© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 4327 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Zhou Zhang, Timothy C. Hughes, Paul A. Gurr, Anton Blencowe, Xiaojuan Hao,* and Greg G. Qiao* Influence of Polymer Elasticity on the Formation of Non-Cracking Honeycomb Films Z. Zhang, Dr. P. A. Gurr, Dr. A. Blencowe, Prof. G. G. Qiao Polymer Science Group Department of Chemical & Biomolecular Engineering University of Melbourne Parkville, VIC 3010, Australia E-mail: gregghq@unimelb.edu.au Z. Zhang, Dr. T. C. Hughes, Dr. X. Hao Materials Science and Engineering Commonwealth Scientific and Industrial Research Organisation (CSIRO) Clayton, VIC 3168, Australia E-mail: Xiaojuan.Hao@csiro.au DOI: 10.1002/adma.201200877 The Breath Figure (BF) technique is a well established, robust and highly efficient approach for the preparation of highly ordered porous films. [1] It was first introduced by François and co-workers, whereby water droplets were utilised as tem- plates for the self-assembly of polymers to form highly ordered honeycomb (HC) films. [2] HC structured films have also been prepared using lithographic and other non-lithographic pat- terning methods, although these methods are expensive and/ or complex. [3,4] Numerous polymer architectures have been employed with the BF technique to prepare ordered macro- porous and microporous HC structures on flat surfaces. [5–11] Highly ordered HC patterned films are of great interest due to their wide range of potential specialised and high value appli- cations including biosensors, [12] membranes, [13] scaffolds for tissue engineering, [10,14] microreactors, [15] photonic band gap devices, [16] and optoelectronic devices. [17] Recently, Fujifilm has developed a technique to produce large areas of HC films with minimal defects on flat substrates. [18] However, the applicability of this technique to non-planar surfaces is unknown. Cracking or fracture of the film remains a major issue during the forma- tion of non-planar HC films, resulting in poor reproducibility. Methodologies that enable HC films to be reproducibly pre- pared over large imperfect surfaces (e.g., non-planar surfaces) will facilitate commercial uptake of this micro-fabrication tech- nique to a wide variety of surfaces. Therefore, the development of strategies for the fabrication of regularly patterned films on rough surfaces and the ability to select precursors from a wide range of polymers with varied composition will signifi- cantly expand the scope of applications where HC films can be applied. In 2006 we reported the first example of HC formation on non-planar surfaces using star polymers with low glass tran- sition temperatures ( T g ’s). [7,8,19] Highly conformal non-planar HC films were successfully prepared without cracking on the surface of transmission electronic microscopy (TEM) grids as well as other non-planar surfaces. [8] During an investigation of a series of star polymers made from poly(dimethylsiloxane) (polyDMS), poly(methyl acrylate) (polyMA), poly( tert-butyl acr- ylate) (poly tBA) and poly(methyl methacrylate) (polyMMA), it was observed that the T g of these polymers played an impor- tant role in the process of non-planar porous film formation. It was found that all these polymers could form regular HC struc- tures on planar surfaces without the occurrence of cracking. However, the polymers with a T g > 48 °C formed HC films on non-planar surfaces with cracks while polymers with a T g ≤ 48 °C formed HC films that contoured to the TEM grid surface without cracking. Similar phenomena were also later reported by Tian and co-workers. [20] Until now it has remained unclear if T g is the only parameter that determines the repetitive forma- tion of conformal HC patterned films on non-planar surfaces for star polymer systems. In this communication, we report that there are parameters other than T g that have a significant influence on the forma- tion of non-cracking HC films when a broad range of polymers are used. More specifically, a polymer’s elasticity, represented as the Young’s modulus, can be used to better predict if a well- defined, conformal and non-cracking HC film will be formed on non-planar surfaces. In this study the Young’s modulus ( E) is used to define the tensile elasticity of the polymeric films and we demonstrate that tuning the E of polymers can regulate the formation of non-cracking HC films on non-planar surfaces; this observation applies to seven of the polymers investigated in this study, including polymers with various T g ’s. With appro- priate design, a polymer with a high T g and suitable E could potentially be employed to form non-cracking HC films on non-planar substrates, which may ultimately lead to more and new industrial applications of this technology. Star polymers were chosen as candidates for the HC film formation based upon previous studies. [7,9] A series of β-cyclodextrin ( β-CD)-based star polymers with varying T g and E were designed and synthesised via atom transfer radical poly- merisation (ATRP) and the ‘core-first’ approach ( Scheme 1). [21] Subsequently, the formation of HC films on planar and non- planar surfaces was investigated using these star polymers and an improved static casting method employing a warm (30 °C) humid environment. [11] Initially, a bromoester functionalised β-CD derivative with 21 initiating sites was prepared from the reaction of β-CD with 2-bromopropanoyl bromide (Scheme 1 and Figure S1). [22] Using this initiator, one-pot ATRP reactions were performed with a range of monomers or combinations of monomers to form star polymers with varying T g and E. These monomers include methyl methacrylate (MMA), methyl acrylate (MA), tert-butyl Adv. Mater. 2012, 24, 4327–4330