Electrospun Janus film: Innovative design of “macro + micro” to achieve thermochromic-anisotropic conductive-fluorescent tri-functionality

Abstract

Multi-functionalized thermochromic materials will greatly expand their application scopes. In this work, two functions of electrical conductivity and fluorescence are innovatively integrated into a thermochromic material by designing specific “macro + micro” Janus structures. Tri-functional thermochromic-anisotropic conductive-fluorescent thermochromic microcapsules-45 °C (TCM-45)/polymethyl methacrylate (PMMA) microfibers non-array&{[polypyrrole (PPy)/PMMA]//[Coumarin/PMMA] Janus microfibers array} up-down Janus film (named as TCFJAJF) is constructed by two-step electrospinning techniques of “single axis + parallel axis”. The up layer of TCFJAJF is TCM-45/PMMA microfibers non-array film (defined as U-TCFJAJF, viz. thermochromic layer) and the down layer of TCFJAJF is [PPy/PMMA]//[Coumarin/PMMA] Janus microfibers array film (denoted as D-TCFJAJF, viz. anisotropic conductive-fluorescent layer). The macroscopic up-down Janus structure eliminates the mutual influences between D-TCFJAJF and U-TCFJAJF. U-TCFJAJF has excellent color conversion and fading/coloring performances, achieving a reversible and multiple colors conversion from blue to white upon heating from 25 to 45 °C. A new concept of “transient thermal printing” is proposed based on fast thermal coloring/fading conversion of thermally printed patterns on film. Meanwhile, the microscopic Janus microfiber structure of D-TCFJAJF largely alleviates absorption to the excitation light and emission light of Coumarin by PPy and influence of Coumarin on conduction of PPy, ensuring the intense fluorescence and high conductivity. Due to the ordered arrangement of Janus microfibers, D-TCFJAJF has excellent anisotropic conductivity. As a demo application, TCFJAJF as a conductor can light up LED bulbs in a microcircuit and emit bright blue fluorescence under UV light excitation, achieving the working state visualization of the conductive microcircuit in dark surroundings. The multi-functionalized thermochromic film will have huge application prospects in fields of temperature monitoring, transient information reading and displaying, flexible electronics, fluorescent labeling and thermal-fluorescent dual encryption anti-counterfeiting, etc.