Average size of cooperative rearranging regions and fragility in a drawn poly(ethylene terephthalate) at the glass transition

Amorphous films of poly(ethylene terephthalate) were uniaxially drawn at temperature above the glass transition for different draw ratio λ in order to study samples with various structural states. When λ is lower than 2, the films remain practically amorphous while for higher λ values, a strain induced crystallization takes place and reaches a maximum. In regard to the Strong Fragile concept proposed by C.A. Angell, the different materials show a behaviour breaking when the draw ratio λ is greater than 3. Indeed, the weakly drawn samples show a fragile behavior, while the highly drawn samples show a strong one. Moreover, the determination of the average size of the Cooperative Rearranging Regions (CRR) proves the crystallite presence drastically reduces the CRR size in the remaining amorphous phase. This is attributed to an amorphous phase confinement effect by the crystallites and the rigid amorphous fraction.