Facile and scalable fabrication of flexible re-attachable ionomer

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Surfaces, Interfaces, and Applications

Facile and scalable fabrication of flexible re-attachable ionomer nanopatterns (RAINs) by continuous multidimensional nanoinscribing and low-temperature roll imprinting Dong Kyo Oh, Dang Thuan Nguyen, Seungjo Lee, Pyeongsam Ko, GiSeok Heo, Changhun Yun, Tae-Won Ha, Hongseok Youn, and Jong G. Ok ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21915 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 16, 2019

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ACS Applied Materials & Interfaces

Facile and Scalable Fabrication of Flexible ReAttachable Ionomer Nanopatterns (RAINs) by Continuous Multidimensional Nanoinscribing and Low-temperature Roll Imprinting Dong Kyo Oh,a‡ Dang Thuan Nguyen,b‡ Seungjo Lee,a‡ Pyeongsam Ko,b Gi-Seok Heo,c Chang-Hun Yun,c Tae-Won Ha,c Hongseok Youn,b* and Jong G. Oka*

aDepartment

of Mechanical and Automotive Engineering, Seoul National University

of Science and Technology, Seoul 01811, Korea.

bDepartment

of Mechanical Engineering, Hanbat National University, Daejeon

34158, Korea.

cCenter

for Nano-Photonics Convergence Technology, Korea Institute of Industrial

technology, Gwangju 61012, Korea.

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KEYWORDS

re-attachable; nanopattern; nanoinscribing; low-temperature roll imprinting; ionomer

ABSTRACT

We develop a facile route to the scalable fabrication of flexible re-attachable ionomer nanopatterns (RAINs) by continuous nanoinscribing and low-temperature roll imprinting, which are repeatedly attachable to and detachable from arbitrarily shaped surfaces. First by sequentially performing continuous nanoinscribing over a polymer substrate along the multiple directions, we readily create the multidimensional nanopattern which otherwise demands complex nanofabrication. After its transferring to an elastomer pad for using as a soft nanoimprinting stamp, we then conduct a low-temperature roll imprinting of the ionomer film to fabricate a flexible and highly transparent RAIN. Reversible loosening of ionic units in the ionomer material at the mild temperature as low as ~60-70 °C enables the faithful nanopatterning over thermosensitive organic compounds and fragile materials under a slight pressure. The excellent adhesion purely emerging from ionic interactions 2 ACS Paragon Plus Environment

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uniquely realizes the conformal attachability and clean detachability of RAINs for universal targets at ambient conditions, particularly beneficial for individual wearable and mobile devices requiring the user-specific ‘on/off’ of the nanopattern-driven functionalities. As one vivid example, we demonstrate that a single light-emitting device can be switched from the focused pointer to the widespread flashlight depending on the RAIN application upon the user’s purpose.

1. INTRODUCTION

Nanoimprint lithography (NIL)1, 2 enables a faithful and practical nanostructure fabrication through the mechanical molding of a desired surface by thermal deformation (thermal NIL) and/or curing of a UV-curable resin coating (UV NIL). UV NIL is typically faster and more suitable to continuous and scalable processing such as roll-to-roll (R2R) NIL.3-5 It however demands a careful choice of the UV-curable resin6 and preparation of its coating7 to secure good substrate adhesion, demolding, and functionality requirements such as transmittance and refractive index, for many applications. Thermal NIL,8, 9 on the other hand, can imprint nanostructures directly onto the target substrate having desired properties and functionalities without the 3 ACS Paragon Plus Environment


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use of interfacial resins, thus can simplify the layer interfaces within the device structure. This would be particularly beneficial for most photonic elements.6, 10-12

Multidimensional nanopatterns (MDNPs) beyond simple one-dimensional (1D) nanogratings have extensively been capitalized in many photonic and sensing applications. MDNPs are possibly more advantageous for many optical components and sensors as they are much less dependent from incident angle of light and possess larger surface area compared to 1D nanopatterns.13-16 However, it must be often challenging to fabricate MDNPs by thermal NIL when the target surface and/or its underlying layers are vulnerable to pressurized heating that should be maintained for a relatively long time. This might be critical, especially for instance in the devices using organic or temperature-sensitive materials.1, 10 Also, it is typically much more difficult to fabricate the NIL molds containing MDNPs compared to simple nanogratings by existing nanofabrication methods including e-beam lithography, laser interferometry, and so on.17, 18 Further, a recent trend focusing on individual and user-specific applications involving wearable, mobile, and internet-of-things products,19, 20 requires a facile and scalable engineering of nanopatterns on a flexible 4 ACS Paragon Plus Environment

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and arbitrary surface. Here, it is highly desirable to support user-specific use of such nanopatterns depending on the individual purpose and environment, namely, by easily attaching and detaching the nanopattern ‘plasters’.

