Glass Transition Temperature Determination of Poly(ethylene terephthalate) Thin Films Using Reflection−Absorption FTIR

By using the in-situ reflection−absorption infrared technique, the glass transition temperatures of poly(ethylene terephthalate) thin films were determined by detecting the abrupt change in the absorbance ratio of two comparable bands at 1340 cm -1 (trans conformation) and 1370 cm -1 (gauche conformation) during heating. It was found that the T g of thin PET films reduces with respect to that of bulk sample and has a great dependence on the film thickness. The conformational energy difference (Δ E ) between trans and gauche states of each film was also calculated. The results indicate that the thickness dependence of calculated Δ E is nearly consistent with that of T g measured. Furthermore, the relaxation behavior of the PET chains in thin films prepared by spin-coating and the influence of annealing on the T g measured were also investigated.