A way to understand the impact of fluorine is to explore the differences between linear polyethylene (PE) and PTFE which is the ultimate fluoropolymer in terms of properties and characteristics.
There are important differences between properties of PE and that of PTFE:
- * PTFE is one of the lowest surface energy polymers
- * PTFE is the most chemically resistant polymer
- * PTFE is one of the most thermally stable polymers
- * Melting point and specific gravity of PTFE are more than double those of polyethylene
PTFE and PE differences are attributable to the differences of CeF and CeH bonds. The differences in the electronic properties and sizes of F and H lead to the following observations:
- * F is the most electronegative of all elements (4 Paulings)
- * F has unshared electron pairs
- * F is more easily converted to Fe
- * Bond strength of CeF is higher than CeH
- * F is larger than H
The electronegativity of carbon at 2.5 Paulings is somewhat higher than that of hydrogen and lower than the electronegativity of fluorine. Consequently, the polarity of the CeF bond is opposite to that of the CeH bond, and the CeF bond is more highly polarized. In the CeF bond, the fluorine end of the bond is negatively charged compared to the CeH bond in which the carbon is negatively charged.
The difference in bond polarity of CeH and CeF affects the relative stability of the conformations of the two polymer chains. Crystallization of polyethylene takes place in a planar and trans conformation. PTFE can be forced into such a conformation at extremely high pressure. PTFE, below 19 C, crystallizes as a helix with 0.169 nm per repeat distance: it takes 13 C atoms for a 180 turn to be completed. Above 19 C, the repeat distance increases to 0.195 nm which means that 15 carbon atoms are required for a 180 turn. At above 19 C the chains are capable of angular displacement, which increases above 30 C until reaching melting point (327 C).
Substitution of F for H in the CeH bond substantially increases the bond strength from 99.5 kcal/mole for the CeH bond to 116 kcal/mole for the CeF bond. Consequently, thermal stability and chemical resistance of PTFE is higher than PE because more energy is required to break the CeF bond. The polarity and strength of the CeF bond render F atom abstraction mechanism for branching difficult. In contrast, highly branched polyethylene (>8 branches per 100 carbon atoms) can be synthesized. Branching mechanism as a tool to adjust crystallinity is not practical for PTFE. Instead comonomers with pendent groups have to be polymerized with TFE.
Crystallinity of never-melted PTFE is in the range of 92e98%, consistent with an unbranched chain structure. FEP, a copolymer of TFE and HFP, has an as-polymerized crystallinity of 40e50%. In FEP, the pendent CF3 group is bonded to a tertiary carbon that is less thermally stable than primary and secondary carbon atoms. Degradation curves indicate degradation onset temperatures of 300 C for FEP (0.02% weight loss) and 425 C for PTFE (0.03% weight loss).
Post time: Sep-25-2020