The role of surface-induced ordering in the crystallisation of PET films

The molecular configuration and long-range order at the surface of spin-cast poly(ethylene terephthalate) (PET) thin films spin-cast onto an etched $\chem{Si(001)}$ surface during crystallisation from the amorphous state has been investigated for a range of annealing temperatures. The structural ordering at the surface of the film was compared directly with that in the bulk of the film by employing grazing-incidence X-ray diffraction with incident angles below, at and above the critical angle for total reflection. Ordering was observed at the surface at annealing temperatures of 80–95$\un{{}^{\circ}C}$, lower than that in the bulk, involving local parallel alignment of chains in the plane of the surface with the benzene rings oriented preferentially with the ring plane parallel to the surface. Comparison with atomic-force microscopy images enables the fold configuration at the surface to be deduced.