuum for 2 h. The

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NFC film was placed horizontally in the Falcon tube.

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Thiol-ene modification of NFC surface. The silane-modified NFC film was positioned on

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a glass slide and then wetted with ca. 100 μL of hydrophobic thiol compounds: 20% (v/v) 1H, 1H,

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2H, 2H-perfluorodecanethiol solution in ethyl acetate or 20% (v/v) 1-butanthiol in ethanol. In order

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to improve the reaction efficiency, the sample was covered with a quartz slide (Science Services

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GmbH, München, Germany). The covered NFC film was immediately irradiated with UV light

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(OAI model 30 deep-UV collimated light source, San Jose, CA, USA) equipped with a 500 W

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HgXe lamp (260 nm, ~9 mW·cm-2) (Ushio GmbH, Steinhöring, Germany) for 5 min. After


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irradiation, the NFC film was thoroughly washed with acetone or ethanol, depending on the thiol

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compound used. Superhydrophilic NFC films were achieved by photo-induced thiol-ene reaction

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with cysteamine or 2-mercaptoethanol.

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Preparation of slippery, lubricant-infused NFC film (SLI-NFC). NFC films carrying

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TCVS-SNFs were reacted with 1H, 1H, 2H, 2H-perfluorodecanethiol solution (20% (v/v) 1H, 1H,

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2H, 2H-perfluorodecanethiol solution in ethyl acetate) under UV light (260 nm, ~9 mW·cm-2) for

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5 min. After washing with acetone and drying under nitrogen flow, the superhydrophobized NFC

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film was impregnated by spreading 20 L·cm-2 lubricant fluid (Krytox 103) with a pipette and

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kept for 2 h to ensure spontaneous, complete wicking throughout the capillary structure of the

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films. Afterwards, the NFC film was positioned vertically overnight to remove by gravity any

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excess of lubricant.

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Preparation of superhydrophobic-superhydrophilic patterns via thiol-ene reaction.

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NFC films modified with the TCVS-SNFs were positioned on a glass slide that was wetted with

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20 % of 1H, 1H, 2H, 2H-perfluorodecanethiol (in ethyl acetate) and covered with a photomask.

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After exposure for 1 minute under UV light (260 nm, ~9 mW·cm-2), the NFC film was rinsed

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immediately with acetone in a dark room and dried under nitrogen flow. In order to prepare

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superhydrophilic areas, the NFC film was re-positioned on the glass slide and covered with a quartz

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slide after addition of 100 µL of cysteamine hydrochloride or 2-mercaptoethanol solution (20% in

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ethanol). After 1 min exposure to UV light, the NFC film was completely rinsed with ethanol and

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dried with a nitrogen flow. The prepared NFC film carrying the hydrophobic-hydrophilic patterns

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was stored until further use.

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Characterization. Surface morphology of TCVS-SNFs grown on the NFC films was

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assessed by using Scanning Electron Microscopy (SEM) (Zeiss Sigma VP). Prior to imaging, the



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samples were sputtered with a ca. 7 nm Au/Pt layer (Emitech K100X). Transmittance of the

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samples in 300~1000 nm was recorded by using a UV-Vis spectrophotometer (Biotek Epoch 2,

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Germany). The static and dynamic water contact angles (advancing and receding water contact

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angles) were measured with a custom-built contact angle system equipped with a UK 1115 digital

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camera. The data were analyzed with Image J software. The static contact angle value and standard

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deviations were obtained from the average of at least three measurements for each sample. Bending

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and abrasion tests under load were carried out in order to investigate the mechanical robustness of

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the NFC films carrying the TCVS-SNFs. Bending was performed for 100 cycles at a bending angle

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of 45° while abrasion tests were conducted by cyclic displacement of 10 g or 100 g loads on the

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surface. The water contact angles were measured before and after the experiments. ToF-SIMS

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measurements were performed with a ToF.SIMS5 instrument (ION-TOF GmbH, Münster,

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Germany) equipped with a Bi cluster primary ion source and a reflectron type analyzer. All the

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ToF-SIMS spectra were normalized by total counts.

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RESULTS AND DISCUSSION

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Reactive NFC films. The NFC films used in this study were prepared in a pilot modular unit

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that uses NFC hydrogels and the solvent casting technique. The NFC films had a thickness of ca.

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20 μm and were flexible, translucent, strong and easy-to-handle (Figure 2a). SEM images in Figure

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2b indicate a smooth NFC film surface that comprised cellulose nanofibers with a width of 5 - 20

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nm and only nanometer scaled roughness (ca. 100 nm).44 Compared with traditional paper, the

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transparency of the NFC films is very high (Figure 3c), with over 60% light transmittance at

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wavelengths above 500 nm. Given that the both surface morphology (micro/nano structure) and

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chemical composition (surface energy) play key roles in controlling surface wettability, we


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modified both characteristics of the NFC films.45 In NFC films, with their characteristic smooth

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surfaces, the development of hydrophobicity is limited to water contact angles (WCA) of < 120°.

