LAUSR

Abstract Crystallization of polymer film is a widely concerned condensed matter phenomenon directly determining the film properties and performances. However, challenging on understanding of the crystal growth process of polymer thin film still remains. Here, we developed an in-situ high-pressure observing system to record the crystal growth process of Poly( l -Lactic Acid) (PLLA) film in pressurized CO2 environment with different film thicknesses, molecular weights, temperatures and CO2 pressures. The preliminary stage of morphological evolution of snowflake-shaped crystal was recorded for the first time. Based on the morphological feature and microscopic structure, we found that the snowflake-shaped crystal with six main branches is constructed by multilayer rhombic terrace crystal originated from screw dislocation, and the six main branches are not formed at the same time. Two of them are preferentially formed on the long-axis directions of rhombic crystal, while the other four main branches are subsequently formed. The subsequently formed branches root from twin crystal induced from self-generated concentration field. The growth process of snowflake-shaped PLLA crystal is quite different from that of snowflake-shaped H2O crystal, but still meets the self-similarity principle and iterative generation principle of the fractal theory.