Fabrication of honeycomb-patterned polyalkylcyanoacrylate films from monomer solution by breath figures method

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Abstract

Honeycomb-patterned polyalkylcyanoacrylate (PACA) films were prepared from the chloroform solutions of alkylcyanoacrylate (ACA) by breath figures (BFs) method. Condensed water droplets on the solution surface acted not only as templates to endow the ordered structure but also as initiators to trigger the polymerization of ACA. After the polymerization started, the in situ formed polymer chains self-assembled around the water droplets, structuring PACA film with a hexagonal arrangement of holes. This was the first time that polymerization was introduced to breath figures method. The formation mechanism and the influencing factors, including substrates, relative humidity, and solution concentrations were investigated. Hela cells were cultured on both flat and honeycomb-like PACA films to investigate their application as biomaterials.

Graphical abstract

Honeycomb-like films were prepared directly from the alkylcyanoacrylate monomer solution via a polymerization involved breath figure method on the hydrophobic polypropylene substrate.

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Research highlights

► Honeycomb-like PACA films are obtained from monomer solution by BFs method ► Water droplets are used as both templates and the initiators ► Ordered porous structure enhances the cell proliferations

Introduction

Highly ordered micro-porous materials have attracted intensive interest due to their potential applications in biology [1], [2], pharmaceuticals [3], tissue engineering [4], [5], micrographics [6], optics [7], [8], [9], and separation [10]. Several strategies such as templating [7], [11], [12], [13], [14], [15], self-assembly [16], [17], [18] and “breath figures” (BFs) method [19], [20], [21], [22], [23], [24], [25], [26], have been developed for fabricating micro-porous materials. Among these methods, BFs is a simple and versatile technique which can dynamically control both the pore formation process and the morphology of the resultant pores [27], [28]. The phenomenon that water droplets were condensed on cold solid surfaces was first investigated by Aitken [29], [30], and the arrays of water droplets on surfaces were termed “breath figures”. Francois et al. firstly used the arrays of water droplets on solution surface as the templates to fabricate honeycomb-patterned polymer films [19], and this method was commonly called as breath figures method. In a typical BFs procedure, a dilute polymer solution is cast on a solid substrate in a humid environment. Because of the evaporative cooling of the volatile solvent, water droplets are condensed on the cold solution surface from the moisture [31]. The as-formed water droplets prefer to arrange hexagonally to reduce free energy [32]. The polymer assembles at the interface of the water and solution, and forms regular pore patterns after the evaporation of water and solvent. Several conditions must be met to generate a BFs film, including high humidity, a water-immiscible volatile solvent and special structured polymer.

Some star polymers [19], rod-coil block copolymers [19], [33], amphiphilic block copolymers [31], [34], [35], linear polymers [36], [37], [38] and polymer-particle system [39], [40], [41] have been used to prepare BFs film. However, there were still many kinds of polymers that were not suitable for BFs method, which hindered the application of this technique. Aimed at exploring new materials to construct ordered porous structure by BFs, we here introduced in situ polymerization into the procedure, by using water droplets as initiators. Alkylcyanoacrylate (ACA) monomers, including ethyl, butyl and octyl α-cyanoacrylate (ECA, BCA, and OCA), are commonly used as super glue [42] or rapid medical adhesive [43]. ACA can perform anionic polymerization rapidly initiated by weak base or even trace amount of water [44], [45], [46]. In this article, polyalkylcyanoacrylate (PACA) was synthesized when water droplets were condensed on the ACA monomer solution, and then effectively supported the arrangement of the water templates. In contrast, PACA polymer solution could not generate ordered structure itself by BFs method. The influences of the substrate, relative humidity and the concentration of the monomer solution on the resultant morphologies of the films were investigated. The potential application of the patterned PACA films in cell culture was studied because of their biodegradable and biocompatible characteristics [47]. We hope this method could set an example for other polymeric systems to widen the application of BFs method.

Section snippets

Materials

Ethyl α-cyanoacrylate (>99%) was purchased from Beijing Chemical Plants. Butyl α-cyanoacrylate (>99%) was received from Beijing Fuaile Science and Technology Development Co., Ltd. and octyl α-cyanoacrylate was purchased from Guangzhou Baiyun Medical Adhesive Co., Ltd. All the monomers were used as received without further purification. Chloroform (CHCl3) of analytical grade was used as received.

Fabrication of PACA films

Alkylcyanoacrylate was dissolved in CHCl3 at different concentrations. Then, the solution was dropped

Fabrication of honeycomb-like polyalkylcyanoacrylate films

ECA solution was cast onto PP substrate and put in a box with RH above 60%. Because of the cooling surface entailed from solvent evaporating, water droplets were condensed on the surface of the monomer solution. The OH in the condensed water droplets initiated the polymerization of ECA at the interface of the water and monomer solution. The –OH heads of synthesized polymer were anchored on the periphery of the water droplets and the PECA chains propagated rapidly (Fig. 1). Water droplets grew

Summary

Ordered porous polymeric films were obtained from ACA monomer solutions by combining in situ polymerization and BFs method. A series of ACA monomers have been approved to be suitable for the fabrication of highly ordered films. On the other hand, the ordered structure cannot be obtained from the PACA polymer solution. The pore size could be adjusted by controlling the relative humidity and the concentration of the monomer solution. Substrate employed should have good affinity with the solution

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 50821062), 863 Project (2009AA033601) and CAS Innovation Project (No. 2007CB936400). We acknowledge Beijing Municipal Commission of Education for the special fund from the Disciplines & Postgraduate Education Construction project.

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