Light-melt adhesive based on dynamic carbon frameworks in a columnar liquid-crystal phase

Abstract

Liquid crystal (LC) provides a suitable platform to exploit structural motions of molecules in a condensed phase. Amplification of the structural changes enables a variety of technologies not only in LC displays but also in other applications. Until very recently, however, a practical use of LCs for removable adhesives has not been explored, although a spontaneous disorganization of LC materials can be easily triggered by light-induced isomerization of photoactive components. The difficulty of such application derives from the requirements for simultaneous implementation of sufficient bonding strength and its rapid disappearance by photoirradiation. Here we report a dynamic molecular LC material that meets these requirements. Columnar-stacked V-shaped carbon frameworks display sufficient bonding strength even during heating conditions, while its bonding ability is immediately lost by a light-induced self-melting function. The light-melt adhesive is reusable and its fluorescence colour reversibly changes during the cycle, visualizing the bonding/nonbonding phases of the adhesive.

Figure 1. Molecular structures of the photoresponsive liquid crystal 1 and its derivative 2.

Rigid anthracene units (blue wings) are fused with a flexible cyclooctatetraene ring (eight-membered ring). Dendritic moieties (orange fans) are attached to the flapping core framework.

Figure 2. Photoinduced melting in a columnar LC phase.

(a) Differential scanning calorimetry (DSC) traces of 1 at 2 °C min^–1 rate of cooling (top) and heating (bottom). (b) Crystal packing structure of a derivative 2 with no dendritic peripheral chain. Interfacial distance of the π-stacked anthracene moieties, d(π–π)=3.50 Å. Intermolecular distance between the photoreactive carbon sites, d(C–C)=4.84 Å. Bent angle of the V-shaped molecule, ϕ=43.8°. (c) POM image of the LC film of 1 (bright) and its photoirradiated area (dark) under the crossed Nicols. Scale bar, 500 μm. (d) Isothermal photoinduced melting of 1 in the range of 70–135 °C (left to right), in which the columnar LC phase of 1 is photochemically transformed into a fluid mixture mainly composed of unreacted 1 and its photodimer product. Heating the melted mixture at 160 °C induces a thermal back reaction of the photodimer into the monomer 1, which recovers the columnar LC phase when the temperature is set again in the range of 70–135 °C (right to left).

Figure 3. Light-melt adhesive properties.

(a) High-temperature resistant bonding and photoinduced separation of two glass plates stuck with the adhesive film of 1. (b) Demonstration of the strong adhesive function of 1. (c,d) Ultimate shear strengths of the 130-μm-thick film of 1 depending on the phase of 1 (c) and on the hydrophilicity of the glass surface (d). See the Methods section for the preparation of the uniform films as well as the hydrophilic and hydrophobic glass surfaces.

Figure 4. Photoresponse and thermal recovery of the film.

(a) Transmittance of 365-nm light dependent on the film thickness of 1. (b) Required irradiation dose for glass separation at 100 °C using the films of 1 with different thickness. (c) Reusability of the adhesive 1. The room-temperature shear strengths before and after 320 mJ cm⁻² ultraviolet exposure at 100 °C in the recycling processes. The inset photographs show the fluorescent film in the corresponding stage. (d,e) Fluorescence spectral change during the ultraviolet irradiation on the 5-μm-thick film of 1 (d) and its thermal restoring steps (e). The film fluorescence at 100 °C before (green line) and after (blue line) light irradiation at 3.2 mW cm⁻² for 100 s (d) and the film fluorescence before (blue line) and after (green line) heating at 160 °C for 30 min (e).

Figure 5. Interpretation of the photoresponse mechanism.

(a) Calculated potential energy diagram for the ground state (S₀) and lowest excited state (S₁) of the carbon framework 3 with fixed bent angle ϕ. The relaxed potential energy surface scan was performed for the S₁ state of 3 at the TD-PBE0/def-SV(P) level. (b) Fluorescence spectra of a thin film of 1 in the liquid phase (160 °C, black line) and in the LC phase (100 °C, red line). Excitation at 365 nm. Fluorescence spectra of 1 in CH₂Cl₂ solution (green dotted line) and in polymethyl methacrylate (PMMA) matrix (blue dotted line) are shown for comparison. The fluorescence in PMMA matrix was measured at 20 °C, which is lower than the glass transition temperature of the PMMA (T_g=105 °C). (c) The reaction process of the photodimerization of 1 in the LC phase.