polymers Article Nanopatterned Polymer Molds Using Anodized Aluminum Templates for Anti-Reflective Coatings Soon Hyuk Lim 1,† , Nguyễn Hoàng Ly 1,† , Jung A. Lee 1 , Ji Eun Kim 1 , Se-Woong La 2 , Vu Thi Huong 2 , Thi-Giang Tran 2 , Ngoc Thanh Ho 2 , Seung Man Noh 3, *, Sang Jun Son 1, * and Sang-Woo Joo 2, *   Citation: Lim, S.H.; Ly, N.H.; Lee, J.A.; Kim, J.E.; La, S.-W.; Huong, V.T.; Tran, T.-G.; Ho, N.T.; Noh, S.M.; Son, S.J.; et al. Nanopatterned Polymer Molds Using Anodized Aluminum Templates for Anti-Reflective Coatings. Polymers 2021, 13, 3333. https://doi.org/10.3390/ polym13193333 Academic Editors: Thanh-Danh Nguyen, Lilia Sabantina and Han-Yu Hsueh Received: 29 August 2021 Accepted: 24 September 2021 Published: 29 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Chemistry, Gachon University, Seongnam 13120, Korea; 2952305@naver.com (S.H.L.); nguyenhoangly2007@gmail.com (N.H.L.); sb52431@naver.com (J.A.L.); wnpwnp81@naver.com (J.E.K.) 2 Department of Chemistry, Soongsil University, Seoul 06978, Korea; sktpdnd9668@daum.net (S.-W.L.); huongvu172018@gmail.com (V.T.H.); trangianghup@gmail.com (T.-G.T.); ngocthanhkd94@gmail.com (N.T.H.) 3 Research Center for Green Fine Chemicals, Korea Research Institute of Chemical Technology, Ulsan 44412, Korea * Correspondence: smnoh@krict.re.kr (S.M.N.); sjson@gachon.ac.kr (S.J.S.); sjoo@ssu.ac.kr (S.-W.J.); Tel.: +82-2-820-0434 (S.-W.J.) † S.H.L. and N.H.L. contributed equally to this work. Abstract: This work introduces a facile geometry-controlled method for the fabrication of embossed and engraved polymeric moth-eye-inspired nanostructures in imprinting molds using anodic alu- minum oxide (AAO) templates, resulting in a novel anti-reflective transparent coating. The moth-eye nanostructures are prepared directly on the surface of a flexible polyethylene terephthalate (PET) substrate. As a prerequisite procedure, a UV-curable polyurethane acrylate resin is spun on the PET. The shape of the moth-eye nanostructures can then be adjusted by controlling the size and shape of the nanopores in the AAO templates. Both embossed and concaved polymer moth-eye nanostructures were successfully mounted on a PET substrate. Embossed polymer replica molds were prepared using the AAO master templates in combination with an imprinting process. As revealed by field-emission electron microscope (FE-SEM) images, conical nanopatterns in the AAO template with a diameter of ~90 nm and a depth of ~100 nm, create a homogeneous embossed mor- phology in the polymer moth-eye nanostructure. The polymeric molds with the depths of 300 and 500 nm revealed the amalgamated structures in their apexes. In addition, a dip-imprinting process of the polymeric layers was implemented to yield a concaved mold by assembly on the surface of the 100 nm embossed polymer mold substrate. Considering that the embossed structures may be crumbled due to their protuberant shapes, the concaved geometries can have an advantage of stability in a certain application concerning physical degradation along with a higher transmission by ~2%, despite somewhat nonuniform structure. The experimental and theoretical results of this study indicate that this polymer layer has the potential for use in anti-reflective coating applications in transparent films. Keywords: nanocone pattern; transparent coating films; anodized aluminum oxide; anti-reflective surfaces; moth-eye structures; shape-controlled fabrication 1. Introduction Nanostructures have been used for various applications in chemistry and materials science [1]. Nanotemplates have recently emerged as potential platforms for the fabrication of functional materials with many uses in multiple fields, including transparent coatings [2] and solar cells [3]. The bionic prototypes and antireflection principles inspired by nature are discussed in the recent review [4]. Hexagonal nipple-array of subwavelength conical nanostructures have a diameter of ca. 100 nm and can provide broadband anti-reflectivity to enhance photon collection capability [5]. The shapes and sizes appeared to affect reflectivity performance [6–8]. Several nanostructures have been developed for efficiently hydrophobic Polymers 2021, 13, 3333. https://doi.org/10.3390/polym13193333 https://www.mdpi.com/journal/polymers