On crystallization of bisphenol-A polycarbonate thin films upon annealing

•Fractional free-volume in PC film decreased of early stage of annealing.•Crystallization of PC film on Si substrate occurred after annealed for ∼48 hours.•Fractional free-volume in PC film increased remarkably before crystallization.•Positron diffusion length and S parameter revealed the variation of free volumes.

Crystallization of polycarbonate (PC) films as a function of annealing time has been investigated by various methods. A distinct diffraction peak at 17.56°, a sharp decrease of film thickness, an increase of refractive index and branch-type structures on the surface are found merely for the film after crystallization. Interestingly, positron annihilation parameters demonstrate fractional free-volumes in PC films vary significantly not only before crystallization but also at the early stage of annealing, which are not found by other methods. The results show that free-volumes in PC film must be increased remarkably before crystallization, which enables the occurrence of molecule rearrangement.