Three specimens from a solution-cast poly （ethylene terephthalate） （PET） film

one being liquid-N

2

quenched from 92℃（Q）

one being slowly cooled from 92℃（SC） and one being quenched and sub-T

g

annealed at 67℃（AN）

have been studied by specimen difference spectra Q-SC and AN-Q and temperature difference spectra T-70 and T

2

-T

1

for every 2℃ steps on heating to 90℃ at 2℃ /min. SC and AN showed more gauche conformers than Q. That means that the PET chain has more trans conformers at higher temperatures and some of these are frozen during quenching through T

g

. A band at 1340 cm

-1

has been found to be complex containing overlapping bands reflecting trans in crystalline regions and trans in amorphous regions. The temperature difference spectra on heating through T

g

showed that the spectral changes in Q are gradual while a rather abrupt change occurs in AN at 80—82℃ for the bands at 1340

1042 and 1020 cm

-1

. No new conformational structure or new vibrational mode is involved. A kind of locking mechanism is suggested which hinders the molecular vibrational changes in AN below T

g

until a sudden release occurs at T

g

. These locking sites can be nothing else than sites of tighter local packing of chain segments. Consequently it is believed that inter-chain van der Waals attraction energy plays a dominating role in the volume relaxation and sub-T

g

annealing of quenched amorphous polymers.