Lamellar crystal orientations biased by crystallization kinetics in polymer thin films

We report dynamic Monte Carlo simulations of polymer crystallization confined in thin films of thicknesses comparable to the polymer-coil sizes. We considered two contrasting affinities of the walls to the polymers, namely sticky walls that arrest the movement of polymers in contact with the substrate (such adsorbed layers allow to avoid dewetting) and slippery walls reflecting neutral repulsion of polymers. We found that at high temperatures slippery walls slightly enhance the crystallization rate with the decrease of film thickness, and the surface-assisted crystal nucleation results in dominant edge-on lamellar crystals (chain axis parallel to the wall); on the contrary, sticky walls significantly depress the crystallization rate, and the random crystal nucleation yields preferentially flat-on lamellar crystals (chain axis normal to the wall). The growth of self-seeded crystals demonstrates that the flat-on dominance is a kinetic phenomenon due to a stronger restriction on the thickening growth of edge-on lamellar crystals.