Exploring the mysteries of the "primordial universe" of expanded polytetrafluoroethylene (e-PTFE)
In 1911, British scientist Rutherford proposed that the atomic structure, interactions, and spatial arrangement of elements determine the properties of materials. So, what is the mystery of the "primordial universe" of expanded polytetrafluoroethylene?
Expanded polytetrafluoroethylene (PTFE) is a modified PTFE material composed of polytetrafluoroethylene (PTFE) biaxially stretched film composite, which is composed of fine fibers and nodes. PTFE is a polymer made by polymerizing tetrafluoroethylene as a monomer, and its polymer molecular chain is shown in Figure 2. There is a column of carbon atoms on the polytetrafluoroethylene chain, each of which is connected to two fluorine atoms. The fluorine atoms surround the carbon atoms, forming a long polymer chain. Its molecular weight is as high as 500000 to 1 million.
The molecular structure of polytetrafluoroethylene is similar to that of polyethylene (Medpor's material, a polymer with a C-H structure). Due to the fact that the radius of fluorine atoms (0.064nm) is much larger than that of hydrogen atoms (0.028nm), and the fluorine atoms have strong binding to the extranuclear and bonding electron clouds, fluorine atoms replace the hydrogen atoms in polyethylene, causing the carbon carbon chain to gradually twist from the planar and fully extended tortuous conformation of polyethylene (see Figure 3) to the helical conformation (see Figure 4).
Figure 3: The planar and fully extended tortuous conformation of polyethylene
The helical conformation surrounds the carbon chain skeleton of polytetrafluoroethylene, which is susceptible to chemical attack, forming a tight and completely fluorinated protective layer. This makes the main chain of polytetrafluoroethylene less susceptible to any external reagents compared to polyethylene, and it has a unique helical conformation.
From the molecular structure of polytetrafluoroethylene mentioned above, it can be inferred that it is polymerized from tetrafluoroethylene monomer. Tetrafluoroethylene monomer is composed of four fluorine atoms symmetrically arranged on two carbon atoms, and each fluorine atom connected to the carbon atom is completely symmetrical.
According to Rutherford's atomic planetary structure model, atomic orbits are equivalent to determined orbits in classical mechanics, just like the Earth revolves around the Sun, with electrons moving uniformly in a circular motion around the atomic nucleus. Fluorine atoms are the smallest atomic radius apart from hydrogen atoms. The first outer orbital of fluorine atoms has 2 electrons, and the outermost second orbital has 7 electrons. In chemical reactions, this type of atom easily gains 1 electron, thus achieving a stable structure. Therefore, fluorine atoms have extremely strong electronegativity and can obtain electrons from other elements. The second orbital of a carbon nucleus has four free electrons, and its ability to lose electrons (reducibility) is equivalent to its ability to gain electrons (oxidizability). Moreover, due to the fact that fluorine and carbon atoms only have two electron layers, their reactivity is stronger than that of their counterparts such as silicon, germanium, tin, and lead. The schematic diagram of fluorine atoms and carbon atoms is shown in Figure 5.
How do carbon atoms form covalent bonds with fluorine atoms? Taking the simplest structure of methane (CH4) as an example, when all hydrogen atoms in methane are replaced by fluorine atoms, the four fluorine atoms form covalent electron pairs with the outermost four electrons of the carbon atom (see Figure 6), thus achieving a stable structure.
When fluorine atoms form covalent bonds with carbon atoms, the electrons in the outermost s orbital of the carbon atom transition to p orbitals, and the electron cloud of one s orbital and three p orbitals "mixes" to form four S P3 hybrid orbitals with bond angles of 109.5 °, located at the four vertices of the tetrahedron, forming a tetrahedral structure (see Figure 7 below). The shared electrons provided by carbon atoms enter the second orbital of fluorine atoms, forming a C-F σ bond with extremely strong bond energy (see Figure 8). So the bond length of C-F bond (139pm) is short, the bond energy is high (389.303KJ/mo), and the electronegativity is high (relative electronegativity is 4.0).
This quantum mechanical characterization of C-F bonds is very strong, not easily broken, and gives polytetrafluoroethylene outstanding chemical properties: good stability, heat resistance, corrosion resistance, acid and alkali resistance, biological inertness, and anti adhesion are all superior.
After understanding the secrets of polytetrafluoroethylene, let's explore the various special properties of expanded polytetrafluoroethylene, such as surface non adhesiveness, excellent chemical properties, mechanical properties, thermal properties, etc., and provide examples of its practical applications in many fields.
