Compton profiles of polytetrafluoroethylene (PTFE) at room temperature
and at higher temperatures up to and above the melting point are repo
rted. For the first time, one has experimental evidence that thermally
induced conformational excitations of the polymer chains are accompan
ied by a charge transfer, related to the sigma- and pi-molecular orbit
als (MO) of the C-F bond. It is argued that localization of the pi-MO
related to the fluorine atom facilitates the translational movements o
f the chains, while the weakening of the C-F bond and the transition f
rom a crystalline to an amorphous phase are associated with delocaliza
tion of sigma-MOs. [S0031-9007(98)06632-0].