A High Number of Adsorption Sites per Polymer Chain Promotes Nucleation and Growth of Edge-on Lamellar Crystals in Ultrathin Films of Poly( l -lactic acid)

Abstract

Using atomic force microscopy, we investigated crystallization in equilibrated layers of adsorbed poly(l-lactic acid) (PLLA) chains on silicon wafers. The resulting lamellar crystals exhibited a predominant edge-on orientation. With increasing molecular weight, this tendency became even more pronounced. Complementary studies on gradually annealed spin-coated films revealed an increasing number NAS of adsorption sites per polymer chain, related to hydrogen bonds between PLLA carbonyl groups and –OH on substrate, which, in turn, caused a progressive shift from a flat-on-dominated to an edge-on-dominated crystalline morphology. Identifying NAS as the key parameter, the underlying mechanisms for preferential formation of edge-on lamellae in equilibrated layers of adsorbed PLLA chains could be recognized: due to multiple adsorption sites per chain, the PLLA chains are anchored strongly to the substrate and their mobility is significantly limited. The intervening nonanchored loop segments are confined within a nanoscopic layer and adopt a preferential orientation parallel to the substrate. Nucleation and growth of edge-on lamellae are kinetically favored because adsorbed chains can be incorporated easily into the crystal lattice through small and predominantly local conformational adjustments. Accordingly, the preferential in-plane orientation of chains is largely preserved, yielding a prevalence of edge-on lamellae scaling with NAS. The link between NAS and the resulting mesoscale crystalline morphology provides fundamental insight into crystallization of polymer chains under nanoconfinement as a function of their adsorption strength characterized by NAS.