The first is because its melting point is about 327 degrees Celsius, which is much higher than that of PE (~130) and PVDF (~177) with similar structures. The key factor that determines the melting point is the force between the chain and the chain molecules. , although fluorine has high electronegativity, but from the chemical structure of PTFE, the dipole moments are canceled each other, so it is a non-polar substance and does not produce obvious orientation force, which means that the dispersion force is the main effect Therefore, it is reasonable to say that the melting point of PTFE should not be so much higher than that of PE, but the high electron density of fluorine makes these closely arranged fluorine atoms have mutual repulsion and steric hindrance effect, so that the molecular chain conformation presents a spirochete structure, rather than the flat zigzag pattern common in saturated polymers, which facilitates the more efficient stacking of molecular chains to form crystals, and the reduction in the separation distance between the stacked chains multiplies the intermolecular forces and thus increases the melting point.
The second is the stability of PTFE at high temperature. For polymers, one of the main reasons for the degradation of performance and toxicity is the decomposition of bonds into small molecules. Since the C-F bonds are very stable, the C-C bonds on the main chain phase Relatively speaking, it is more prone to breakage, but due to the existence of C-F, it produces a hyperconjugation effect on its nearby C-C (the anti-bonding orbital of C-F and the bonding orbital interaction of C-C), which also improves the main chain's resistance to high temperature/light. At the same time, the hyperconjugation effect brings about the lower potential energy of the ortho position (gauche) than the anti position (anti), which also explains why although there is electrostatic repulsion between fluorines, they are still affected by the potential energy. The advantages of fluorine atoms are offset so that the fluorine atoms can be arranged very closely in the PTFE.
Those closely arranged fluorine atoms have a complete octahedral structure, so they are not good electron donors or electron acceptors. They tightly wrap the relatively fragile main chain in the middle, like forming a protective film. Resistant to chemical attack, giving PTFE excellent chemical resistance. Another consequence of the tight arrangement is the low surface energy, so that PTFE does not have much attraction to foreign molecules, which is why it is not sticky.
Post time: Aug-29-2022