Optimization of the cross-linker and fluorophore chain length in liquid crystal material mechanoluminescence

Abstract

Currently, there are various anisotropic materials with singular behaviors, whose mechanical, electrical or optical properties vary when an external stimulus is applied in a different orientation. Within this group of materials, we find those called Smart Materials (SM). These, when an external stimulus is applied to them, are capable of triggering a response, that is, they give rise to reversible changes in their properties, often associated with memory effects. Due to their peculiarity and their potential applications, these materials are studied in depth. Our research has been based on the study of monodomain Liquid Single Crystal Elastomers (LSCEs) and on the analysis of the variation of their fluorescent response when an external mechanical force is applied to them. Furthermore, the effect of two different synthesis methods on the final optical properties possessed by the synthesized elastomers in each method has been compared. The static pool method proposed by our research group and the method reported by Küpfer and Finkelmann, which is based on polymerization in a rotating PTFE mould. The elastomers that have been synthesized are composed of different monomers, among them, a fluorophore based on a carbazole functionalized with an aliphatic chain with a terminal double bond which is responsible for the fluorescence emission and a cross-linking agent derived from a hydroquinone substituted in the para position with two identical aliphatic chains that links the polymer chains. In this research, elastomers have been generated by combining fluorophores and cross-linkers with different aliphatic chain lengths in order to study their influence on the emitted mechanoluminescence. The obtained results show that elastomers synthesized using the pool method do not exhibit fluorescence due to the quenching phenomenon of the fluorophore caused by its proximity to the mesogen. In contrast, elastomers synthesized using the centrifugation method trigger a mechanoluminescent response, since during the synthesis process, the centrifugal force induces an orientation of the monomers that prevents quenching. In addition, the use of cross-linkers with longer aliphatic chains, generates elastomers more readily. Finally, the synthesized elastomers have been characterized using three techniques, a mechanoluminescence measurement and the swelling-deswelling process, which is used to determine the cross-linking state based from the degree of anisotropic swelling in different solvents and differential scanning calorimetry analysis.