Surface non sticky
Expanded polytetrafluoroethylene has outstanding non stick properties and is an excellent anti stick material. The water contact angle of the swollen body is about 132 °, which is the largest among various materials. According to the Young's equation, the wettability of material interfaces can be determined based on the water contact angle. When the water contact angle is 0 °, water completely wets the solid surface; When the contact angle of water is greater than 90 °, the water partially wets the solid surface and does not wet it; When the contact angle of water is 180 °, water only has point contact with the solid surface and is in a completely non wetting state. For the same liquid, the larger the contact angle, the less wetting the material will be.
Chemical properties
The two fluorine atoms connected to each carbon atom in expanded polytetrafluoroethylene are completely symmetrical, and the carbon and fluorine atoms are covalently bonded together, so there are no free electrons in the molecule, and the entire molecule is electrically neutral, making it a completely non-polar polymer. Therefore, the swollen body does not conduct electricity and has good electrical insulation properties. There are no hydrophilic or photosensitive groups in its macromolecules, so it has good moisture resistance and weather resistance. The bond energy of C-F bonds in polytetrafluoroethylene molecules is high and stable, and the van der Waals radius of fluorine atoms is larger than that of hydrogen atoms. The substitution of fluorine atoms causes polytetrafluoroethylene to form a helical structure, and fluorine atoms form a cylindrical shell. It is precisely because of the inert spiral perfluorinated "shell" and the non-polar and crystalline structure of the polymer itself that the expanded body has excellent chemical corrosion resistance. All strong acids, bases, oxidants, and salts have no effect on the expanded body, even under heating conditions, and are stable in boiling aqua regia.
mechanical property
The tensile strength of expanded polytetrafluoroethylene is generally between 10-30MPa, and the expanded material has excellent fatigue resistance. Unlike other plastics, the expanded material does not undergo permanent fatigue damage. Even if it is damaged due to fatigue, it can still maintain its physical integrity and maintain a "residual" fatigue strength. Therefore, the mechanical properties of the swollen body also have a certain degree of stability.
Thermal Properties
Due to the shielding effect of fluorine atoms on skeleton carbon atoms and the high bond energy of F-C bonds, expanded polytetrafluoroethylene has excellent high and low temperature resistance, with a wide working temperature range of about -200 ° C~260 ° C for a long time.
It is precisely because of its non adhesiveness, chemical resistance, aging resistance, high temperature resistance, and low temperature resistance that PTFE is known as the 'king of plastics' and widely used in fields such as chemical, textile, medical, mechanical, construction, and aerospace.
1. Used as a sealing material
Expanded polytetrafluoroethylene, as a rapidly developing fluorine material in recent years, has great applications in the sealing industry. Traditional sealing materials are mainly rubber. Although rubber has good compressibility, its temperature resistance and aging resistance are poor. Once aged, it will lose elasticity and crack, losing its sealing function. Compared with traditional sealing materials, e-PTFE material has a lower density and lighter weight. At the same time, due to its wide temperature range, it has good aging resistance, creep resistance, and corrosion resistance, making construction and maintenance easier and more convenient.
Chemical industry: Expanded polytetrafluoroethylene is used as a sealing material for pipelines, valves, storage tanks and other equipment, which can prevent liquid and gas leakage; The application of emission pipelines in the semiconductor production field has become large-scale;
Transportation industry: Expanded polytetrafluoroethylene is used as a sealing ring and gasket in automotive braking systems, which can effectively reduce friction and noise, and improve braking performance; It is also widely used for sealing key equipment on high-speed trains and airplanes.
2. Used as insulation material
Electronics industry: Expanded polytetrafluoroethylene, as an insulation material for wires and cables, has good high temperature resistance and electrical insulation performance, and can provide safe and reliable power transmission.
3. Used as medical material
Expanded polytetrafluoroethylene has good biocompatibility and chemical inertness. Human tissue cells can grow into its micropores and form tissue connections, just like autologous tissue. At the same time, due to the non adhesiveness of the swelling material itself, it is difficult to adhere to human blood tissue cells. It has been successfully applied in various medical products such as artificial blood vessels, medical sutures, scaffolds, and facial implants.
4. Used as textile fabric
Expanded polytetrafluoroethylene exhibits strong hydrophobicity and is not easily wetted by water; On the other hand, the pore diameter of the expanded film is extremely small (about 2um), which is about 5000 times the diameter of water vapor molecules and 1/200 of the diameter of small water droplet molecules. Water droplets cannot pass through the film, but water vapor can pass through. Therefore, expanded fabric is waterproof and breathable, making it the preferred fabric for outdoor sports clothing. It can be waterproof and wind resistant, and can timely discharge sweat secreted by human skin, with excellent comfort and practicality.
Expanded polytetrafluoroethylene, as a new material, has continuously improved its comprehensive performance and expanded its application range. I believe that with the continuous in-depth research on expanded polytetrafluoroethylene, its application prospects will become increasingly broad.
