Contents lists available at ScienceDirect Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat Spontaneous formation of microporous poly(lactic acid) coatings Ozgun Can Onder, Muhammed Anwaar Nazeer, Emel Yilgör, Iskender Yilgör ⁎ Chemistry Department, KUYTAM Surface Science and Technology Center, Koc University, Istanbul, Turkey ARTICLE INFO Keywords: Poly(lactic acid) coating Microporous surface Spin-coating ABSTRACT Microporous poly(lactic acid) (PLA) coatings with fairly homogeneous pore size and distribution were obtained through spin-coating process. Critical effects of the thermal conductivity of the substrate (copper, glass, insulated glass), solvent type (dichloromethane, chloroform and tetrahydrofuran), polymer concentration (3–15% by weight) and relative humidity of the environment (0–85% relative humidity) on the formation and topography of microporous surfaces were investigated. Characterization of the surfaces was achieved via scanning electron microscopy, white light interferometry and static water contact angle measurements. By tuning the production parameters, PLA coatings displaying microporous surfaces with uniform pore sizes and distribution were ob- tained. Pore sizes varied from 1 to 7 μm depending on the experimental conditions. 1. Introduction Polymeric coatings with controlled surface porosity are interesting materials that may find uses in various applications including super- hydrophobic and self-cleaning surfaces, tissue scaffolds, foul release coatings, photonics, and template for functional materials, catalysts and membranes. A wide range of methods such as thermally induced phase separation [1–3], immersion precipitation [4], non-solvent induced phase separation [5,6], lithography [7], microcontact printing [8,9], salt leaching [10], micromolding [11], freeze-drying [12], gas foaming [13] and utilization of colloidal assemblies as templates [14] have been reported for the preparation of polymeric structures with micro and/or nanoscale pores on the air surface or in bulk/core structures [7,9,15,16]. Another widely utilized approach, termed as the breath figures, which involves the utilization of condensed water droplets from humid air during solvent evaporation of a polymer solution, also re- ported to produce well-ordered microporous films with honeycomb structures [15–23]. A wide range of polymeric materials, including polystyrene [19], poly(methyl methacrylate) [7], polycarbonate [22], poly(lactic acid) [6,24] and various others [2,10,12] have been used for the production of microporous surfaces by various methods. Poly(lactic acid) (PLA) is a highly crystalline and biodegradable polyester that can be used as a polymeric carrier for drug release ap- plications and template for cell growth and tissue scaffolding [25]. Three dimensional (3D) scaffolds with controlled porosity play critical roles in tissue engineering applications, where they function as tem- plates that provide temporary structural support to the cells and allow new tissue growth [26]. For this purpose a high degree of porosity with controlled pore size and distribution are necessary to facilitate the cell seeding, adhesion and diffusion through the structure both for the cells and the nutrients. To obtain 3D scaffolds with controlled porosity some of the most widely used techniques include 3D printing and electro- spinning or the combination of both. In this study we investigated the preparation of microporous PLA surfaces via breath figures method which was developed by Francois [17]. In this process, polymer is dissolved in a highly volatile solvent, fast evaporation of which leads to cooling of the solution surface. This cooling results in the condensation of the water vapor onto the solution surface to form water droplets. Through convection and capillary forces, condensed droplets are self-organized throughout the surface. In the later stages of evaporation, solution viscosity increases and polymer molds around water droplets, which leads to surface cavities after complete removal of the solvent and water. In this work we utilized a fairly simple, high speed spin-coating process for casting polymer so- lutions in various solvents with different polarities, boiling points and evaporation rates. Homogenous microporous PLA thin films with con- trolled pore sizes were produced by optimization and tuning of the following process parameters; (i) thermal conductivity of the substrate, (ii) polymer concentration, (iii) relative humidity of the environment, and (iv) solvent type. 2. Experimental methods 2.1. Materials PLA (M n = 109,000 g/mol, PDI = 1.76, 8% D content) was https://doi.org/10.1016/j.porgcoat.2018.09.016 Received 25 May 2018; Received in revised form 4 September 2018; Accepted 4 September 2018 ⁎ Corresponding author. E-mail address: iyilgor@ku.edu.tr (I. Yilgör). Progress in Organic Coatings 125 (2018) 249–256 0300-9440/ © 2018 Elsevier B.V. All rights reserved. T