Influence of growth kinetics on crystal morphology of polylactide stereocomplex in ultrathin films

Atomic force microscopy (AFM) and optical microscopy (OM) were utilized to study the growth kinetics and morphology of the flat-on stereocomplex lamellae in poly(d-lactide)/poly(l-lactide) (PDLA/PLLA) blend ultrathin films at various crystallization temperatures (T_c) and mass ratios. As the T_c increases from 170 to 195 °C, the crystal morphology of PLA stereocomplex (SC-PLA) lamellae transitions from dendrites with curved main and side branches to triangular crystals without curvature, accompanied by a decrease in the early growth rate (G_e). Similarly, increasing the mass ratio of PDLA to PLLA from 1.5:1 to 9:1 results in a decrease in G_e, while the curvature radius of the main branch (ρ_b) increase. By measuring the width (w_d) of the depletion zone ahead of the growing lamellae, we made an intriguing observation that ρ_b correlates with w_d, through a power law relationship: ρ_b ∼ w_d^α (α = 1–2, depending on the molecular weight). Furthermore, w_d increases over time at higher T_c, leading to a non-linear growth of crystals (G_e continuously decays with time), confirmed a diffusion-controlled growth mechanism. Concurrently, the thickness of stereocomplex lamellar crystals begins to expand as growth rate diminishes to approximately 40 % of G_e. This indicates that the non-equilibrium growth kinetics is the primary factor driving the changes in ρ_b and thickness of main branches of SC-PLA dendritic crystals within ultrathin films.