Supramolecular systems refer to assemblies constructed through secondary interactions (non-covalent bonds), since their dynamic interactions with reversible of supramolecular, it has promising application in intelligent recognition, controllable release, orderly assembly, and intelligent. Aerogel is a type of nanoporous material prepared by sol-gel chemical transformation and supercritical drying, its unique structural features, such as large specific surface area, low density, high porosity, and large pore volume, has brought low thermal conductivity, low sound conductivity, low dielectric constant, high transparency, and other properties, which widely used in thermal insulation, catalysis, energy storage, oil-water separation, drug carriers and other fields. However, the preparation of aerogels requires a sol-gel transformation process, and its microstructure is uncontrollable, and its performance is difficult to control. Therefore, the structure and performance control of aerogels are key scientific issues facing this field. If the orderly assembly of supramolecules is introduced into the preparation of aerogels, it is expected that the structure of aerogels can be precisely controlled, and the performance control can be achieved at the same time. However, the interaction between supramolecules is weak, and their functions are usually displayed in a solution state. Once prepared into a dry porous material, whether its function and mechanical strength can meet actual needs is still a key question to be answered.
In order to solve the above problems, our team used supramolecular system as the building unit to prepare aerogels with controllable structure and adjustable performance. Firstly, the supramolecular aerogel with double crystal structure was prepared by self-assembly with cyclodextrin (CD) as the main molecule and polyethylene glycol (PEG) as the guest molecule. The aerogel is a hydrophilic two-dimensional nano-sheet structure material, which organically integrates heat insulation and phase change energy storage for the first time, related work was published in ACS Nano 2015, 9, 11389. However, the mechanical strength of aerogel is weak and fragile with small specific surface area, because of their entirely composed of hydrogen bond interactions. For this reason, we further use cyclodextrin or cyclodextrin polyrotaxane as the skeleton structure, and then introduce some chemical crosslinking groups, and successfully obtain aerogels with molecular cavities, micropores, mesopores and macropores. This type of aerogel has simple preparation, controllable structure, excellent mechanical strength, up to 160 MPa, and a specific surface area of up to 200 m2/g. It has potential great application value in the fields of thermal insulation, phase change energy storage, and organic dye adsorption (Figure 1), related work was published in J. Mater. Chem. A 2017, 5, 4308.
Figure 1. Hierarchical porous structure and high-strength CD aerogel block prepared with cyclodextrin as the building unit.
Recently, our group has prepared a dual temperature-responsive polyrotaxane with poly(N-isopropylacrylamide) (PNIPAAm) as the end-capping group through the atom transfer free polymerization method (ATRP). Then, polyrotaxane is used as the building unit, and the cyclodextrin in the polyrotaxane is chemically cross-linked by a chemical cross-linking agent to prepare supramolecular polyrotaxane aerogel with high mechanical strength and temperature stimulus response. The specific surface area of the aerogel is as high as 232 m2/g, and the Young's modulus is as high as 74.7 MPa. Due to the introduction of PNIPAAm at both ends of polyrotaxane, the supramolecular aerogel exhibits a temperature-responsive hydrophilic-hydrophobic transition (as shown in Figure 2), related work was recently published in ACS Appl. Mater. Interfaces 2018, 10, 1468. In short, the cyclodextrin-based supramolecular system was introduced into the preparation of aerogels, and aerogels with controllable structure and stimulus response were successfully prepared. The supramolecular system of mechanical strength has increased to 10-160MPa through the introduction of chemical bonds. These works provide a new way for the structural design and functionalization of aerogels.
Figure. 2. Temperature-responsive, high-strength supramolecular aerogels prepared with supramolecular polyrotane as construction unit.
