Direct Observation of Early-Stage Polymer Crystallization Driven by Surface Wrinkling and Compressive Stress in Thin Films

The early stages of crystallization and occurrence of surface wrinkling were investigated using poly(butadiene)-block-poly(ε-caprolactone) with an ordered lamellar structure. Direct evidence has demonstrated that surface wrinkling precedes nucleation and crystal growth. This study examined the relationship between surface wrinkling, nucleation, and the formation of crystalline supramolecular structures using atomic force microscopy (AFM) and X-ray scattering measurements. Surface wrinkling is attributed to curving induced by accumulated stresses, including residual stress from the sample preparation and thermal stress during cooling. These stresses cause large-scale material flow and corresponding changes in the molecular conformations, potentially reducing the nucleation barrier. This hypothesis is supported by the rapid crystal growth observed following the spread of surface wrinkles. Additionally, the surface curving of the polymer thin film creates local minima of the free energy, facilitating nucleation. The nuclei subsequently grow into crystalline supramolecular structures by incorporating polymer molecules from the melt. This mechanism highlights the role of localized structural inhomogeneity in the early stages of crystallization and provides new insights into structure formation processes.