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Fabrication and Post-Modification of Nanoporous Liquid Crystalline Networks via Dynamic Covalent Chemistry Dirk J. Mulder, Luc M.W. Scheres, Jingjin Dong, Giuseppe Portale, Dirk J. Broer, and Albertus P. H. J. Schenning Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b01334 • Publication Date (Web): 13 Jul 2017 Downloaded from http://pubs.acs.org on July 14, 2017
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Chemistry of Materials
Fabrication and Post-Modification of Nanoporous Liquid Crystalline Networks via Dynamic Covalent Chemistry Dirk J. Mulder,†,‡ Luc M. W. Scheres,†,§ Jingjin Dong,ǁ Giuseppe Portale,ǁ Dirk J. Broer,†,¶ and Albertus P. H. J. Schenning∗,†,¶ †
Department of Functional Organic Materials and Devices, Chemical Engineering and Chemistry, Eindhoven University of Technology, Den Dolech 2, 5612 AZ, Eindhoven ‡ Dutch Polymer Institute (DPI), PO Box 902, 5600 AZ, Eindhoven, The Netherlands ǁ Zernike Institute for Advanced Materials, Department of Macromolecular Chemistry and New Polymeric Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands ¶ Institute for Complex Molecular Systems (ICMS), Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands Abstract: We herein present a novel approach to obtain nanoporous smectic liquid crystalline polymer networks by dynamic covalent chemistry. A binary mixture of reactive smectic liquid crystalline monomers, of which one contains a bisimine template, is used to fabricate polymer networks. The bisimine is subsequently hydrolyzed leading to the formation of two-dimensional nanopores having aldehyde functionalities. By post-modification of these moieties with a variety of amines, the size and chemical nature of the pores can be tuned in a facile fashion. These results can be used to control the size and chemical nature of the pores and the set of molecules and ions that can be separated by polymers.
Well-defined nanoporous polymers which exhibit a high porosity, low density, high specific surface area, and permeability are of great current interest due to their applications in areas as filtration, adsorption, ion conductivity and drug-release.1–4 The small pore size in these materials makes discrimination between molecules and ions based on size and shape possible. In order to tune the performance of these materials, efforts are made to control pore size, pore morphology, and pore chemistry.5,6 Straight and well-defined pores are often desired for fast diffusion and size selectivity while tunable pore chemistry offers additional tools for enhancing selectivity. In the last decade, self-assembled liquid crystalline systems have proven to be useful to prepare porous materials with pore sizes around 1 nm.7,8 Both lyotropic and thermotropic liquid crystal polymers have been employed by incorporating polymerizable groups to the liquid crystalline moieties using columnar, lamellar or bicontinuous cubic phases.9,10 In lyotropic liquid crystals, pores are created and size controlled by ions and the solvent, i.e. water.11,12 In thermotropic liquid crystals, nanopores have been created by so-called porogen templates which are hydrogen bonded to the polymer network and washed away to create the pores structured.13–16 So far, most of the pores that have been made, are based on negatively charged carboxylates6,14,17 but there are some examples of neutral13 or positively charged18 pores. These prefunctionalization approaches limit the control over the size and chemical nature of the pores and the set of molecules and ions that can be separated.6,17 Therefore the control of pore size and the chemical tunability of the porous material remains a challenge. Herein we report on a facile method to fabricate and postmodify nanoporous liquid crystal polymer materials by using dynamic covalent chemistry. Dynamic covalent
bonds can form, break, and exchange under an external stimulus, like the addition of a catalyst19, change of environment (solvent)20,21, or heat. For this reason, they are appealing to produce, e.g. adaptive and self-healing materials.21,22 However, so far the use of dynamic covalent chemistry in the fabrication and functionality of nanoporous materials is limited.23 We describe the photopolymerization of a reactive thermotropic smectic LC monomer containing a bisimine porogen (Figure 1a, compound 1) and a reactive crosslinker (Figure 1a, compound 2) to yield a polymer film that after acid treatment contains reactive pore surface aldehyde functionalities.17 By postmodification of these moieties with a variety of amines, the size and chemical nature of the pores can be tuned in a facile fashion (Figure 1e). The newly developed reactive bisimine 1 was prepared in a 2-step synthesis route starting from 4-(6hydroxyhexyloxy)benzaldehyde. First, a methacrylate ester was prepared, whereafter 1 was obtained from the condensation reaction of the aldehyde with 4-(2aminethyl)aniline. A methacrylate was used instead of acrylate to prevent side reactions while synthesizing the bisimine. The bisimine based on 4-(2-aminethyl)aniline (compound 1) exhibits a smectic phase in a suitable working temperature range (
