Lamellar orientation of polyamide 6 thin film crystallization on solid substrates

We studied the lamellar orientation of polyamide 6 (PA6) thin films with the thickness ranged from ∼5 μm to ∼20 nm on different solid substrates. The lamellar morphology was observed using atomic force microscopy and transmission electron microscopy, and the lamellar orientation was characterized using two-dimensional (2D) grazing incidence wide-angle X-ray diffraction (GI-WAXD) and selected area electron diffraction (SAED). The 2D GI-WAXD results indicate that in the films thicker than ∼50 nm PA6 lamellae of the α-form are preferentially edge-on with the ab-planes (b is the chain axis) parallel to the substrate. Decrease of film thickness leads to higher order of the orientation. This edge-on orientation is insensitive to the surface properties of the substrates changed from hydrophilic to hydrophobic, suggesting that the low energy hydrogen bonded sheets of PA6 are prone to lying down in the film. For the films on the hydrophilic substrate with thickness less than ∼50 nm, the edge-on orientation remains. On the other hand, on a more hydrophobic substrate, flat-on lamellae can be detected by SAED when the film thickness is reduced to be less than 50 nm. An unexpected result is that in the ∼30 nm-thick thin films the strong confinement effect can result in flat-on lamellae of PA6ʹs γ-form, although the crystallization temperature applied did not favor the γ-form formation. We consider that in comparison with that of the α-form lamellae with a lath-like shape, the growth of more symmetrical lamellae of the γ-form with large lateral dimensions can reduce more the free energy of the very thin film.