2023 Author: Bryan Walter | [email protected]. Last modified: 2023-05-21 22:24
Scientists from Japan, Switzerland and Saudi Arabia have created three polymers containing rotaxane molecules that can reversibly activate their own fluorescent properties when stretched. This allows you to create a polymer with the desired fluorescence color without affecting the mechanical and other properties of the material, say the authors of the article in ACS Central Science.
Typically, when creating a structure, engineers calculate the workloads on its elements in advance, but the actual load can vary greatly. To monitor the load, you can use various additional sensors, visually monitor the elongation or change in tensile resistance, or use other methods. In addition, scientists have long been working on the creation of materials capable of changing their externally observed properties under mechanical stress.
For example, in 2018, scientists led by Christoph Weder from the University of Friborg in Switzerland created an unusual mechanophore (a material that changes its optical properties when exposed to mechanical stress) that can reversibly activate its fluorescent properties under mechanical stress without breaking weak covalent connections, as is the case in most mechanophores. The basis of this material is polyurethane, in the structure of which scientists have included a molecule of the class of rotaxanes, known for its unusual properties. In fact, rotaxane consists of two molecules - a dumbbell-shaped one, in which most of the molecule is linear, and at its ends there are large groups of atoms, and also ring-shaped. The latter is “put on” on a dumbbell-shaped one and these two molecules are not chemically linked, but are held together mechanically thanks to the edges of the “dumbbell”.
The principle of changing polymer properties
The researchers chemically attached a fluorescent functional group to the ring-shaped molecule. In the normal state, the ring-shaped molecule and the center of the linear molecule form a charge transfer complex, which stabilizes the position of the ring in the center and "turns off" the fluorescence. However, tensile stress forces the ring molecule to move to the edge of the linear and activates the fluorescent properties. In their new work, Weder and colleagues showed that the color of fluorescence can be controlled by changing only the fluorescent functional group attached to the ring molecule. They selected three molecules that emit blue, green or orange light when exposed to ultraviolet radiation.
One of three fluorescent-attached rotaxane molecules (top) and the general structure of polyurethane with a marked rotaxane functional group (bottom)
Despite the slightly different activation energy of the transition of the rotaxane ring between two states in three polymers, at the macrolevel, the properties of the three materials are virtually the same. This, according to scientists, allows you to set the desired fluorescence parameters without changing the macroscopic properties of the polymer. Scientists have demonstrated the properties of three separate fluorescent polymers and combined all three types of rotaxanes in one material to create a polymer that emits white in response to ultraviolet radiation:
Last year we talked about another polymer that changes color when stretched, but its properties are due to a different mechanism. The material created by the scientists consists of an array of layers with different properties, forming a Bragg reflector that reflects light in a very narrow wavelength range, and changes this range when the distance between the array elements changes.