Proton-conductive materials formed by coumarin photocrosslinked ionic liquid crystal dendrimers
Abstract: In this work, we have successfully examined for the first time the use of ionic dendrimers as building blocks for the preparation of 1D and 2D proton conductive materials. For this purpose, a new family of liquid crystalline dendrimers has been synthesized by ionic self-assembly of poly(amidoamine) (PAMAM) dendrimers bearing 4, 8, 16, 32 or 64 NH2 terminal groups and a coumarin-containing bifunctional dendron. The noncovalent architectures were obtained by the formation of ionic salts between the carboxylic acid group of the dendron and the terminal amine groups of the PAMAM dendrimer. The liquid crystal properties have been investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). All the compounds exhibited mesogenic behavior with smectic A or hexagonal columnar mesophases depending on the generation of the dendrimer. Coumarin photodimerization was used as a crosslinking reaction to obtain liquid crystalline polymer networks. All the materials showed good proton conductive properties as the LC arrangement leads to the presence of ionic nanosegregated areas (formed by the ion pairs) that favor proton conduction.
Summary (1 min read)
- Ion transport is an important phenomenon in biological processes, batteries and separation technologies.
- LCs showing columnar mesophases can be used to create 1D ion conductors, with ion conduction taking place in the direction of the columnar axes.
- Crosslinking of polymerizable LC monomers in their mesophase can yield nanostructured, thermally and mechanically stable membrane materials with permanent pathways for ion transport.
Synthesis and Characterization of Ionic dendrimers
- The carboxylic acid dendron was prepared by the synthetic route and the experimental details given in the Supporting Information.
- The mixture was ultrasonicated for 5 min, then the THF was slowly evaporated at room temperature and the sample was dried in vacuum at 40 ᵒC until the weight remained constant.
- The formation of ionic interactions between the PAMAM dendrimer and the dendron acids was studied by infrared spectroscopy (IR) and by nuclear magnetic resonance (NMR).
- In the spectrum of PAMAM16-ChCou the signals at 1686 and 1741 were replaced by two new bands at around 1550 and 1400 cm1 due to the asymmetric and symmetric stretching modes of the carboxylate group.
Proton Conduction Properties
- The proton conductivity was measured using electrochemical impedance spectroscopy in samples consisting of films sandwiched between ITO-coated electrodes.
- The typical impedance response (Nyquist plots) consisted of a suppressed semicircle in the high-frequency region and an incline straight line in the low-frequency range .
- (b) FTIR spectra of PAMAM16-ChCou before (blue) and after (red) coumarin photodimerization.
- (b) Proton conduction through the nanochannels generated in the ionic dendrimers.
- A new strategy for the preparation proton conductive materials has been developed using ionic LC dendrimers combined with a crosslinking reaction based on coumarin photodimerization.
- 1D and 2D ionic nanosegregated assemblies can be obtained in a modular approach using different generation dendrimers.
- The use of coumarin photodimerization impart a new tool to fabricate mechanical stable ionic materials.
- All ionic materials showed good proton conductivity and it is expected that macroscopic alignment will enhance this.
- The proton conduction in these ionic LC dendrimers may open a new path in the search of electrolyte materials for the preparation of electrochemical devices.
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