Every time I see insulated cups, aerospace thermal insulation materials, or even some high-tech wound dressings and moisturizing masks, I feel there's a magical laboratory behind them. Today, let's talk about those materials that are all the rage in the scientific research community - hydrogels and aerogels.
First, let's talk about hydrogels. They are like bouncy "water babies" and are experts at retaining water.
The secret lies in a transformation technique called "cross-linking polymerization". The most common type is acrylamide-based hydrogels, with acrylamide as the monomer, N,N'-methylenebisacrylamide as the cross-linking agent, and ammonium persulfate + TEMED as the initiator. In just a few minutes, a hydrogel baby is born.
If you want it to be temperature-sensitive, just replace acrylamide with N-isopropylacrylamide. Moreover, with a photoinitiator, it can be formed by shining ultraviolet light on it, which is super cool!
Natural hydrogels are also amazing. For instance, gelatin, when dissolved in warm water and mixed with glutaraldehyde, can be cross-linked; sodium alginate, when encountering calcium chloride, instantly turns into a gel and can be printed into threads; chitosan, in a slightly acidic solution, can be cross-linked with glutaraldehyde or sodium phosphate to form a miraculous hydrogel.
Now, let's talk about aerogels. They are the pinnacle of porous materials, as light as clouds and excellent at thermal insulation!
The key is to build a porous network and then remove the liquid inside without causing the structure to collapse.
Silica aerogels are the most typical. Tetraethyl orthosilicate (TEOS) is used as the precursor, mixed with ethanol and water, and slowly hydrolyzed with hydrochloric acid. Then, ammonia water is added to promote the condensation of silanol groups, forming a Si-O-Si framework.
The most challenging part is the drying process. Supercritical drying involves replacing ethanol with liquid CO2, heating and pressurizing it in a high-pressure reactor to reach a supercritical state, and then slowly depressurizing to obtain a perfect aerogel. Freeze-drying is also fascinating. The gel is frozen to -80°C, and the water is sublimated, which is particularly suitable for organically modified or biologically derived gels.
These two materials have a wide range of applications. Hydrogels can be used as wound dressings and moisturizing masks, while aerogels can be used as aerospace thermal insulation materials and insulated cups. Next time you see these high-tech products, you'll know how they are made!