Nonmonotonic Variation of the Preferential Orientation of Lamellar Crystals in Films of Cold-Crystallized Poly(l-lactide) of Increasing Thickness and Molecular Weight

Using infrared reflection–absorption spectroscopy and atomic force microscopy, we revealed that polymer–substrate interactions, chain length, and film thickness critically influenced the morphology and orientation of lamellar crystals of poly(l-lactide) (PLLA) grown at a constant temperature. In contrast to the often-observed monotonic change from preferentially flat-on to edge-on crystals upon increasing film thickness, an unprecedented nonmonotonic change in the dominating orientation of lamellar crystals from edge-on to flat-on and back to edge-on was observed. In the thinnest films, size confinement and adsorption-induced alignment of the chains favored the nucleation and growth of edge-on lamellar crystals. Enhanced polymer–substrate interactions promoted the formation of edge-on lamellar crystals in the thinnest films with a higher molecular weight. Upon increasing film thickness, cross-nucleation of flat-on lamellar crystals at the fold surfaces of first nucleated edge-on lamellar crystals, enhanced by a difference in growth rate caused by transport of the chain confined in the plane of the substrate, progressively favored the formation of flat-on lamellar crystals. Intriguingly, in the thickest films, the high mobility of chains at the free surface, preferentially aligned parallel to the surface, caused an increase in the number density of edge-on lamellar crystals. With increasing Mw, the thickness window sustaining lamellar crystals with a flat-on orientation narrowed. Our findings revealed the combined influence of substrate interactions, confinement of polymer transport, and chain length on the lamellar orientation of PLLA films. Together, these parameters control the observed nonmonotonic change in preferential orientation of lamellar crystals of PLLA in films of increasing thickness.