To this end, developing an advanced nanoarchitecturing methodology that enables facile and scalable MDNP fabrication on a flexible and re-attachable material at low temperature and gentle pressure, without resorting to complex nanofabrication, additional resin coating, and time-taking heating, is called for.21, 22 In this study, we present that the MDNP structure can be created on a flexible and highly transparent thermoplastic ionomer film through the combination of continuous and scalable multidimensional nanoinscribing and low-temperature roll imprinting, which can be conformally attached to and cleanly detached from arbitrary surfaces.

2. RESULT AND DISCUSSION

First, we create MDNPs on a polycarbonate (PC) substrate by two-dimensional (2D) dynamic nanoinscribing (DNI) (Figure 1a). A full experimental detail is described elsewhere.23, 24 Briefly, the well-cleaved rigid (e.g., SiO2) nanograting mold makes



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conformal contact to and slides over the PC substrate surface at proper temperature and force (e.g., 140-150 °C and 3-4 N, respectively) to continuously inscribe the 1D nanograting pattern (Figure 1b). Since DNI is based on the plastic deformation of a substrate material, the resulting nanograting structure inscribed by the rectangular grating mold edge turns into the smooth sinusoidal shape during elastic recovery, which is indeed favorable for many photonic and electronic applications. By multidimensional combination of such 1D strokes in a sequential fashion, a welldefined MDNP can be scalably made (Figure 1c). Here in this work, we representatively fabricate and use a 700 nm-period, 250 nm-deep, and 1:1 duty cycle 2D nanopattern (2DNP), while the period, depth, and duty can be fully controlled by modulating the inscribing mold shape, processing temperature, and force. Then we make a flexible elastomer mold for the upcoming low-temperature thermal NIL on an ionomer film, by transferring the DNI-fabricated 2DNP structure to the polydimethylsiloxane (PDMS) pad (Figure 1e). Compared to the as-DNIed PC film, this PDMS mold can help conduct more conformal and accurate NIL to the ionomer film at lower temperature and pressure due to its elasticity25, 26.


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We now perform the low-temperature, low-pressure NIL on a flexible and transparent ionomer film using the PDMS 2DNP stamp. The ionomer, copolymer of the neutral polymeric segments and the ionized units, behaves like an elastomer (i.e., crosslinked polymer) at room temperature, while it becomes softer and easily deformed at elevated temperature like a thermoplastic; this change in physical characteristics is reversible depending on the temperature. This is because the ionic bonds in ionized parts which bind the polymer parts well at ambient temperature are loosened as the temperature increases, thereby allowing the polymeric units to deform more easily upon slight mechanical force. In this work we use the ionomer comprising repeated units of polymerized ethylene and NaOH-neutralized methacrylic acid (poly(ethylene-co-methacrylic acid)) as the NIL target substrate (see Figure S1 in the Supporting Information). This ionomer film can be rapidly imprinted at the very low temperature of ~70 °C (i.e., sufficiently higher than its vicat softening point of 60 °C) with a slight force (~1-2 N) provided by smooth rolling as schematically depicted in Figure 1d. Once cooled down with the contact with the PDMS mold maintained, the deformed ionomer nanopattern becomes fixed due to



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revitalized ionic bonds.27-29 Figure 1e shows that the 2DNP structure is clearly transferred onto the ionomer film after cooldown and demolding, as such a consequence.

The fabricated flexible and transparent ionomer nanopattern (Figure 2a) offers versatile and practically useful features attributed to the unique properties of ionomer; it is repeatedly attachable and detachable. As schematically illustrated in Figure 2c, it can be conformally attached to any arbitrary surface either flat or curved (Figure 2d-e) at ambient or mildly heated condition (

Facile and scalable fabrication of flexible re-attachable ionomer

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