Nanoconfinement Dominates the Formation of Edge-On Lamellar Crystals in Ultrathin Poly(l-lactide) Films

The crystallization morphologies and orientations of poly(l-lactide) (PLLA) in ultrathin films supported on hydroxylation-modified gold-coated substrates with molecular weights (M w) between 71 and 2100 kg/mol were investigated by atomic force microscopy and infrared reflection absorption spectroscopy. The results show that the predominant orientation of the crystals in the ultrathin PLLA films varies from flat-on to edge-on with increasing M w or decreasing film thickness (h). These phenomena were mainly attributed to the difference in chain conformation for various M w and h, which was caused by nanoconfinement. With the enhancement of the nanoconfinement, the molecular chains prefer to line up parallel to the substrate as prepared by spin-coating and have a stronger memory effect to resist the variation of chain conformation during annealing. Therefore, the crystals with kinetically favored edge-on orientation were easier to be formed. These results provide a new insight into controlling the lamellar orientation in thin crystalline polymer films.