Thickness of the Surface Mobile Layer with Accelerated Crystallization Kinetics in Poly(ethylene terephthalate) Films: Direct Measurement and Analysis

Ultrathin polymer films exhibit a liquidlike mobile surface layer near the free surface, which plays an important role in the size-dependent physical properties of polymer nanomaterials. Most research has focused on the thickness of the surface layer related to segmental relaxation, while few studies have focused on that related to large-scale segmental rearrangement, such as crystallization and diffusion. In this paper, a simple measurement of the surface mobile layer thickness with accelerated crystallization kinetics in a poly­(ethylene terephthalate) film was performed by examining the stepwise crystallization behavior. The thickness of the surface crystallization layer (<i>h</i>_<i>s</i>^cry) was determined by ellipsometry or X-ray reflection and was taken as the surface mobile layer thickness. <i>h</i>_<i>s</i>^cry was observed to increase from 4 to 13 nm when the temperature increased from 343 to 373 K without any dependence on the molecular weight or film thickness, which fits well with the theoretical prediction by the cooperative strings model