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 (Mw) 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 Mw or decreasing film thickness (h). These phenomena were mainly attributed to the difference in chain conformation for various Mw 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.