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Hence, here we took advantage of the growth of reactive silicone nanofilaments, as reported by Li

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et al.41 The active TCVS-SNFs micro/nanostructures were grown on the surface of the NFC films

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(1.5 cm * 1.5 cm) by simple immersion in the TCVS solution (containing toluene and water) of a

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given composition. TCVS condensing with water undergoes hydrolysis and polycondensation to

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form a cross-linked porous polymeric silicone nanofilament layer on the NFC film (vertical

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polymerization) (Figure S1). For instance, SEM images of the NFC films reveal full coverage with

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TCVS-silicone nanofilaments (TCVS-SNFs) after immersion in 40 mL toluene containing 625

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ppm TCVS and 125 ppm water (Figure 2c). As a result of the polycondensation process, TCVS

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formed a porous TCVS-SNFs bearing photo-reactive vinyl groups. The full coverage and the

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introduction of vinyl functional groups were further confirmed by time-of-flight secondary ion

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mass spectroscopy (ToF-SIMs) (Figure 2d and Figure S2). The structure and properties of the

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TCVS-SNFs coating layer is affected by several parameters, for instance reaction time and the

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composition of the precursor TCVS solution (TCVS, solvent and water).

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Figure 2. (a) Photographs of a printed A4 paper sheet covered with a NFC film to illustrate its transparency

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and flexibility (inset). The scale bar corresponds to 20 mm, (b) SEM image of the surface of a NFC film to

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reveal the surface topography. Scale bars correspond to 2 µm and 200 nm, respectively (inset). (c) SEM

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image revealing the surface topography of a NFC film after growing TCVS-SNFs on the NFC surface, such

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as that shown in (b). Scale bars correspond to 2 µm and 200 nm (inset). (d) ToF-SIMS of chemical 2D

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scans of the functionalized NFC film in (c) showing the distribution of C2H3+ fragments.

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Superhydrophobic/hydrophilic NFC films via thiol-ene click reaction. Figure 3a includes

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the values of the water contact angle (WCA) of NFC films after modification. The initial static

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WCA (θst) of NFC film was 33° owing to the hydrophilic characteristics of cellulose which

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comprises hydroxyl and oxygen-containing groups. TCVS-SNFs grown on the surface of the NFC

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films produced a micro/nanostructured layer that resulted in an increased θst of 123° (Table S1).

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In order to further investigate the possibility for the NFC films coated with TCVS-SNFs to

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develop superhydrophobicity or superhydrophilicity, the prepared hydrophobic and reactive NFC 10 ACS Paragon Plus Environment

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surface was modified with various alkylthiols via the photo-induced thiol-ene reaction.

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Functionalization of the reactive TCVS-NFC film with hydrophobic 1-butanthiol (named therein

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“alkylated NFC” surface) or with 1H,1H,2H,2H-perfluorodecanethiol (named thereafter as

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“fluorinated NFC” surface) endowed the surface with superhydrophobicity (see supporting video

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V1). The θst for the alkylated and the fluorinated NFC were 166° and 167°, respectively. The WCA

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hysteresis for both surfaces were < 3°. We note that after installation of the fluorinated compound,

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the θst of both sides of the NFC film was > 155° (and WCA hysteresis < 3°). The “fluorinated NFC”

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surface showed pinning of droplets of solvents, including glycerol ( = 64.0 mN·m-1) or n-hexane

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( = 18.4 mN·m-1) even at 90o (Figure 4 a). Functionalization of the reactive TCVS-NFC film with

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hydrophilic cysteamine or 2-mercaptoethanol resulted in a decrease of θst from 123° to ca. 8° in

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both cases (Figure 3a).

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In order to verify the superhydrophobic character of the fluorinated NFC films, a water

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droplet was deposited on films that were positioned horizontally. As the film was moved in one

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direction at a constant velocity (0.4 cm·s-1), it was observed that the water droplet did not adhere

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on the surface, confirming a very low WCA hysteresis (Figure 3b and supporting video V2). It is

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well known that a hierarchical micro/nano scale roughness is required for producing

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superhydrophobic surfaces; however, this can cause a reduction in transparency due to increased

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scattering of light. Thus, it is difficult to create superhydrophobic surfaces that at the same time

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are transparent.41 Remarkably, functionalization of the NFC films with TCVS-SNFs did not

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significantly affect their transparency (Figure 3a and Figure S3). From the UV-Vis data, we found

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a reduction by only 130° (Figure S4).

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Omniphobic and slippery lubricant-infused hydrophobic NFC films (SLI-NFC). Liquid

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infused porous polymer surfaces (SLIPS) introduced by Aizenberg et al. have attracted attention

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in the scientific community due to their simple fabrication, highly repellent properties towards

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various liquids,46-48 as well as antiicing,49 antibiofouling,50-53 excellent eukaryotic cell54 and blood

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repellency.55 Omniphobic slippery membranes and films possessing the above mentioned

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properties could find numerous applications in biomedical, biotechnological and other industries.56

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Hydrophobic NFC films are uniquely suited for this application due to the transparency, good

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mechanical properties, availability and low costs of cellulose. To create a slippery lubricant-

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infused NFC membrane (SLI-NFC), we impregnated a fluorinated NFC membrane with

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perfluoropolypropylene oxide lubricant (Krytox 103). The Krytox 103 was infiltrated into the

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roughened nanostructure formed by TCVS-SNFs on the NFC film surface. Thus, we created

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surfaces with high roughness (TCVS-SNFs growth) and matched the surface chemistry

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(fluorination) to facilitate the spontaneous spreading of Krytox 103 in the porous NFC film. The

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produced SLI-NFC film showed excellent omniphobic liquid repellent properties (Figure 4 a) and

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enhanced optical transparency (Figure 3 c). Thus, the SLI-NFC films possessed high contact angles

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with very low contact angle hysteresis (α

Superhydrophobic and Slippery Lubricant-Infused Flexible

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